Iron Catalysis in Reduction and Hydrometalation Reactions

Review pubs.acs.org/CR

Cite This: Chem. Rev. XXXX, XXX, XXX−XXX

Iron Catalysis in Reduction and Hydrometalation Reactions Duo Wei and Christophe Darcel*

Downloaded via KAROLINSKA INST on January 23, 2019 at 15:06:13 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.

Univ Rennes, CNRS, ISCR-UMR 6226, F-35000 Rennes, France ABSTRACT: The last two decades have seen an impressive improvement of the use of iron as a fascinating and valuable alternative transition metal in homogeneous catalysis in terms of sustainability and economy. It was efficiently used in catalytic organic synthetic transformations, which in particular include the reduction of unsaturated bonds. This review summarizes the fast development and the recent advances in selective reductions of olefins, alkynes, carbonyl and carboxylic derivatives, imines, and nitro compounds promoted by iron catalysts. The topical hydrogen-borrowing reactions and hydroboration of unsaturated compounds are also reported. It is hoped that this account not only provides an overview of the state of the art in iron catalysis but also stimulates the development of superior greener catalytic systems in the near future.

CONTENTS 1. Introduction 2. Hydrogenation and Transfer Hydrogenation 2.1. Alkynes and Alkenes 2.2. Carbonyl Derivatives 2.2.1. Hydrogenation of Aldehydes and Ketones 2.2.2. Transfer Hydrogenation of Aldehydes and Ketones 2.3. Imines, Nitro Derivatives, and Reductive Amination of Carbonyl Compounds 2.3.1. Imines 2.3.2. Nitro Derivatives 2.3.3. Direct Reductive Amination (DRA) of Carbonyl Compounds 2.4. Carboxylic Acid Derivatives and Carbon Dioxide 2.4.1. Amides 2.4.2. Nitriles 2.4.3. Carboxylic Esters 2.4.4. Carbon Dioxide and Carbonates 3. Hydrosilylation 3.1. Alkynes and Alkenes 3.2. Aldehydes and Ketones 3.3. Imines, Nitro Derivatives 3.3.1. Imines 3.3.2. Nitro Derivatives 3.4. Hydrosilylation of Carboxylic Acid Derivatives and Carbon Dioxide 3.4.1. Carboxamides 3.4.2. Nitriles 3.4.3. Carboxylic Esters 3.4.4. Carboxylic Acids 3.4.5. Ureas 3.4.6. Carbon Dioxide and Formic Acid 4. Reduction of Sulfoxides © XXXX American Chemical Society

5. Hydrogen-Borrowing Reactions 6. Hydroboration 6.1. Hydroboration of Alkenes and Alkynes 6.2. Dehydrogenative Borylation 6.3. Carbonyl Derivatives 6.4. Carbon Dioxide 7. Conclusion Author Information Corresponding Author ORCID Notes Biographies Acknowledgments Abbreviations References

A B B E E J O O Q Q R R R S T W W AA AH AH AI

AO AQ AQ AU AU AV AV AW AW AW AW AW AW AW AX

1. INTRODUCTION The development of more efficient and sustainable methodologies for the construction of diversely functionalized molecules using homogeneous transition-metal catalysis is nowadays a cornerstone in green chemistry, mainly explained by the outstanding regio-, chemo-, and stereoselectivity observed. Indeed, regarding the current important concerns about climate changes and the associated green chemistry principles, the replacement of noble transition metals by more benign ones, such as the first-row transition metals, is absolutely required and is definitively one of the important challenges of the beginning of this millennium. Thus, the beginning of the 21st century has witnessed interest in the use of iron in homogeneous catalysis which is a highly valuable

AJ AJ AJ AK AL AN AN AN

Special Issue: First Row Metals and Catalysis Received: June 14, 2018

A

DOI: 10.1021/acs.chemrev.8b00372 Chem. Rev. XXXX, XXX, XXX−XXX

Chemical Reviews

Review

alternative to classical precious metals such as platinum, rhodium, or palladium for catalyzing a wide range of organic transformations, including the efficient and chemoselective reduction processes and hydrofunctionalization of unsaturated C−C or C−heteroatom bonds.1−3 Although this area of research is now well established, it is amazing that until recently there were only scarce examples of large-scale applications of iron catalysts, such as the classical Fischer− Tropsch and Haber−Bosch processes. Thus, the development of new efficient catalytic systems is still attractive and necessary for both academic and industrial applications. Several reviews have treated iron-catalyzed transformations4−12 and reduction1,2,13−17 in the past decade. In addition, numerous reviews, book chapters, and accounts have been published focusing on thematic aspects of iron catalysis such as hydrogenation18−33 and hydrofunctionalization.34−37 This review focuses on the fast and impressive improvement of numerous iron-catalyzed selective transformations from the early examples through the most recent reports of the beginning of 2018 in the areas of hydrogenation, transfer hydrogenation, and hydrosilylation of various unsaturated compounds (olefins, alkynes, carbonyl and carboxylic derivatives, imines, nitro, sulfoxides). Following an overview of the iron-catalyzed hydrogen-borrowing reactions and hydroboration is discussed.

Scheme 1. Selective Iron-Catalyzed Hydrogenation of Alkynes to Alkenes

protonolysis. Additionally, the catalytic reductive dimerization of HCCSiMe3 to 1,4-bis(trimethylsilyl) butadiene prevails over alkene formation at reflux temperature. In 2012, Beller et al. reported a similar active catalytic system in-situ generated from Fe(BF4)2·6H2O and the ligand L1 to perform the selective catalytic transfer hydrogenation of terminal arylacetylenes to styrenes by hydrogen transfer using formic acid as the hydrogen donor under base-free conditions at 40 °C for 5 h (Scheme 1). 52 A huge scope of phenylacetylenes was selectively reduced to the corresponding styrene derivatives, and impressive chemoselectivities can be obtained starting with arylethynes bearing reducible functional groups such as ketones or esters. This catalytic system was also used for the reduction of heteroarylethynes and aliphatic terminal alkynes but failed for the reduction of internal alkynes. Noticeably, it was shown that the catalyst C6 was efficient for the chemoselective reduction of α,β-unsaturated ketones leading to the corresponding saturated ketones in hydrogen transfer conditions using secondary alcohols, such as cyclopentanol, as the hydrogen donor (Scheme 2).53 Similarly,

2. HYDROGENATION AND TRANSFER HYDROGENATION 2.1. Alkynes and Alkenes

Numerous pioneering contributions on the hydrogenation of alkenes and alkynes were reported in the early 1960s using iron species as catalysts, including [Fe(CO)5] (C1)38−41 and Fe(acac)3 (C2)42 for the hydrogenation of polyunsaturated oils and fats. However, the main limitations of these reactions were often harsh reaction conditions, the lack of chemoselectivity, and the rather narrow scope of the applications.43−46 In addition to hydrogenation reactions performed with gaseous hydrogen in the appropriate high-pressure apparatus, transfer hydrogenation reactions were conducted using easy-to-handle and cheap liquid hydrogen-donor substitutes such as alcohols and formic acid and are thus helpful alternative procedures for the reduction of unsaturated compounds.47 The first iron-catalyzed transfer hydrogenations of alkenes and alkynes were reported in the early 1970s using [FeBr2(PPh3)2] (C3)48 and [FeCl2(PPh3)2] (C4)48,49 as catalysts for the reduction of 1,5-cyclooctadiene (COD). Noticeably, dihydroxybenzenes such as catechol (catechol = cat) derivatives have been used as hydrogen donors under harsh conditions (160−240 °C) in both protocols, making such procedures less interesting in molecular synthesis. In the early 1990s, Bianchini and co-workers made the first real breakthrough, describing the selective hydrogenation of terminal alkynes to the corresponding alkenes catalyzed by iron(II)−hydride precursors associated with tetraphosphine ligands [(L1)FeH(N2)]BPh4 (C5) and [(L1)FeH(H2)]BPh4 (C6) [L1 = P(CH2CH2PPh2)3] under very mild conditions (THF, room temperature, 1 atm of H2, Scheme 1).50,51 Interestingly, under a mechanistic point of view, the selectivity of the reaction was rationalized by the insertion of the coordinated alkyne into the Fe−H bond giving a Fe−vinyl intermediate, which was then cleaved via an intramolecular

Scheme 2. Iron-Catalyzed Chemoselective TH of α,βUnsaturated Ketones

Bhanage reported in 2008 the same chemoselectivity for the reduction of α,β-unsaturated ketones and esters using a watersoluble catalytic system based on Fe(SO4)2·7H2O and ethylenediaminetetraacetic acid disodium dihydrate (EDTANa2·2H2O) (Scheme 2). Performing the reaction in water at 100 °C under 27.5 bar of hydrogen, the corresponding saturated analogues were obtained in 22−95% GC yields.54 B


Chemical Reviews

Review

Similarly, several in-situ-generated catalysts based on multidentate organophosphorus ligands have been developed in combination with iron to perform such hydrogenation reactions. In 2004, Peters et al. reported a family of tri(phosphino)borate (L2) supported iron(II) alkyl (C7− C8) and iron(IV) trihydride complexes (C9−C10)55 used as precatalysts for the hydrogenation of several nonfunctionalized olefins and 2-pentyne (Scheme 3). Under mild conditions

Scheme 4. Iron Pincer-Type Catalytic Systems for the Hydrogenation of Alkenes

Scheme 3. Iron Complexes Containing Multidentate Organophosphorus Ligands for the Hydrogenation of Alkenes and Alkynes

(room temperature, 1−4 atm of H2), the corresponding alkanes were obtained with TOFs up to 24 h−1. Similarly, the complex C11 (3.33 mol %) achieved efficiently the hydrogenation of alkenes and phenylacetylene conducting to the respective alkanes under ambient conditions (1 atm of H2, room temperature) with TOFs up to 15 h−1 for ethylene.56 In 2004, Chirik made another important breakthrough in this area of research dealing with hydrogenation of alkenes and alkynes developing a series of highly active iron(0, II) complexes bearing NNN, CNC, and PNP tridentate pincertype ligands. Thus, olefins can be efficiently reduced in the presence of only 0.33 mol % of [(iPrPDI)Fe(N2)2] (C12), at 22 °C under 4 atm of H2, exhibiting TOFs up to 1814 h−1 (Scheme 4).57 As a representative example, the hydrogenation of 1-hexene took only 12 min to reach >98% conversion. Noticeably, longer reaction times were required to reduce internal alkenes, and noticeably, only the gem-disubstituted CC bond of (+)-(R)-limonene was selectively hydrogenated. Furthermore, the full reduction of alkynes such as diphenylacetylene to 1,2-diphenylethane can be performed via the earlier formation of cis-stilbene intermediate. Importantly, the authors demonstrated the crucial role of the electron reservoir of the ligand being able to accept up to three electrons from the iron center.57 Huge and interesting functional group tolerance was shown for the hydrogenation of linear olefins bearing reducible functional groups such as ketones, ethers, esters, amides, or unprotected amines using 5.0 mol % of diamidopyridine pincer-type complex (C13) as the catalyst at 23 °C under 4 atm of H2 (Scheme 4). Additionally, α,β-unsaturated esters were selectively reduced to the corresponding saturated esters with TOFs up to 240 h−1, unlike α,β-unsaturated ketones which provoked deactivation of the iron catalyst.58 In 2012, Chirik reported the use of pyridine-di(NHC)-based iron complexes (NHC = N-heterocyclic carbene) such as C14 and C15 for the hydrogenation of the most challenging nonfunctionalized, tri- and tetrasubstituted alkenes such as trans-methyl stilbene, 2-methyl-2-butane, or methylcyclohexene59 (Scheme 4). In particular, complex C14 was shown to

promote the hydrogenation of tetrasubstituted alkenes such as 2,3-dimethyl-1H-indene in moderate conversion (68%) and ratio (3:1 cis/trans diastereomers) after 48 h. 2,6-Diisopropylphenyl (dipp) substituted analog C15 was identified as the best catalyst for the hydrogenation of ethyl 3,3-dimethyl acrylate, giving 95% conversion after 1 h of reaction. The same authors also described the analogous bis(diisopropylphosphino)pyridine pincer-type ligand (iPrPNP) iron(II) hydride complex cis-[(iPrPNP)Fe(H)2N2] (C16)60 for the efficient hydrogenation of 1-hexene and cyclohexene under mild conditions (room temperature, 4 atm H2, Scheme 4). Using a MACHO-type iron complex, [(PNPiPr)Fe(H)(CO)] (PNPiPrN(CH2CH2PiPr2)2) (C17), Jones reported the hydrogenation of alkenes with a polarized CC bond in ambient conditions (23 °C, 1 atm of H2)61 (Scheme 4). Noticeably, para-substituted styrenes with electron-withdrawing groups were more reactive than those with an electrondonating substituent. Furthermore, reducible functionalities such as ester, cyano, and N-heterocycles were tolerated, whereas α,β-unsaturated ketones such as benzylideneacetone led to the fully reduced alcohols. Noticeably, the mechanism operated via a metal−ligand cooperative pathway with first a Fe−H hydride transfer and then a N−H proton transfer from the pincer ligand to form the hydrogenated product. Gade described a high-pin carbazolide-based PNP pincer ligand iron(II) complex (C18, 1.0 mol %) able to catalyze the hydrogenation of alkenes including styrenes, β-methylsyrene, and internal disubstituted alkenes when performing the reduction under 8 bar of H2 at 29 °C for 4−72 h in C6D662 (Scheme 4). By contrast, tetrasubstituted alkenes were not C


Chemical Reviews

Review

reduced. Interestingly, it was shown that the ferrous alkyl complex was converted by hydrogenation into a high-spin iron(II) hydrido dimer complex with two bridging hydrido ligands between the iron(II) centers. Similarly, in 2013, Milstein and co-workers published the semihydrogenation of internal alkynes to E-alkenes using a new iron pincer complex (C19) bearing an acridine-based PNP ligand: using 0.6−4.0 mol % of C19, the semihydrogenation is conducted in THF at 90 °C for 11−72 h under 4−10 atm of H2, with notably the production of small amounts of overreduced alkanes as the byproduct63 (Scheme 5). Interestingly,

Scheme 6. Bidentate Iron Complexes for the Hydrogenation of Olefins

Scheme 5. Iron PNP-Pincer-Type Complex C19 for Catalyzed Selective Semi-Hydrogenation of Alkynes to EAlkenes

alkenes such as 2,3-dimethyl-2-butene and 2,3-dimethyl-1Hindene with TON up to 100. In 2014, Nakazawa described an interesting example of an iron-catalyzed transfer hydrogenation reaction of alkynes in the presence of alcohols using a new series of bifunctional pianostool iron complexes (C24) bearing both a Fe−H and a Si−H bond tethered to the Cp ligand68 (Scheme 7). These iron Scheme 7. Transfer Hydrogenation of Alkynes Utilizing a Bifunctional Piano-Stool Iron Complex

using this catalytic system, functional groups such as esters, nitriles, ketones, and trimethylsilyl were tolerated. It must be also underlined that terminal alkynes such as phenylacetylene were selectively reduced to the corresponding styrene in quantitative yield. Iron complexes bearing bidentate ligands are also able to successfully hydrogenate unfunctionalized alkenes. In 2005, Chirik described a series of iron(0) complexes bearing bidentate α-diimine ligands such as C20,64 which can hydrogenate 1-hexene under 4 atm of H2 at 22 °C with TOF up to 90 h−1 (Scheme 6). In 2017, Jacobi von Wangelin reported a family of similar bis(imino)acenaphthene iron complexes (C21) able to successfully hydrogenate terminal or geminated CC bond under mild conditions (2 bar of H2, 20 °C, 3 h) when activated with 3.0 equiv of n-BuLi65 (Scheme 6). Noticeably, to reduce more challenging tri- and tetrasubstituted CC bonds, harsher conditions have to be applied (10 bar of H2, 80 °C, 16 h). Furthermore, iron complexes bearing diphosphine ligands66 are also able to hydrogenate unfunctionalized alkenes. As a representative example, using [Fe(L)(CH2SiMe3)2] [(C22), L = (R)-1(SP)-2-[di(2-furyl)phosphine]ferrocenyl]ethyl-di-tertbutylphosphine)] as the catalyst (5.0 mol %), under 4 atm of H2 at 23 °C for 24 h, alkenes were reduced to the corresponding alkanes (Scheme 6). Unfortunately, no significant enantioselectivity was detected when prochiral alkenes were used, thus suggesting that the reaction proceeded heterogeneously. Recently, Nagashima described a new disilaferracyclic complex (C23, 0.05−1.0 mol %) able to hydrogenate mono-, di-, tri-, and tetrasubstituted alkenes at 80 °C for 2−8 h under 1−20 bar of H2.67 Notably, C23 can reduce tetrasubstituted

complexes served as catalysts in the reduction of ptolylacetylene leading in the presence of iPrOH to a mixture of p-methylstyrene and p-methylethylbenzene with conversion up to 44% after 24 h at 75 °C. In the same year, Jacobi von Wangelin developed a new heteroatom-free arene−iron(-I) catalyst (C25) based on anthracene ligands for hydrogenation of styrenes and terminal alkenes under 1 bar of H2 at 20 °C for 3 h in toluene. Nevertheless, C25 has a lower activity than the Co analogue (Scheme 8). This homoleptic arene-based catalyst is a nice complement to the use of complexes bearing heteroatom donor ligands for reduction reactions with TOF up to 33 h−1.69,70 Jacobi von Wangelin also described a simple and practical catalytic system in-situ generated from FeCl3 (5.0 mol %) and LiAlH4 (10 mol %) in THF at room temperature for 10 min. This catalyst was then used for the hydrogenation of styrene derivatives (1 bar of H2, 18 °C, 6 h, 26 examples, 18− 99% yields) and allylbenzenes (1 bar of H2, 18 °C, 18 h, 8 examples, 79−99% yields). For the reduction of functional D


Chemical Reviews

Review

decades, there has been increasing interest in an economic and sustainable point of view in the replacement of expensive noble transition-metal catalysts with iron-based ones. In 1983, Markó described the first pioneering report on iron-catalyzed hydrogenation of aldehydes and ketones using 10 mol % of [Fe(CO)5] (C1)79,80 under drastic conditions (triethylamine, at 150 °C under 100 bar of a mixture H2/CO (98.5/1.5)). In 2007, Casey81 reported the use of the Knölker complex C2682 as the catalyst to perform one of the first valuable ironcatalyzed hydrogenations of aldehydes and ketones under mild conditions (3 atm of H2 at room temperature for 1−68 h) with TOF up to 273 h−1 (Scheme 9). Noteworthy, CC and C

Scheme 8. Heteroatom-Free Arene−Iron(-I) Catalyst for Hydrogenation of Olefins

linear alkenes and alkynes, the catalytic system was generated from FeCl3 (5.0 mol %) and LiAlH4 (5.0 mol %) and the reaction conducted at 18 °C for 3 h. Fe(0) homogeneous species and/or nanoparticles were postulated as the active species.71 Particularly worth mentioning is that some scarce reports dealing with Fe−nanoparticles (NPs) catalyzed hydrogenation reactions were published. De Vries reported a catalytic hydrogenation of alkenes and alkynes using Fe NPs72,73 obtained by the protocol developed by Bedford74 via the reduction of a solution of FeCl3 with 3.0 equiv of EtMgCl in THF or Et2O. Thus, terminal, 1,2-cis-, and 1,1-disubstituted alkenes were hydrogenated quantitatively after 15 h at room temperature under 1 atm of H2. To be able to hydrogenate 1,2trans-disubstituted and cyclic cis-alkenes, harsher conditions (100 °C) were applied. Noteworthy, tri- and tetrasubstituted alkenes were not reduced. To achieve the full reduction of alkynes to alkanes, more drastic conditions were required (20 atm of H2 at 25 °C for 15 h). Similarly, in 2015, NPs formed from FeCl3−LiAlH4 (5.0 mol %) were reported to catalyze hydrogenation of styrenes, alkenes, and alkynes under 1 bar of H2.71 Fe NPs supported on chemically derived graphene are also efficient for similar hydrogenations (20 atm of H2, 100 °C, 24 h). Interestingly, the catalyst can easily be separated by simple magnetic decantation.75 Moores and Uozumi et al. developed an amphiphilic, polymer-stabilized Fe(0) NPs catalyst active in the hydrogenation of alkenes, styrenes, and alkynes in ethanol under 20−40 atm of H2 at 80−100 °C in a flow reactor.76 It is important to underline the high chemoselectivity of the reduction as functionalities such as ketones, esters, arenes, and nitro were not altered. Beller and Chaudret also described the use of well-defined ultrasmall iron(0) NPs as catalysts for the selective hydrogenation of various alkenes and alkynes to alkanes under mild conditions (2.4 mol % of NPs, room temperature, 10 bar of H 2 for 20 h). 77 Noticeably, monodisperse iron NPs (about 2 nm size) were prepared by the decomposition of [Fe[N(SiMe3)2]2]2 under 3 bar of H2 at 150 °C for 2 h. Recently, Jacobi von Wangelin reported new Fe low-valent nanocluster architectures generated by reaction of Fe[N(SiMe3)2]2, Dibal-H (2.0 equiv/Fe),78 able to catalyze hydrogenation of a huge variety of olefins including tri- and tetrasubstituted alkenes under mild conditions (catalytic loading 5.0 mol %, 1−4 bar of H2, 20 °C) with TOF up to 600 h−1 for the hydrogenation of 1-octene.

Scheme 9. Knölker-Type Complexes for the Hydrogenation of Ketones

C located remotely from the carbonyl were tolerated, and unsaturated alcohols were selectively obtained, whereas the reduction of α,β-unsaturated ketones led to a mixture of compounds due to the reduction of the CO and/or the C C bond after 6 days. Additionally, C26 can also catalyze the transfer hydrogenation of acetophenone using 2-propanol as the hydrogen donor (87% isolated yield using 1.0 mol % of catalyst at 75 °C in 16 h). Beller et al.83 used a series of air-stable Knölker-type complexes such as C2784−86 able to catalyze the hydrogenation of aldehydes and ketones under 30 atm of H2 at 100 °C in a mixture of iPrOH/water (Scheme 9). It must be underlined that the similar efficiency (TOF up to 217 h−1) and the nice functional group tolerance as esters, amides, and heterocycles remained intact under such conditions. Furthermore, α,βunsaturated aldehydes were selectively reduced to the corresponding allylic alcohols. C27 was also an efficient catalyst for the reduction of aldehydes under water−gas shift conditions [10 atm of CO, DMSO/water, 100 °C,87 or paraformaldehyde (10 equiv), Na2CO3 (3 equiv), DMSO/ water, 120 °C, 24 h].88 Renaud simultaneously designed Knölker-type complexes bearing ionic fragments such as ammonium salts (C28)89 which were active in the catalytic hydrogenation of carbonyl derivatives in water under 10 atm of H2 at 85 °C, even if the efficiency decreased as TOFs up to 6.1 h−1 were observed (Scheme 9). Even if cyano moieties were tolerated, by contrast,

2.2. Carbonyl Derivatives

2.2.1. Hydrogenation of Aldehydes and Ketones. Selective hydrogenation and hydrogen transfer reactions of carbonyl derivatives (aldehydes and ketones) are relevant transformations in both bulk and fine chemistry. For a few E


Chemical Reviews

Review

the hydrogenation of α,β-unsaturated ketones led in the majority of the cases to a mixture of compounds resulting of simultaneous reduction of the CC and CO bonds. Additionally, the hydrogenation of CN bonds of watersoluble imines was also performed in water at 100 °C leading to the corresponding amines with 61−98% isolated yields. Noticeably, in 2018, the same authors have shown that the nature of the catalytic system has a drastic effect on the chemoselectivity of the reduction of α,β-unsaturated ketones into saturated ketones using similar iron complexes. Indeed, using 2 mol % of C30 in isopropanol at 70 °C for 16 h in the presence of 2.5 mol % of Cs2CO3, aromatic unsaturated ketones Ar−CO-CC−Ar led to the corresponding saturated ketones (11 examples, 48−99% yields) (Scheme 10). To promote the reaction with aliphatic unsaturated ketones R− CO−CC−R, the reaction has to be conducted at 90 °C (11 examples including steroidic derivatives, 38−99% yields).90

Scheme 11. Knölker-Type Complexes for the Asymmetric Hydrogenation of Acetophenone

Scheme 10. Knölker-Type Complexes for the Reduction of α,β-Unsaturated Ketones

In order to explain the catalytic transformation, an outersphere mechanism was proposed (Scheme 12).96,97 Scheme 12. Proposed Outer-Sphere Mechanism for Hydrogenation Catalyzed by Knölker-Type Catalysts

Recently, Piarulli, Berkessel, and Gennari reported the preparation of new [bis(hexamethylene)cyclopentadienone] iron tricarbonyl (C29) by the reaction of cyclooctyne with [Fe(CO)5] (C1) and showed that upon in-situ activation with Me3NO (2.0 mol %) C29 (1.0 mol %) promoted the catalytic hydrogenation of various ketones and aldehydes at 70 °C under 30 bar of H2 in a mixture iPrOH/H2O (5:2) (Scheme 9). Noticeably, C29 exhibited better TOF than the Knölker parent complex C27 under similar conditions (35.9 vs 7.5 h−1).91 Interestingly C29 can also reduce activated trifluoromethyl esters as well as catalyze the transfer hydrogenation of ketones. Using chiral Knölker’s iron complexes, asymmetric hydrogenation of ketones can be also efficiently conducted. In 2011 the group of Berkessel designed a chiral catalyst (C31), modifying Casey’s complex by a substituting one of the carbonyl ligands under UV-light irradiation by a chiral phosphoramidite ligand (Scheme 11). Using 10 mol % of C31 under 10 bar of H2 at room temperature, only moderate enantiomeric excesses were obtained (up to 31% ee).92 Chiral information can be also located on the cyclopentadienone ligand. Thus, Wills described a series of complexes derived from C2-symmetric diols, including C32. The active catalytic species in-situ generated from 1.0 mol % of C32 and 1.0 mol % of Me3NO was able to promote the hydrogenation of acetophenone with ee up to 20% when performing the reaction at 80 °C in a mixture of isopropanol/water93 (Scheme 11). The highest asymmetric inductions (up to 77%) so far recorded for ketone reduction bearing Knölker-type catalyst were performed by Pignataro/Piarulli/Gennari using a complex bearing binaphthyl-scaffold-substituted cyclopentadienone ligand C3394,95 (Scheme 11).

On the basis of a concerted hydride/proton transfer from the hydroxyl and the iron hydride to the carbonyl moiety, the species I-1 is obtained. I-1 then leads to I-2 and the alcohol. The 16-electron species I-2 can be also generated from C27 by the labilization of CO. Finally, the catalytic active species C26 is reformed by heterolytic H2 cleavage via I-3. This mechanism was supported by density functional theory (DFT) calculations and kinetic studies.98 In 2008, based on his previous works on the design of ruthenium catalysts devoted to reduction reactions,99 Morris described a series of chiral tetradentate diiminophosphine (PNNP) iron(II) complexes.100 He first showed that the complex [Fe(NCMe)2[(R,R)-cyP2N2]][BF4]2 (C34) (0.44 F


Chemical Reviews

Review

mol %), in combination with 6.7 mol % of KOtBu, catalyzed the hydrogenation of acetophenone leading to 1-phenylethanol with a moderate conversion (40%) and ee (27%) when performing the reaction in iPrOH under 25 atm of H2 after 18 h (TOF = 5 h−1 at 50 °C, Scheme 13).

conversion, TONs up to 1880, and TOF up to 425 h−1 were observed for a huge scope of several aromatic and aliphatic ketones (Scheme 14). Noticeably, inhibition of the reaction was observed with cyano or amino groups, and the reduction of α,β-unsaturated ketones was not chemoselective.

Scheme 13. PNNP Iron(II) Complexes for Hydrogenation Reactions

Scheme 14. Hydrogenation of Ketones Catalyzed by Iron Pincer Complexes

The fine design of the hydrocarbon bridge (N−CHR− CHR−N) of the ligand in chiral iron(II)−PNNP complexes has crucial effects on the activity with notable dependence of the nature of the substituents on the diamine moieties on the activity.101 Using analogous reaction conditions, the complex C35 having an ethylene bridge (R = H) displayed similar moderate catalytic performances (conversion = 70−95%, Scheme 13). Noteworthy, slightly higher activities were observed using the diaminodiphosphino iron complex C35 (conversion = 99%, TOF = 12 h −1) instead of the diiminodiphosphino iron complex C35, both complexes bearing an ethylene bridge. By contrast, when chiral complexes C36 and C37 having more sterically hindered substituents on the bridge (R = iPr or Ph) were used, the activity decreased dramatically (conversion lower than 4%, Scheme 13). Under a mechanistic point of view, both catalysts based on an imine-type ligand (e.g., C35) and an amine-type one (e.g., C38) exhibited similar activity, which suggested that a similar, common active iron hydride intermediate may be involved in an outer-sphere mechanism via a secondary amino moiety of the ligand and the iron hydride (Scheme 13). The tetradentate amine(imine)diphosphine P−NH−N−P iron complex C39 was also active in hydrogenation of arylmethylketones with TOF up to 33 h−1 and ee up to 70% at 50 °C and 20 atm of H2.102,103 In 2011, in a continuation of his previous work on ruthenium−pincer complexes highlighting an original mode of cooperation ligand−metal center involving aromatization− dearomatization of the ligand,104−108 Milstein described an elegant contribution on the hydrogenation of ketones catalyzed by a novel iron(II) diphosphino−pyridine pincer complex [Fe(Br)(H)(CO)(iPrPNP)] (C40).109 The complex C40 exhibited high activity when the reaction was performed at 40 °C under 4.1 atm of hydrogen in ethanol; 39−97%

Milstein et al. also described an iron(II) pincer complex bearing a hydride and a borohydride ligand [Fe(η1-BH4)(H)(CO)(iPrPNP)] (C41) showing catalytic activities comparable to C40 in the hydrogenation of acetophenones, notably with very low catalyst loading (0.05 mol %)110 (Scheme 14). Importantly, the hydrogenation of ketones can be promoted without the use of additional base. The complex C41 is also highly active and exhibited a high TON of 1780 for the reduction of 2-acetylpyridine in ethanol at 40 °C under 4.1 bar of H2. Nevertheless, C41 is less active than the parent complex C40 as the TOF was up to 296 h−1 (vs 425 h−1). Modifying the backbone and using NH instead of CH2 linkers, Kirchner and co-workers developed a series of complexes such as C42, which showed rather good activities for the hydrogenation of aldehydes (TOF up to 120 h−1) and ketones (0.5 mol % cat., 1.0 mol % KOt-Bu, 5 atm of H2, EtOH, room temperature; TOF up to 770 h−1)111,112 (Scheme 14). Interestingly, the analogous complex bearing a NMe spacer (X = NMe) led to a drastic loss of activity in ketone hydrogenation activity but exhibited high activity for the hydrogenation of aldehydes with TON and TOF up to 80 000 and 20 000 h−1, respectively, with a very low loading of catalyst (down to 12.5 ppm).113 Notably, this catalyst permitted one to perform selective reduction of aldehydes in the presence of ketones, epoxides, and other reducible groups. Noticeably, Kirchner reported a supported ionic-liquid-phase (SILP) system containing this analogous complex bearing a NMe spacer used as catalyst in the hydrogenation of aldehydes to alcohols.114 This SILP catalyst was used with low loadings (0.1−0.05 mol %) at 25 °C under 50 bar of hydrogen in heptane in the presence of 5 mol % of DBU exhibiting TONs and TOFs of up to 1000 and 4000 h−1, respectively. No significant leaching of both the complex and the IL was detected. G


Chemical Reviews

Review

The same authors designed a series of chiral unsymmetrical aminodiphosphine PNP iron complexes, including the most selective one (C46) for the hydrogenation of prochiral aryl ketones under mild conditions (0.1 mol % catalyst, 1.0 mol % KOtBu, 5−10 bar of H2, 50 °C for 1−2 h) in ee up to 96% (Scheme 15).121 Furthermore, similar chiral complexes bearing a planar chiral ferrocene scaffold were also reported: using 1.0 mol % of C47 in the presence of 4.0 mol % of KOtBu in isopropanol under 5−10 bar of H2, at room temperature for 1−16 h, secondary alcohols were obtained in 48−96% conversion with ee up to 81%.122 Recently, Mezzetti reported the synthesis of the tridentate Pstereogenic, C2-symmetric PNP pincer ligand (SP,SP)-2,6bis((cyclohexyl-(methyl)phosphanyl)methyl) pyridine and its iron(II) complex C48123 (Scheme 15). In the presence of KOtBu (5.0 mol %), C48 (1.0 mol %) catalyzed the hydrogenation of acetophenone with H2 (50 bar) in EtOH at 20 °C leading to (S)-1-phenylethanol with up to 49% ee. Similarly, he prepared P-stereogenic PN(H)P pincer complexes C49−50 based on [R(Me)P-CH2−CH2)2NH] ligands which exhibited under similar conditions ee up to 44% (Scheme 15).124 To rationalize the effect of such PNP ligands on the high reactivity of the corresponding iron complexes in hydrogenation, a first plausible mechanism via an aromatization− dearomatization process of the diphosphine pyridine ligand, which assisted the activation of dihydrogen in a synergistic metal−ligand fashion, was illustrated with the complex C40 (Scheme 16).125

In a similar fashion, Hu developed 2,6-bis(phosphinito)pyridine iron pincer complexes such as C43 as a catalyst (10 mol %) able to selectively hydrogenate aldehydes with 60− 90% yields under 8 bar of H2 in methanol at room temperature for 24 h115 (Scheme 14). Noticeably, the hydrogenation was accelerated when using 10 mol % of sodium formate as an additive and 5.0 mol % of C43. Furthermore, it has been shown that the pincer phosphino(imino)pyridine complex (C44)116 catalyzed the hydrogenation of ketones at room temperature under 4 atm of H2 in the presence of a catalytic amount of base with good activities (TOF up to 300 h−1, Scheme 14). Jones and Schneider also described an active hydride amido pincer iron complex C17 able to efficiently catalyze the reduction of ketones without an additional base (TOF up to 500 h−1)117 (Scheme 14). In 2014, Morris reported the use of the chiral unsymmetrical PNP’ iron pincer complex mer-trans-[Fe(Br)(CO)2(P−CH N−P′)][BF4] (C45) for asymmetric hydrogenation of ketones (Scheme 15).118−120 Noticeably, the precatalyst was obtained Scheme 15. Iron Pincer Complexes for the Asymmetric Hydrogenation of Ketones

Scheme 16. Proposed Mechanism for the Hydrogenation of Ketones Catalyzed by Diphosphine Pyridine Iron Complexes

A reactive, dearomatized species II-1 was first produced by action of KOtBu and then coordinated the carbonyl generating the intermediate II-2. Noteworthy, the use of tert-butyloxide is an interesting and general activation pathway of iron halides in order to generate active catalytic species.126 The insertion of the coordinated CO moiety into the Fe−H bond gave the species II-3 which has then the ability to activate efficiently H2 leading to the hydrido alkoxy complex II-4. In a last step, elimination of the alcohol permitted the regeneration of the dearomatized species II-1. Another plausible mechanism is based on the attack of the hydride on the substrate in the outer-sphere process (Scheme 17). The reaction of the starting complex with a base generates a hydride amide complex III-1 via deprotonation of the NH of

as a mixture of iron hydride complexes mer-[Fe(OR)(H)(CO)(Cy2P−CH2−NH-PPh2)] by the reaction of the bromo iron complex C45 with LiAlH4 and then an alcohol (ROH). It catalyzed the hydrogenation of a huge variety of (hetero)arylalkylketones with 20−96% conversion, ee up to 85%, TONs up to 5000, and TOFs up to 1980 h−1 when conducted under mild conditions (in THF at 50 °C under 5 atm H2 in the presence of 1.5 mol % of KOtBu). H


Chemical Reviews

Review

asymmetric hydrogenation of various (hetero)arylalkyl ketones with ee up to 99% and good functional substituent tolerance (iodo, bromo, cyano, α,β-CC, Scheme 19).128 Noteworthy, β-ketoesters are also reduced to the corresponding chiral hydroxyesters when the hydrogenation was performed under 50 bar of hydrogen at 65 °C for 30 h in ethanol.

