Hydride Transfer Reactions Catalyzed by Cobalt Complexes

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

Hydride Transfer Reactions Catalyzed by Cobalt Complexes Wenying Ai,‡ Rui Zhong,‡ Xufang Liu, and Qiang Liu*

Chem. Rev. Downloaded from pubs.acs.org by YORK UNIV on 12/19/18. For personal use only.

Center of Basic Molecular Science (CBMS), Department of Chemistry, Tsinghua University, Beijing 100084, China ABSTRACT: Cobalt has become increasingly attractive in homogeneous catalysis because of its unique characteristics and outstanding catalytic performance in addition to being cheap and earth-abundant. Hydride transfer processes are involved in a broad range of organic transformations that allow the facile preparation of various useful chemicals and synthetic building blocks. These reactions have continuously received great attention both from academia and industry. In this perspective, we review homogeneous cobalt-catalyzed hydride transfer reactions according to the classified reaction types and provide a comprehensive overview of the design, synthesis, and reactivity of cobalt catalysts, their catalytic applications, and reaction mechanisms.

CONTENTS 1. Introduction 2. Synthesis, Structure, And Reactivity of Cobalt Precatalysts 2.1. Synthesis and Structure of Cobalt Complexes 2.1.1. Cobalt(−1) Complexes 2.1.2. Cobalt(I) Complexes 2.1.3. Cobalt(II) Complexes 2.1.4. Cobalt(III) Complexes 2.2. Reactivity Features of Cobalt Complexes 2.2.1. Cobalt(−1) Complexes 2.2.2. Cobalt(I) Complexes 2.2.3. Cobalt(II) Complexes 2.2.4. Cobalt(III) Complexes 3. Cobalt-Catalyzed Hydride Transfer Reactions 3.1. Hydrogenation Reactions 3.1.1. Hydrogenation of Alkenes 3.1.2. Hydrogenation of Alkynes 3.1.3. Hydrogenation of Aldehydes and Ketones 3.1.4. Hydrogenation of Imines 3.1.5. Hydrogenation of Esters and Carboxylic Acids 3.1.6. Hydrogenation of Nitriles 3.1.7. Hydrogenation of Heterocycles 3.1.8. Hydrogenation of Carbon Dioxide 3.2. Transfer Hydrogenation Reactions 3.2.1. Transfer Hydrogenation of Alkenes and Alkynes 3.2.2. Transfer Hydrogenation of Aldehydes, Ketones, and Imines 3.2.3. Transfer Hydrogenation of Nitriles and Heterocycles 3.3. Dehydrogenative Transformations 3.3.1. Dehydrogenation Reactions 3.3.2. Dehydrogenative Condensation 3.4. Hydrogen-Borrowing Reactions 3.4.1. N-Alkylation Reactions 3.4.2. C-Alkylation Reactions © XXXX American Chemical Society

3.5. Hydrofunctionalization Reactions 3.5.1. Hydroboration Reactions 3.5.2. Silylation Reactions 3.5.3. Hydrocarbon Functionalizations 3.6. Olefin Isomerization Reactions 3.6.1. Isomerization of 1-Alkenes 3.6.2. Isomerization of Dienes 3.6.3. Cycloisomerization Reactions 4. Conclusions and Outlook Author Information Corresponding Author ORCID Author Contributions Notes Biographies Acknowledgments Abbreviations References

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AK AK AQ BC BH BH BK BL BM BN BN BN BO BO BO BO BO BO

1. INTRODUCTION Hydride transfer reactions constitute a broad class of chemical reactions involving transfer processes of hydrogen in the form of hydride. Such reactions are commonly catalytic and involve a metal hydride species as an indispensable catalytically active intermediate.1−7 Classic hydride transfer reactions include (de)hydrogenation, hydroformylation, hydroboration, and hydrosilylation. These transformations play essential roles in both organic synthesis and chemical industry, providing convenient access to a variety of chemical products and synthetic building blocks. Most hydride transfer reactions rely on precious metal catalysts composed of metals such as ruthenium (Ru),8−13 rhodium (Rh),14−16 palladium (Pd),15,17 iridium (Ir),18,19 and platinum (Pt).20−22 However, use of these metals is limited by

T V W X AA AB AD AE AF AF AG AI AI AJ

Special Issue: First Row Metals and Catalysis Received: June 26, 2018

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DOI: 10.1021/acs.chemrev.8b00404 Chem. Rev. XXXX, XXX, XXX−XXX

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their high cost, low abundance, and biological compatibility. In contrast, use of first-row transition metals like iron (Fe), manganese (Mn), and cobalt (Co) as catalysts is much more sustainable and addresses the disadvantages of precious metals. For example, the reserve of elemental Co is over 1000 times larger than the total content of platinum-group metals in the Earth’s crust, highlighting the relative abundance of first-row transition metals.23 As a result, substitution of precious metal catalysts in hydride transfer reactions with economical and ecologically friendly earth-abundant metal catalysts is highly desirable in terms of sustainability.24−31 In addition, the multiple oxidation and spin states accessible for nonprecious metals enable unprecedented reactivity, distinct mechanistic pathways, and unique substrate scope. Therefore, the development of catalyst systems based on first-row transition metals offers an opportunity to develop hydride transfer reactions in a new chemical space. Since the discovery of hydroformylation reactions,32,33 cobalt has been established as an efficient catalyst for a variety of hydride transfer reactions.24,34−41 The utility of this earthabundant metal has been markedly expanded in the past decade along with the development of a series of well-defined cobalt complexes as effective homogeneous catalysts for hydride transfer processes. The representative cobalt complexes are displayed in Scheme 1. The specific structural and electronic properties of these complexes result in exceptional reactivity, such as the ability to activate strong chemical bonds42 to facilitate efficient and selective catalytic hydride transfer processes. In this respect, it is worth mentioning that related reviews have only given a partial outline of this research field by collating important improvements. A global overview covering a wide range of reaction types with a specific emphasis on cobalt catalysts has been lacking. This review aims to provide a comprehensive outline of homogeneous cobalt-catalyzed hydride transfer reactions and highlight the rapid development of this field. The review is structured as follows. Section 2 introduces the synthesis, electronic structure, and reactivity of important cobalt complexes that act as efficient hydride transfer catalysts. Important catalytic applications in hydride transfer reactions and the corresponding mechanistic studies are discussed in detail in section 3. Section 3 is classified into different types of reactions, including hydrogenation reactions (section 3.1), transfer hydrogenation (TH) reactions (section 3.2), dehydrogenative transformations (section 3.3), hydrogen-borrowing reactions (section 3.4), hydrofunctionalization reactions (section 3.5), and olefin isomerization reactions (section 3.6). Considering that the existing reviews summarizing the hydroformylation well and the rather minor development in recent years, the Co-catalyzed hydroformylation was excluded in this review.43 Finally, the major achievements and current challenges of this chemistry are highlighted, and useful perspectives for the further development of this exciting area are provided in section 4.

Scheme 1. Representative Homogenous Cobalt Catalysts for Hydride Transfer Reactions

compared with those of noble metals is another important reason for their complicated synthesis. For example, metal hydride complexes are important catalytically active species for hydride transfer reactions. There are a variety of commercially available precious metal precursors with hydride ligands (e.g., Ru(CO)ClH(PPh3)3, Ru(CO)(H)2(PPh3)3, RhH(PPh3)4, [RhH(cod)]4, and IrH(CO)(PPh3)3), which can be easily coordinated with various strong field ligands to afford the desired precious metal hydride complexes. In contrast, the similar cobalt hydride complexes have to be prepared in multiple steps from very simple precursors, such as CoCl2 or CoCl(PPh3)3, including coordination with supporting ligands and subsequent reduction steps. In this section, we describe the synthetic routes to representative cobalt complexes that have found important catalytic applications in hydride transfer reactions. These complexes can be classified into four categories according to the valence of the cobalt metal center. 2.1.1. Cobalt(−1) Complexes. An interesting cobalt(−1) complex in the absence of any heteroatom-based ligand, potassium bis(anthracene) cobaltate Co-8, is a useful catalyst in hydrogenation reactions.44 The complex Co-8 was prepared by reduction of cobalt bromide with three equivalents of potassium anthracene in 1,2-dimethoxyethane (DME) followed by recrystallization from dimethyl ether/diethyl ether (Scheme

2. SYNTHESIS, STRUCTURE, AND REACTIVITY OF COBALT PRECATALYSTS 2.1. Synthesis and Structure of Cobalt Complexes

The synthesis of cobalt complexes is usually more sophisticated than that of the corresponding precious metal complexes. Apart from the distinct coordination chemistry of 3d transition metals, the limited availability of cobalt precursors for complex synthesis B


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2). The single crystal structure of Co-8 confirmed the η4coordination of both anthracene ligands. This complex can be

An alternative synthetic route for the preparation of cobalt(I) complexes uses CoCl(PPh3)3 as the cobalt precursor. For example, a bulky N-heterocyclic carbene (NHC) ligand, 1,3diadamantylimidazol-2-ylidene (IAd) L3, reacted with CoCl(PPh3)3 to generate [Co(IAd)(PPh3)Cl] Co-18. The alkylation of Co-18 with LiCH2TMS produced the low-coordinate Co(I) alkyl complex Co-5 as a precatalyst for the selective hydrosilylation of terminal alkynes (Scheme 4).48

Scheme 2. Synthesis of Potassium Bis(anthracene) Cobaltate Co-8

Scheme 4. Synthesis of Low-coordinate Co(I) Alkyl Complex Co-5 from CoCl(PPh3)3 regarded as a precursor of quasi-“naked” anionic metal species because of the presence of labile arene ligands. 2.1.2. Cobalt(I) Complexes. Cobalt(I) complexes are often prepared by the reduction of the corresponding cobalt(II) dihalide complexes. A typical example is the synthesis of complex Co-1, which is an effective catalyst for the enantioselective hydrogenation of geminal-disubstituted olefins.45 Treatment of the bis(imino)pyridine ligand L1 with CoCl2 in THF at room temperature resulted in the formation of bis(imino)pyridine cobalt dichloride complex Co-16. Co-16 was reduced by NaBEt3H to afford the cobalt(I) monochloride complex Co-17, which was converted to the corresponding cobalt methyl complex by direct methylation with MeLi (Scheme 3).

2.1.3. Cobalt(II) Complexes. Divalent cobalt complexbased catalysts are less oxygen-sensitive than their cobalt(I) counterparts and are highly desired for practical usage. The cobalt(II) complexes that act as efficient catalysts for hydride transfer reactions can be mainly classified into two types: cobalt(II) alkyl complexes and cobalt(II) halide complexes. The synthesis of cobalt(II) halide complexes is quite straightforward. However, these complexes need to be activated by a strong base and/or sensitive reductants to generate catalytically active lowvalent species when they are used as catalyst precursors in various hydride transfer reactions.49 2.1.3.1. Cobalt(II) Alkyl Complexes. In 2012, Hanson and coworkers developed the first cobalt-catalyzed hydrogenation of polar unsaturated bonds using a 15-electron d7-cobalt(II) alkyl complex, [(PNHPCy)Co-(CH2SiMe3)]BArF4 Co-2.50 Co-2 was prepared from anhydrous CoCl2 in four steps and was very sensitive to air and moisture (Scheme 5). More specifically, coordination of pyridine with CoCl2 produced (Py)4CoCl2 Co19,51 which was alkylated with LiCH2SiMe3 to give an expedient

Scheme 3. Synthesis of Pincer Cobalt(I) Complexes Co-1 and Co-3 via Reduction of Co(II) Species

Scheme 5. Synthesis of 15-Electron d7-Cobalt(II) Complex [(PNHPCy)Co(CH2SiMe3)]BArF4 Co-2

The solid-state structure of the (S)-enantiomer of Co-1 was characterized by X-ray diffraction (XRD), revealing that it possessed a distorted square planar structure, in which the CoMe lift angle ranged between 14.6° and 17.3° out of the idealized plane and the Me group was directed away from the cyclohexyl substituent. Similarly, complexation of Nozaki’s bisphosphinohydridoborane ligand L246 with CoBr2 followed by reduction using sodium amalgam in THF under N2 afforded cobalt(I) pincer complex Co-3 as a green solid (Scheme 3). This diamagnetic complex possessed a pseudosquare-planar geometry with a metalated three-coordinate boron center and was an efficient catalyst for olefin hydrogenation.47 C


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source of “CoR2”, (Py)2Co(CH2SiMe3)2 Co-20.52 Reaction of PNHPCy L4 with Co-20 generated the cobalt(II) alkyl complex [(PNPCy)Co(CH2SiMe3)] Co-21 as dark-yellow crystals. Finally, one equivalent of the acid H[BArF4]·(Et2O)2 reacted with Co-21 to afford the protonated complex Co-2, which is paramagnetic (μeff = 2.8 μB) and has a distorted square-planar geometry according to the results of XRD, nuclear magnetic resonance (NMR) spectroscopy, infrared (IR) spectroscopy, and elemental analysis. The bis(phosphine)cobalt(II) dialkyl complex Co-4 was synthesized in a similar manner (Scheme 5). Co-4 catalyzed the hydrogenation of disubstituted alkenes and more hindered trisubstituted alkenes through hydroxyl group activation.53 Magnetic measurements confirmed that Co-4 possessed an S = 1/2 ground state (μeff = 2.0 μB). X-band electron paramagnetic resonance (EPR) spectroscopy confirmed that Co-4 was a low-spin Co(II) d7 complex. The solid-state structure determined by XRD revealed that two large SiMe3 groups were situated on opposite sides of the square-planar metal−ligand plane. Weix/Holland and co-workers reported a β-diketiminatesupported cobalt(II) alkyl complex Co-6 as an efficient catalyst for Z-selective alkene isomerization.54 Thallium(I) salts of the ligand L6 were coordinated with CoCl2(THF)1.5, taking advantage of the insolubility of TlCl as a driving force.55 Addition of a hexyl Grignard reagent to LCoCl complex Co-22 gave the desired three-coordinate 13-electron cobalt alkyl complex Co-6, which was extremely sensitive to air and moisture (Scheme 6).54 Co-6 was characterized by various spectroscopic techniques, which established its high-spin d7 (S = 3/2) electronic configuration.

Scheme 7. Synthesis of Cobalt(II) Dichloride Complexes

ments of these cobalt complexes revealed that Co-10 and Co-15 have d7 metal centers with one unpaired electron (S = 1/2 ground state). In contrast, Co-13 is a high-spin complex with three unpaired electrons, which is a rare example of a (de)hydrogenation catalyst based on cobalt in a high spin state. 2.1.4. Cobalt(III) Complexes. Use of low-valent cobalt complexes in catalytic reactions is heavily restricted by their high air sensitivity compared with that of platinum-group metal complexes. To overcome this limitation, the stable catalyst precursor (MesCCC)-CoCl2py Co-14 was developed for the catalytic hydrogenation of nitriles.66 The cobalt(III) complex Co-14 was synthesized from CoCl2 in three steps: (1) Co[N(SiMe3)2]2 Co-23 was prepared by the addition of NaN(SiMe3)2 to CoCl2 in THF;67,68 (2) Co[N(SiMe3)2]2 in hexane was treated with pyridine to produce Co[N(SiMe3)2]2py2 Co-24;69 and (3) LiN(SiMe3)2, Co-24, and one equivalent of ClCPh3 as an oxidant were added sequentially to a THF solution of benzimidazolium salt L7 to afford the C2-symmetric monoanionic bis(carbene) pincer complex Co-14 containing a cobalt(III) center with octahedral geometry (Scheme 8).70

Scheme 6. Synthesis of β-Diketiminate-Supported Cobalt(II) Alkyl Complex Co-6

Scheme 8. Synthesis of Monoanionic Bis(carbene) Pincer Cobalt(III) Complex Co-14

2.1.3.2. Cobalt(II) Dichloride Complexes. The most easily accessible cobalt catalyst precursors are cobalt(II) dichloride complexes, which can be conveniently prepared by treatment of CoCl2 with the corresponding ligands. Very recently, complexes such as Co-7,56,57 Co-9,58,59 Co-10,60−62 Co-13,63,64 and Co-15 have been used as effective catalysts for hydrogenation and dehydrogenation reactions (Scheme 7).65 Moreover, these fivecoordinate cobalt(II) complexes exhibit different configurations and spin states. Specifically, XRD indicated that Co-7 and Co-10 possess distorted square-pyramidal coordination, whereas the Co centers of Co-9, Co-13, and Co-15 exhibit distorted trigonal-bipyramidal geometry. The effective magnetic mo-

2.2. Reactivity Features of Cobalt Complexes

Understanding of the reactivity features of cobalt complexes related to the activation of precatalyst, bond activation of the substrates, and catalyst deactivation is the basis for the rational design of cobalt catalytic processes. In this section, we summarize studies on the reactivity of cobalt catalysts used in hydride transfer reactions to deliver pivotal mechanistic insights. 2.2.1. Cobalt(−1) Complexes. Monitoring the reaction of Co-8 with 20 equiv of styrene by 1H NMR spectroscopy D


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revealed that anthracene is rapidly exchanged by styrene (Scheme 9). During the catalytic hydrogenation reaction, the

Scheme 11. Reactivity of Bis(arylimidazol-2ylidene)pyridine Cobalt(I) Methyl Complex Co-28

Scheme 9. Reactivity of Potassium Bis(anthracene) Cobaltate Co-8

free anthracene in the reaction system is slowly hydrogenated only after the substrate (styrene) is entirely consumed, which is crucial for maintaining the stability of the catalytically active species and the homogeneous nature of the reaction system.44 These studies uncovered the mechanism for the activation of precatalyst Co-8 by the substrate and the effect of the free anthracene ligand on the catalyst system. 2.2.2. Cobalt(I) Complexes. Exposure of a benzene solution of bis(imino)pyridine cobalt(I) complex Co-1 to 4 atm of H2 resulted in the rapid formation of the cobalt hydride complex Co-26. Removal of excess H2 from the reaction system afforded cyclometalated compound Co-27, which could regenerate Co-26 in the presence of H2 (Scheme 10). Therefore, Co-27 might be an off-cycle species that is in equilibrium with cobalt hydride species Co-26 in alkene hydrogenation.45 Scheme 10. Reactivity of Bis(imino)pyridine Cobalt(I) Complex Co-1

initiated by hydrogen atom transfer (HAT) from cobalt hydride Co-29 was also a possible mechanism for the formation of Co33. The observation of the hydride and alkyl migration raised the possibility of the pyridine-centered chelating ligand possessing radical character. Structural, spectroscopic, and computational studies provided definitive evidence for the formation of bis(arylimidazol-2-ylidene)pyridine radicals in the ground state of Co-28, Co-29, and Co-31. In addition, spin density calculations revealed that [iPrCNC]•− radical anions were mostly localized on the pyridine ring, accounting for the observed hydride and alkyl migration processes. Exposure of the bis(phosphino)boryl cobalt(I) complex Co-3 to 1 atm of H2 in toluene resulted in quantitative formation of η2,η2-dihydridoboratocobalt dihydride complex Co-34 via a net two electron process [Scheme 12, eq (1)]. Co-3 served as a highly efficient catalyst for the hydrogenation of 1-octene at room temperature under 1 atm of H2 and was converted into Co-34 in the reaction along with the production of octane. Moreover, Co-3 reacted with HMe2N−BH3 to quantitatively form the hydridoborane cobalt tetrahydridoborate complex Co35 [Scheme 12, eq (2)], both of which could catalyze the dehydrogenation of HMe2N−BH3 into (Me2N-BH2)2. The MLC of the cobalt boryl subunit facilitates the reversible addition of H2 and HMe2N−BH3 across the Co−B bond.47 Low-coordinate Co(I) alkyl complex Co-5 reacted with H2SiPh2 and 1,2-diphenylethyne to yield the three-coordinate Co(I) silyl complex Co-36 and Co(I) η2-alkyne complex Co-37 (Scheme 13). Notably, Co-36 is the first three-coordinate cobalt silyl complex. Using Co-36 as the catalyst in alkyne hydro-

The bis(arylimidazol-2-ylidene)pyridine cobalt(I) methyl complex Co-28 was found to be an active catalyst for the hydrogenation of sterically hindered tri- and tetrasubstituted alkenes.71 The corresponding cobalt hydride Co-29, a likely incycle catalytically active species, was formed upon exposure of Co-28 to 1 atm of H2 (Scheme 11). Standing a benzene solution of Co-29 at room temperature under a N2 atmosphere resulted in Co-30 via the migration of the cobalt hydride to the 4position of the pyridine ring, suggesting ligand-radical character. Moreover, addition of one equivalent of Ph3CCl to a toluene solution of Co-30 at −35 °C led to the formation of (iPrCNC)CoCl Co-31 and a stoichiometric quantity of Ph3CH, which revealed metal−ligand cooperative (MLC) reactivity to realize a synergetic C−H bond cleavage and Co− Cl bond formation process. Subsequently, the reactivity of cobalt hydride Co-29 with alkenes was studied to gain insight into alkene insertion, an important step in catalytic hydrogenation (Scheme 11). Addition of 1-butene to a benzene solution of Co-29 furnished a 1,2-insertion product, cobalt butyl complex Co-32. The migration of the butyl group to the 4-position of pyridine was not observed. In contrast, the more sterically hindered olefin 1,1diphenylethylene reacted with Co-29 under N2 atmosphere to give the apparent 2,1-insertion/migration product (4CPh2CH3-iPrCNC)CoN2 Co-33. An alternative pathway E


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alcohols.73 The diamagnetic cobalt(III) (acetylphenyl)hydride complex Co-40 was produced from the reaction of Co-2 with 1phenylethanol at 120 °C (Scheme 14). The formation of complex Co-40 might involve reduction of the cobalt(II) complex Co-2 to a cobalt(I) species by 1-phenylethanol followed by the dehydrogenation of a second molecule of 1phenylethanol by the cobalt(I) intermediate. Finally, the oxidative addition of a C−H bond of acetophenone with the cobalt(I) species would generate the cobalt(III) hydride complex Co-40. This complex could act as an effective precatalyst for alcohol dehydrogenation, indicating that Co-40 is an off-cycle species in the dehydrogenation reaction involving a Co(I)/(III) catalytic cycle.74 The reaction of bis(phosphine)cobalt(II) dialkyl complex Co-4 with five equivalents of 1,5-cyclooctadiene (COD) produced d9 cobalt(0) complex Co-41 in 93% yield, which is the first example of a Co(0) COD complex (Scheme 15).53 The

Scheme 12. Reactivity of Bis(phosphino)boryl Cobalt(I) Complex Co-3

Scheme 13. Reactivity of Low-Coordinate Co(I) Alkyl Complex Co-5

Scheme 15. Reactivity of Bis(phosphine)cobalt Dialkyl Complex Co-4

silylation led to similar yield and selectivity to those of Co-5, whereas using the alkyne complex Co-37 as the catalyst gave a much lower yield.48 These results indicate that Co(I) silyl complex Co-36 is likely an active on-cycle species and the essential role of PPh3 to achieve high catalytic efficiency. 2.2.3. Cobalt(II) Complexes. A THF solution of paramagnetic cationic cobalt(II) alkyl complex Co-2 was treated with 1 atm of H2, generating tetramethylsilane (TMS) and an NMR-silent cobalt-containing product. Subsequent addition of CHCl3 led to the formation of cobalt(II) chloride complex Co39 and CH2Cl2, which suggested the paramagnetic cobalt(II) hydride species Co-38 was formed as an intermediate (Scheme 14).50 A THF solution of proposed cobalt hydride complex Co38 could catalyze rapid isomerization of 1-octene at room temperature via a typical metal hydride insertion/β-hydride elimination mechanism, which was verified by a crossover deuteration experiment.72 The complex Co-2 not only served as a precatalyst for hydrogenation of ketones and alkenes but also catalyzed the acceptorless dehydrogenation of secondary

catalytic activity of Co-41 in alkene hydrogenation was similar to that for Co-4. The isolation and catalytic activity of (dppe)Co(COD) Co-41 supported the occurrence of a Co(0)/(II) catalytic cycle for this cobalt-catalyzed olefin hydrogenation reaction. β-Diketiminate-supported cobalt(II) alkyl complex Co-6 displayed poor catalytic performance toward the isomerization of aromatic alkenes.54 Interestingly, the η2-allylbenzenecoordinated cobalt(I) complex Co-42 was isolated from the reaction of Co-6 with allylbenzene (Scheme 16). The following reaction mechanism for this transformation was proposed: β-hydride elimination of Co-6 generates cobalt hydride species Co-43, which reacts with allylbenzene to afford 3-phenylpropyl cobalt(II) complex Co-44 and the η2-olefinScheme 16. Reactivity of β-Diketiminate-Supported Cobalt(II) Alkyl Complex Co-6

