Cobalt Complex-Catalyzed Hydrosilylation of Alkenes and Alkynes

Nov 30, 2015 - By screening a series of 2,6-iminopyridine-cobalt complexes, the cobalt complexes (MesPDI)CoCH3 (E in Table 1) and (MesPDI)Co(N2) prove...
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Cobalt Complex-Catalyzed Hydrosilylation of Alkenes and Alkynes Jian Sun, and Liang Deng ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.5b02308 • Publication Date (Web): 30 Nov 2015 Downloaded from http://pubs.acs.org on December 6, 2015

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Cobalt Complex-Catalyzed Hydrosilylation of Alkenes and Alkynes Jian Sun and Liang Deng* State Key Laboratory of Organometallic Chemistry, Shanghai Institute of Organic Chemistry, Chinese Academy of Sciences, 345 Lingling Road, Shanghai, 200032, People’s Republic of China ABSTRACT: The demanding for economical and environmentally benign catalysts for important chemical transformations has intrigued great recent efforts on non-precious metal-catalyzed hydrosilylation reactions. The special chemical properties of cobalt enable the development of diverse cobalt complex-based catalysts for hydrosilylation reactions. This paper reviews the significant advances of cobalt complex-catalyzed hydrosilylation of alkenes and alkynes from the early study in the 1960s to now, with the aim to provide readers with the status of the field and the underlying late 3d metal chemistry that is meaningful for new non-precious metal catalyst design. Progress, problems, and perspectives in this vibrant field are discussed.

Key Words: Cobalt, Hydrosilylation, Alkene, Alkyne, Selectivity Introduction Hydrosilylation, the addition of a Si-H bond across a multiple bond, is a very useful reaction for the production of organosilicon compounds that are widely used in organic synthesis, polymer chemistry and material science.1,2 As hydrosilanes are generally unreactive toward common unsaturated bonds, UV light-irradiation, heat, or a catalyst has to be used to promote the addition reaction.1 The latter method, namely catalytic hydrosilylation, is the most prevailing one used in laboratories and industry for its high selectivity and applicability under mild reaction conditions. Up to now, noble metal complexes are the dominant catalysts used for hydrosilylation reactions,1 and among them, platinum compounds are the most efficient and widely used ones. Linked with the huge production of organosilicon compounds, the uptake of platinum in hydrosilylation reactions is no little. It was estimated that the silicone industry worldwide depleted ca. 5.6 tons of platinum in 2007, which is unrecyclable.3 The high cost of platinum urges research efforts to develop non-precious metal replacements. The late 3d transition-metals, iron, cobalt, and nickel, with their earth-abundant, low-cost, and low-toxicity features are promising candidates for non-precious metalbased hydrosilylation catalysts. Early explorations showed that certain late 3d metal complexes are capable of facilitating alkene hydrosilylation, although the catalytic efficiency is not as high as the noble metal catalysts.4 The continuing efforts in the recent decades have led to the discovery of efficient iron-based hydrosilylation catalysts. Some of them exhibit high activity even comparable to

that of platinum catalysts.5 The development of ironbased hydrosilylation catalysts has been summarized in several reviews.6 Along with the progress on iron catalysts, the explorations on cobalt-based hydrosilylation catalysts are also fruitful, and cobalt-catalyzed hydrosilylation reactions have shown diversified substrate scope and high selectivity. The lack of pertinent review for this vibrant field thus prompts us to summarize the advances of cobalt complex-catalyzed hydrosilylation reactions. In this review, we will at first present the chemical features that test cobalt complexes’ potential as efficient hydrosilylation catalysts. Then, the advances of cobalt complex-catalyzed hydrosilylation of alkenes and alkynes will be summarized. In the last section, a brief summary of the major progresses, the unsolved problems in the field and their possible solutions will be presented. The intention of this review is to provide readers with not only the status of cobalt complex-catalyzed hydrosilylation reactions, but also the underlying late 3d metal chemistry that is meaningful for new non-precious metal catalyst design. It should be mentioned that, in addition to cobalt-catalyzed hydrosilylation of carbon-carbon multiple bonds, significant progress in cobalt-catalyzed hydrosilylation of carbon-heteroatom multiple bonds has also been achieved.7 Since the progress in this area has been reviewed,7a,b it is not included in the article. The Features of Cobalt and Its Complexes Cobalt comprises 0.0029% (in mass) of the Earth's crust.8 Its abundance is lower than iron, but is much higher than the heavier platinum group metals, e.g. ca. 40000 and 70000 folds excess than rhodium and iridium,