Scheme 17. Proposed Mechanism for the Hydrogenation of Ketones Catalyzed by Diphosphine Amino Iron Complexes

Scheme 19. Fe3(CO)12/N4P2-Macrocyclic Ligand-Based Catalyst for Asymmetric Hydrogenation of Ketones

ligand. The addition of dihydrogen to III-1 trans to the hydride leads to the trans-dihydride species III-3. III-3 transfers a proton from the nitrogen and a hydride from the iron to the carbonyl of the ketone to lead to the alcohol and regenerates the active amide complex III-1.119 Iron complexes associated to multidentate ligands are also good candidates for hydrogenation of carbonyl compounds. The cationic tetraphosphine iron fluoride complex C51 (0.2− 1.0 mol %) promoted the hydrogenation of aromatic and aliphatic aldehydes in the presence of 1.0−5.0 mol % of trifluoroacetic acid (TFA) in isopropanol under 20 atm of H2 at 120 °C for 2−5 h in 95−99% yields and TOF up to 360 h−1 127 (Scheme 18). Interestingly, the transformation

Beller succeeded to reduce selectively α-ketoesters to produce the corresponding α-hydroxyesters using in a consecutive way two different iron precatalysts, [Fe3(CO)12] C52 (6.7 mol %) and Fe(OTf)2 (7.2 mol %) in the presence of phenanthridine 1 (20 mol %) (Scheme 20).129 [Fe3(CO)12]

Scheme 18. Iron-Catalyzed Selective Hydrogenation of α,βUnsaturated Aldehydes

Scheme 20. Fe3(CO)12/Fe(OTf)2-Catalyzed Hydrogenation of α-Ketoesters

tolerated reducible functional substituents such as CC bonds, esters, and even ketones. Furthermore, allylic alcohols were selectively obtained in 94−99% yields by reduction of α,β-unsaturated aldehydes. Supported by NMR and DFT investigations, a mechanism was proposed suggesting the formation of a catalytically active species [(P4)Fe−H][BF4] generated from the iron fluoride complex C51 via an oxidative addition of H2 followed by a reductive elimination of HF. Xiao and Gao described a new in-situ-generated iron catalyst (0.5 mol %) prepared from the chiral N4P2-22-membered macrocyclic ligand L3 and Fe3(CO)12 precursor C52 which was active in the presence of 20 mol % of KOH to perform the

first catalyzed the hydrogenation of 1 to the corresponding hydrogenated reagent 2 and then used as the hydride source to selectively reduce the α-keto CO moiety via a Fe(OTf)2catalyzed hydrogen transfer reaction. A useful application of the iron-catalyzed hydrogenation is the conversion of levulinic acid to γ-valerolactone. Using the pincer iron catalyst C42, Song described an efficient hydrogenation of both methyl levulinate and levulinic acid performing the reaction in methanol at 100 °C for 2−12 h under 100 bar of H2 in the presence of 1 equiv of KOH I


Chemical Reviews

Review

activity was obtained in the transfer hydrogenation of numerous ketones including α-substituted alkoxyketones (22−99% conversion and TOFs up to 642 h−1, Scheme 22).134,135 Noteworthy, in the light of reports demonstrating that simple bases such as NaOH, KOH or KOtBu are able to promote the transfer hydrogenation of aldehydes and ketones at lower temperatures,136,137 the high reaction temperatures (80−100 °C) and the base-dependent reactivity are the main drawbacks of these catalytic systems. The combination of diaminodiphosphine ligands (P2N2) and iron precursors permitted us to perform this reaction under milder conditions. In a first report in 2004, Gao et al. developed an asymmetric transfer hydrogenation (ATH) of ketones catalyzed an in-situ-generated active species (1.0 mol %) from [NHEt3][Fe3H(CO)11] (C53) and tetradentate P− NH−NH−P ligands, such as (S,S)-Ph-ethP2(NH)2 (L7) and (S,S)-CyP2(NH)2 (L8) (Scheme 23).138 The best results in

(Scheme 21). GVL is then obtained in high TON and TOF of 23 000 and 1917 h−1, respectively.130 Scheme 21. Iron-Catalyzed Hydrogenation of Levulinate Derivatives to γ-Valerolactone

Scheme 23. Gao’s Diaminodiphosphine Ligands for ATH of Ketones 2.2.2. Transfer Hydrogenation of Aldehydes and Ketones. In the 1980s, Vancheesen et al. described pioneering contributions dealing with iron-catalyzed transfer hydrogenation of ketones, mainly using iron carbonyl complexes. The most efficient system was based on the use of [Fe3(CO)12] C52 (4.0 mol %) in the presence of 1phenylethanol or isopropanol as the hydride source and benzyltriethylammonium chloride and 18-crown-6 as phase transfer agents. The reaction performed at 28 °C for 2.5 h led to the corresponding alcohols with moderate 20−60% conversion and moderate TOFs up to 13 h−1.131,132 The combination of the commercially available triphenylphosphine, 2,2′:6′,2′′-terpyridine (terpy, L4) and [Fe3(CO)12] C52 or FeCl2 led to efficient catalytic systems (1.0 mol %) active for transfer hydrogenation of aliphatic and aromatic ketones in the presence of NaOiPr (5.0 mol %) in isopropanol at high temperature (100 °C) (Scheme 22).133 In a similar fashion, with in-situ catalysts prepared from either [Fe3(CO)12] C52 or FeCl2 in association with porphyrins (L5−L6), very good

terms of enantioselectivity were observed for the reduction of Ph2CHCOCH3 with ee up to 98% but with poor yields (18%) when conducting the reaction with 80 mol % of KOH in isopropanol at 65 °C for 21 h. As a representative example, the ATH of classical acetophenone led to 1-phenylethanol with 56% ee and 92% yield. Using similar ligand architecture, Morris et al. reported in 2008 the design of well-defined iron(II) complexes trans(R,R)-[Fe(CyP2N2)(NCMe)(CO)][(BF4)2] (C54) and trans(R,R)-[Fe(CyP 2 N 2 )(NCMe)(tBuNC)][(BF 4 ) 2 ] (C55) (which were inactive in hydrogenation reactions), which were efficient catalysts to perform the ATH of ketones with 18−61% ee (Scheme 24).100 The complex C54 was efficient for the reduction at room temperature of benzaldehyde (94% conversion, 2.4 h) and N-benzylideneaniline (>99% conversion, 17 h). By contrast, it did not promote the reduction of more challenging substrates such as ketimine (PhC(Me) CPh, conversion < 5% after 17 h) and cyclohexanone. Furthermore, even if the reduction of aromatic ketones can be performed with good conversions, the obtained enantiomeric excesses were moderate. C54 also catalyzed the transfer hydrogenation of α,β-unsaturated ketones such as E-PhCH CHCOMe giving a mixture of the allylic alcohol (18% conversion, 45% ee) and the saturated alcohol (82% conversion, 27% ee). Noteworthy, with the complex C55, a better enantioselectivity but a lower activity was observed. By contrast, the efficiency of C54 significantly increased compared to those previously described at room temperature (e.g., acetophenone, TOF = 2600 h −1, 35% ee at 24 °C).139

Scheme 22. Nitrogen-Containing Ligand Iron-Catalyzed Transfer Hydrogenation of Ketones

J


Chemical Reviews

Review

Scheme 24. Morris’ P2N2 Ligand Fe Catalysts Active in ATH from 2-Propanol

major compound (82% conversion, 60% ee) with only a trace amount of the saturated alcohol (4% conversion, 25% ee). It must be underlined that an optimized TOF of 21 000 h−1 was obtained using C57 at 28 °C. To activate the precatalyst, a high amount of base (8.0 equiv of KOtBu with respect to the iron precatalyst) was required. The nature of the phosphorus moiety also had a crucial effect on the activity. Such effect can be highlighted using the architecture of the complex (C58, R′ = Ph, Scheme 24). Indeed, substituting the phenyl on the phosphine by ethyl groups led to the corresponding complex, which showed a moderate catalytic activity at 25 °C (TOF: 563 h−1). By contrast, the complexes bearing a (c-C6H11)2P or (i-Pr)2P moiety were inactive. For complexes with phosphino moieties bearing ortho-, para-, and meta-substituted phenyl groups, whereas the o-tolylphosphino complex was inactive, the ptolylphosphino complex exhibited the highest activity in this series (TOF 30 000 ± 1500 h−1) with similar enantioselectivity (acetophenone 82% ee, at 28 °C and 0.1 mol % loading). Interestingly, the m-xylylphosphino complex also showed good activity (TOF 26 000 ± 1000 h−1) and slightly better enantioselectivity (90% ee). On the other hand, the nature of the diamino bridge was also crucial: although the diphenylphosphino iron complexes with a chiral 1,2-diamino1,2-diarylethylene bridge (R′ = Ph, p-OMe-C6H4) showed

On the basis of these results, Morris then reported analogues of C54 targeting an increase of the enantiomeric induction. With C56139 (0.17−0.5 mol %), similar catalytic activity (TOFs up to 2000 h−1) under mild conditions and ee up to 96% were observed, more particularly with more sterically hindered ketones (Scheme 24). It must be underlined that the reaction time was crucial in ATH reaction and had to be optimized. As a representative example, the best enantiomeric excess obtained for the reduction of acetophenone to 1phenylethanol was 63% after 15 min of reaction at room temperature. After 12 h, racemization was observed as ee of 1phenylethanol dropped to 44%, highlighting that TH in isopropanol involves equilibria ketones−alcohols which can lower the enantioselectivity. Morris then described a systematic study designing a new family of diiminophosphine iron(II) complexes.140−143 As a significant example, the complex C57144 gave higher activity and selectivity than with the previously reported complexes C54−56. The TH of acetophenone led to 90% conversion and ee up to 82% (22 °C, 30 min, 0.05 mol % C57, TOF of 3600 h−1). The TH of other ketones conducted at room temperature for 8−200 min led to 35−90% conversion and 14−99% ee, t-BuCOPh giving the best ee. Additionally, the reduction of (E)-PhCHCHCOMe exhibited a high chemoselectivity; the α,β-unsaturated alcohol was produced as the K


Chemical Reviews

Review

similar activities (TOF = 21 000 h−1), complex with a chiral 1,2-diamino-1,2-dicyclohexyl bridge and with a 1,2-ethylene bridge (R = H) exhibited lower activities (TOF 4900 and 2100 h−1, respectively), which clearly demonstrate the crucial influence of the hindrance of the substituents on the backbone and the requirement of an adequate balance between sterics and electronics at phosphino moieties to target the right iron complex associating both activity and enantioselectivity (Scheme 24). Morris also described a series of amine(imine)diphosphine chloro iron complexes efficient in ATH of ketones. Using 0.016−0.05 mol % of C59 and C39 in isopropanol in the presence of 0.033−0.4 mol % of KOtBu, acetophenone was reduced with ee up to 90% and TOFs up to 119 s−1 (for C39, Scheme 24).145 Using the complex C60 (1.0−2.0 mol %) in the presence of tetrabutylammonium tetrafluoroborate (1.0− 2.0 mol %), KHCO2 (10−20 equiv) in a mixture of 1:1 water:2-Me-THF at 65 °C, Morris described a biphasic asymmetric transfer hydrogenation of ketones with ee up to 76%.146 Morris also developed unsymmetrical tetradentate P−NH− N−P′ iron complexes such as C61 bearing a dicyclohexylphosphino group on the amine arm and a diphenylphosphino group on the imine part (Scheme 24). C61 was successfully evaluated in asymmetric TH of ketones with 38−99% ee and TON up to 4300. In comparison with the parent complexes C39, C59, and C60, C61 exhibited better enantioselectivities but lower activities.147,148 Interestingly, C61 crystallized in the cis-β configuration with the more flexible ethyldiphenylphosphino group in the apical position.149 With this structural information in mind, Morris then developed two families of new cis-β iron(II) PNNP′’ complexes with an orthophenylene arm: one with an amino−imino bridge (C62) and the other one with a bisamino bridge (C63). For reaction performed in isopropanol with 0.2 mol % of C62 or C63 and 1.6 mol % of KOtBu, at room temperature for 1 h, the catalytic activity was moderate (1−80%) and the enantiomeric excess of the 1phenylethanol obtained ranged from 94% (R) to 95% (S) depending on the nature of the PR2 moiety and the amino− imino or diamino nature of the bridge. The significant difference in ee between C62 and C63 complexes was rationalized by the formation of different kinetic iron hydride (on the opposite sides of the plane of the PNNP’).150 With the target to rationalize the role of base in the transformation, Morris prepared a new series of neutral ene− amido pentacoordinate iron(II)−N2P2 complexes C66−C67 obtained by deprotonation at the carbon α to the phosphorus center of the complexes C64−C65 using KOtBu (Scheme 25).151 Noteworthy, under base-free conditions, in TH of acetophenone, C66−C67 led to a similar activity than that of their parent complexes. Such results underlined that ene− amido compounds are intermediates in the base activation of iron(II)−N2P2 complexes in catalytic transfer hydrogenations. Thanks to detailed kinetic151 and DFT152 studies, a plausible mechanism was given.153 Starting from C64−C65, an imino/ enamido intermediate complex IV-1 was obtained and reacted slowly with KOiPr to lead to the active catalytic species IV-2 (Scheme 26). An outer-sphere process with isopropanol permitted one to generate the iron hydride species IV-4 which then reacted with acetophenone to produce the 1phenylethanol. Noteworthy, under deprotonation conditions, isomerization of the double bond can produce the deactivated

Scheme 25. Neutral Pentacoordinate Fe(II)−N2P2 Complexes for TH of Ketones

Scheme 26. Proposed Outer-Sphere Mechanism for the Transfer Hydrogenation of Ketones

species IV-6 with the CC located between the amine donors of the tetradentate ligand.154 From a mechanistic point of view it is always important to know the true nature of the active catalytic species, and more particularly, it is crucial to make a clear differentiation between homogeneous and nanoparticle (asymmetric) transformation. In this field, scarce examples of iron nanoparticle-catalyzed hydrogenation of alkenes and alkynes have been described.73,75−77,155 In the area of TH of ketones, Morris did a nice demonstration selecting chosen examples that Fe(0) nanoparticles functionalized with chiral and achiral PNNP ligands prepared from C56 and C38 (with L = MeCN, L′ = CO), respectively, were active catalysts in TH of ketones with ee up to 64% with the chiral Fe nanoparticles. Importantly, the method of preparation of nanoparticles has a drastic influence on their activity.156 It must be pointed out that two structurally similar complexes can lead to different catalytic species: whereas the trans-[Fe(NCMe)CO(PPh 2 C 6 H 4 CH NCHPh)2][(BF4)2] precatalyst C56 produced Fe nanoparticles during the reduction, the trans-[Fe(CO)Br(PR2CH2CHNCHPh)2][BF4] C38 (with L = MeCN, L′ = CO) generated homogeneous species.157 Noteworthy, Morris L


Chemical Reviews

Review

isopropanol in the presence of 28 mol % of KOH and 20 mol % of KCl. Good to high ee’s (up to 96%) were obtained at moderate yields (12−40% in most of cases).160 Mezzetti and co-workers developed an efficient methodology to obtain a chiral macrocyclic P2N2 ligand, more particularly bearing P-chirogenic centers, and their related iron complexes for applications in TH reactions (Scheme 29).161

reported helpful methodologies to try to distinguish between homogeneous and nanoparticle asymmetric iron catalysis. Le Floch finely designed a series of related iron complexes bearing tetradentate ligands bearing two iminophosphorane moieties with two phosphines, thiophosphino, and phosphine oxide substituents for TH of acetophenone (Scheme 27).158 Scheme 27. Iminophosphorane Diorganophosphorus-Based Iron Complexes for Catalytic TH of Acetophenone

Scheme 29. Mezzetti’s Macrocyclic P-Chirogenic N2P2 IronBased Catalysts for ATH of Ketones

The versatile coordination of these ligands to iron(II) precursor such as [FeCl2(THF)1.5] (C68) gave the corresponding complexes [FeCl2(P2N2)] (C69), [FeCl(O2N2)] (C70), and [FeCl2(N2)] (C71). Using 0.1 mol % of those complexes C69−C71, acetophenone was reduced in 80−91% conversion in the presence of 4.0 mol % of NaOiPr in isopropanol at 82 °C for 6−8 h. Like for hydrogenation reactions, using 0.5 mol % of a catalyst in-situ prepared from the chiral 22-membered macrocycle P2N4 ligand L3 and [Fe3(CO)12] C52, the ATH of ketones was performed at 65 °C in isopropanol in the presence of 12 mol % of KOH and 6.0 mol % of NH4Cl (Scheme 28).159 TOFs up to 1940 h−1, high conversion up to 99%, and excellent enantioselectivities (up to 99% ee) were observed. Li was inspired by the structure of this macrocyclic ligand to design open multidentate chiral aminophosphine ligand L9 (Scheme 28). Associated to [Fe3(CO)12] (2 mol %), the ATH of aromatic ketones was performed at 50 °C in Scheme 28. In-Situ Catalyst Obtained from [Fe3(CO)12]/ Chiral Multidentate Aminophosphine Ligands for ATH of Ketones

Using 1.0 mol % of bis(tert-butylisonitrile) P2N2 complex (C72, cis-β:trans = 13:1), ATH of ketones can be achieved in iPrOH at 75 °C in the presence of 4.0 mol % of NaOtBu giving the corresponding alcohols with moderate to high enantioselectivity (44−91% ee).162 In comparison to the bis(imino)based analogue C72, cis-β-bis(amino)macrocyclic iron complex C73 exhibited higher activity. Indeed, using 0.1 mol % of C73 in the presence of 1.0 mol % of NaOtBu in iPrOH at 60 °C, ATH of various arylalkylketones was efficiently performed with high ee (87−99%) and TOF up to 1960 h−1, which is one of the highest observed at iron. However, ATH with dialkyl ketones such as methyl isopropyl ketone was less stereoselective as only 24% ee was obtained.163 The cis-βbis(amino)macrocyclic iron complex bearing two tert-butylisonitrile ligands (C74) gave outstanding results, more particularly with ketones bearing electron-poor or orthoM


Chemical Reviews

Review

substituted aromatic substituents. Indeed, as an illustrative example, the TH of acetophenone led to 1-phenylethanol with 91% conversion and 98% ee. Furthermore, the enantioselectivity remained constant even at substrate-to-catalyst ratios as high as 10 000:1.164 The larger 15-membered macrocycle bearing a propanediyl diphosphine bridge-based iron complexes such as C75 (0.1 mol %) gave the highest conversions (up to 99.9%) and ee (up to 99.6%) and exhibited TOF up to 3950 h−1.165 Additionally, Mezzetti described a hydride isonitrile complex C76) able to catalyze base-free ATH of ketones in 2-propanol with high TOFs up to 9430 h−1 and similar enantioselectivity than the ones observed with the parent catalyst catalytic system C75 in the presence of NaOtBu (Scheme 29).166 It must be noted that with this catalytic system the authors succeeded in performing unprecedented hemihydrogenation of 1,2-diketones (benzils) to benzoins with up to 83% yields and 95% ee. Cyclopentadienyl (Cp) functionalized NHC ligand-based iron(II) complexes were also active precatalysts for TH of ketones. In 2010, Royo described tethered Cp-NHC iron complexes (e.g., C67)167,168 as efficient catalysts for transfer hydrogenation of ketones (e.g., acetophenone, benzophenone, and cyclohexanone) in the presence of a stoichiometric amount of KOH in isopropanol at 80 °C for 2−18 h (Scheme 30). [Fe(Cp)(NHC)(CO)2][I] complexes C78−C80169 with

Scheme 31. Knölker-Type Catalysts for TH of Ketones

Alternatively, using Knölker’s iron-type complexes such as C83174 and C32,93 Wills reported ATH of ketones using a mixture formic acid/triethylamine as hydride/proton donors with good conversion but modest enantioselectivity (e.g., acetophenone: 90% conversion, 24% ee, Scheme 31). A useful application of the hydrogen transfer reaction catalyzed by iron catalysts is the conversion of ethyl levulinate to γ-valerolactone (GVL). In 2014, Fu showed that using 5.0 mol % of the in-situ-prepared catalyst from Fe(OTf)2 and the tetraphos ligand [P(CH2CH2PPh2)3] L1, GVL can be obtained in 99% yield after reaction with 2 equiv of formic acid in dioxane at 140 °C for 24 h (TON 24).175 In 2015, Metzker and Burtuloso reported the same reaction starting from [Fe3(CO)12] catalyst (4 mol %) in the presence of 4 equiv of formic acid and 4 equiv of imidazole in water at 180 °C for 15 h (92% yield, TON 23).176 The use of Casey’s complex C26 permitted one to perform the reaction in milder conditions (Scheme 32). Using 1.0 mol % of C26 in the presence of 5 mol % of NaHCO3 in isopropanol at 100 °C for 19 h, γ-valerolactone was obtained in 95% yield and TON of

Scheme 30. NHC−Fe Piano-Stool Complexes for Catalytic TH of Ketones

1,3-dialkylated NHC were also used as efficient catalysts for transfer hydrogenation of cyclohexanone under similar conditions (Scheme 30). Noteworthy, the active species (0.5 mol %) can be obtained in situ by reaction of imidazolium salts with the [CpFe(CO)2I] precursor and used for the TH of various ketones in 21−99% conversion under similar conditions. In comparison to the hydrogenation reaction, Knölker complexes were less used in TH of ketones. Nitrile-ligated complexes previously developed by Knölker170 were shown to be efficient catalysts for TH as demonstrated by Funk. The best activity in the TH of aldehydes and ketones was obtained performing the reaction at 80 °C for 18 h using the acetonitrile complex C81 (aldehyde, 2.0 mol %; ketones, 5.0 mol %, Scheme 31). Interestingly, the catalyst C81 showed similar activities to the air-sensitive iron hydride complex C26 (1.0 mol %, 75 °C, 16 h).171,172 The same author reported a [2,5bis(3,5-dimethylphenyl)-3,4-diphenylcyclopentadienone]iron tricarbonyl (C72, 2.0 mol %) with Me3NO (2 mol %) at 80 °C for 24 h in both transfer hydrogenations and dehydrogenations.173 It is worth mentioning that C82 was as active as or more active than C27 in the carbonyl reductions and alcohol oxidations (Scheme 31).

Scheme 32. Iron-Catalyzed Reduction of Ethyl Levulinate to γ-Valerolactone

N


Chemical Reviews

Review

95 from ethyl levulinate.177 De Wildeman recently described the reduction of levulinic acid using the acetonitrile-ligated Knölker catalyst C81 (4.0 mol %) in the presence of 25 equiv of isopropanol in toluene at 80 °C for 20 h yielding GVL in 38% (Scheme 32). Noticeably, C81 (0.1 mol %) can perform the hydrogenation of levulinic acid to GVL in ethanol under 60 bar of H2 at 100 °C for 20 h with 57% yield (TON 570).178 Tridentate PSiP ligand-based iron pincer complexes C84 and C85 can be used in transfer hydrogenation of aldehydes (Scheme 33). Sun reported the reduction of aldehydes using

Scheme 34. Selective TH of Aldehydes Catalyzed by Fe(BF4)2·6H2O/L1

Scheme 33. Pincer Iron Catalyst for the TH of Aldehydes and Ketones Scheme 35. Knölker Complex/Chiral Phosphoric Brønsted Acids for the Catalytic Asymmetric Hydrogenation of Imines

2.0 mol % of catalyst in the presence of 2.0 mol % of KOtBu in isopropanol at 60 °C for 24 h. The reduction can tolerate functional groups such as halides or cyano. Additionally, chemoselective reduction of α,β-unsaturated aldehydes led to the corresponding allylic alcohols in 70−82% yields.179 Noticeably, ketones were not reduced under such conditions. Using another pincer complex, 2,6-bis(pyrazolyl)pyridine iron(II) complex (C86, 1.0 mol %), TH of ketones can be also achieved using 1 equiv of KOH as the base in isopropanol at 82 °C for 48 h (Scheme 33). C86 has a better activity with aromatic ketones than with aliphatic ketones.180 Efficient and highly selective transfer hydrogenation of aromatic, heteroaromatic, and aliphatic aldehydes using in-situgenerated catalytically active species (0.4 mol %) from Fe(BF4)2·6H2O/tetraphosphine L1 in the presence of 1.1 equiv of formic acid as the hydrogen source in THF at 60 °C for 2 h (20 examples, GC yields: 96−99%) were described by Beller (Scheme 34).181 Chloro, bromo, ketone, ester, and styryl functionalities were not reduced under these conditions. Additionally, chemoselective reduction of α,β-unsaturated aldehydes to the corresponding allylic alcohols was conducted under base-free conditions.

corresponding amines in 60−94% isolated yields and 67−98% ee. On the basis of this methodology, chiral amines were prepared from terminal alkynes and primary anilines using a consecutive hydroamination/hydrogenation sequence: (i) a gold-catalyzed hydroamination of terminal alkynes using 1.0 mol % of [Au(o-BiPh)(t-Bu)2P][BF4] leading to imines which were then in situ involved in (ii) an enantioselective hydrogenation [5.0 mol % of C26/2.0 mol % of L10 or L11 under the same conditions, yields up to 93%, ee of 70− 94%].183 The same catalytic system (3.0−5.0 mol % of C26 and 1.0− 2.0 mol % of L10 or L11) was also used to promote the enantioselective hydrogenation of quinoxalines and 2H-1,4benzoxamines, conducting to the corresponding tetrahydroquinoxalines with enantiomeric ratios (er) up to 97:3 and to 3,4-dihydro-2H-1,4-benzoxamines with er up to 87:13, respectively (Scheme 36).184 Recently, Benaglia and co-workers reported an ironcatalyzed diastereoselective reduction of chiral imines, using an achiral cyclopentadienone-based iron complex (C87) (10.0 mol %) in the presence of 0.4 equiv of Me3NO (Scheme 37). Performing the reduction under 30 bar of H2 at 70 °C in EtOH, enantiomerically pure amines were often produced with dr up to 98:2 in moderate conversions, yields, and activity (TOF up to 0.53 h−1).185 Noteworthy, using a water-soluble Knölker-type complex C28 (Scheme 9, 2.5 mol %) in the presence of 3.75 mol % of Me3NO in water under 10 bar of H2 at 100 °C for 14 h,

2.3. Imines, Nitro Derivatives, and Reductive Amination of Carbonyl Compounds

2.3.1. Imines. To date, there are only scarce reports on the reduction of imines catalyzed by iron-based species. In 2011, Beller and co-workers developed the first catalytic hydrogenation of imines to amines,182 associating in a synergic manner the Knölker complex C26 and a chiral phosphoric Brønsted acid (L10−L11, Scheme 35). The resulting in-situ catalytic system was active at 50 atm of H2 at 65 °C, resulting in the hydrogenation of numerous N-arylketimines to the O


Chemical Reviews

Review

under harsher conditions (20 atm H2 at 50 °C), Morris also described the hydrogenation of imines with TOF = 5 h−1.118 Transfer hydrogenation conditions can be also used for the reduction of imines. In 2011, the first iron-catalyzed enantioselective TH was performed using activated ketimines.187 Thus, using an in-situ-formed catalyst (0.33 mol %) from [NHEt3][Fe3H(CO)11] C53 and the diiminophosphine tetradentate chiral ligand CyN2P2 (L12), aromatic and heteroaromatic N-phosphonyl ketimines were reduced in 67− 98% yields and 89−98% ee after a reaction in the presence of a catalytic amount of a base at 45 °C for 30 min in isopropanol (Scheme 39).