Scheme 14. Reactivity of 15-Electron d7-Cobalt(II) Alkyl Complex Co-2

F


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concomitant formation of H2. Co-49 could also be obtained via the reduction of Co-14 with KC8. After isolating active cobalt(I) complex Co-49, the coordination of the substrate (nitriles) to the cobalt center was further investigated. Addition of 4methoxybenzonitrile to Co-49 yielded ( Mes CCC)Co(pNCC6H4−OCH3) complex Co-50, which could further coordinate with one equivalent of PPh3 to furnish fivecoordinate 18-electron cobalt(I) complex Co-51. The results of these stoichiometric experiments supported a Co(I)/(III) catalytic cycle for this nitrile hydrogenation reaction.66

coordinated cobalt(II) complex Co-45. The disproportionation of these two cobalt(II) species via HAT could lead to the formation of the isolated cobalt(I) complex Co-42 and the phenylpropyl cobalt(III) hydride complex Co-46. The reductive elimination of the latter compound would produce propylbenzene, which was detected in the reaction mixture by gas chromatography (GC)−mass spectrometry (MS). In this reaction pathway, two cobalt species have to react with each other. Therefore, it was envisioned that lowering the concentration of cobalt catalyst should inhibit the bimolecular reaction and thus prevent catalyst decomposition. Consistent with this hypothesis, decreasing the catalyst concentration obviously improved the conversion and selectivity of the isomerization reaction of allylbenzene. 2.2.4. Cobalt(III) Complexes. The reactivity between monoanionic bis(carbene) pincer cobalt(III) complex Co-14 and NaHBEt3 was explored to probe the activation of the precatalyst. An active cobalt(I) species (MesCCC)Co−N2 complex Co-47 was afforded by treatment of a solution of Co14 in benzene with two equivalents of NaHBEt3 (Scheme 17). It

3. COBALT-CATALYZED HYDRIDE TRANSFER REACTIONS 3.1. Hydrogenation Reactions

As one of the most widely studied class of reactions enabled by homogeneous catalysis, hydrogenation is a pivotal step in a wide range of laboratory- and industrial-scale processes. Hydrogenation is used to produce numerous fine chemicals as well as compounds employed in agriculture and pharmaceuticals.75−79 The use of earth-abundant metals like Co, Mn, and Fe instead of precious metals such as Ru, Rh, Ir, Pd, and Pt, which are present in the main catalysts currently employed in catalytic hydrogenation, is a promising solution to reach a major goal of sustainable chemistry.24,29,36,80−83 In this respect, development of cobalt-catalyzed homogeneous hydrogenation reactions has progressed rapidly in the past five years, indicating the great potential of cobalt-based homogeneous hydrogenation catalysts. In this section, we summarize the reported examples according to the substrates evaluated and the asymmetric cases are discussed individually in each subsection. It should be noted that heterogeneous cobalt catalytic systems (e.g., nanoparticles84−88 and metal−organic frameworks89−92) are not included in this review, despite the burgeoning development of these fields over the past few years. 3.1.1. Hydrogenation of Alkenes. Alkene hydrogenation is a type of metal-catalyzed reaction with broad applications in organic synthesis and receives constant attention from both academia and industry.75 Iguchi76 reported cobalt-catalyzed homogeneous hydrogenation of alkenes as early as 1942 (Scheme 18). [CoH(CN)5]3− (Co-52), which was generated in cobalt cyanide aqueous solution upon absorption of hydrogen, was found to hydrogenate conjugated dienes to monoenes with high selectivity. Later research by the groups led by Funabiki, Reger, and Alper in the 1970s and 1980s to 1990 continued the investigation of pentacyanocobaltate-catalyzed hydrogenation of conjugated dienes (e.g., cyclopentadiene, 1,3-cyclohexadiene, and 2,4hexadienoic acid) in aqueous solution including the reaction mechanisms, effect of phase transfer conditions, and substrate scopes.93−99 Reger and co-workers also reported that K3[CoH(CN)5] was an effective water-soluble hydrogenation catalyst to convert α,β-unsaturated ketones to saturated ketones under 1 atm of H2 at room temperature with the aid of phase-transfer reagents.96 Ziegler-type catalysts with aluminum trialkyls have also been used for the hydrogenation of alkenes.100−102 In 1963, Breslow’s group described an effective in situ catalytic system Co-53, which was generated by the activation of Co(acac)3 with Al(iC4H9)3, for the hydrogenation of cyclohexene and 1-hexene with high conversion under mild conditions (30 °C, 2.4−3.7 atm of H2).100 In 1968, Stern and Sajus reported the use of Co(acac)2 or Co(OAc)2 together with LiA1H(OR)3 (Co-54) to catalyze

Scheme 17. Reactivity of Monoanionic Bis(carbene) Pincer Cobalt(III) Complex Co-14

was hypothesized that this transformation proceeded through (MesCCC)Co(III) dihydride intermediate Co-48, followed by reductive elimination of H2. Although H2 was not detected, the addition of styrene to this reaction resulted in the production of ethylbenzene, which is indicative of the formation of cobalt hydride intermediate Co-48. Furthermore, using Cp2ZrHCl (Cp = cyclopentadienyl) as the reductant afforded another cobalt(I) complex, (MesCCC)Co−py complex Co-49, with G


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hydrogenation of alkenes via rational design of the ligands. A number of cobalt complexes bearing different supporting ligands have been prepared and used for the hydrogenation of monoand disubstituted alkenes (Scheme 19).

Scheme 18. Early Examples of Cobalt Catalysts for Hydrogenation of Alkenes

Scheme 19. Homogenous Cobalt Catalysts for Hydrogenation of Mono- and Disubstituted Alkenes

the hydrogenation of cyclopentene with quantitative conversion under 1 atm of H2 at 20 °C.101 Cobalt carbonyl complexes, which are known to catalyze hydroformylation reactions, have been employed for the hydrogenation of alkenes and arenes. For example, CoH(CO)(PPh3)3 (Co-55),103 CoH(CO)4‑n[P(nC4H9)3]n (n = 2, 3) (Co-56),104,105 and CH3COCo(CO)2P(OCH3)2 (Co-57)106 were used for the hydrogenation of disubstituted terminal, internal, and cyclic olefins during the 1970s and 1980s. In most cases, terminal olefins exhibited the highest activities of the various types of olefins and high conversions were achieved. Additionally, Muetterties’s group107 described the use of C3H5Co[P(OCH3)3]3 (Co-58) for the hydrogenation of benzene as well as n-hexene under mild conditions (20 °C, 1 atm of H2) in 1974. The same year, BaloghHergovich and co-workers reported that CoH(N2)(PPh3)3 (Co-59) could catalyze the hydrogenation of cyclohexene at room temperature under 1 atm of H2.109 Those early examples clearly demonstrated the promising catalytic activity of cobalt catalysts for alkene hydrogenation. However, such catalytic systems are quite limited in their substrate scope and show restricted possibilities for modification of the catalysts by ligand variation. Since 2000, much effort has been dedicated to developing cobalt complexes for the

In 2005, Budzelaar’s group110 used cobalt(I) diimine pyridine complex Co-60 for the hydrogenation of alkenes. They revealed that Co-60 was active in the hydrogenation of monosubstituted (e.g., 1-octene) and disubstituted alkenes (e.g., 2-pentene) but inactive for more hindered trisubstituted alkenes (e.g., 7-methyl1,6-octadiene) under 5−50 bar of H2 at 50 °C. Considering the findings of density functional theory (DFT) calculations, the authors proposed a catalytic hydrogenation cycle starting with cobalt monohydride complex LCoH rather than a dihydride Co(III) intermediate LCo(alkyl)(H2). Nevertheless, they also considered that a small amount of paramagnetic compound H


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active for hydrogenation of a variety of alkenes including terminal and internal olefins (>98% yields) such as 4fluorostyrene, 1-octene, cis-cyclooctene, and norbornene under 1 atm of H2 at room temperature. This indicates that an MLC interaction involving the N−H group of the PNP ligand is not crucial for hydrogenation of alkenes by Co-2. Cobalt complexes Co-40 and Co-67 with various coordination ligands or counterions were also effective for the hydrogenation of styrene, whereas Co-68 with a diphenyl phosphorus substituent showed no reactivity. Furthermore, a cobalt(II) hydride complex Co-38, which was generated in situ upon the reaction of Co-2 with H2, was found to catalyze the isomerization of 1-octene to internal octene isomers in THF-d8 solution. Additionally, H/D scrambling was observed when 1-pentene and cyclohexene-d10 were mixed together in the presence of Co-38 (Scheme 20b). The authors proposed an inner-sphere alkene insertion mechanism to explain this observation. More specifically, the cobalt(II) hydride complex Co-38, which was initially formed by hydrogenolysis of Co-2, was considered to be the active catalytic species (Scheme 20c). The following insertion of an alkene into the Co−H bond resulted in the formation of cobalt(II) alkyl intermediate Co-69. Co-69 then reacted with H2 to release the alkane product and regenerate the cobalt(II) hydride species Co-38. In 2014, von Wangelin and co-workers developed the heteroatom-free arene cobaltate catalyst Co-8 for alkene hydrogenation under mild conditions without any activation reagents.44 Co-8 may benefit from rapid ligand exchange between π ligands and π-acidic substrates, thus preventing the aggregation and deactivation of the catalyst during the hydrogenation. A number of terminal and internal alkenes (33 examples) were reduced by 1−5 mol % of Co-8 at 20−60 °C under 1−10 bar of H2 in good-to-excellent yields (76%−99%). Notably, the trisubstituted olefin 4-ethylidenecyclohexyl could also be reduced in moderate yields (60%) by this catalytic system using a high catalyst loading of 5 mol % and temperature of 80 °C. However, Co-8 was unable to hydrogenate aryl chlorides and bromides, which showed no conversion. Follow-up research on the hydrogenation mechanism of these cobalt arene complex catalysts was conducted by the same group in 2017.111 They prepared five more arene cobaltate complexes Co-70−Co-74 and evaluated their ability to catalyze hydrogenation of styrene and 1-dodecene (Scheme 21). It was found that complex Co-70 with one COD ligand exhibited the highest catalytic activities for both substrates (99% and 93%). The complexes Co-73 and Co-74 with dibenzo[a,e]cyclooctatetraene (dct) ligands showed decreased activities compared with that of Co-8 in the hydrogenation of styrene. This may be attributed to the strong coordination of dct, which resulted in slow ligand exchange of the catalysts with the alkene substrates. Furthermore, the hydrogenation of styrene by Co-8 was monitored through NMR spectroscopy experiments (Scheme 22a and b). The results indicated that redox-neutral π-ligand exchange occurred prior to styrene hydrogenation. In addition, kinetic studies of Co-8-catalyzed hydrogenation of styrene showed no detectable induction period, thus ruling out the formation of nanoparticles and nanoclusters. Poisoning studies involving addition of mercury or dct to the Co-8-catalyzed hydrogenation of styrene supported the homogeneity of the catalyst. On the basis of these experimental results, the authors postulated that the catalytic mechanism of Co-8-catalyzed alkene hydrogenation involved initial formation of 18-electron

generated during the hydrogenation processes might also be the catalytically active species. In 2012, Hanson and co-workers developed the pincer (PNP) ligand-supported cobalt(II) alkyl complex Co-21, which could react with Brookhart’s acid H[BArF4]·(Et2O)2 (BArF4B(3,5(CF3)2C6H3)4) to generate Co-2 in situ to allow hydrogenation of alkenes under very mild conditions.50 This catalytic system exhibited high hydrogenation activities (12 examples, 80%− 99%) under 1−4 atm of H2 at 25−60 °C with a catalyst loading of 2 mol % and displayed a good functional group tolerance for both terminal and internal alkenes. However, this catalytic system was not effective for trisubstituted alkenes, as revealed by the selective hydrogenation of the terminal CC bond of (R)(+)-limonene rather than the internal trisubstituted CC bond (4k). The same group conducted a detailed mechanistic investigation to elucidate the different reaction pathways for the hydrogenation of olefins and ketones by Co-2.74 Initially, a set of cobalt analogue complexes (Co-40 and Co-66 to Co-68) of Co-2 were prepared and evaluated in styrene hydrogenation (Scheme 20a). The N-Me-substituted derivative Co-66 was Scheme 20. Derivative Complexes of Co-2 for Hydrogenation of Styrene and the Proposed Mechanism of Co-2-Catalyzed Hydrogenation of Alkenes

I


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of a series of alkenes bearing different functional groups, such as ketone, aldehyde, anhydride, and hydroxyl, and pyridine functionalities, achieving quantitative conversions under 4 atm of H2 at room temperature. Selective hydrogenation of terminal alkenes over internal alkenes was achieved by variation of the reaction temperature, as evidenced by the reduction of the terminal CC bond of 4-vinylcyclohexene catalyzed by 2 mol % of Co-62 at room temperature. With the successful preparation of Co-78 from Co-62 under H2 atmosphere (Scheme 23a), the catalytic mechanism of alkene

Scheme 21. Derivative Cobalt Complexes of Co-8 for Hydrogenation of Styrene and 1-Dodecene

Scheme 23. Mechanistic Investigation of Co-62-Catalyzed Hydrogenation of Alkene Substrates

Scheme 22. Mechanism Investigation of Co-8-Catalyzed Hydrogenation of Alkene Substrates

bis(alkene) complex Co-75 in the reaction mixture (Scheme 22, panels b and c). The bis(alkene) complex Co-75 was presumably the resting state and could lose an alkene ligand to form the unsaturated 16-electron monoalkene complex Co-76. Co-76 might undergo oxidative addition to produce the alkylated cobalt hydride complex Co-77, which then releases the alkane products and regenerates complex Co-76 via reductive elimination. Regarding hydrogenation reactions using Co(I)/Co(III) catalysis, Fout and co-workers reported the hydrogenation of alkenes catalyzed by the Co(I) complex Co-62, which was supported by an electron-rich monoanionic bis(carbene) ligand (Scheme 19).112 Co-62 was able to catalyze the hydrogenation

hydrogenation was investigated through parahydrogen-induced polarization (PHIP) transfer NMR experiments (ALTADENA and SABRE methods) as well as deuterium studies (Scheme 23, panels b and c). The PHIP NMR studies during the hydrogenation of styrene using Co-62 with parahydrogen (pH2) suggested that Co-62 can be converted into Co-80 via ligand exchange with alkene substrates (Co-78 and Co-79) as J


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well as the consequent oxidative addition of H2 onto the cobalt(I) center (Scheme 23d). The alkylated cobalt complexes Co-81 and Co-82 can be then produced by migratory insertion of the coordinated alkene to the Co−H bond of Co-80. A deuteration experiment involving addition of D2 to the hydrogenation of styrene using Co-62 produced a partially deuterated substrate. This indicates that the alkylated cobalt complexes Co-81 and Co-82 can regenerate Co-80 via βhydride elimination. Moreover, the detection of HD and H2 confirmed that HD exchange occurs through Co-81 and Co-82. Eventually, Co-78 is reproduced through reductive elimination of the alkane and recoordination of PPh3 to the cobalt metal center. In addition to the cobalt catalysts mentioned above, several other cobalt complexes have also been reported to display catalytic activity for the hydrogenation of alkenes (Scheme 19). Hapke and co-workers113 reported that cobalt(I) complex Co63 (2 mol %) was active for the hydrogenation of 1-octene (39%) under 20 atm of H2 at 80 °C. Peters and co-workers developed a series of a meridional bis-phosphinoboryl ligandsupported cobalt complexes that exhibited reversible H2 addition properties.47,114 Among these catalysts, the cobalt(I) complexes Co-3 and Co-61 were active for catalytic hydrogenation of alkenes under 1 atm of H2 at room temperature. Co61 reduced styrene, 1-octene, cis-cyclooctene, and norbornene in excellent yields (92%−99%) with a catalyst loading of 2 mol %, whereas Co-3 was inactive for internal olefins such as ciscyclooctene and norbornene. Paul and co-workers conducted a theoretical study on the mechanism of Co-3-catalyzed olefin hydrogenation.115 Stryker’s group116 prepared the homoleptic ancillary-ligand-free tetrametallic Co(I) cluster complex Co-64, which can hydrogenate allylbenzene in quantitative yield under 1 atm of H2 at room temperature with a catalyst loading of just 0.5 mol % in THF. Bai and co-workers reported that the homoleptic cobalt(II) bis(phosphoranimide) complex Co-65 is capable of catalyzing the hydrogenation of allylbenzene (82% NMR yield) under 1 atm of H2 at room temperature with the assistance of pinacolborane (HBpin).117 Hydrogenation of tri- and tetrasubstituted alkenes catalyzed by cobalt complexes is more challenging than hydrogenation of simpler alkenes, and only a few examples have been reported to date. For example, von Wangelin’s group44 used complex Co-8 for the hydrogenation of the trisubstituted alkene 4-ethylidenecyclohexyl, and Chirik’s group realized general and efficient cobalt-catalyzed homogeneous hydrogenation of hindered alkenes (Scheme 24).53,71,118 In 2013, Chirik and co-workers reported that the bis(arylimidazol-2-ylidene)pyridine (CNC) cobalt(I) methyl complex Co-28 was an active catalyst for the hydrogenation of sterically hindered tri- and tetrasubstituted alkenes (Scheme 24).71 In this work, eight bulky alkenes were hydrogenated under 4 atm of H2 at 22 °C in the presence of 5 mol % of Co-28. All seven trisubstituted alkenes investigated, including 2-methyl2-butene, (E)-α-methylstilbene, 1-methyl-1-cyclohexene, and 3,3-dimethyl acrylate, were successfully converted to the corresponding alkanes in excellent yields (>95%, 4r-4u), whereas the tetrasubstituted alkene 2,3-dimethyl-2-butene exhibited a moderate conversion of 69% at an elevated reaction temperature of 50 °C. These results indicate that the CNC cobalt complex Co-28 shows superior reactivity over its analogous iron complex for the hydrogenation of sterically hindered alkenes.119 In view of the high catalytic hydrogenation activity of Co-28, the authors further conducted a set of

Scheme 24. Cobalt-Catalyzed Hydrogenation of Tri- and Tetrasubstituted Alkenes

experiments (see section 2.2.2) combining spectroscopic and computational studies to elucidate potential catalytic pathways. The CNC ligand was established as a redox-active ligand, undergoing one-electron reduction chemistry. Thus, chelate radicals may be present in the electronic structure of (CNC)CoX (X = H, halide, alkyl) compounds, which are likely in-cycle catalytically active species, rendering the unique catalytic hydrogenation activity of Co-28. In other work by Chirik’s group, they revealed that bis(phosphine)cobalt dialkyl complex Co-4 is active for the hydrogenation of disubstituted alkenes as well as hydroxylated tri- and tetrasubstituted alkenes.53 They found that Co-4 can effectively hydrogenate both cyclic and acyclic disubstituted alkenes with different functionalities such as sulfolane and pyridine in good-to-excellent yields (80%−99%) under 4 atm of H2 at 25 °C. Interestingly, this catalyst is highly active for the hydrogenation of hydroxylated tri- and tetrasubstituted alkenes (seven examples, 81%−99%), such as 4-hydroxy-2,3-dimethyl2-butene (86%), 3,3-dimethylallyl alcohol (99%), and 4terpineol (99%). In contrast, the analogous alkenes without hydroxyl functionalities displayed almost no conversion (99% for the majority of cases). It should be noted that the terminal alkyne was incompatible with this catalytic system, as revealed by the unsuccessful hydrogenation of phenylacetylene. A catalytic reaction mechanism for this cobalt-catalyzed alkyne hydrogenation reaction was proposed (Scheme 37). The alkyne dihydrogen complex Co-78 (generated from Co-62 under H2, see Scheme 23) is initially coordinated by the alkyne substrate (Co-106, cycle I). Further oxidative addition of H2 onto the cobalt center results in the Co(III) dihydride intermediate Co-107, which could then undergo migratory insertion of the alkyne to generate the Co(III)-(η1-vinyl) hydride/dihydrogen species Co-108. Eventually, the catalytically active species Co-78 is regenerated via reductive elimination and then recoordination of PPh3 along with the production of the Z-alkene. Meanwhile, the isomerization process commences with the coordination of the cis-olefin to Co-78 by replacement of PPh3 to afford an alkene-coordinated dihydrogen intermediate compound Co-109, which could then convert to dihydride Co(III) intermediate Co-110 via oxidative

addition of H2 in a reversible manner (cycle II). Subsequent migratory insertion affords two possible Co(III) alkyl dihydrogen/hydride species Co-111 and Co-112, which are in fast equilibrium. Finally, two competing pathways, β-hydride elimination (Scheme 37, pathway a) and reductive elimination (Scheme 37, pathway b), produce the trans-alkene and alkane products, respectively. Unlike Co-62 employed for (E)-selective hydrogenation of alkynes, Zhang and co-workers134 developed the readily available cobalt catalytic system Co-105 for (Z)-selective hydrogenation of alkynes in 2017 (Scheme 36). Co-105, which was generated in situ by mixing Co(OAc)2·(H2O)4 (1 mol %), NaBH4 (2 mol %), and ethylenediamine (8 mol %), could effectively catalyze (Z)-selective hydrogenation of a series of aryl- and alkyl-substituted internal alkynes with very high yields (77%−98%) and good selectivities (11:1:1 to 99:1:1 Z/ E/alkane) under 2 bar of H2 at room temperature in a mixture of THF, EtOH, and H2O. A number of functionalized alkenes, such as (Z)-3-substituted protected allylic amines and (Z)-3substituted allylic alcohols, were obtained with distinctly higher selectivities than those of nonfunctionalized alkynes. This was probably because the amine and alcohol moieties of the functionalized alkynes could behave as weak coordinating groups, thereby slowing down the isomerization and overreduction processes. Furthermore, this protocol could tolerate functional groups such as acetyl and cyano moieties, whereas alkynes featuring strongly coordinating functionalities like dimethyl amine and thiophene groups only resulted in low conversions. Control experiments were also conducted to P


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determine the hydrogen source in the hydrogenation of 1,2diphenylacetylene using CD3OD, D2O, and D2. Products containing deuterium were only observed upon using D2, suggesting that H2 was the only hydrogen source and the TH pathway could be excluded. The tetrametallic Co(I) cluster complex Co-64 was also tested in the hydrogenation of diphenylacetylene.116 Quantitative yields of the alkane products were obtained under 1 atm of H2 at room temperature with a catalyst loading of just 0.5 mol % in THF (Scheme 38). Moreover, the homoleptic cobalt(II)

Scheme 39. Cobalt Catalysts for Homogenous Hydrogenation of Carbonyl Compounds

Scheme 38. Hydrogenation of Alkynes by Co-64 and Co-65

bis(phosphoranimide) complex Co-65 was used for the complete hydrogenation of terminal and internal alkynes (six examples) into the corresponding alkanes, achieving high NMR yields (80%−99%) under 1 atm of H2 at room temperature with the assistance of HBpin.117 3.1.3. Hydrogenation of Aldehydes and Ketones. The first hydrogenation of carbonyl compounds by a homogeneous cobalt catalyst dates back to the 1950s (Scheme 39).135,136 Orchin and co-workers found that [Co2(CO)8] Co-113, a common hydroformylation catalyst, was capable of promoting aldehyde hydrogenation.135 In their report, complete reduction of the carbonyl group and double bond of the α, β-unsaturated aldehyde substrate was observed under 200−300 atm of synthesis gas (H2/CO = 1:1) at 180−185 °C. Five aldehydes were tested in this reduction, producing the corresponding alcohols in yields of 31%−72% in the presence of 5 mol % catalyst. Acetone could also be reduced to isopropanol in 41% yield at a slightly increased temperature of 185−190 °C. Later work by Slaugh and Mullineaux in 1968 revealed that addition of tertiary phosphines to Co-113 during the hydroformylation of olefins led to the generation of alcohols.137 In this context, Ugo and co-workers104 evaluated the cobalt carbonyl tertiary phosphine catalytic system Co-56 in hydrogenation of aldehydes at 30 atm and 130 °C. They compared the initial reaction rates of hydrogenation of four different aldehydes using CoH(CO)2[P(n-C4H9)3]2 and CoH(CO)[P(n-C4H9)3]3, and it was found that CoH(CO)[P(n-C4H9)3]3 exhibited a greater initial rate than CoH(CO)2[P(n-C4H9)3]2. Furthermore, a significant drop of the reactivity was observed in the hydrogenation of n-butanal when a high pressure of carbon monoxide (≥15 atm) was introduced by using an in situ catalytic system of Co2(CO)8 and P(n-C4H9)3 (cobalt to phosphine ratio >1:5). The authors therefore postulated that the dissociation of carbon

monoxide from the cobalt catalysts could be an important step for the hydrogenation process. Although these early examples showed quite limited substrate scopes and usually required harsh reaction conditions, more recent work by Hanson’s group50 in 2012 demonstrated that cobalt complexes could serve as effective catalysts for the hydrogenation of carbonyl compounds under mild conditions (Scheme 39). In addition to its high activity for hydrogenation of CC bonds, Co-2 was also capable of catalyzing the hydrogenation of CO bonds under mild conditions (1−4 atm H2, 25−60 °C). Twelve ketones and aldehydes including aliphatic and aromatic substrates were tested using 2 mol % of Co-2, affording the desired products with GC yields of >92%.50 Hydrogenation of 2-acetylpyridine, 2-hydroxy benzaldehyde, and 4-hydroxy-3-methoxy acetophenone with Co-2 was unsuccessful. The authors proposed that it was probably in Q