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respectively.8 Cobalt is inexpensive and its price is ca. 27500 US dollars/ton according to the recent London Metal Exchange data.9 For organisms, cobalt is a trace dietary element to all mammals, and it occurs in metalloenzymes, such as coenzyme B12-dependent enzymes, methionine aminopeptidase 2, and nitrile hydratase.10,11 Cobalt has the common features of the first-row transition-metals relative to those of the 4d and 5d metals, such as small electronegativity, small atomic radii, and “harder” according to the Soft-Hard Acid-Base theory.12 Calculation studies on RM(CO)3 (R = CH3, H; M = Co, Rh, Ir) revealed the lower bond energies of the Co-CH3 and Co-H bonds than those of their heavier metal analogs.13 It has also been proposed that 3d metal species have high density of states as compared to the analog 4d and 5d metal species.14 Endowed by these characters, cobalt complexes could exhibit different reactivity pattern over their rhodium and iridium analogues. A notable example is the C-H and Si-H bond-activation reactions of the 16e- species CpM(CO) (M = Co, Rh). Studies by Bergman, Harris, Siegbahn, Harvey et al. revealed that the 16e- cobalt(I) species CpCo(CO) has a triplet ground spin-state and interacts weakly with C-H bonds, whereas, the rhodium species CpRh(CO) has a low-spin ground spin-state and can undergo facile oxidative addition reactions with C-H bonds.15 This difference might make the triplet cobalt species hard to be trapped in alkyl-solvated intermediates. However, when encountering a more coordinating Si-H bond, the triplet cobalt species can readily undergo spincrossover to a singlet state, leading to subsequent oxidative addition with the Si-H bond.15e The redox chemistry of cobalt is quite rich. The common oxidation states of cobalt observed in its organometallic compounds are broad, including +3, +2, +1, 0 and -1.16 Comparing with its neighboring metals, cobalt displays the ease to form low valent complexes over iron, and to form high valent compounds over nickel. In redox reactions, both one-electron and two-electron redox processes are common for cobalt complexes. By taking advantage of its rich redox chemistry, versatile cobalt-mediated catalytic cycles can be envisioned.

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kynes with high catalytic efficiency and high selectivity have been developed. Chart 1 lists the representative cobalt complexes developed for the catalytic hydrosilylation of alkenes (A-F) and alkynes (A, G-I).

Chart 1. Examples of Cobalt Catalysts Used in Hydrosilylation of Alkenes and Alkynes Dicobalt octacarbonyl, Co2(CO)8, is the most extensively studied cobalt-based hydrosilylation catalyst. In 1960’s, Chalk and Harrod reported that, under a dinitrogen atomosphere, Co2(CO)8 can catalyze the hydrosilylation of alkyl substituted terminal alkenes (1-octene, 1-hexene, and 1-pentene) with tertiary silanes (HSiEt3, HSi(OMe)3, and HSiPhCl2) to selectively produce the anti-Markovnikov addition products n-alkyl silanes in excellent yields (Scheme 1).4b, 17 The catalytic reaction is solvent-free, and can be conducted at ambient temperature (0 to 60 oC) with low catalyst loading (0.01 to 0.001 M Co2(CO)8 equaling to ca. 0.6 to 0.06 mol%). One shortcoming of these reactions is the use of an excess amount of alkenes (3 equiv relative to the hydrosilanes) due to the coexistence of cobalt-catalyzed isomerization of terminal alkenes to internal alkenes. The study noted that this side reaction is faster than the hydrosilylation reaction and more pronounced than the reactions with platinum and rhodium catalysts.4b

Cobalt Complex-Catalyzed Alkene Hydrosilylation Ever since Chalk and Harrod’s finding of Co2(CO)8catalyzed hydrosilylation of terminal alkenes in the 1960s,4b there has been long-lasting research interest on cobalt-catalyzed hydrosilylation reactions. While the early studies did not result in highly efficient cobalt-based hydrosilylation catalyst, it was from these pioneering studies that the two most important mechanisms accounting for late transition-metal-catalyzed hydrosilylation of alkenes, Chalk-Harrod mechanism and modified Chalk-Harrod mechanism were proposed. The more recent efforts have been devoted to new catalyst design. Upon the introduction of new ligand sets and tuning metal center’s oxidation state, structurally well-defined cobalt catalysts for the hydrosilylation of alkenes and al-

Scheme 1. Co2(CO)8-Catalyzed Hydrosilylation of Alkenes In addition to the hydrosilylation of the alkyl substituted alkenes, Kalinin et al. investigated Co2(CO)8-catalyzed hydrosilylation of styrene, vinyltrimethylsilane, 1-vinyl-ocarborane, and allyl di(triethoxysilyl)amine with HSi(OEt)3 (Scheme 1).18 These reactions can operate with an equimolar ratio of the substrates and selectively furnish the anti-Markovnikov addition products. The reac-

