and CâAlkylation Reactions via Borrowing Hydrogen or Hydrogen

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

3d-Metal Catalyzed N- and C‑Alkylation Reactions via Borrowing Hydrogen or Hydrogen Autotransfer Torsten Irrgang and Rhett Kempe*

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Inorganic Chemistry II − Catalyst Design, University of Bayreuth, 95440 Bayreuth, Germany ABSTRACT: The conservation of our element resources is a fundamental challenge of mankind. The development of alcohol refunctionalization reactions is a possible fossil carbon conservation strategy since alcohols can be obtained from indigestible and abundantly available biomass. The conservation of our rare noble metals, frequently used in key technologies such as catalysis, might be feasible by replacing them with highly abundant metals. The alkylation of amines by alcohols and related C−C coupling reactions are early examples of alcohol refunctionalization reactions. These reactions follow mostly the borrowing hydrogen or hydrogen autotransfer catalysis concept, and many 3d-metal catalysts have been disclosed in recent years. In this review, we summarize the progress made in developing Cu, Ni, Co, Fe, and Mn catalysts for C−N and C−C bond formation reactions with alcohols and amines using the borrowing hydrogen or hydrogen autotransfer concept. We expect that the findings in this field will inspire others to develop new efficient and selective earth-abundant metal catalysts for borrowing hydrogen or hydrogen autotransfer applications or to develop novel alcohol refunctionalization reactions that can be mediated by such metals.

CONTENTS 1. Introduction 2. Nickel-Catalyzed C−N and C−C Coupling Reactions 2.1. N-Alkylation by Alcohols 2.2. N-Alkylation by Amines 2.3. C-Alkylation by Alcohols 3. Iron-Catalyzed C−N and C−C Coupling Reactions 3.1. N-Alkylation by Alcohols 3.2. C-Alkylation by Alcohols 4. Cobalt-Catalyzed C−N and C−C Coupling Reactions 4.1. N-Alkylation by Alcohols 4.2. N-Alkylation by Amines 4.3. C-Alkylation by Alcohols 5. Manganese-Catalyzed C−N and C−C Coupling Reactions 5.1. N-Alkylation by Alcohols 5.2. C-Alkylation by Alcohols 6. Other 3d-Metal Catalyzed C−N and C−C Coupling Reactions 6.1. N-Alkylation by Alcohols 6.2. N-Alkylation by Amines 6.3. C-Alkylation by Alcohols 7. Conclusions Author Information Corresponding Author ORCID Notes Biographies © XXXX American Chemical Society

Acknowledgments References

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1. INTRODUCTION The conservation of our element resources is a key issue regarding sustainable living on our planet. An element intensively used in the chemical industry is carbon. Unfortunately, most of the chemical compounds containing carbon are currently produced from finite fossil resources, such as oil or coal.1 A sustainable alternative is biomass. A biomass to produce chemicals should ideally be a barely used waste material, be indigestible to avoid food chain competition, and be abundantly available, since we have a high and increasing demand for chemicals. Lignocellulose fulfills these criteria.2 Since lignocellulose can be processed to alcohols,3,4 the development of reactions converting alcohols to the diversity of bulk and fine chemicals containing carbon is a central topic in sustainable or green chemistry.5 The alkylation of amines by alcohols is an early and chemically important example of an alcohol refunctionalization reaction.6−35 It can proceed as shown in Scheme 1.6,36 The alcohol starting material is oxidized via dehydrogenation. The two hydrogen atoms, which were removed from the alcohol, are parked at the catalyst. The so-formed carbonyl compound (aldehyde or ketone) can undergo a condensation reaction (Schiff base reaction) with amines forming an imine intermediate. In the final step, the

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

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DOI: 10.1021/acs.chemrev.8b00306 Chem. Rev. XXXX, XXX, XXX−XXX

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Scheme 1. Alkylation of Amines by Alcohols Following the Borrowing Hydrogen or Hydrogen Autotransfer (BH/HA) Concept

Scheme 3. C-Alkylation by Alcohols Following the BH/HA Concept

imine is reduced using the hydrogen atom equivalents gained in the oxidation/dehydrogenation step. The reaction was discovered by Winans and Adkins in 1932 employing a heterogeneous nickel catalyst.37 Grigg,38 Watanabe,39 and coworkers introduced the first homogeneous catalysts in 1981. Inspired by this pioneering work, a variety of groups such as Beller,40−42 Grigg,43−45 Fujita and Yamaguchi,46−48 Williams,49−51 Yus52−54 and us,55−57 have developed this synthesis concept into an elegant and broadly applicable method for the selective N-alkylation of amines by alcohols. The complexes shown in Scheme 2 were used as catalysts or precatalysts. If ammonia is used as the nucleophile, as first demonstrated by the groups of Baiker140 (heterogeneous), Milstein,58 and Fujita and Yamaguchi59 (homogeneous), the OH group of the alcohol starting material is converted directly into a NH2 group or a primary aliphatic amine.60−66 Similarly, C-alkylation reactions can proceed6−34 with the central C−C coupling step being an aldol type of condensation (Scheme 3). The first example was the Guerbet reaction.67,68 In recent years, a variety of elegant catalytic syntheses permitting alkyl−alkyl C− C coupling reactions have been developed.69−82 More recently, the selective dimerization of ethanol has been disclosed.83,84 Nearly all these early developments have been mediated by rare noble metal catalysts, based on Ru or Ir mostly. It would be highly desirable to replace them by abundantly available transition metals, such as Cu, Ni, Co, Fe, and Mn (base, nonprecious, or earth-abundant metals). Since catalysis is a key technology, such a replacement would contribute to the conservation of our rare element resources. In addition, novel selectivity patterns might be accessible. In this review, we summarize the recently made progress in N- and C-alkylation

reactions employing alcohols and amines as alkylating agents mediated by Cu, Ni, Co, Fe, and Mn catalysts following the borrowing hydrogen or hydrogen autotransfer (BH/HA) concept. This combination of alcohol refunctionalization chemistry and earth-abundant metal catalysis is especially attractive regarding the development of a more sustainable chemistry, since it contributes to the conservation of our rare noble metals and our fossil carbon resources. Quintard and Rodriguez recently published a highlight8 and Corma and coworkers very recently published “Advances in One-Pot Synthesis through Borrowing Hydrogen Catalysis”.6 In addition, Quintard and co-workers summarized “Recent Achievements in Enantioselective Borrowing Hydrogen by the Combination of Iron- and Organocatalysis”.85 A review with the focus of our work has not been published to the best of our knowledge.6−35 The catalyst systems discussed here are ordered by catalytically active metal and in chronological order. We always describe the N-alkylation first followed by Calkylation catalysis. N- and C-alkylation reactions that take place in a hydrogen atmosphere (reductive alkylation) are not discussed here.