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virtue of the strong coordination of these substrates to the cobalt center suppressing further catalysis. A follow-up mechanistic investigation using a series of analogues of Co-2 (Co-40 and Co-66−Co-68) for the hydrogenation of acetophenone was conducted (Scheme 20).74 Since the N-Me-substituted derivative Co-66 and aryl hydride derivative Co-40 (2 mol %) showed no catalytic activity in this transformation, MLC catalysis involving an outer-sphere mechanism was postulated for this Co-2-catalyzed hydrogenation of carbonyl compounds. In contrast, the hydrogenation of alkenes by Co-2 follows an inner-sphere mechanism involving an olefin insertion pathway. In 2015, Kempe and co-workers developed the air-stable and easily tunable PNP cobalt catalyst Co-10, which was active for the hydrogenation of ketones and aldehydes in 2-methyl-2butanol or THF as the solvent under 20 bar of H2 at room temperature (Scheme 39).60 Notably, complex Co-10 bearing a triazine ring showed superior catalytic activity (>99%) in the hydrogenation of acetophenone over that of its analogous complex bearing a pyridine moiety (23%). This marked difference in activity might be ascribed to the N atoms of the triazine moiety, which altered the electron-donating ability of the coordinating N atom and could stabilize the proton shuttle chain through hydrogen bonding. Co-10 displayed a broad substrate scope (25 examples) with catalyst loadings ranging of 0.25−3 mol %, achieving excellent yields (>95%) for most of the aromatic and aliphatic substrates featuring an N-heteroatom, halide, or unsaturated CC bonds. It is worth mentioning that Co-10 could selectively reduce CO bonds in the presence of remote or conjugated CC bonds. This behavior was opposite to that of Co-2, which only reduced the CC bond in 2-methyl5-(prop-1-en-2-yl) cyclohexanone rather than the CO bond. In 2014, von Wangelin and co-workers44 tested arene cobalt complex Co-8 in the hydrogenation of CO bonds (Scheme 39). Although this catalyst was mainly used for hydrogenation of olefins, its reactivity for the hydrogenation of ketones was also observed. Both aromatic and aliphatic ketones (five examples) were successfully reduced to the corresponding alcohols in good-to-excellent yields (71%−99%) at 60 °C and 10 bar of H2 in the presence of 5 mol % Co-8, whereas aldehyde substrates tended to undergo condensation as well as oligomerization; no reduced products were generated. In a subsequent study by the same group, they tested five other arene cobaltate complexes Co-70−Co-74 (Scheme 21) as catalysts for the hydrogenation of dibenzyl ketone.111 Co-8 was demonstrated to be the best catalyst, affording the reduction product dibenzyl methanol in excellent yield, whereas the other complexes only exhibited low-to-moderate activities (5%− 65%). Notably, it was found that Co-8 could catalyze direct SET reduction of acetophenone to generate the pinacol product in moderate yields. Moreover, the generation of dihydrogen was also detected from the stoichiometric reaction between Co-8 and 1,3-diphenyl-2-propanol, indicating that a dehydrogenative side reaction could also occur once alcohol products were generated from the hydrogenation process. Therefore, the authors postulated that Co-8 could transformed into a higher oxidation manifold of the Co metalate species (Co-114) via these two types of side reactions (Scheme 40). And this newly formed Co species Co-114 exhibited a lower activity than Co-8, thus requiring a higher reaction temperature. The asymmetric hydrogenation of prochiral ketones is one of the most useful synthetic methods to prepare optically active alcohols.34,78 However, only a few examples using a chiral cobalt

Scheme 40. Proposed Mechanism of Co-8 in Hydrogenation of Polar Substrates

catalyst for the asymmetric hydrogenation of ketones have been reported to date. Early work by Ohgo and co-workers as well as Waldron and Weber in the 1970s and early 1980s described the use of two analogous quadridentate N-coordinated cobalt complexes Co-95 and Co-115 (Scheme 41), respectively, for Scheme 41. Use of Cobalt Complexes Co-95 and Co-115 for Asymmetric Hydrogenation of Benzil

the hydrogenation of diketone substrates (e.g., benzil) in the presence of the chiral alkaloid quinine.125,138−141 Co-95 catalyzed the asymmetric reduction of benzil to benzoin in 62% ee under 1 atm of H2 at room temperature, whereas Co-115 achieved a higher ee value of 79% for the same reaction under similar conditions. It was proposed that quinine associated with the diketone substrate, making it more susceptible to nucleophilic attack by the cobalt metal center. Thus, the chirality-determining site of this cocatalytic system is attached to the substrate rather than to the metal catalyst. In 1997, Lemaire and co-workers used (1R,2R)-cyclohexyldiamine as a ligand combined with Co(acac)2 or CoC12 for the hydrogenation of methyl acetoacetate under 50 bar of H2 at room temperature.142 However, no notable ee values and only low-to-moderate yields (17%−77%) of the product were obtained using 5 mol % catalyst. In 2016, Li and co-workers reported that the new cobalt complex Co-116 bearing a chiral tetradentate (PNNP) ligand was active for the enantioselective hydrogenation of a series of aromatic ketones (Scheme 42).143 Cobalt complex Co-116 was prepared in air; as a result, one phosphine atom of the PNNP ligand was oxidized and eventually formed Co-116 with a PNNO coordination geometry. This cobalt catalyst (2 mol %) R


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substituted N-benzylideneaniline substrates (six examples) at 10 bar of H2 and 60 °C in the presence of 5−7.5 mol % of the catalyst.44 Four imines including N-benzylideneaniline and its derivatives with methyl or methoxyl substituents were reduced to the corresponding amines with excellent yields (96%−99%), whereas bromide-substituted N-benzylideneaniline showed no conversion. Chiral amines are important intermediates in the syntheses of numerous useful molecules in the pharmaceutical and fine chemical industries.144,145 In this context, the asymmetric hydrogenation of imines to produce chiral amines represents an attractive direct approach to such valuable building blocks. An early example reported by Okamoto and co-workers146 in 1986 described the use of a Co-95/quinium salt catalytic system for the asymmetric hydrogenation of methyl Np-toluenesulfonyl-1-imino-1-phenylacetate to methyl Np-toluenesulfonyl-1phenylglycinate. The reduction proceeded under 1 atm of H2 at room temperature with 25 mol % Co-95, producing the desired product in 82% chemical yield and 20% optical yield. In 2013, Amézquita-Valencia’s group147 used Co2(CO)8/ (R)-BINAP (2:1) Co-117 as an efficient catalytic system for asymmetric hydrogenation of a series of N-aryl benzophenone ketimines (Scheme 44). The optimized reduction conditions for

Scheme 42. Chiral Cobalt Complex Co-116 for Asymmetric Hydrogenation of Aromatic Ketones

was then applied to the asymmetric hydrogenation of various ketones in MeOH under 60 bar of H2 at 100 °C. Most of the ketone substrates (19 examples) afforded good-to-high yields (77%−99%) and moderate-to-high ee values (54%−95%). Particularly high enantioselectivities were observed for the αsubstituted acetophenones (83%−95%), whereas the aryl methyl ketones were mostly produced with relatively low enantioselectivities (35%−79%). Notably, phenyl ring substituents appeared to have no marked electronic effect on the enantioselectivity of products; instead, enantioselectivity varied with the substitution position. 3.1.4. Hydrogenation of Imines. Only a few cobalt complexes able to catalyze the homogeneous hydrogenation of imines have been reported (Scheme 43). Hanson’s group50 used Co-2 generated in situ for the hydrogenation of three imine substrates, affording the corresponding amines in moderate-togood GC yields (70%−98%) under 4 atm of H2 at 60 °C with 2 mol % of Co-2. In another example, von Wangelin and coworkers employed Co-8 for the hydrogenation of aryl-

Scheme 44. Asymmetric Hydrogenation of Imines with Co117

Scheme 43. Cobalt Catalysts for Hydrogenation of Imines

a broad range of substrates were 31 bar of mixed gas H2/CO (1:3) at 120 °C with 1 mol % of Co-117. Initially, various imines derived from anilines bearing different aryl substituents (attached to the N atom of the imine) were tested. The corresponding products were obtained in excellent yields (94%−98%), but good enantioselectivities (70%−83% ee) were only afforded in the presence of p-MeO or p-Me groups on the amine moiety as well as p-F or p-Cl groups attached to the benzophenone moiety. A number of benzylamine derivatives were also applied to this methodology. All substrates tested were reduced to corresponding amines in excellent yields (93%− 98%) with moderate-to-good enantioselectivities (36%−99% ee). The presence of p-Me, p-Cl, or p-F groups attached to the benzophenone moiety gave good-to-excellent enantioselectivities (73%−99% ee), whereas m-CF3, p-CF3, or p-Cl groups on S


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performance of the investigated catalysts, achieving a higher yield of cyclohexanol (85%) than the other two complexes (27% and 67%). A variety of primary, secondary, and tertiary aliphatic ester substrates were tested under the optimized reaction conditions (2−4 mol % of Co-9, 8 mol % of NaHBEt3, 25 mol % of KOtBu, 50 bar of H2, and 130 °C), furnishing the desired alcohol products in moderate-to-excellent yields (50%−87%). Next, aromatic and fluorinated esters were also examined; however, no reduction of the substrates was observed. On the basis of this unexpected catalytic behavior as well as the need for a large amount of base (25 mol % of KOtBu) in this catalysis, the authors postulated a plausible mechanism for Co-9-catalyzed ester hydrogenation, which is shown in Scheme 46. Initially, the

the benzylamine moiety exerted a negative influence on the enantioselectivities (41%−72% ee). 3.1.5. Hydrogenation of Esters and Carboxylic Acids. The reduction of esters is a fundamental chemical transformation for the production of alcohols in both industry and academic research.148 Because homogeneous catalytic hydrogenation can be conducted under mild conditions with high selectivity and broad functional group tolerance, there has been growing demand for homogeneous catalytic ester hydrogenation in recent years.11,12,149 However, the majority of homogeneous catalysts for hydrogenation of esters reported so far are based on precious metals, usually ruthenium.149 Very recently, development of several cobalt catalytic systems for homogeneous hydrogenation of esters has been reported (Scheme 45).36,150

Scheme 46. Proposed Mechanism of Co-9 Catalyzed Ester Hydrogenation via an Ester Enolate Intermediate

Scheme 45. Homogenous Cobalt Catalysts for Hydrogenation of Esters

ester was in equilibrium with its enolate formed under the basic conditions. The generated enolate then underwent hydrogenation to afford an aldehyde and alkoxide via a hemiacetal salt intermediate. Subsequently, the resulting aldehyde could undergo further hydrogenation to produce the corresponding alcohol product. The proposed mechanism explains why nonenolizable esters were not hydrogenated using this catalytic system. In 2017, Jones’ group tested complex Co-2, which was originally developed by Hanson’s group, for the hydrogenation of esters without any additives (Scheme 45).151 The pressure of H2 was found to strongly influence the reaction efficiency, because a considerable yield increase of benzyl alcohol from 54% to 90% was achieved when the H2 pressure was raised from 35 to 55 bar in the hydrogenation of benzyl benzoate. Moreover, Co-2 was found to decompose at high temperature (150 °C). Thus, the catalytic tests to determine substrate scope were conducted under optimized conditions of 55 bar of H2 and 120 °C in THF with 2 mol % of Co-2. Both aromatic and aliphatic esters (11 examples) were reduced to the corresponding alcohols in moderate-to-excellent yields (67%−97%) except for the methyl ester substrates, which were obtained in low yields of 8%−35%. The successful isolation of cobalt complex Co-119 from the hydrogenation of methyl benzoate might explain this phenomenon because the CH3−O bond of methyl esters can be cleaved under these reaction conditions, consequently generating inactive cobalt benzoate complex Co-119 (Scheme 47). Biomass-derived γ-valerolactone 10b was also tested on a gram scale, providing 1,4-pentanediol with a high turnover number (TON) of 3890. Further comparison of Co-2 with its N-methylated counterpart in hydrogenation of benzyl benzoate as well as γvalerolactone 10b indicated that ester hydrogenation had a nonbifunctional mechanism because these two complexes showed quite similar catalytic activities. Considering the poor activity of methyl esters in this hydrogenation protocol, the

Milstein and co-workers reported the first cobalt catalyst Co-9 with a lutidine-based pincer ligand for homogeneous ester hydrogenation in 2015 (Scheme 45).58 Complex Co-9 with a secondary amine side arm (PNNH) coordinated to the cobalt center was initially compared with its tridentate biphosphine (PNP) or tertiary amine (PNN) pincer analogue complexes in the hydrogenation of cyclohexyl hexanoate 10d under 50 bar of H2 at 130 °C. Complex Co-9 showed the best catalytic T


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this protocol, providing the desired alcohol products in moderate-to-excellent yields (62%−99%) for the majority of cases, although benzoates bearing bulky ester groups such as tBu or Bn only gave low yields of the reduced product (26%−35%). Fluorinated (2-F, 4-F, and 4-CF3) aromatic esters were tolerated in this reduction, giving the alcohol products in excellent yields (85%−90%), whereas chlorinated or brominated aromatic esters underwent partial dehalogenation. Next, a variety of aliphatic and cyclic esters were tested in this hydrogenation, generating the respective alcohols in moderate-to-excellent yields (52%−94%). Three lactone substrates were also successfully reduced to the corresponding diols in yields of 83%−94%. Methyl cyclohex-3-ene-1-carboxylate 10e was reduced to cyclohexenylmethanol in 55% yield without affecting the CC double bond. To elucidate a possible reaction mechanism, several control experiments were then conducted. Poisoning experiments involving addition of mercury or PPh3 (2.5 or 1 mol %) to the Co-118-catalyzed hydrogenation of methyl benzoate supported the homogeneity of this catalytic system. Moreover, comparison of Co-118 with its N-methylated analogue in hydrogenation of benzyl benzoate and methyl hexanoate indicated an outer-sphere mechanism involving an NH moiety, because the N-methylated complex showed absolutely no catalytic activity in the hydrogenation reactions. Compared with hydrogenation of esters, direct hydrogenation of carboxylic acids is generally a more challenging transformation.9 On the one hand, the carbonyl carbon of carboxylic acids has lower electrophilicity than that of esters. On the other hand, catalyst deactivation may be caused by dissociation of the acid-protonated ligand as well as overcoordination of the metal center by the (bidentate) carboxylate moiety. The only example of cobalt-catalyzed hydrogenation of carboxylic acids to date was reported by de Bruin and coworkers in 2015 (Scheme 45).152 In this seminal work, the combination of Co(BF4)2·6H2O and a tridentate phosphine ligand was found to provide the efficient catalytic system Co-11 for hydrogenation of a broad range of carboxylic acids as well as esters under relatively mild reaction conditions (80 bar of H2, 100 °C). Initially, ester substrates (seven examples) were tested in this catalytic system. It was found that both aromatic and aliphatic esters could be effectively converted to corresponding alcohols in good-to-excellent yields (72%−98%). Next, γvalerolactone (10b), triglyceride, and a long-chain methyl ester were found to be smoothly hydrogenated to the respective alcohols in good yields (72%−88%). Notably, Co-11 could effectively reduce the sterically hindered substrate tertbutylbenzoate 10f to the corresponding alcohol in an excellent yield of 98% within 5 h, whereas Co-118 only transformed the same substrate to the alcohol in 35% yield after 24 h.150 In addition, the CC double bond was not preserved in this reduction. Further investigation of more challenging carboxylic acid substrates revealed that hydrogenation of carboxylic acids with Co-11 (0.1−10 mol %) outcompeted the hydrogenation of the respective carboxylic esters, as evidenced by the full conversion of benzoic acid 10g in only 4 h rather than the 22 h required for the complete reduction of methyl benzoate. Subsequent investigation of substrate scope indicated that Co-11 was capable of hydrogenating various aromatic and long- and shortchain aliphatic acids, the majority of which were effectively reduced to the corresponding alcohol products in good yields (71%−99%). Levulinic acid 10h and succinic acid 10i could be directly reduced to the synthetically useful starting chemicals

Scheme 47. Formation of Co-119 from Co-2 in Hydrogenation of Methyl Benzoate

authors proposed an inner-sphere mechanism as shown in Scheme 48.58 In this case, the metal-bound hemiacetal Co-120 Scheme 48. Proposed Mechanism of Ester Hydrogenation by Co-2

is initially formed by insertion of the ester carbonyl into a cobalt−hydride bond. The hemiacetal is then protonolyzed by the cleavage of dihydrogen coordinated to the cobalt center, thus generating Co-122 with an aldehyde with one bound oxygen and an alcohol molecule. Next, the aldehyde C−O bond inserts into the Co−H bond to produce the alkoxidecoordinated cobalt complex Co-123, which can again cleave dihydrogen and thus protonate the alkoxide to generate a second alcohol molecule. Very recently, Beller and co-workers described the use of Co118 as a more generally applicable catalyst for the hydrogenation of a large number of esters compared with the previous examples reported by Milstein’s group and Jones’s group (Scheme 45).150 In this study, three other analogous cobalt complexes bearing different substituents on the phosphorus atoms (iPr, Cy, or Ph) of the pincer ligand as well as on the metal center (Cl or Br) were initially prepared and compared with Co-118 in hydrogenation of methyl benzoate. The preliminary catalytic results indicated that Co-118 exhibited the highest catalytic activity of the complexes and that the amount as well as the kind of base used clearly influenced the reaction efficiency. Thus, the following evaluation of substrate scope was conducted with 5 mol % of Co118 under optimized reaction conditions of 20 mol % NaOMe, 50 bar of H2, and 120 °C. Fifteen aromatic esters tested using U


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dedicated to developing precious metal-catalyzed homogeneous hydrogenation of nitriles; homogeneous catalytic systems based on earth-abundant metals were only achieved recently (Scheme 50).36,153,154

1,4-pentanediol and 1,4-butanediol in moderate yields (47% and 61%), respectively. Trifluoroacetic acid, which is highly polar, was fully hydrogenated at only 125 ppm of Co-11, producing trifluoroethanol in 50% yield. Moreover, this catalytic system tolerated various functional groups, such as hydroxide, chloride, fluoride, and trifluoromethyl moieties but was incompatible with more labile groups like boronate, bromide, iodide, and methoxy functionalities. In addition, the hydrogenation of butyric acid and acetic acid proceeded under solvent-free conditions with a low catalyst loading of 0.1 mol %. To elucidate the reaction mechanism of this catalyst system, the authors conducted a set of experiments using various metal precursors and ligands as well as poisoning studies. Considering the results of high-resolution electrospray ionization (ESI) MS, in situ EPR spectroscopy, XRD, and DFT calculations, a homogeneous catalytic mechanism with a Co(triphos)(κ2alkanoate) Co-126 resting state was proposed (Scheme 49).

Scheme 50. Cobalt-Catalyzed Hydrogenation of Nitriles

Scheme 49. Proposed Mechanism for Co-11-Catalyzed Hydrogenation of Carboxylic Acids

In 2015, the first cobalt-catalyzed homogeneous hydrogenation of nitriles was reported by Milstein and co-workers (Scheme 50).155 The PNNH cobalt complex Co-9, which has also been employed for the hydrogenation of esters, was found to effectively hydrogenate nitriles under 30 bar of H2 at 135 °C in benzene. NaBHEt3 (2 mol %) and NaOEt (4.4 mol %) are both required in this transformation to activate the cobalt precatalyst. Substrate scope investigation indicated that Co-9 can reduce a wide range of nitrile substrates (25 examples), including (hetero)aromatic, benzylic, and aliphatic nitriles, to corresponding primary amines in good-to-excellent yields (65%−99%) for most cases. Benzonitriles bearing functional groups such as amino, chloride, and fluoride were effectively hydrogenated to benzylamines by this catalytic system (78%− 93%), whereas 4-bromobenzonitrile produced a mixture including only 6% of 4-bromobenzylamine (11c) and 24% of the corresponding dibenzylmethanamine side product. Additionally, formation of a large amount of the corresponding secondary amine (41%) was observed in the case of 4(trifluoromethyl)benzonitrile. This unusual reactivity might be caused by the strongly electron-withdrawing trifluoromethyl group, which rendered the imine intermediate susceptible to nucleophilic substitution by the generated primary amine. In contrast, 4-nitrobenzyl cyanide was inactive under such catalytic conditions, retaining its nitro functionality. With respect to aliphatic nitriles, the hydrogenation of benzylic nitriles with Co9 produced the corresponding phenethylamines in excellent yields (85%−99%), whereas other nonactivated aliphatic nitriles could only be reduced with moderate yields (65%−67%).

In this catalytic cycle, Co-126 is initially formed by alkanoate substitution of the hydroxyl ligand of [Co2(triphos)2(μ− OH)2](BF4)2 Co-125, which forms spontaneously when Co(BF4)2·6H2O and triphos are mixed. Co-126 then splits H2 over the Co−O bond in a heterolytic manner, followed by hydride migratory insertion into the carbonyl group to generate the cobalt(II) hydroxyalkanolate intermediate Co-128. The cobalt(II) species Co-128 then undergoes a second heterolytic H2 splitting step, which expels one molecule of water and produces the [Co(triphos)(H)(aldehyde)]+ species Co-129. Finally, Co-129 undergoes a hydride migration step followed by ligand exchange with another carboxylic acid molecule, thus generating the desired alcohol and the catalytic resting state species Co-126. 3.1.6. Hydrogenation of Nitriles. Hydrogenation of nitriles is a synthetically useful and atom-efficient method to prepare amines, which are valuable compounds in the bulk and fine chemical industries. For a long period, major effort has been V


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Later work by Beller’s group156 in 2017 described the combination of Co(acac)3 and tris[2-(dicyclohexylphosphino) ethyl]phosphine (1:1.1) as an in situ catalytic system (denoted as Co-131) for effective hydrogenation of a series of (hetero)aromatic and aliphatic nitriles (Scheme 50). In this study, the amounts of catalyst and base were initially evaluated in the hydrogenation of benzonitrile to benzylamine. It was found that 10 mol % or higher of KOtBu could prevent the formation of secondary amines as side products, and 4 mol % of Co-131 was required to ensure high conversions. Next, a series of (hetero)aromatic nitrile substrates were hydrogenated under 30 bar of H2 at 80−120 °C in the presence of 4 mol % of Co-131 and 10 mol % KOtBu in tBuOH, generating primary amines in good-to-excellent yields (76%−99%). Both electron-donating and -withdrawing substituted benzonitriles could be effectively reduced, and a variety of functional groups such as amino, chloride, bromide, fluoride, methylthio, trifluoromethyl, and amide could be tolerated by this catalytic system, whereas bromide- and trifluoromethyl-substituted benzonitriles were not suitable for Co-9 (vide supra). Furthermore, N-containing heteroaromatic nitriles including pyridinecarbonitrile, quinolinecarbonitriles, and 1H-indole-5-carbonitrile were also successfully transformed into corresponding amines in high yields (79%−99%) at elevated reaction temperatures (120 or 140 °C). Terephthalodinitrile could be selectively hydrogenated to singly or doubly reduced amine compounds through a variation of the reaction conditions. Additionally, various aliphatic nitriles bearing long, short, branched, or cyclic alkyl chains, or benzylic or phenylpropyl groups could also be reduced smoothly, affording the desired amine products in good-to-excellent yields (70%−99%, 12 examples). Fout and co-workers used an NHC-based cobalt complex Co14 as an effective catalyst for the hydrogenation of nitriles to primary amines with the assistance of Lewis acids (Scheme 50).66 To achieve complete conversion of benzonitrile to benzylamine in toluene in the presence of 2 mol % of Co-14 at 115 °C under 4 atm of H2, addition of KOtBu (6 mol %) and NaHBEt3 (4 mol %) was required. A series of aromatic and aliphatic nitriles (20 examples) were then examined under these reaction conditions, affording the corresponding primary amines in high yields (78%−99%). It was found that aromatic substrates bearing electron-donating groups, including methoxy, methyl, and naphthyl moieties, showed higher reactivity than substrates with electron-withdrawing groups, such as fluoro, chloro, and trifluoromethyl, toward hydrogenation. Moreover, anilines and sterically hindered substrates bearing ortho-substituents were well-tolerated. However, substrates featuring phenolic hydroxyl, nitro, amide, bromide, and formyl functionalities were not amenable to hydrogenation. Aliphatic substrates, such as 3phenylpropionitrile, t-butylnitrile, and cyclohexylnitrile, gave good yields ranging from 78% to 90%. Acetonitrile was reduced to ethylamine in 39% yield by Co-14. Using PHIP transfer NMR spectroscopy, the authors probed the reaction mechanism for hydrogenation of 4-methoxybenzonitrile 12a (Scheme 51). The observation of the PASADENA (para-hydrogen and synthesis allow dramatically enhanced nuclear alignment) effect in the NMR spectra confirmed the pairwise hydrogenation of the nitrile group to form an imine. Additional stoichiometric studies successfully provided the possible intermediate complex Co-49 (see section 2.2.4, Scheme 17), which helped to elucidate the role of the Lewis acid in the hydrogenation process. Co-49 was active only in the presence of a Lewis acid (e.g., BEt3, BPh3, LiOTf, or Ca(OTf)2) during the

Scheme 51. NHC Cobalt Complex-Catalyzed Hydrogenation of Nitriles with the Assistance of Lewis Acids

hydrogenation of 4-methylbenzonitrile 12b to 4-methylbenzylamine. Moreover, the addition of excess amounts of the Lewis acid BEt3 to the hydrogenation of 4-acetylbenzonitrile 12c resulted in a high yield (95%) of the desired primary amine with the ketone functionality preserved. In contrast, the same substrate was unamenable toward hydrogenation with only a catalytic amount of Lewis acid, possibly because the Lewis acid might interact with the ketone functionality to inhibit catalytic turnover. Consistent with literature precedents, the authors thus proposed that the Lewis acid aided side-on coordination of the nitrile to the cobalt metal center, allowing a pairwise transfer of H2 via a Co(I/III) redox process. 3.1.7. Hydrogenation of Heterocycles. Because of their high stability derived from their aromaticity, hydrogenation of arenes is a challenging transformation. Heterocycles are ubiquitous scaffolds in natural products as well as many bioactive compounds, therefore, hydrogenation of heterocycles as a direct approach to produce their saturated counterparts has attracted considerable interest.157−159 In 1978, Muetterties and co-workers used η3-C3H5Co[P(OCH3)3]3 Co-132 to catalyze homogeneous hydrogenation of arenes at low hydrogen pressure (1−3 atm) and room temperature (Scheme 52). In this case, heterocyclic substrates furan and benzofuran were also tested and were reduced to THF (5%) and perhydrobenzofuran (19%), respectively.108 In 1982, Fish and co-workers employed Co2(CO)6(PPh3)2 Co-133 as a catalyst precursor for the hydrogenation of acridine and quinoline to 9,10-dihydroacridine (turnover frequency (TOF) = 10 h−1) and 1,2,3,4-tetrahydroquinoline (TOF = 14 h−1), respectively, under 24 bar of syngas at 200 °C in THF for 2 h.160 Kispert and co-workers subsequently reported the hydrogenation of quinoline, isoquinoline, and their derivatives by Co(stearate)2·AlEt3 catalyst Co-134 under 50 bar of H2.161 Using this catalytic system, isoquinoline was reduced to 1,2,3,4tetrahydroisoquinoline in 70% yield at 90 °C, whereas quinoline was transformed into 1,2,3,4-tetrahydroquinoline in 82% yield at an elevated temperature of 150 °C. Unfortunately, this Zieglertype cobalt catalytic system was unable to tolerate chloride- or nitro-substituted quinolines; no conversion of the starting materials was observed. Following the above studies, the cobaltcatalyzed homogeneous hydrogenation of heterocycles remained unexplored for a long period. W


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with good yields (63%−95%). This protocol was also extended to other heterocycles such as naphthyridine, quinoxaline, acridine, isoquinoline, and benzoquinoline, producing the corresponding products in high yields (79%−93%). However, this catalytic system was ineffective for indole and pyridine derivatives, with only moderate or low yields of the reduced products generated (19%−48%). 3.1.8. Hydrogenation of Carbon Dioxide. Use of CO2 as a renewable and economical C1 building block has received increasing attention from the scientific community in recent years.80,82,163−166 The catalytic transformation of CO2 with H2 is a useful pathway to afford a broad range of compounds and provides potential access to numerous essential organic chemicals. Research on cobalt-catalyzed homogeneous hydrogenation of CO2 (Scheme 53) has experienced consistent growth since 2012.