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tion yields were found depending on the alkenes. The former two reactions gave the products in near quantitative yields, whereas, the yields of the reactions with 1vinyl-o-carborane and allyldi(triethoxysilyl)amine are only moderate probably effected by the coordinating carboranyl and amine groups. A comparative study on the reactions of vinyltrimethylsilane with HSi(OEt)3 using Co2(CO)8 (0.023 M) and Rh4(CO)12 (0.0011 M) as catalysts revealed that both reactions can furnish the antiMarkovnikov addition product in comparable yields. However, the rate of the cobalt-catalyzed reaction is slower.18 An interesting application of Co2(CO)8-catalyzed hydrosilylation of alkenes is the preparation of polymersupported organosilicon protecting groups.19 Darling et.al found that, in the presence of 2-4 mol% Co2(CO)8, the residual vinyl groups on (vinyl)polystyrene can slowly react with the hydrochlorosilanes, HSiMe2Cl, HSiPri2Cl, and HSiMeCl2, to yield chlorosilyl group-functionalized polymer beans.19 The conversion of the vinyl groups can reach ca. 50% after 5 days. The author mentioned that this cobalt-catalyzed method enables the hydrosilylation of (vinyl)polystyrene with HSiMe2Cl, which could not be achieved effectively via free radical-initiated hydrosilylation. The hydrosilylation of 2,3-dimethyl-1,3-butadiene might produce three types of products: 1,2-addition, 1,4-addition, and di-addition, and the selectivity control is challenging. Lappert and coworkers investigated the Co2(CO)8catalyzed hydrosilylation of 2,3-dimethyl-1,3-butadiene with HSi(OSiMe3)2Me in detail.20 While the reactions generally gave mixtures of the hydrosilylation products with the net yields varying from 69% to 93% (Scheme 2), the study found that a higher diene/hydrosilane molar ratio can result in a higher mono-addition/di-addition ratio of the products, and the use of the ethereal solvents, such as Et2O, THF, and 1,4-dioxane, can lead to the increase in the amount of mono-addition products, especially that of the 1,4-addition product. In contrast to the hydrosilylation of the linear diene, the hydrosilylation of 1,3-cyclohexadiene, and 1,3-cyclooctadiene with HSi(OEt)3 could selectively afford the 1,4-addition products in good yields (Scheme 2).20 This result is in line with Kalinin’s study on Co2(CO)8-catalyzed hydrosilylation of cyclopentadiene with HSi(OEt)3.18

Scheme 2. Co2(CO)8-Catalyzed Hydrosilylation of Dienes There are also attempts to use Co2(CO)8 as catalyst for hydrosilylation of alkenes bearing polar unsaturated functionalities. An early paper reported by Chalk noted that the Co2(CO)8-catalyzed hydrosilylation of acrylonitrile and methyl acrylonitrile by HSiMe2Cl produced the αaddition product Me(ClMe2Si)HCCN and the 1,4-addition product Me2C=CHN(SiMe2Cl)2, respectively, in low yields.21 Later, Seki, Murai, and their coworkers reported Co2(CO)8-catalyzed reactions of α,β-unsaturated esters with hydrosilanes.22 The study revealed that, in the presence of 4 mol% Co2(CO)8, the reaction of the hydrosilanes, HSiEt2Me, HSiMe3, HSiMe2Ph, and HSi(OEt)3, with 5 equiv of H2C=CHCOOR (R = Me, Et, Bun) in benzene at 25 oC can afford the dehydrogenative silylation products (E)-3-silylacrylates in 43-86% yields (based on the hydrosilanes), β-silylesters in trace amounts, and the alkene hydrogenation products in high yields (Scheme 3). The Co2(CO)8-catalyzed dehydrogenative silylation reaction shows a higher yield and higher selectivity than the reactions with rhodium, ruthenium, and iridium catalysts.22 The study also revealed that the substituents on the C=C bond and the molar ratio of α,β-unsaturated ester to hydrosilanes affect the reaction outcome. When the ratio of methyl acrylate/HSiEt2Me was changed to 2.5, the βsilylacrylate was obtained as the major product in 94% yield. On the other hand, the catalytic reactions of HSiEt2Me with five-fold excess of methyl crotonate and methyl methacrylate produced silyl substituted esters and silylmethacrylate in low yields (Scheme 3).22

Scheme 3. Co2(CO)8-Catalyzed Reactions of α,βUnsaturated Esters with Hydrosilanes

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The mechanism of Co2(CO)8-catalyzed hydrosilylation of alkenes was subjected to detailed examination. Chalk and Harrod found that Co2(CO)8 can react with hydrosilanes to form R3SiCo(CO)4 and HCo(CO)4, and in the presence of hydrosilanes, the hydride complex can convert to R3SiCo(CO)4 and H2.17 Catalytic studies also revealed that the reaction mixture of Co2(CO)8 with HSi(OEt)3 is capable of facilitating the isomerization of alkenes and the deuterium-scrambling of 1-heptene-3-d2.17 Moreover, infrared spectroscopy studies indicated that, as the catalytic reaction proceeds, the concentration of the silyl cobalt species Et3SiCo(CO)4 increases and the rate of hydrosilylation reaction decreases. Based on these observations, they proposed the cycle involving cobalt hydride species as key intermediate (Scheme 4).17 In the ChalkHarrod mechanism, the precatalyst Co2(CO)8 initially reacts with HSiR3 to yield cobalt silyl and cobalt hydride. Then, the hydride species interacts with an alkene molecule to produce alkylcobalt carbonyl intermediate. The further interaction of the alkylcobalt carbonyl species with HSiR3 can produce the oxidative addition product (R3Si)(H)(R)Co(CO)n that undergoes reductive elimination reaction to form the hydrosilylation product and the cobalt hydride species, or the hydrogenation product and cobalt silyl species. The Chalk-Harrod mechanism explains the side reaction of alkene isomerization (Scheme 4), but it fails to explain the formation of alkenylsilanes.22 The latter reaction, however, can be accounted by the modified Chalk-Harrod mechanism that has cobalt silyl species as the in-cycle intermediate.