2. NICKEL-CATALYZED C−N AND C−C COUPLING REACTIONS 2.1. N-Alkylation by Alcohols

The first example of an N-alkylation reaction following the borrowing hydrogen or hydrogen autotransfer concept was reported in 1932. Winans and Adkins used a nickel catalyst and alkylated aliphatic amines with primary alcohols (Scheme 4).37

Scheme 2. Selected Homogeneous Noble Metal Catalysts or Precatalysts Used for the N-Alkylation of Amines by Alcohols

B


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Scheme 4. First Ni-Catalyzed N-Alkylation with Alcohols

Scheme 5. Raney Ni Catalyzed N-Alkylation of Aniline and Benzidine with Alcohols

Scheme 6. Alkylation of Amines Using Raney Ni

corresponding carbonate and reduced in a stream of hydrogen at 450 °C (Scheme 4).86 Based on the observations of other groups on N-alkylation of amines with alcohols catalyzed by Raney nickel as a side reaction,87−89 Rice and Kohn developed the Raney nickel catalyzed N-alkylation of aniline and benzidine with alcohols in 1955 (Scheme 5).90 The reactions proceed under reflux with

The reactions were carried out in a hydrogen atmosphere. The authors, however, described that the same yields of alkylated amines were obtained in a nitrogen, hydrogen-free, atmosphere. This is the first report using nickel as a catalyst for the alkylation of amines with alcohols following the borrowing hydrogen mechanism. Their nickel catalyst was prepared starting from nickel nitrate, which was converted into the C


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activity under the given conditions. Mechanistic studies suggested that the active sites are low coordinated Ni0 atoms adjacent to alumina. Shimizu and co-workers also used this catalyst system for the successful alkylation of primary and secondary alcohols employing ammonia (Scheme 9A).99 In 2014 they introduced another heterogeneous nickel catalyst for this type of reactions (Scheme 9B).100 This catalyst, nickel nanoparticles loaded onto calcium silicate (Ni/CaSiO3), was prepared by ion-exchange method followed by in situ H2 reduction of the calcined precursor. With this catalyst material similar results were obtained in the N-alkylation of amines and ammonia under similar reaction conditions as described above. In 2013, Shi’s group introduced a multi-metal catalyst.101 The air- and moisture-stable NiCuFeOx catalyst can be recovered easily due to its magnetic property and reused without obvious deactivation. The catalytic activity for the synthesis of N-substituted primary, secondary, tertiary, and cyclic amines by using primary or secondary amines in the absence of bases is remarkable (Scheme 10). The reactions were carried out in a pressure tube in an argon atmosphere, with xylene as the solvent under reflux. In addition, the monoalkylation and double alkylation of ammonia, which was used in gaseous form or was obtained by thermal decomposition of (NH4)2CO3, was successful under the optimized conditions. A total of 107 amines could be synthesized with up to 98% yields of isolated products. It is not clear which metal is responsible for the BH/HA activity. In 2017, Tang and Zhou and co-workers introduced a homogeneous Ni catalyst system for the direct N-alkylation of hydrazides and arylamines with alcohols (Scheme 11).102 Hydrazides were alkylated in the presence of 2 mol % Ni(OTf)2, 2.5 mol % dcpp (1,3-bis(dicyclohexylphosphino)propane), and molecular sieves in a 1 to 1 mixture of tert-amyl alcohol and HFIP (1,1,1,3,3,3-hexafluoroisopropanol) at 120− 150 °C. Products (24 examples) were obtained in good to high yields (Scheme 11A). Furthermore, double alkylation of ophenylenediamine with diols was carried out with this catalyst system and 2-substituted tetrahydroquinoxalines (two examples) were obtained in high yields (Scheme 11D). The alkylation of arylamines (five examples) was achieved using nickel/dcpe (1,2-bis(dicyclohexylphosphino)ethane) in tertamyl alcohol at 120 °C (Scheme 11C). A nickel catalyst based on 5 mol % Ni(OTf)2 and 6 mol % of the chiral diphosphine (S)-binapine was used for an asymmetric N-alkylation of benzohydrazide with racemic alcohols (Scheme 11B). Benzylamines (13 examples) with enantioselectivities up to 96% enantiomeric excess (ee) were obtained. Acetic acid is an important additive for promoting the condensation step. Banerjee and co-workers developed an efficient homogeneous nickel catalyst system, a 1 to 2 ratio of NiBr2 and 1,10phenanthroline, for the monoalkylation of aniline derivatives with various primary alcohols (Scheme 12).103 Many functional groups such as hydroxyl, alkene, nitrile, nitro, and trifluoromethyl functionalities are tolerated by the described protocol, and 41 products could be isolated in moderate to high yields (Scheme 12A). They have also demonstrated the double alkylation of diamines (four examples; Scheme 12B) and the intramolecular cyclization of 2-(2-aminophenyl)ethanols to 1H-indoles (two examples; Scheme 12C). Recently, Banerjee and co-worker presented their results on the N-alkylation of amides with this catalyst system and K3PO4 as base (Scheme 13).104 A broad scope of monoalkylated amides (37 examples) could be synthesized in good to high

an excess of the alcohol. The products for the aniline alkylation were obtained in 41−83% yields, and those for the benzidine alkylation were obtained in 52 and 63% yields. Aniline did not react with methanol, isopropyl alcohol, or sec-butyl alcohol under the used reaction conditions. In 1977, Nicoletti and co-workers reported on the nickel catalyzed N-alkylation of amines with alcohols (Scheme 6A).91 They used a Raney nickel/(t-C4H9O)3Al catalyst system for the alkylation of primary and secondary aromatic as well as aliphatic amines with primary and secondary alcohols to get the corresponding products in excellent yields. This catalyst system is also well-suited for the N-alkylation of indole with secondary amines in toluene under reflux.92 Garcia Ruano and co-workers described the monoalkylation of primary amines with primary and secondary alcohols (Scheme 6B).93 The reactions were catalyzed by Raney Ni at room temperature. The primary alcohols ethanol and nbutanol and the secondary alcohol isopropanol were used as alkylating reagents. The corresponding monoalkylated products (12 examples) were obtained in 65−89% yields. Nandan and co-workers prepared W4 grade Raney nickel and used it for the N-alkylation of primary and secondary amines with alcohols (Scheme 6C).94,95 In total, 23 secondary and tertiary amines were synthesized in 62−90% yields. The prepared catalyst can be recycled and reused. The synthesis of drugs such as Pyrilamine (82% yield) and Piribedil (85% yield) was demonstrated (Scheme 6). In 2017, Wei and co-workers demonstrated the synthesis of furfurylamine from furfuryl alcohol and ammonia over Raney nickel (Scheme 7).96 The reaction takes place in THF at an Scheme 7. Amination of Furfuryl Alcohol with NH3 Using Raney Ni

ammonia pressure of 3.5 bar. The yield of furfurylamine dropped sharply from 77% in the first run to 53% in the second run if the catalyst was reused. In 2012, Li and co-workers introduced the Ni−Cu/γ-Al2O3catalyzed N-alkylation of amines with alcohols (Scheme 8).97 Scheme 8. Heterogeneous Ni−Cu-Catalyzed N-Alkylation with Alcohols

The recyclable bimetallic catalyst was prepared by the electroless plating method and shows excellent selectivity in the presence of base and a catalytic amount of Lewis acid. High to excellent yields were obtained in the N-alkylation of amines with both aliphatic and aromatic alcohols. Shimizu’s group studied nickel nanoparticles loaded onto various supports for the N-alkylation of anilines and aliphatic amines with benzyl and aliphatic alcohols (Scheme 9A).98 The reusable heterogeneous catalyst Ni/θ-Al2O3 was prepared by in situ H2-reduction of NiO/θ-Al2O3 and shows the highest D


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Scheme 9. Ni Nanoparticle Catalyzed N-Alkylation with Alcohols

In 2017, Pera-Titus and Ponchel and co-workers synthesized a series of Ni−Al2O3 catalysts with nickel loadings between 2 and 20 wt % (Scheme 14).105 The catalysts were prepared by aqueous wet impregnation of γ-Al2O3 using nickel(II) nitrate hexahydrate and native cyclodextrins as metal complex hosts and by subsequent drying and calcination at 400 °C. The use of cyclodextrin afforded a much higher Ni dispersion and a narrow distribution of Ni particles. The resulting higher availability of reduced surface Ni species leads to enhanced catalytic activity in the amination of benzyl alcohol with aniline and 1-octanol with ammonia. In a follow-up work, Pera-Titus and Michel and co-workers examined the amination of noctanol by ammonia in the gas phase through a combined experimental−computational approach to provide a deeper understanding of the factors determining the activity of Ni catalysts (Scheme 14).106 The catalyst was prepared by incipient wetness impregnation of γ-Al2O3 using an aqueous solution of Ni(NO3)2·6H2O and a H2 reduction at 500 °C. Hellgardt and Hii and co-workers demonstrated the selective synthesis of primary amines from alcohols and ammonia using a commercially available Ni catalyst (65 wt % Ni−Al2O3/SiO2) (Scheme 15).107 The reaction proceeds at 160 °C by using an improved batch reactor system or a continuous flow platform.