Scheme 52. Cobalt-Catalyzed Homogeneous Hydrogenation of Heterocycles

Scheme 53. Cobalt-Catalyzed Homogeneous Hydrogenation of CO2 to Formate

Then in 2015, Jones and co-workers described the cobaltcatalyzed acceptorless dehydrogenation and hydrogenation reactions of N-heterocycles (Scheme 52).162 In this study, Co2 (5−10 mol %) was used for the hydrogenation of quinolone and its related heterocycles under 10−20 atm of H2 at 120 °C in THF. Three quinoline derivatives bearing phenyl or methyl substituents were successfully reduced in quantitative yields under 10 atm of H2 in the presence of 5 mol % of Co-2 for 48 h, whereas only 11% of isoquinoline was converted to 1,2,3,4tetrahydroisoquinoline (13c) under the same reaction conditions. Further increasing the H2 pressure to 20 atm as well as the catalyst loading to 10 mol % gave a higher yield of 13c of 53%. However, Co-2 was unable to hydrogenate 2,6-lutidine even at an elevated temperature of above 120 °C. A more general protocol for the chemoselective hydrogenation of heterocycles was reported by Beller’s group159 in 2017 (Scheme 52). The combination of a tetradentate phosphine [tris(2-(diphenylphosphino)phenyl)phosphine)] ligand with Co(BF4)2·6H2O was employed as an effective catalytic system (denoted as Co-135) for hydrogenation of quinolines and related heterocycles. After a systematic evaluation of the reaction conditions, Co-135 (3 mol %) was found to be able to effectively hydrogenate quinolone under 10 bar of H2 at 60 °C in THF. Further investigation of the substrate scope using the optimized reaction conditions revealed that Co135 tolerated various electron-donating and -withdrawing groups and reduced a number of substituted quinolines (>20 examples) in good-to-excellent yields (78%−98%). More challenging substrates such as 8-aminoquinoline, 4-methylquinoline, and some sterically hindered quinolines were also hydrogenated in good-to-excellent yields when higher catalyst loadings and reaction temperatures were used. The natural product (±)-galipinine bearing a quinoline functionality was successfully prepared by this protocol. Surprisingly, this catalytic system also tolerated various reducible groups such as ester, carboxylic acid, cyclic imide, alkene, alkyne, and even aldehyde moieties, as evidenced by the successful hydrogenation of a variety of quinoline substrates bearing such reducible groups

Although an early work by Jessop and co-workers167 in 2003 referred to the use of Co(OAc)2 and phosphine ligands in combinatorial catalyst screening for the hydrogenation of CO2, the real endeavor in this field was initiated by Beller’s group in 2012.168 They reported the first well-defined cobalt dihydride catalyst [Co(H2)PP3]+ BPh4− [PP3 = P(CH2CH2PPh2)3] Co136, which was generated in situ from P(CH2CH2PPh2)3 and Co(BF4)2·6H2O, for the reduction of CO2 to formate and formamide (Scheme 53). The catalytic system Co-136 gave similar results to the well-defined complex [Co(H2)PP3]+ BPh4− for the preparation of formates from CO2 (or bicarbonate) with a good TON of up to 3877 and 71% yield under 60 bar of CO2/ H2 at 120 °C in a sodium bicarbonate solution containing NEt3 as the base. This catalytic system was also active under a comparably low CO2/H2 pressure of 5 bar, making it one of the best nonprecious metal-based homogeneous catalysts at the time. Although the mechanism was not investigated in detail, a cobalt dihydride species was considered to be involved in the pivotal step of CO2 insertion. A computational study on this catalytic process was conducted by Chen and co-workers in X


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rate constant was found to be a combination of the rate constant of step A as well as the equilibrium constant of step C (Scheme 54). This was because the TOF appeared to be first order with respect to the CO2 pressure in the presence of the strong base Vkd, whereas it showed first-order dependence on the base concentration in the presence of weaker bases such as DBU. Ke and co-workers recently reinvestigated the catalytic mechanism of Co-137-catalyzed CO2 hydrogenation in a DFT study, the results of which suggested that both the reductive elimination mechanism and direct hydride transfer mechanism are possible pathways during step A (Scheme 54), and the formate anion itself can act as a base to abstract one proton of the cobalt dihydride complex to generate the cobalt monohydride complex.172 To further elucidate the relationship between kinetics and thermodynamics in hydrogenation of CO2, research based on this type of diphosphine cobalt catalytic system was conducted by Linehan and co-workers in 2017.173 In this investigation, a set of diphosphine ligands were used to prepare the corresponding cobalt monohydride complexes. By studying the free energy for H2 addition to the metal cationic complexes M(P2)2+ (ΔG°H2), the hydricity of the monohydride complexes M(P2)2H (ΔG°H−) and the pKa of metal dihydride complexes M(P2)2H2+, the correlation between the catalytic TOFs and the thermodynamic driving force of the rate-determining step of catalysis was established. The complex Co-137 bearing a dmpe ligand was found to be the most active catalyst of the series because of its most efficiently balanced linear free-energy correlations of each step in the catalytic cycle. On the basis of these prior studies on Co-137 and to avoid the use of the strong base Vkd, Linehan and co-workers recently developed the tetraphosphine ligand-supported cobalt(I) complex Co-140 for the hydrogenation of CO2 to formate under mild conditions using the weaker base 2-tert-butyl-1,1,3,3tetramethylguanidine (tBuTMG).174 The same catalytic mechanism as that of Co-137 was also proposed for this catalytic system. Because the cobalt(III) dihydride complex obtained from Co-140 displayed a lower pKa relative to that of the dihydride complex generated from Co-137 by 7 pKa units, tBuTMG (pKa BH+ = 24.3) was successfully used instead of Vkd (pKa BH+ = 33.4) for the catalytic hydrogenation of CO2 under 1.7 atm of H2/CO2 (1:1) at room temperature, obtaining a TOF of 150 h−1 and TON of 270. This result is comparable to that of Co-137-catalyzed CO2 hydrogenation (140 h−1) with DBU as the base under 20 atm of H2/CO2 (1:1) at room temperature. In 2016, Bernskoetter and co-workers used cobalt complex Co-139 featuring another type of pincer phosphine ligand (PNP) for the hydrogenation of CO2 to formate (Scheme 53).175 Co-139 achieved a quite high TON of 29000 (TOF = 5700 h−1) with DBU as the base and LiOTf as the Lewis acid. The Lewis acid was proposed to facilitate the dissociation of the formate anion from the metal center,176 and a marked decrease in catalytic activity was observed in the absence of LiOTf (TON = 460). Poisoning experiments using mercury and PMe3 confirmed the homogeneous catalytic nature of Co-139. Moreover, addition of one equivalent of the PNP ligand to the catalytic reaction resulted in an increased TON, suggesting that a PNP ligand stabilized cobalt catalyst rather than Co2(CO)8 performed as the catalytic active species. A hypothetical catalytic mechanism for this Co-139-catalyzed hydrogenation of CO2 was then proposed (Scheme 55). In this mechanism, initial coordination of dihydrogen to Co-139 forms the intermediate complex Co-145 with one coordinated dihydrogen, which could

2015, which revealed that a dihydride catalytic pathway was more favorable than a monohydride catalytic pathway.169 In 2013, Linehan’s group170 used the cobalt monohydride complex Co(dmpe)2H (dmpe = 1,2-bis(dimethylphosphino)ethane) Co-137 as a catalyst for the hydrogenation of CO2 to formate, affording a much higher TOF of 74000 h−1 (TON 9400) under 20 atm of CO2/H2 (1:1) at 21 °C with a low catalyst loading of 0.04 mM (Scheme 53). Notably, a TOF of 3400 h−1 (TON 2000) was still obtained when the H2 pressure was decreased to 1 atm in the presence of a higher catalyst loading of 0.28 mM. Stoichiometric amounts of Verkade’s base (Vkd) with a high pKa (BH+ 33.6) are required for this catalytic system to ensure the deprotonation of the dihydride species Co(dmpe)2(H)2 to regenerate the monohydride species during the catalytic cycle. The high cost of Vkd limits the practical application of this reaction system. Other weaker bases (pKa 18.8−28.4) such as Et3N, 1,8-diazabicyclo-[5.4.0]undec-7-ene (DBU), and the phosphazene base P1tBu also showed certain reactivity; however, TOF decreased from 830 to 0 h−1 as the basicity weakened. A subsequent mechanistic investigation of Co-137-catalyzed hydrogenation of CO2 was reported by the same group in 2014.171 In this study, three essential reaction steps as well as the pathways for catalyst deactivation were successfully established based on a set of experiments, including stoichiometric reactions, kinetics, and NMR spectroscopy (Scheme 54). The monohydride species Co-137 initially Scheme 54. Proposed Catalytic Cycle for CO2 Hydrogenation to Formate Using Co-137

transfers a hydride to CO2 to generate the formate product as well as a cobalt(I) cationic complex with empty coordination sites. This is the rate-determining step (step A) because Co-137 was the only cobalt species observed by operando NMR spectroscopy during the catalysis. The oxidative addition of H2 to Co-141 affords the dihydride species Co-142 (step B), which can then be deprotonated by a base to regenerate the monohydride species Co-137 (step C). Moreover, the cobalt(I) cationic complex Co-141 may undergo deactivation to form catalytically inactive species [(μ-dmpe)-(Co(dmpe)2)2]2+ Co143 and [Co(dmpe)2CO]+ Co-144 when the CO2/H2 ratio is greater than 1, as revealed by a series of stoichiometric reactions monitored by NMR spectroscopy. The reverse water−gas shift reaction is most likely the pathway to generate CO, which deactivates the catalytic species, because the formation of CO only occurred at high CO2/H2 ratios. Additionally, the overall Y


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Scheme 55. Proposed Mechanism for Co-139-Catalyzed CO2 Hydrogenation

displayed much lower catalytic activity, which was ascribed to the poor thermal stability of the cobalt complex. Nevertheless, this example provides us with an alternative to employing phosphine ligands for cobalt-catalyzed CO2 hydrogenation. A DFT study on aqueous-phase CO2 hydrogenation catalyzed by hydroxylated bipyridine ligand-supported half-sandwich Co, Ir, and Rh complexes was conducted by Zhao and co-workers in 2014.181 In accordance with their study, the different catalytic efficiencies displayed by these complexes could be ascribed to the back-donation ability of the different metals, which strongly influenced H2 heterolytic cleavage. Because the Co complex has a higher activation free energy than those of the Rh and Ir complexes, it shows the lowest catalytic efficiency of the complexes. Converting CO2 into other useful chemicals such as methanol and dimethoxymethane (DMM) under hydrogenation conditions (Scheme 57) is an even more challenging and less-

then be deprotonated by DBU to afford the cobalt monohydride complex Co-146. Monohydride complex Co-146 could reversibly convert to the cobalt complex Co-147 with one carbonyl by loss of one molecule of CO or directly react with CO2 to generate the formate-coordinated cobalt intermediate complex Co-148 via insertion pathways. Finally, dissociation of the formate anion regenerates Co-139 for another catalytic cycle. To further elucidate the potential benefit brought by the bifunctional ligands on this cobalt-catalyzed hydrogenation of CO2, two analogues of Co-139, namely Co-149 and Co-150, were used in a comparative study (Scheme 56).177 The complex

Scheme 57. Synthesis of MeOH and Dimethoxymethane from CO2 by Cobalt Catalysis

Scheme 56. Cobalt-Catalyzed Homogeneous Hydrogenation of CO2 to Formate developed transformation than that to formate.182,183 The Co(BF4)2/Triphos [(Triphos = 1,1,1-tris(diphenylphosphinomethyl)ethane)] catalytic system, which de Bruin and co-workers152 successfully applied to the hydrogenation of esters as well as carboxylic acids, was also found to be active for the hydrogenation of CO2 to methanol (TON = 3) by Beller and co-workers in 2017 (Scheme 57).184 Further optimization of this transformation revealed that the catalytic system Co-151 ([Co(acac)3]/Triphos/HNTf2 (Tf = trifluoromethanesulfonyl)) exhibited the highest reaction TON of 78 under 70/20 bar of H2/CO2 at 100 °C in a mixture of THF and EtOH. In combination with a kinetic study, in situ highpressure NMR spectroscopy, and high-resolution ESI-MS, the authors proposed a reaction pathway for the generation of the catalytically active species, as shown in Scheme 58. Initially, formation of the [Co(acac)2(Triphos)]+ species Co-153 occurs by coordination of Triphos to Co(acac)3. Next, gradual removal of the remaining acac ligands generates the active cobalt Triphos

Co-149 with a cyclohexyl-substituted PNP ligand displayed similar catalytic performance to that of Co-139 (TON of 29000) with a TON of 24000, whereas a much lower TON of 450 was observed using the N−H analogue complex Co-150. These results indicate that the bifunctional ligand present in Co-139 exerted a negative effect on the catalytic hydrogenation of CO2 to formate, which was inconsistent with the relative activity previously observed for bifunctional/nonbifunctional iron analogue catalysts in the same reaction.176 This behavior is opposite to that of the hydrogenation of carbonyl compounds, in which catalysts with a N−H functionality usually show superior activity to that of their N−Me analogues.74 Instead of phosphine-based cobalt complexes, Fujita and coworkers178 used dihydroxyl-substituted bipyridine ligandsupported cobalt complex Co-138 for CO2 hydrogenation under aqueous conditions in 2013. Co-138 was able to catalyze aqueous-phase CO2 hydrogenation under 40−50 bar of CO2/ H2 (1:1) at 100 °C with a TOF of 39 h−1 (TON = 39). Compared with its iridium counterparts,179,180 Co-138

Scheme 58. Proposed Pathway for the Generation of the Catalytically Active Species from [Co(acac)3] and Triphos

Z


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cationic species [Co(Triphos)(L)n]n+ (L = ligand) Co-155, which is stabilized by NTf2. Additional experiments involving removal of acac ligands prior to the catalytic reactions were also conducted, which gave increased reaction TONs of 8 and 12 compared with that of the standard catalytic reaction (TON = 0.9) after 5.5 h. These results suggested that an inner-sphere mechanism might be involved in this Co-151-catalyzed hydrogenation of CO2 to MeOH. Using a similar catalytic system to Co-151, Klankermayer and Schieweck achieved the selective transformation of CO2 to dialkoxymethane ethers under hydrogenation conditions (Scheme 57).185 The combination of Co(BF4)2·6H2O, Triphos, and HNTf2 (Co-152) provided a catalytic system that was able to catalyze the transformation of CO2 to DMM under 60/20 bar of H2/CO2 at 100 °C in a mixture of THF and MeOH with a TON of 92. This catalytic performance was comparable to that of a reported Ru catalytic system (TON = 71−214).186 Furthermore, catalytic system Co-152 was extended to other alcohol substrates such as ethanol, n-butanol, t-butanol, and isopropanol to prepare corresponding dialkoxymethane ethers with TONs ranging from 16 to 109. Preliminary results indicated the variation of the Triphos ligand sphere could possibly enhance the activity of this cobalt Triphos catalytic system in the transformation of CO2 to DMM. N-Formylation of amines using a mixture of CO2 and H2 as a formylating agent is an attractive atom-economical route in organic synthesis and can be viewed as an extension of CO2 reduction.183 To date, only two examples of cobalt-catalyzed homogeneous N-formylation of amine with CO2 and H2 have been reported (Scheme 59).187,188 In 2017, Milstein’s group188

yields (90%−99%), whereas less-nucleophilic anilines (e.g., 3,4dimethylaniline) were inactive under the same reaction conditions. Notably, DMF could be prepared in 99% yield using this method with dimethylamine as substrate. Moreover, the following plausible mechanism was postulated based on the results of stoichiometric reactions, IR spectroscopy, and X-ray crystallography (Scheme 60). Co-156 is first reduced by one Scheme 60. Proposed Mechanism for the Co-156-Catalyzed N-Formylation of Amines with CO2 and H2

Scheme 59. Cobalt-Catalyzed N-Formylation of Amines with CO2 and H2

equivalent NaHBEt3 to generate Co(I) complex Co-158, which can then react with KOtBu to form the intermediate complex Co-159 with empty coordination sites. Under H2, Co-159 further generates the Co(I) monohydride complex Co-160, which allows insertion of CO2 into the cobalt−hydride bond to prepare the η1-formato complex Co-161. Once Co-161 is formed, the presence of an excess amount of amine may lead to the formation of formate salt, which then loses one molecule of H2O to afford the formamide product and regenerate the active species Co-159. It should be noted that addition of 4 Å molecular sieves to the catalytic reactions increased their TONs. Another example of cobalt-catalyzed N-formylation was reported by Jessop and co-workers.187 In their study, various nonprecious metal complexes generated in situ such as FeCl2/ dmpe, NiCl2/dmpe, and Co(OAc)2·4H2O/dmpe were tested for the N-formylation of amines with CO2 and H2. Although the Co(OAc)2·4H2O/dmpe catalytic system Co-157 was not the most active catalyst, it still achieved a TON of 40 under 60/40 bar of H2/CO2 at 135 °C for the N-formylation of 2ethylhexylamine. In addition to the experimental results described above, Yang’s group also reported a series of computational studies on the cobalt-catalyzed homogeneous hydrogenation of CO2 include using cyclopentadienone-, PNP pincer ligand-, and pendant amine-containing cobalt catalytic systems.189−192

screened a number of pincer phosphine ligand-supported Co(II) complexes for amine formylation with CO2 and H2 in the presence of KOtBu and NaHBEt3. The PNP complex Co156 was found to be the most active catalyst for the Nformylation of a set of primary and secondary amines (14 examples, yields of 60%−99%) under 30/30 bar of H2/CO2 at 150 °C and 5 mol % catalyst loading. Cyclic secondary amines were effectively converted to the desired products in excellent

3.2. Transfer Hydrogenation Reactions

As an alternative to direct hydrogenation with H2, transfer hydrogenation (TH) offers a feasible and powerful tool to access numerous hydrogenated compounds without using highpressure H2 gas and generally only requires simple operational setup.1,16,34,36,80,193−197 Although cobalt-catalyzed homogeneAA


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ous TH has made some progress since 2013,198 it is still in its infancy compared with precious metal-catalyzed TH reactions. 3.2.1. Transfer Hydrogenation of Alkenes and Alkynes. Early work by Chung in 1979 described the use of CoBr2 and NaBH4 for the reduction of mono- and disubstituted alkenes.199 However, this protocol, which required one equivalent of CoBr2 and an excess of NaBH4, could hardly be regarded as an efficient catalytic process. In 2016, RajanBabu’s group developed an experimentally feasible method for the selective reduction of monosubstituted terminal alkenes without the use of an oxidant like TBHP (Scheme 61).200 In this

Scheme 62. Co-162-Catalyzed Transfer Hydrogenation of Alkynes with (EtO)2SiMeH

observed. In this context, the authors proposed the possible mechanism of this silane-mediated reduction process depicted in Scheme 63. Initially, Co-162 is activated by two equivalents of

Scheme 61. Cobalt-Catalyzed Transfer Hydrogenation of Alkenes

Scheme 63. Proposed Mechanism of Co-162-Catalyzed Transfer Hydrogenation of Alkenes with (EtO)2SiMeH

NaBHEt3 to form the Co(I) hydride intermediate Co-165. The alkene substrate then coordinates to the cobalt center to give Co-166, followed by insertion of the alkene into the Co−H bond, thus generating cobalt alkyl complex Co-167. Subsequently, Co-167 reacts with the silane to produce the reduced product and the silica cobalt intermediate Co-168, which could regenerate the Co(I) hydride intermediate Co-165 for use in the next run. In 2015, Waterman and co-workers201 reported that the sandwich cobalt complexes Co-163 and Co-164, which can catalyze the dehydrogenation of ammonia borane, were active in TH of alkenes with NH3BH3 as a hydrogen donor (Scheme 61). With these two complexes, alkenes such as styrene, 2vinylpyridine, t-butylethylene, phenylacetylene, cis-cyclooctene, and diphenylacetylene could be reduced to the corresponding hydrogenated products in almost quantitative yields. A control reaction under 1 atm of H2 supported the TH mechanism instead of a hydrogenation process with the ambient H2 generated from dehydrocoupling of NH3BH3. The use of isopropanol as a hydrogen donor is advantageous because of its ready availability. In 2016, Zhang and co-workers extended the application of the well-established hydrogenation catalyst Co-2 in TH using isopropanol as a hydrogen donor (Scheme 61).202 Using Co-2 (2 mol %), a series of alkenes including styrene and its derivatives as well as aliphatic alkenes (e.g., 1-octene, cyclooctene, and norbornylene) were success-

protocol, the 2,6-bis(arylimino)pyridine cobalt complex Co162 activated by two equivalents of NaBHEt3 was employed as the catalyst (0.001−0.05 equiv) and (EtO)2SiMeH (1.1 equiv) was used as the hydrogen donor to reduce the alkene substrates at room temperature in toluene (Scheme 61). A number of vinyl arenes and linear aliphatic alkenes bearing bromide, amino, hydroxyl, carbonyl, and epoxy functionalities were successfully reduced to the corresponding saturated products with excellent yields (89%−99%). Moreover, high selectivity for the terminal double bond was observed during the reduction of 1,3-dienes, 1,4-dienes, and silyloxydienes. This protocol was also applied to the reduction of terminal alkynes to either the half-reduced alkene products or the completely reduced alkane products, depending on the stoichiometry of (EtO)2SiMeH used (Scheme 62). Notably, a stepwise reduction of alkyne could be realized by controlling the amount of the silane employed, because one equivalent of silane only produced the intermediate alkene product. Further mechanistic investigation including stoichiometric reactions and NMR experiments indicated that the silane was the only source of hydrogen. Besides, the HAT reaction pathway was precluded from the reduction of 3-arylbutenes, in which no rearrangement to more stable conjugated products was AB


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alkynes using three cobalt dichloride complexes Co-169−Co171 bearing different phosphine pincer ligands. Both Z- and Ealkenes were selectively prepared using a designated cobalt complex (1−2 mol %) with ammonia borane (1−1.5 equiv) as the hydrogen source under mild conditions (MeOH, 50 °C, 16− 20 h). Co-169 featuring two bulky tert-butyl substituents on the phosphine ligand displayed quite high reactivity in Z-selective semireduction of numerous alkynes with different steric and electronic natures. Twenty alkynes (mostly unsymmetrical) bearing different functionalities, such as methoxyl, fluoro, chloro, trifluoromethyl, cyano, ester, amino, hydroxyl, and heterocyclic groups, were successfully reduced to the corresponding Z-alkenes in good isolated yields (72%−99%) with high Z/E selectivity (8:1 to >99:1). The more challenging Eselective semireduction of alkynes was realized using Co-170 and Co-171 featuring less bulky phosphine ligands than that of Co-169. Most of the alkyne substrates tested with Co-169 were also successfully reduced to the respective E-alkene products by Co-171 in excellent yields (90%−98%) with high E/Z selectivity (99:1). Co-170 showed good reactivity for phenyltrimethylsilyl acetylene (3aj), providing a 90% yield of the E-alkene product. Notably, a high TON of 460 was achieved in a gram-scale synthesis of trans-stilbene using 0.2 mol % of Co-171. Furthermore, Co-170 was found to be more effective than the other two cobalt complexes in semireduction of terminal alkynes with the target terminal alkenes (six examples) prepared in good yields (62%−89%). In addition, kinetic studies and control experiments were conducted to investigate the reaction mechanism. The kinetic profile of the E-selective semireduction of 2-tolyl-phenylacetylene by Co-171 suggested that a Z- to Eisomerization was involved in this catalytic process. Further isomerization of cis-stilbene with Co-171 provided transstilbene quantitatively in the presence of a catalytic amount of ammonia borane, thus confirming the hypothesis of a Co-171catalyzed Z- to E- isomerization. To identify the hydrogen source of the reduced product, isotopic labeling experiments were conducted (Scheme 65). Using CD3OH or CD3OD as the

fully reduced to the corresponding saturated products under the standard TH reaction conditions (isopropanol/THF, 24 h, 100 °C) in excellent yields (11 examples, 92%−99%), whereas cisand trans-stilbenes displayed only moderate reactivity (yields of 24% and 35%, respectively). Moreover, functionalities such as carboxyl (Scheme 61, 4ae) and ester groups were tolerated in this protocol, and the carbonyl group of 5-hexen-2-one was reduced to a hydroxyl group. The alkyne substrate diphenylacetylene was also tested in this protocol; however, no reduction of the carbon−carbon triple bond was observed. Apart from TH of alkenes,200 the groups of Liu and Balaraman also reported the successful application of cobalt complexes to the TH of alkynes (Scheme 64).63,203 In 2016, Liu and coworkers63 reported the first cobalt-catalyzed selective TH of Scheme 64. Highly Selective Transfer Hydrogenation of Alkynes Catalyzed by Co-169−Co-172 Using NH3BH3 as a Hydrogen Donor

Scheme 65. Isomerization of Alkenes and Deuterium Labeling Experiments Using Co-169, Co-170, and Co-171

solvent revealed a methanol-mediated protonation of the alkenyl cobalt intermediate, supporting a monohydride mechanism. Moreover, the TH of deuterated phenylacetylene was found to give the deuterated product with 19% deuterium incorporation at the C2 position, suggesting that the alkynyl-type and vinylidene-type mechanisms are also potential minor reaction AC


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pathways for the reduction of terminal alkynes.204,205 A series of dehydrogenation experiments was also conducted to confirm that ammonia borane can undergo cobalt-catalyzed dehydrogenation to generate B(OMe)3 and H2 gas. Finally, a plausible reaction mechanism for this TH of alkynes was proposed based on the experimental observations (Scheme 66). It was postulated that the catalytically active cobalt hydride

alkyne substrate over the cis-alkene intermediate inhibited Z/E alkene isomerization when using complex Co-169 bearing bulky tert-butyl substituents on the phosphine ligand. Using ammonia borane as a hydrogen source, Balaraman’s group203 achieved the TH of alkynes to Z-alkenes catalyzed by the phosphine-free cobalt pincer complex Co-172 (Scheme 64). Moreover, terminal alkynes and alkenes could also be reduced to the corresponding alkene (six examples, 78%−99%) and alkane (eight examples, 81%−99%) products, respectively, in good yields. 3.2.2. Transfer Hydrogenation of Aldehydes, Ketones, and Imines. In 1997, Lemaire’s group142 reported the first cobalt-catalyzed TH of acetophenone by the chiral diamine ligand-supported dichloride complex Co-180 in isopropanol (Scheme 67). However, this catalytic process only produced the desired product with a moderate ee and low conversion of 8% after 6 days of reaction.