Scheme 4. The Chalk-Harrod Mechanism Co2(CO)8-Catalyzed Hydrosilylation of Alkenes

for

Wrighton and his coworkers did the seminal study supporting the modified Chalk-Harrod mechanism.23 Different from Chalk and Harrod’s proposal that Et3SiCo(CO)4 is catalytically irrelevant, Wrighton et al. found that, with a catalyst loading of 0.054 mol% Et3SiCo(CO)4, the reaction of 1-pentene with an equimolar of HSiEt3 under the irradiation of 355±20 nm light for 39 h at 25 oC can afford n-C5H11SiEt3 in 60% yield.23a Later, by using cryogenic infrared spectroscopy techniques, Wrighton et al. established the conversion of Et3SiCo(CO)4 to the coordinatively unsaturated species Et3SiCo(CO)3 by photolysis, the coordination of Et3SiCo(CO)3 with ethylene to form Et3SiCo(CO)3(CH2=CH2), the conversion of the alkenecomplex to HCo(CO)3 and CH2=CHSiEt3 at elevated temperature, and the reaction of MeCo(CO)3 with HSiMe3 to produce methane and Me3SiCo(CO)3.23b According to

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these stoichiometric conversions, the modified ChalkHarrod cycle was built (Scheme 5). In the modified ChalkHarrod mechanism, the migratory insertion of alkene into a Co-Si bond to form β-silylalkyl cobalt species and the oxidative addition of the alkyl complex with a hydrosilane molecule followed by C-H bond-forming reductive elimination constitute the key steps.23b The formation of alkenylsilanes was explained by the β-hydride elimination reaction of the β-silylalkyl cobalt species (Scheme 5). In line with Wrighton’s study, Seki and Murai also found that the silyl complex (Et2MeSi)Co(CO)4 can catalyze the dehydrogenative silylation of methyl acrylate.22

Scheme 5. The Modified Chalk-Harrod Mechanism In addition to the cobalt carbonyl catalysts, cobaltbased hydrosilylation catalysts bearing phosphine, cyclopentadienyl, N-heterocyclic carbene (NHC), 2,6diiminopyridine, and β-diketimine ligands also scatter in literature. Haszeldine et al. found that the phosphinecobalt-hydride complexes, CoH(N2)(PPh3)3 and CoH3(PPh3)3, can react reversibly with HSi(OEt)3 to form the cobalt(III) silyl complex CoH2(Si(OEt)3)(PPh3)3, and that all these three cobalt complexes can catalyze the hydrosilylation of 1-hexene with HSi(OEt)3 at room temperature (Scheme 6).24 The study noted the rate of alkene’s isomerization was about half of that of hydrosilylation in these phosphine-cobalt complex-catalyzed reactions. In 2004, Hilt and coworkers reported that a lowvalent cobalt-phosphine species generated in-situ from the interaction of CoBr2, PBu3, Bu4NBH4, and ZnI2 can catalyze the hydrosilylation of isoprene with HSi(OEt)3 to selectively produce the 1,4-addition product Me2C=CHCH2Si(OEt)3 in 90% yield. The high selectivity is comparable to those of noble metal-catalyzed reactions.25

Scheme 6. Cobalt-Phosphine Complex-Catalyzed Hydrosilylation of 1-Hexene Brookhart and Grant found that the pentamethylcyclopentadienyl cobalt(III) alkyl complex [Cp*Co(P(OMe)3)(CH2CH3)][BArF4] (ArF = 3,5ditrifluoromethylphenyl) can catalyze the hydrosilylation of 1-hexene with HSiEt3.26 In the presence of 1 mol% of the cobalt(III) complex, the equimolar reaction can afford the anti-Markovnikov addition product in high yield (Scheme 7). Mechanistic studies using low temperature NMR tech-

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niques revealed a modified Chalk-Harrod mechanism for this cobalt-catalyzed hydrosilylation reaction, and more intriguingly, the existence of fast β-H elimination/migratory insertion processes from the β-silylalkyl cobalt(III) intermediate [Cp*CoCH(Bun)CH2(SiEt3)(P(OMe)3)]+ (Scheme 7). The substrate scope of the catalytic system is unknown yet.

ate with the special silyl-NHC ligands, whose role has to wait for disclosure.