Scheme 10. Heterogeneous Catalyzed Amination of Alcohols with Amines and Ammonia

yields. Furthermore, they demonstrated the synthesis of bisamides (three examples) starting from 1,4-phenylenedimethanol and the synthesis of the antiemetic drug Tigan and the dopamine D2 receptor antagonist Itopride (Scheme 13).

Scheme 11. Ni-Catalyzed (Enantioselective) N-Alkylation with Alcohols

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Scheme 12. Ni-Catalyzed Direct N-Alkylation of Anilines with Alcohols

Scheme 13. Ni-Catalyzed N-Alkylation of Amides with Alcohols

2.2. N-Alkylation by Amines

Scheme 14. Al2O3 Supported Ni Catalysts for the Amination of Alcohols

The N-alkylation of amines with amines through BH/HA is an interesting alternative for the synthesis of secondary amines (Scheme 16). One equivalent of ammonia is liberated during such a reaction. The reaction of primary amines to get secondary amines catalyzed by Raney nickel was described by Matthies and coworkers in 1961 (Scheme 17A).108 The reactions were carried out under reflux in different solvents at different reaction times depending on the amine used. Scheme 16. “Hydrogen-Borrowing” Strategy for the Alkylation of an Amine with Another Amine

Scheme 15. Ni-Catalyzed Synthesis of Primary Amines from Alcohols and NH3

Seven primary amines were obtained in good yields and selectivities. The amination results were strongly improved by using anhydrous NH3 under continuous flow conditions to avoid the accumulation of water and amine products that can inhibit the catalyst. F


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Scheme 17. Raney Nickel Catalyzed Synthesis of Secondary Amines

Scheme 18. Self-N-Alkylation of Primary Amines

carbon (AC) supported metal loaded catalysts. The AC was first impregnated with various alkaline compound solutions using incipient wetness method. Afterward, the material was loaded with 10−30 mol % Ni using nickel acetate and treated in H2 at 400 °C. This nickel-containing and alkali metal salt modified activated carbon catalyst was shown to give high productivity and selectivity. Butanol was the main product. Longer chain primary aliphatic alcohols, having even numbers of carbon atoms, were also obtained.

In 1979, Nicoletti and co-workers reported the synthesis of secondary amines using the amines themselves as alkylating agents (Scheme 17B).109 The reaction was catalyzed by Raney nickel in xylene under reflux. The air- and moisture-stable NiCuFeOx catalyst introduced by Shi and co-workers for the alkylation of amines and ammonia by alcohols can mediate the self-N-alkylation of primary amines in the absence of bases too (Scheme 18).101 Both purely aliphatic and benzylic amines could be transformed into the corresponding secondary amines in 82−93% yields of isolated products (Scheme 18). Again, it is not clear which of the metals of the multimetallic catalyst system is active in BH/HA-based transformations.

3. IRON-CATALYZED C−N AND C−C COUPLING REACTIONS Bauer and Knölker recently published a review which provides a comprehensive overview of “Iron Catalysis in Organic Synthesis”.114 However, the iron-catalyzed N- and Calkylations by alcohols following the BH/HA concept is still a relatively young field of research, which has been investigated intensively and is developing rapidly.

2.3. C-Alkylation by Alcohols

In 2007 and 2008, Alonso and Yus and co-workers introduced the α-alkylation of methyl ketones with primary alcohols promoted by Ni nanoparticles (Scheme 19).110,111 The Scheme 19. Ni Nanoparticles Catalyzed α-Alkylation of Ketones with Primary Amines

3.1. N-Alkylation by Alcohols

The first example of an iron-catalyzed N-alkylation of amines, which could follow a BH/HA mechanism, has been known since the pioneering work of Ramón and Yus and co-workers in 2009 (Scheme 21A). Aromatic amines were alkylated with benzylic alcohols using a heterogeneous catalyst. This group also introduced the term “hydrogen autotransfer”. They showed that unmodified commercial magnetite can be used as a recyclable catalyst (eight cycles, yield range 83−93%) in an alkylation of amines using alcohols.115 The reaction proceeds slowly (7 days) in dioxane and in the presence of 2 equiv of potassium tert-butoxide. The corresponding amines were obtained in yields of 33 to >99%. Aliphatic alcohols do not react under these conditions, and aliphatic amines showed only a low reactivity and led preferentially to the corresponding imines. In 2010, Luque’s group introduced supported iron oxide nanoparticles as a highly active catalyst system in the microwave-assisted N-alkylation of amines with alcohols (Scheme 21B).116 The heterogeneous and reusable catalyst Fe−HMS (HMS = hexagonal mesoporous silica) was prepared using FeCl2·4H2O and the support in EtOH under microwave irradiation (200 W) for 15 min. The resulting material has a specific surface area of 525 m2 g−1. The catalytic reactions

nickel(0) nanoparticles were synthesized from nickel(II) compounds by reduction with lithium powder and a catalytic amount of an arene in THF at room temperature.112 The αalkylations proceed in the absence of a base under mild conditions and in short reaction times (2−24 h). In total, 15 alkylated ketones were received in good to high yields. Very recently, Onyestyák studied the Guerbet alkylation of ethanol (Scheme 20).113 The dimerization of ethanol was carried out in a flow-through tube microreactor using activated Scheme 20. Heterogeneous Ni-Catalyzed Guerbet Reaction of Ethanol

G


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Scheme 21. Fe (Pre-)catalysts for the N-Alkylation of Amines with Alcohols

Scheme 22. N-Alkylation of Amines with Alcohols Catalyzed by an Iron/Amino Acid System

Scheme 23. Homogeneously Fe-Catalyzed Direct N-Alkylation of Amines with Alcohols

duced by Singh and co-workers in 2013 (Scheme 21C).118 The products desired were obtained in good to high isolated yields by using an alcohol to aminoheterocycle ratio of 1 to 1. In addition, 2 equiv of sodium tert-butoxide was required and the reactions were carried out in toluene at 100 °C. The coupling of amines with alcohols catalyzed by an acid− base cooperative catalysis was introduced by Saito and coworkers (Scheme 22).119 The catalyst system, a combination of FeBr3, DL-pyroglutamic acid, and Cp*H (Cp*H = 1,2,3,4,5pentamethylcyclopentadiene), was generated in situ, and a broad substrate scope and good functional group tolerance were observed. The N-alkylation was carried out in 1,2,4trimethylbenzene (1,2,4-TMB), and almost neutral pH conditions and higher reaction temperatures up to 200 °C are required to obtain good yields of products. Regarding the underlying mechanism, the authors postulate a substitution at the sp3 carbon atom bearing the hydroxyl group of the alcohol. The α/γ (>99%) and E/Z (>99%) selectivities observed of the reaction with allylic alcohols and crossover experiments using

proceed under microwave irradiation (300 W, 1−2 h) with an iron loading of 0.39 wt % using 2 equiv of DABCO (1,4diazabicyclo[2.2.2]octane) as a base and an alcohol to amine ratio of 1 to 1.2. Under these conditions, 19 substituted anilines, cyclic amines, and aliphatic amines were alkylated with benzyl alcohols and good to high isolated yields were obtained. In the same year, Shi and Deng and co-workers developed a FeCl2/K2CO3 catalyst system for the environmentally benign N-alkylation of sulfonamides with benzylic alcohols (Scheme 21D).117 The reactions were carried out at 135 °C for 20 h, and 21 examples with high to excellent yields of isolated products were obtained. X-ray photoelectron spectroscopic analysis of the catalyst system indicated the presence of Fe(II) and Fe(0) species. Mechanistic studies indicated the presence of a BH/HA process. An efficient and versatile iron(II) phthalocyanine catalyst for the N-alkylation of aminobenzothiazoles, aminopyridines, and aminopyrimidines with readily available alcohols was introH


