Advances in Base-Metal-Catalyzed Alkene Hydrosilylation - ACS

Review pubs.acs.org/acscatalysis

Advances in Base-Metal-Catalyzed Alkene Hydrosilylation Xiaoyong Du and Zheng Huang* 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: This review covers the advance in the development of Fe, Co, and Ni catalysts for the alkene hydrosilylation reaction, as well as the related dehydrogenative silylation reaction. The hydrosilylation of alkene is an important reaction for the synthesis of alkylsilanes that has widespread applications in numerous siliconbased materials, and for decades, this transformation has been relying on the use of Pt catalysts. Recently, the high abundance and low cost, coupled with the environmentally benign nature of the base metals have stimulated enormous research on the development of first-row transition-metal catalysts as replacements for the precious Pt catalysts. Several base-metal catalysts which have emerged during the past 5 years offer high activity, broad substrate scope, and excellent regioselectivity. Both of the antiMarkovnikov and the unusual Markovnikov additions can be achieved in a high degree of regioselectivity. The reactions of acyclic internal olefins catalyzed by the base-metal catalysts reported to date yield linear alkylsilanes via a tandem olefin-isomerization and hydrosilylation process. A few catalysts enable the dehydrogenative silylation of alkenes to form vinylsilanes and/or allylsilanes. KEYWORDS: iron, cobalt, nickel, alkene, hydrosilylation, dehydrogenative silylation

1. INTRODUCTION The alkene hydrosilylation is an atom-economic reaction to produce alkylsilanes, which can be further converted to highvalue silicon-based materials including silicon rubbers, paperreleasing coatings, molding implants, and pressure-sensitive adhesives.1−5 In addition, the alkene hydrosilylation has become a useful method to access organosilane intermediates for fine-chemical synthesis.2−5 The H−Si addition process can be promoted by UV light-irradiation, heat, or a catalyst.1 Among these methods, the transition-metal-catalyzed hydrosilylations are the most widely used approaches because of high activity and mild reaction conditions (Scheme 1). In fact, the alkene hydrosilylation is currently one of the largest volume reactions utilizing homogeneous metal catalysts in industry.2,4,6,7 The catalytic hydrosilylation reactions can be complicated by side reactions including dehydrogenative silylation, silane redistribution, and silane dehydrocoupling (Scheme 1). The dehydrogenative silylations often occur with trialkylsilanes, but recent works showed that such silylation reactions can occur with tertiary alkoxysilanes and siloxysilanes,8−10 and secondary and primary silanes (vide infra).10 Overhydrosilylation (double or triple hydrosilylation) can also compete with the monohydrosilylation event when using primary and secondary hydrosilanes as the reagents. α-Olefin isomerization is another common side reaction, although the resultant internal olefins may undergo hydrosilylation to afford alkylsilane products. In the majority of the hydrosilylation processes, the complexes of platinum group metals,11−13 in particular, the Pt-based complexes,14−16 have been extensively explored. Two Pt complexes, Speier’s catalyst (H2PtCl6·6H2O/iPrOH)14 and © XXXX American Chemical Society

Scheme 1. Transition-Metal-Catalyzed Alkene Hydrosilylation Reaction

Karstedt’s catalyst15 (Figure 1), are the most prevailing catalyst systems for the commercial synthesis of silicones. In addition to their extremely high activity and high anti-Markovnikov selectivity, these Pt catalysts offer broad functional-group compatibility and ease of manipulation. There are a wide variety of alkene and hydrosilane reagents that can be used in the Pt-catalyzed hydrosilylation, allowing for its application in synthesis of many different silicon compounds. Side reactions, including alkene isomerization and dehydrogenative silylation, Received: October 20, 2016 Revised: December 22, 2016

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DOI: 10.1021/acscatal.6b02990 ACS Catal. 2017, 7, 1227−1243

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ACS Catalysis

in alkene hydrosilylations are described. The relevant alkene dehydrogenative silylation reactions catalyzed by base-metal complexes are also included. The last section highlights the challenges and opportunities remaining in this field.

2. IRON-CATALYZED ALKENE HYDROSILYLATION Iron is the most abundant transition metal in the earth’s crust, and it is also one of the most versatile metals operative in metalloenzymes.25 The redox chemistry of iron is very rich with oxidation states ranging from −2 to +6. The iron center in the absence of strong-field ligands is prone to undergo one-electron redox processes, rather than two-electron changes operating in many noble metal-catalyzed transformations. However, recent research on iron complexes with noninnocent ligands, which are able to participate in the redox events, has shown promising applications of such complexes in catalysis.26−28 In fact, iron catalysis has experienced remarkable processes over the past several decades, and molecular Fe catalysts have now been applied in various transformations, such as cross-coupling reactions and olefin polymerization/oligomerization reactions.20−22,29−31 A number of Fe complexes have been also developed for the alkene hydrosilylation reactions. The first example of Fe-catalyzed alkene hydrosilylation was reported by Nesmeyanov and co-workers in 1960 using Fe(CO)5 as the catalyst precursor (Scheme 2).32,33 Mechanistic