Scheme 36. Knölker Complex/Chiral Phosphoric Brønsted Acids for the Catalytic Asymmetric Hydrogenation of the CN Bond

Scheme 39. ATH of Imines Promoted by Tetradentate Chiral Ligands Scheme 37. Knölker Complex for Catalyzed Diastereoselective Reduction of Chiral Imines

Renaud described the reduction of water-stable and -soluble imines (4 examples) in 50−98% yields.89 In addition to Knölker-type complexes, some other welldefined iron complexes were shown to perform hydrogenation of CN bonds. Thus, using the bifunctional iron pincer complex (C88) as the catalyst (3.0 mol %), Jones performed the hydrogenation of tetrahydroquinoxaline N-heterocycles under 5−10 atm of H2 in the presence of 10 mol % of KOtBu in THF at 80 °C for 24 h, and the corresponding reduced derivatives were isolated in 60−92% yields (Scheme 38). Noteworthy, the same type of pincer catalyst [e.g., (C89)] also promotes the reverse dehydrogenation of N-heterocycles when refluxing in xylene for 30 h, whereas C88 was inactive.186 Similarly, using the chiral PNP pincer iron complex C45 (Scheme 15, 1.0 mol %) in the presence of 10 mol % of base

In a similar manner, using diimino−diphosphine iron complexes, Morris also reported the transfer hydrogenation of N-(diphenylphosphinoyl)- and N-(tolylsulphonyl)-ketimines. The corresponding amines were then obtained with 29−94% conversion and 95−99% ee using the most efficient complex (C90) (1.0 mol %) in combination with 0.8 mol % of KOtBu in iPrOH at 40 °C for 40−120 min (Scheme 39).188 It must be pointed out that the amine(imine)diphosphine analogue (C91) exhibited an enhanced activity for transfer hydrogenation of imines with TOF up to 10 s−1.145 Interestingly, Funk’s catalyst can also catalyze transfer hydrogenation of N-aryl and N-alkyl imines. Indeed, Zhao succeeded to perform such a reaction using the combination of the Knölker’s nitrile-ligated complex (C92, 5.0 mol %) and Fe(acac)3 C2 (10 mol %) in isopropanol in the absence of base at 110 °C for 48 h (Scheme 40). The key for the success of this catalytic system is the use of Fe(acac)3 as a Lewis acid, as in its absence the reaction led to only 9% conversion.189 Recently, Piarulli, Gennari, and Pignataro reported TH of nonactivated imines promoted by a Knölker complex C29 (2.0 mol %) for the reduction of a number of N-aryl and N-alkyl imines in the presence of Me3NO (4.0 mol %) in iPrOH at 100 °C for 18 h (Scheme 40).190

Scheme 38. Pincer Iron Complexes for the Catalytic Hydrogenation of N-Heterocycles

P


Chemical Reviews

Review

styryl and halides (including iodides) were tolerated. The same group reported alternative reduction of nitroarenes by TH using 4.5 equiv of formic acid in the absence of additional base and the in-situ-formed catalyst (4.0 mol %) from Fe(BF4)2· 6H2O and the ligand P(CH2CH2PPh2)3 L1 at 40 °C in ethanol for 1−2 h194 (86−99% yields). Even though less active (TOF up to 23.3 h−1), the main advantage of this last catalytic reduction is the lower temperature used (40 vs 120−150 °C). 2.3.3. Direct Reductive Amination (DRA) of Carbonyl Compounds. Among the main preparative pathways for the production of amines, DRA is undoubtedly one of the most powerful and useful ones.195 Notwithstanding this reaction has been extensively studied with stoichiometric alkali reducing agents. Over the past decade, applications with iron as a catalyst have been developed.196 In 2008, using 10 mol % of FeSO4·7H2O and 25 mol % of Na2EDTA, Bhanage et al. performed the DRA of aldehydes and ketones with primary and secondary amines in water under 28 bar of hydrogen at 150 °C for 12 h197 (Scheme 42). Under these drastic conditions, small amounts of alcohols due to the reduction of the carbonyl compounds were also detected.

Scheme 40. Iron-Catalyzed TH of Imines

2.3.2. Nitro Derivatives. In synthetic molecular synthesis, the efficient and selective preparation of primary amines via reduction processes is still a challenging area of research. Only scarce protocols dealing with homogeneous iron-catalyzed hydrogenation of nitroarenes are reported. In 1976, Knifton used 0.5 mol % of [Fe(CO)3(PPh3)2] (C93) to catalyze the hydrogenation of nitrobenzene under 80 bar of H2 at 125 °C for 8.3 h in benzene/ethanol mixture giving aniline in 87% yield191 (Scheme 41). With an in-situ-formed catalyst from

Scheme 42. Iron-Catalyzed DRA Reactions

Scheme 41. Iron-Catalyzed Reduction of Nitroarenes to Aniline Derivatives

DRA can be performed under milder conditions using 4.0 mol % of [Fe3(CO)12] C52 under 50 bar of H2 in toluene at 65 °C for 24 h: by reaction with both aldehydes and ketones, anilines were transformed to the corresponding alkylated anilines in 68−97% yields (Scheme 42). In order to have a complete condensation between anilines and ketones, the use of molecular sieves is required. Noticeably, the reduction is chemoselective when aldehydes bearing ketone or ester moieties were used.198 Molecular-defined Knölker complexes C27 and C94 also efficiently catalyzed this transformation under mild conditions: with 5.0 mol % of C27 and 5.0 mol % of Me3NO, under low hydrogen pressure (5 bar) at 85 °C in ethanol, aldehydes reacted with alkylamines producing the corresponding alkylated amines in 38−94% yields (Scheme 42). A slight modification of the reaction conditions is required to perform the DRA of ketones: the reaction has to be performed in methanol with a catalytic amount of NH4PF6.199,200 Interestingly, the catalyst C29 (5.0 mol %) in combination with Me3NO (5.0 mol %) can also promote the transfer hydrogenative reductive amination of aldehydes and ketones in the presence of 3 Å molecular sieves at 100 °C for 18 h in good yields (Scheme 43).190 More recently, reductive amination/cyclization of levulinic acid via transfer hydrogenation with HCO2H as hydrogen source was reported by Burtoloso (Scheme 44).201 Using 4.0

FeSO4·7H2O (0.75 mol %) and ethylenediaminetetraacetic acid disodium salt (Na2EDTA, 3.75 mol %), Chaudhari et al. succeeded to hydrogenate efficiently nitroarenes with TOF up to 529 h−1, conducting the reaction in water under 28 bar of H2 at 150 °C. The tolerance of ketone, carboxylic acid, and cyano functionalities has to be underlined (Scheme 41).192 The hydrogenation of nitroarenes can be also performed using 2.0−5.0 mol % of the iron−tetraphos complex C51 in the presence of 1.0 equiv of trifluoroacetic acid (TFA) in tAmylOH in milder conditions (20 atm of H2 at 120 °C for 2 h, Scheme 41). The corresponding anilines were obtained in 78− 99% yields.193 Noticeably, the in-situ-generated catalytic species from Fe(BF4)2·6H2O and the corresponding tetradentate tris[2-(diphenylphosphino)phenyl]-phosphine L1 led to similar results (Scheme 41). Furthermore, such catalytic systems exhibited nice chemoselectivities as nitriles, esters, Q


Chemical Reviews

Review

Scheme 43. Iron-Catalyzed Transfer Hydrogenative Reductive Amination of Carbonyl Compounds

Scheme 45. Hydrogenation of Amides with Iron Pincer Catalysts

mol % of [Fe3(CO)12] C52 with 2.2 equiv of amine and 2.2 equiv of HCO2H in water at 180 °C, levulinic acid was converted to numerous pyrrolidones in 40−91% yields. Scheme 44. Cascade Reaction Involving an Iron-Catalyzed DRA

mol %, Scheme 45). As a representative example, the treatment of N-phenylbenzamide with 50 bar of H2 at 70−100 °C in dry THF after 24 h results in the clean formation of alcohol and amine, whereas the CO bond cleavage product was not detected by GC analysis.204 Similarly, Sanford used an analogous pincer complex (C96) bearing PCy2 moieties to catalyze the hydrogenation of amides (including formamides) in quite harsher conditions (110−130 °C) but at lower hydrogen pressure (20−50 bar) in 3 h (Scheme 45). TON up to 300 can be reached under such conditions. Notably, N,Ndimethylformamide was hydrogenated with TON up to 1000.205 In 2017, Bernskoetter used a PNP iron hydride complex as the catalyst to promote the hydrogenation of amides. The use of 0.07−0.018 mol % of C17 in THF under 30 atm of H2 for 4 h mainly permitted one to hydrogenate secondary formamide derivatives with 4−99% conversion and TON up to 4430 (Scheme 45). Under such conditions, acetamides and benzamides exhibited reduced activities. Interestingly, the addition of cocatalyst such as formanilide HCON(H)Ph and LiOTf enhanced the productivity of the reaction with acetamide and benzamides derivatives when performed at 120 °C for 16 h under 60 bar of H2.206 2.4.2. Nitriles. The catalytic hydrogenation of nitriles leading selectively to the corresponding primary amines is still a challenging transformation, mainly with iron catalysts. In his continuous effort to develop new iron-catalyzed transformations, Beller reported the use of 1.0 mol % of the iron PNP pincer complexes C89207 and C96208 for the selective hydrogenation of nitriles to primary amines under 30 bar of H2 in isopropanol at 70−100 °C for 3 h (Scheme 46). Importantly, aromatic, heteroaromatic, alkyl, and dinitriles (including the industrial relevant adiponitrile) can be reduced

2.4. Carboxylic Acid Derivatives and Carbon Dioxide

Iron-catalyzed reduction of polarized CX bonds such as aldehydes or ketones is a flourishing research area in which some complexes start to compete with the noble metal ones in terms of activity. By contrast, catalytic hydrogenation of less reactive derivatives such as carboxylic acids, amides, or esters in the presence of iron complexes is still in its infancy compared to the hydrogenation with noble transition metal complexes such as ruthenium. Nevertheless, some recent results demonstrate the high potential of this chemistry at iron.202 2.4.1. Amides. In the series of the carboxylic acid derivatives, carboxamides are undoubtedly among the most difficult ones to selectively reduce. Even if numerous transition metals have already efficiently been used for the catalytic reduction of amides, hydrogenations using iron catalysts were scarcely reported. Additionally, one of the challenges is the selective cleavage of the C−N bond leading to alcohols and amines or the CO bond furnishing the corresponding amines. In early 2016, Milstein described the first example of homogeneous iron-catalyzed hydrogenation of amides using an iron PNP pincer complex C41 as the catalyst (Scheme 45). The reaction can be performed in the presence of 6.0 mol % of KHMDS under drastic conditions (60 bar of H2 at 140 °C) with a range of activated secondary and tertiary N-substituted 2,2,2-trifluoroacetamides leading to the corresponding amines and trifluoroethanol with moderate to good yields.203 In 2016, Langer reported the hydrogenation of nonactivated amides and lactams with an iron pincer catalyst (C95, 2.0−10 R


Chemical Reviews

Review

Scheme 46. Iron PNP Pincer Complexes in the Catalytic Hydrogenation of Nitriles to Primary Amines

in 40−95% yields with C89. Notably, TOFs up to 250 h−1 at 100 °C were obtained. Under such reaction conditions, several functional groups such as halides, esters, ethers, and acetamido groups and α,β-unsaturated CC bonds were tolerated. Noticeably, modifying only the nature of the solvent by switching from iPrOH in nitrile reduction to THF used in ester hydrogenation209 (see below) under quite similar reaction conditions led to an unexpected change of reactivity and selectivity. Subsequently, Milstein’s group reported the synthesis of a novel complex C97 and its application in the catalytic homogeneous hydrogenation of (hetero)aromatic, benzylic, and aliphatic nitriles to selectively form primary amines (Scheme 46). The catalytically active species (1.0−5.0 mol %) prepared in situ by reaction of C97 with 1.0 equiv of NaHBEt3 and 3.0 equiv of KHMDS was applied for the reduction of (hetero)aromatic and alkyl nitriles under 60 bar of H2 in THF at 140 °C for 16−60 h (78−99% conversion, 63− 99% NMR yields).210 Interestingly, they described an ironcatalyzed hydrogenation of nitriles leading selectively to secondary imines. The reduction proceeded under relatively mild conditions (90 °C, 30 bar H2) using 1.0−8.0 mol % of the pincer complex [(iPrPNP)Fe(H)Br(CO)] (C98) and KOt-Bu as a base (in an equimolar amount to Fe precatalyst), leading to the corresponding imines in 11−99% NMR yields (Scheme 47). Noticeably, no products resulting from the full hydro-

genation (primary or secondary amines) were observed.211 This reaction resulted from the condensation of the reduced derivatives the reactive imine intermediate with the amine. Using the same catalyst C98, cross-secondary imines were selectively formed through hydrogenative cross-coupling of nitriles and amines under similar conditions but at a lower temperature (60 vs 90 °C) and pressure of H2 (10−20 vs 30 bar, Scheme 48).212 The reaction was particularly efficient with aromatic nitriles.

Scheme 47. Selective Hydrogenation of Nitriles to Secondary Imines Catalyzed by C98

2.4.3. Carboxylic Esters. In organic synthesis, one of the most important but also the most challenging task is the hydrogenation of carboxylic acids or esters in an efficient and chemoselective manner to obtain alcohols, ethers, or aldehydes. In terms of large-scale and industrial applications, reduction of esters to alcohols via hydrogenation is an important task. In the area of iron catalysis, there are only a few reports until recently, with 2014 being a productive year in this area. First, Milstein described a selective iron-catalyzed hydrogenation of activated trifluoroacetic acid esters F3C−CO2R leading to 2,2,2-trifluoroethanol and the corresponding alcohols RCH2OH in 52−99% NMR yields. The iron dihydrido pincer complex (C99) (1.0 mol %) was used as catalyst in the presence of 5.0 mol % of NaOMe as the base in

Scheme 48. Selective Reductive Cross-Coupling of Nitriles and Amines To Form Secondary Aldimines Catalyzed by C98

S


Chemical Reviews

Review

dioxane under 25 bar of hydrogen at 40 °C for 16 h (Scheme 49).213 It must be pointed out that no reduction activity was observed with difluoroacetic acid ester derivatives.

were well tolerated, whereas cyano functionalities were hydrogenated to the corresponding amines. Noteworthy, lactones led selectively to the corresponding diols. Crude industrial samples of a mixture of C12−C16 esters were also reduced under solvent-free conditions. Beller then had shown that the sterically less hindered Et2PNP analogous iron complex C95 gave superior catalytic activity working at lower temperatures (60−100 vs 100−120 °C).218 A catalytic cycle was proposed based on DFT calculations and experimental studies including an observation indicating that the −NH moiety on the PNP pincer backbone was an important key as no activity was detected using PNP ligand bearing a N-Me substituent (Scheme 50).219 Thus, after BH3 elimination from C89, simultaneous hydrogen transfers from the metal center (V-1) to the carbonyl moiety (without any previous coordination) and from the NH function produced the hemiacetal and an iron− amido complex via an outer-sphere mechanism. Hydrogenation of the iron complex (V-2) regenerated the dihydride species V-1. The solvolysis of the acetal led to methanol and aldehydes, which is then reduced to alcohol via the same catalytic cycle.207 Apart from pincer iron complexes, Lefort and Pignataro reported the use of the (cyclopentadienone)iron complexes C27 for the hydrogenation of activated trifluoroacetate esters (Scheme 49). With 1.0 mol % of C27 in the presence of 2.0 mol % of Me3NO and 20 mol % of trimethylamine, the reaction proceeded at 90 °C under 70 bar of H2 and led quantitatively to the corresponding alcohols, with TON up to 336. One main limitation of this methodology is its substrate dependence as only trifluoroacetate esters could be reduced.220 An iron-catalyzed hydrogenation of biomass-derived ethyl levulinate to γ-valerolactone has been also developed using a catalytic species (5.0 mol %) in-situ generated from iron(II) triflate and the tris[2-(diphenylphosphino)phenyl]phosphine L1 with formic acid as hydrogen source in 1,4-dioxane at 140 °C for 24 h.175 2.4.4. Carbon Dioxide and Carbonates. In terms of sustainability, the use of carbon dioxide as an attractive and

Scheme 49. Iron Dihydrido Pincer Complexes for the Hydrogenolysis of Esters to Alcohols

Simultaneously, Beller,207 Guan, and Fairweather214,215 reported the hydrogenation of a wide variety of aliphatic and aromatic esters catalyzed by the bifunctional PNP iron pincer complexes C89 and C95 under base-free conditions (Scheme 49). Such PNP ligand was initially described by Gusev in the osmium-catalyzed hydrogenation of esters.216,217 With this catalytic system, carboxamides, heteroaromatic motifs (e.g., furans, pyridines, benzothiazoles), and remote alkenyl moieties

Scheme 50. Proposed Mechanism for Ester Hydrogenolysis Catalyzed by C89

T


Chemical Reviews

Review

abundant C1 feedstock is nowadays a challenging purpose in bulk chemical industry (methanol, formic acid, formaldehyde, urea, etc.).221,222 In the field of reductions, the direct hydrogenation of CO247,223−229 to formaldehyde, methanol, methane, formamides, and amines is a crucial but challenging goal, with a particular focus on the CO2 hydrogenation to afford formic acid (or formate). Pioneering contribution made by Evans in 1978 using [NR4][HFe(CO)4] (C100) demonstrated that iron complexes could be nice candidates for the reduction of CO2 into formate derivatives, although harsh reaction conditions were used to reach very low yields (up to 6%).230 In 2003, thanks to a highpressure combinatorial catalyst discovery technique, Jessop et al. identified that the combination of FeCl3 and 1,2bisdicyclohexylphosphinoethane (dcpe) in the presence of 0.5 equiv of DBU (1,8-diazabicyclo[5.4.0]undec-7-ene) catalyzed the direct hydrogenation of CO2 to formic acid with a TON of 113 and a TOF of 15.1 h−1 under 40 bar H2 and 60 bar CO2 at 50 °C for 7.5 h.231 Noticeably, this in-situformed catalyst was more active than [NR4][HFe(CO)4] C100 when used with DBU. In 2010, Beller, Laurenczy, and co-workers described the hydrogenation of carbon dioxide and bicarbonates (HCO3−) to formates, alkyl formates, and formamides (when the reaction was performed in the presence of alcohols and amines, respectively). Indeed, using an in-situ-formed catalyst [(L1)FeH][BF 4 ] (C101) from Fe(BF 4 ) 2 ·6H 2 O, [P(CH2CH2PPh2)3] L1, and H2, the reduction of sodium bicarbonate led to formate with 88% yield and a TON of 610 when performing the reaction at 80 °C under 60 bar H2 for 20 h (Table 1).232 Using the same catalyst, methyl formate was also obtained by hydrogenation of CO2 in the presence of methanol and an excess of triethylamine with 56% yield and a TON of 585 and dimethylformamide from dimethylamine with 75% yield and a TON of 727 [PH2/CO2 = 60/30 bar, 100 °C, 20 h]. When performing the hydrogenation in the presence of ethanol, propanol, and diethylamine, ethyl and propyl formates and diethylformamide were produced, respectively, in lower yields (9−28%). Additionally, the in-situ catalyst obtained from L1 and FeCl2 was also efficient in the hydrogenation of CO2.233 Furthermore, the in-situ catalytic system Fe(BF4)2·6H2O/L1 can also promote the reversible reaction by dehydrogenation of formic acid to CO2 and H2 without base in high catalytic activity (TOFs up to 9425 h−1 and TONs up to 92 417 at 80 °C with 0.005 mol % of catalyst, Table 1).234,235 Noticeably, using a meta-trisulfonated-tris[2(diphenylphosphino)ethyl] phosphine sodium salt P[(CH2CH2P(Ph)(m-NaO3S−C6H4)]3 as a ligand associated to Fe(II) salts, Laurenczy demonstrated that the hydrogenation of CO2 can be also promoted in water at room temperature.236 By reaction of the tetradentate tris[2-(diphenylphosphino)phenyl]-phosphine L13 and Fe(BF4)2·6H2O, the air- and thermally-stable complex [(L13)Fe(F)][BF4] C51 was shown to be one of the most active iron catalysts for the hydrogenation of carbon dioxide and bicarbonates, affording formates and formamides with TONs up to 7500 for the hydrogenation of sodium bicarbonate (Table 1).237 Milstein described that trans-[Fe(H)2(CO) (tBuPNP)] (C99, 0.1 mol % loading) was efficient for the hydrogenation of CO2 and sodium bicarbonate to formate salts in a 10:1 H2O/THF mixture at 80 °C at low pressures (Table 1). The highest TON of 320 (32% formate yield) was obtained for the hydro-

Table 1. Iron-Catalyzed Hydrogenation of Carbon Dioxide and Bicarbonate

genation of bicarbonate performing at 80 °C under 8.3 bar of H2. The catalytic activity in CO2 hydrogenation was increased for the reaction under 10 bar pressure (H2/CO2 = 2:1) in an aqueous NaOH/THF solution (TONs up to 788 and TOFs up to 156 h−1).238 A similar complex based on the pyrazine backbone [(2,6-bis(di(tert-butyl)-phosphinomethyl)pyrazine)Fe(H)(Cl)(CO)] (C102) was evaluated in the catalytic hydrogenation of CO2 at 55 °C but exhibited lower activity (TON = 388, Table 1).239 Iron PNP pincer complexes (C103, C104), which were previously shown to exhibit remarkable activities in formic acid dehydrogenation, were also tested for the reverse CO2 hydrogenation in the presence of a Lewis acid (Table 1).240 The transformation was realized in a DBU/THF solution at 80 °C under 69 bar H2/CO2 (1:1). The addition of lithium triflate (LiOTf) was shown to have a beneficial effect and provided the best results with a TON of 58 990. Hazari and Bernskoetter also described similar pincer complexes (C105) U


Chemical Reviews

Review

Scheme 51. Proposed Mechanism for C99-Catalyzed CO2 Hydrogenation

Peters showed that triphosphinoiron chloride complexes including [(SiPR3)Fe(Cl)(H)] (C107, 0.1 mol %) also reacted with CO2 under elevated pressures of CO2 (29 atm) and H2 (29 atm), leading to formate and methylformate (Table 1). When conducting the reaction in the presence of NEt3 in MeOH at 100 °C for 20 h, (Et3NH)(OCHO) and MeOCHO were obtained in a 2:1 ratio with TON = 200.246 Gonsalvi and co-workers studied both a well-defined iron complex and the in-situ analogous one prepared from Fe(BF4)2·6H2O bearing the tetradentate 1,1,4,7,10,10-hexaphenyl-1,4,7,10-tetraphosphadecane phosphine (L14) for bicarbonate hydrogenation (Scheme 52).247,248 With the catalyst in-situ prepared from the commercial ligand L14 and Fe(BF4)2·6H2O, the hydrogenation of sodium bicarbonate in methanol under 60 bar H2 at 80 °C for 24 h was performed leading to the formate in 15% conversion (TON = 154). Under the aforementioned reaction conditions in the presence of propylene carbonate (PC), [Fe(rac-L14)BF4][BF4] (C108),

with an aryl isonitrile ligand instead of a carbonyl which exhibited lower activity in the CO2 hydrogenation (TON = 613, Table 1).241 Supported by computational, including the initial DFT calculations by Yang,242 and experimental studies with C99,238,242 a possible reaction mechanism for the hydrogenation of carbon dioxide was proposed and is outlined in Scheme 51. Resulting from the direct attack of CO2 on the hydride ligand of C99, the oxygen-bound formate complex VI1 is obtained. The formate ligand in VI-1 is then easily substituted by a molecule of water, leading to the cationic complex VI-2. Under hydrogen pressure, the η2-H2-coordinated species VI-3 may be formed. Finally, for VI-3, by reaction with −OH, the active species C99 is then regenerated by heterolytic cleavage of the coordinated H2 in VI-4 or by dearomatization and subsequent proton migration in VI-4′. Similar mechanism pathways were also proposed using [FeH(PP3)] (PP3 = L1 = (P-(CH2CH2PPh2)3).243 Noticeably, theoretical studies showed that the reaction was very dependent on the choice of the solvent used, with protic solvent with a higher ability to solvate the bicarbonate being the best.244 Similarly, the groups of Kirchner and Gonsalvi demonstrated that C42 and C106 were good candidates to catalyze the CO2/ bicarbonate reduction under comparable reaction conditions (Table 1).245 In the case of bicarbonate reduction, high TONs of 1964 (98% conversion) and 4560 (23% conversion) were detected after 24 h under 90 bar of H2 with catalyst loadings of 0.05 and 0.005 mol %, respectively. The hydrogenation of CO2 in basic conditions (0.5 M NaOH) proceeded with TONs of up to 1220 and quantitative yields when catalyzed by C42 under 80 bar at 80 °C. The N-methylated aminophosphino pincer complex C106 was less active than C42 in the presence of NaOH. Noteworthy, at a lower catalyst loading (0.01 mol %), quantitative formation of formate was still observed at 80 °C, which corresponds to a TON of approximately 10 000.

Scheme 52. Structure of L14 and Its Iron Catalysts Tested for Bicarbonate Hydrogenation

V


Chemical Reviews

Review

the authors succeeded to obtain the formate in 58% conversion (TON = 575). C109 led to an improvement in the bicarbonate conversion up to 76% in the absence of PC, whereas the TON reached a maximum value of 1229 at a higher substrate-tocatalyst ratio of 10 000 (12% conversion). High-pressure NMR experiments suggested the formation of [Fe(rac-L14)H]+ as a key intermediate species whatever the starting complex C108 or C109. As an alternative to the iron pincer catalyst discussed above, the group of Zhou developed a phosphine-free, air- and moisture-stable iron catalyst C27 active for bicarbonate hydrogenation in basic media (Table 1).249 The highest activity (TON of 447 after 24 h) was obtained for the reduction of sodium bicarbonate in ethanol/water (1:2) under 30 bar of H2 at 120 °C (yield = 45%). Noticeably, even at lower H2 pressure (5 bar), a TON of 163 could still be obtained. By contrast, with CO2 in the presence of NaOH, only traces of sodium formate were formed whatever the pressure applied. It is worth mentioning that the complex C27 in association with an iridium complex, [Ir(dF(CF3)ppy)2(dtbbpy)][PF6] (IrPS), used as a photosensitizer and triethanolamine used as a electron/proton donor, can promote the photochemical reduction of CO2 to carbon monoxide under visible light at room temperature with an initial TOF up to 22.2 min−1 250 (dF(CF3)ppy = cyclometalated 2-(2,4difluorophenyl)-5-trifluoro-methylpyridine and dtbbpy = 4,4′di-tert-butyl-2−2′-bipyridyl). Using the well-defined PNP pincer complex C103 (0.02 mol %), Hazari and Bernskoetter succeeded recently to perform the selective N-formylation of various alkylamine derivatives via CO2 hydrogenation (Scheme 53). Performing the reaction in

Scheme 54. Iron-Catalyzed Hydrogenation of Carbon Dioxide to Methanol

3. HYDROSILYLATION 3.1. Alkynes and Alkenes

The catalyzed hydrosilylation of alkenes is of great importance in the production of silicon derivatives and represents one of the largest industrial-scale applications of homogeneous catalysis.36 Accordingly, there are a lot of methods to conduct such transformations using precious transition metal-based complexes including the use of Speier’s catalyst,253 [H2PtCl6]· 6H2O/iPrOH, and the more active and selective Karstedt catalyst,254,255 [Pt2(Me2SiCHCH2)2O]3. During the last decades, the development of iron-based catalysts for hydrosilylation has seen an important increase of interest. In 1962, Nesmeyanov et al. reported pioneering results on the reaction of olefins with hydrosilanes in the presence of [Fe(CO)5]256 C1 or colloidal iron as the catalysts (Scheme 55). Mixtures of alkylsilanes (resulting from hydrosilylation Scheme 55. C1-Catalyzed Hydrosilylation of Alkenes

Scheme 53. Iron-Catalyzed N-Formylation of Amines by Hydrogen and Carbon Dioxide

reactions) and vinylsilanes (from dehydrogenative silylation processes) were then obtained. Only the reaction of ethylene with triethylsilane could occur selectively depending on the amount of triethylsilane used: with 0.2 equiv of triethylsilane, triethylvinylsilane was produced specifically with 92% yield, whereas with 3.0 equiv of triethylsilane, tetraethylsilane was obtained with 79% yield (Scheme 55). Under photoirradiation conditions, C1 catalyzed the reactions of alkenes with trialkylsilanes giving mixtures of alkanes, alkylsilanes, and alkenylsilanes.257 Catalyzed by [Fe3(CO)12] C52, the reaction of styrene with triethylsilane generated selectively the dehydrogenative hydrosilylation adduct (E)-β-(triethylsilyl)styrene Ph−CHCH−SiEt3 (yields up to 89%). It must be underlined that under such conditions no traces of the resulting hydrosilylation adduct of the styrenes were detected.258 Marciniec described iron carbonyl species associated with multivinylsilicon ligands which are also able to catalyze hydrosilylation of styrene. As a representative example, 1.0 mol % of Fe(CO)3[CH2CH−Si(Me)2−O− Si(Me)2−CHCH2] (C111) catalyzed the hydrosilylation of styrene in the presence of 1 equiv of PhMe2SiH at 0 °C for 24 h leading to the corresponding silane Ph−CH2−CH2− SiMe2Ph in 68% yield. Noticeably, the selectivity changed when working at 20 °C with 0.5 equiv of PhMe2SiH as (E)-

THF at 120 °C for 4−16 h under 35 bar of CO2 and 35 bar of H2, a large variety of primary and secondary alkylamines led to the corresponding compounds with 8−92% yields and TONs up to 8900.251 Noticeably, mechanistic studies indicated that the transformation proceeded first via the reversible reduction of CO2 to ammonium formate and then by its dehydration to formamide. Another interesting target in CO2 reduction is the production of methanol. Martins and Pombeiro described this reaction using an iron(II) scorpionate catalyst [FeCl2{κ3HC(pz)3}] (C110) (pz = pyrazol-1-yl) in solvent-free conditions at 80 °C for 24 h (Scheme 54). Even if the transformation can be promoted in amine-free conditions (yields up to 28%), the addition of pentaethylenehexamine increases the efficiency of the transformation. Notably, TONs up to 2.3 × 103 were obtained, making this catalyst one of the most efficient ones for this reaction.252 W


Chemical Reviews

Review

Ph−CHCH−SiMe2Ph is quantitatively obtained, resulting from a dehydrogenative silylation process.259,260 In his continuous work on well-defined diiminopyridine iron(0) dinitrogen complexes, in 2004, Chirik also described their use for efficient catalyzed hydrosilylations of alkynes and alkenes.64,261 Thus, the hydrosilylation of terminal or cyclic olefins using 1.0 equiv of phenylsilane gave exclusively the antiMarkovnikov products with TOFs up to 364 h−1. Noticeably, the reaction was conducted with a low loading of C12 (0.33 mol %) under mild experimental conditions (22 °C for 1−18.5 h, Scheme 56). The hydrosilylation of internal olefins such as

Scheme 58. Proposed Mechanism for the Olefin Hydrosilylation Promoted by (PDI)Fe-Type Precatalyst

Scheme 56. Pincer−Iron Complex-Catalyzed Hydrosilylation of Alkenes

the complexes involved in the mechanism can adopt a variety of electronic configurations. After generation of the active species (VII-1) from the precatalyst, the olefin and then the hydrosilane are coordinated to the iron center leading to VII-2 and VII-3, respectively. The key difference in this mechanism is the direct antiMarkovnikov hydride migration from the (σ-silane)−Fe into Fe-bound olefin without prior Si−H oxidative addition at the metal center. This is the rate-determining step in the mechanism. The Si−C reductive elimination of the obtained (PDI)Fe(alkyl)(SiR3) species (VII-4) then regenerates the active species VII-1 and leads to the alkylsilane. Notably, the regioselectivity is explained by a lower activation enthalpy for the anti-Markovnikov hydride migration vs Markovnikov migration (Scheme 58). In a similar manner, an iron(0) catalytic species in-situ formed from FeCl2 (1.0 mol %) and the bis-iminopyridine ligand L15 (1.0 mol %) in the presence of NaBHEt3 (2.0 mol %) can be used (Scheme 56).263 Interestingly, aniline, ester, ketone, aldimine, and nitrile functional groups were tolerated. Furthermore, this catalytic system was efficient for the hydrosilylation of internal alkynes conducting to (E)-vinylsilanes and terminal alkynes yielding (Z)-vinylsilanes (Scheme 57). Using the nitrogen-bridged complex (C112),264 a huge increase of the activity of the hydrosilylation of terminal olefins

trans-2-hexene generated mixtures of hexylsilanes resulting from the isomerization of the CC bond before hydrosilylation after 70 h at 22 °C. Diphenylacetylene reacted also with phenylsilane to yield quantitatively and selectively the corresponding (E)-vinylsilane (Scheme 57). From a mechanistic point of view, the stoichiometric reaction of C12 with phenylsilane led to the bis(σ-silane)iron(0) complex, whose structure was clearly established by X-ray analysis. Additionally, this complex was also highly efficient in the hydrosilylation of 1-hexene.57 For Chirik (PDI)Fe-type precatalyst in olefin hydrosilylation, DFT calculations seem to support a mechanism similar to the Chalk−Harrod one (Scheme 58).262 Noticeably, due to the redox-noninnocent bis(imino)pyridine (PDI) ligand, the iron center can adopt a variety of spin states, and