Scheme 8. The Silyl-NHC-Cobalt Complex-Catalyzed Hydrosilylation of 1-Octene

Table 1. (MesPDI)CoCH3-Catalyzed Dehydrogenative Silylation of Terminal Alkenes

Scheme 7. [Cp*(P(OMe)3)Co(CH2CH3)][BArF4]Catalyzed Hydrosilylation of 1-Hexene and Its Proposed Catalytic Cycle The strong σ-donating nature and tunable steric property of NHCs 27 make them ideal candidate ligands for the development of high efficient transition-metal hydrosilylation catalysts. Markó and coworkers had shown that the NHC-platinum(0) complex-catalyzed hydrosilylation of alkenes can selectively produce anti-Markovnikov addition products with low amounts of alkene isomers.28 The study on cobalt-NHC-based hydrosilylation catalysts was reported quite recently. In 2013, Deng and coworkers reported the use of silyl-anchored NHC-Co(II) complexes for the hydrosilylation of 1-octene with H3SiPh (Scheme 8).29 All the three silyl-anchored NHC-Co(II) complexes can promote the hydrosilylation reaction at room temperature to yield the secondary hydrosilane with the antiMarkovnikov addition product being dominant and the amounts of alkene isomerization and hydrogenation products being trace. The catalyst system shows very fast initial rate and high turnover number. For example, the reaction using a 0.1 mol% of the hydrosilyl-archored NHC-Co(II) catalyst in 5 min can furnish the hydrosilylation products in 89% GC yield, and the reaction using 0.005 mol% of the catalyst can afford the products in 75% yield in 24 h, which suggests a high turnover number of 15000 (Scheme 8). Control experiments indicated the higher activity and higher selectivity of the silyl-anchored NHC-Co(II) catalysts versus Co2(CO)8 and trans(IPr2Me2)2CoPh2. This observation, in addition to the different activity observed for the three silyl-archored NHC cobalt(II) complexes, suggests that the fine performance of the silyl-anchored NHC-Co(II) catalysts should associ-

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ble 1). Interestingly, the outcome of the cobalt-catalyzed dehydrogenative silylation reaction is substratedependent as the majority of the alkenes subjected to study were converted to alkyl silanes and the reactions of 3,3-dimethylbutene and isobutene generate vinylsilanes as the major products. The formation of allyl silanes is unusual as the reported dehydrogenative silylation reactions generally have vinyl silanes as the major products. The catalyst can also promote the dehydrogenative silylation of internal linear alkenes to form terminal functionalized allylsilanes (Scheme 9), which can be viewed as a remote C-H functionalized strategy via cobalt-mediate alkeneisomerization and dehydrogenative silylation processes. Using this strategy, the elegant cobalt-catalyzed crosslinking of polymeric silylhyrides with 1-octene and 1octadecene was achieved (Scheme 9).30

Scheme 9. (MesPDI)CoCH3-Catalyzed Dehydrogenative Silylation of Internal Alkenes

A milestone in non-precious metal-based hydrosilylation catalysts is Chirik’s 2,6-iminopyridine-irondinitrogen complex that exhibits comparable activity in alkene hydrosilylation reactions as the traditional platinum catalysts.5 The further study on 2,6-iminopyridinecobalt complexes by the same group has led to the discovery of new cobalt catalysts for dehydrogenative silylation of alkenes. By screening a series of 2,6iminopyridine-cobalt complexes, the cobalt complexes (MesPDI)CoCH3 (E in Table 1) and (MesPDI)Co(N2) proved the most active dehydrogenative silylation catalysts for the reaction of HSi(OSiMe3)2Me with 1-octene (two equiv), wherein allylsilanes were formed in more than 98% yield as a 3:1 E/Z mixture along with one equiv n-octane.30 Substrate scope study using 0.5 mol% (MesPDI)CoCH3 as catalyst indicated the applicability of the catalytic system to a wide range of hydrosilanes (HSi(OSiMe3)2Me, HSi(OEt)3, HSiEt3, H2SiPh2, H3SiPh) and linear terminal alkenes (Ta-

The authors performed the stoichiometric reactions of the catalyst, in-situ NMR experiments, and deuteriumlabeling reactions to probe the reaction mechanism. The precatalyst (MesPDI)CoCH3 is unreactive toward 1-octene at room temperature, but it can quickly interact with HSi(OSiMe3)2Me to form CH4. In-situ NMR experiments indicated the cobalt alkyl species (MesPDI)Co(C8H17-n) as a probable catalyst resting state. A deuterium kinetic isotope effect of 2.6 was observed from a series of deuterium-labeling reactions with DSi(OSiMe3)2Me, showing that the step including the broken Si-H bond should be rate-limiting. Referring to these observations, a proposed mechanism that involves cobalt silyl, cobalt β-silylalkyl, cobalt hydride, and cobalt alkyl species as the key intermediates is proposed (Scheme 10).30 The full cycle is reminiscent of that proposed for the Co2(CO)8-catalyzed dehydrogenative silylation of α,β-unsaturated esters by Seki and Murai.22 Considering the low-spin cobalt(II) nature of the cobalt center in (MesPDI)CoCH3,31 the author mentioned that the capability of (MesPDI)CoCH3 in promoting the dehydrogenative silylation reaction might be due to the presence of a more labile cobalt(II) center that might be prone to alkene-dissociation after β-hydride elimination.30

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Scheme 10. Proposed Mechanism for the CobaltCatalyzed Dehydrogenative Silylation of Alkenes Table 2. (MePDI)CoCH2SiMe3-Catalyzed Hydrosilylation of Terminal Alkenes