Scheme 66. Catalytic Mechanism of Selective Transfer Hydrogenation of Alkynes Catalyzed by Cobalt Complexes Co-169−Co-171

Scheme 67. Transfer Hydrogenation of Ketones and Imines Catalyzed by Co-180 and Co-2 with Isopropanol

species Co-173 is initially generated by reduction of the cobalt dichloride complexes (Co-169-Co-171) with ammonia borane.58,155,206 With respect to catalytic cycle I in Scheme 66, an alkyne substrate would initially coordinate to the cobalt hydride complex Co-173, followed by insertion of the alkyne into the Co−H bond, thus forming alkenyl cobalt intermediate complex Co-175. The Z-alkene intermediate and methoxyl cobalt complex Co-176 are subsequently produced via the methanolmediated protonation of the Co−C bond of alkenyl cobalt complex Co-175. The methoxyl cobalt complex Co-176 would then react with ammonia borane to regenerate the active cobalt hydride species Co-173 along with the formation of B(OMe)3. In catalytic cycle II in Scheme 66, a Z- to E-isomerization would occur through the initial insertion of the Z-alkene into the Co− H bond to afford the cobalt alkyl complex Co-178, which could undergo a subsequent β-hydride elimination to eventually produce the more thermodynamically stable E-alkene product. β-Hydride elimination of the alkyl cobalt complex needs to be much faster than its protonation by methanol to achieve such high chemoselectivity for the E-selective semireduction of alkyne. The same group together with that of Lan recently conducted a detailed mechanistic study on this reaction.207 Considering DFT calculation and experimental results revealed that catalyst deactivation and the much higher reactivity of the

More recent progress was made by Hanson and Zhang in the TH of polar multiple bonds including aldehydes, ketones, and imines using Co-2 with isopropanol as the hydrogen source (Scheme 67).198 A number of ketones and aldehydes were successfully reduced by 2 mol % of Co-2 in excellent yields (13 examples, 92%−99%) at room temperature or 80 °C in a mixture of THF and isopropanol. Both aromatic and aliphatic ketones were compatible with this protocol, but only aromatic aldehydes were evaluated in this research. It should be noted that both the CC and CO bonds of the conjugated substrates cinnamaldehyde and trans-4-phenyl-3-buten-2-one AD


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were reduced. Moreover, three aldimines were also successfully transformed into respective amines in high yields (95%−98%) at an elevated temperature of 80 °C. This is the only example of cobalt-catalyzed homogeneous TH of imines achieved so far. Additionally, deuterium labeling experiments using isopropanold8 and isopropanol-OD as the reaction media were conducted to gain further insights into the reaction mechanism (Scheme 68).

Scheme 69. Transfer Hydrogenation of Nitriles Catalyzed by Co-171 and Co-181−Co-183

Scheme 68. Deuterium Labeling Experiments Conducted to Investigate the Transfer Hydrogenation of Acetophenone by Co-2

A monohydride pathway was confirmed because no scrambling of the deuterium into both the O−H and C−H positions was observed.195 Finally, by comparison with the analogous N−Mesubstituted complex Co-66 for the TH of acetophenone, it was found that MLC was not essential for the reduction process. Ke’s group208 conducted a computational study on this PNP-cobalt catalytic system for TH of ketones in 2015. They revealed that this reaction tends to follow an inner-sphere nonbifunctional reaction pathway, consistent with the experimental observations, because of the high deformation energy of the catalyst caused by the change of the ligand field during the bifunctional process. 3.2.3. Transfer Hydrogenation of Nitriles and Heterocycles. The only example of cobalt-catalyzed homogeneous TH of nitriles was reported by Liu and co-workers64 in 2016 (Scheme 69). In their study, a chemodivergent synthesis of primary and secondary amines was realized by using different solvents and pincer cobalt catalysts with ammonia borane as a hydrogen source under mild conditions. A series of nitriles (26 examples) bearing different functionalities such as fluoro, chloro, bromo, iodo, methoxyl, trifluoromethyl, ester, and nitrogen- and sulfur-containing heterocycles were successfully reduced to the corresponding primary amine products in good yields (71%−96% for most cases) using Co-181 as the catalyst (1 mol %) at 50 °C in hexane. Several dinitriles were also transformed into industrially interesting diamines by this protocol. Using complex Co-171 (0.5 mol %) as the catalyst and hexafluoroisopropanol (HFIP) as the solvent, a number of nitrile substrates with different steric and electronic natures were selectively converted into respective secondary amines in high yields (20 examples, 76%−95% for most cases) at room temperature. By employing this protocol, two alkyl nitriles were also successfully reduced to their respective secondary aliphatic amines, albeit in only moderate yields (31%−52%). In addition, mono-N-alkylation of a variety of aromatic and aliphatic primary amines with nitriles was also realized in high yields (21 examples, 73%−98% for most cases) using 0.5 mol % of Co-171 in HFIP at room temperature. The difficult transformation of secondary amine substrates into tertiary amine products was also achieved in good yields (three examples, 55%−89%). Four pharmaceutical molecules featuring other reducible functional groups were successfully functionalized by this cobalt-catalyzed reductive amination of nitriles,

and one asymmetric tertiary amine was prepared via sequential TH of three aromatic nitriles using cobalt catalysts Co-171 and Co-181. Therefore, this methodology represents a complementary approach to conventional reductive amination reactions using aldehydes as the alkylation reagent. To further probe the reaction mechanism, the N-methylsubstituted analogue complexes (Co-182 and Co-183 are analogues of Co-171 and Co-181, respectively) were also prepared and used for the TH of benzonitrile (Scheme 69). The comparable catalytic results achieved for the N-methylsubstituted cobalt catalysts suggested an inner-sphere mechanism of this cobalt catalyzed TH of nitriles instead of MLC. The first example of cobalt-catalyzed homogeneous TH of heterocycles was realized by Beller’s group in 2017 (Scheme 70).209 The catalytic system Co-135, which was initially reported by the same group for the hydrogenation of Nheterocycles,159 was found to be capable of catalyzing TH of heterocycles using formic acid as the hydrogen donor under base-free conditions at 80 °C in isopropanol. A number of 6- and 8-substitued quinoline substrates (16 examples) bearing electron-donating and -withdrawing substituents were reduced AE


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Scheme 70. Transfer Hydrogenation of Heterocycles Catalyzed by Co-135 Using Formic Acid as a Hydrogen Donor

Scheme 71. Co-2-Catalyzed Acceptorless Dehydrogenation of Alcohols

in good-to-excellent yields (76%−98%) using ten equivalents of formic acid and 2.5−8 mol % of Co-135. Moreover, reducible functionalities such as carboxylic acid, ester, and alkene groups were found to be tolerated by this protocol, thus providing a potential method to prepare further functionalized tetrahydroquinolines. Additionally, more challenging quinaldines could also be converted into the corresponding reduced products in high yields (three examples, 78%−90%) using 5 mol % of Co135 at an elevated temperature of 100 °C. This example is also the first base metal catalyzed homogeneous TH of heterocycles. 3.3. Dehydrogenative Transformations

ligand, N-Me-substituted cobalt complex Co-66 was synthesized. The catalytic activity of Co-66 was compared with that of Co-2. The yield for the dehydrogenation of 1-phenylethanol catalyzed by Co-66 was 95%, which was similar to that of Co-2. This result suggests that MLC is not crucial in the alcohol dehydrogenation reaction. The reaction of 1-phenylethanol with Co-2 led to the formation of new diamagnetic cobalt complex Co-40, which was detected by IR and NMR spectroscopies and elemental analysis (also see Scheme 14).74 In addition, the isolated complex Co-40 displayed high activity for the acceptorless dehydrogenation of 1-phenylethanol, indicating that Co-40 is a catalyst resting state in the dehydrogenation reaction. On the basis of the experimental observations, the authors suggested a possible mechanism involving a cobalt(I)/ (III) catalytic cycle. Starting from complex Co-40, reductive elimination of acetophenone provides the cobalt(I) species Co184, which is transformed into cobalt(III) species Co-185 through sequential ligand exchange and oxidative addition of the O−H bond of the alcohol. Subsequent β-hydride elimination of Co-185 generates acetophenone and cobalt(III) dihydride species Co-186. The liberation of H2 and the coordination of acetophenone regenerates complex Co-184 and completes the catalytic cycle. DFT calculations by Yang’s group suggested that oxidative addition of the O−H bond is the rate-determining step of this catalytic cycle.213 Considering that the tetradentate tripodal ligand may stabilize the reactive intermediates and display different catalytic reactivity compared with that of tridentate pincer ligand systems, Ding’s group designed a tetradentate tripodal ligand, which is featured with an N−H linker connecting a phosphino binding moiety and pyridine ring. Its cobalt complex Co-15

Catalytic dehydrogenation is a powerful synthetic protocol, which has broad applications in fine chemicals, fuels, and pharmaceutical industries.29,36,210 The important utility of this transformation is particularly illustrated in two aspects: (1) construction of other functional group or complex molecules by dehydrogenation and tandem condensation or coupling reactions18,81 and (2) generation of valuable hydrogen gas from potentially renewable resouces.211,212 This section focuses on the reactions that involve dehydrogenative steps and cobalthydride intermediate, for example, the acceptorless dehydrogenation of alcohols/amines and the related tandem reactions by cobalt catalysis. 3.3.1. Dehydrogenation Reactions. Acceptorless catalytic dehydrogenation of organic molecules is the formal reverse process of hydrogenation, generating oxidized species with hydrogen liberation.18 Because of the microscopic reversibility principle, the catalysts used for hydrogenation reactions can also potentially induce dehydrogenation processes. For example, the pincer catalyst [(PNHPCy)Co(CH2SiMe3)]BArF4 Co-2, which is an effective hydrogenation catalyst, is also capable of promoting the dehydrogenation of alcohols and amines. In 2013, Hanson and Zhang used this cobalt catalyst for the acceptorless dehydrogenation of secondary alcohols.73 As shown in Scheme 71, both secondary benzylic and other aliphatic alcohols smoothly reacted to afford the corresponding ketone products in good yields. However, Co-2 was less effective for the dehydrogenation of primary alcohols, such as 4methoxybenzyl alcohol 16c, producing the dehydrogenated aldehyde product in low yield. To investigate the possible influence of MLC originating from the N−H group on the AF


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([(iPrPPPNHPyMe)CoCl]Cl) was synthesized and found to be effective for the dehydrogenation of secondary alcohols (Scheme 72).65 A mechanistic study suggested that MLC originating from the N−H linker does not play a critical role in this reaction system.

Scheme 74. Co-3-Catalyzed Dehydrogenation of Amine Borane

Scheme 72. Co-15-Catalyzed Acceptorless Dehydrogenation of Alcohols

Remarkably, the cobalt complex Co-2 can also catalyze the dehydrogenation of N-heterocycles, including tetrahydroquinolines, 2-methylindoline, and 1,2,3,4-tetrahydroquinoxaline (Scheme 73).162 Because valuable H2 gas is released during Scheme 73. Co-2-Catalyzed Acceptorless Dehydrogenation of N-Heterocycles

followed by coordination of an amine borane to afford species Co-188. Then, the N−H proton of Co-188 transfers to the cobalt center and occupies a bridging position between Co and B atoms to form intermediate Co-189, which can be seen as an LCo(H2) complex coordinated by NMe2=BH2. Thus, complex Co-189 can dissociate into LCo(H2) species Co-192. For pathway (2), the dehydrogenation is initiated through association of amine borane with Co-3 prior to N2 dissociation. Next, a concerted proton and hydride shift process can occur to form species Co-191, which undergoes N2 dissociation to give species Co-192. Subsequently, Co-192 can bind another NHMe2BH3 to produce intermediate Co-193, which can provide the dihydrogenated cobalt species Co-194 through a concerted proton and hydride transfer process. Transformation of complex Co-194 to Co-195 via the formation of intermediate Co-34 is a facile pathway, and self-dehydrogenation of complex Co-195 reforms Co-192 and releases H2 gas. Alternatively, the self-dehydrogenation of species Co-194 can regenerate intermediate Co-192 with liberation of H2. 3.3.2. Dehydrogenative Condensation. Another important utility of alcohol dehydrogenation by cobalt catalysis is based on the higher reactivity of carbonyl products (ketones or aldehydes) versus the starting alcohols.29,36 This allows for a tandem reaction pathway to form imines via condensation with amines, or to afford various N-heterocyclic aromatic products through aromatization process. Notably, dehydrogenative condensation is an environmentally friendly and atomeconomical transformation because the only byproducts are H2 and H2O. The well-defined cobalt complex [(PNHP C y )Co(CH2SiMe3)]BArF4 Co-2 can also catalyze dehydrogenative condensation of amines with alcohols to produce imines along with H2 and water liberation.73 Various aliphatic and benzylic

this reaction, dehydrogenation of N-heterocycles is applicable to the field of H2 storage as a liquid organic hydrogen carrier.214 Under 150 °C, several N-heterocycle substrates, except for 2,6dimethylpiperidine (13n), were converted into the corresponding dehydrogenated aromatic products in good yields along with production of H2. Preliminary mechanistic studies suggested that an MLC effect might be involved in this transformation. Because of the potential application of amine boranes as hydrogen storage/transfer materials, the dehydrogenation process of amine boranes by transition-metal catalysts has been extensively studied.215,216 In 2013, Peters and co-workers studied the dehydrocoupling reaction of amine boranes (AB) catalyzed by a cobalt(I)-N2 complex (Scheme 74a).47 The reaction of amine-borane HMe2N-BH3 with 2 mol % of cobalt(I)-N2 complex Co-3 in C6D6 under N2 initially resulted in the linear dehydrogenation product HMe2N-BH2-Me2NBH3, which was fully converted into (Me2N-BH2)2 with the evolution of H2 gas within 6 h. Paul’s group conducted a DFT calculation to elucidate the mechanism of LCo(N2)-catalyzed dehydrogenation of amine boranes (Scheme 74b).115 Their study revealed that this process involved the active catalytic species LCo(H2) Co-192, which could be generated through two distinct pathways starting from the cobalt(I)−N2 complex Co-3: (1) a N2 dissociative process and (2) an amine borane associative process. For pathway (1), the reaction is initiated by the N2 dissociation reaction of the cobalt(I)−N2 complex Co-3 AG


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alcohols react with primary amines under dehydrogenative condensation conditions to deliver a number of imines in moderate-to-good yields (Scheme 75). This catalytic reaction

Scheme 76. Co-9-Catalyzed Acceptorless Dehydrogenative Condensation of Alcohols and Amines


has been proposed to proceed via an initial alcohol dehydrogenation step, affording a ketone or aldehyde intermediate, and subsequent condensation with the amine to produce the imine product. Milstein and co-workers developed the first synthesis of Nheterocyclic aromatics via cobalt-catalyzed dehydrogenative condensation (Scheme 76).59,217 The reaction of 1,4-diols with primary amines catalyzed by complex Co-9 in the presence of NaHBEt3, KOtBu, and 4 Å molecular sieves as additives gave a series of 1,2,5-substituted pyrroles.59 Preliminary mechanistic studies suggested that the cobalt(I) chloride pincer complex Co196, which could be generated by the treatment of complex Co9 with one equivalent of NaHBEt3, was involved in the catalytic process. Co-196 displayed similar catalytic activity to the precatalyst Co-9 in dehydrogenative condensation reactions without including NaHBEt3 as an activator. However, they could not isolate the active Co(I) complex by treating Co-196 with one equivalent of KOtBu. In addition, attempts to isolate an in situ formed intermediate also failed. Nevertheless, they believe that a monodeprotonated Co(I) complex is formed under the reaction conditions by dehydrohalogention via either the methylene or the N−H proton of the N-arm of the ligand. The possibly generated unsaturated Co(I) species could potentially be stabilized by binding with alcohol furnishing an alkoxy-Co(I) intermediate, which could undergo dehydrogenation to give the diketone, liberating H2. Subsequently, the formed diketone could couple with the present amine, generating the N-substituted pyrrole and water through a Paal-Knorr condensation process. Furthermore, various 2substituted benzimidazole derivatives were readily generated from primary alcohols and 1,2-diaminobenzene derivatives

under base-free conditions.217 A hypothetical mechanism for this Co-catalyzed dehydrogenative condensation was then proposed. Initially, formation of aldehyde occurs by dehydrogenation of the primary alcohol. Next, a condensation reaction of the generated aldehyde with amine forms an imine intermediate, which undergoes cyclization and dehydrogenation process to deliver the corresponding benzimidazole derivatives. Although phosphine ligands play an essential role in homogeneous catalysis, their preparation is often tedious, requiring multistep synthesis and handling under inert atmosphere.218,219 To overcome these limitations, Balaraman and co-workers recently developed the sulfur-based SNScobalt(II) pincer complex Co-197, which promotes the dehydrogenative condensation/annulation of unprotected amino alcohols with secondary alcohols under phosphine ligand-free conditions.220 As shown in Scheme 77, various asymmetric 2,5-disubstituted 1H-pyrroles could be prepared in moderate-to-good yields from different β-amino alcohols and secondary alcohols. Later, they further expanded the substrate scope from β-amino alcohols to γ-amino alcohols and 2AH


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catalyst used for the monoalkylation of aromatic amines with primary alcohols.62 In addition, diverse unsymmetrically alkylated diamines could also be accessed through two alkylation steps of diamines with different primary alcohols (Scheme 79a).


Scheme 79. Co-198 and Co-2-Catalyzed N-Alkylation of Amines with Alcohols

aminobenzyl alcohols, forming diverse 2-substituted pyridines and quinoline products. 3.4. Hydrogen-Borrowing Reactions

The hydrogen-borrowing reaction (also called hydrogen autotransfer) has become a useful synthetic tool. This reaction combines the TH process with other intermediate reactions to construct more complex molecules, obviating the need to introduce additional H2 gas.29,197 Generally, this transformation involves three steps: (1) catalytic dehydrogenation, (2) intermediate bond formation, and (3) catalytic hydrogenation. As shown in Scheme 78, the reaction usually starts with a Scheme 78. General Reaction Pathway for HydrogenBorrowing Reactions

Shortly after, Zheng/Zhang and co-workers reported that the cobalt catalyst Co-2 could be used for the monoalkylation of both aliphatic and aromatic amines with primary alcohols.221 In this work, no base additive was required, and the addition of 4 Å molecular sieves was crucial for amine production to avoid the generation of imine products. Moreover, cyclohexanol, a secondary alcohol substrate, reacted with aniline and afforded N-cyclohexylaniline in a moderate yield under the same conditions (Scheme 79b). Later, Kirchner’s group reported the N-alkylation of aromatic amines with primary alcohols to produce diverse N-alkylated amines using Co(II) pincer complexes (Co-12 and Co-199) that were supported by an anionic PCP ligand.222 The chloride complex Co-12 needed to be activated by the strong base tBuOK to initiate the catalytic process. In contrast, the reactions catalyzed by alkyl complex Co-199 bearing a basic anionic ligand −CH2SiMe3 could be performed under base-free conditions in the presence of 3 Å molecular sieves as an additive (Scheme 80). Apart from alcohols, aliphatic amines can also be used as a source of electrophilic intermediates triggered by a dehydrogenation process. Zhang and co-workers found that the well-defined cobalt catalyst Co-2 was active for the monoalkylation of amines

transition metal-catalyzed dehydrogenation of an alcohol or amine substrate to generate a more reactive carbonyl or imine intermediate. Further transformations of this reaction intermediate could give a new unsaturated compound, which is reduced by the metal hydride formed in the initial dehydrogenation step. In this section, two types of bond formation (Nalkylation and C-alkylation reactions) are discussed. 3.4.1. N-Alkylation Reactions. Homogeneous cobalt catalysts have shown catalytic activity in (de)hydrogenation reactions, which are the main steps of hydrogen-borrowing reactions. Therefore, it was envisioned that hydrogen-borrowing reactions could also be realized by cobalt catalysis. In 2015, Kempe and co-workers reported a cobalt complex supported by a PN5P ligand (Co-198), which was the first non-noble metal AI


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3.4.2. C-Alkylation Reactions. The hydrogen-borrowing strategy also offers a sustainable methodology for carbon− carbon bond formation using alcohols as the alkylation reagents of carbon nucleophiles.197 This approach is initiated by a catalytic dehydrogenation of the alcohols to form carbonyl compounds. A subsequent condensation process with acidic C− H bonds at the α-position of the carbonyl groups under basic conditions results in α,β-unsaturated carbonyl intermediates, which are reduced in situ by the metal hydride species generated from the initial dehydrogenation step to form the final Calkylation products. In this respect, direct alkylation of unactivated amides and esters with alcohols is a challenging process for the following reasons: (1) amides and esters display low C−H acidity because of resonance stabilization and (2) esters can easily undergo side reactions such as transesterification.224 Nevertheless, Kempe and co-workers61 reported the first cobalt-catalyzed C-alkylation of amides and esters with alcohols in 2016. The alkylation of amides was realized in the presence of 2.5 mol % of Co-198 in THF at 100 °C. Esters were alkylated using 5 mol % of Co-200 as the catalyst in toluene at 80 °C. In these two reactions, 1.2− 1.5 equiv of t-BuOK were required to promote the alkylation processes. Mechanistic studies indicated that the rate-limiting step of this transformation is dehydrogenation of alcohols (Scheme 82).

Scheme 80. PCP-Cobalt(II) Pincer Complex-Catalyzed NAlkylation of Amines with Alcohols

with aliphatic amines.223 A range of aromatic and aliphatic secondary amines were generated in moderate-to-high yields via cross- or homocoupling of amine substrates. In addition, cyclic sec-amines were also formed from diamine precursors by this hydrogen-borrowing strategy (Scheme 81).