Scheme 57. Pincer−Iron Complex-Catalyzed Hydrosilylation of Internal Alkynes

X


Chemical Reviews

Review

which are formed in situ by reaction of C115−C117 with NaHBEt3 were able to perform the hydrosilylation of 1-octene with 1.0 equiv of phenylsilane under neat conditions at 100 °C for 24 h (Scheme 60). Interestingly, unsymmetrically

was observed: indeed, the complete hydrosilylation of 1-octene was observed using only 0.004 mol % of C112 with (Me3SiO)2MeSiH (MD′M) under neat reaction conditions at 23 °C for 15 min (Scheme 56). To highlight the high efficiency, only 0.007 mol % of C112 was needed to fully convert 1-octene with (OEt)3SiH in 15 min and only 0.02 mol % of C112 with Et3SiH after 45 min. Noteworthy, C112 also catalyzed the anti-Markovnikov hydrosilylation of styrene, N,N-dimethylallylamine, and allylpolyethers. In 2016, Thomas developed a catalytic system based on the air- and moisture-stable iron complex C113 (2.0 mol %) and iPr2NEt (20 mol %), which was efficient for alkene and alkyne hydrosilylation (Scheme 56).265 In the presence of PhSiH3 (1.2 equiv) at room temperature for 1 h under neat condition, a series of functionalized and unfunctionalized alkenes, styrenes, and alkynes was hydrosilylated in 35−95% NMR yields, with TOFs up to 47.5 h−1. Noticeably, terminal alkynes gave selectively the (Z)-vinylsilanes (Scheme 57). Significantly, these reactions proceed in equal yield under both air and inert reaction conditions. The high activity of C113 should be noted, as with only 0.25 mol % hydrosilylation of 1-octene can be conducted on a gram scale in only 10 min under air. Additionally, complex C12 had an interesting reactivity for the hydrosilylation of 1,2,4-trivinylcyclohexane with tertiary alkoxysilanes, which is an important industrial process for the manufacture of tires with low rolling resistance. Indeed, only the hydrosilylation of one of the three CC bonds was performed when the reaction was conducted at 23 °C for 16 h. It must be mentioned that using C12 an unprecedented regioselectivity for the monohydrosilylation of the desired 4vinyl moiety was obtained (conversion of MD′M 70%; 33% of monosilylated compound with 98% of C4-selectivity after 3 h at 23 °C). Noticeably, the modified complex C114266 led to better results under similar conditions (conversion of MD′M 83%; 60% of monosilylated compound with 98% of C4-selectivity, Scheme 59). If the reaction time is extended to 16 h, full

Scheme 60. Terpyridine-Based Iron Complex for Hydrosilylation of Alkenes

substituted terpyridine complexes were identified to be more efficient than the symmetrical one with TONs at 100 °C up to 1530 for C116 and C117 vs 34 for C115.267 Concomitantly, Chirik identified the terpyridine iron dialkyl complex C118 as another active catalyst (0.5 mol %) for the hydrosilylation of 1octene using tertiary hydrosilanes such as Et3SiH or MD′M at 60 °C for 1 h (Scheme 60).268 C118 successfully performed the chemoselective transformation of vinylcyclohexene oxide without alteration of the oxirane moiety. Mixed structures bearing bipyridine or iminopyridine moieties were also designed. In 2013, Huang reported a novel family of phosphinite−iminopyridine pincer iron complexes efficient for the chemoselective hydrosilylation of functionalized olefins. In the presence of 1.0 equiv of either PhSiH3 or Ph2SiH2, using an in-situ-generated catalyst from 1.0 mol % of C119 and 2.0 mol % of NaBHEt3, numerous alkenes were reduced in toluene at 23 °C for 3 h269 with a good functional group tolerance (ketones, esters, and amides, Scheme 61). It must be noted that styrenes and internal CC bonds were not hydrosilylated. Iminobipyridine iron complexes were also studied for this reaction. Nakazawa synthesized and applied [(aldiminobipyridine)FeBr2] (C120) when associated with NaBHEt3 in the hydrosilylation of 1-octene with primary, secondary, and tertiary silanes (Scheme 61). C120 exhibited high catalytic activity in organic solvents with TONs up to 12 038 for the reaction between 1-octene and Ph2SiH2.270 The substitution on the imino moiety is crucial for the activity. Indeed, starting from the [(ketiminobipyridine)FeBr2] (C121) associated with NaHBEt3, the hydrosilylation of olefins was performed using various primary, secondary, and tertiary silanes with low catalyst loadings (down to 0.01 mol %). Primary or secondary silanes were converted into not only monoalkylated but also dialkylated silanes. Functionalized olefins such as 6-chloro-1-hexene, N,N-dimethylallylamine, and vinylpentamethyldisiloxane can also be reacted selectively with diphenylsilane. Remarkably, one of the highest TONs reported in iron-catalyzed hydrosilylation of olefins, up to 42 000, was detected for the reaction of diphenylsilane with 1-octene using 0.001 mol % of C121 (Scheme 61).271

Scheme 59. Iron-Catalyzed Selective Hydrosilylation of 1,2,4-Trivinylcyclohexane

conversion of the silane was observed and the desired C4silylated product obtained in 83%. Importantly, these iron complexes exhibited better reactivity than those obtained with commercially used platinum compounds. Terpyridine-based iron complexes were successfully used at low catalytic loading in the hydrosilylation of alkenes. Nakazawa et al. designed a series of substituted terpyridine iron halide complexes including some exhibiting catalytic activity such as C115−C117: the catalytically active species Y


Chemical Reviews

Review

Scheme 61. Unsymmetrical Pincer−Iron Complexes for Hydrosilylation of Olefins

Scheme 63. Asymmetric Iron-Catalyzed Hydrosilylation of Olefins

(97.1−99.8% ee) were observed. Noteworthy, the regioselectivity of the addition was high in most of the examples, the ratio linear/branch being lower than 3:97. This catalytic system exhibited excellent chemoselectivity as functional groups such as chloro, trimethylsilyl, tosylate, mesylate, esters, ethers, acetal, heterocycles, or hydroxyl protecting group (TBS, benzyl) were not altered. Noticeably, a geminated CC bond was not hydrosilylated.273 In 2018, Peng and Zhu reported the use of a series of iron catalysts such as C124 based on 2,9-diaryl-1,10-phenanthroline ligands for alkene hydrosilylation (Scheme 64). Using 2.0 mol In 2015, Lu designed a chiral oxazoline-substituted iminopyridine iron chloride complex C105 for highly regioand enantioselective anti-Markovnikov hydrosilylation of styrenes. When activated by NaHBEt3 (3.0−15 mol %), complex C122 (1.0−5.0 mol %) catalyzed the addition of diphenylsilane to 1,1-disubstituted alkenes at room temperature for 12 h, leading to the corresponding silylated products with 79−99% yields and high enantioselectivities up to 99% ee (Scheme 62). Good to excellent enantioselectivities (78−99%

Scheme 64. 2,9-Diaryl-1,10-phenanthroline-Based Iron for Catalyzed Hydrosilylation of Olefins

Scheme 62. Iron-Catalyzed Asymmetric Hydrosilylation of Olefins

% of C124 in the presence of 4.4 mol % of EtMgBr as the reductant, under solvent-free conditions at 30 °C for 24 h, various β-alkylstyrenes reacted with 1.1 equiv of phenylsilane leading to the corresponding silane derivatives in 68−95% yields. The reaction can be also performed with styrenes working in THF at 30−35 °C for 10 h (88−95% yields). Noticeably, the reaction exhibited an exclusive benzylic selectivity. When starting from 1,1-disubstituted buta-1,3dienes, the corresponding allylsilanes were obtained in 91− 95% yields, resulting from a Markovnikov selectivity. Finally, the hydrosilylation can be also performed with terminal linear alkenes when the reaction was conducted in toluene at 30−35 °C for 24 h leading to the terminal silanes with 72−98% yields and TON up to 9800.274 DFT calculations suggested mechanism pathways involving a high-spin Fe(I) catalytically active species.

ee) were observed for Ar(Alkyl)CH2 olefins, whereas low ee’s were obtained for 1,1-dialkyl alkenes. This catalytic system showed moderate chemoselectivity, and fluoro, chloro, methoxy, trifluoromethyl, and ketimine groups were tolerated, whereas aldehyde, ketone, ester, and alcohol groups reacted with silane faster than the alkene moiety.272 Recently, Lu also developed an iron-catalyzed highly Markovnikov selective and enantioselective hydrosilylation of terminal aliphatic alkenes. Using a chiral unsymmetrical oxazolinyl−imino pyridine iron complex (C123, 2.0 mol %) associated to 6 mol % of NaOtBu, terminal aliphatic alkenes were reduced in the presence of 1.2 equiv of arylsilane in THF at 0 °C for 2 h (Scheme 63). Excellent enantioselectivities Z


Chemical Reviews

Review

More particularly, complexes with 2,9-diaryl-1,10-phenanthroline ligands exhibited unexpected reactivity and selectivity for hydrosilylation of alkenes, including unique benzylic selectivity with internal alkenes, Markovnikov selectivity with terminal styrenes and 1,3-dienes, and excellent activity toward aliphatic terminal alkenes. According to the mechanistic studies, the unusual benzylic selectivity of this hydrosilylation initiates from π−π interaction between the phenyl of the alkene and the phenanthroline of the ligand. This ligand scaffold and its unique catalytic model will open possibilities for base metal-catalyzed hydrosilylation reactions. In 2016, Nagashima showed that the simple combination of Fe(OPiv)2 (1.0 mol %) associated with 1-adamantyl isocyanide (CN-Ad, 2.0 mol %) was able to catalyze the hydrosilylation of styrene and allylic ether derivatives with 1.3 equiv of PMDS (Me2HSi-O-SiMe3) at 80 °C for 3−23 h (84−99% yields). Noticeably, functional groups such as epoxide, O-benzyl, or ester were tolerated.275 Using 5.0 mol % of molecular-defined iron precatalyst C125 in association with 5.0 mol % of iminopyridine ligand L16, Ritter reported the first 1,4-hydrosilylation of 1,3-dienes by reaction with 1.2 equiv of triethoxysilane at 23 °C for 15 h, conducting to allyl silanes in high regio- and stereoselectivities (linear/branch ratio from 95:5 to 99:1, E/Z ratio > 99:1, Scheme 65). By contrast, a low regioselectivity (38:62) was detected with C126 as precatalyst.276

Scheme 66. Iron-Catalyzed Hydrosilylation of Alkynes to Alkenes

iron(0) [Fe(CO)4(L18 or L19)] complexes were synthesized and characterized. Under optimized conditions, the in-situprepared catalyst exhibited higher yields and better selectivities than the defined complexes. Recently, Albrecht reported similar results on the selective semihydrogenation of alkynes to alkenes using a triazolylidene iron(II) piano-stool complex (C128, 7.0 mol %) in the presence of 2 equiv of (EtO)2MeSiH in 1,2-dicloroethane (1,2-DCE) at 60 °C for 8−24 h (Scheme 66). It must be underlined that both terminal and internal alkynes were reduced selectively in the corresponding alkenes. In most cases, the Z-alkenes were obtained starting from internal alkynes with E/Z selectivity up to 1/99. Furthermore, such conditions were selective to alkynes as alkenes were not converted.279 Using 1.0 mol % of the complex [Fe(H)(CO)(NO)(PPh3)2] (C129) in the presence of triethylamine as an additive (50 mol %), Plietker succeeded to promote the selective hydrosilylation of internal alkynes to vinylsilanes (Scheme 67).280 Noticeably, the nature of the silane had a crucial effect on the E- or Z-selectivity obtained. Indeed, the hydrosilylation of diphenylacetylene with phenylsilane led selectively to Z-vinylsilanes (E/Z up to 1/20; yields up to 98%), although tertiary silanes (e.g., methylphenylvinylsilane, [Ph(Me)(CHCH2)Si−H]) produced the E-vinylsilane with a E/Z ratio of >20:1 and yields up to 97%. The structure of the internal alkyne also strongly influenced the regioselectivity: as a representative example, dialkylacetylene led to the E-alkene whatever the nature of the hydrosilane was [PhSiH3 or Ph(Me)(CHCH2)Si−H] (Scheme 67). Noticeably, in 2018, Bhat showed that FeCl3·6H2O (20.0 mol %) in the presence of 1.02 equiv of Et3SiH was able to promote the selective reduction of the CC bond of Michael acceptor alkylidene-β-keto esters and alkylidene1,3-diketones in dichloromethane at room temperature (Scheme 68). The corresponding saturated β-keto esters and 1,3-diketones were obtained in 85−96% yields. Noticeably, only a trace amount of derivatives resulting from the reduction of the carbonyl group was detected.281

Scheme 65. Iron-Catalyzed Hydrosilylation of 1,3-Dienes

In order to perform specifically the hydrosilylation of alkynes leading to vinylsilanes, new iron complexes were designed. In 2011, Enthaler reported a selective reduction of alkynes to olefins in the presence of 1.1 equiv of (EtO)3SiH using an insitu-prepared catalyst from [Fe2(CO)9] (C127, 5.0 mol %) and tributylphosphine (L17, 10 mol %) at 60 °C for 48 h (Scheme 66). Notably, starting from internal alkynes, (Z)alkenes were stereospecifically obtained, except for carboxylatesubstituted alkynes. Additionally, this catalytic system tolerated numerous functional groups (e.g., aldimines, esters, amides, alkenes, nitriles, epoxides).277 Similarly, the same authors identified catalytic species based on [Fe2(CO)9] C127 and monodentate pyrrole−phosphine ligands [N,N-diphenyl-1H-pyrrol-1-amine-2-di-R-phosphine; R = Ph, (L18); R = tBu, (L19)], also able to reduce stereoselectively alkynes in the presence of (EtO)3SiH as the reducing reagent (Scheme 66).278 It must be pointed out that from the reaction of [Fe2(CO)9] C127 with L18 or L19, two

3.2. Aldehydes and Ketones

Even if the catalyzed hydrogenation or transfer hydrogenation is a powerful methodology for the reduction of carbonyl derivatives, for chemoselectivity issues, the hydrosilylation can AA


Chemical Reviews

Review

Scheme 67. Stereoselective C129-Catalyzed Reduction of Alkynes

Scheme 68. FeCl3·6H2O-Catalyzed Reduction of α,βUnsaturated Derivatives

Scheme 70. Initial Iron-Based Catalysts for Hydrosilylation of Ketones

be an interesting alternative pathway, more particularly when using inexpensive hydrogen sources such as PMHS and TMDS (1,1,3,3-tetramethyldisiloxane). During the past decade, the area of iron-catalyzed hydrosilylation of aldehydes and ketones has seen an amazing development. The early example of the hydrosilylation of ketones catalyzed by iron complexes was reported in 1990 by Brunner and Fisch.282 Indeed, 0.5−1.0 mol % of [Fe(Cp)(CO)(X)(L)] (C130) promoted the reaction of acetophenone with 1.0 equiv of diphenylsilane at 50−80 °C for 24 h, yielding quantitatively the silylated ether, without the formation of the silylated enol ether (Scheme 69). Scheme 69. Pioneering Iron-Catalyzed Hydrosilylation of Acetophenone by Brunner

Two decades later, one of the first efficient and scalable methodologies using iron-based catalysts for the hydrosilylation of carbonyl derivatives was described by Beller. Thus, the in-situ-formed catalyst from Fe(OAc)2 (5.0 mol %) and PCy3 (L20, 10.0 mol %) permitted one to conduct the reduction of functionalized (hetero)aromatic and alkyl aldehydes in the presence of 3.0 equiv of PMHS in THF at 65 °C for 16 h (35 examples, 60−99% yields, Scheme 70).283 It also catalyzed the reduction of ketones (21 examples, 60− 96% yields) after 20 h at 65 °C.284 Interestingly, ester, amino,

cyano, and α,β-CC moieties were tolerated. Using the same methodology, an enantioselective version was then developed associating Fe(OAc)2 and (S,S)-Me-Duphos (L21) as a chiral diphosphine ligand. Thus, using stoichiometric amounts of (EtO)2MeSiH or PMHS at room temperature or 65 °C, aromatic ketones were reduced affording the corresponding AB


Chemical Reviews

Review

alcohols with yields and ee up to 99% (Scheme 70).285 In concomitant contributions, Nishiyama showed that the nitrogen-based ligands such as N,N,N′,N′-tetramethylethylene-diamine (TMEDA, L22)286 or sodium thiophenecarboxylate (L23)287 (10 mol %) associated to Fe(OAc)2 (5.0 mol %) gave an efficient catalyst for the hydrosilylation of ketones under similar conditions (2.0 equiv of (EtO)2MeSiH; 65 °C; 20−24 h; 50−94% yields, Scheme 70). Asymmetric hydrosilylation of ketones was experimentally achieved using N,N,Nbis(oxazolinylphenyl)-(Bopa) ligands such as Bopa-dpm (L24, 3.0 mol %) in combination with Fe(OAc)2 (2.0 mol %) and (EtO)2MeSiH as the hydride source (88−99% yields and 50− 88% ee, Scheme 70).288 It must be also underlined that when using the preformed complex [(L24)FeCl2] (5.0 mol %) in association with zinc powder (6.0 mol %) with 2 equiv of (EtO)2MeSiH under similar conditions the other enantiomer of the alcohol was obtained.289 N1-Alkylated 2-(pyrazol-3-yl)pyridines such as L25 (10 mol %) by reaction with iron octanoate [Fe(O2C8H15)2] (5.0 mol %, C131) led also to suitable catalyst precursors for the hydrosilylation of aldehydes and ketones at 80 °C for 20 h in the presence of PMHS with yields up to 99% (Scheme 70).290 Furthermore, Plietker et al. described a catalyst prepared in situ from PCy3 L18 and the [Bu4N][Fe(CO)3(NO)] complex291 C132 (which is used with success in allylic substitution reactions292), also highly active for the hydrosilylation of numerous functionalized aldehydes and ketones in the presence of PMHS (Scheme 70). The corresponding alcohols were produced in moderate to excellent yields [aldehydes (65−99%); ketones (92−99%)] at low catalyst loadings [C132 (1.0−2.5 mol %); PCy3 L20 (1.1 mol %)] at 30−50 °C for 14 h.293 In terms of activity, Tilley achieved a breakthrough. Indeed, the simple low-valent but highly air-sensitive iron silylamide complex [Fe(N(SiMe3)2)2] (C133, 0.01−2.7 mol %) catalyzed efficiently the hydrosilylation of various aldehydes and ketones in the presence of 1.6 equiv of Ph2SiH2 at 23 °C for 0.3−20 h (Scheme 70). Remarkably, TOFs up to 2400 h−1 can be reached for the reduction of 3-pentanone, and the reaction tolerated functional groups such as nitrile, cyclopropyl, or a CC bond (Scheme 70).294 The related iron(II) bis(trimethylsilyl)amido complexes C134 coordinated to a Nphosphinoamidate ligand (0.015−1.0 mol %) promoted the hydrosilylation of a range of aldehydes and ketones in the presence of 1.0 equiv of phenylsilane (Scheme 70).295 Noteworthy, a significant beneficial influence of the ligand on the activity was increased with TOFs up to 23 600 h−1 for the reduction of acetophenone (to be compared to 1266 h−1 with C133). Togni has shown that an in-situ-formed catalyst from cyclopentadienyl-bearing chiral diamine (L26) and Fe(acac)2 promoted the reduction of ketones affording the corresponding alcohols with conversion up to 99% but with low to moderate ee up to 37% (Scheme 70).296 Pincer-type iron complexes, which are usually more stable and efficient in hydrogenation, were also extensively reported in hydrosilylation of carbonyl compounds. The first contribution by Chirik in 2008 described that a family of highly active bis(imino)pyridine(PDI) iron complexes such as [Fe(iPrPDI)(N2)2] C12, already identified as catalysts for the hydrogenation and hydrosilylation of alkenes, was also active in the hydrosilylation of p-tolualdehyde and acetophenone with Ph2SiH2 in pentane at 23 °C in 1 h (Scheme 71).297 Using the iron dialkyl complexes C135 (0.1

Scheme 71. Pincer Iron Catalysts for Hydrosilylation of Carbonyl Derivatives

mol %) or C136 (1.0 mol %) under similar conditions (2.0 equiv of Ph2SiH2, pentane, 23 °C, 3 h), numerous ketones including cyclohexanones were efficiently reduced (Scheme 71).297 Noteworthy, cyclohexenones were chemoselectively reduced to the corresponding unsaturated alcohols, although acyclic enones led to the allylic alcohols in a less selective manner. Furthermore, this catalytic system is highly active exhibiting one of the highest TOFs of up to 23 600 h−1. In 2011, Guan designed new iron hydride complexes coordinated to phosphinite-based pincer ligands (POCOP) such as C137 applicable in the catalytic hydrosilylation of aldehydes and ketones (Scheme 71).298 Full conversion of benzaldehyde was reached at 50 °C for 1 h using 1.0 mol % of catalyst C137 and 1.1 equiv of (EtO) 3SiH. For the AC


Chemical Reviews

Review

hydrosilylation of ketones such as acetophenone, a higher temperature (80 °C for 4.5 h) was necessary to observe full conversion; the corresponding alcohols were then obtained in up to 88% yield. This system was appropriate for the hydrosilylation of aromatic and aliphatic aldehydes (80−92% yields at 50−65 °C for 1−36 h) and aromatic ketones (0−88% yields at 50−80 °C for 4.5−48 h). Noticeably, the decoordination of PMe3 or CO ligand in an initial step seems to be a crucial step in order to generate the active catalytic species. Noticeably, similar activities were obtained with cationic pincer iron complexes C138−C139.299,300 Similarly, tridentate PSiP (C140)301 and PCP C141302 and C142303 ligand-based iron pincer complexes can be used under mild conditions in hydrosilylation (Scheme 71). Thus, using 1.0 mol % of C140 and 1.5 equiv of (EtO)3SiH, the reduction of aldehydes and ketones can be performed at 60 °C in 1 and 6 h, respectively. Slightly higher activities are obtained with the complex C141 at 50 °C (aldehydes 0.3−1.0 mol % C141, 1− 13 h, 85−90% yields; ketones 1.0 mol % C141, 16 h, 21−90% yields). Findlater prepared similar structural iron complexes, (2,6-bis(di-tert-butyl-phosphinito)pyridine) [(tBuPONOP)FeCl2] (C143) and (2,6-bis(di-tert-butyl-phosphinomethyl)pyridine) [(tBuPNP)FeCl2] (C144), which were more active in the hydrosilylation of aldehydes and ketones by reaction with 1.5 equiv of (EtO)3SiH at room temperature for 24 h.304 Noticeably, unusual pincer iron complexes were also reported in catalytic hydrosilylation. Driess and Oestreich reported a disilylene pyridine SiNSi pincer complex C145 which exhibited catalytic activity toward the hydrosilylation of ketones (Scheme 71). Noticeably, it was shown that the transition metal in the catalytic species does not seem to be directly involved in the catalytic process. By contrast, activation of the ketone and silane should happen in the ligand sphere, i.e., at silicon.305,306 Recently, Lee described the use of a low-coordinate iron(II) complex bearing an NNN-pincer ligand (C146, 1.0 mol %) for the hydrosilylation of aldehydes and ketones using phenylsilane (1 equiv) at room temperature for 24 h (Scheme 71). TOFs up to 13 min−1 were observed showing the good efficiency of such catalyst.307 On the other hand, chiral pincer iron complexes have been developed for asymmetric hydrosilylation of ketones. In 2008, Gade reported a [Fe(tetraphenyl-carbpi)(OAc)] complex (C147, 5.0 mol %) able to promote the reduction using (EtO)2MeSiH or PHMS as the hydrosilane at 40−65 °C leading to the corresponding alcohols in 50−93% ee (Scheme 72).308 To date, it is still one of the most efficient catalysts in terms of enantioselectivity and activity. In 2009, Chirik described a chiral tridentate bis(oxazolinyl) ligated [(S,S)-(iPrpybox)-Fe(CH2SiMe3)2] complex C148 (0.3 mol %) able to catalyze the asymmetric hydrosilylation of ketones with 2.0 equiv of PhSiH3 and 0.95 equiv of B(C6F5)3 at 23 °C for 1 h, exhibiting ee up to 54% (Scheme 72).309,310 Similarly, in 2015, Nishiyama and Ito reported a bis(oxazolinyl)-ligated iron complex C149 with lower enantioselectivity (up to 44% ee) under relatively similar conditions (Scheme 72).311 Modifying the structure of one arm of the Pybox ligand, Huang designed an unsymmetrical oxazolinylimino pyridine iron complex (C150) able to enhance the enantioselectivity of the hydrosilylation (Scheme 72). On activating complex C150 (1.0 mol %) with 2.0 mol % of NaHBEt3 in the presence of 1.0 equiv of Ph2SiH2 at 25 °C for 3 h ee values increased up to 93%.312

Scheme 72. Chiral Pincer Iron Catalysts for Asymmetric Hydrosilylation of Ketones

In 2015, Gade described a general and highly efficient asymmetric hydrosilylation of ketones using 5.0 mol % of chiral iron alkoxide pincer complex C151 in the presence of 2.0 equiv of (EtO)2MeSiH in toluene in a temperature range from −78 °C to room temperature for 6 h (Scheme 72). Importantly, unprecedented activities and stereoselectivities were obtained in hydrosilylation area with TOF up 240 h−1 at −40 °C with 73−99% ee for numerous alkyl aryl ketones.313 A detailed mechanism study of the catalytic cycle demonstrated that the rate-determining step is a σ-bond metathesis of the alkoxide complex VIII-1 with the hydrosilane, leading to an iron hydride species VIII-2 and generating the alkoxysilane compounds (Scheme 73). The subsequent coordination and then insertion of the ketone to the iron hydride bond then regenerated the catalytic alkoxy species VIII-1.314 Cyclopentadienyl piano-stool iron(II) complexes are another family of molecular-defined iron complexes widely studied in hydrosilylation reactions. Following the pioneering work of Brunner in the early 1990s,282 in 2008, Nikonov reported an AD


Chemical Reviews

Review

Scheme 73. Mechanistic Proposal for Iron-Catalyzed Hydrosilylation of Ketones

acetophenone was the neutral iron complex [CpFe(CO)(I)(PPh3)] (C157), which exhibited higher activity in the presence of 1.2 equiv of phenylsilane under neat conditions and visible light activation and slightly harsher conditions (70 °C, 30 h). Noticeably, the tetrafluoroborate complex C137 (5.0 mol %) exhibited similar or superior activities compared to the iodo- or hexafluorophosphate analogs (e.g., acetophenone 98% conversion, visible light activation, 70 °C, 16 h with 1.2 equiv of PhSiH3 or 72 h with 4.0 equiv of PMHS). It is important to notice that visible light activation is required in order to favor the decoordination of one CO ligand and then generate an unsaturated active species. Cyclopentadienyl piano-stool iron(II) complexes, particularly when associated with N-heterocyclic carbene ligands (NHC), are another important series of efficient catalysts extensively developed for the hydrosilylation of carbonyl derivatives.317 Following the pioneering contribution of Brunner, in 2010, Royo reported the activity of tethered CpNHC iron complexes (e.g., C77, 1.0 mol %) for the hydrosilylation of activated aldehydes (6 examples) with 1.2 equiv of (EtO)2MeSiH in acetonitrile at 80 °C for 1−18 h (Scheme 75).167 Our group designed a series of NHC-piano-stool iron complexes including the cationic complex [Fe(CO)2(IMes)][I] (C159) and the neutral complex [Fe(I)(CO)(IMes)] (C139), which was selectively obtained by visible photoirradiation of C159 in CH2Cl2 (Scheme 75). They were efficient in the reduction of both aldehydes and ketones using 1.0 equiv of PhSiH3 (aldehydes, THF, 30 °C, 3 h, 88−99% conversion; ketones, toluene, 70 °C, 16 h, 50−99% conversion).318 Notably, lower activities were obtained with ketones, electron-deficient acetophenone derivatives being easier to reduce. It must be underlined that the visible light activation is required to generate the active catalyst from the cationic complex C159, albeit neutral complex C160 was active at 30 °C without activation. When conducting the transformation under solvent-free conditions and light irradiation, significant rate enhancements were obtained as the reactions proceeded with higher conversions and yields and at lower temperatures (50 versus 70 °C).319 Interestingly, nitrile, amine, and alkene groups were tolerated. Furthermore, NHC ligands such as IMes exhibited a significant influence on the activity, and this effect was clearly

unusual and new iron silyl dihydride complex C153 (5.0 mol %) active for the hydrosilylation of benzaldehyde with H2SiMePh at 50 °C for 12 h (Scheme 74).315 Noteworthy, the parent cationic complex C152 (5.0 mol %) was also efficient in the hydrosilylation of benzaldehyde with PhSiH3 at 22 °C for 3 h. Scheme 74. Cyclopentadienyl Iron Phosphine-Based Complexes

Our group developed an analogous series of cationic carbonyl complexes [Fe(Cp)(CO)2(PR3)][X]316 (C154− C158, Scheme 74) which successfully catalyzed the hydrosilylation of both aldehydes and ketones. Using 5.0 mol % of catalysts C154−C156 and 1.1 equiv of phenylsilane at 30 °C for 16 h under continuous visible light irradiation, the reduction of benzaldehyde derivatives proceeded with excellent conversions either in THF (92−98%) or under neat conditions (91−97%). Importantly, PMHS (4.0 equiv) could also be used in the presence of 5.0 mol % of complexes C154−C156 under similar conditions in THF: the best conversion reached 95%. In this series, the best catalyst identified for the reduction of AE


Chemical Reviews

Review

(CO)2(NHC)][I] (e.g., C163) has shown moderate activity as full conversions were obtained only at 100 °C (PhSiH3, 0.5−4 h, neat conditions, without light activation) (Scheme 75).321 By contrast, the complex [Fe(Cp)(NHC)(CO)2] (C164, 1.0 mol %) with a cyclic, six-membered N-heterocyclic carbene ligand incorporating a malonate backbone efficiently performed the hydrosilylation of aromatic aldehydes using 1.0 equiv of diphenylsilane at 30 °C (full conversion after 1−3 h) and for reduction of acetophenone derivatives in the presence of 1.0 equiv of phenylsilane at 70 °C (37−98% conversions after 16 h), thus exhibiting comparable activity to C159 (Scheme 75).322 Likely, benzimidazole-based NHC iron complexes such as C165 (2.0 mol %) showed analogous activity for the hydrosilylation of benzaldehyde at 30 °C within 3 h and acetophenone at 70 °C within 17 h (1.2 equiv PhSiH3, visible light irradiation, neat, Scheme 75).323 In 2017, Albrecht developed a new series of 1,2,3triazolylidene iron(II) piano-stool complexes such as C166 (an analog of C128), which exhibited high activity in the hydrosilylation of aldehydes and ketones: under relatively mild conditions (1,2-dichloroethane, 60 °C), in the presence of 1.2 equiv of phenylsilane, TOFs up to 14 400 h−1 at 0.1 mol % catalyst loading (for the hydrosilylation of 4-bromobenzaldehyde) were obtained. Notably an induction period of 20−30 min was observed (Scheme 75). Preliminary mechanistic investigations suggested the formation of radical species and the involvement of a SET-type mechanism for hydrosilanes activation.324 Adolfsson has shown that an in-situ-prepared iron complex, starting from iron(II) acetate salt and the imidazolium salt precursors IPr·HCl (L27) or N-hydroxyethyl-imidazolium (L28) in the presence of a base such as n-BuLi, can be used efficiently for the hydrosilylation of ketones with 3.0 equiv of PHMS in THF at 65 °C (Scheme 76).325,326 Importantly, the

Scheme 75. Representative Cp-NHC Iron Complexes for Catalyzed Hydrosilylation Reactions

Scheme 76. Adolfsson’s Catalytic System for Hydrosilylation of Ketones

demonstrated: when using C159 versus [Fe(Cp)(CO)2]2 (C161) or [Fe(Cp)(I)(CO)2] (C162) precursors as the catalysts for benzaldehyde reduction, after 3 h at 30 °C, the conversions are >97% for C159 vs 20:1 to 3:1 (Scheme 119).420 Noticeably, a good functional group tolerance was observed (amino, sulphide, silylether, halides, remote CC bonds, thienyl). Attempts to perform asymmetric transformation of vinylcyclopropanes were also performed using the complex

Scheme 117. Iron-Catalyzed Hydroboration of Alkynes

Scheme 119. Iron-Catalyzed Hydroboration of Vinylcyclopropanes

then obtained with high stereoselectivity. Starting from terminal alkynes, (E)-vinylboronate derivatives were obtained selectively (17 examples, 40−99% yields), whereas more complex mixtures of regioisomers were obtained with internal nonsymmetric alkynes.416 Using FeCl3 or iron nanoparticles from Fe3O4 (5.0 mol %) in the presence of 1.2 equiv of B2pin2 and 2.0 equiv of Cs2CO3 in acetone at 60 °C, terminal alkynes led selectively to (E)-vinylboronates in 65−98% yields (Scheme 117). It is interesting to note that NP Fe3O4 can be recycled up to 6 times without significant loss of activity.417 AT


Chemical Reviews

Review

C210, and excellent enantioselectivities were obtained (77− 98% ee).