In a recent patent, Chirik et al. described the catalytic performance of a series of N-alkyl 2,6-diiminopyridinecobalt complexes in promoting the reactions of alkenes with hydrosilanes.32 Contrast to the aforementioned Nmesityl 2,6-diiminopyridine-cobalt methyl complex that promotes the dehydrogenative silylation reaction, the Nmethyl 2,6-diiminopyridine-cobalt alkyl complex (PDIMe)Co(CH2SiMe3) turns out an effective catalyst for the hydrosilylation of alkenes with HSi(OEt)3 (Table 2). Among these reactions, the ones with 1-octene and allyl(poly)ether can selectively afford the anti-Markovnikov product in more than 98% GC yield with HSi(OEt)3, and the catalyst loading in the reaction with 1-octene can even be reduced to 0.022 mol%. The patent also noted the type of the hydrosilane used affects reaction selectivity. In the (PDIMe)Co(CH2SiMe3)-catalyzed reactions of 1-octene with HSi(OSiMe3)2Me, HSiMe2OSiMe2CH2CH2SiMe3, and HSiMe2CH2CH2SiMe3, mixtures of the hydrosilylation, dehydrogenative silylation, and hydrogenation products of the alkene were formed.32 Table 3. Cobalt(I) β-Diketiminato Complex-Catalyzed Hydrosilylation of Terminal Alkenes

More recently, Holland and coworkers applied cobalt(I) β-diketiminato complexes to the hydrosilylation of alkenes.33 After screening a series of the cobalt(I) complexes bearing different β-diketiminate ligands, the cobalt(I)benzene complex featuring the N-mesityl-3(mesitylimino)prop-1-en-1-amino ligand (F in Table 3) was found the most effective one for the hydrosilylation reactions of a variety of terminal alkenes with H3SiPh or HSi(OEt)3. The catalytic reactions can be solvent-free, and afford the hydrosilylation products in high yields (67-94%) with high linear selectivities (up to 98%) and alkenes bearing silyl ether, chloro, tertiary amine, ester, and amide groups can all be used (Table 3).33 In addition to the hydrosilylation of terminal alkenes, the study demonstrated a novel tandem strategy of cobalt-catalyzed internal alkene hydrosilylation to form terminal alkyl silanes,33 by taking advantage of the capability of a β-diketiminatecobalt(I)-toluene complex in promoting alkene isomerization. The combined use of the cobalt(I)-toluene complex with the cobalt(I)-benzene complex as catalysts enables the regioconvergent hydrosilylation of internal hexenes to yield 1-hexylsilane(Scheme 11). The reaction mechanism of this β-diketiminato cobalt(I)-catalyzed hydrosilylation reaction has not been disclosed. However, as no vinyl or allyl silane was detected, the system might proceed in a modified Chalk-Harrod mechanism. On the other hand, noting the tautomerization reaction of N-(2’,6’diisopropylphenyl) β-diketiminato cobalt(I)-THF complex,34 the ligand tautomerization from the bidentate βdiketiminato ligand to a ƞ1-(β-diketiminato) ligand or the dissociation of C6H6 might occur.

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mations on dicobaltcarbonyl species (Scheme 13), rather than mononuclear cobalt carbonyl species.39b While the validation of this interesting mechanism needs further mechanistic study, similar bimetallic synergism has been elucidated in cobalt-rhodium mixed-metal carbonylcatalyzed alkyne silylformylation reaction by density functional calculation.40 Table 4. The Hydrosilylation of Phenylthio Alkynes Catalyzed by Co2(CO)8 Scheme 11. Cobalt(I) β-Diketiminato ComplexesCatalyzed Hydrosilylation of Internal Alkenes

Cobalt Complex-Catalyzed Alkyne Hydrosilylation Transition-metal catalyzed hydrosilylation of alkynes is an ideal method for the preparation of vinylsilanes that are valuable synthetic reagents in organic synthesis.35 A serious issue hampering the widespread use of the method is the difficulty in regio- and stereocontrol as even the hydrosilylation of a terminal alkyne might give rise to three vinylsilanes, namely the β-(Z), β-(E), and α-isomers (Scheme 12). In addition to the regio- and stereoselectivity issue, the side reactions of alkyne oligmerization and hydrogenation could complicate the catalytic system. In the past decades, a series of effective noble metal catalysts for the regio- and stereoselective hydrosilylation of alkynes have been developed.1 However, the success using late 3d metal-based catalysts is very limited.36, 37

Scheme 12. The Hydrosilylation of Terminal Alkynes Co2(CO)8 can readily react with alkynes to form Co2(CO)6(C2R2). The reaction is used in organic synthesis as a protecting method for C-C triple bonds. During the course of the study toward a reductive decomplexation of alkyne dicobalt hexacarbonyl species, Isobe et al. found that tertiary hydrosilanes can react with acetylene biscobalt hexacarbonyl complexes to form vinylsilanes.38 On the basis of this finding, they then revealed that Co2(CO)8 and (2-methylbut-3-yn-2-ol)biscobalthexacarbonyls, which is relatively air-stable over Co2(CO)8, can serve as effective catalysts for the hydrosilylation of alkynes with tertiary hydrosilanes, although the cobalt-catalyzed reactions necessitate higher catalyst loadings than the Na2PtCl6-catalyzed reaction.39 The catalytic reactions employing 3-10 mol% Co2(CO)6(HC≡CCMe2OH) (G in Table 4) show high regio- and stereoselectivity in the hydrosilylation of phenylthio alkynes with HSiEt3 or HSiPhMe2 to afford the cis-addition products, α-phenylthio-α-silyl alkenes, as the sole or dominant products (Table 4).39 The high selectivity achieved in these hydrosilylation reactions was ascribed to the directing groups, phenylthio and hydroxyl groups, on the C-C triple bond. Interestingly, the authors proposed a catalytic cycle involving transfor-