Scheme 82. Co-198 and Co-200-Catalyzed C-Alkylation of Amides and Esters with Alcohols

Scheme 81. Co-2-Catalyzed N-Alkylation of Amines with Aliphatic Amines

Recently, Zhang/Zheng and co-workers developed a cobaltcatalyzed α-alkylation of ketones with primary alcohols.225 In this work, a series of ketone and alcohol substrates were welltolerated using Co-2 as the catalyst and a catalytic amount of tBuOK as a base additive, leading to the formation of the corresponding α-alkylated ketone products in good yields (Scheme 83a). Even more recently, the cobalt-catalyzed βalkylation of secondary alcohols by primary alcohols was reported by Kempe and co-workers.226 The catalyst Co-200 displayed high activity in this transformation (Scheme 83b). AJ


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Scheme 83. Co-2 and Co-200-Catalyzed C-Alkylation of Ketones and sec-Alcohols with Primary Alcohols

Scheme 84. Cobalt-Catalyzed Anti-Markovnikov Hydroboration of Terminal Olefins

3.5. Hydrofunctionalization Reactions

The direct catalytic H−E bond (E = B, Si, N, O, C, H etc.) addition to an unsaturated bond, namely, hydrofunctionalization, is one of the most powerful tools for the construction of new C−E and C−H bonds.227 In this review, we outline the advances in cobalt-catalyzed hydrofunctionalization of unsaturated organic compounds, except for hydroformylation, which is the addition of synthesis gas to an olefin leading to the formation of a formyl group and has been discussed in great detail along its 80-year journey.43 Four main subareas of this topic, hydroboration, hydrosilylation, hydroamination, and hydrocarbon functionalization, are discussed here. 3.5.1. Hydroboration Reactions. Catalytic hydroboration of unsaturated bonds is a direct, powerful, and atom-economical method to provide alkyl or alkenyl organoboron compounds that can serve as important synthetic intermediates in chemical synthesis.228 In 1956, Brown developed the hydroboration of alkenes using sodium borohydride and aluminum chloride, representing a milestone in the development of organoborane chemistry.229 Since then,229,230 most hydroboration reactions of alkenes have been found to occur readily to form antiMarkovnikov products. Considerable progress has been made in recent years, demonstrating that the chemo-, regio-, and enantioselectivity of catalytic hydroboration reactions can be effectively controlled by the ligands, metal center, and substrate structures.228 The corresponding alkene hydroboration reactions with Markovnikov selectivity as well as asymmetric versions have also been realized. Pioneered by the work of Chirik’s and Huang’s groups, cobalt catalysts have been found to be some of the most effective transition-metal catalysts for selective alkene hydroboration reactions.231−233 In 2014, Huang and co-workers developed the first cobaltcatalyzed anti-Markovnikov hydroboration of terminal alkenes with HBpin.233 The PNN-pincer cobalt catalyst Co-201 was used as the precatalyst in combination with NaBHEt3 as an activator, which offers excellent regio- and chemo-selectivity as well as broad functional-group tolerance (Scheme 84a). In addition, a similar hydroboration reaction catalyzed by Co-2 with high anti-Markovnikov selectivity without any additives was reported by Zheng and co-workers very recently (Scheme 84b).234 A more practical cobalt catalytic system was developed by Thomas and co-workers. The robust moisture- and air-stable precatalyst MesBIPCoCl2 Co-202 was activated by NaOtBu in the presence of HBpin.49 The resulting catalytically active species enabled the highly efficient anti-Markovnikov hydroboration of terminal olefins. Mechanistic investigations

indicated that sodium tert-butoxide behaved as a masked reducing agent to produce an “ate” species with HBpin, which then acted as a hydride donor for the precatalyst activation (Scheme 84c). The cobalt complex Co-203 based on an electron-rich CCC pincer carbene ligand is also an efficient catalyst for antiMarkovnikov hydroboration of terminal olefins.235 A wide range of functional groups were well-tolerated by Co-203, including epoxide, amino, ketone, and ester groups. Deuterium-labeling experiments suggested both 1,2- and 2,1-insertion were involved in this reaction. The 2,1-insertion pathway is reversible and does not produce the branched hydroboration product. The following possible mechanism was proposed: (1) oxidative addition of HBpin with a cobalt(I) species to generate a cobalt(III) hydride complex; (2) coordination of the terminal alkene substrate to the cobalt center; (3) alkene insertion into the Co−H bond to form an alkyl cobalt(III) intermediate; and (4) C−B reductive elimination to furnish the desired antiMarkovnikov hydroboration product and regenerate the catalytically active cobalt(I) species (Scheme 84d). The anti-Markovnikov hydroboration of 1,1-disubstituted alkenes provides a possibility to create a chiral carbon center at the β-position of the organoborane products, but it is more challenging than the reaction of terminal alkenes because of the steric hindrance of substrates. In 2014, Huang’s group57 and Lu’s group236 independently developed chiral iminopyridine oxazoline (IPO) ligand-supported cobalt catalysts, which were very effective for the anti-Markovnikov asymmetric hydroAK


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boration of 1,1-disubstituted aryl alkenes. In Huang’s work, the prepared cobalt(I) methyl complex was used as the catalyst without any additives. In contrast, Lu and co-workers employed the stable IPO cobalt dichloride complex as a precatalyst and NaBHEt3 as an activator to generate the catalytically active lowvalent cobalt species in situ. Both catalytic systems afforded excellent regio- and enantioselectivity. However, the reactions of ortho-substituted α-methylstyrenes only gave moderate enantioselectivity (Scheme 85).

Scheme 86. Anti-Markovnikov Hydroboration of Sterically Hindered Styrenes

Scheme 85. Anti-Markovnikov Hydroboration of 1,1Disubstituted Aryl Alkenes

Scheme 87. Proposed Mechanism for Anti-Markovnikov Hydroboration of Sterically Hindered Styrenes

Later, Lu and co-workers developed a cobalt-catalyzed dualstereocontrol asymmetric hydroboration of sterically hindered styrene derivatives using a flexible oxazoline aminoisopropylpyridine (OAP) ligand and rigid IPO ligand (Scheme 86).237 The combination of OPA ligand L12 with CoCl2 could efficiently promote the hydroboration of ortho-substituted α-methylstyrenes, giving up to 92% ee. Using the IPO-CoCl2 complex, the other enantiomer could be produced from a single chiral source with up to 95% ee. The primary mechanistic studies suggested that the two reactions followed distinct pathways (Scheme 87). The IPO cobalt-catalyzed alkene hydroboration was proposed to proceed through cycle I. In cycle I, the IPO cobalt precatalyst is reduced by NaBH(sBu)3 to form an in-cycle catalytically active IPO cobalt hydride species. Alkene coordination with the cobalt hydride species followed by alkene insertion into the Co− H bond and ligand exchange delivers the product and reforms the cobalt hydride species. Using OAP as a ligand (cycle II), a low-valent OAP cobalt species might be the in-cycle active catalyst, which could be generated through a deprotonation pathway. The OAP cobalt species then coordinates with an alkene followed by oxidative addition of HBPin, alkene insertion

into the Co−B bond, and reductive elimination to afford the hydroboration product and regenerate the OAP cobalt species. Very recently, the cobalt-catalyzed hydroboration of alkenes with Markovnikov selectivity was also realized along with ligand development for cobalt catalysis. In 2016, Hollis and co-workers AL


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with excellent selectivity. Mechanistic investigations indicated that a 1,1-diborylalkene intermediate was generated through two cobalt-catalyzed dehydrogenative borylations. The subsequent hydroboration of this intermediate with HBpin generated in situ led to the formation of the 1,1,1-tris(boronate) product (Scheme 89).

reported that the air-stable cobalt complex Co-217 supported by an anionic pincer carbene ligand in combination with the hydride additive LiBHEt3 behaved as a selective catalytic system for the Markovnikov hydroboration of styrene (Scheme 88a).238 Scheme 88. Markovnikov Hydroboration of Aromatic Terminal Alkenes

Scheme 89. Cobalt-Catalyzed Dehydrogenative Borylations− Hydroboration of Vinylarenes

Ge’s group further extended the cobalt-catalyzed hydroboration of alkenes to the selective diborylation of 1,1disubstituted vinylarenes with HBpin in the presence of Co(acac)2/xantphos as the catalyst.241 Preliminary mechanistic studies demonstrated that vinylboronates were not intermediates for this transformation, which is different from the above triborylation reaction of styrenes (Scheme 90). Activation of the cobalt catalyst Co(acac)2/xantphos with HBpin generates the in-cycle catalytically active Co−H species Co-223. The reaction of Co-223 with norbornene and HBpin produces the Co(I)Bpin species Co-225, which then undergoes alkene insertion into the Co−B bond to furnish the sterically hindered alkyl cobalt intermediate Co-226. Subsequently, valence tautomerization of Co-226 gives the less hindered alkyl cobalt species Co228 via a cyclic σ,π-transition state. The following reaction of Co-228 with HBpin provides the gem-bis(boryl)alkane product and reforms the Co−H species Co-223. Isomerization−hydroboration of internal alkenes to produce terminal borylation products has become an efficient method for direct functionalization at the remote positions of the original CC double bonds. In 2013, Chirik’s group developed the first cobalt-catalyzed isomerization−hydroboration of internal alkenes catalyzed by the bis(imino)pyridine cobalt complex Co229.232 However, this catalyst displayed low activity for the reactions of more hindered tri- or tetrasubstituted olefin substrates. To overcome this limitation, the more reactive catalyst (4-pyrr-MesPDI)CoCH3 Co-230 was prepared by

Shortly after, Thomas’s group239 and Zheng’s group234 also reported the cobalt-catalyzed Markovnikov hydroboration of aromatic terminal alkenes. In the reaction developed by Thomas and co-workers, the bipyridiyl-oxazoline cobalt complex Co-218 was used as the Markovnikov hydroboration catalyst and sodium tert-butoxide served as an activator. Zheng’s group234 employed the dinuclear cobalt alkyl complex Co-219 supported by a flexible NNN ligand as the catalyst for Markovnikov hydroboration of alkenes under additive-free conditions. Both catalytic systems promoted hydroboration of various aromatic terminal alkenes but were less effective for the Markovnikov hydroboration of alkyl alkenes (Scheme 88, panels b and c). Multiborylated compounds are important building blocks in organic synthesis. In 2015, Huang and co-workers achieved cobalt-catalyzed triborylation of vinylarenes with B2pin2 to synthesize 1,1,1-tris(boronates).240 This reaction proceeded in the presence of cobalt dichloride complex Co-220 as the catalyst and NaBHEt3 as the precatalyst activator at room temperature, affording diverse 1,1,1-tris(boronate) products in high yields AM


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Scheme 90. Cobalt-Catalyzed Diborylation of 1,1Disubstituted Vinylarenes

Scheme 91. Co-229 and Co-230-Catalyzed Isomerization− Hydroboration of Alkenes

introducing an electron-donating pyrrolidinyl group to the 4position of the pyridine on the ligand, which proved very effective for the isomerization−hydroboration of hindered internal alkenes. Deuterium labeling experiments with DBpin catalyzed by Co-229 generated the deuterated 1-octylboronate ester products with deuterium located in the interior (2−7) positions of the carbon chain, providing evidence for the occurrence of a fast and reversible double bond migration process. On the basis of these observations, the following plausible mechanism was proposed. The precatalyst cobalt methyl complex reacts with DBpin to furnish the in-cycle catalytically active cobalt deuteride species. The subsequent olefin insertion into the Co−D bond results in a secondary cobalt alkyl intermediate. A fast and reversible chain walking process along with the borylation of the Co−C bond at the less hindered terminal position generates the terminal alkylboronic ester (Scheme 91). Later, Chirik’s group developed cobalt alkyl complexes Co234 and Co-235, which were supported by redox-active and readily available 2,2′:6′,2″-terpyridine and α-diimine ligands, respectively.242 These two cobalt alkyl complexes displayed high activity in the terminal-selective isomerization−hydroboration of sterically hindered alkenes. The terpyridine chelate was exceptionally active for the hydroboration of olefins containing diene or arene groups. For more hindered substrates like tri-, tetra-, and geminally substituted olefins, Co-235 was a more effective catalyst than Co-234. The N-phosphinoamidinateligated cobalt catalyst Co-236 also showed pronounced activity for the terminal-selective hydroboration of germinal and internal alkenes with HBpin (Scheme 92).243,244 A nonterminal-selective isomerization−hydroboration of alkenes using a cobalt catalyst has also been developed by Chirik and co-workers.245 They found that the cobalt phosphine complex (PPh3)3CoHN2 (Co-59), which can be generated in situ from cobalt precursor Co(OAc)2 or synthesized by reacting (PPh3)2CoCl2 with NaHBEt3, showed prominent activity for the

Scheme 92. Cobalt Catalytic Systems and Substrate Scopes for Terminal-Selective Isomerization−Hydroboration of Olefins

hydroboration of terminal alkenes. Unlike cobalt complexes bearing redox-active supporting ligands, which promote boron incorporation at terminal positions, Co-59 places the boron substituent adjacent to π-systems (Scheme 93). Vinylboronate esters are very useful building blocks because of their applications in C−C bond formation. In most cases, hydroboration of terminal alkynes affords vinylboronate esters with E-stereoselectivity arising from anti-Markovnikov, syn addition of the B−H bond to the CC bond. Trovitch’s group developed the α-diimine (DI) cobalt hydride catalyst Co-238, (Ph2PPrDI)CoH, which could efficiently catalyze the hydroAN


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Scheme 93. Co-59-Catalyzed Isomerization−Hydroboration of Alkenes

methyl complex and alkyne. The subsequent oxidative addition of HBpin to the cobalt acetylide complex generates the cobalt(III) hydride intermediate, which then undergoes reductive elimination to afford the alkynylboronate estercoordinated cobalt(I) hydride complex. The syn-hydrometalation of this complex produces the pro-(Z) cobalt vinyl intermediate. The protonation of the cobalt vinyl complex with a terminal alkyne liberates the final product (Z)vinylboronate ester and regenerates the cobalt acetylide species (Scheme 94, panels c to e). Recently, the iminopyridine oxazoline-ligated cobalt complex Co-244 was found to be effective for the regioselective sequential hydroboration of terminal alkynes with HBpin to form the dual hydroboration products.247 This transformation offers high conversion and broad functional-group tolerance under mild reaction conditions (Scheme 95).

boration of terminal alkynes under mild conditions.246 A range of corresponding E-alkenyl boronate esters was generated with TOFs of up to 900 h−1. DFT calculations and XRD analysis suggested that this complex possesses a low-spin Co(II) center and a radical monoanionic DI chelate (Scheme 94, panels a to

Scheme 95. Co-244-Catalyzed Dual Hydroboration of Terminal Alkynes

Scheme 94. Cobalt-Catalyzed Hydroboration of Terminal Alkynes

The synthetic utility of Co-catalyzed hydroboration of alkynes was further extended to regio- and enantioselective hydroboration/hydrogenation of internal alkynes by Lu’s group. Initially, they chose a series of oxazoline iminopyridine (OIP) cobalt complexes as precatalysts; however, the corresponding alkyl boronic ester product was obtained either in poor yield or with low ee. Subsequently, they developed a novel pincer cobalt complex containing an electron-rich chiral imidazoline iminopyridine ligand, which acted as an excellent catalyst for the highly regio- and enantioselective conversion of various internal alkynes to the corresponding chiral secondary organoboronates in one-pot in the presence of NaBHEt3, HBpin, and a hydrogen balloon (Scheme 96).248 Aryl alkyl alkynes could be Scheme 96. Co-245-Catalyzed Hydroboration/ Hydrogenation of Internal Alkynes

b). The synthesis of the (Z)-isomer via alkyne hydroboration is more challenging than that of the (E)-isomer. The bis(imino)pyridine cobalt methyl complex Co-239 developed by Chirik’s group205 is an active catalyst for the hydroboration of terminal alkynes with HBpin to form the (Z)-vinylboronate esters with high yields and selectivity. Mechanistic studies illustrated that the (Z)-selectivity resulted from syn-hydrometalation of the alkynylboronate ester intermediate. The reaction is initiated by the formation of the cobalt acetylide species from the cobalt AO


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was initiated by the cobalt-catalyzed regioselective hydroboration of alkynes followed by the HBpin-promoted and cobalt-catalyzed enantioselective hydrogenation of alkenylboronates. The catalytically active Co−H species in this reaction could be regenerated through three possible pathways: (1) chiral interconvertible alkyl cobalt species Co-250 or Co-251, which is afforded through alkene insertion and may slowly react with H2 gas to produce chiral secondary organoboronate 26 and regenerate the Co−H species. The mostly likely process may be the σ-bond metathesis of alkyl cobalt species Co-250 or Co-251 with HBpin to give 26 and cobalt(I) boryl species Co-252. (2) Cobalt(I) boryl species Co-252 undergoes σ-bond metathesis with water to regenerate the cobalt hydride species, and (3) the cobalt hydride species could be regenerated by reductive elimination of cobalt(III) boryl dihydrides species Co-253, which is formed from cobalt(I) boryl species Co-252 through oxidative addition of H2 (Scheme 97b). Selective borylative cyclization of 1,6-enynes represents a powerful approach to obtain cyclic organoborane compounds.17,249−252 In 2017, Lu’s group reported a cobaltcatalyzed ligand-controlled chemo-divergent hydroboration/ cyclization of 1,6-enynes to produce cyclic alkylboronates and alkenylboronates.253 When OIP was used as a ligand, the cobaltcatalyzed cyclization reaction of 1,6-enynes with HBpin delivered alkylboronate products in high yields. In contrast, the use of the bidentate IP as the ligand instead of OIP led to the formation of alkenylboronate products with good selectivity. However, the enantioselective cyclization reaction was not realized using the chiral OIP ligand. In accordance with the results of deuterium labeling and control experiments, the authors proposed that the reaction is initiated by alkyne insertion into the Co−H bond followed by alkene insertion to afford cobalt alkyl species Co-258 in the presence of IP. When using OIP as a ligand, the alkene insertion into the Co−H bond might occur initially followed by alkyne insertion to produce cobalt vinyl species Co-257. Intermediates Co-259 and Co-260 undergo σ-bond metathesis with HBpin to deliver the alkylboronate and alkenylboronate products, respectively (Scheme 98). Later, Ge and co-workers developed the corresponding asymmetric cobalt-catalyzed hydroboration/cyclization of 1,6enynes.254 The catalytically active species were generated in situ from cobalt precursor Co(acac)2 and chiral bisphosphine ligands. Using (R,R)-QuinoxP* as the ligand, enynes containing para- or meta-substituted aryl groups and aliphatic 1,6-enynes were converted to vinyl boronate esters with excellent enantioselectivity and modest-to-high yields. In contrast, the Co(acac)2/(R,R)-QuinoxP*-catalyzed cyclization of enynes with ortho-substituted arenes and O-tethered substrates bearing two substituent groups at the propargylic position produced alkylboronates in high yields and enantioselectivities. Moreover, using (R,R)-L15 as a ligand, the asymmetric hydroboration/ cyclization of N- and C-tethered substrates afforded alkylboronate products with high enantioselectivities (Scheme 99). As well as alkene and alkyne substrates, the cobalt-catalyzed hydroboration of polar unsaturated bonds has also been reported. In 2017, Fout’s 235 and Trovitch’s246 groups independently developed the hydroboration of nitriles via cobalt catalysis using the electron-rich cobalt pincer complex (DIPPCCC)CoN2 Co-203 and radical monoanion α-diimine chelate (ph2PPrDI)CoH Co-238 as catalysts, respectively. Both of these catalytic systems delivered N,N-diborylated products with modest-to-high yields (Scheme 100).

transformed smoothly into the corresponding products with yields of 36%−97% and ee of 84%−99%. Poly- and heterocycles were also tolerated, furnishing chiral boronic esters or alcohols after oxidation with yields of 52%−85% and ee of 60%−98%. When an ortho-methyl-substituted aromatic internal alkyne and symmetric dec-5-yne were used, the sole alkyne hydroboration products (24g and 24h, respectively) were generated with yields of 70% and 73%, respectively. To elucidate the possible reaction process, several control reactions were conducted (Scheme 97a). The hydroboration reaction of (Z)-but-1-en-1-ylbenzene Scheme 97. Proposed Mechanism for Co-245-Catalyzed Hydroboration/Hydrogenation of Internal Alkynes

(3ap) delivered (S)-26a in high yield but with 17% ee. Therefore, the possibility of a tandem hydrogenation of alkynes and hydroboration pathway could be ruled out as a major pathway to achieve high enantioselectivity. Under the catalytic conditions without HBpin, the alkenylboronic ester 24i could not be hydrogenated. The use of 0.3 equiv of HBpin or B2(pin)2 promoted the hydrogenation process to afford chiral boronic ester 26d with similar results (55% yield, 93% ee). When 0.5 equiv of HBpin were added to the system, 26d was generated in 99% yield and 95% ee. This demonstrated that the hydrogenation process might be promoted by the HBpin or boronic group. These control experiments suggested that the reaction AP


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Scheme 98. Co-254 and Co-255-Catalyzed Hydroboration/ Cyclization of 1,6-Enynes

Scheme 100. Cobalt-Catalyzed Hydroboration of Nitriles

261) for the enantioselective hydroboration of aryl ketones with HBpin.257 A variety of aryl ketones were hydroborated to afford chiral alcohols with high yields and enantioselectivities. Notably, a wide range of functional groups were well-tolerated by this catalytic system (Scheme 101). Scheme 101. Co-261-Catalyzed Asymmetric Hydroboration of Aryl Ketones

Scheme 99. Co(acac)2-Catalyzed Asymmetric Hydroboration/Cyclization of 1,6-Enynes

3.5.2. Silylation Reactions. 3.5.2.1. Hydrosilylation Reactions. The hydrosilylation of unsaturated bonds, such as C−C multiple bonds, imine moieties, and carbonyl groups, is an efficient and straightforward method to produce functionalized organosilane compounds, which are very useful building blocks in organic synthesis.4,258,259 In the 1960s, Chalk and Harrod260 reported the Co2(CO)8-catalyzed hydrosilylation of terminal alkenes, which greatly accelerated the development of cobaltbased hydrosilylation catalysts. Over the last three years, several comprehensive reviews on the cobalt- and iron-catalyzed hydrosilylation of alkenes and alkynes have been published by the groups of Deng,37 Huang,38 and Lu.41 In this section, we briefly introduce cobalt-catalyzed hydrosilylation of C−C multiple bonds and mainly focus on the hydrosilylation of other unsaturated substrates by cobalt catalysis. The well-defined cobalt(0) complex Co2(CO)8 has been extensively studied in hydrosilylation reactions and found to be effective for the anti-Markovnikov hydrosilylation of terminal alkenes. In the 1960s, Chalk and Harrod260 disclosed that Co2(CO)8 was capable of promoting the anti-Markovnikov hydrosilylation of alkyl-substituted terminal alkenes with tertiary silanes ((MeO)3SiH, Et3SiH, and HSiPhCl2) to produce n-alkyl silanes in high yields. However, an excess of alkenes was required in these reactions because of the side reaction of Co-catalyzed isomerization of 1-alkenes to inactive internal alkenes. Later, Kalinin’s group261 expanded the substrate scope from aliphatic terminal alkenes to styrene and several functionalized alkenes, such as vinyltrimethylsilane, allyl di(triethoxysilyl)amine, and 1vinyl-o-carborane. In another pioneering study, the utility of Co2(CO)8-catalyzed anti-Markovnikov alkene hydrosilylation was extended to the formation of chlorosilyl-functionalized polystyrene by Darling and co-workers.262 Silane-modified

Asymmetric hydroboration of prochiral ketones is an efficient method to produce chiral secondary alcohols.255,256 In 2015, Lu’s group developed a chiral IPO-ligated cobalt catalyst (CoAQ


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polymers have important industrial applications (Scheme 102a). More recently, Lappert’s group studied the hydrosilylation of

Scheme 103. Proposed Mechanism for Co2(CO)8-Catalyzed Anti-Markovnikov Hydrosilylation of Terminal Alkenes and Dienes