Scheme 121. Iron-Catalyzed Dehydrogenative Borylation of Alkynes

6.2. Dehydrogenative Borylation

An attractive alternative way for preparation of vinyl boronate esters can be performed by the catalytic dehydrogenative borylation of styrene derivatives. In 2016, Ge,421 Sortais, and Darcel422 reported such selective dehydrogenative borylation of vinylarenes with pinacolborane catalyzed by molecular defined iron complexes. Thus, using [Fe(PMe3)4] (C211, 3.0− 5.0 mol %) in the presence of 1.2 equiv of monosubstituted and disubstituted vinylarenes and 2.0 equiv of norbornene in hexane at 50 °C for 18 h, E-vinyl boronate esters were selectively produced in high yields of 60−94% (Scheme 120).421 Good functional group tolerance was shown as acetal,

Scheme 122. Iron-Catalyzed Hydroboration of Aldehydes and Ketones

Scheme 120. Iron-Catalyzed Dehydrogenative Borylation of Alkenes

imines, furyl, amino, boronate, and Ph2P were not altered. It must be noticed that norbornene is crucial for the success of the transformation: indeed, when the hydroboration was conducted in the absence of hydrogen acceptors, only a trace amount of the desired product was detected. Our group developed the synthesis of a series of dicarbonyl PCP−iron hydride complexes422 and has shown that they are active and selective catalytic precursors for the dehydrogenative borylation of styrene with HBpin. Using C212 or C213 (5.0 mol %), HBpin (1.0 equiv) reacted with styrene at room temperature under neat conditions and UV irradiation (350 nm) for 72 h, giving the corresponding vinyl boronate ester with up to 80% isolated yields (Scheme 120). Noticeably, no hydrogen scavenger was required to promote the reaction. Recently, a catalytic system based on Fe(OTf)2 (2.5 mol %) and DABCO (1.0 mol %) that selectively promoted the dehydrogenative borylation of both aromatic and aliphatic terminal alkynes was reported by us to afford alkynylboronate derivatives in the presence of 1 equiv of pinacolborane. This methodology was applicable to a variety of terminal alkynes (16 examples, yield 62−93%, Scheme 121) at 100 °C in toluene.423 6.3. Carbonyl Derivatives

Even if it is an important reaction in molecular synthesis and it is well exemplified with numerous transition metals, hydroboration of aldehydes and ketones has been performed with iron since 2017. Findlater then reported the first contribution using Fe(acac)3 C2 (10 mol %) as precatalyst in combination with NaHBEt3 (10 mol %) in the presence of HBpin (1.5 equiv) in THF at room temperature within 24 h (Scheme 122).424 The hydroboration of aldehydes and ketones gave

after hydrolysis primary and secondary alcohols, respectively, with 62−99% yields. Noticeably, the hydroboration of transAU


Chemical Reviews

Review

cinnamaldehyde led to a mixture of α,β-unsaturated 3-phenyl2-propen-1-ol and 2-phenylethanol in a 5.1:1 ratio. Bai then described the use of a low-coordinated homoleptic tri-tert-butyl-phosphoranimido−iron(II) dimer (C214) for the same reaction. Using a lower catalytic loading of C214 (2.5 mol %) without additive, in the presence of 1.1 equiv of HBpin in benzene at room temperature for 13 h, various aldehydes and ketones led to the corresponding alcohols in 59−95% yields after a simple acidic quench (Scheme 122).425 Tong and Wang also performed the hydroboration of carbonyl derivatives using an iron(II) hydride complex bearing a tetradentate PSNP ligand (C215, 1.0 mol %) with 1.1 equiv of HBpin in C6D6 at room temperature for 20−40 min.426 In 2018, Hein and Baker reported the use of an imine-coupled iron complex [Fe−(N2S2)]2 (C216) able to catalyze the hydroboration of aliphatic and aromatic aldehydes (Scheme 122). Using low catalyst loading (0.1 mol %) and 1 equiv of pinacolborane, aldehydes led to borylated ethers by reaction at room temperature in C6D6 for 30 min with 26−99% yields. Under such conditions, ketones, nitriles, alkenes, amines, and halides were tolerated.427 These catalytic systems are to date the more active ones for this transformation. Wang reported the use of a N2-bridged diiron complex [[Cp*-(Ph2PC6H4S)Fe]2(μ-N2)] (C217, 1.0 mol %) for the catalytic hydroboration of N-heteroarenes (Scheme 123). In

(Scheme 124). If the reduction is conducted in THF for 47 min with 9-BBN (9-borabicyclo[3.3.1]nonane), bis(boryl)Scheme 124. Iron-Catalyzed Hydroboration of CO2

Scheme 123. Iron-Catalyzed Hydroboration of Heterocycles

acetal was produced in 85% yield and 8% methoxyborane was detected as a byproduct.431 More interestingly, this bis(boryl)acetal derived from 9-BBN was used as a methylene source as a surrogate of formaldehyde. As representative examples, the bis(boryl)acetal, generated in situ starting from carbon dioxide, reacted with 2,6diisopropylaniline giving the corresponding methylene imine in 83% yield, with 2-methylaminophenol affording the corresponding hemiaminal in 67% yield, and with N,Ndimethyl-1,2-benzenediamine yielding the corresponding aminal (77%, Scheme 125). The hydroboration of CO2 can be catalyzed by the iron triphosphine (L34) complex (C219, Scheme 126). One atmosphere of carbon dioxide reacted with 5.0 equiv of 9BBN in the presence of 1.5 mol % of C219 in CD3CN at 60 °C for 24 h, leading to a 95:5 mixture methoxyborane/ bis(boryl)acetal. Noticeably, after 40 h reaction time, the methoxyborane was specifically obtained in 99% conversion.432

the presence of 2 equiv of pinacolborane in C6D6 at 50 °C for 24 h, the N-borylated 1,2-reduced derivatives were obtained with high regioselectivity in 61−99% yields.428 Mechanistic studies suggested that the catalytic cycle started by the coordination of the heterocyclic substrate to the iron center, followed by B−H bond cleavage favored by coordination of the boron to the sulfur atom.

7. CONCLUSION This review highlights the main advances made in the area of iron-catalyzed, chemoselective reduction of alkenes, alkynes, carbonyl derivatives, carboxylic compounds, and CO2. In particular, it was shown that an accurate design of the catalytic iron system is crucial to perform highly chemoselective transformations. Furthermore, these reported results demonstrate without any doubts the high potential of the earthabundant, inexpensive, benign iron transition metal to realize reduction, hydroelementation, and hydrogen-borrowing reactions under mild conditions. Since two decades, a rebirth began in iron-catalyzed transformations which represent an impressive potential for future academic or industrial applications. Even if many of the iron-catalyzed reactions became competitive as the ones conducted with noble metals, they did not outperform their activities. Thus, further improvements in terms of activity, productivity, and catalyst cost have to be done to become attractive. Notably, based on mechanistic aspects, a fine design of the catalyst has to be achieved to decrease significantly catalyst loadings and to perform the catalytic transformation at ambient conditions. In terms of catalyst design, innocent as well as noninnocent redox-active and tri- or tetradentate cooperative ligands have already shown impressive and promising results. NHC-, cyclopentadienyl-, and organophosphorus-based complexes have also revealed outstanding activities and selectivities. Notably, in-depth mechanistic studies will be crucial to

6.4. Carbon Dioxide

The reduction of carbon dioxide can be also usefully performed via hydroboration. This transformation was already efficiently done using several systems including transition metals.429,430 By contrast, only scarce examples involving ironbased catalysts were recently reported. Sabo-Etienne and Bontemps paved the way using an iron dihydride complex [(dmpe)2Fe(H)2] (C218, 5.0 mol %, analogue of C180) to reduce selectively CO2 (1 atm) into methoxyborane in 59% yield in the presence of 1.0 equiv of HBcat in C6D6 for 3 h AV


Chemical Reviews

Review

Scheme 125. Sequential Iron-Catalyzed Hydroboration/Functionalization of CO2

Scheme 126. Iron Triphosphine Complex Catalyzed Hydroboration of CO2

performed his master’s research training in Prof. Christophe Darcel’s team; he currently works towards his Ph.D. degree from the University of Rennes 1 under the supervision of Prof. Christophe Darcel and Prof. Jean-Baptiste Sortais. His work focuses on the homogeneous iron- and manganese-catalyzed reduction and dehydrogenation reactions. Christophe Darcel grew up in the north coast of Britany and studied chemistry at the University of Rennes 1, where he received his Ph.D. degree in 1995 under the supervision of Dr. C. Bruneau and Prof. P. H. Dixneuf on ruthenium- and palladium-catalyzed transformations of functional alkynes. He first spent 1 year as a postdoctoral fellow in Geneva (Switzerland) with Prof. W. Oppolzer. He then obtained a Humboldt fellowship and joined Prof. P. Knochel group in Marburg (Germany). He was then appointed at the University of CergyPontoise and at the University of Burgundy as Associate Professor. In 2007, he was appointed Full Professor at the University of Rennes 1. His current research interests concentrate on homogeneous catalysis with particular emphasis on well-defined iron complexes in catalysis.

elucidate reaction pathways, important steps in the elaboration of finely designed iron catalysts able to reach the best activity and selectivity. In terms of catalytic reactions, several transformations have to be developed: (i) the use of carbon dioxide as a C1 building block for catalytic methylation reactions and the efficient formation of formic acid and methanol, (ii) the selective reduction of carboxylic derivatives at lower catalyst loading and at ambient conditions, (iii) the achievements of highly efficient and general asymmetric reduction of CC and CO moieties, and (iv) the selective hydroborylation of alkynes and alkenes, notably in an asymmetric version. These initial achievements, in particular, for the challenging reductions of carboxylic acid derivatives and carbon dioxide, have already allowed very impressive advances and should stimulate the utilization of iron-catalyzed methodologies in large-scale synthesis and fine chemistry.

ACKNOWLEDGMENTS We gratefully acknowledge funding from the University de Rennes 1, the Centre National de la Recherche Scientifique (CNRS), the CNRS Federation Increase, and the CNRS International Associated Laboratory ChemSusCat. ABBREVIATIONS ATH = asymmetric transfer hydrogenation 9-BBN = 9-borabicyclo[3.3.1]nonane cat = catechol COD = 1,5-cyclooctadiene Cp = cyclopentadienyl CPME = cyclopentyl methyl ether DBU = 1,8-diazabicyclo[5.4.0]undec-7-ene DCE = 1,2-dichloroethane dcpe = 1,2-bisdicyclohexylphosphinoethane DFT = density functional theory Dipp = 2,6-diisopropylphenyl DMC = dimethyl carbonate dr = diastereomeric ratio DRA = direct reductive amination ee = enantiomeric excess equiv = equivalent er = enantiomeric ratio FA = formic acid IMes = 1,3-bis(2,4,6-trimethylphenyl) imidazole-2-ylidene IPr = 1,3-bis(2,6-diisopropylphenyl) imidazole-2-ylidene KHMDS = potassium hexamethyldisiloxane MD′M = (Me3SiO)2MeSiH

AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected]. ORCID

Duo Wei: 0000-0002-5928-3151 Christophe Darcel: 0000-0001-6711-5978 Notes

The authors declare no competing financial interest. Biographies Duo Wei was born in Hebei Province, People’s Republic of China, in 1991. He received his B.S. degree in 2014 from Zhejiang Sci-Tech University under the supervision of Prof. Xiao-Feng Wu. Then he joined the international master’s program “Catalysis, Molecules and Green Chemistry” in the University of Rennes 1 (France). He AW


Chemical Reviews

Review

(18) Morris, R. H. Asymmetric Hydrogenation, Transfer Hydrogenation and Hydrosilylation of Ketones Catalyzed by Iron Complexes. Chem. Soc. Rev. 2009, 38, 2282−2291. (19) van der Vlugt, J. I. Cooperative Catalysis with First-Row Late Transition Metals. Eur. J. Inorg. Chem. 2012, 2012, 363−375. (20) Sues, P. E.; Demmans, K. Z.; Morris, R. H. Rational Development of Iron Catalysts for Asymmetric Transfer Hydrogenation. Dalton Trans 2014, 43, 7650−7667. (21) Ito, J.-i.; Nishiyama, H. Recent Topics of Transfer Hydrogenation. Tetrahedron Lett. 2014, 55, 3133−3146. (22) Guo, N.; Zhu, S. F. Iron-Catalyzed Hydrogenation Reactions. Youji Huaxue 2015, 35, 1383−1398. (23) Chirik, P. J. Iron- and Cobalt-Catalyzed Alkene Hydrogenation: Catalysis with Both Redox-Active and Strong Field Ligands. Acc. Chem. Res. 2015, 48, 1687−1695. (24) Morris, R. H. Exploiting Metal-Ligand Bifunctional Reactions in the Design of Iron Asymmetric Hydrogenation Catalysts. Acc. Chem. Res. 2015, 48, 1494−1502. (25) Li, Y.-Y.; Yu, S.-L.; Shen, W.-Y.; Gao, J.-X. Iron-, Cobalt-, and Nickel-Catalyzed Asymmetric Transfer Hydrogenation and Asymmetric Hydrogenation of Ketones. Acc. Chem. Res. 2015, 48, 2587− 2598. (26) Wang, D.; Astruc, D. The Golden Age of Transfer Hydrogenation. Chem. Rev. 2015, 115, 6621−6686. (27) Renaud, J.-L.; Gaillard, S. Recent Advances in Iron- and CobaltComplex-Catalyzed Tandem/Consecutive Processes Involving Hydrogenation. Synthesis 2016, 48, 3659−3683. (28) Š tefane, B.; Požgan, F. Metal-Catalysed Transfer Hydrogenation of Ketones. In Hydrogen Transfer Reactions; Guillena, G., Ramón, D., Eds.; Springer, 2016; pp 1−67. (29) Mezzetti, A. Iron Complexes with Chiral N/P Macrocycles as Catalysts for Asymmetric Transfer Hydrogenation. Isr. J. Chem. 2017, 57, 1090−1105. (30) Gorgas, N.; Kirchner, K. Isoelectronic Manganese and Iron Hydrogenation/Dehydrogenation Catalysts: Similarities and Divergences. Acc. Chem. Res. 2018, 51, 1558−1569. (31) Filonenko, G. A.; van Putten, R.; Hensen, E. J.; Pidko, E. A. Catalytic (de)Hydrogenation Promoted by Non-Precious Metals−Co, Fe and Mn: Recent Advances in an Emerging Field. Chem. Soc. Rev. 2018, 47, 1459−1483. (32) Zell, T.; Langer, R. From Ruthenium to Iron and Manganese-A Mechanistic View on Challenges and Design Principles of Base-Metal Hydrogenation Catalysts. ChemCatChem 2018, 10, 1930−1940. (33) Zhang, Z.; Butt, N. A.; Zhou, M.; Liu, D.; Zhang, W. Asymmetric Transfer and Pressure Hydrogenation with EarthAbundant Transition Metal Catalysts. Chin. J. Chem. 2018, 36, 443−454. (34) Zhang, M.; Zhang, A. Iron-Catalyzed Hydrosilylation Reactions. Appl. Organomet. Chem. 2010, 24, 751−757. (35) Nakajima, Y.; Shimada, S. Hydrosilylation Reaction of Olefins: Recent Advances and Perspectives. RSC Adv. 2015, 5, 20603−20616. (36) Greenhalgh, M. D.; Jones, A. S.; Thomas, S. P. Iron-Catalysed Hydrofunctionalisation of Alkenes and Alkynes. ChemCatChem 2015, 7, 190−222. (37) Du, X.; Huang, Z. Advances in Base-Metal-Catalyzed Alkene Hydrosilylation. ACS Catal. 2017, 7, 1227−1243. (38) Frankel, E. N.; Emken, E. A.; Peters, H. M.; Davison, V. L.; Butterfield, R. O. Homogeneous Hydrogenation of Methyl Linoleate Catalyzed by Iron Pentacarbonyl. Characterization of Methyl Octadecadienoate-Iron Tricarbonyl Complexes. J. Org. Chem. 1964, 29, 3292−3297. (39) Frankel, E. N.; Peters, H. M.; Jones, E. P.; Dutton, H. J. Homogeneous Catalytic Hydrogenation of Unsaturated Fats: Iron Pentacarbonyl. J. Am. Oil Chem. Soc. 1964, 41, 186−191. (40) Frankel, E.; Emken, E.; Davison, V. Homogeneous Hydrogenation of Methyl Linolenate Catalyzed by Iron Pentacarbonyl. Formation of Methyl Octadecatrienoate-Iron Tricarbonyl Complexes. J. Org. Chem. 1965, 30, 2739−2745.

Mes = 2,4,6-Me3C6H2 Na2EDTA = ethylenediaminetetraacetic acid disodium salt NHC = N-heterocyclic carbene NPs = nanoparticles OPPA = oxazolinylphenyl picolinamide PC = propylene carbonate Pin = pinacol PMHS = polymethylhydrosiloxane t-PBO = trans-4-phenyl-but-3-en-2-one terpy = 2,2′: 6′,2′′-terpyridine TFA = trifluoroacetic acid TH = transfer hydrogenation TMANO = trimethylamine N-oxide TMDS = 1,1,3,3-tetramethyldisiloxane TMEDA = N,N,N′,N′-tetramethyethylene-diamine TMPP = tris(2,4,6-trimethoxyphenyl)phosphine TOF = turnover frequency TON = turnover number

REFERENCES (1) Darcel, C.; Sortais, J.-B. Iron-Catalyzed Reduction and Hydroelementation Reactions. In Iron Catalysis II; Bauer, E., Ed.; Springer, 2015; Vol. 50, pp 173−216. Darcel, C.; Sortais, J.-B. Ironcatalyzed Reduction and Hydroelementation Reactions. Top. Organomet. Chem. 2015, 50, 173−216. (2) Nakazawa, H.; Itazaki, M. Fe-H Complexes in Catalysis. In Iron Catalysis; Plietker, B., Ed.; Springer: Berlin, Heidelberg, 2011; Vol. 33, pp 27−81. (3) Misal Castro, L. C.; Li, H.; Sortais, J.-B.; Darcel, C. When Iron Met Phosphines: a Happy Marriage for Reduction Catalysis. Green Chem. 2015, 17, 2283−2303. (4) Bolm, C.; Legros, J.; Le Paih, J.; Zani, L. Iron-Catalyzed Reactions in Organic Synthesis. Chem. Rev. 2004, 104, 6217−6254. (5) Bauer, E. B. Recent Advances in Iron Catalysis in Organic Synthesis. Curr. Org. Chem. 2008, 12, 1341−1369. (6) Plietker, B. Iron Catalysis in Organic Chemistry: Reactions and Applications; Wiley-VCH: Weinheim, Germany, 2008. (7) Plietker, B. Iron Catalysis: Fundamentals and Applications; Springer-Verlag: Berlin Heidelberg, 2011; Vol. 33. (8) Riener, K.; Haslinger, S.; Raba, A.; Högerl, M. P.; Cokoja, M.; Herrmann, W. A.; Kühn, F. E. Chemistry of Iron N-Heterocyclic Carbene Complexes: Syntheses, Structures, Reactivities, and Catalytic Applications. Chem. Rev. 2014, 114, 5215−5272. (9) Bauer, I.; Knölker, H.-J. Iron Catalysis in Organic Synthesis. Chem. Rev. 2015, 115, 3170−3387. (10) Ollevier, T.; Keipour, H. Enantioselective Iron Catalysts. In Iron Catalysis II; Bauer, E., Ed.; Springer-Verlag: Berlin, 2015; Vol. 50, pp 259−309. (11) He, X.; Hu, X.; Tao, J.; Han, G.; Shang, Y. Progress in Iron Complexes-Catalyzed Organic Reactions. Youji Huaxue 2016, 36, 1465−1483. (12) Dai, H.; Guan, H. Iron Dihydride Complexes: Synthesis, Reactivity, and Catalytic Applications. Isr. J. Chem. 2017, 57, 1170− 1203. (13) Gaillard, S.; Renaud, J.-L. Iron-Catalyzed Hydrogenation, Hydride Transfer, and Hydrosilylation: An Alternative to PreciousMetal Complexes? ChemSusChem 2008, 1, 505−509. (14) Junge, K.; Schröder, K.; Beller, M. Homogeneous Catalysis Using Iron Complexes: Recent Developments in Selective Reductions. Chem. Commun. 2011, 47, 4849−4859. (15) Le Bailly, B. A. F.; Thomas, S. P. Iron-Catalysed Reduction of Carbonyls and Olefins. RSC Adv. 2011, 1, 1435−1445. (16) Bauer, G.; Kirchner, K. A. Well-Defined Bifunctional Iron Catalysts for the Hydrogenation of Ketones: Iron, the New Ruthenium. Angew. Chem., Int. Ed. 2011, 50, 5798−5800. (17) Lopes, R.; Royo, B. Iron N-Heterocyclic Carbenes in Reduction Reactions. Isr. J. Chem. 2017, 57, 1151−1159. AX


Chemical Reviews

Review

Hydrogenation of Hindered, Unfunctionalized Alkenes. ACS Catal. 2012, 2, 1760−1764. (60) Trovitch, R. J.; Lobkovsky, E.; Chirik, P. J. Bis(diisopropylphosphino)pyridine Iron Dicarbonyl, Dihydride, and Silyl Hydride Complexes. Inorg. Chem. 2006, 45, 7252−7260. (61) Xu, R.; Chakraborty, S.; Bellows, S. M.; Yuan, H.; Cundari, T. R.; Jones, W. D. Iron-Catalyzed Homogeneous Hydrogenation of Alkenes under Mild Conditions by a Stepwise, Bifunctional Mechanism. ACS Catal. 2016, 6, 2127−2135. (62) Ott, J. C.; Blasius, C. K.; Wadepohl, H.; Gade, L. H. Synthesis, Characterization, and Reactivity of a High-Spin Iron(II) Hydrido Complex Supported by a PNP Pincer Ligand and Its Application as a Homogenous Catalyst for the Hydrogenation of Alkenes. Inorg. Chem. 2018, 57, 3183−3191. (63) Srimani, D.; Diskin-Posner, Y.; Ben-David, Y.; Milstein, D. Iron Pincer Complex Catalyzed, Environmentally Benign, E-Selective Semi-Hydrogenation of Alkynes. Angew. Chem., Int. Ed. 2013, 52, 14131−14134. (64) Bart, S. C.; Hawrelak, E. J.; Lobkovsky, E.; Chirik, P. J. LowValent α-Diimine Iron Complexes for Catalytic Olefin Hydrogenation. Organometallics 2005, 24, 5518−5527. (65) Villa, M.; Miesel, D.; Hildebrandt, A.; Ragaini, F.; Schaarschmidt, D.; Jacobi von Wangelin, A. Synthesis and Catalysis of Redox-Active Bis (imino) acenaphthene (BIAN) Iron Complexes. ChemCatChem 2017, 9, 3203−3209. (66) Hoyt, J. M.; Shevlin, M.; Margulieux, G. W.; Krska, S. W.; Tudge, M. T.; Chirik, P. J. Synthesis and Hydrogenation Activity of Iron Dialkyl Complexes with Chiral Bidentate Phosphines. Organometallics 2014, 33, 5781−5790. (67) Sunada, Y.; Ogushi, H.; Yamamoto, T.; Uto, S.; Sawano, M.; Tahara, A.; Tanaka, H.; Shiota, Y.; Yoshizawa, K.; Nagashima, H. Disilaruthena-and Ferracyclic Complexes Containing Isocyanide Ligands as Effective Catalysts for Hydrogenation of Unfunctionalized Sterically Hindered Alkenes. J. Am. Chem. Soc. 2018, 140, 4119−4134. (68) Kamitani, M.; Nishiguchi, Y.; Tada, R.; Itazaki, M.; Nakazawa, H. Synthesis of Fe−H/Si−H and Fe−H/Ge−H Bifunctional Complexes and Their Catalytic Hydrogenation Reactions toward Nonpolar Unsaturated Organic Molecules. Organometallics 2014, 33, 1532−1535. (69) Gärtner, D.; Welther, A.; Rad, B. R.; Wolf, R.; Jacobi von Wangelin, A. Heteroatom-Free Arene-Cobalt and Arene-Iron Catalysts for Hydrogenations. Angew. Chem., Int. Ed. 2014, 53, 3722−3726. (70) Bü s chelberger, P.; Gär tner, D.; Reyes-Rodriguez, E.; Kreyenschmidt, F.; Koszinowski, K.; Jacobi von Wangelin, A.; Wolf, R. Alkene Metalates as Hydrogenation Catalysts. Chem. - Eur. J. 2017, 23, 3139−3151. (71) Gieshoff, T. N.; Villa, M.; Welther, A.; Plois, M.; Chakraborty, U.; Wolf, R.; Jacobi von Wangelin, A. Iron-Catalyzed Olefin Hydrogenation at 1 bar H2 with a FeCl3−LiAlH4 Catalyst. Green Chem. 2015, 17, 1408−1413. (72) Phua, P.-H.; Lefort, L.; Boogers, J. A. F.; Tristany, M.; de Vries, J. G. Soluble Iron Nanoparticles as Cheap and Environmentally Benign Alkene and Alkyne Hydrogenation Catalysts. Chem. Commun. 2009, 3747−3749. (73) Rangheard, C.; de Julián Fernández, C.; Phua, P.-H.; Hoorn, J.; Lefort, L.; de Vries, J. G. At the Frontier between Heterogeneous and Homogeneous Catalysis: Hydrogenation of Olefins and Alkynes with Soluble Iron Nanoparticles. Dalton Trans 2010, 39, 8464−8471. (74) Bedford, R. B.; Betham, M.; Bruce, D. W.; Davis, S. A.; Frost, R. M.; Hird, M. Iron Nanoparticles in the Coupling of Alkyl Halides with Aryl Grignard Reagents. Chem. Commun. 2006, 1398−1400. (75) Stein, M.; Wieland, J.; Steurer, P.; Tölle, F.; Mülhaupt, R.; Breit, B. Iron Nanoparticles Supported on Chemically-Derived Graphene: Catalytic Hydrogenation with Magnetic Catalyst Separation. Adv. Synth. Catal. 2011, 353, 523−527. (76) Hudson, R.; Hamasaka, G.; Osako, T.; Yamada, Y. M.; Li, C.-J.; Uozumi, Y.; Moores, A. Highly Efficient Iron(0) Nanoparticle-

(41) Lynch, T. J.; Banah, M.; Kaesz, H. D.; Porter, C. R. Hydrogenation of Anthracene and Nitrogen Heterocycles Catalyzed by Iron Pentacarbonyl under Water-Gas Shift Conditions. J. Org. Chem. 1984, 49, 1266−1270. (42) Emken, E. A.; Frankel, E. N.; Butterfield, R. O. Homogeneous Catalytic Hydrogenation of Unsaturated Fats: Metal Acetylacetonates. J. Am. Oil Chem. Soc. 1966, 43, 14−18. (43) Schroeder, M. A.; Wrighton, M. S. Pentacarbonyliron(0) Photocatalyzed Hydrogenation and Isomerization of Olefins. J. Am. Chem. Soc. 1976, 98, 551−558. (44) Inoue, H.; Suzuki, M. Catalytic Function of the PhenylLithium-Treated Fe4S4Cl42-Cluster in the Hydrogenation of cis- and trans-Stilbenes. J. Chem. Soc., Chem. Commun. 1980, 817−818. (45) Inoue, H.; Sato, M. Substrate Selectivity of the Phenyl-Lithium Activated Fe4S4Cl42-Cluster as a Catalyst in the Hydrogenation of Octenes with Hydrogen Gas. J. Chem. Soc., Chem. Commun. 1983, 983−984. (46) Vol’pin, M. E.; Kolomnikov, I. S. Homogeneous Hydrogenation. Russ. Chem. Rev. 1969, 38, 273. (47) Sordakis, K.; Tang, C.; Vogt, L. K.; Junge, H.; Dyson, P. J.; Beller, M.; Laurenczy, G. Homogeneous Catalysis for Sustainable Hydrogen Storage in Formic Acid and Alcohols. Chem. Rev. 2018, 118, 372−433. (48) Nishiguchi, T.; Fukuzumi, K. Homogeneous Transfer-Hydrogenation of Olefins Catalysed by Fe, Co, and Ni Complexes: o- and pDihydroxybenzene as Hydrogen Donors. J. Chem. Soc. D 1971, 139− 140. (49) Nishiguchi, T.; Fukuzumi, K. Transfer-Hydrogenation and Transfer-Hydrogenolysis. I. Transfer-Hydrogenolysis of Cyclooctadienes Catalyzed by Dichlorobis(triphenylphosphine) Iron(II). Bull. Chem. Soc. Jpn. 1972, 45, 1656−1660. (50) Bianchini, C.; Meli, A.; Peruzzini, M.; Vizza, F.; Zanobini, F.; Frediani, P. A Homogeneous Iron(II) System Capable of Selectively Catalyzing the Reduction of Terminal Alkynes to Alkenes and Buta1,3-dienes. Organometallics 1989, 8, 2080−2082. (51) Bianchini, C.; Meli, A.; Peruzzini, M.; Frediani, P.; Bohanna, C.; Esteruelas, M. A.; Oro, L. A. Selective Hydrogenation of 1-Alkynes to Alkenes Catalyzed by an Iron(II) cis-Hydride η2-Dihydrogen Complex. A Case of Intramolecular Reaction between η2-H2 and σVinyl Ligands. Organometallics 1992, 11, 138−145. (52) Wienhöfer, G.; Westerhaus, F. A.; Jagadeesh, R. V.; Junge, K.; Junge, H.; Beller, M. Selective Iron-Catalyzed Transfer Hydrogenation of Terminal Alkynes. Chem. Commun. 2012, 48, 4827−4829. (53) Bianchini, C.; Farnetti, E.; Graziani, M.; Peruzzini, M.; Polo, A. Chemoselective Hydrogen-Transfer Reduction of α,β-Unsaturated Ketones Catalyzed by Isostructural Iron(II), Ruthenium(II), and Osmium(II) cis-Hydride (η2-Dihydrogen) Complexes. Organometallics 1993, 12, 3753−3761. (54) Bhor, M. D.; Panda, A. G.; Jagtap, S. R.; Bhanage, B. M. Hydrogenation of α, β-Unsaturated Carbonyl Compounds Using Recyclable Water-Soluble FeII/EDTA Complex Catalyst. Catal. Lett. 2008, 124, 157−164. (55) Daida, E. J.; Peters, J. C. Considering FeII/IV Redox Processes as Mechanistically Relevant to the Catalytic Hydrogenation of Olefins by [PhBPiPr3] Fe−Hx Species. Inorg. Chem. 2004, 43, 7474−7485. (56) Fong, H.; Moret, M.-E.; Lee, Y.; Peters, J. C. Heterolytic H2 Cleavage and Catalytic Hydrogenation by an Iron Metallaboratrane. Organometallics 2013, 32, 3053−3062. (57) Bart, S. C.; Lobkovsky, E.; Chirik, P. J. Preparation and Molecular and Electronic Structures of Iron(0) Dinitrogen and Silane Complexes and Their Application to Catalytic Hydrogenation and Hydrosilation. J. Am. Chem. Soc. 2004, 126, 13794−13807. (58) Trovitch, R. J.; Lobkovsky, E.; Bill, E.; Chirik, P. J. Functional Group Tolerance and Substrate Scope in Bis(imino)pyridine IronCatalyzed Alkene Hydrogenation. Organometallics 2008, 27, 1470− 1478. (59) Yu, R. P.; Darmon, J. M.; Hoyt, J. M.; Margulieux, G. W.; Turner, Z. R.; Chirik, P. J. High-Activity Iron Catalysts for the AY