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Scheme 24. Iron-Catalyzed Amination of Alcohols

Scheme 25. Amination of Benzyl Alcohols with Primary and Secondary Amines

related to Knölker’s iron complex.123 In comparison to the work of Feringa and Barta and co-workers, benzylamines cannot be used as coupling partners. They inhibited the catalyst. In a follow-up work, Wills and co-workers tested a series of (cyclopentadienone)iron tricarbonyl complexes for C−N bond formation reactions between amines, including unsaturated amines, and alcohols (Scheme 24B).124 They observed that the variation of the substituents embedding the CO group of the cyclopentadienone influence the efficiency of the catalysis. In 2015, Zhao and co-workers introduced the efficient amination of primary (six examples) and secondary (13 examples) alcohols catalyzed by Knölker’s iron complex and assisted by a Lewis acid in good to excellent yields (Scheme 24C).125 Silver fluoride (AgF) was identified as a highly effective additive, whereby, for example, the amination of symmetrical secondary alcohols (isopropanol and cyclohexanol) using aniline leads to higher yields of the corresponding products (77 and 79%, respectively) than reported by Feringa and Barta and co-workers (12 and 14%, respectively). Zhao and co-workers postulate that AgF serves as the Lewis acid to facilitate imine condensation and activate the imine intermediate toward reduction by the iron hydride species in the catalytic cycle. A follow-up report of their work published in 2014 was described by Barta and co-workers in 2016.126 They introduced the direct amination of benzylic alcohols with primary (12 examples) and secondary (15 examples) amines to

deuterium-labeled alcohols supported the proposed SN2 pathway. A homogeneous bifunctional Fe catalyst can be Knölker’s iron complex. 120,121 This air- and moisture-stable Fe precatalyst was used for the first time for the alkylation of amines with alcohols by Feringa and Barta and co-workers (Scheme 23).122 The N-alkylation takes place under base-free conditions by using the solvent cyclopentyl methyl ether. The key step was the screening of conditions under which the species [Fe−H] formed from the initial alcohol dehydrogenation is able to reduce the imine formed. Therefore, one CO ligand was abstracted from [Fe]-1 by the addition of trimethylamine N-oxide as an oxidant. The [Fe]-2 generated in situ reacts readily with an alcohol to give the corresponding [Fe−H] (Scheme 23). The broad scope of the methodology presented includes the selective monoalkylation of functionalized anilines with various alcohols (23 examples), the Nalkylation of aliphatic amines with aliphatic alcohols (three examples) and the reaction of various benzylamines with pentan-1-ol (seven examples). The usage of diols leads to the formation of five-, six-, and seven-membered N-heterocycles (six examples). The synthesis of a pharmaceutically relevant molecule, Piribedil (Scheme 23), a dopamine antagonist used in the treatment of Parkinson’s disease, was accomplished in a 54% isolated yield. The work of Wills and co-workers in 2015 focused on the optimization of the reaction of anilines with benzyl alcohols using an iron cyclopentadienone precatalyst (Scheme 24A) I


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Scheme 26. Fe-Catalyzed Allylic Amination from Allylic Alcohols

method shows good selectivity and facilitates an access to nonsymmetric tertiary amines. A mechanistic proposal has been made based on experimental work and density functional theory calculations (Scheme 28). The N-monomethylation of amines and sulfonamides was recently reported by Morrill and co-workers (Scheme 29).129

get secondary and tertiary amines, respectively, with up to 91% yields of isolated products (Scheme 25). Knölker’s iron complex was again used as the precatalyst. They also succeeded in the one-pot synthesis of nonsymmetric tertiary benzylic amines (three examples) and the sequential functionalization of a diol. Sundararaju’s group also used the catalytic potential of Knölker’s iron complex for their investigations.127 They used allylic alcohols as an alcohol source to obtain allylic amines (18 examples) with moderate to good (up to 81%) yields of isolated products (Scheme 26). With this method, they also succeeded in the one-pot synthesis of Cinnarizine (antihistaminic drug) and Nafetifine (antifungal drug) from cinnamyl alcohol in isolated yields of 62 and 61%, respectively (Scheme 26). Gandon and Bour and co-workers extended the BH/HA methodology to the Fe-based reductive ethylation of imines with ethanol.128 An air-stable Knö lker-type precatalyst mediates the reaction, and imines bearing electron-rich aryl or alkyl groups at the nitrogen could be efficiently alkylated without the need for molecular hydrogen (Scheme 27). The

Scheme 29. N-Monomethylation of Amines and Sulfonamides

The reactions were catalyzed by Knölker-type iron carbonyl complexes, and the monomethylated amines (11 examples) and sulfonamides (five examples) were obtained in isolated yields between 54 and 95%. In 2016, Kirchner and co-workers showed that the PN3−5P ligand-stabilized iron complexes can also catalyze the Nalkylation of amines by alcohols. In a first report, they introduced a Fe(II) PN3P (2,6-diaminopyridine scaffold) pincer complex which catalyzed the selective monoalkylation of amines with alcohols (Scheme 30A).130 Both aromatic and aliphatic amines were converted efficiently, and the resulting 12 products were isolated in yields between 61 and 93%. This

Scheme 27. Fe-Catalyzed Reductive Ethylation of Iminesa

a

Ts = p-toluenesulfonyl.

Scheme 28. Mechanistic Proposala

a

Ar = 4-MeOC6H4. J


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Scheme 30. Coupling of Alcohols and Amines Catalyzed by Fe(II)−PN3−5P Complexes

Scheme 31. Fe-Catalyzed β-Alkylation of Alcohols with Alcohols

Scheme 32. Enantioselective Functionalization of Allylic Alcohols by Multicatalyst Systems

conversion of primary alcohols and aromatic or benzylic amines into the corresponding N-alkylation products (29 examples). Good to excellent isolated yields were obtained. The precatalyst was activated in situ by a base and an alcohol:amine:base ratio of 1:1.2:1.3 led to the best results.

coupling reaction proceeds under base-free conditions and required the addition of molecular sieves. In another study, Kirchner and co-workers presented welldefined [Fe(PN3−5P)Br2] and [Fe(PN3−5P)(CO)Br2] complexes. The PN3−5P ligands have a triazine or a pyridine backbone. The complexes were tested for the selective alkylation of amines with alcohols (Scheme 30B).131 Only the diamagnetic d6 low-spin complexes of the type [Fe(PN3−5P)(CO)Br2] are catalytically active. These complexes bear a carbonyl and two bromides as coligands. From this screening, an air-stable iron complex with a diaminotriazine scaffold (Scheme 30B) shows the highest activity for the

3.2. C-Alkylation by Alcohols

A Fe-catalyzed example of an alcohol-based C-alkylation was introduced by Sun and co-workers.132 They linked secondary alcohols with primary alcohols catalyzed by a homogeneous commercially available catalyst (ferrocenecarboxaldehyde) and a catalytic amount of sodium hydroxide (Scheme 31) and received 24 products in yields between 54 and 97%. K


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Scheme 33. α-Alkylation of Ketones with Alcohols Catalyzed by Knölker-Type Catalysts

Scheme 34. C-Mono- and Dimethylations of Ketones, Indoles, and Oxindoles

Scheme 35. Proposed Mechanisma

a

Reproduced from ref 129. Copyright 2018 American Chemical Society.