Scheme 102. Co2(CO)8-Catalyzed Anti-Markovnikov Hydrosilylation of Terminal Alkenes and Dienes

conjugated dienes.263 They found that the hydrosilylation of linear dienes generally gave a mixture of the 1,2- and 1,4addition products as well as dihydrosilylation products. Using cyclodiene as the substrate selectively afforded the 1,4hydrosilylation products in good yields (Scheme 102b). Apart from hydrosilylation reactions, Co2(CO)8 is also capable of catalyzing dehydrogenative silylation of α,β-unsaturated esters to provide alkenylsilane products.264 On the basis of their experimental observations, Chalk and Harrod proposed a catalytic cycle involving a hydride migration insertion pathway.265 The proposed cycle could explain the coexistence of alkene isomerization products but not the formation of dehydrogenative silylation products. Wrighton’s group then proposed a modified Chalk−Harrod mechanism involving a silyl migration insertion pathway in which dehydrogenative silylation could occur through the β-hydride elimination of β-silylalkyl cobalt species (Scheme 103).266,267 As well as Co2(CO)8, cobalt-based alkene-hydrosilylation catalysts supported by pentamethylcyclopentadienyl, N-heterocyclic carbene, β-diketimine, phosphine, and 2,6-diiminopyridine ligands have also been reported (Scheme 104). Deng and co-workers used the strong σ-donating ligand NHC in an NHC Co system (Co-273) for the anti-Markovnikov hydrosilylation of 1-octene with primary silane PhSiH3.268 In addition, Holland’s group evaluated the catalytic performance of cobalt(I) β-diketimine complex Co-274 in the selective anti-Markovnikov hydrosilylation of functionalized aliphatic alkenes with PhSiH3 and hydrosiloxane (EtO)3SiH.269 In 2017, Ge’s group developed a cobalt/bisphosphine system for the anti-Markovnikov hydrosilylation of alkenes.270 The hydrosilylation reactions of vinylarenes with secondary silane Ph2SiH2 were carried out using 2 mol % of Co(acac)2/dppf at 50 °C and produced the corresponding linear organosilanes with high isolated yields. For aliphatic alkenes, the linear hydrosilylation products could be obtained with good yields and excellent regioselectivities in the presence of 2 mol % of Co(acac)2/xantphos and primary silane PhSiH3 at room temperature. Mechanistic studies indicated that these two

transformations promoted by the cobalt/bisphosphine catalytic system followed a Chalk−Harrod-type mechanism involving a cobalt hydride migration insertion pathway (Scheme 104a). A challenging selective hydrosilylation of terminal double bonds in skipped 1,4-diene and 1,3-diene was described by RajanBabu’s group.271 With the use of (iPrPDI)CoCl2 (Co-162, PDI = 2,6-bis(2,6-alkylphenyl-iminoethyl)pyridine) as a precatalyst that was activated with NaBHEt3, the 1,3- and 1,4-dienes reacted with primary and secondary silanes (PhSiH3, Ph2SiH2, and PhSi(Me)H2) to give 1,2-hydrosilylation products in goodto-excellent yields. On the basis of the experimental observations, this reaction was proposed to proceed through the classical Chalk−Harrod mechanism involving a cobalt hydride species that was generated in situ (Scheme 104a). Cobalt catalysts that are effective for hydrosilylation reactions using primary and secondary silanes are often less efficient or even inactive for hydrosilylation of tertiary silanes and hydrosiloxanes. The groups of Grant,272 Holland,269 Nagashima,273 Chirik,274 Fout,275 Deng,276 and Lee277 have developed powerful Co-based catalysts to address this limitation (Scheme 104b). Grant and co-workers developed a cyclopentadienylcobalt complex for the anti-Markovnikov hydrosilylation of 1-hexene with Et3SiH.272 Nagashima’s group used the tertiary hydrosiloxane Me3SiOSiHMe2 in the carboxylate complex Co(OPv)2/CNAd-catalyzed hydrosilylation of styrene derivatives.273 Chirik and co-workers developed a pyridine diimine cobalt bis(carboxylate) complex (Co-279)-catalyzed alkene hydrosilylation with (EtO)3SiH.274 The groups of Fout and Deng recently independently developed the cobalt-catalyzed hydrosilylation of functionalized terminal alkenes with tertiary silanes. More specifically, Fout’s group275 employed the well-defined bis(carbene) cobalt(I) dinitrogen complex Co-203 as a catalyst featuring a broad substrate scope. Alkenes bearing nitrile, amino, hydroxyl, epoxide, ester, formyl, and ketone groups were selectively hydrosilylated by a tertiary silane (Me 2 PhSiH or (TMSO)2MeSiH). This transformation was proposed to AR


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Scheme 104. Cobalt Catalysts for the Anti-Markovnikov Hydrosilylation of Terminal Alkenes

not regioselective; a branched/linear ratio of 1:1 was obtained. Stoichiometric experiments revealed that the CoII dichloride complex can be reduced by PhSiH3 to result a catalytically active CoI species (iPrPCNNMe)CoCl and PhSiH2Cl. In the presence of Co-282, the hydrosilylation of the deuterated alkene 1dodecene-d2 with PhSiH3 yieled 34c-d2 selectively and the reaction of 1-octene with PhSiD3 gave 34c-d3. No deuterium was detected for the α-carbon position in either case (Scheme 106c). These observations stand against the formation of a Co−D or Co−H species in the catalytic cycle because the deuterium scrambling between α- and β-carbon would otherwise occur through β-H elimination and olefin reinsertion into the Co-D bond. On the basis of the deuterium labeling experimental results, the authors proposed that this reaction could proceed through a silyl migration process involving a putative CoI silyl species. The CoI complex (iPrPCNNMe)CoCl reacts with PhSiH2Cl to form a Co−H intermediate, which could further convert to the CoI silyl species and release hydrogen gas (Scheme 106d). For apolar olefins, the Markovnikov products were formed through a selective 1,2-insertion (presumably because of steric effects). An alternative, but less likely fashion, involves 2,1-insertion of the alkene to furnish the antiMarkovnikov product. For vinylarenes, due to the marked electronic difference between the two olefinic carbon atoms, 2,1insertion may compete with the 1,2-insertion pathway (Scheme 106e). Ge’s group reported efficient protocols for the cobaltcatalyzed Markovnikov hydrosilylation of alkenes using catalytic systems generated in situ.270 The combination of xantphos with Co(acac)2 could efficiently promote the hydrosilylation of vinylarenes with PhSiH3 with high regioselectivity (branched/ linear ratio of 96:4 to >99:1). However, the steric hindrance originating from the ortho-substituents of aryl groups strongly influenced the regioselectivities. For example, the hydrosilylation of 2,4,6-trimethylstyrene afforded the linear silane 35a exclusively. In contrast, by employing the mesPDI ligand, aliphatic alkenes could react with PhSiH3 with high efficiency. It was found that the steric properties of silanes strongly affected the regioselectivity of these hydrosilylation reactions. Increasing the steric hindrance around Si−H favored the formation of

proceed via a Chalk−Harrod-type mechanism. Deng and coworkers used the NHC cobalt(II) amide complex Co-280 ([(NHC)Co(N(SiMe3)2)2]) to catalyze hydrosilylation of aliphatic alkenes with tertiary silanes (EtO)3SiH (Scheme 104b).276 Later, Lee reported the cobalt (aminomethyl)pyridine complex Co-281, which was capable of promoting the challenging anti-Markovnikov hydrosilylation of siloxy- or alkoxy(vinyl) silanes with siloxy- or alkoxy-hydrosilanes in the presence of alkylating agent TMSCH2Li (Scheme 104b and 105).277 Scheme 105. Co-281-Catalyzed Anti-Markovnikov Hydrosilylation of Siloxy- or Alkoxy(vinyl) Silanes with Siloxy- or Alkoxy-Hydrosilanes

Although the anti-Markovnikov-selective hydrosilylation of alkenes using cobalt catalysts has been well-established, Markovnikov alkene hydrosilylation is more challenging. In 2016, Huang’s group reported a phosphine iminopyridine iron complex (tBuPCNNiPr)FeCl2 that behaved as a selective catalyst for anti-Markovnikov hydrosilylation of terminal alkenes with PhSiH3.278 Interestingly, by changing the metal center from iron to cobalt to obtain (iPrPCNNMe)CoCl2 Co-282, high Markovnikov selectivity for the hydrosilylation of aliphatic terminal alkenes was achieved. Transformation of a wide variety of functionalized alkenes proceeded smoothly, furnishing the branched hydrosilylation products with high yields and regioselectivities. However, the hydrosilylation of styrene was AS


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Scheme 106. Co-282-Catalyzed Racemic Markovnikov Hydrosilylation of Terminal Alkenes

Scheme 107. Co(acac)2-Catalyzed Racemic Markovnikov Hydrosilylation of Terminal Alkenes

linear products (35b). To elucidate the mechanism of these two transformations, deuterium labeling experiments were carried out. They tested the hydrosilylation of 4-tert-butylstyrene-β,β-d2 with PhSiH3 and the reaction of 4-tert-butylstyrene with PhSiD3 catalyzed by Co(acac)2/xantphos. The deuterium incorporation on the α-carbon and silicon atom of the products in these two cases support the formation of a Co-H intermediate and indicate a Chalk−Harrod-type mechanism for alkene hydrosilylation Co(acac)2/xantphos. They also conducted the hydrosilylation of 1-dodecene-1,1-d2 with PhSiH3 and the reaction of 1dodecene with PhSiD3 in the presence of Co(acac)2/ mesPDI. Different from hydrosilylation of 4-tert-butylstyrene catalyzed by Co(acac)2/xantphos, these two reactions afforded the products with no deuterium localized on the α-carbon atom. These observations argue strongly against the generation of a Co−H species in the catalytic cycle and imply that the Co(acac)2/mesPDI-catalyzed Markovnikov hydrosilylation follows the modified Chalk−Harrod mechanism (Scheme 107). Ge’s group also reported a cobalt-catalyzed stereoconvergent Markovnikov 1,2-hydrosilylation of functionalized conjugated

(E/Z)-dienes (Scheme 108).279 In the presence of 1 mol % of Co(acac)2/xantphos (Co-276) and PhSiH3, various (E)allylsilanes were produced smoothly from trans-dienes or a mixture of (E/Z)-isomeric 1,3-dienes with high yields as well as excellent stereo- and regioselectivities (E/Z = > 99:1; branched/ linear = > 99:1). On the basis of their experimental observations, they proposed that the stereoconvergence arose from a σ−π−σ isomerization process of the allyl cobalt species formed by the hydrometalation of (Z)-dienes (Scheme 108a). Moreover, they identified that the cobalt complex Co(acac)2/BINAP was only active for Markovnikov 1,2-hydrosilylation of the (E)-isomer; it did not promote the conversion of the (Z)-isomer. The separation of a (Z)-isomer from (Z/E)-diene isomeric mixtures AT


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Scheme 108. Co-276-Catalyzed Markovnikov 1,2Hydrosilylation of 1,3-Dienes

Scheme 109. Co-296-Catalyzed Asymmetric Markovnikov Hydrosilylation of Alkenes

could also be achieved by this method (Scheme 108b). Furthermore, they studied the asymmetric Markovnikov 1,2hydrosilylation of (E)-1-aryl-1,3-dienes using chiral cobalt catalyst Co(acac)2/(R)-difluorphos; the corresponding allylsilanes were obtained with good enantioselectivity (Scheme 108c). Asymmetric Markovnikov hydrosilylation of alkenes via cobalt catalysis has been recently developed by Lu’s group. When a sterically hindered OIP cobalt complex Co-296 was employed as the precatalyst and sodium tert-butoxide as the activator, a wide range of chiral benzyl silanes were obtained in high yields with excellent regio- and enantioselectivities (>96:4 rr; > 98% ee) in the presence of PhSiH3. Under the standard conditions, unactivated aliphatic alkenes were also suitable substrates for this transformation, affording the corresponding chiral aliphatic dihydrosilanes with 81%−87% ee. In addition, the enantiomerically opposite (R)-silane products could be easily obtained by using the opposite enantiomer as a ligand (Scheme 109).280 Transition metal-catalyzed hydrosilylation of alkynes is one of the best methods to access vinylsilanes, which are valuable and versatile starting materials in organic synthesis.281,282 The hydrosilylation of terminal alkynes can produce three possible isomers, (E)-β-, (Z)-β-, and α-vinylsilanes, and thus one challenge for developing this reaction is to control both the regio- and stereochemistries. Among these isomeric vinylsilanes, the thermodynamically favorable (E)-β-isomers are usually generated with high selectivity. In 2006, Butenschön’s group developed the di-tert-butylphosphanylethylcyclopentadienyl

cobalt complex Co-297, which promoted the hydrosilylation of terminal alkynes with HSiEt3 with high syn stereoselectivity, resulting in 1:1 mixtures of α-vinylsilanes and (E)-β-isomers.283 A few years later, Deng and co-workers found that the threecoordinate cobalt(I) complex Co-5 could catalyze the hydrosilylation of terminal alkynes with high (E)-β-selectivity.48 Very recently, Thomas and co-workers49 described an (E)-β-selective hydrosilylation of 1-hexyne with PhSiH3 catalyzed by the (MesBIP)CoCl2 complex Co-202 and sodium tert-butoxide (Scheme 110a). The (Z)-selective hydrosilylation of terminal alkynes is more challenging than the synthesis of (E)-isomers. Using the sterically congested pyridine-2,6-diimine ligands MesPDI and iPr PDI, Ge group’s achieved the cobalt-catalyzed (Z)-selective anti-Markovnikov hydrosilylation of terminal alkynes (Scheme 110b).284 Using the MesPDI-Co(OAc)2 catalyst Co-298 generated in situ, hydrosilylation of terminal aromatic alkynes with PhSiH3 afforded the corresponding (Z)-vinylsilanes. The reactions of terminal aliphatic alkynes required the use of iPrPDI as the ligand in combination with Co(OAc)2 (Co-299). Furthermore, they found that the addition of phenol was crucial to obtain high (Z)-selectivity because it could effectively suppress the (Z/E)-isomerization of (Z)-isomers (Scheme 111). On the basis of the results of deuterium labeling and kinetic experiments, they proposed a catalytic cycle initiated by a low-valent Co(I) silyl intermediate. Silylmetalation of the terminal alkyne forms a cobalt vinyl intermediate (Z)-Co-304, which is followed by isomerization to the less sterically demanding complex (E)-Co-307 through Ojima−Crabtree type rearrangement. The intermediate (E)-Co-307 then reacts with PhSiH 3 to deliver (Z)-vinylsilane along with the regeneration of the Co silyl species. At almost the same time, Huang’s group developed a similar cobalt-catalyzed hydrosilylation reaction of terminal alkynes with high (Z)-β-selectivity.285 With the use of the (PNN)CoCl2 (Co-300)/NaBEt3H catalytic system, a variety of aliphatic terminal alkynes underwent hydrosilylation with Ph2SiH2 to afford the (Z)-β-vinylsilanes in high yields. However, aryl terminal alkynes were inactive under these reaction conditions. Notably, unsymmetrical internal alkynes were suitable substrates for this transformation, achieving high regioselectivity. More specifically, dialkyl-substituted alkynes produced vinylsilanes containing the silyl group at the olefinic carbon with the lowest steric hindrance. The reactions of arylalkyl disubstituted AU


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Scheme 110. Cobalt Catalysts for the Hydrosilylation of Terminal Alkynes

Scheme 111. Co-298 and Co-299-Catalyzed (Z)-β-Selective Hydrosilylation of Terminal Alkynes

Scheme 112. Co-300-Catalyzed (Z)-β-Selective Hydrosilylation of Alkynes

alkynes afforded hydrosilylation products with the silyl unit adjacent to the alkyl group (Scheme 112). Huang’s and Lu’s groups independently reported the cobaltcatalyzed Markovnikov (α-selective) hydrosilylation of terminal alkynes. A pyridinebis(oxazoline) cobalt complex, (tBuPyBox)CoCl2 (Co-301), was developed by Huang and co-workers as a precatalyst to promote the hydrosilylation of terminal alkynes with Ph2SiH2, affording α-vinylsilanes with broad functional

group tolerance (Scheme 110c and 113).286 A silyl migration mechanistic pathway involving the Co(I) silyl intermediate Co308 was proposed. Most likely because of a steric effect, the cobalt alkenyl species Co-310 was formed through a selective 1,2-insertion (Scheme 113). Meanwhile, Lu’s group287 used the oxazoline iminopyridine cobalt complex Co-302 (2,4DMBOIP·CoBr2) as a catalyst for Markovnikov hydrosilylation of alkynes with Ph2SiH2 (Schemes 110c and 114). Aryl terminal alkynes were converted into the corresponding α-vinylsilane products with excellent regioselectivity. However, silylalkynes AV


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Scheme 113. Co-301-Catalyzed α-Selective Hydrosilylation of Terminal Alkynes

substituted closer to the smaller substituents (also see Scheme 112). In Lu’s group’s previous study on the α-selective hydrosilylation of terminal alkynes,287 they found that the hydrosilylation of phenylacetylene with three equivalents of Ph2SiH2 in the presence of a chiral cobalt catalyst afforded the unexpected chiral silane product 44a in 35% yield with 68% ee. This result encouraged them to develop a cobalt-catalyzed one-pot regio- and enantioselective hydrosilylation/hydrogenation of alkynes.288 In the presence of cobalt complex OIP· CoCl2 (Co-311), Ph2SiH2, NaBHE3, and a balloon of H2, terminal aryl alkynes were converted into chiral organosilanes with high yields and enantioselectivities. However, the reaction of alkyl-substituted terminal alkynes only afforded vinylsilane products (41i and 39d) instead of the desired hydrosilylation/ hydrogenation products. Preliminary mechanistic studies indicated that the cobalt-catalyzed regioselective hydrosilylation of alkynes occurred as an initial step, and the subsequent enantioselective hydrogenation of the generated vinylsilanes furnished the final chiral organosilane products (Scheme 115). Scheme 115. Co-311-Catalyzed Regio- and Enantioselective Hydrosilylation/Hydrogenation of Terminal Alkynes

Scheme 114. Co-302-Catalyzed α-Selective Hydrosilylation of Alkynes

Although there are many synthetic methods for the formation of chiral silanes containing an α-carbon stereocenter,280,288−290 the asymmetric synthesis of silicon-stereogenic silanes is much more challenging. Very recently, a catalytic strategy involving desymmetrization of dihydrosilanes for the construction of silicon-stereogenic compounds was reported.291 Huang’s group developed the new chiral PyBox cobalt catalyst Co-312 for the regio- and enantioselective hydrosilylation of alkynes with diaryl dihydrosilanes Ph(Ar)SiH2, which provided a class of siliconstereogenic vinylhydrosilanes. Terminal aryl alkynes bearing either electron-rich or -deficient groups on the aryl rings were converted into the Markovnikov products in high efficiency with good enantioselectivity (59%−97% yield, branched/linear >99:1, 82%−91% ee). The Co-catalyzed hydrosilylation of terminal alkyl alkynes occurred smoothly, albeit with slightly lower regioselectivity (93:7−97:3). Importantly, this process was not limited to terminal alkynes; challenging internal alkynes were also suitable substrates. Aryl/alkyl disubstituted internal alkynes gave the products with high regioselectivities, favoring the products with the silyl unit adjacent to the aryl group. The unsymmetrical dialkyl alkyne 45g was converted to the vinylsilane containing the silyl group at the sterically more hindered carbon atom (Scheme 116).

and terminal alkyl alkynes gave products with lower selectivity. Internal alkynes were also suitable for this method, providing syn-addition products in yields of 33%−97%. Arylalkyl alkynes yielded trisubstituted olefins with the silyl unit adjacent to the aryl group, which is opposite to the regioselectivity in Huang’s work (see Scheme 112). For unsymmetrical dialkyl-substituted alkynes, such as 1-tert-butyl-1-propyne 42c, the results were similar to those of Huang’s group with the Si atom being AW


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fluoroalkylated alkynes with triethylsilicane.292 Various αtrifluoromethyl-α-silyl alkenes were afforded as the major product in this transformation. Isobe’s group initially reported the Co2(CO)6(HCCCMe2OH)-catalyzed hydrosilylation of unsymmetrical internal alkynes, which suffered from poor regioselectivity.293−295 In 2014, the same group described the syn-α-hydrosilylation of internal phenylthioalkynes using the same catalyst, affording cis-α-silyl-α-thioaryl-substituted olefins with high selectivity.296 The low-coordinate Co(I) alkyl complex Co-5 was found to be an effective catalyst to facilitate terminal alkyne hydrosilylation.48 Remarkably, the same cobalt catalyst is also active for the hydrosilylation of internal alkynes.48 Substrate scope studies revealed the excellent stereoselectivity of the reactions of diverse internal symmetrical alkynes with H2SiPh2, from which the syn-adducts were obtained in high yields. With respect to the internal unsymmetrical alkyne substrates, the syn-adducts were solely produced and the regioselectivity was proposed to be controlled by the steric properties of the substituents. For example, the hydrosilylation of alkynes bearing a TMS substituent could selectively afford (Z)-α,α-disilylalkenes (Scheme 118).

Scheme 116. Asymmetric Synthesis of Silicon-Stereogenic Vinylhydrosilanes

Scheme 118. Co-5-Catalyzed Hydrosilylation of Internal Alkynes In the past decades, several groups have explored the hydrosilylation of internal alkynes using cobalt catalysis (Scheme 117). In 2006, Butenschön and co-workers described the hydrosilylation of unsymmetrical alkynes using the aforementioned di-tert-butylphosphanylethylcyclopentadienyl cobalt complex Co-297 as the catalyst, albeit with poor regioselectivity.283 A few years later, Konno and co-workers demonstrated the catalytic activity of complex Co2(CO)8 toward the highly stereo- and regioselective hydrosilylation of Scheme 117. Cobalt Catalysts for the Hydrosilylation of Internal Alkynes

To shed light on the mechanism, stoichiometric reactions of complex Co-5 with hydrosilanes and alkynes, which generated the three-coordinate Co(I) silyl complex Co-36 and Co(I) alkyne complex Co-37, respectively, were investigated (see Scheme 13).48 The Co(I) silyl species Co-36 showed catalytic activity and selectivity in the hydrosilylation of 1-octyne comparable to those of complex Co-5. In contrast, Co(I) alkyne complex Co-37 gave a much lower yield. These results implied the in-cycle catalytically active nature of the Co(I) silyl species Co-36 and the importance of the PPh3 ligand to obtain high reaction efficiency. On the basis of these observations, the following modified Chalk−Harrod mechanism was proposed for this transformation. The ligand exchange of Co(I) silyl species Co-36 with an alkyne molecule generates the alkyne Co(I) silyl species Co-315, which then undergoes a migratory insertion to form the alkenyl cobalt species Co-316. Finally, cobalt complex Co-316 reacts with a hydrosilane to close the catalytic cycle, leading to the formation of the vinylsilane product (Scheme 119). For the TMS-substituted internal alkynes, the silacobaltation step is controlled by the bulky ligand 1,3diadamantylimidazol-2-ylidene (IAd), which determines the high regioselectivity of the catalytic reaction. AX


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carbocyclic and heterocyclic compounds, in which the newly formed reactive silyl group could potentially undergo further transformations.249,251 In 2016, Lu’s group developed the iminopyridine (IP) cobalt dichloride catalyst Co-255 for the hydrosilylation/cyclization of 1,6-enynes with silanes (Scheme 121).298 The reaction of various 1,6-enynes with Ph2SiH2 in the

Scheme 119. Proposed Mechanism for the Co-5-Catalyzed Hydrosilylation of Internal Alkynes

Scheme 121. Co-255-Catalyzed Hydrosilylation/Cyclization of 1,6-Enynes

Another interesting cobalt complex used in catalytic internal alkyne hydrosilylation is the low-valent cobalt(I) complex [HCo(PMe3)4]. The hydrosilylation of 1,2-diphenylethyne with various hydrosilanes (aryl/alkyl tertiary silanes and alkoxysilanes) catalyzed by this complex furnished syn-adducts in good yields with high (E/Z) ratios.297 Moreover, the hydrosilylation of sterically differentiated unsymmetrical alkynes led to the synadducts with the Si atom located close to the smaller substituents. For example, the TMS-substituted internal alkynes yielded (E)-α,β-disilylalkenes as the sole products, which is different from Deng’s group, who obtained (Z)-α,α-disilylalkenes as major products.48 A Chalk−Harrod-type mechanism was proposed, in which a hydrocobaltation step controlled the regioselectivity (Scheme 120). Scheme 120. HCo(PMe3)4-Catalyzed Hydrosilylation of Internal Alkynes presence of the precatalyst Co-255 and NaBHEt3 as a reductant at room temperature afforded the alkyl silane products in yields of 51%−80%. The following Chalk−Harrod-type mechanism was proposed for this transformation. Initially, the cobalt dichloride precatalyst could be reduced by NaBHEt3 to provide an in-cycle catalytically active cobalt hydride species. The subsequent alkyne insertion into the Co−H bond affords the vinyl cobalt intermediate, which is followed by alkene insertion to give an alkyl cobalt species. This intermediate could then react with the silane to release the cyclization product. Hydrosilylation of allenes is one of the most straightforward and atom-economical methods to generate synthetically valuable vinylsilanes and allylsilanes.299−302 However, this reaction can produce six isomeric organosilane products, so the major difficulty is controlling both regio- and stereoselectivity. Most transition metal-catalyzed terminal allene hydrosilylations show high selectivity for β,γ-hydrosilylation, affording vinylsilanes or allylsilanes. There have been few reports on the α,β-hydrosilylation of terminal allenes. In this case, four isomeric allylsilane and vinylsilane products can potentially be formed, and control of the E/Z selectivity is rather difficult (Scheme 122). In 2017, Ge’s group reported a cobalt-catalyzed α,βhydrosilylation reaction of allenes to (Z)-allylsilanes (Scheme 123).303 With the use of a catalytic system generated in situ from the bench-stable cobalt precursor Co(acac)2 and phosphinebased ligand rac-BINAP or xantphos, diverse mono- and disubstituted allenes reacted with PhSiH3 to provide the corresponding disubstituted (Z)-allylsilanes with high yields (74%−92%) and excellent stereoselectivity (Z:E = 99:1). To probe the mechanism of this transformation, deuterium-labeling

The aforementioned cobalt catalysts (PCNN)CoCl2 (Co300), 2,4-DMBOIP·CoBr2 (Co-302), and PyBox·CoCl2 (Co312) developed by the groups of Huang285,291 and Lu287 for terminal alkyne hydrosilylation also displayed remarkable reactivity and selectivity in the hydrosilylation of internal alkynes (Schemes 112, 114, and 116). Hydrosilylation/cyclization of 1,6-enynes provides efficient and convenient access to silicon-containing five-membered AY


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deuterated (Z)-allylsilane with a deuterium atom positioned trans to the phenyl group. Subsequently, they reacted Co(acac)2 and PhSiH3 with various bisphosphine ligands. With the use of two equivalents of the dppbz ligand, this reaction afforded the well-defined cobalt hydride species (dppbz)2CoH in 70% yield. This hydride species showed comparable catalytic activity and selectivity to those of the cobalt catalyst formed in situ. On the basis of these experimental observations, it was proposed that a cobalt(I) hydride intermediate could be involved in this catalytic process. The corresponding allene adduct Co-326 undergoes migratory insertion to form the η 1-bound allyl cobalt intermediate Co-327. Because of the steric repulsion between the larger group of the allene substrate and ligand of the cobalt complex, the formation of η3-bound allyl cobalt intermediate Co-328 is unfavorable and high (Z)-selectivity is realized. The η1-bound allyl cobalt intermediate then reacts with hydrosilane to furnish the (Z)-allylsilane product (Scheme 124). At almost the same time, Ma and Huang and co-workers also reported a cobalt-catalyzed (Z)-selective allene hydrosilylation.304 With the use of (tBuPCNNiPr)CoCl2 as the catalyst and NaBHEt3 as the precatalyst activator, both mono- and 1,1disubstituted allenes were smoothly transformed and a variety of functional groups were well-tolerated (Scheme 125).