Chemical Reviews

Review

Catalyzed Hydrogenation in Water in Flow. Green Chem. 2013, 15, 2141−2148. (77) Kelsen, V.; Wendt, B.; Werkmeister, S.; Junge, K.; Beller, M.; Chaudret, B. The Use of Ultrasmall Iron(0) Nanoparticles as Catalysts for the Selective Hydrogenation of Unsaturated C−C Bonds. Chem. Commun. 2013, 49, 3416−3418. (78) Gieshoff, T. N.; Chakraborty, U.; Villa, M.; Jacobi von Wangelin, A. Alkene Hydrogenations by Soluble Iron Nanocluster Catalysts. Angew. Chem., Int. Ed. 2017, 56, 3585−3589. (79) Markó, L.; Radhi, M. A.; Ö tvös, I. Catalytic and Stoichiometric Reduction of Ketones and Aldehydes by the Hydridotetracarbonyl Ferrate Anion. J. Organomet. Chem. 1981, 218, 369−376. (80) Markó, L.; Palágyi, J. Hydrogenation of Aldehydes and Ketones with Molecular Hydrogen Using Iron Pentacarbonyl as Catalyst Precursor. Transition Met. Chem. 1983, 8, 207−209. (81) Casey, C. P.; Guan, H. An Efficient and Chemoselective Iron Catalyst for the Hydrogenation of Ketones. J. Am. Chem. Soc. 2007, 129, 5816−5817. (82) Knö lker, H.-J.; Baum, E.; Goesmann, H.; Klauss, R. Demetalation of Tricarbonyl(cyclopentadienone)iron Complexes Initiated by a Ligand Exchange Reaction with NaOHX-Ray Analysis of a Complex with Nearly Square-Planar Coordinated Sodium. Angew. Chem., Int. Ed. 1999, 38, 2064−2066. (83) Fleischer, S.; Zhou, S.; Junge, K.; Beller, M. General and Highly Efficient Iron-Catalyzed Hydrogenation of Aldehydes, Ketones, and α,β-Unsaturated Aldehydes. Angew. Chem., Int. Ed. 2013, 52, 5120− 5124. (84) Knölker, H.-J.; Heber, J.; Mahler, C. H. Transition Metal-Diene Complexes in Organic Synthesis, Part 14. Regioselective IronMediated [2+2+1] Cycloadditions of Alkynes and Carbon Monoxide: Synthesis of Substituted Cyclopentadienones. Synlett 1992, 1002− 1004. (85) Knölker, H.-J.; Heber, J. Transition Metal-Diene Complexes in Organic Synthesis, Part 18. Iron-Mediated [2+2+1] Cycloadditions of Diynes and Carbon Monoxide: Selective Demetalation Reactions. Synlett 1993, 924−926. (86) Knölker, H.-J.; Baum, E.; Heber, J. Transition Metal-Diene Complexes in Organic Synthesis, Part 25. Cycloadditions of Annulated 2,5-Bis (trimethylsilyl) cyclopentadienones. Tetrahedron Lett. 1995, 36, 7647−7650. (87) Tlili, A.; Schranck, J.; Neumann, H.; Beller, M. Discrete Iron Complexes for the Selective Catalytic Reduction of Aromatic, Aliphatic, and α,β-Unsaturated Aldehydes under Water−Gas Shift Conditions. Chem. - Eur. J. 2012, 18, 15935−15939. (88) Natte, K.; Li, W.; Zhou, S.; Neumann, H.; Wu, X.-F. IronCatalyzed Reduction of Aromatic Aldehydes with Paraformaldehyde and H2O as the Hydrogen Source. Tetrahedron Lett. 2015, 56, 1118− 1121. (89) Mérel, D. S.; Elie, M.; Lohier, J. F.; Gaillard, S.; Renaud, J. L. Bifunctional Iron Complexes: Efficient Catalysts for CO and CN Reduction in Water. ChemCatChem 2013, 5, 2939−2945. (90) Lator, A.; Gaillard, S.; Poater, A.; Renaud, J. L. Iron-Catalyzed Chemoselective Reduction of α, β-Unsaturated Ketones. Chem. - Eur. J. 2018, 24, 5770−5774. (91) Vailati Facchini, S.; Neudörfl, J. M.; Pignataro, L.; Cettolin, M.; Gennari, C.; Berkessel, A.; Piarulli, U. Synthesis of [Bis(hexamethylene)cyclopentadienone]iron Tricarbonyl and its Application to the Catalytic Reduction of CO Bonds. ChemCatChem 2017, 9, 1461−1468. (92) Berkessel, A.; Reichau, S.; von der Höh, A.; Leconte, N.; Neudörfl, J.-M. Light-Induced Enantioselective Hydrogenation Using Chiral Derivatives of Casey’s Iron−Cyclopentadienone Catalyst. Organometallics 2011, 30, 3880−3887. (93) Hodgkinson, R.; Del Grosso, A.; Clarkson, G.; Wills, M. Iron Cyclopentadienone Complexes Derived from C2-Symmetric BisPropargylic Alcohols; Preparation and Applications to Catalysis. Dalton Trans 2016, 45, 3992−4005. (94) Gajewski, P.; Renom-Carrasco, M.; Facchini, S. V.; Pignataro, L.; Lefort, L.; de Vries, J. G.; Ferraccioli, R.; Forni, A.; Piarulli, U.;

Gennari, C. Chiral (Cyclopentadienone)iron Complexes for the Catalytic Asymmetric Hydrogenation of Ketones. Eur. J. Org. Chem. 2015, 1887−1893. (95) Gajewski, P.; Renom-Carrasco, M.; Facchini, S. V.; Pignataro, L.; Lefort, L.; de Vries, J. G.; Ferraccioli, R.; Piarulli, U.; Gennari, C. Synthesis of (R)-BINOL-Derived (Cyclopentadienone)iron Complexes and Their Application in the Catalytic Asymmetric Hydrogenation of Ketones. Eur. J. Org. Chem. 2015, 5526−5536. (96) Casey, C. P.; Guan, H. Cyclopentadienone Iron Alcohol Complexes: Synthesis, Reactivity, and Implications for the Mechanism of Iron-Catalyzed Hydrogenation of Aldehydes. J. Am. Chem. Soc. 2009, 131, 2499−2507. (97) Bullock, R. M. An Iron Catalyst for Ketone Hydrogenations under Mild Conditions. Angew. Chem., Int. Ed. 2007, 46, 7360−7363. (98) Zhang, H.; Chen, D.; Zhang, Y.; Zhang, G.; Liu, J. On the Mechanism of Carbonyl Hydrogenation Catalyzed by Iron Catalyst. Dalton Trans 2010, 39, 1972−1978. (99) Rautenstrauch, V.; Hoang-Cong, X.; Churlaud, R.; AbdurRashid, K.; Morris, R. H. Hydrogenation versus Transfer Hydrogenation of Ketones: Two Established Ruthenium Systems Catalyze Both. Chem. - Eur. J. 2003, 9, 4954−4967. (100) Sui-Seng, C.; Freutel, F.; Lough, A. J.; Morris, R. H. Highly Efficient Catalyst Systems Using Iron Complexes with a Tetradentate PNNP Ligand for the Asymmetric Hydrogenation of Polar Bonds. Angew. Chem., Int. Ed. 2008, 47, 940−943. (101) Sui-Seng, C.; Haque, F. N.; Hadzovic, A.; Pütz, A.-M.; Reuss, V.; Meyer, N.; Lough, A. J.; Zimmer-De Iuliis, M.; Morris, R. H. Synthesis and Characterization of Iron(II) Complexes with Tetradentate Diiminodiphosphine or Diaminodiphosphine Ligands as Precatalysts for the Hydrogenation of Acetophenone. Inorg. Chem. 2009, 48, 735−743. (102) Zuo, W.; Tauer, S.; Prokopchuk, D. E.; Morris, R. H. Iron Catalysts Containing Amine(imine)diphosphine P-NH-N-P Ligands Catalyze both the Asymmetric Hydrogenation and Asymmetric Transfer Hydrogenation of Ketone. Organometallics 2014, 33, 5791−5801. (103) Zuo, W.; Prokopchuk, D. E.; Lough, A. J.; Morris, R. H. Details of the Mechanism of the Asymmetric Transfer Hydrogenation of Acetophenone Using the Amine(imine)diphosphine Iron Precatalyst: the Base Effect and the Enantiodetermining Step. ACS Catal. 2016, 6, 301−314. (104) Zhang, J.; Leitus, G.; Ben-David, Y.; Milstein, D. Facile Conversion of Alcohols into Esters and Dihydrogen Catalyzed by New Ruthenium Complexes. J. Am. Chem. Soc. 2005, 127, 10840− 10841. (105) Zhang, J.; Leitus, G.; Ben-David, Y.; Milstein, D. Efficient Homogeneous Catalytic Hydrogenation of Esters to Alcohols. Angew. Chem., Int. Ed. 2006, 45, 1113−1115. (106) Balaraman, E.; Gnanaprakasam, B.; Shimon, L. J.; Milstein, D. Direct Hydrogenation of Amides to Alcohols and Amines under Mild Conditions. J. Am. Chem. Soc. 2010, 132, 16756−16758. (107) Gunanathan, C.; Milstein, D. Metal-Ligand Cooperation by Aromatization-Dearomatization: A New Paradigm in Bond Activation and “Green” Catalysis. Acc. Chem. Res. 2011, 44, 588−602. (108) Zell, T.; Milstein, D. Hydrogenation and Dehydrogenation Iron Pincer Catalysts Capable of Metal-Ligand Cooperation by Aromatization/Dearomatization. Acc. Chem. Res. 2015, 48, 1979− 1994. (109) Langer, R.; Leitus, G.; Ben-David, Y.; Milstein, D. Efficient Hydrogenation of Ketones Catalyzed by an Iron Pincer Complex. Angew. Chem., Int. Ed. 2011, 50, 2120−2124. (110) Langer, R.; Iron, M. A.; Konstantinovski, L.; Diskin-Posner, Y.; Leitus, G.; Ben-David, Y.; Milstein, D. Iron Borohydride Pincer Complexes for the Efficient Hydrogenation of Ketones under Mild, Base-Free Conditions: Synthesis and Mechanistic Insight. Chem. - Eur. J. 2012, 18, 7196−7209. (111) Gorgas, N.; Stöger, B.; Veiros, L. F.; Pittenauer, E.; Allmaier, G.; Kirchner, K. Efficient Hydrogenation of Ketones and Aldehydes Catalyzed by Well-Defined Iron(II) PNP Pincer Complexes: AZ


Chemical Reviews

Review

Biomimetic Reduction of α-Keto-/α-Iminoesters. Angew. Chem., Int. Ed. 2013, 52, 8382−8386. (130) Yi, Y.; Liu, H.; Xiao, L.-P.; Wang, B.; Song, G. Highly Efficient Hydrogenation of Levulinic Acid into γ-Valerolactone using an Iron Pincer Complex. ChemSusChem 2018, 11, 1474−1478. (131) Jothimony, K.; Vancheesan, S. Mechanism for Transfer Hydrogenation of Ketones to Alcohols Catalysed by HydridotriIronundecacarbonylate Anion under Phase Transfer Conditions. J. Mol. Catal. 1989, 52, 301−304. (132) Jothimony, K.; Vancheesan, S.; Kuriacose, J. Phase TransferCatalysed Transfer Hydrogenation of Ketones Using Iron Carbonyls as Catalysts. J. Mol. Catal. 1985, 32, 11−16. (133) Enthaler, S.; Hagemann, B.; Erre, G.; Junge, K.; Beller, M. An Environmentally Benign Process for the Hydrogenation of Ketones with Homogeneous Iron Catalysts. Chem. - Asian J. 2006, 1, 598−604. (134) Enthaler, S.; Erre, G.; Tse, M. K.; Junge, K.; Beller, M. Biomimetic Transfer Hydrogenation of Ketones with Iron Porphyrin Catalysts. Tetrahedron Lett. 2006, 47, 8095−8099. (135) Enthaler, S.; Spilker, B.; Erre, G.; Junge, K.; Tse, M. K.; Beller, M. Biomimetic Transfer Hydrogenation of 2-Alkoxy-and 2-Aryloxyketones with Iron−Porphyrin Catalysts. Tetrahedron 2008, 64, 3867−3876. (136) Polshettiwar, V.; Varma, R. S. Revisiting the Meerwein− Ponndorf−Verley Reduction: a Sustainable Protocol for Transfer Hydrogenation of Aldehydes and Ketones. Green Chem. 2009, 11, 1313−1316. (137) Ouali, A.; Majoral, J.-P.; Caminade, A.-M.; Taillefer, M. NaOH-Promoted Hydrogen Transfer: Does NaOH or Traces of Transition Metals Catalyze the Reaction? ChemCatChem 2009, 1, 504−509. (138) Chen, J.-S.; Chen, L.-L.; Xing, Y.; Chen, G.; Shen, W.-Y.; Dong, Z.-R.; Li, Y.-Y.; Gao, J.-X. Asymmetric Transfer Hydrogenation of Ketones Catalyzed by Chiral Carbonyl Iron Systems. Acta Chim. Sin. 2004, 62, 1745−1750. (139) Meyer, N.; Lough, A. J.; Morris, R. H. Iron(II) Complexes for the Efficient Catalytic Asymmetric Transfer Hydrogenation of Ketones. Chem. - Eur. J. 2009, 15, 5605−5610. (140) Mikhailine, A. A.; Kim, E.; Dingels, C.; Lough, A. J.; Morris, R. H. Template Syntheses of Iron(II) Complexes Containing Chiral PN-N-P and P-N-N Ligands. Inorg. Chem. 2008, 47, 6587−6589. (141) Lagaditis, P. O.; Lough, A. J.; Morris, R. H. Iron Complexes for the Catalytic Transfer Hydrogenation of Acetophenone: Steric and Electronic Effects Imposed by Alkyl Substituents at Phosphorus. Inorg. Chem. 2010, 49, 10057−10066. (142) Mikhailine, A. A.; Morris, R. H. Effect of the Structure of the Diamine Backbone of P−N−N−P ligands in Iron(II) Complexes on Catalytic Activity in the Transfer Hydrogenation of Acetophenone. Inorg. Chem. 2010, 49, 11039−11044. (143) Sues, P. E.; Lough, A. J.; Morris, R. H. Stereoelectronic Factors in Iron Catalysis: Synthesis and Characterization of ArylSubstituted Iron(II) Carbonyl P-N-N-P Complexes and Their Use in the Asymmetric Transfer Hydrogenation of Ketones. Organometallics 2011, 30, 4418−4431. (144) Mikhailine, A.; Lough, A. J.; Morris, R. H. Efficient Asymmetric Transfer Hydrogenation of Ketones Catalyzed by an Iron Complex Containing a P-N-N-P Tetradentate Ligand Formed by Template Synthesis. J. Am. Chem. Soc. 2009, 131, 1394−1395. (145) Zuo, W.; Lough, A. J.; Li, Y. F.; Morris, R. H. Amine(imine)diphosphine Iron Catalysts for Asymmetric Transfer Hydrogenation of Ketones and Imines. Science 2013, 342, 1080−1083. (146) Demmans, K. Z.; Ko, O. W. K.; Morris, R. H. Aqueous Biphasic Iron-Catalyzed Asymmetric Transfer Hydrogenation of Ketones. RSC Adv. 2016, 6, 88580−88587. (147) Smith, S. A.; Morris, R. H. An Unsymmetrical Iron Catalyst for the Asymmetric Transfer Hydrogenation of Ketones. Synthesis 2015, 47, 1775−1779. (148) Smith, S. A. M.; Prokopchuk, D. E.; Morris, R. H. Asymmetric Transfer Hydrogenation of Ketones Using New Iron(II) (P-NH-N-

Evidence for an Insertion Mechanism. Organometallics 2014, 33, 6905−6914. (112) Schröder-Holzhacker, C.; Gorgas, N.; Stöger, B.; Kirchner, K. Synthesis and reactivity of BINEPINE-based chiral Fe(II) PNP pincer complexes. Monatsh. Chem. 2016, 147, 1023−1030. (113) Gorgas, N.; Stöger, B.; Veiros, L. F.; Kirchner, K. Highly Efficient and Selective Hydrogenation of Aldehydes: A Well-Defined Fe(II) Catalyst Exhibits Noble-Metal Activity. ACS Catal. 2016, 6, 2664−2672. (114) Brünig, J.; Csendes, Z.; Weber, S.; Gorgas, N.; Bittner, R. W.; Limbeck, A.; Bica, K.; Hoffmann, H.; Kirchner, K. Chemoselective Supported Ionic-Liquid-Phase (SILP) Aldehyde Hydrogenation Catalyzed by an Fe(II) PNP Pincer Complex. ACS Catal. 2018, 8, 1048−1051. (115) Mazza, S.; Scopelliti, R.; Hu, X. Chemoselective Hydrogenation and Transfer Hydrogenation of Aldehydes Catalyzed by Iron (II) PONOP Pincer Complexes. Organometallics 2015, 34, 1538− 1545. (116) Butschke, B.; Feller, M.; Diskin-Posner, Y.; Milstein, D. Ketone Hydrogenation Catalyzed by a New Iron(II)-PNN Complex. Catal. Sci. Technol. 2016, 6, 4428−4437. (117) Chakraborty, S.; Lagaditis, P. O.; Förster, M.; Bielinski, E. A.; Hazari, N.; Holthausen, M. C.; Jones, W. D.; Schneider, S. WellDefined Iron Catalysts for the Acceptorless Reversible Dehydrogenation-Hydrogenation of Alcohols and Ketones. ACS Catal. 2014, 4, 3994−4003. (118) Lagaditis, P. O.; Sues, P. E.; Sonnenberg, J. F.; Wan, K. Y.; Lough, A. J.; Morris, R. H. Iron(II) Complexes Containing Unsymmetrical P-N-P’ Pincer Ligands for the Catalytic Asymmetric Hydrogenation of Ketones and Imines. J. Am. Chem. Soc. 2014, 136, 1367−1380. (119) Sonnenberg, J. F.; Wan, K. Y.; Sues, P. E.; Morris, R. H. Ketone Asymmetric Hydrogenation Catalyzed by P-NH-P′ Pincer Iron Catalysts: An Experimental and Computational Study. ACS Catal. 2017, 7, 316−326. (120) Sonnenberg, J. F.; Lough, A. J.; Morris, R. H. Synthesis of Iron PNP′ and P-NH-P′ Asymmetric Hydrogenation Catalysts. Organometallics 2014, 33, 6452−6465. (121) Smith, S. A.; Lagaditis, P. O.; Lüpke, A.; Lough, A. J.; Morris, R. H. Unsymmetrical Iron P-NH-P′ Catalysts for the Asymmetric Pressure Hydrogenation of Aryl Ketones. Chem. - Eur. J. 2017, 23, 7212−7216. (122) Zirakzadeh, A.; Kirchner, K.; Roller, A.; Stöger, B.; Widhalm, M.; Morris, R. H. Iron (II) Complexes Containing Chiral Unsymmetrical PNP′ Pincer Ligands: Synthesis and Application in Asymmetric Hydrogenations. Organometallics 2016, 35, 3781−3787. (123) Huber, R.; Passera, A.; Mezzetti, A. Iron (II)-Catalyzed Hydrogenation of Acetophenone with a Chiral, Pyridine-Based PNP Pincer Ligand: Support for an Outer-Sphere Mechanism. Organometallics 2018, 37, 396−405. (124) Huber, R.; Passera, A.; Gubler, E.; Mezzetti, A. P-Stereogenic PN(H) P Iron(II) Catalysts for the Asymmetric Hydrogenation of Ketones: The Importance of Non-Covalent Interactions in Rational Ligand Design by Computation. Adv. Synth. Catal. 2018, 360, 2900. (125) Yang, X. Unexpected Direct Reduction Mechanism for Hydrogenation of Ketones Catalyzed by Iron PNP Pincer Complexes. Inorg. Chem. 2011, 50, 12836−12843. (126) Docherty, J. H.; Peng, J.; Dominey, A. P.; Thomas, S. P. Activation and Discovery of Earth-Abundant Metal Catalysts Using Sodium tert-Butoxide. Nat. Chem. 2017, 9, 595−600. (127) Wienhoefer, G.; Westerhaus, F. A.; Junge, K.; Ludwig, R.; Beller, M. A Molecularly Defined Iron-Catalyst for the Selective Hydrogenation of α,β-Unsaturated Aldehydes. Chem. - Eur. J. 2013, 19, 7701−7707. (128) Li, Y.; Yu, S.; Wu, X.; Xiao, J.; Shen, W.; Dong, Z.; Gao, J. Iron-Catalyzed Asymmetric Hydrogenation of Ketones. J. Am. Chem. Soc. 2014, 136, 4031−4039. (129) Lu, L. Q.; Li, Y.; Junge, K.; Beller, M. Iron-Catalyzed Hydrogenation for the In Situ Regeneration of an NAD (P) H Model: BA


Chemical Reviews

Review

Macrocyclic Iron(II) Hydride. Angew. Chem., Int. Ed. 2017, 56, 11949−11953. (167) Kandepi, V. V. K. M.; Cardoso, J. M. S.; Peris, E.; Royo, B. Iron(II) Complexes Bearing Chelating Cyclopentadienyl-N-Heterocyclic Carbene Ligands as Catalysts for Hydrosilylation and Hydrogen Transfer Reactions. Organometallics 2010, 29, 2777−2782. (168) Cardoso, J. M. S.; Fernandes, A.; Cardoso, B. D. P.; Carvalho, M. D.; Ferreira, L. P.; Calhorda, M. J.; Royo, B. Cationic HalfSandwich Iron(II) and Iron(III) Complexes with N-Heterocyclic Carbene Ligands. Organometallics 2014, 33, 5670−5677. (169) Bala, M. D.; Ikhile, M. I. Application of Three-Legged PianoStool Cyclopentadienyl-N-Heterocyclic Carbene Iron(II) Complexes as in situ Catalysts for the Transfer Hydrogenation of Ketones. J. Mol. Catal. A: Chem. 2014, 385, 98−105. (170) Knölker, H.-J.; Goesmann, H.; Klauss, R. A Novel Method for the Demetalation of Tricarbonyliron−Diene Complexes by a Photolytically Induced Ligand Exchange Reaction with Acetonitrile. Angew. Chem., Int. Ed. 1999, 38, 702−705. (171) Plank, T. N.; Drake, J. L.; Kim, D. K.; Funk, T. W. Air-Stable, Nitrile-Ligated (Cyclopentadienone)iron Dicarbonyl Compounds as Transfer Reduction and Oxidation Catalysts. Adv. Synth. Catal. 2012, 354, 597−601. (172) Plank, T. N.; Drake, J. L.; Kim, D. K.; Funk, T. W. Corrigendum: Air-Stable, Nitrile-Ligated (Cyclopentadienone)iron Dicarbonyl Compounds as Transfer Reduction and Oxidation Catalysts. Adv. Synth. Catal. 2012, 354, 1179−1179. (173) Funk, T. W.; Mahoney, A. R.; Sponenburg, R. A.; Zimmerman, K. P.; Kim, D. K.; Harrison, E. E. Synthesis and Catalytic Activity of (3, 4-Diphenylcyclopentadienone) Iron Tricarbonyl Compounds in Transfer Hydrogenations and Dehydrogenations. Organometallics 2018, 37, 1133−1140. (174) Hopewell, J. P.; Martins, J. E. D.; Johnson, T. C.; Godfrey, J.; Wills, M. Developing Asymmetric Iron and Ruthenium-Based Cyclone Complexes; Complex Factors Influence the Asymmetric Induction in the Transfer Hydrogenation of Ketones. Org. Biomol. Chem. 2012, 10, 134−145. (175) Fu, M.-C.; Shang, R.; Huang, Z.; Fu, Y. Conversion of Levulinate Ester and Formic Acid into γ-Valerolactone Using a Homogeneous Iron Catalyst. Synlett 2014, 25, 2748−2752. (176) Metzker, G.; Burtoloso, A. C. B. Conversion of Levulinic Acid into γ-Valerolactone Using Fe3(CO)12: Mimicking a Biorefinery Setting by Exploiting Crude Liquors from Biomass Acid Hydrolysis. Chem. Commun. 2015, 51, 14199−14202. (177) Dai, N.; Shang, R.; Fu, M.; Fu, Y. Transfer Hydrogenation of Ethyl Levulinate to γ-Valerolactone Catalyzed by Iron Complexes. Chin. J. Chem. 2015, 33, 405−408. (178) van Slagmaat, C. A. M. R.; De Wildeman, S. M. A. A Comparative Study of Structurally Related Homogeneous Ruthenium and Iron Catalysts for the Hydrogenation of Levulinic Acid to γValerolactone. Eur. J. Inorg. Chem. 2018, 694−702. (179) Zhang, P.; Li, X.; Qi, X.; Sun, H.; Fuhr, O.; Fenske, D. Transfer Hydrogenation of Aldehydes Catalyzed by Silyl Hydrido Iron Complexes Bearing a [PSiP] Pincer Ligand. RSC Adv. 2018, 8, 14092−14099. (180) Magubane, M. N.; Nyamato, G. S.; Ojwach, S. O.; Munro, O. Q. Structural, Kinetic, and DFT Studies of the Transfer Hydrogenation of Ketones Mediated by (pyrazole)pyridine iron(II) and nickel(II) complexes. RSC Adv. 2016, 6, 65205−65221. (181) Wienhöfer, G.; Westerhaus, F. A.; Junge, K.; Beller, M. Fast and Selective Iron-Catalyzed Transfer Hydrogenations of Aldehydes. J. Organomet. Chem. 2013, 744, 156−159. (182) Zhou, S.; Fleischer, S.; Junge, K.; Beller, M. Cooperative Transition-Metal and Chiral Brønsted Acid Catalysis: Enantioselective Hydrogenation of Imines To Form Amines. Angew. Chem. 2011, 123, 5226−5230. (183) Fleischer, S.; Werkmeister, S.; Zhou, S.; Junge, K.; Beller, M. Consecutive Intermolecular Reductive Hydroamination: Cooperative Transition-Metal and Chiral Brønsted Acid Catalysis. Chem. - Eur. J. 2012, 18, 9005−9010.

P′) Catalysts: Changing the Steric and Electronic Properties at Phosphorus P′. Isr. J. Chem. 2017, 57, 1204−1215. (149) Smith, S. A. M.; Lough, A. J.; Morris, R. H. Bromidocarbonyl{(1S,2S)-N-[2-(dicyclohexylphosphanyl)ethylidenyl]-N’-[2(diphenylphosphanyl)ethyl]-1,2-diphenylethane-1,2-diamine}iron(II) Tetraphenylborate. IUCrData 2017, 2, x170452. (150) Demmans, K. Z.; Seo, C. S. G.; Lough, A. J.; Morris, R. H. From Imine to Amine: An Unexpected Left Turn. Cis-β Iron(ii) PNNP′ Precatalysts for the Asymmetric Transfer Hydrogenation of Acetophenone. Chem. Sci. 2017, 8, 6531−6541. (151) Lagaditis, P. O.; Lough, A. J.; Morris, R. H. Low-Valent Ene− Amido Iron Complexes for the Asymmetric Transfer Hydrogenation of Acetophenone without Base. J. Am. Chem. Soc. 2011, 133, 9662− 9665. (152) Prokopchuk, D. E.; Sonnenberg, J. F.; Meyer, N.; Zimmer-De Iuliis, M.; Lough, A. J.; Morris, R. H. Spectroscopic and DFT Study of Ferraaziridine Complexes Formed in the Transfer Hydrogenation of Acetophenone Catalyzed Using trans-[Fe(CO)(NCMe)(PPh2C6H4CHNCH2−)2-κ4P,N,N,P](BF4)2. Organometallics 2012, 31, 3056−3064. (153) Mikhailine, A. A.; Maishan, M. I.; Lough, A. J.; Morris, R. H. The Mechanism of Efficient Asymmetric Transfer Hydrogenation of Acetophenone Using an Iron(II) Complex Containing an (S,S)Ph2PCH2CHNCHPhCHPhN = CHCH2PPh2 Ligand: Partial Ligand Reduction Is the Key. J. Am. Chem. Soc. 2012, 134, 12266− 12280. (154) Lagaditis, P. O.; Sues, P. E.; Lough, A. J.; Morris, R. H. Exploring the Decomposition Pathways of Iron Asymmetric Transfer Hydrogenation Catalysts. Dalton Trans 2015, 44, 12119−12127. (155) Phua, P.-H.; Lefort, L.; Boogers, J. A.; Tristany, M.; de Vries, J. G. Soluble Iron Nanoparticles as Cheap and Environmentally Benign Alkene and Alkyne Hydrogenation Catalysts. Chem. Commun. 2009, 3747−3749. (156) Sonnenberg, J. F.; Coombs, N.; Dube, P. A.; Morris, R. H. Iron Nanoparticles Catalyzing the Asymmetric Transfer Hydrogenation of Ketones. J. Am. Chem. Soc. 2012, 134, 5893−5899. (157) Sonnenberg, J. F.; Morris, R. H. Distinguishing Homogeneous from Nanoparticle Asymmetric Iron Catalysis. Catal. Sci. Technol. 2014, 4, 3426−3438. (158) Buchard, A.; Heuclin, H.; Auffrant, A.; Le Goff, X. F.; Le Floch, P. Coordination of Tetradentate X2N2 (X = P, S, O) Ligands to Iron(II) Metal Center and Catalytic Application in the Transfer Hydrogenation of Ketones. Dalton Trans 2009, 1659−1667. (159) Yu, S.; Shen, W.; Li, Y.; Dong, Z.; Xu, Y.; Li, Q.; Zhang, J.; Gao, J. Iron-Catalyzed Highly Enantioselective Reduction of Aromatic Ketones with Chiral P2N4-Type Macrocycles. Adv. Synth. Catal. 2012, 354, 818−822. (160) Wu, F.; Zhang, W.; An, D.; Li, Y.; Gao, J. Asymmetric Transfer Hydrogenation of Ketones Catalyzed by Chiral Multidentate Aminophosphine Ligands/Iron Cluster. Youji Huaxue 2017, 37, 1295−1299. (161) Bigler, R.; Huber, R.; Mezzetti, A. Iron Chemistry Made Easy: Chiral N2P2 Ligands for Asymmetric Catalysis. Synlett 2016, 27, 831− 847. (162) Bigler, R.; Mezzetti, A. Isonitrile Iron (II) Complexes with Chiral N2P2 Macrocycles in the Enantioselective Transfer Hydrogenation of Ketones. Org. Lett. 2014, 16, 6460−6463. (163) Bigler, R.; Huber, R.; Mezzetti, A. Highly Enantioselective Transfer Hydrogenation of Ketones with Chiral (NH)2P2 Macrocyclic Iron (II) Complexes. Angew. Chem., Int. Ed. 2015, 54, 5171−5174. (164) Bigler, R.; Mezzetti, A. Highly Enantioselective Transfer Hydrogenation of Polar Double Bonds by Macrocyclic Iron (II)/ (NH)2P2 Catalysts. Org. Process Res. Dev. 2016, 20, 253−261. (165) Bigler, R.; Huber, R.; Stöckli, M.; Mezzetti, A. Iron (II)/ (NH)2P2 Macrocycles: Modular, Highly Enantioselective Transfer Hydrogenation Catalysts. ACS Catal. 2016, 6, 6455−6464. (166) De Luca, L.; Mezzetti, A. Base-Free Asymmetric Transfer Hydrogenation of 1,2-Di- and Monoketones Catalyzed by a (NH)2P2BB


Chemical Reviews

Review

(184) Fleischer, S.; Zhou, S.; Werkmeister, S.; Junge, K.; Beller, M. Cooperative Iron−Brønsted Acid Catalysis: Enantioselective Hydrogenation of Quinoxalines and 2 H-1,4-Benzoxazines. Chem. - Eur. J. 2013, 19, 4997−5003. (185) Brenna, D.; Rossi, S.; Cozzi, F.; Benaglia, M. Iron-Catalyzed Diastereoselective Hydrogenation of Chiral Imines. Org. Biomol. Chem. 2017, 15, 5685−5688. (186) Chakraborty, S.; Brennessel, W. W.; Jones, W. D. A Molecular Iron Catalyst for the Acceptorless Dehydrogenation and Hydrogenation of N-Heterocycles. J. Am. Chem. Soc. 2014, 136, 8564−8567. (187) Zhou, S.; Fleischer, S.; Junge, K.; Das, S.; Addis, D.; Beller, M. Enantioselective Synthesis of Amines: General, Efficient IronCatalyzed Asymmetric Transfer Hydrogenation of Imines. Angew. Chem., Int. Ed. 2010, 49, 8121−8125. (188) Mikhailine, A. A.; Maishan, M. I.; Morris, R. H. Asymmetric Transfer Hydrogenation of Ketimines Using Well-Defined Iron(II)Based Precatalysts Containing a PNNP Ligand. Org. Lett. 2012, 14, 4638−4641. (189) Pan, H.-J.; Ng, T. W.; Zhao, Y. Iron-Catalyzed Transfer Hydrogenation of Imines Assisted by an Iron-Based Lewis Acid. Org. Biomol. Chem. 2016, 14, 5490−5493. (190) Facchini, S. V.; Cettolin, M.; Bai, X.; Casamassima, G.; Pignataro, L.; Gennari, C.; Piarulli, U. Efficient Synthesis of Amines by Iron-Catalyzed CN Transfer Hydrogenation and CO Reductive Amination. Adv. Synth. Catal. 2018, 360, 1054−1059. (191) Knifton, J. Homogeneous Catalyzed Reduction of Nitro Compounds. IV. Selective and Sequential Hydrogenation of Nitroaromatics. J. Org. Chem. 1976, 41, 1200−1206. (192) Deshpande, R. M.; Mahajan, A. N.; Diwakar, M. M.; Ozarde, P. S.; Chaudhari, R. Chemoselective Hydrogenation of Substituted Nitroaromatics Using Novel Water-Soluble Iron Complex Catalysts. J. Org. Chem. 2004, 69, 4835−4838. (193) Wienhöfer, G.; Baseda-Krüger, M.; Ziebart, C.; Westerhaus, F. A.; Baumann, W.; Jackstell, R.; Junge, K.; Beller, M. Hydrogenation of Nitroarenes Using Defined Iron−Phosphine Catalysts. Chem. Commun. 2013, 49, 9089−9091. (194) Wienhöfer, G.; Sorribes, I.; Boddien, A.; Westerhaus, F.; Junge, K.; Junge, H.; Llusar, R.; Beller, M. General and Selective IronCatalyzed Transfer Hydrogenation of Nitroarenes without Base. J. Am. Chem. Soc. 2011, 133, 12875−12879. (195) Jagadeesh, R. V.; Murugesan, K.; Alshammari, A. S.; Neumann, H.; Pohl, M.-M.; Radnik, J.; Beller, M. MOF-Derived Cobalt Nanoparticles Catalyze a General Synthesis of Amines. Science 2017, 358, 326−332. (196) Watanabe, Y.; Yamashita, M.; Mitsudo, T.-A.; Tanaka, M.; Takegami, Y. The Selective Amination of Carbonyl Compounds Using Iron Pentacarbonyl. Tetrahedron Lett. 1974, 15, 1879−1880. (197) Bhor, M. D.; Bhanushali, M. J.; Nandurkar, N. S.; Bhanage, B. M. Direct Reductive Amination of Carbonyl Compounds with Primary/Secondary Amines Using Recyclable Water-Soluble FeII/ EDTA Complex as Catalyst. Tetrahedron Lett. 2008, 49, 965−969. (198) Fleischer, S.; Zhou, S.; Junge, K.; Beller, M. An Easy and General Iron-catalyzed Reductive Amination of Aldehydes and Ketones with Anilines. Chem. - Asian J. 2011, 6, 2240−2245. (199) Pagnoux-Ozherelyeva, A.; Pannetier, N.; Mbaye, M. D.; Gaillard, S.; Renaud, J.-L. Knölker’s Iron Complex: an Efficient in situ Generated Catalyst for Reductive Amination of Alkyl Aldehydes and Amines. Angew. Chem., Int. Ed. 2012, 51, 4976−4980. (200) Moulin, S.; Dentel, H.; Pagnoux-Ozherelyeva, A.; Gaillard, S.; Poater, A.; Cavallo, L.; Lohier, J.-F.; Renaud, J.-L. Bifunctional (Cyclopentadienone) Iron−Tricarbonyl Complexes: Synthesis, Computational Studies and Application in Reductive Amination. Chem. Eur. J. 2013, 19, 17881−17890. (201) Metzker, G.; Dias, R. M.; Burtoloso, A. C. Iron-Catalyzed Reductive Amination from Levulinic and Formic Acid Aqueous Solutions: An Approach for the Selective Production of Pyrrolidones in Biorefinery Facilities. ChemistrySelect 2018, 3, 368−372.