Quintard and Rodriguez and co-workers combined Knölker’s iron complex with a chiral organocatalyst for the enantioselective functionalization of allylic alcohols (Scheme 32A).133,134 Fe-catalyzed BH/HA is a central step. The authors

propose a dehydrogenation of the allylic alcohol followed by iminium salt formation with the organocatalyst [Im] to enable the enantioselective C−C bond formation step. Subsequent hydrolysis of the iminium salt and reduction of the released L


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Scheme 36. Heterogeneous Co-Catalyzed Amination of Bifunctional Secondary Alcohols with NH3

by reduction with H2 at 335−420 °C. The amination proceeds in a continuous fixed-bed reactor in the pressure range 50−150 bar. Application of supercritical ammonia doubled the selectivity to amino alcohol and diamine. Promotion of the Co catalyst with iron or lanthanum improved the diamine selectivity significantly, and a Co/Fe weight ratio of 19 gave the best catalyst material. In follow-up works, Baiker and coworkers used this Co−Fe catalyst for the amination of 1,3cyclohexanediol and 2,4-pentanediol in supercritical ammonia141 and for the amination of 1,4-cyclohexanediol in supercritical ammonia142 under nearly identical reaction conditions as described above (Scheme 36B). In case of the amination of 1,3-cyclohexanediol and 2,4-pentanediol, application of supercritical NH3 as a solvent and reactant suppressed catalyst deactivation and improved selectivity to amino alcohol intermediates. The reductive amination of 2-propanol to monoisopropylamine using Co/γ-Al2O3 catalysts with 4−27 wt % cobalt loadings was introduced by Shin and co-workers (Scheme 37).143 The catalysts were prepared by incipient wetness

carbonyl function leads to functionalized allylic alcohols with high enantioselectivities (up to 95:5 enantiomeric ratio (er)). A catalyst system consisting of three components was disclosed for the redox neutral enantioselective functionalization of allylic alcohols in a follow-up work (Scheme 32B,135C136). Mechanistically, a similar BH/HA based enantioselective C−C bond formation step has been proposed. The Fe-catalyzed α-alkylation of ketones with primary alcohols was disclosed by Sortais and Darcel and co-workers (Scheme 33A).137 The catalyst was generated in situ from Knölker’s complex and PPh3 in a 1 to 1 ratio. In the presence of a catalytic amount of Cs2CO3, 22 alkylated ketones were synthesized in moderate isolated yields. In 2017, Poater and Renaud and co-workers designed several cyclopentadienone iron carbonyl complexes, which also catalyze this reaction efficiently (Scheme 33B).138 Both aliphatic and aromatic ketones and alcohols could be coupled under mild reaction conditions, and the resulting products (34 examples) were isolated in good to excellent yields. In addition to the methylation of amines and sulfonamides described in section 3.1, a recent publication by Morrill and coworkers reported on the C-methylation of ketones, indoles, and oxindoles using methanol as a sustainable C1 building block (Scheme 34).129 The used Knölker-type precatalyst was activated with trimethylamine N-oxide. A variety of ketones undergo C-monomethylation (27 examples) and C-dimethylation (seven examples) in moderate to excellent isolated yields. In addition, the catalysis protocol involves the C(3)methylation of indoles (seven examples) and oxindoles (seven examples) in high isolated yields (78 and 77% average yields). Based on the monomethylation of propiophenone as a model reaction, mechanistic experiments were performed (Scheme 35).

Scheme 37. Co/γ-Al2O3-Catalyzed Amination of 2-Propanol

impregnation, and 23 wt % Co loading resulted in the highest catalytic activity and a long-term stability of up to 100 h on stream. Our group introduced PN5P ligand-stabilized Co complexes for BH/HA applications. The complexes are easy to synthesize in almost quantitative yields and are easy to handle. Due to the modular design of the PN5P ligand, their steric and electronic properties are highly variable, and hence, these ligands are appropriate for an easy identification of highly active Co catalysts.144 The precatalyst can be activated by just adding 2 equiv of a base. We first used this catalyst system for the alkylation of aromatic amines by alcohols (Scheme 38).145 The mild reaction conditions allow the selective monoalkylation of (hetero)aromatic amines and the synthesis of unsymmetrically alkylated diamines. In 2016, the groups of Zhang146 and Kirchner147 published further results on the Co-catalyzed N-alkylation of amines with alcohols (Scheme 39). Zhang and co-workers reported on the selective, direct N-alkylation of both aromatic and aliphatic amines by alcohols using a cationic Co(II) alkyl complex, originally developed by Hanson and Zhang as a catalyst139 for

4. COBALT-CATALYZED C−N AND C−C COUPLING REACTIONS Most of the Co catalysts falling in the scope of this review are based on tridentate PNP ligand systems. The precatalysts used are neutral (di)chloro Co(II) complexes (Scheme 38, right) which undergo self-activation in the presence of a base or the cationic Co(II) alkyl complex introduced by Hanson and coworkers139 (Scheme 39A). 4.1. N-Alkylation by Alcohols

In 1999, Baiker and co-workers introduced the cobalt-catalyzed amination of 1,3-propanediol(s) (Scheme 36A).140 Unsupported Co-based catalysts were used. The catalyst materials were prepared starting from aqueous solutions of the metal nitrates. After adding (NH4)2CO3 until pH 7 was reached, the resulting material was calcinated in air at 400 °C and activated M


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Scheme 38. Selective Alkylation of Aromatic Amines and Diamines by Alcohols Catalyzed by a PN5P−Co(II) Complex

Scheme 39. Coupling of Alcohols and Amines Catalyzed by Co Complexes of the Groups of Zhang and Kirchner

Scheme 40. Co(II)-Catalyzed N-Alkylation of Amines and Diamines

Scheme 41. Alkylation of Amines by Amines Catalyzed by a Cationic Co(II) Alkyl Complex

conditions with added 3 Å molecular sieves and needs a higher reaction temperature. In 2018, Madhu and Balaraman and co-workers introduced an air-stable, phosphine-free cobalt(II)−NNN complex, which catalyzes the N-alkylation of amines and diamines with alcohols (Scheme 40).148 A representative number of anilines were monoalkylated in moderate to high yields using a wide range of benzylic alcohols, aliphatic alcohols, and heterocyclic alcohols. Furthermore, the N-alkylation of phenylenediamines with benzylic alcohols was investigated and the formation of dialkylated amines as major products and monoalkylated amines as minor products was observed.

the hydrogenation of alkenes, aldehydes, ketones, and imines (Scheme 39A). The N-alkylation proceeds under base-free conditions. The high chemoselectivity (amine vs imine formation) could be addressed by simply adding 4 Å molecular sieves. Kirchner and co-workers described PCP ligand-stabilized Co(II) complexes for the alkylation of aromatic amines with primary alcohols (Scheme 39B). The PCP ligand is based on a 1,3-diaminobenzene scaffold and acts as a tridentate monoanionic ligand. A chloro complex and an alkyl complex were used successfully (Scheme 39B). The corresponding alkyl complex, bearing a CH2SiMe3 group, works under base-free N


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Scheme 42. C-Alkylation of Amides, Esters, and Secondary Alcohols by Alcohols Catalyzed by PN5P Ligand-Stabilized Co(II) Complexes

Scheme 43. α-Alkylation of Ketones with Primary Alcohols

Scheme 44. Mn-Catalyzed N-Alkylation of Aromatic Amines with Alcohols

4.2. N-Alkylation by Amines

excess of amide regarding the alcohol is required, a 4-fold excess of ester is needed for the ester alkylation. In summary, 34 examples, comprising 22 alkylated amides and 12 alkylated esters, have been synthesized. Based on these PN5P pincer ligand stabilized catalysts, we introduced the first Co-catalyzed version of the alkylation of secondary alcohols with primary alcohols (Scheme 42B).159 The catalyst system is applicable to a broad substrate scope (27 examples) with yields of isolated products up to 80%. Even the heterocoupling of different purely aliphatic alcohols was demonstrated. In the same year, Zhang and co-workers reported the Cocatalyzed α-alkylation of ketones with primary alcohols (Scheme 43).160 A broad range of alkylated ketones were synthesized with yields of isolated products up to 98%. The modified Friedländer annulation reaction of 2-aminobenzyl alcohol with ketones by using the standard conditions for the α-alkylation of ketones, providing the corresponding quinoline derivatives, has been disclosed, too.