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ACS Catalysis ent fluoroalkyl groups (CF3, CF2H, and CF2CF2CF2H) and/or aryl, benzyl, propargyl alcohol groups can be hydrosilylated to form cis-addition products in good yields with the ratio of the α-fluoroalkyl-α-silyl alkene/αtrifluoromethyl-β-silyl alkene varying from 71:29 to 100:0 (Table 5).37 Different from Isobe’s mechanism,39b Konno proposed a mononuclear cobalt carbonyl speciesfacilitated cycle that parallels to the modified ChalkHarrod mechanism for alkene hydrosilylation. Table 6. Cyclopentadienylcobalt(I) Catalyzed Hydrosilylation of Alkynes

Complex-

Scheme 13. The Dinuclear Mechanism Proposed for (2-methylbut-3-yn-2-ol)biscobalthexacarbonylCatalyzed Alkyne Hydrosilylation Reaction

Table 5. Co2(CO)8-Catalyzed Hydrosilylation of Fluoro–Containing Alkynes

Konno and coworkers reported the study on Co2(CO)8catalyzed hydrosilylation of fluoroalkylated alkynes.41 Investigating the catalytic reactions of F3CC≡CC6H4-Cl-p with different tertiary hydrosilanes revealed that the reaction with HSiEt3 affords α-trifluoromethyl-α-silyl alkene as the major product, whereas, the reactions with HSiPhMe2 and HSi(OEt)3 have α-trifluoromethyl-β-silyl alkenes being the major ones (Table 5). With Et3SiH as the hydrosilane source and a 5 mol% Co2(CO)8 catalyst loading, a series of fluoroalkylated alkynes bearing differ-

Another interesting cobalt complex employed in catalytic alkyne hydrosilylation is the cyclopentadienylcobalt(I) ethylene complex bearing an appended phosphine tether (H in Table 6).42 Butenschön et al. had found that the cobalt(I) complex can perform oxidative addition reactions with hydrosilanes to furnish cobalt(III) hydrido silyl complexes.43 This cobalt(I) complex subsequently proved an effective alkyne hydrosilylation catalysts. Using a 5 mol% catalyst loading, the catalytic hydrosilylation reactions of symmetrical internal alkynes with HSiEt3 selectively produce cis-addition products in moderate to good yields (Table 6). Remarkably, the reactions using HSi(OEt)3 as the hydrosilane source lead to the predominant formation of anti-addition products (Table 6).42 The hydrosilylation reactions of unsymmetrical internal alkynes and terminal alkynes, however, suffer from poor regio-selectivity (Table 6). For the former cases, the reactions using HSiEt3 and HSi(OEt)3 all afford the cisaddition products as mixtures of two regio-isomers in moderate yields. On the other hand, the catalytic reactions of terminal alkynes with HSiEt3 give the 1:1 mixtures of β-(E) and α-isomers, and those using HSi(OEt)3 result in the trimerization of alkynes with no hydrosilylation products. The catalytic reaction was proposed to operate in a typical Chalk-Harrod or modified Chalk-Harrod

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mechanism, during which the hemilabile phosphine sidearm plays an important role in creating open coordination site and promoting reductive elimination.42 The anti-addition products could be formed via either the isomerization of the initially formed cis-addition product, alkenyl cobalt intermediate through the Crabtree-Ojima mechanism,44 or via a concerted oxidative additionhydride insertion step as proposed in rutheniumcatalyzed alkyne hydrosilylation reactions.45 The exact mechanism has not been elucidated yet.

tion of internal symmetrical alkynes, yielding the synadducts in more than 80% yields (Table 8). As for unsymmetrical alkynes, the syn-adducts were exclusively formed and the regio-selectivity is depending on the steric nature of the substituents. By tuning the steric property of the substituents, regio- and stereo-selective hydrosilylation of unsymmetric internal alkynes were achieved in the reactions of trimethylsilyl substituted alkynes, wherein the (Z)-α,α-disilylalkenes were exclusively formed (Table 8).

Table 7. Catalytic Performance of Some Metal Complexes in Hydrosilylation of 1-Octyne