(202) Mérel, D. S.; Do, M. L. T.; Gaillard, S.; Dupau, P.; Renaud, J.L. Iron-Catalyzed Reduction of Carboxylic and Carbonic Acid Derivatives. Coord. Chem. Rev. 2015, 288, 50−68. (203) Garg, J. A.; Chakraborty, S.; Ben-David, Y.; Milstein, D. Unprecedented Iron-catalyzed Selective Hydrogenation of Activated Amides to Amines and Alcohols. Chem. Commun. 2016, 52, 5285− 5288. (204) Schneck, F.; Assmann, M.; Balmer, M.; Harms, K.; Langer, R. Selective Hydrogenation of Amides to Amines and Alcohols Catalyzed by Improved Iron Pincer Complexes. Organometallics 2016, 35, 1931−1943. (205) Rezayee, N. M.; Samblanet, D. C.; Sanford, M. S. Ironcatalyzed Hydrogenation of Amides to Alcohols and Amines. ACS Catal. 2016, 6, 6377−6383. (206) Jayarathne, U.; Zhang, Y.; Hazari, N.; Bernskoetter, W. H. Selective Iron-Catalyzed Deaminative Hydrogenation of Amides. Organometallics 2017, 36, 409−416. (207) Bornschein, C.; Werkmeister, S.; Wendt, B.; Jiao, H.; Alberico, E.; Baumann, W.; Junge, H.; Junge, K.; Beller, M. Mild and Selective Hydrogenation of Aromatic And Aliphatic (Di)Nitriles with a WellDefined Iron Pincer Complex. Nat. Commun. 2014, 5, 4111. (208) Lange, S.; Elangovan, S.; Cordes, C.; Spannenberg, A.; Jiao, H.; Junge, H.; Bachmann, S.; Scalone, M.; Topf, C.; Junge, K. Selective Catalytic Hydrogenation of Nitriles to Primary Amines Using Iron Pincer Complexes. Catal. Sci. Technol. 2016, 6, 4768− 4772. (209) Werkmeister, S.; Junge, K.; Wendt, B.; Alberico, E.; Jiao, H.; Baumann, W.; Junge, H.; Gallou, F.; Beller, M. Hydrogenation of Esters to Alcohols with a Well-Defined Iron Complex. Angew. Chem., Int. Ed. 2014, 53, 8722−8726. (210) Chakraborty, S.; Leitus, G.; Milstein, D. Selective Hydrogenation of Nitriles to Primary Amines Catalyzed by a Novel Iron Complex. Chem. Commun. 2016, 52, 1812−1815. (211) Chakraborty, S.; Milstein, D. Selective Hydrogenation of Nitriles to Secondary Imines Catalyzed by an Iron Pincer Complex. ACS Catal. 2017, 7, 3968−3972. (212) Chakraborty, S.; Leitus, G.; Milstein, D. Iron-Catalyzed Mild and Selective Hydrogenative Cross-Coupling of Nitriles and Amines To Form Secondary Aldimines. Angew. Chem., Int. Ed. 2017, 56, 2074−2078. (213) Zell, T.; Ben-David, Y.; Milstein, D. Unprecedented IronCatalyzed Ester Hydrogenation. Mild, Selective, and Efficient Hydrogenation of Trifluoroacetic Esters to Alcohols Catalyzed by an Iron Pincer Complex. Angew. Chem., Int. Ed. 2014, 53, 4685−4689. (214) Chakraborty, S.; Dai, H.; Bhattacharya, P.; Fairweather, N. T.; Gibson, M. S.; Krause, J. A.; Guan, H. Iron-Based Catalysts for the Hydrogenation of Esters to Alcohols. J. Am. Chem. Soc. 2014, 136, 7869−7872. (215) Fairweather, N. T.; Gibson, M. S.; Guan, H. Homogeneous Hydrogenation of Fatty Acid Methyl Esters and Natural Oils under Neat Conditions. Organometallics 2015, 34, 335−339. (216) Spasyuk, D.; Gusev, D. G. Acceptorless Dehydrogenative Coupling of Ethanol and Hydrogenation of Esters and Imines. Organometallics 2012, 31, 5239−5242. (217) Acosta-Ramirez, A.; Bertoli, M.; Gusev, D. G.; Schlaf, M. Homogeneous Catalytic Hydrogenation of Long-Chain Esters by an Osmium Pincer Complex and Its Potential Application in the Direct Conversion of Triglycerides into Fatty Alcohols. Green Chem. 2012, 14, 1178−1188. (218) Elangovan, S.; Wendt, B.; Topf, C.; Bachmann, S.; Scalone, M.; Spannenberg, A.; Jiao, H.; Baumann, W.; Junge, K.; Beller, M. Improved Second Generation Iron Pincer Complexes for Effective Ester Hydrogenation. Adv. Synth. Catal. 2016, 358, 820−825. (219) Qu, S.; Dai, H.; Dang, Y.; Song, C.; Wang, Z.-X.; Guan, H. Computational Mechanistic Study of Fe-Catalyzed Hydrogenation of Esters to Alcohols: Improving Catalysis by Accelerating Precatalyst Activation with a Lewis Base. ACS Catal. 2014, 4, 4377−4388. (220) Gajewski, P.; Gonzalez-de-Castro, A.; Renom-Carrasco, M.; Piarulli, U.; Gennari, C.; de Vries, J. G.; Lefort, L.; Pignataro, L. BC


Chemical Reviews

Review

Expanding the Catalytic Scope of (Cyclopentadienone) iron Complexes to the Hydrogenation of Activated Esters to Alcohols. ChemCatChem 2016, 8, 3431−3435. (221) Quadrelli, E. A.; Centi, G.; Duplan, J. L.; Perathoner, S. Carbon Dioxide Recycling: Emerging Large-Scale Technologies with Industrial Potential. ChemSusChem 2011, 4, 1194−1215. (222) Wang, W.; Wang, S.; Ma, X.; Gong, J. Recent Advances in Catalytic Hydrogenation of Carbon Dioxide. Chem. Soc. Rev. 2011, 40, 3703−3727. (223) Jessop, P. G.; Ikariya, T.; Noyori, R. Homogeneous Hydrogenation of Carbon Dioxide. Chem. Rev. 1995, 95, 259−272. (224) Leitner, W. Carbon Dioxide as a Raw Material: the Synthesis of Formic Acid and Its Derivatives From CO2. Angew. Chem., Int. Ed. Engl. 1995, 34, 2207−2221. (225) Jessop, P. G.; Joó, F.; Tai, C.-C. Recent Advances in the Homogeneous Hydrogenation of Carbon Dioxide. Coord. Chem. Rev. 2004, 248, 2425−2442. (226) Federsel, C.; Jackstell, R.; Beller, M. State-of-the-Art Catalysts for Hydrogenation of Carbon Dioxide. Angew. Chem., Int. Ed. 2010, 49, 6254−6257. (227) Dong, K.; Razzaq, R.; Hu, Y.; Ding, K. Homogeneous Reduction of Carbon Dioxide with Hydrogen. Top. Curr. Chem. 2017, 375, 23. (228) Wang, W.-H.; Feng, X.; Bao, M. Transformation of Carbon Dioxide to Formic Acid and Methanol; Springer: Singapore, 2018. (229) Li, B.; Sortais, J.-B.; Darcel, C. Amine Synthesis via Transition Metal Homogeneous Catalysed Hydrosilylation. RSC Adv. 2016, 6, 57603−57625. (230) Evans, G.; Newell, C. Conversion of CO2, H2, and Alcohols into Formate Esters Using Anionic Iron Carbonyl Hydrides. Inorg. Chim. Acta 1978, 31, L387−L389. (231) Tai, C.-C.; Chang, T.; Roller, B.; Jessop, P. G. High-Pressure Combinatorial Screening of Homogeneous Catalysts: Hydrogenation of Carbon Dioxide. Inorg. Chem. 2003, 42, 7340−7341. (232) Federsel, C.; Boddien, A.; Jackstell, R.; Jennerjahn, R.; Dyson, P. J.; Scopelliti, R.; Laurenczy, G.; Beller, M. A Well-Defined Iron Catalyst for the Reduction of Bicarbonates and Carbon Dioxide to Formates, Alkyl Formates, and Formamides. Angew. Chem., Int. Ed. 2010, 49, 9777−9780. (233) Drake, J. L.; Manna, C. M.; Byers, J. A. Enhanced Carbon Dioxide Hydrogenation Facilitated by Catalytic Quantities of Bicarbonate and Other Inorganic Salts. Organometallics 2013, 32, 6891−6894. (234) Boddien, A.; Mellmann, D.; Gärtner, F.; Jackstell, R.; Junge, H.; Dyson, P. J.; Laurenczy, G.; Ludwig, R.; Beller, M. Efficient Dehydrogenation of Formic Acid Using an Iron Catalyst. Science 2011, 333, 1733−1736. (235) Yang, X. Mechanistic Insights into Iron-Catalyzed Dehydrogenation of Formic Acid: β-Hydride Elimination vs. Direct Hydride Transfer. Dalton Trans 2013, 42, 11987−11991. (236) Montandon-Clerc, M.; Laurenczy, G. Additive free, Room Temperature Direct Homogeneous Catalytic Carbon Dioxide Hydrogenation in Aqueous Solution Using an Iron(II) Phosphine Catalyst. J. Catal. 2018, 362, 78−84. (237) Ziebart, C.; Federsel, C.; Anbarasan, P.; Jackstell, R.; Baumann, W.; Spannenberg, A.; Beller, M. Well-Defined Iron Catalyst for Improved Hydrogenation of Carbon Dioxide and Bicarbonate. J. Am. Chem. Soc. 2012, 134, 20701−20704. (238) Langer, R.; Diskin-Posner, Y.; Leitus, G.; Shimon, L. J.; BenDavid, Y.; Milstein, D. Low-Pressure Hydrogenation of Carbon Dioxide Catalyzed by an Iron Pincer Complex Exhibiting Noble Metal Activity. Angew. Chem., Int. Ed. 2011, 50, 9948−9952. (239) Rivada-Wheelaghan, O.; Dauth, A.; Leitus, G.; Diskin-Posner, Y.; Milstein, D. Synthesis and Reactivity of Iron Complexes with a New Pyrazine-Based Pincer Ligand, and Application in Catalytic LowPressure Hydrogenation of Carbon Dioxide. Inorg. Chem. 2015, 54, 4526−4538. (240) Zhang, Y.; MacIntosh, A. D.; Wong, J. L.; Bielinski, E. A.; Williard, P. G.; Mercado, B. Q.; Hazari, N.; Bernskoetter, W. H. Iron-

Catalyzed CO2 Hydrogenation to Formate Enhanced by Lewis Acid Co-Catalysts. Chem. Sci. 2015, 6, 4291−4299. (241) Smith, N. E.; Bernskoetter, W. H.; Hazari, N.; Mercado, B. Q. Synthesis and Catalytic Activity of PNP-Supported Iron Complexes with Ancillary Isonitrile Ligands. Organometallics 2017, 36, 3995− 4004. (242) Yang, X. Hydrogenation of Carbon Dioxide Catalyzed by PNP Pincer Iridium, Iron, and Cobalt Complexes: a Computational Design of Base Metal Catalysts. ACS Catal. 2011, 1, 849−854. (243) Sánchez-de-Armas, R.; Xue, L.; Ahlquist, M. S. G. One Site Is Enough: A Theoretical Investigation of Iron-Catalyzed Dehydrogenation of Formic Acid. Chem. - Eur. J. 2013, 19, 11869−11873. (244) Marcos, R.; Xue, L.; Sanchez-de-Armas, R.; Ahlquist, M. S. G. Bicarbonate Hydrogenation Catalyzed by Iron: How the Choice of Solvent Can Reverse the Reaction. ACS Catal. 2016, 6, 2923−2929. (245) Bertini, F.; Gorgas, N.; Stöger, B.; Peruzzini, M.; Veiros, L. F.; Kirchner, K.; Gonsalvi, L. Efficient and Mild Carbon Dioxide Hydrogenation to Formate Catalyzed By Fe(II) Hydrido Carbonyl Complexes Bearing 2,6-(Diaminopyridyl)diphosphine Pincer Ligands. ACS Catal. 2016, 6, 2889−2893. (246) Fong, H.; Peters, J. C. Hydricity of an Fe−H Species and Catalytic CO2 Hydrogenation. Inorg. Chem. 2015, 54, 5124−5135. (247) Bertini, F.; Mellone, I.; Ienco, A.; Peruzzini, M.; Gonsalvi, L. Iron(II) Complexes of the Linear rac-Tetraphos-1 Ligand as Efficient Homogeneous Catalysts for Sodium Bicarbonate Hydrogenation and Formic Acid Dehydrogenation. ACS Catal. 2015, 5, 1254−1265. (248) Marcos, R.; Bertini, F.; Rinkevicius, Z.; Peruzzini, M.; Gonsalvi, L.; Ahlquist, M. S. Mechanistic Studies on NaHCO3 Hydrogenation and HCOOH Dehydrogenation Reactions Catalysed by a FeII Linear Tetraphosphine Complex. Chem. - Eur. J. 2018, 24, 5366−5372. (249) Zhu, F.; Zhu-Ge, L.; Yang, G.; Zhou, S. Iron-Catalyzed Hydrogenation of Bicarbonates and Carbon Dioxide to Formates. ChemSusChem 2015, 8, 609−612. (250) Rosas-Hernández, A.; Alsabeh, P. G.; Barsch, E.; Junge, H.; Ludwig, R.; Beller, M. Highly Active and Selective Photochemical Reduction of CO2 to CO Using Molecular-Defined Cyclopentadienone Iron Complexes. Chem. Commun. 2016, 52, 8393−8396. (251) Jayarathne, U.; Hazari, N.; Bernskoetter, W. H. Selective IronCatalyzed N-Formylation of Amines using Dihydrogen and Carbon Dioxide. ACS Catal. 2018, 8, 1338−1345. (252) Ribeiro, A. P. C.; Martins, L. M. D.; Pombeiro, A. J. L. Carbon Dioxide-to-Methanol Single-Pot Conversion Using a C-scorpionate Iron(II) Catalyst. Green Chem. 2017, 19, 4811−4815. (253) Speier, J. L. Homogeneous Catalysis of Hydrosilation by Transition Metals. In Advances in Organometallic Chemistry; Stone, F. G. A., West, R., Eds.; Elsevier, 1979; Vol. 17, pp 407−447. (254) Karstedt, B. US Patent 3 715 334, 1973. (b) Hitchcock, P. B.; Lappert, M. F.; Warhurst, N. J. W. Angew. Chem., Int. Ed. Engl. 1991, 30, 438−440. (255) Hitchcock, P. B.; Lappert, M. F.; Warhurst, N. J. Synthesis and Structure of a rac-Tris(divinyldisiloxane) diplatinum(0) Complex and its Reaction with Maleic Anhydride. Angew. Chem., Int. Ed. Engl. 1991, 30, 438−440. (256) Nesmeyanov, A.; Freidlina, R. K.; Chukovskaya, E.; Petrova, R.; Belyavsky, A. Addition, Substitution, and Telomerization Reactions of Olefins in The Presence of Metal Carbonyls or Colloidal Iron. Tetrahedron 1962, 17, 61−68. (257) Schroeder, M. A.; Wrighton, M. S. Pentacarbonyliron(0) Photocatalyzed Reactions of Trialkylsilanes with Alkenes. J. Organomet. Chem. 1977, 128, 345−358. (258) Kakiuchi, F.; Tanaka, Y.; Chatani, N.; Murai, S. Completely Selective Synthesis of (E)-β-(Triethylsilyl)styrenes by Fe3(CO)12Catalyzed Reaction of Styrenes with Triethylsilane. J. Organomet. Chem. 1993, 456, 45−47. (259) Marciniec, B.; Kownacka, A.; Kownacki, I.; Hoffmann, M.; Taylor, R. Hydrosilylation vs. Dehydrogenative Silylation of Styrene Catalysed by Iron(0) Carbonyl Complexes with Multivinylsilicon BD


Chemical Reviews

Review

Ligands−Mechanistic Implications. J. Organomet. Chem. 2015, 791, 58−65. (260) Marciniec, B.; Kownacka, A.; Kownacki, I.; Taylor, R. Hydrosilylation Cross-Linking of Silicon Fluids by a Novel Class of Iron(0) Catalysts. Appl. Catal., A 2014, 486, 230−238. (261) Archer, A. M.; Bouwkamp, M. W.; Cortez, M.-P.; Lobkovsky, E.; Chirik, P. J. Arene Coordination in Bis(imino)pyridine Iron Complexes: Identification of Catalyst Deactivation Pathways in IronCatalyzed Hydrogenation and Hydrosilation. Organometallics 2006, 25, 4269−4278. (262) Lam, Y. C.; Nielsen, R. J.; Goddard, W. A., III; Dash, A. K. The Mechanism for Catalytic Hydrosilylation by Bis(imino)pyridine Iron Olefin Complexes Supported by Broken Symmetry Density Functional Theory. Dalton Trans 2017, 46, 12507−12515. (263) Greenhalgh, M. D.; Frank, D. J.; Thomas, S. P. Iron-Catalysed Chemo-, Regio-, and Stereoselective Hydrosilylation of Alkenes and Alkynes using a Bench-Stable Iron(II) Pre-Catalyst. Adv. Synth. Catal. 2014, 356, 584−590. (264) Tondreau, A. M.; Atienza, C. C. H.; Weller, K. J.; Nye, S. A.; Lewis, K. M.; Delis, J. G.; Chirik, P. J. Iron Catalysts for Selective Anti-Markovnikov Alkene Hydrosilylation Using Tertiary Silanes. Science 2012, 335, 567−570. (265) Challinor, A. J.; Calin, M.; Nichol, G. S.; Carter, N. B.; Thomas, S. P. Amine-Activated Iron Catalysis: Air-and MoistureStable Alkene and Alkyne Hydrofunctionalization. Adv. Synth. Catal. 2016, 358, 2404−2409. (266) Hojilla Atienza, C. C.; Tondreau, A. M.; Weller, K. J.; Lewis, K. M.; Cruse, R. W.; Nye, S. A.; Boyer, J. L.; Delis, J. G.; Chirik, P. J. High-Selectivity Bis(imino)pyridine Iron Catalysts for the Hydrosilylation of 1,2,4-Trivinylcyclohexane. ACS Catal. 2012, 2, 2169− 2172. (267) Kamata, K.; Suzuki, A.; Nakai, Y.; Nakazawa, H. Catalytic Hydrosilylation of Alkenes by Iron Complexes Containing Terpyridine Derivatives as Ancillary Ligands. Organometallics 2012, 31, 3825−3828. (268) Tondreau, A. M.; Atienza, C. C. H.; Darmon, J. M.; Milsmann, C.; Hoyt, H. M.; Weller, K. J.; Nye, S. A.; Lewis, K. M.; Boyer, J.; Delis, J. G. Synthesis, Electronic Structure, and Alkene Hydrosilylation Activity of Terpyridine and Bis(imino)pyridine Iron Dialkyl Complexes. Organometallics 2012, 31, 4886−4893. (269) Peng, D.; Zhang, Y.; Du, X.; Zhang, L.; Leng, X.; Walter, M. D.; Huang, Z. Phosphinite-Iminopyridine Iron Catalysts for Chemoselective Alkene Hydrosilylation. J. Am. Chem. Soc. 2013, 135, 19154− 19166. (270) Hayasaka, K.; Kamata, K.; Nakazawa, H. Highly Efficient Olefin Hydrosilylation Catalyzed by Iron Complexes with Iminobipyridine Ligand. Bull. Chem. Soc. Jpn. 2016, 89, 394−404. (271) Toya, Y.; Hayasaka, K.; Nakazawa, H. Hydrosilylation of Olefins Catalyzed by Iron Complexes Bearing Ketimine-type Iminobipyridine Ligands. Organometallics 2017, 36, 1727−1735. (272) Chen, J.; Cheng, B.; Cao, M.; Lu, Z. Iron-Catalyzed Asymmetric Hydrosilylation of 1, 1-Disubstituted Alkenes. Angew. Chem., Int. Ed. 2015, 54, 4661−4664. (273) Cheng, B.; Liu, W.; Lu, Z. Iron-Catalyzed Highly Enantioselective Hydrosilylation of Unactivated Terminal Alkenes. J. Am. Chem. Soc. 2018, 140, 5014−5017. (274) Hu, M.-Y.; He, Q.; Fan, S.-J.; Wang, Z.-C.; Liu, L.-Y.; Mu, Y.J.; Peng, Q.; Zhu, S.-F. Ligands with 1,10-Phenanthroline Scaffold for Highly Regioselective Iron-Catalyzed Alkene Hydrosilylation. Nat. Commun. 2018, 9, 221. (275) Noda, D.; Tahara, A.; Sunada, Y.; Nagashima, H. NonPrecious-Metal Catalytic Systems Involving Iron or Cobalt Carboxylates and Alkyl Isocyanides for Hydrosilylation of Alkenes with Hydrosiloxanes. J. Am. Chem. Soc. 2016, 138, 2480−2483. (276) Wu, J. Y.; Stanzl, B. N.; Ritter, T. A Strategy for The Synthesis of Well-Defined Iron Catalysts and Application to Regioselective Diene Hydrosilylation. J. Am. Chem. Soc. 2010, 132, 13214−13216.

(277) Enthaler, S.; Haberberger, M.; Irran, E. Highly Selective IronCatalyzed Synthesis of Alkenes by the Reduction of Alkynes. Chem. Asian J. 2011, 6, 1613−1623. (278) Haberberger, M.; Irran, E.; Enthaler, S. Synthesis, Characterization and Catalytic Application of Iron Complexes Modified by Monodentate Phosphane Ligands. Eur. J. Inorg. Chem. 2011, 2797− 2802. (279) Johnson, C.; Albrecht, M. Z-Selective Alkyne Semi-Hydrogenation Catalysed by Piano-Stool N-Heterocyclic Carbene Iron Complexes. Catal. Sci. Technol. 2018, 8, 2779−2783. (280) Belger, C.; Plietker, B. Aryl−Aryl Interactions as Directing Motifs in the Stereodivergent Iron-Catalyzed Hydrosilylation of Internal Alkynes. Chem. Commun. 2012, 48, 5419−5421. (281) Syamala, L. V. R. B.; Mete, T. B.; Bhat, R. G. FeCl3·6H2OCatalyzed Selective Conjugate Reduction of Alkylidene-β-Keto Esters and Alkylidene-1,3-diketones. Tetrahedron Lett. 2018, 59, 3288−3291. (282) Brunner, H.; Fisch, K. Catalytic Hydrosilylation or Hydrogenation at One Coordination Site of [Cp’Fe(CO)(X)] Fragments. Angew. Chem., Int. Ed. Engl. 1990, 29, 1131−1132. (283) Shaikh, N. S.; Junge, K.; Beller, M. A Convenient and General Iron-Catalyzed Hydrosilylation of Aldehydes. Org. Lett. 2007, 9, 5429−5432. (284) Addis, D.; Shaikh, N.; Zhou, S.; Das, S.; Junge, K.; Beller, M. Chemo-and Stereoselective Iron-Catalyzed Hydrosilylation of Ketones. Chem. - Asian J. 2010, 5, 1687−1691. (285) Shaikh, N. S.; Enthaler, S.; Junge, K.; Beller, M. IronCatalyzed Enantioselective Hydrosilylation of Ketones. Angew. Chem., Int. Ed. 2008, 47, 2497−2501. (286) Nishiyama, H.; Furuta, A. An Iron-Catalysed Hydrosilylation of Ketones. Chem. Commun. 2007, 760−762. (287) Furuta, A.; Nishiyama, H. Highly Efficient Catalytic System for Hydrosilylation of Ketones With Iron(II) Acetate−Thiophenecarboxylate. Tetrahedron Lett. 2008, 49, 110−113. (288) Inagaki, T.; Phong, L. T.; Furuta, A.; Ito, J.-i.; Nishiyama, H. Iron-and Cobalt-Catalyzed Asymmetric Hydrosilylation of Ketones and Enones with Bis(oxazolinylphenyl)amine Ligands. Chem. - Eur. J. 2010, 16, 3090−3096. (289) Inagaki, T.; Ito, A.; Ito, J.-i.; Nishiyama, H. Asymmetric IronCatalyzed Hydrosilane Reduction of Ketones: Effect of Zinc Metal upon the Absolute Configuration. Angew. Chem., Int. Ed. 2010, 49, 9384−9387. (290) Muller, K.; Schubert, A.; Jozak, T.; Ahrens-Botzong, A.; Schünemann, V.; Thiel, W. R. Electronic Effects in the Catalytic Hydrosilylation with In-Situ Generated Iron(II)-Catalysts. ChemCatChem 2011, 3, 887−892. (291) Plietker, B.; Dieskau, A. The Reincarnation of the Hieber Anion [Fe(CO)3(NO)] - a New Venue in Nucleophilic Metal Catalysis. Eur. J. Org. Chem. 2009, 2009, 775−787. (292) Holzwarth, M.; Dieskau, A.; Tabassam, M.; Plietker, B. Preformed π-Allyl Iron Complexes as Potent, Well-Defined Catalysts for the Allylic Substitution. Angew. Chem., Int. Ed. 2009, 48, 7251− 7255. (293) Dieskau, A. P.; Begouin, J. M.; Plietker, B. Bu4N[Fe(CO)3(NO)]-Catalyzed Hydrosilylation of Aldehydes and Ketones. Eur. J. Org. Chem. 2011, 2011, 5291−5296. (294) Yang, J.; Tilley, T. D. Efficient Hydrosilylation of Carbonyl Compounds with the Simple Amide Catalyst [Fe{N(SiMe3)2}2]. Angew. Chem., Int. Ed. 2010, 49, 10186−10188. (295) Ruddy, A. J.; Kelly, C. M.; Crawford, S. M.; Wheaton, C. A.; Sydora, O. L.; Small, B. L.; Stradiotto, M.; Turculet, L.; NPhosphinoamidinate. Iron Pre-Catalysts for the Room Temperature Hydrosilylation of Carbonyl Compounds with Broad Substrate Scope at Low Loadings. Organometallics 2013, 32, 5581−5588. (296) Flückiger, M.; Togni, A. Iron (II)-Catalyzed Asymmetric Hydrosilylation of Acetophenone. Eur. J. Org. Chem. 2011, 4353− 4360. (297) Tondreau, A. M.; Lobkovsky, E.; Chirik, P. J. Bis(imino)pyridine Iron Complexes for Aldehyde and Ketone Hydrosilylation. Org. Lett. 2008, 10, 2789−2792. BE


Chemical Reviews

Review

Nonclassical Iron Silyl Dihydride. J. Am. Chem. Soc. 2008, 130, 3732−3733. (316) Zheng, J.; Misal Castro, L. C.; Roisnel, T.; Darcel, C.; Sortais, J.-B. Iron Piano-Stool Phosphine Complexes for Catalytic Hydrosilylation Reaction. Inorg. Chim. Acta 2012, 380, 301−307. (317) Bézier, D.; Sortais, J.-B.; Darcel, C. N-Heterocyclic Carbene Ligands and Iron: An Effective Association for Catalysis. Adv. Synth. Catal. 2013, 355, 19−33. (318) Jiang, F.; Bézier, D.; Sortais, J.-B.; Darcel, C. N-Heterocyclic Carbene Piano-Stool Iron Complexes as Efficient Catalysts for Hydrosilylation of Carbonyl Derivatives. Adv. Synth. Catal. 2011, 353, 239−244. (319) Bézier, D.; Jiang, F.; Roisnel, T.; Sortais, J.-B.; Darcel, C. Cyclopentadienyl−NHC Iron Complexes for Solvent-Free Catalytic Hydrosilylation of Aldehydes and Ketones. Eur. J. Inorg. Chem. 2012, 1333−1337. (320) Jung, T. C.; Argouarch, G.; van de Weghe, P. Cyclopentadienyliron Dicarbonyl Dimer: a Simple Tool for the Hydrosilylation of Aldehydes and Ketones under Air. Catal. Commun. 2016, 78, 52−54. (321) Demir, S.; Gökçe, Y.; Kaloğlu, N.; Sortais, J.-B.; Darcel, C.; Ö zdemir, I.̇ Synthesis of New Iron−NHC Complexes as Catalysts for Hydrosilylation Reactions. Appl. Organomet. Chem. 2013, 27, 459− 464. (322) César, V.; Misal Castro, L. C.; Dombray, T.; Sortais, J.-B.; Darcel, C.; Labat, S.; Miqueu, K.; Sotiropoulos, J.-M.; Brousses, R.; Lugan, N. (Cyclopentadienyl)iron (II) Complexes of N-Heterocyclic Carbenes Bearing a Malonate or Imidate Backbone: Synthesis, Structure, and Catalytic Potential in Hydrosilylation. Organometallics 2013, 32, 4643−4655. (323) Kumar, D.; Prakasham, A.; Bheeter, L. P.; Sortais, J.-B.; Gangwar, M.; Roisnel, T.; Kalita, A. C.; Darcel, C.; Ghosh, P. Cationic Iron (II) Complexes of the Mixed Cyclopentadienyl (Cp) and the NHeterocyclic Carbene (NHC) Ligands as Effective Precatalysts for the Hydrosilylation of Carbonyl Compounds. J. Organomet. Chem. 2014, 762, 81−87. (324) Johnson, C.; Albrecht, M. Triazolylidene Iron(II) Piano-Stool Complexes: Synthesis and Catalytic Hydrosilylation of Carbonyl Compounds. Organometallics 2017, 36, 2902−2913. (325) Buitrago, E.; Zani, L.; Adolfsson, H. Selective Hydrosilylation of Ketones Catalyzed by in situ-Generated Iron NHC Complexes. Appl. Organomet. Chem. 2011, 25, 748−752. (326) Buitrago, E.; Tinnis, F.; Adolfsson, H. Efficient and Selective Hydrosilylation of Carbonyl Compounds Catalyzed by Iron Acetate and N-Hydroxyethylimidazolium Salts. Adv. Synth. Catal. 2012, 354, 217−222. (327) Hashimoto, T.; Urban, S.; Hoshino, R.; Ohki, Y.; Tatsumi, K.; Glorius, F. Synthesis of Bis(N-Heterocyclic Carbene) Complexes of Iron(II) and Their Application in Hydrosilylation and Transfer Hydrogenation. Organometallics 2012, 31, 4474−4479. (328) Grohmann, C.; Hashimoto, T.; Fröhlich, R.; Ohki, Y.; Tatsumi, K.; Glorius, F. An Iron(II) Complex of a Diamine-Bridged Bis-N-Heterocyclic Carbene. Organometallics 2012, 31, 8047−8050. (329) Warratz, S.; Postigo, L.; Royo, B. Direct Synthesis of Iron(0) N-Heterocyclic Carbene Complexes by Using Fe3(CO)12 and Their Application in Reduction of Carbonyl Groups. Organometallics 2013, 32, 893−897. (330) Liang, Q.; Liu, N. J.; Song, D. Constructing Reactive Fe and Co Complexes from Isolated Picolyl-Functionalized N-Heterocyclic Carbenes. Dalton Trans 2018, 47, 9889−9896. (331) Zuo, Z.; Sun, H.; Wang, L.; Li, X. Hydrosilylation of Aldehydes and Ketones Catalyzed by Hydrido Iron Complexes Bearing Imine Ligands. Dalton Trans 2014, 43, 11716−11722. (332) Ren, S.; Xie, S.; Zheng, T.; Wang, Y.; Xu, S.; Xue, B.; Li, X.; Sun, H.; Fuhr, O.; Fenske, D. Synthesis of Silyl Iron Hydride via Si-H Activation and Its Dual Catalytic Application in the Hydrosilylation of Carbonyl Compounds and Dehydration of Benzamides. Dalton Trans 2018, 47, 4352−4359.