Only a few homogeneous noble metal based catalyst systems (Pt, Pd, Ru, Ir) have been described for the synthesis of secondary amines by selective monoalkylation of amines with alkyl amines.149−156 The first and, so far, only homogeneous nonprecious metal catalyst was introduced by Zheng and Zhang and co-workers (Scheme 41).157 They used a cationic Co alkyl catalyst, which shows a similar catalytic activity compared to the known Ru and Ir catalysts. A broad range of aryl−alkyl amines and cyclic sec-amines could be synthesized. 4.3. C-Alkylation by Alcohols

An especially challenging alkylation to construct carbon− carbon bonds is that of unactivated esters and amides with alcohols. In 2016, our group introduced the first earthabundant metal-catalyzed example of such a reaction. Cobalt(II) complexes stabilized with PN5P ligands catalyze these reactions very efficiently (Scheme 42A).158 The method is characterized by mild reaction conditions and a good functional group tolerance. Amide alkylation products were obtained with a catalyst loading of 2.5 mol % in up to 93% yields of isolated products. A twice as high catalyst loading is needed for the ester alkylation (Scheme 42A). While a double O


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Scheme 45. Mn-Catalyzed Coupling of Hydrazine and Alcohols

Scheme 46. Proposed Mechanism for the Coupling of Hydrazine and Alcohols

Scheme 47. Mn-Catalyzed Synthesis of 1,2-Disubstituted Benzimidazoles

inexpensive methanol as C1 source (Scheme 44B).168 This novel catalyst is strongly related to Mn catalysts developed by Milstein and co-workers.169 In comparison with the catalyst described previously (Scheme 44A), this lutidine-based catalyst gives better yields (up to 95%) under milder conditions. Functional groups, such as ketones, C−C double bonds, and amides, have been tolerated. Sortais and co-workers presented a cationic Mn precatalyst for the selective mono-Nmethylation of a large variety of aniline derivatives with methanol (Scheme 44C) in good to excellent yields.170 The cationic Mn precatalyst was stabilized by a tridentate PN3P ligand (2,6-diaminopyridine backbone) and three CO coligands. Milstein and co-workers introduced very recently a Mn pincer complex for the one-step synthesis of N-substituted hydrazones by the coupling of hydrazine with alcohols in the presence of catalytic amounts of KOtBu (Scheme 45).171 It is noteworthy that the reaction involves both BH/HA and ADC (ADC = acceptorless dehydrogenative condensation) in one system. They provided a proposed mechanism (Scheme 46). The molecular structures of the Mn precatalyst 1, the dearomatized species 2, and the alkoxo complex 3 were confirmed by X-ray diffraction. The active catalyst 2, which was formed by deprotonation of 1 with KOtBu, catalyzed the

5. MANGANESE-CATALYZED C−N AND C−C COUPLING REACTIONS 5.1. N-Alkylation by Alcohols

The development of Mn-based complexes as efficient catalysts for C−N and C−C coupling reactions via BH/HA is still a very young field of research.161−164 Pioneering work on this topic has been carried out by Beller and co-workers. They reported on a Mn-catalyzed N-alkylation (Scheme 44A).165 An air-stable [Mn(I)(CO)2Br] complex stabilized by a tridentate PNHP ligand166 and activated in situ by a base catalyzed the N-alkylation of amines with alcohols very efficiently. An amine to alcohol ratio of only 1 to 1.2 is sufficient. The catalytic system shows an excellent chemoselectivity and a large variety of functional groups, such as olefins, halides, thioether, benzodioxane, and heteroaromatic, are tolerated. The high chemoselectivity in the presence of vinyl groups enabled the synthesis of five resveratrol derivatives, which are known to be active for the treatment of Alzheimer’s disease,167 in excellent isolated yields up to 97%. In addition, the N-methylation of various amines using methanol was demonstrated successfully, the first example of such a C−N coupling reaction using a nonnoble metal catalyst under mild conditions. Beller and coworkers used a different Mn PNP pincer complex to improve the selective monomethylation of aromatic amines with P


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Scheme 48. Mn-Catalyzed and Base-Switchable Synthesis of Amines or Imines

Scheme 49. Manganese-Catalyzed Reaction of Ketones with Primary Alcohols

Scheme 50. Plausible Mechanism for the Mn-Catalyzed α-Alkylation of Ketones with Alcohols

Scheme 51. Synthesis of Tetrasubstituted Pyrimidines by a One-Pot or Consecutive Four-Component Reaction

reaction with no added base. Heating of 3 for 5 h at 110 °C resulted in the formation of the hydrido complex 4. In 2018, the Srimani group introduced a tridentate NNS ligand based manganese(I) complex, which catalyzed the reaction of 1,2-diaminobenzenes with primary alcohols to synthesize the desired 1,2-disubstituted benzimidazoles (20 examples) in moderate to high yields (Scheme 47).172 The catalysis proceeds under neat conditions at 140 °C with 10 mol % of the manganese precatalyst and a stoichiometric amount of base. The authors discuss the possibility of N-alkylation steps in the course of this benzimidazole synthesis. Very recently, our group reported on the manganesecatalyzed base-switchable synthesis of N-alkylated amines or imines (Scheme 48).173 The presence of an alkali metal base determines the product, with potassium bases giving selectively

N-alkyl amines via borrowing hydrogen and sodium bases giving selectively imines via dehydrogenative condensation. We found that the used manganese hydride precatalyst reacts with KOtBu or NaOtBu via double deprotonation to form the corresponding potassium or sodium manganate hydride. Mechanistic investigations revealed that the potassium manganate hydride reacts, under identical conditions, about 40 times faster with an imine to give the corresponding amine than the sodium manganate hydride. This base-switchable reaction has a broad scope (22 examples of amines), an attractive functional group tolerance, and a very good amine/ imine selectivity. 5.2. C-Alkylation by Alcohols

In 2016, Beller and co-workers presented the Mn-catalyzed αalkylation of ketones and related compounds with primary Q


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Scheme 52. Mn-Catalyzed Guerbet Reaction of Ethanol

Scheme 53. Mn-Catalyzed β-Alkylation of Secondary Alcohols with Primary Alcohols

Scheme 54. Mn-Catalyzed α-Alkylation of Nitriles with Primary Alcohols

alcohols (Scheme 49).174 The precatalyst used is the complex described in section 5.1 for the Mn-catalyzed N-alkylation of aromatic amines with alcohols (Scheme 44A). The activation in situ by a catalytic amount of Cs2CO3 in tert-amyl alcohol (instead of KOtBu in toluene) catalyzes this reaction efficiently. A total of 35 ketones, including 2-oxindole, estrone 3-methyl ether, and testosterone, were alkylated successfully with isolated yields of up to 94%. Mechanistic investigations of this Mn-catalyzed transformation suggested an intramolecular ligand-assisted mechanism (Scheme 50). The Mn hydride III, which hydrogenates the α,β-unsaturated aldol condensation product 2, yields the ketones desired 3 and regenerates the catalytic active species I. In 2016, our group reported on a Mn-catalyzed version of the multicomponent reaction of alcohols and amidines to form pyrimidines (Scheme 51),175 which was first developed by our group applying an iridium complex catalyst.176 The Mn precatalyst is stabilized by a PN5P ligand (triazine backbone) bearing a phenyl substituent in the 4-position and catalyzed a consecutive four-component process to give fully substituted pyrimidines in a one-pot procedure. In this process, Mncatalyzed β-secondary alcohols with primary alcohols via BH/ HA were combined with the dehydrogenative condensation concept permitting selective C−N and C−C bond formations. The transformation of biomass-derived alcohols, such as ethanol, into longer chain alcohols which can be used as high quality biofuels is a great challenge. Recently, Liu and coworkers177 and Jones and co-workers178 have independently reported their results on the 3d-metal catalyzed Guerbet reaction (Scheme 52). Both groups used the same homogeneous [PNHP−Mn(CO) 2 Br] complex to catalyze the dimerization of ethanol to 1-butanol. The activity and productivity of this catalyst system are similar to those of noble metal catalysts.83 The reaction, developed by the Liu group (Scheme 52A), can proceed at a very low catalyst loading of only 0.0001 mol % with a reaction time of 168 h. Jones and co-workers performed their catalytic reactions at 150 °C for 24 h with a catalyst loading of 0.5 mol % (Scheme 52B).