Table 8. (IAd)(PPh3)Co(CH2SiMe3)-Catalyzed Hydrosilylation of Alkynes

Recently, Deng et.al reported unique low-coordinate cobalt(I) catalysts that promote the hydrosilylation of terminal and internal alkynes with high stereo- and regioselectivity.46 The group had found that bulky NHCs can stabilize three- and two-coordinate cobalt(I) complexes in the forms of (NHC)2CoCl and [(NHC)2Co][BAr4].47 The continuing study then led to the preparation of the threecoordinate cobalt(I) alkyl complex (IAd)(PPh3)Co(CH2SiMe3) upon the reaction of (PPh3)3CoCl with IAd followed by alkylation with LiCH2SiMe3.46 The low-coordinate alkyl complex (2 mol%) can catalyze the reaction of 1-octyne with 1.2 equiv of H2SiPh2 at room temperature to selectively afford (E)octenyldiphenylsilane in 96% yield (Table 7). The decreased selectivity and yields with Rh(PPh3)3Cl, Pt2(dvtms)3, and other cobalt complexes as catalysts signify the fine catalytic performance of the three-coordinate cobalt(I) alkyl complex (Table 7).46 Substrate scope studies indicated the high regio- and stereo-selectivity of the catalytic reactions of a series of phenyl- and alkyl-substituted terminal alkynes with H2SiPh2, wherein the β-(E)-adducts were obtained as the sole hydrosilylation products in high yields and the functionalities of tertiary amine, ester, chloro groups are all tolerated (Table 8).46 The catalytic hydrosilylation of 1,6dialkynes with H2SiPh2 can also be achieved, producing the silylated 1,2-dialkylidenecyclopentanes in moderate yields with high selectivity (Table 8). The catalytic system also shows excellent stereo-selectivity in the hydrosilyla-

To take insights into the mechanism, the stoichiometric reactions of (IAd)(PPh3)Co(CH2SiMe3) with PhC≡ CPh and H2SiPh2 were investigated, which led to the isolation of the low-coordinate alkyne complex (IAd)(ƞ2PhC ≡ CPh)Co(CH2SiMe3) and the silyl complex (IAd)(PPh3)Co(SiHPh2), respectively (Scheme 14).46 With the silyl complex as the catalyst, the catalytic hydrosilylation reaction of 1-octyne with H2SiPh2 shows comparable activity and selectivity as those using the alkyl complex (IAd)(PPh3)Co(CH2SiMe3) as the catalyst. These observations point out a modified Chalk-Harrod mechanism for

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the low-coordinate cobalt(I) complex-catalyzed hydrosilylation reaction (Scheme 14).46 The regio- and stereoselectivity achieved in the catalytic reactions was proposed to be governed by the intricate balance of steric repulsion among the IAd ligand, the silyl group and the coordinated alkyne of the alkyne-cobalt(I)-silyl intermediate as the study found that the cobalt-catalyzed reactions with terminal alkynes exhibit different regio-selectivity versus those with the silyl-substituted unsymmetrical internal alkynes, and that the reaction of 1-octyne with H3SiPh produces both 1,2- and 2,1-addition products.46

Scheme 14. Proposed Mechanism for (IAd)(PPh3)Co(CH2TMS)-Catalyzed Hydrosilylation Reactions

terms of the application for the production of organosilanes in fine chemical synthesis, the reported cobalt catalysts are still dwarfed by the traditional noble metal catalysts for their narrow substrate scope and/or poor selectivity. For examples, many of the cobalt-catalyzed hydrosilylation of alkenes require the use of a large excess amount of alkenes to compensate the alkene expense in side reactions, few of the cobalt complex catalyst can promote the hydrosilylation of di- or poly-substituted alkenes, the selectivity control of cobalt-catalyzed hydrosilylation of alkynes with tertiary hydrosilanes is far from ideal, and cobalt catalyst that effects enantioselective hydrosilylation of alkenes has remained unknown. Undoubtedly, the solution to these challenges should rely on a deeper knowledge on cobalt chemistry and the continuing exploration on new cobalt complexes that enable new substrate-activation mode. For the latter point, substrate activation by metal-ligand cooperation that has been successfully applied in the design of nonprecious metal-based hydrogenation catalyst seems a promising method.48 In addition to new ligand design, evaluation of the existing ligand arsenal using high throughput method could be a useful way to speed up the catalyst searching process. The success of this method has been demonstrated by the finding of suitable phosphine ligands for cobalt-based asymmetric alkene hydrogenation catalysts.49 Encouraged by the achievements and upon the continuing exploration, one can foresee the vide vista of cobalt-based catalysts as alternatives for the traditional noble metal-catalysts in catalytic hydrosilylation reactions.

AUTHOR INFORMATION Summary and Perspective

Corresponding Author * E-mail: [email protected]

The economic concern on precious metal-catalyzed hydrosilylation reactions urges the development of their non-precious metal alternatives. The light group 9 metal cobalt, referring from its abundance in earth and special chemical properties, seems an ideal candidate. Indeed, the aforementioned advances have demonstrated the potential of cobalt complexes as a new generation of transition-metal catalyst for hydrosilylation reactions of alkenes and alkynes. While the known cobalt complex-based hydrosilylation catalysts are still limited in numbers, some of them have shown high catalytic efficiency and high regio- and/or stereo-selectivity in promoting the hydrosilylation of alkenes and alkynes with versatile hydrosilanes. Despite the progress, the study on cobalt-based hydrosilylation catalysts is still in the initial stage. From the view of practical usage in industry, an ideal hydrosilylation catalyst should be moisture- and air-stable and be highly efficient. The known cobalt catalysts, however, are generally moisture- and air-sensitive, and most of them have to be used in high catalyst loadings (more than 1 mol%). In

Notes The authors declare no competing financial interests.

ACKNOWLEDGMENT We thank the financial support from the National Natural Science Foundation of China (Nos. 21222208, 21421091, and 21432001).

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