(298) Bhattacharya, P.; Krause, J. A.; Guan, H. Iron Hydride Complexes Bearing Phosphinite-Based Pincer Ligands: Synthesis, Reactivity, and Catalytic Application in Hydrosilylation Reactions. Organometallics 2011, 30, 4720−4729. (299) Bhattacharya, P.; Krause, J. A.; Guan, H. Activation of Dihydrogen and Silanes by Cationic Iron Bis(phosphinite) Pincer Complexes. Organometallics 2014, 33, 6113−6121. (300) Chakraborty, S.; Bhattacharya, P.; Dai, H.; Guan, H. Nickel and Iron Pincer Complexes as Catalysts for the Reduction of Carbonyl Compounds. Acc. Chem. Res. 2015, 48, 1995−2003. (301) Wu, S.; Li, X.; Xiong, Z.; Xu, W.; Lu, Y.; Sun, H. Synthesis and Reactivity of Silyl Iron, Cobalt, and Nickel Complexes Bearing a [PSiP]-Pincer Ligand via Si-H Bond Activation. Organometallics 2013, 32, 3227−3237. (302) Zhao, H.; Sun, H.; Li, X. Synthesis and Catalytic Property of Iron Pincer Complexes Generated by Csp3−H Activation. Organometallics 2014, 33, 3535−3539. (303) Huang, S.; Zhao, H.; Li, X.; Wang, L.; Sun, H. Synthesis of [POCOP]-pincer Iron and Cobalt Complexes via Csp3−H Activation and Catalytic Application of Iron Hydride in Hydrosilylation Reactions. RSC Adv. 2015, 5, 15660−15667. (304) Smith, A. D.; Saini, A.; Singer, L. M.; Phadke, N.; Findlater, M. Synthesis, Characterization and Reactivity of Iron-and Cobaltpincer Complexes. Polyhedron 2016, 114, 286−291. (305) Metsänen, T. T.; Gallego, D.; Szilvási, T.; Driess, M.; Oestreich, M. Peripheral Mechanism of a Carbonyl Hydrosilylation Catalysed by an SiNSi Iron Pincer Complex. Chem. Sci. 2015, 6, 7143−7149. (306) Gallego, D.; Inoue, S.; Blom, B.; Driess, M. Highly ElectronRich Pincer-Type Iron Complexes Bearing Innocent Bis(metallylene)pyridine Ligands: Syntheses, Structures, and Catalytic Activity. Organometallics 2014, 33, 6885−6897. (307) Lin, H.-J.; Lutz, S.; O’Kane, C.; Zeller, M.; Chen, C.-H.; Al Assil, T.; Lee, W.-T. Synthesis and Characterization of an Iron Complex Bearing a Hemilabile NNN-Pincer for Catalytic Hydrosilylation of Organic Carbonyl Compounds. Dalton Trans 2018, 47, 3243−3247. (308) Langlotz, B. K.; Wadepohl, H.; Gade, L. H. Chiral Bis(pyridylimino)isoindoles: A Highly Modular Class of Pincer Ligands for Enantioselective Catalysis. Angew. Chem., Int. Ed. 2008, 47, 4670−4674. (309) Tondreau, A. M.; Darmon, J. M.; Wile, B. M.; Floyd, S. K.; Lobkovsky, E.; Chirik, P. J. Enantiopure Pyridine Bis(oxazoline) ″Pybox″ and Bis(oxazoline) ″Box″ Iron Dialkyl Complexes: Comparison to Bis(imino)pyridine Compounds and Application to Catalytic Hydrosilylation of Ketones. Organometallics 2009, 28, 3928−3940. (310) Hosokawa, S.; Ito, J.-i.; Nishiyama, H. A Chiral Iron Complex Containing a Bis (oxazolinyl) phenyl Ligand: Preparation and Asymmetric Hydrosilylation of Ketones. Organometallics 2010, 29, 5773−5775. (311) Zuo, Z.; Zhang, L.; Leng, X.; Huang, Z. Iron-catalyzed Asymmetric Hydrosilylation of Ketones. Chem. Commun. 2015, 51, 5073−5076. (312) Ito, J.-i.; Hosokawa, S.; Khalid, H. B.; Nishiyama, H. Preparation, Characterization, and Catalytic Reactions of NCN Pincer Iron Complexes Containing Stannyl, Silyl, Methyl, and Phenyl Ligands. Organometallics 2015, 34, 1377−1383. (313) Bleith, T.; Wadepohl, H.; Gade, L. H. Iron Achieves Noble Metal Reactivity and Selectivity: Highly Reactive and Enantioselective Iron Complexes as Catalysts in the Hydrosilylation of Ketones. J. Am. Chem. Soc. 2015, 137, 2456−2459. (314) Bleith, T.; Gade, L. H. Mechanism of the Iron (II)-Catalyzed Hydrosilylation of Ketones: Activation of Iron Carboxylate Precatalysts and Reaction Pathways of the Active Catalyst. J. Am. Chem. Soc. 2016, 138, 4972−4983. (315) Gutsulyak, D. V.; Kuzmina, L. G.; Howard, J. A.; Vyboishchikov, S. F.; Nikonov, G. I. Cp(iPr2MeP)FeH2SiR3: BF


Chemical Reviews

Review

Nitro Compounds to Amines. Tetrahedron Lett. 2010, 51, 1939− 1941. (352) Pehlivan, L.; Métay, E.; Laval, S.; Dayoub, W.; Demonchaux, P.; Mignani, G.; Lemaire, M. Alternative Method for the Reduction of Aromatic Nitro to Amine Using TMDS-Iron Catalyst System. Tetrahedron 2011, 67, 1971−1976. (353) Zhu, K.; Shaver, M. P.; Thomas, S. P. Chemoselective Nitro Reduction and Hydroamination Using a Single Iron Catalyst. Chem. Sci. 2016, 7, 3031−3035. (354) Gui, J.; Pan, C.-M.; Jin, Y.; Qin, T.; Lo, J. C.; Lee, B. J.; Spergel, S. H.; Mertzman, M. E.; Pitts, W. J.; La Cruz, T. E. Practical Olefin Hydroamination with Nitroarenes. Science 2015, 348, 886− 891. (355) Shevlin, M.; Guan, X.; Driver, T. G. Iron-Catalyzed Reductive Cyclization of o-Nitrostyrenes Using Phenylsilane as the Terminal Reductant. ACS Catal. 2017, 7, 5518−5522. (356) Werkmeister, S.; Junge, K.; Beller, M. Catalytic Hydrogenation of Carboxylic Acid Esters, Amides, and Nitriles with Homogeneous Catalysts. Org. Process Res. Dev. 2014, 18, 289−302. (357) Zhou, S.; Junge, K.; Addis, D.; Das, S.; Beller, M. A Convenient and General Iron-Catalyzed Reduction of Amides to Amines. Angew. Chem., Int. Ed. 2009, 48, 9507−9510. (358) Tsutsumi, H.; Sunada, Y.; Nagashima, H. New Catalyst Systems for Iron-Catalyzed Hydrosilane Reduction of Carboxamides. Chem. Commun. 2011, 47, 6581−6583. (359) Bézier, D.; Venkanna, G. T.; Sortais, J.-B.; Darcel, C. WellDefined Cyclopentadienyl NHC Iron Complex as the Catalyst for Efficient Hydrosilylation of Amides to Amines and Nitriles. ChemCatChem 2011, 3, 1747−1750. (360) Volkov, A.; Buitrago, E.; Adolfsson, H. Direct Hydrosilylation of Tertiary Amides to Amines by an In Situ Formed Iron/NHeterocyclic Carbene Catalyst. Eur. J. Org. Chem. 2013, 2066−2070. (361) Blom, B.; Tan, G.; Enthaler, S.; Inoue, S.; Epping, J. D.; Driess, M. Bis-N-Heterocyclic Carbene (NHC) Stabilized η6-Arene Iron(0) Complexes: Synthesis, Structure, Reactivity, and Catalytic Activity. J. Am. Chem. Soc. 2013, 135, 18108−18120. (362) Zhou, S.; Addis, D.; Das, S.; Junge, K.; Beller, M. New Catalytic Properties of Iron Complexes: Dehydration of Amides to Nitriles. Chem. Commun. 2009, 4883−4885. (363) Xue, B.; Sun, H.; Wang, Y.; Zheng, T.; Li, X.; Fuhr, O.; Fenske, D. Efficient Reductive Dehydration of Primary Amides to Nitriles Catalyzed by Hydrido Thiophenolato Iron(II) Complexes under Hydrosilation Conditions. Catal. Commun. 2016, 86, 148−150. (364) Das, S.; Wendt, B.; Möller, K.; Junge, K.; Beller, M. Two Iron Catalysts Are Better than One: a General and Convenient Reduction of Aromatic and Aliphatic Primary Amides. Angew. Chem., Int. Ed. 2012, 51, 1662−1666. (365) Ito, M.; Itazaki, M.; Nakazawa, H. Selective Double Hydrosilylation of Nitriles Catalyzed by an Iron Complex Containing Indium Trihalide. ChemCatChem 2016, 8, 3323−3325. (366) Bézier, D.; Venkanna, G. T.; Misal Castro, L. C.; Zheng, J.; Roisnel, T.; Sortais, J.-B.; Darcel, C. Iron-Catalyzed Hydrosilylation of Esters. Adv. Synth. Catal. 2012, 354, 1879−1884. (367) Junge, K.; Wendt, B.; Zhou, S.; Beller, M. Iron-Catalyzed Reduction of Carboxylic Esters to Alcohols. Eur. J. Org. Chem. 2013, 2061−2065. (368) Das, S.; Li, Y.; Junge, K.; Beller, M. Synthesis of Ethers from Esters via Fe-Catalyzed Hydrosilylation. Chem. Commun. 2012, 48, 10742−10744. (369) Li, H.; Misal Castro, L. C.; Zheng, J.; Roisnel, T.; Dorcet, V.; Sortais, J.-B.; Darcel, C. Selective Reduction of Esters to Aldehydes under the Catalysis of Well-Defined NHC−Iron Complexes. Angew. Chem., Int. Ed. 2013, 52, 8045−8049. (370) Quintero-Duque, S.; Li, H.; Misal Castro, L. C.; Dorcet, V.; Roisnel, T.; Clot, E.; Grellier, M.; Sortais, J.-B.; Darcel, C. NHeterocyclic Carbene Iron Silyl Hydride Complexes. Isr. J. Chem. 2017, 57, 1216−1221.

(333) Zheng, T.; Li, J.; Zhang, S.; Xue, B.; Sun, H.; Li, X.; Fuhr, O.; Fenske, D. Selective C−F and C−H Activation of Fluoroarenes by Fe(PMe3)4 and Catalytic Performance of Iron Hydride in Hydrosilylation of Carbonyl Compounds. Organometallics 2016, 35, 3538− 3545. (334) Xue, B.; Sun, H.; Li, X. Syntheses and Catalytic Application of Hydrido Iron(II) Complexes with [P,S]-Chelating Ligands in Hydrosilylation of Aldehydes and Ketones. RSC Adv. 2015, 5, 52000−52006. (335) Xue, B.; Sun, H.; Niu, Q.; Li, X.; Fuhr, O.; Fenske, D. Catalytic Hydrosilylation of Carbonyl Compounds by Hydrido Thiophenolato Iron(II) Complexes. Catal. Commun. 2017, 94, 23− 28. (336) Wang, Y.; Ren, S.; Zhang, W.; Xue, B.; Qi, X.; Sun, H.; Li, X.; Fuhr, O.; Fenske, D. Syntheses of Hydrido Selenophenolato Iron(II) Complexes and Their Catalytic Application in Hydrosilylation of Aldehydes and Ketones. Catal. Commun. 2018, 115, 1−5. (337) Wekesa, F. S.; Arias-Ugarte, R.; Kong, L.; Sumner, Z.; McGovern, G. P.; Findlater, M. Iron-Catalyzed Hydrosilylation of Aldehydes and Ketones under Solvent-Free Conditions. Organometallics 2015, 34, 5051−5056. (338) Zhu, K.; Shaver, M. P.; Thomas, S. P. Stable and Easily Handled Fe(III) Catalysts for Hydrosilylation of Ketones and Aldehydes. Eur. J. Org. Chem. 2015, 2119−2123. (339) Raya-Barón, Á .; Ortuño, M. A.; Oña-Burgos, P.; RodríguezDiéguez, A.; Langer, R.; Cramer, C. J.; Kuzu, I.; Fernández, I. Efficient Hydrosilylation of Acetophenone with a New Anthraquinonic AmideBased Iron Precatalyst. Organometallics 2016, 35, 4083−4089. (340) Raya-Barón, Á .; Galdeano-Ruano, C. P.; Oña-Burgos, P.; Rodríguez-Diéguez, A.; Langer, R.; López-Ruiz, R.; Romero-González, R.; Kuzu, I.; Fernández, I. A New Anthraquinoid Ligand for the IronCatalyzed Hydrosilylation of Carbonyl Compounds at Room Temperature: New Insights and Kinetics. Dalton Trans 2018, 47, 7272−7281. (341) Misal Castro, L. C.; Bézier, D.; Sortais, J.-B.; Darcel, C. Iron Dihydride Complex as the Pre-catalyst for Efficient Hydrosilylation of Aldehydes and Ketones Under Visible Light Activation. Adv. Synth. Catal. 2011, 353, 1279−1284. (342) Nesbit, M. A.; Suess, D. L.; Peters, J. C. E−H. Bond Activations and Hydrosilylation Catalysis with Iron and Cobalt Metalloboranes. Organometallics 2015, 34, 4741−4752. (343) Dal Zotto, C.; Virieux, D.; Campagne, J.-M. FeCl3-Catalyzed Reduction of Ketones and Aldehydes to Alkane Compounds. Synlett 2009, 276−278. (344) Misal Castro, L. C.; Sortais, J.-B.; Darcel, C. NHC-Carbene Cyclopentadienyl Iron Based Catalyst for a General and Efficient Hydrosilylation of Imines. Chem. Commun. 2012, 48, 151−153. (345) Bhunia, M.; Hota, P. K.; Vijaykumar, G.; Adhikari, D.; Mandal, S. K. A Highly Efficient Base-Metal Catalyst: Chemoselective Reduction of Imines to Amines Using an Abnormal-NHC−Fe(0) Complex. Organometallics 2016, 35, 2930−2937. (346) Enthaler, S. Synthesis of Secondary Amines by Iron-Catalyzed Reductive Amination. ChemCatChem 2010, 2, 1411−1415. (347) Jaafar, H.; Li, H.; Misal Castro, L. C.; Zheng, J.; Roisnel, T.; Dorcet, V.; Sortais, J. B.; Darcel, C. Phosphane-Pyridine Iron Complexes: Synthesis, Characterization and Application in Reductive Amination through the Hydrosilylation Reaction. Eur. J. Inorg. Chem. 2012, 3546−3550. (348) Li, H.; Achard, M.; Bruneau, C.; Sortais, J.-B.; Darcel, C. IronCatalysed Tandem Isomerisation/Hydrosilylation Reaction of Allylic Alcohols with Amines. RSC Adv. 2014, 4, 25892−25897. (349) Sunada, Y.; Kawakami, H.; Imaoka, T.; Motoyama, Y.; Nagashima, H. Hydrosilane Reduction of Tertiary Carboxamides by Iron Carbonyl Catalysts. Angew. Chem., Int. Ed. 2009, 48, 9511−9514. (350) Junge, K.; Wendt, B.; Shaikh, N.; Beller, M. Iron-Catalyzed Selective Reduction of Nitroarenes to Anilines Using Organosilanes. Chem. Commun. 2010, 46, 1769−1771. (351) Pehlivan, L.; Métay, E.; Laval, S.; Dayoub, W.; Demonchaux, P.; Mignani, G.; Lemaire, M. Iron-Catalyzed Selective Reduction of BG


Chemical Reviews

Review

(371) Cong, C.; Fujihara, T.; Terao, J.; Tsuji, Y. Iron Oxide Catalyzed Reduction of Acid Chlorides to Aldehydes with Hydrosilanes. Catal. Commun. 2014, 50, 25−28. (372) Misal Castro, L. C.; Li, H.; Sortais, J.-B.; Darcel, C. Selective Switchable Iron-Catalyzed Hydrosilylation of Carboxylic Acids. Chem. Commun. 2012, 48, 10514−10516. (373) Pouessel, J.; Jacquet, O.; Cantat, T. Pushing Back the Limits of Hydrosilylation: Unprecedented Catalytic Reduction of Organic Ureas to Formamidines. ChemCatChem 2013, 5, 3552−3556. (374) Frogneux, X.; Jacquet, O.; Cantat, T. Iron-Catalyzed Hydrosilylation of CO2: CO2 Conversion To Formamides and Methylamines. Catal. Sci. Technol. 2014, 4, 1529−1533. (375) Enthaler, S. A. Facile and Efficient Iron-Catalyzed Reduction of Sulfoxides to Sulfides. ChemCatChem 2011, 3, 666−670. (376) Cardoso, J. M.; Royo, B. Unprecedented Synthesis of Iron− NHC Complexes by C−H Activation of Imidazolium Salts. Mild Catalysts for Reduction of Sulfoxides. Chem. Commun. 2012, 48, 4944−4946. (377) Corma, A.; Navas, J.; Sabater, M. J. Advances in One-Pot Synthesis through Borrowing Hydrogen Catalysis. Chem. Rev. 2018, 118, 1410−1459. (378) Cui, X.; Shi, F.; Zhang, Y.; Deng, Y. Fe(II)-catalyzed Nalkylation of Sulfonamides with Benzylic Alcohols. Tetrahedron Lett. 2010, 51, 2048−2051. (379) Bala, M.; Verma, P. K.; Sharma, U.; Kumar, N.; Singh, B. Iron Phthalocyanine as an Efficient and Versatile Catalyst for N-alkylation of Heterocyclic Amines with Alcohols: One-pot Synthesis of 2Substituted Benzimidazoles, Benzothiazoles and Benzoxazoles. Green Chem. 2013, 15, 1687−1693. (380) Yan, T.; Feringa, B. L.; Barta, K. Iron Catalysed Direct Alkylation of Amines with Alcohols. Nat. Commun. 2014, 5, 5602. (381) Yan, T.; Feringa, B. L.; Barta, K. Benzylamines via Ironcatalyzed Direct Amination of Benzyl Alcohols. ACS Catal. 2016, 6, 381−388. (382) Emayavaramban, B.; Roy, M.; Sundararaju, B. Iron-Catalyzed Allylic Amination Directly from Allylic Alcohols. Chem. - Eur. J. 2016, 22, 3952−3955. (383) Brown, T. J.; Cumbes, M.; Diorazio, L. J.; Clarkson, G. J.; Wills, M. Use of (Cyclopentadienone)iron Tricarbonyl Complexes for C−N Bond Formation Reactions between Amines and Alcohols. J. Org. Chem. 2017, 82, 10489−10503. (384) Pan, H.-J.; Ng, T. W.; Zhao, Y. Iron-catalyzed Amination of Alcohols Assisted by Lewis Acid. Chem. Commun. 2015, 51, 11907− 11910. (385) Yan, T.; Feringa, B. L.; Barta, K. Direct N-alkylation of Unprotected Amino Acids with Alcohols. Sci. Adv. 2017, 3, No. eaao6494. (386) Emayavaramban, B.; Sen, M.; Sundararaju, B. Iron-catalyzed Sustainable Synthesis of Pyrrole. Org. Lett. 2017, 19, 6−9. (387) Elangovan, S.; Sortais, J.-B.; Beller, M.; Darcel, C. IronCatalyzed α-Alkylation of Ketones with Alcohols. Angew. Chem., Int. Ed. 2015, 54, 14483−14486. (388) Seck, C.; Mbaye, M. D.; Coufourier, S.; Lator, A.; Lohier, J. F.; Poater, A.; Ward, T. R.; Gaillard, S.; Renaud, J.-L. Alkylation of Ketones Catalyzed by Bifunctional Iron Complexes: From Mechanistic Understanding to Application. ChemCatChem 2017, 9, 4410− 4416. (389) Di Gregorio, G.; Mari, M.; Bartoccini, F.; Piersanti, G. IronCatalyzed Direct C3-Benzylation of Indoles with Benzyl Alcohols through Borrowing Hydrogen. J. Org. Chem. 2017, 82, 8769−8775. (390) Burgess, K.; Ohlmeyer, M. J. Transition-Metal Promoted Hydroborations of Alkenes, Emerging Methodology for Organic Transformations. Chem. Rev. 1991, 91, 1179−1191. (391) Hartwig, J. F. Regioselectivity of the Borylation of Alkanes and Arenes. Chem. Soc. Rev. 2011, 40, 1992−2002. (392) Miyaura, N. Hydroboration, Diboration, Silylboration, and Stannylboration. In Catalytic Heterofunctionalization; Togni, A., Grützmacher, H., Eds.; Wiley-VCH Verlag GmbH: Weinheim, 2001; pp 1−45.

(393) Mkhalid, I.; Barnard, J.; Marder, T.; Murphy, J.; Hartwig, J. CH. Activation for the Construction of C-B Bonds. Chem. Rev. 2010, 110, 890−931. (394) Doucet, H. Suzuki−Miyaura Cross-Coupling Reactions of Alkylboronic Acid Derivatives or Alkyltrifluoroborates with Aryl, Alkenyl or Alkyl Halides and Triflates. Eur. J. Org. Chem. 2008, 2013− 2030. (395) Hall, D. G. Boronic Acids: Preparation, Applications in Organic Synthesis and Medicine; Wiley-VCH Verlag GmbH & Co. KGaA: Weinheim, 2006. (396) Jana, R.; Pathak, T. P.; Sigman, M. S. Advances in Transition Metal (Pd,Ni,Fe)-Catalyzed Cross-Coupling Reactions Using Alkylorganometallics as Reaction Partners. Chem. Rev. 2011, 111, 1417− 1492. (397) Miyaura, N.; Suzuki, A. Palladium-Catalyzed Cross-Coupling Reactions of Organoboron Compounds. Chem. Rev. 1995, 95, 2457− 2483. (398) Brown, J. Modern Rhodium-Catalysed Organic Reactions. In Modern rhodium-catalysed organic reactions; Evans, P., Ed.; WileyVCH: Weinheim, 2005; pp 33−54. (399) Coyne, A.; Guiry, P. Modern Reduction Methods. In Modern Reduction Methods; Andersson, P., Munslow, I., Eds.; Wiley-VCH: Weinheim, 2008; p 65. (400) Hartwig, J. F.; Huber, S. Transition-Metal Boryl Complexes: Structure and Reactivity of CpFe(CO)2Bcat and CpFe(CO)2BPh2. J. Am. Chem. Soc. 1993, 115, 4908−4909. (401) Waltz, K. M.; He, X.; Muhoro, C.; Hartwig, J. F. Hydrocarbon Functionalization by Transition Metal Boryls. J. Am. Chem. Soc. 1995, 117, 11357−11358. (402) Wu, J. Y.; Moreau, B.; Ritter, T. Iron-Catalyzed 1,4Hydroboration of 1,3-Dienes. J. Am. Chem. Soc. 2009, 131, 12915− 12917. (403) Cao, Y.; Zhang, Y.; Zhang, L.; Zhang, D.; Leng, X.; Huang, Z. Selective Synthesis of Secondary Benzylic (Z)-Allylboronates by FeCatalyzed 1,4-Hydroboration of 1-Aryl-Substituted 1,3-Dienes. Org. Chem. Front. 2014, 1, 1101−1106. (404) Zhang, L.; Peng, D.; Leng, X.; Huang, Z. Iron-Catalyzed, Atom-Economical, Chemo-and Regioselective Alkene Hydroboration with Pinacolborane. Angew. Chem. 2013, 125, 3764−3768. (405) Obligacíon, J. V.; Chirik, P. J. Highly Selective Bis(imino)pyridine Iron-Catalyzed Alkene Hydroboration. Org. Lett. 2013, 15, 2680−2683. (406) Greenhalgh, M. D.; Thomas, S. P. Chemo-, Regio-, and Stereoselective Iron-Catalysed Hydroboration of Alkenes and Alkynes. Chem. Commun. 2013, 49, 11230−11232. (407) Zheng, J.; Sortais, J.-B.; Darcel, C. [(NHC)Fe(CO)4] Efficient Pre-Catalyst for Selective Hydroboration of Alkenes. ChemCatChem 2014, 6, 763−766. (408) Gilbert-Wilson, R.; Chu, W.-Y.; Rauchfuss, T. B. Phosphineiminopyridines as Platforms for Catalytic Hydrofunctionalization of Alkenes. Inorg. Chem. 2015, 54, 5596−5603. (409) Tseng, K.-N. T.; Kampf, J. W.; Szymczak, N. K. Regulation of Iron-catalyzed Olefin Hydroboration by Ligand Modifications at a Remote Site. ACS Catal. 2015, 5, 411−415. (410) Espinal-Viguri, M.; Woof, C. R.; Webster, R. L. Iron-Catalyzed Hydroboration: Unlocking Reactivity through Ligand Modulation. Chem. - Eur. J. 2016, 22, 11605−11608. (411) Ogawa, T.; Ruddy, A. J.; Sydora, O. L.; Stradiotto, M.; Turculet, L. Cobalt-and Iron-Catalyzed Isomerization−Hydroboration of Branched Alkenes: Terminal Hydroboration with Pinacolborane and 1, 3, 2-Diazaborolanes. Organometallics 2017, 36, 417−423. (412) Liu, Y.; Zhou, Y.; Wang, H.; Qu, J. FeCl2-catalyzed Hydroboration of Aryl Alkenes with Bis(pinacolato)diboron. RSC Adv. 2015, 5, 73705−73713. (413) Chen, X.; Cheng, Z.; Lu, Z. Iron-Catalyzed, MarkovnikovSelective Hydroboration of Styrenes. Org. Lett. 2017, 19, 969−971. (414) MacNair, A. J.; Millet, C. R.; Nichol, G. S.; Ironmonger, A.; Thomas, S. P. Markovnikov-selective, Activator-free Iron-catalyzed Vinylarene Hydroboration. ACS Catal. 2016, 6, 7217−7221. BH


Chemical Reviews

Review

(415) Chen, J.; Xi, T.; Lu, Z. Iminopyridine Oxazoline Iron Catalyst for Asymmetric Hydroboration of 1, 1-Disubtituted Aryl Alkenes. Org. Lett. 2014, 16, 6452−6455. (416) Haberberger, M.; Enthaler, S. Straightforward Iron-Catalyzed Synthesis of Vinylboronates by the Hydroboration of Alkynes. Chem. Asian J. 2013, 8, 50−54. (417) Rawat, V. S.; Sreedhar, B. Iron-Catalyzed Borylation Reactions of Alkynes: an Efficient Synthesis of E-Vinyl Boronates. Synlett 2014, 25, 1132−1136. (418) Nakajima, K.; Kato, T.; Nishibayashi, Y. Hydroboration of Alkynes Catalyzed by Pyrrolide-Based PNP Pincer−Iron Complexes. Org. Lett. 2017, 19, 4323−4326. (419) Gorgas, N.; Alves, L. G.; Stöger, B.; Martins, A. M.; Veiros, L. F.; Kirchner, K. Stable, Yet Highly Reactive Nonclassical Iron(II) Polyhydride Pincer Complexes: Z-Selective Dimerization and Hydroboration of Terminal Alkynes. J. Am. Chem. Soc. 2017, 139, 8130− 8133. (420) Chen, C.; Shen, X.; Chen, J.; Hong, X.; Lu, Z. Iron-Catalyzed Hydroboration of Vinylcyclopropanes. Org. Lett. 2017, 19, 5422− 5425. (421) Wang, C.; Wu, C.; Ge, S. Iron-Catalyzed E-Selective Dehydrogenative Borylation of Vinylarenes with Pinacolborane. ACS Catal. 2016, 6, 7585−7589. (422) Jiang, S.; Quintero-Duque, S.; Roisnel, T.; Dorcet, V.; Grellier, M.; Sabo-Etienne, S.; Darcel, C.; Sortais, J.-B. Direct Synthesis of Dicarbonyl PCP-Iron Hydride Complexes and Catalytic Dehydrogenative Borylation of Styrene. Dalton Trans 2016, 45, 11101−11108. (423) Wei, D.; Carboni, B.; Sortais, J.-B.; Darcel, C. Iron-Catalyzed Dehydrogenative Borylation of Terminal Alkynes. Adv. Synth. Catal. 2018, 360, 3649. (424) Tamang, S. R.; Findlater, M. Iron-Catalyzed Hydroboration of Aldehydes and Ketones. J. Org. Chem. 2017, 82, 12857−12862. (425) Bai, T.; Janes, T.; Song, D. Homoleptic Iron (ii) and Cobalt (ii) Bis(phosphoranimide) Complexes for the Selective Hydrofunctionalization of Unsaturated Molecules. Dalton Trans 2017, 46, 12408−12412. (426) Liu, J.; Zhang, A.; Song, H.; Tong, Q.; Tung, C.-H.; Wang, W. Iron (II) Hydrides Bearing a Tetradentate PSNP Ligand. Chin. Chem. Lett. 2018, 29, 949−953. (427) Das, U. K.; Higman, C. S.; Gabidullin, B.; Hein, J. E.; Baker, R. T. Efficient and Selective Iron-Complex-Catalyzed Hydroboration of Aldehydes. ACS Catal. 2018, 8, 1076−1081. (428) Zhang, F.; Song, H.; Zhuang, X.; Tung, C.-H.; Wang, W. IronCatalyzed 1,2-Selective Hydroboration of N-Heteroarenes. J. Am. Chem. Soc. 2017, 139, 17775−17778. (429) Chong, C. C.; Kinjo, R. Catalytic Hydroboration of Carbonyl Derivatives, Imines, and Carbon Dioxide. ACS Catal. 2015, 5, 3238− 3259. (430) Bontemps, S. Boron-Mediated Activation of Carbon Dioxide. Coord. Chem. Rev. 2016, 308, 117−130. (431) Jin, G.; Werncke, C. G.; Escudié, Y.; Sabo-Etienne, S.; Bontemps, S. Iron-Catalyzed Reduction of CO2 into Methylene: Formation of C-N, C-O, and C-C Bonds. J. Am. Chem. Soc. 2015, 137, 9563−9566. (432) Aloisi, A.; Berthet, J.-C.; Genre, C.; Thuéry, P.; Cantat, T. Complexes of the Tripodal Phosphine Ligands PhSi(XPPh2)3 (X = CH2, O): Synthesis, Structure and Catalytic Activity in the Hydroboration of CO2. Dalton Trans 2016, 45, 14774−14788.

BI


Iron Catalysis in Reduction and Hydrometalation Reactions

Recommend Documents