Very recently, Yu and co-workers introduced a manganese(I) complex, stabilized by a pyridyl-supported pyrazolyl− imidazolyl ligand, which catalyzes the β-alkylation of secondary alcohols with primary alcohols (Scheme 53).179 A large number of secondary and primary alcohols could be successfully converted to the corresponding coupling products (35 examples) in moderate to good isolated yields. The βalkylation of cholesterol derivatives (two examples) and the diβ-alkylation of cyclopentanol (five examples) were also efficiently achieved. The α-alkylation of nitriles with primary alcohols was introduced by Maji and co-workers in 2018 (Scheme 54).180 The manganese precatalyst was stabilized by a phosphine-free ligand and can be generated in situ. α-Branched nitriles (36 examples) were synthesized in yields between 45 and 88% using both benzylic and aliphatic alcohols, and a broad functional group tolerance was observed.

6. OTHER 3d-METAL CATALYZED C−N AND C−C COUPLING REACTIONS 6.1. N-Alkylation by Alcohols

In 2009, a heterogeneous copper−aluminum hydrotalcite catalyst (CuAl-HT) was reported for the amination of alcohols with both aliphatic and aromatic amines by Likhar and coworkers (Scheme 55).181 The CuAl-HT/K2CO3 catalyst system shows an excellent selectivity, and 25 alkylated amines were obtained in good to high yields. Cu/Al ratios of 3:1 and 2.5:1 were found to be optimal. In addition, the CuAl-HT Scheme 55. Heterogeneous Cu-Catalyzed Amination of Alcohols

R


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Scheme 56. Cu(II) Acetate Catalyzed N-Alkylation of Amines and Amides with Alcohols

catalyst can be easily separated and reused for several cycles with consistent activity. The Cu(II) acetate catalyzed N-alkylation of sulfonamides with alcohols was demonstrated by Shi and Beller and coworkers in 2009 (Scheme 56A).182 The reaction was catalyzed by a simple Cu(OAc)2/K2CO3/air system, and the corresponding secondary amines were obtained in high to excellent yields. Subsequently, Shi and Deng and co-workers presented a mechanistic study concerning the N-alkylation of sulfonamides with alcohols.183 These studies, in which 29 alkylated sulfonamides were synthesized, confirmed the borrowing hydrogen mechanism and the dehydrogenation of the alcohol is the rate-determining step. This catalyst system (Cu(OAc)2/base) was also used by Yus and co-workers for the N-alkylation of sulfonamides (Scheme 56B) and poor nucleophilic amines (Scheme 56C) with primary alcohols.184 The corresponding products were obtained in high to excellent yields after relatively long reaction times (1−6 days) using KOtBu as a base. In a followup work, this group presents a broader product scope and mechanistic considerations concerning the N-alkylation of amines and diamines and the N-alkylation of amides with alcohols catalyzed by a Cu(OAc)2/KOtBu system (Scheme 56D).185 In 2014, DFT calculations were performed by Liu and Huang and co-workers to study the mechanism of the Cu(OAc)2/KOtBu-catalyzed N-alkylation of amino derivatives with primary alcohols.186 A catalytic cycle was found, which contains three sequential steps: alcohol oxidation, which proceeds according to the outer-sphere hydrogen transfer pathway, imine formation, and imine reduction. The N-alkylation of primary amines and ammonia with alcohols, catalyzed by supported copper hydroxide catalysts, was developed by Mizuno and co-workers (Scheme 57).187 The reactions proceeded in good to excellent yields without any cocatalysts. Ammonia was also in situ generated from urea. The Cu(OH)x/Al2O3 catalyst could be reused without significant loss of catalytic performance.

Scheme 57. Supported Copper Hydroxide Catalysts for the N-Alkylation with Alcohols

In 2011, the Shimizu group introduced a silver cluster promoted heterogeneous copper catalyst for the N-alkylation of amines with alcohols (Scheme 58).188 The catalyst was Scheme 58. Heterogeneous Cu-Catalyzed N-Alkylation of Amines with Alcohols

prepared by an impregnation method, followed by H 2 reduction at 600 °C. A bimetallic material with a Cu/Ag molar ratio of 95/5 was found to be the most effective catalyst. Structural studies indicate that small Ag nanoclusters are supported on Cu nanoparticles. The alkylation of anilines and aliphatic amines with both benzyl and aliphatic alcohols proceeds with moderate to excellent yields, and 19 different amines were isolated. The paper was included in our review due to the high Cu−Ag ratio. In the same year, Li and co-workers published on the regioselective copper(I) catalyzed N-alkylation of 2-aminobenzothiazoles with benzylic alcohols (Scheme 59).189 The reaction was carried out using a CuCl/NaOH catalyst system, and 19 N-alkylated 2-aminobenzothiazoles were obtained in high to excellent yields. S


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Scheme 59. Cu(I)-Catalyzed N-Alkylation of 2-Aminobenzothiazoles

Ravasio and co-workers introduced a heterogeneous Cu catalyst for the N-alkylation of aniline with both secondary and benzylic alcohols (Scheme 60).190,191 The reusable Cu/Al2O3

Cu catalyst under solvent-free and base-free conditions. For the C-alkylation of cyclohexanone, mono- and dialkylated products were obtained.

Scheme 60. Heterogeneous Cu-Catalyzed N-Alkylation of Aniline

7. CONCLUSIONS The Ni-based BH/HA catalysis has been explored for decades. Early work from 1932 (see section 2.1) indicated the potential of earth-abundant metal catalysts in such transformations. Most of the heterogeneous catalysis has been carried out with Raney Ni type catalysts. Unfortunately, Raney Ni is difficult to handle and its reusability especially if unsupported is limited. Here, the development of novel nanostructured Ni catalysts with high reactivity and good functional group tolerance seems appealing.98−100 Interesting progress has been reported with regard to homogeneous Ni catalysts and selective chiral transformations.102 In general, homogeneous Ni-based BH/ HA catalysis is not yet well-developed. The Fe-based BH/HA catalysis is dominated by the use of catalysts based on Knölker’s complex. The potential of this catalyst (class) is well-explored and impressive with regard to low or no base loading and a very broad substrate scope. The development of novel homogeneous iron catalysts, other than Knölker-type complexes, might be a key to further development of homogeneous Fe-based BH/HA catalysis. We also see a great potential with regard to the development of reusable Fe-based nanocatalysts for BH/HA applications. The pioneering work of Yus and Ramón and co-workers indicates potential.115 Co-based BH/HA catalysis is still strongly limited by available catalysts. The Hanson−Zhang catalyst is sensitive to basic environments which are often considered beneficial in BH/HA catalysis. The catalyst family developed by our group is easy to synthesize and simple to activate but limited with regard to thermal robustness. There is a general problem in addition with regard to the development of Co-based hydrogenation and dehydrogenation catalysts, namely the missing analogy to noble metal catalysts or catalyst classes. Fe catalyst development strongly profits from the Fe−Ru coordination chemistry analogy and Mn from the Mn−Fe isolobal relationship. Reusable nanostructured Co catalysts might also be an interesting direction to step forward to and are right now essentially not developed. Interestingly, Co catalysts seem to show a better functional group tolerance than noble metals; for instance, aryl iodides survive N-alkylation reactions.145 Mn-based hydrogenation and dehydrogenation catalysis is a very young field of research. Consequently, future developments can go in many directions. The available catalysts should be extended significantly. An impressive tolerance of functional groups has been observed in Mn hydrogenation catalysis.194 Thus, Mn catalysts may have a good potential for outstanding functional group tolerance in BH/HA catalysis. There is also evidence that Mn complexes are thermally more robust than Fe and Co catalysts able to mediate similar reactions.162 Thus, Mn catalysts might be the choice if higher temperatures are required. In addition, very low catalyst loadings seem

catalyst shows a high activity, and 20 secondary amines were synthesized in moderate to high yields. The reaction does not require any additive, and the aniline and alcohol were used in stoichiometric amounts. In 2013, Mishra and co-workers investigated the catalytic activity of Mg−Al hydrotalcite supported copper catalyst for N-alkylation of amines with alcohols (Scheme 61).192 The HT Scheme 61. Cu−HT-Catalyzed N-Alkylation of Amines with Alcohols