Scheme 122. Transition Metal-Catalyzed Hydrosilylation of Allenes

Scheme 123. Co(acac)2-Catalyzed Hydrosilylation of Allenes

Scheme 125. (tBuPCNNiPr)CoCl2-Catalyzed Hydrosilylation of Allenes

experiments were carried out (Scheme 124). The hydrosilylation of buta-2,3-dien-2-ylbenzene with PhSiD3 gave the Scheme 124. Proposed Mechanism for the Co(acac)2Catalyzed Hydrosilylation of Allenes

Catalytic hydrosilylation is also a practical protocol for the reduction of carbonyl compounds into alcohols because of its manipulative simplicity and mild reaction conditions.305,306 In 2013 and 2015, Li and co-workers reported the sulfurcoordinated acyl(hydrido)cobalt(III) complex Co-330 and hydrido CNC pincer cobalt complex CoH(PMe3)2[(C6H4)CHN(C10H6)] Co-331, respectively, for the hydrosilylation of ketones and aldehydes.307,308 Compared with aldehyde hydrosilylation, the reduction of ketones requires a higher temperature and longer reaction time using the same cobalt catalyst (Scheme 126). In 2015, Peters’ group investigated the catalytic activity of a (DPB)Co(N2) complex Co-332 that can serve as an active precatalyst for the hydrosilylation of aldehydes and ketones.309 They observed rapid hydrosilylation of aldehydes by Co-332, whereas hydrosilylation of ketones was sluggish. Increasing the steric hindrance at the carbonyl accounted for the decrease in rate, as evidenced by comparing the results for hydrosilylation of 3,3-dimethylbutan-2-one and pentan-3-one. In addition, the reaction of 100 equiv of Ph2SiH2 with Co-332 led to the formation of the new paramagnetic species (DPBH)Co(SiHPh2) Co-333, which was detected by IR and EPR spectroscopies. On the basis of the experimental results, they AZ


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Scheme 126. (Hydrido)cobalt(III) Complexes Co-330 and Co-331-Catalyzed Hydrosilylation of Carbonyl Compounds

Scheme 127. Co-332-Catalyzed Hydrosilylation of Carbonyl Compounds

speculated that the initial step in the catalytic cycle is the ligand exchange of a Co−N2 complex with a silane molecule along with Si−H bond activation to generate species Co-333. The subsequent ketone insertion into the Co−Si bond affords a borohydrido−siloxylalkyl intermediate, which is followed by a reductive elimination process to release the hydrosilylation product (Scheme 127). The first cobalt-catalyzed asymmetric ketone hydrosilylation was reported by Brunner and Amberger in 1991, in which chiral 1-phenylethanol with 56% ee was produced from acetophenone and PhSiH3 in the presence of 5 mol % of Co(py)6BF4 along with a chiral monooxazolinylpyridine ligand.310 Over the past decade, several cobalt-based asymmetric ketone hydrosilylation catalysts with high enantioselectivity have been developed. In 2010, Nishiyama and co-workers311 used the chiral N,N,Nbis(oxazolinylphenyl)amine (Bopa) ligand with Co(OAc)2 as the catalyst for the asymmetric hydrosilylation of aryl ketones to attain (R)-alcohols with high enantioselectivity. Shortly after, Chan’s group reported a cobalt-catalyzed asymmetric hydrosilylation of electron-withdrawing aryl ketones in the presence of PhSiH3, 10 mol % of Co(OAc)2·4H2O, and dipyridylphosphine ligand (S)-Xyl-P-Phos to afford the corresponding (S)-alcohols with up to 96% ee and 5%−99% yields.312 Using a 1,3-bis(2pyridylimino)isoindoline (BPI)-supported cobalt alkyl complex as the catalyst, Gade’s group achieved the asymmetric hydrosilylation reaction of aryl ketones with (EtO)2MeSiH, affording chiral alcohols with up to 91% ee (Scheme 128).313 Very recently, Lu’s group reported a cobalt-catalyzed asymmetric hydrosilylation of aryl ketones using chiral iminophenyl oxazolinylphenylamine (IPOPA) as the ligand.314 This catalytic process starts with the activation of IPOPA cobalt dichloride complex (Co-339) using NaBHEt3 and (EtO)3SiH to form the proposed cobalt hydride species Co-340. Subsequent coordination of a ketone to the cobalt center of the hydride species gives rise to π(C = O)-coordinated intermediate Co341, which undergoes CO insertion to form complex Co-342. Complex Co-342 then reacts with (EtO)3SiH to deliver the chiral hydrosilylation products and close the catalytic cycle. Finally, the hydrosilylation products undergo a hydrolysis process to afford the corresponding chiral alcohols. During the migration insertion step of the ketone into the Co−H bond, because of the steric repulsion between the isopropyl group of the imine ligand and aryl group of the ketone, it is unfavorable

for the cobalt hydride species to approach from the Re-face of the substrates. In contrast, when the hydride species approached from the Si-face of the substrates, a weaker steric effect was exhibited, leading to the (R)-alcohol products with high enantioselectivity (Scheme 129). Apart from aldehydes and ketones, cobalt catalysis has also been used to promote the hydrosilylation of CO2 by Chirik’s group.315 In the presence of 0.5 mol % of (tBuPNP)CoH (Co343) and four equivalents of PhSiH3, a mixture of oligomers containing silyl ether, bis(silyl)acetyl, and silyl formate subunits was obtained via CO2 hydrosilylation (Scheme 130). 3.5.2.2. Dehydrogenative Silylation Reactions. Dehydrogenative silylation of alkenes provide a strategy for C−Si bond formation but maintain the unsaturation of the olefin substrate in the final product. In 2014, the group of Chirik described a bis(imino)pyridine cobalt-catalyzed methodology for the dehydrogenative silylation of linear α-alkenes to selectively form the allysilane products with the silyl unit located at terminal position of the hydrocarbon chain (Scheme 131a).316 In addition, this method was further extended to internal alkene substrates and produced a functionalization sequence for remote BA


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Scheme 128. Cobalt-Catalyzed Asymmetric Hydrosilylation of Ketones

Scheme 130. Co-343-Catalyzed CO2 Hydrosilylation

Scheme 131. (MesPDI)CoCH3-Catalyzed Dehydrogenative Silylation of Alkenes

Scheme 129. Co-339-Catalyzed Asymmetric Hydrosilylation of Aryl Ketones

C−H bond (Scheme 131b). In the presence of (MesPDI)CoCH3 (0.5−1 mol %), stirring a 2:1 mixture of alkenes and (Me3SiO)2MeSiH resulted in high conversion to the terminal allylsilanes. An equivalent of corresponding alkane was also formed and leads to the balance of the dihydrogen. Similar catalytic performace was obtained with the combination of

pyridine-diimine cobalt chloride complex (MesPDI)CoCl2 and MeLi or NaBEt3H. To shed light on the mechanism, the activation mode of (MesPDI)CoCH3 was examined under catalytic conditions. Addition of 1 equiv of (Me3SiO)2MeSiH to a benzene-d6 BB


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solution of (MesPDI)CoCH3 procured liberation of a stoichiometric quantity of CH4 immediately; however, the putative bis(imino)pyridine cobalt silyl product was too reactive to be observed under these conditions. Catalytic dehydrogenative silylation of 1-octene with (Me3 SiO) 2MeSiD was also conducted in the presence of (MesPDI)CoCH3, generating octane-1-d1 along with the expected allylsilane (Scheme 131c). A mechanism consistent with the experimental results was presented in Scheme 131d by Chirik and co-workers. The sequence starts with activation of (MesPDI)CoCH3 with the tertiary silane to produce methane and the putative cobalt silyl species, which undergoes 2,1-olefin insertion followed by selective β-H elimination from the carbon away from the large tertiary sily unit to furnish a cobalt hydride and the allylsilane. Subsequently, the insertion of a second molecule of olefin into the Co−H species followed by the reaction with silane regenerates the putative cobalt silyl and completes the catalytic cycle. Recently, Chen’s group showed that the combination of NaOtBu and pyridine-diimine cobalt chloride complex (MesPDI)CoCl2 (Co-202) could also effectively promote the dehydrogenative silylation of 1-octene with various silanes with very high yield (Scheme 132). Then, by using a α-diimine nickel catalyst, the allylsilanes are copolymerized with ethylene and furnish branched polyolefins.317

provide products with differentiated alkene functionalities capable of further derivatization.319−322 Vogt and co-workers reported a highly selective cobalt-catalyzed hydrovinylation of styrenes using a combination of CoCl2 and diphosphine as the catalyst precursor. AlClEt2 served as an activator to promote the formation of the catalytically active Co−H species. The ligand effect in this transformation is outlined in Scheme 134.323,324 A Scheme 134. Cobalt-Catalyzed Hydrovinylation of Styrenes

Scheme 132. Co-202-catalyzed Dehydrogenative Silylation of 1-Octene

In 2018, Rauchfuss and co-workers introduced a new P−N− N ligand platform. Labeled iPrPQpy, the tridentate PNN ligand embraces the cobalt by two five-membered chelate rings. On the basis of several crystallographic studies, the cobalt complex CoCl2(iPrPQpy) (Co-348) features highly planar CoPNN core. To seek applications of this cobalt complex, the authors sought to exploit the dehydrogenative silylation of ethylene with various hydrosilanes, leading to the formation of vinylsilanes (Scheme 133).318 3.5.3. Hydrocarbon Functionalizations. 3.5.3.1. Hydrovinylation Reactions. Hydrovinylation is a synthetically useful transformation that can enhance structural complexity and

series of chiral ligands were also tested with the aim of developing an asymmetric version of this reaction. Promising enantioselectivity (up to 50% ee) was achieved, although a relatively high pressure of ethylene (30 bar) was required.323,324 A further improvement in asymmetric hydrovinylation of styrenes at low ethylene pressure (1.2 bar) was made by Schmalz and co-workers by introducing a chiral phenol-derived phosphine−phosphite ligand (Scheme 135).325 This reaction showed a broad substrate scope, including functionalized styrenes, heterocyclic vinylarenes and vinylferrocene, to furnish the chiral-branched products in high yields along with up to >99% ee. With regard to the hydrovinylation of 1,3-dienes, selectivity control is complicated because both 1,2-addition and 1,4addition might occur. Nevertheless, inspired by pioneering work by Hilt’s group, many successful examples of 1,3-diene hydrovinylation using cobalt catalysis have been reported.326−338 In 2001, Hilt and co-workers discovered that the catalytic system consisting of CoBr2(dppe)/ZnI2/Bu4NBH4 efficiently promoted the 1,4-hydrovinylation of 1,3-dienes to afford either linear or branched 1,4-diene products depending on the vinyl coupling partners (Scheme 136).331 The reaction of acrylates such as ethyl acrylate or tert-butyl acrylate led to predominantly linear 1,4-hydrovinylation products. On the contrary, using β, γ- and γ,δ-unsaturated carbonates or allyl

Scheme 133. Co-348-Catalyzed Dehydrogenative Silylation of Ethylene

BC


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Scheme 135. Cobalt-Catalyzed Asymmetric Hydrovinylation of Styrenes

Scheme 137. Cobalt-Catalyzed Asymmetric Hydrovinylation of 1,3-Dienes

Scheme 136. Cobalt-Catalyzed Hydrovinylation of 1,3Dienes

vinylcycloalkenes [Scheme 137, eq (2)]340 and the synthesis of functionalized chiral enolates [Scheme 137, eq (3)].341 In the former case, excellent regioselectivity toward 1,4-addition products was obtained, distinguishing it from the Ni-catalyzed protocol, which mainly produced the 1,2-adducts.342 Mechanistic investigations of this transformation revealed that the product distribution is strongly related to the nature of the ligand and reaction temperature.343 Using ligands with narrow bite angles led to predominantly 1,2-hydrovinylation, and the Econfiguration of the original diene was maintained. Most other ligands led to exclusively the branched (Z)-1,4-hydrovinylation products at low temperature. As the temperature rose, an increasing amount of branched (E)-1,4-adducts was generated. The proposed reaction mechanism is outlined in Scheme 138. The catalyst precursor diphosphine cobalt(II) dichloride reacted with Me3Al via transmetalation and halide abstraction to give the cationic methyl cobalt intermediate. Subsequent alkene insertion and β-hydride elimination generate the catalytically active cationic cobalt hydride species. A 1,3-diene could then coordinate to the cobalt hydride to afford a η4complex. Then, an allyl cobalt intermediate is formed through alkene moiety insertion into the Co−H bond. The subsequent ethylene insertion into the Co−C4 bond leads to the intermediate Co-358. Finally, β-hydride elimination in complex Co-358 delivers the branched 1,4-hydrovinylation product and regenerates the cobalt hydride species. In 2017, RajanBabu and co-workers disclosed a novel enantioselective hydrovinylation of 1,3-dienes using acrylates, which are fundamental feedstocks (Scheme 139).344 A variety of functional groups were well-tolerated in this heterodimerization process, such as halides, isolated mono- and disubstituted double bonds, esters, silyl ethers, and silyl enol ethers. It was found that the counterion played an important role in facilitating the formation of the cationic Co(I) complex, which was identified as the catalytically active species. Preliminary mechanistic studies suggested the following Co(I)/(III) catalytic cycle. A Co(I) complex is readily generated from the

ethers selectively provided branched 1,4-dienes. The researchers then described a ligand-controlled regioselective hydrovinylation scenario.333 With the use of dppe as a ligand, a high level of branched regioselectivity was obtained, whereas with the SchmalzPhos ligand, the regioselectivity was directed toward linear 1,4-dienes. Cobalt-catalyzed asymmetric hydrovinylation of 1,3-dienes has been systematically studied by RajanBabu’s group. High regio- and enantioselectivity were obtained using (chiral diphosphine)CoCl2/AlMe3 as the catalyst [Scheme 137, eq (1)].339 The hydrovinylation of alkyl-substituted 1,3-dienes led to (Z)-1,4-diene products with excellent enantioselectivity. Unfortunately, the substrate bearing a Lewis basic benzyloxyl group gave a low yield of the desired product, and the aryl conjugated diene afforded an almost racemic hydrovinylation product. The same group subsequently expanded the utility of this methodology to the asymmetric hydrovinylation of 1BD


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Scheme 138. General Mechanism for Hydrovinylation of 1,3Dienes Catalyzed by a Cobalt(II)/Alkylaluminum Reagent System

Scheme 139. Enantioselective Hetero-Dimerization of Acrylates and 1,3-Dienes

Co(II) precursor by treatment with a reductant (Zn/Mn), which then reacts with a Lewis acid or NaBARF (sodium tetrakis-3,5-(bis-3,5-trifluoromethylphenyl)borate) to form the putative cationic Co(I) intermediate. The subsequent oxidative cyclometalation and β-hydride elimination lead to the allyl cobalt intermediate Co-364, which undergoes reductive elimination to furnish the final hydrovinylation product. 3.5.3.2. Hydroarylation Reactions. Hydroarylation reactions involving a metal-assisted C−H activation process have been widely studied and are a powerful tool for C−H functionalization.35,345−347 Representative results for hydroarylation via cobalt catalysis are summarized in Scheme 140. In 1994, Kisch’s group developed an ortho-alkenylation of azobenzene derivatives in the presence of Co(H)N2(PPh3)3 as the catalyst, which was the pioneering work in this area.348 Since then, a large number of catalytic systems containing cobalt(II) or cobalt(III) precursor, ligand, and Grignard reagent have been developed for the hydroarylation of alkenes or alkynes. Yoshikai’s group has made great contributions to this field by identifying a variety of aromatic substrates that are amenable to cobalt-catalyzed hydroarylation reactions. Most of the aromatic substrates possess a directing group such as an imido,349−356 pyridyl,357,358 or 2-pyrimidyl359 moiety, which induces C−H activation selectively at its ortho position. Amide can also serve as a directing group, as reported by Nakamura and co-workers.360 Indoles361,362 and azoles363 are also appropriate for this transformation, leading to C2-functionalized products. As well as alkenes and alkynes, imines can also undergo hydroarylation with 2-arylpyridines in a similar manner.364 In addition, intramolecular hydroarylation of alkenes has been investigated by Yoshikai’s group.365

A general mechanism for cobalt-catalyzed hydroarylation is outlined in Scheme 141. It has been speculated that the catalytically active cobalt(0) complex can be generated from CoBr2 under reducing conditions, which initiates the catalytic cycle for the hydroarylation process. Hydroarylation involves four steps: (1) alkyne coordination to the cobalt center; (2) imine-directed oxidative addition of the C−H bond; (3) alkyne insertion into the Co−H bond; and (4) reductive elimination to release the hydrovinylation product and regenerate the cobalt(0) species. In 2013, Kanai and co-workers developed a novel strategy to achieve C4-selective alkylation of pyridines (Scheme 142).366 They presumed that hydrometalation of an alkene with a metal hydride species could generate an alkyl metal intermediate. If the nucleophilicity of the alkyl metal complex is adequate, its addition to the C4-position of pyridine would afford amino metal complex Co-371. The subsequent rearomatization of complex Co-371 could liberate the alkylated pyridine product and regenerate the metal hydride species. With the use of CoBr2 as the catalyst with LiBEt3H and Et3B as activators, the preconceived C4-alkylation reaction proceeded smoothly to give branched hydroarylation products of styrene derivatives. Linear products were obtained with good-to-high selectivity from the reactions of aliphatic alkenes. A highly chemo- and stereoselective cobalt-catalyzed tandem cyclization−hydroarylation reaction was developed by Cheng and co-workers [Scheme 143, eq (1)].367 WIth the use of a CoBr2/dppp (1,3-bis-diphenylphosphinopropane) /Zn, ZnI2 system, 1,6-enynes were reacted with aromatic ketones or aromatic esters to afford pyrrolidine and dihydrofuran BE


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Scheme 140. Cobalt-Catalyzed Hydroarylation Reactions

Scheme 141. General Mechanism for Cobalt-Catalyzed Hydroarylation Reactions

Scheme 142. Cobalt-Catalyzed C4-Selective Alkylation of Pyridines

derivatives in high yields with excellent selectivities. Later, the same group described a cobalt-catalyzed ligand-controlled regiodivergent C−H functionalization of aromatic aldehydes with 1,6-enynes [Scheme 143, eq (2)].368 The regioselectivity of this reaction is determined by the nature of the ligand. Using dppen (1,2-bis (diphenylphosphino) ethene) as the ligand, hydroarylation−cyclization of enynes occurred exclusively via ortho C−H activation on the aromatic ring. In contrast, using dppp as the ligand led to hydroacylation−cyclization of enynes

via C−H activation of the aldehyde group. Cheng and coworkers proposed a possible mechanism that included two distinct reaction pathways. First, a low-valent cobalt species could undergo oxidative cyclometalation with an enyne to give five-membered cobaltacycle Co-374. When dppen is used as the ligand, the sequence of aldehyde complexation and deprotonative metalation of the ortho C−H bond would furnish the intermediate Co-376. Subsequent reductive elimination proBF


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Scheme 143. Cobalt-Catalyzed Ligand-Controlled Regiodivergent C−H Functionalization

Scheme 144. Cobalt-Catalyzed Intermolecular and Intramolecular Hydroacylation Reactions

Scheme 145. Cobalt-Catalyzed C1-Regioselective Hydroacylation of 1,3-Dienes

duces the hydroarylation cyclization product. Alternatively, when dppp is used as the ligand, the carbonyl group could insert into the Co−C bond to afford intermediate Co-377. Subsequent β-hydride elimination from complex Co-377 results in cobalt hydride species Co-378, which undergoes reductive elimination to deliver the hydroacylation cyclization product. 3.5.3.3. Hydroacylation Reactions. Hydroacylation is formal addition of an aldehyde C−H bond to an unsaturated bond, which is an attractive approach to provide functionalized ketones.369 Both intramolecular and intermolecular hydroacylation reactions enabled by cobalt catalysis have been developed over the past two decades. Brookhart’s group reported the first example of a cobalt-catalyzed hydroacylation reaction.370,371 Using a cobalt bis-olefin complex as the catalyst, a variety of aromatic and aliphatic aldehydes were reacted with vinylsilanes to give anti-Markovnikov adducts [Scheme 144, eq (1)]. They also demonstrated an intramolecular hydroacylation with aromatic aldehydes [Scheme 144, eq (2)]. The reaction was initiated by C−H bond oxidative addition to form a cobalt hydride species, followed by alkene insertion into the Co−H bond and subsequent reductive elimination to give the hydroacylation product. Recently, the cobalt-catalyzed hydroacylation of 1,3 dienes has also been developed. In 2014, Dong’s group described a cobalt-catalyzed C1-regioselective hydroacylation of 1,3-dienes (Scheme 145).372 The hydroacylation of aromatic aldehydes tends to provide 1,4-addition products. Interestingly, the regioselectivity was reversed to provide 1,2-adducts as the major isomer for the reactions of aliphatic aldehydes. The

authors proposed an oxidative cyclometalation mechanism, which was different from the traditional hydroacylation mechanism. It was supposed that this reaction was initiated by oxidative cyclometalation of a catalytically active cobalt(I) species with a 1,3-diene and aldehyde to furnish cyclic cobalt(III) allyl intermediate Co-385. This species could be in equilibrium with the seven-membered cobaltacycle Co-386 and five-membered cobaltacycle Co-388 via π−σ−π isomerization. Cobaltacycle intermediates Co-386 and Co-388 would then undergo β-hydride elimination and subsequent reductive BG


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catalysts exhibit superior reactivity and selectivity to those of non-noble metal catalysts. The first cobalt-catalyzed olefin isomerization was reported in the 1960s, and considerable progress has been made in recent years.2,72,396−400 In this section, we focus on olefin isomerization reactions enabled by cobalt catalysis involving hydride transfer; other cobaltcatalyzed olefin isomerization reactions that proceed through the HAT mechanism are not discussed in this review.401,402 3.6.1. Isomerization of 1-Alkenes. The cobalt hydride complex [HCo(CO)4], which is capable of promoting hydroformylation of olefins, was also found to be active for olefin isomerization.403 Orchin’s group later used this cobalt catalyst for the isomerization of allylic alcohols into carbonyl compounds [Scheme 148, eq (1)].404 However, the yields of

elimination to afford the corresponding 1,4-addition and 1,2addition products, respectively. In 2015, Yoshikai and co-workers reported a directed intermolecular formal hydroacylation of alkenes with aldimines as aldehyde equivalents, which gave ketone products upon further acid hydrolysis (Scheme 146).373 With the use of a Scheme 146. Cobalt-Catalyzed Intermolecular Formal Hydroacylation of Alkenes

Scheme 148. Co(CO)4H-Catalyzed Olefin Isomerization catalytic system of CoBr2/diphosphine ligand/Zn, a wide range of olefins including styrenes, vinylsilanes, and aliphatic alkenes were suitable substrates in this transformation, displaying high linear selectivity. Cobalt-catalyzed asymmetric intramolecular hydroacylation was first reported by Yoshikai’s group [Scheme 147, eq (1)].374 Scheme 147. Cobalt-Catalyzed Asymmetric Intramolecular Hydroacylation of Ketones and Alkenes

carbonyl compounds were quite low (3.0%−21%), even though high catalyst loadings (20−50 mol %) were used. In accordance with the results of deuterium labeling experiments using Co(CO)4D, an intermolecular 1,3-H shift mechanism was proposed because the deuterium atom was completely transferred to the β-position of the propanal. The same group also studied the isomerization of propene by crossover experiments [Scheme 148, eq (2)].405 They performed the reaction using propene-d6 (C3D6) and one equivalent of 1-butene in the presence of HCo(CO)4. Several deuterated propene compounds (C3D6, C3D5H, C3D4H3, and C3D3H3) were detected from this reaction, indicating an intermolecular hydride transfer scenario. In this case, the authors proposed a sequential [M−H] insertion/β-hydride elimination reaction pathway. A similar mechanism was also proposed by Rosenberg’s group.406 Isomerization of 3-phenylpropene-3,3-d2 catalyzed by HCo(CO)4 afforded trans-1-phenylpropene in 80% yield [Scheme 148, eq (3)]. Appreciable deuterium at the β-vinyl position was observed by NMR spectroscopy as well as a trace amount of PhCDCHCH3. These findings are consistent with the [M− H] insertion/β-hydride elimination mechanism, which is incompatible with an intramolecular 1,3-H shift process previously reported for allyl benzene isomerization.407 Another cobalt hydride catalyst, HCo(N2)(PPh3)3,408 was found to be effective for the isomerization of allylic amines to the corresponding enamines (Scheme 149).409 N,N-Diethylgeranylamine and N,N-diethylnerylamine could be isomerized by 1.0 mol % of this cobalt catalyst to generate the trans-enamine product in 85% yield along with a small amount (

Hydride Transfer Reactions Catalyzed by Cobalt Complexes

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