(HT = Mg−Al hydrotalcite, Mg/Al molar ratio of 2) supported Cu (Cu-HT) catalyst was synthesized by wet impregnation of HT with aqueous solution of copper acetate. The N-alkylation of amines with different alcohols proceeds solvent-free and base-free, giving conversion of amines in the range 80−99%. 6.2. N-Alkylation by Amines

In 2015, Liu and Jaenicke and co-worker developed the selfcoupling of amines catalyzed by a highly active and selective supported copper catalyst (Scheme 62).193 The recyclable Scheme 62. Heterogeneous Cu-Catalyzed Self-Coupling of Amines

catalyst was prepared by a sol−gel method. It allows the entrapping of copper nanoparticles in an alumina matrix. The catalysis proceeds using 2 mmol of amine and 5 wt % Cu/ Al2O3 catalyst at 150 °C. Moderate to high yields were observed. Ring-substituted benzylamines coupled at a slower rate than benzylamine, and gave the corresponding imines as the main products after 24 h. 6.3. C-Alkylation by Alcohols

The C-alkylation of ketones using alcohols as alkylating agent was introduced by Mishra and co-workers in 2013 (Scheme 63).192 This reaction was catalyzed by a hydrotalcite supported T


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Scheme 63. Cu-HT Catalyzed C-Alkylation of Ketones with Alcohols

feasible.177 The recently published base-switchable amine− imine synthesis indicates novel BH/HA selectivity profiles not yet seen with noble metals.173 Cu-based BH/HA catalysis is a rather young field of research. Most of the reactions, heterogeneous and homogeneous, procced at rather high temperatures (130 °C and above). Here, the development of catalysts able to perform under milder reaction conditions could be of interest. In addition, the design of homogeneous catalysts seems in its infancy; especially ligand design work is needed. In general, most of the 3d-metal catalysts introduced so far simply repeat the BH/HA scope of Ru and Ir catalysts. Thus, the most attractive and also the more challenging future is the development of novel BH/HA type of reactions or a combination of BH/HA catalysis with other catalytic methodology to discover novel direct chemical transformations.171 The expected and partially seen better functional group tolerance might also open access to novel direct transformations involving BH/HA steps.195 In addition, the gaining of a deeper mechanistic insight would be desirable and due to the diamagnetism of many of the applied catalysts well feasible. We also feel that 3d-metals other than the ones discussed in this review could give rise to interesting BH/HA transformations.

the group of J. Sieler. Subsequently, he worked as a postdoc with R. R. Schrock at MIT and C. Krüger at the Max-Planck-Institut für Kohlenforschung. From 1994 to 1998, he carried out his habilitation in amido metal chemistry with U. Rosenthal at the Catalysis Institute in Rostock. He received a call as Professor of Inorganic Chemistry at the University of Oldenburg in 2001. In 2002, he moved to Bayreuth University and took over the Chair of Inorganic Chemistry II. His research interests are catalysis (homogeneous and heterogeneous) and metal−metal bonding.

ACKNOWLEDGMENTS We thank the Deutsche Forschungsgemeinschaft for supporting our work in earth-abundant metal catalysis, KE 756/29-1 and 31-1. REFERENCES (1) Arpe, H.-J. In Industrielle Organische Chemie: Bedeutende Vor- und Zwischenprodukte, 6th ed.; Wiley: 2007. (2) Tuck, C. O.; Perez, E.; Horvath, I. T.; Sheldon, R. A.; Poliakoff, M. Valorization of Biomass: Deriving more Value from Waste. Science 2012, 337, 695−699. (3) Vispute, T. P.; Zhang, H.; Sanna, A.; Xiao, R.; Huber, G. W. Renewable Chemical Commodity Feedstocks from Integrated Catalytic Processing of Pyrolysis Oils. Science 2010, 330, 1222−1227. (4) Sun, Z.; Bottari, G.; Afanasenko, A.; Stuart, M. C. A.; Deuss, P. J.; Fridrich, B.; Barta, K. Complete Lignocellulose Conversion with Integrated Catalyst Recycling Yielding Valuable Aromatics and Fuels. Nat. Catal. 2018, 1, 82−92. (5) Michlik, S.; Kempe, R. A Sustainable Catalytic Pyrrole Synthesis. Nat. Chem. 2013, 5, 140−144. (6) Corma, A.; Navas, J.; Sabater, M. J. Advances in One-Pot Synthesis through Borrowing Hydrogen Catalysis. Chem. Rev. 2018, 118, 1410. (7) Huang, F.; Liu, Z.; Yu, Z. C-Alkylation of Ketones and Related Compounds by Alcohols: Transition-Metal-Catalyzed Dehydrogenation. Angew. Chem., Int. Ed. 2016, 55, 862−875. (8) Quintard, A.; Rodriguez, J. A Step into an Eco-Compatible Future: Iron- and Cobalt-Catalyzed Borrowing Hydrogen Transformation. ChemSusChem 2016, 9, 28−30. (9) Yang, Q.; Wang, Q.; Yu, Z. Substitution of Alcohols by NNucleophiles via Transition Metal-Catalyzed Dehydrogenation. Chem. Soc. Rev. 2015, 44, 2305−2329. (10) Gunanathan, C.; Milstein, D. Applications of Acceptorless Dehydrogenation and Related Transformations in Chemical Synthesis. Science 2013, 341, 1229712. (11) Pan, S.; Shibata, T. Recent Advances in Iridium-Catalyzed Alkylation of C−H and N−H Bonds. ACS Catal. 2013, 3, 704−712. (12) Baiker, A.; Kijenski, J. Catalytic Synthesis if Higher Aliphatic Amines from the Corresponding Alcohols. Catal. Rev.: Sci. Eng. 1985, 27, 653−697. (13) Bähn, S.; Imm, S.; Neubert, L.; Zhang, M.; Neumann, H.; Beller, M. The Catalytic Amination of Alcohols. ChemCatChem 2011, 3, 1853−1864.

AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected]. ORCID

Rhett Kempe: 0000-0002-9138-4155 Notes

The authors declare no competing financial interest. Biographies Torsten Irrgang was born in Bergen (Rügen Island) in 1968 and obtained his high school degree at the Ernst-Moritz-Arndt-Erweiterte Oberschule. He studied chemistry in Greifswald at the Ernst-MoritzArndt-University and finished his studies with a Ph.D. in organic chemistry in the group of A. Hetzheim. After completing his doctorate, he joined the group of R. Kempe at the Catalysis Institute in Rostock as a postdoc. He moved with R. Kempe to the University of Oldenburg in 2001 and to the University of Bayreuth in 2002. There he heads the working group Asymmetric Catalysis and Sustainable Organic Synthesis as Akademischer Oberrat at the Chair of Inorganic Chemistry II. Rhett Kempe was born in Dresden in 1964 and obtained his high school degree at the Kreuzschule. He studied chemistry in Leipzig and finished his studies with a Ph.D. degree in organo-nickel chemistry in U


Chemical Reviews

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