Figure 1. Karstedt’s and modified Karstedt’s catalysts.

may compete with the hydrosilylation. Hence, efforts have been devoted to circumvent such limitations. For example, the modified Karstedt Pt catalyst containing carbene and divinyltetramethylsiloxane ligands (Figure 1) has shown superior catalytic behavior with respect to selectivity and catalyst stability in comparison with the parent Karstedt’s complex.16 The major concern regarding the Pt-based hydrosilylations catalysts is the sustainability. Platinum is a rare metal, occurring at an abundance of only 0.005 ppm in the earth’s crust.17 On the other hand, the demand of the Pt metal in the alkene hydrosilylation process is massive; it has been reported that approximately 5.6 t of Pt is consumed annually in the silicone industry,18 most of which unfortunately cannot be recovered. Other major applications including vehicle emission control and petroleum refining also consume a large amount of Pt (>100 t per year). The low abundance is connected to its high cost. In this regard, the development of the earth-abundant, inexpensive base-metal alternatives to the Pt catalysts is of great interest. In addition, the toxicity of the precious metal makes the use of environmentally friendly base-metal catalysts of importance, in particular, for the manufacture of health-related silicon-based materials. In the recent 20 years, continuing efforts on developing nonprecious metal alkene hydrosilylation catalysts have led to tremendous progress in this field. Iron, cobalt, and nickel catalysts (Figure 2) have emerged as the most promising

Scheme 2. Fe(CO)5-Catalyzed Alkene Hydrosilylation

studies by Graham and Wrighton revealed that the dissociation of the CO ligand(s) was required to generate the catalytically active [Fe(CO)4] or [Fe(CO)3] species.34,35 The catalyst activation required continuous photoirradiation or thermolysis at high temperatures (100−140 °C). The hydrosilylation reactions of α-olefins occurred with 1.5−3 mol % of Fe(CO)5 to form the anti-Markovnikov hydrosilylation products in moderate to good yields. Several tertiary silanes, including trichlorosilane (Cl3SiH), proved to be suitable hydrosilylation reagents. The successful hydrosilylation with Cl3SiH is of particularly interest because this silane is an industrially important reagent, and the transformation utilizing Cl3SiH has been hardly reported by using other iron catalysts developed later. The limitation of this catalyst system is that several undesired side reactions compete with the hydrosilylation (Scheme 2). Side-products resulting from the dehydrogenative silylation and the hydrogenation of alkenes were consistently observed.36,37 In 2013, a modified iron carbonyl complex containing two weakly coordinating η2-(Si−H) bonds and a disilyl ligand was reported by Nagashima and co-workers.37 In contrast to the parent catalyst system Fe(CO)5 that necessitates the catalyst activation under harsh conditions, this disilyl dicarbonyl Fe complex (1) enables the alkene hydrosilyaltion reaction at room temperature (Scheme 3). Ethylene was hydrosilylated with several tertiary hydrosilanes and hydrosiloxanes using 0.01−1 mol % of 1 in toluene with turnover numbers (TONs) up to 2000. Treatment of 1-octene with 1,1,1,3,3-pentamethyldisiloxane (PMDS) and the catalyst 1 in toluene at ambient temperature resulted in olefin isomerization. However,

Figure 2. Groups 8, 9, and 10 transition metals used for alkene hydrosilylation.

candidates.11,19−23 Although these first-row transition-metal catalysts developed so far are less general and practical in comparison to the classical Speier and Karstedt’s catalysts, in some cases, the Fe complexes have exhibited activities comparable to the Pt catalysts in hydrosilylations with industrially important tertiary silanes.24 Furthermore, some Co complexes provided high Markovnikov regioselectivity in the hydrosilylation of terminal alkenes and some Fe complexes offered high enantioselectivity in the anti-Markovnikov hydrosilylation of 1,1-disubstituted vinylarenes, which have been barely reported using the precious-metal catalysts. In this review, the advances employing Fe, Ni, and Co catalyst systems 1228


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stoichiometric reaction of the bis(dinitrogen) complex 2 with PhSiH3 resulted in the formation of an Fe bis(silane) σcomplex (iPrPDI)Fe(η2-SiH3Ph)2 (3). Noteworthily, further studies of the electronic structures of the Fe complex 2 by spectroscopic and computational methods revealed a doubly reduced PDI diradical and a ferrous ion.41,42 Building on this work, the Chirik group explored the alkene hydrosilylation using tertiary hydrosilanes and hydrosiloxanes, which are commercially relevant hydrosilylation reagents (Scheme 5).24 They found that reducing the steric hindrance

Scheme 3. Disilyl Dicarbonyl Iron Complex-Catalyzed Alkene Hydrosilylation

Scheme 5. [(MePDI)Fe(N2)2](μ2-N2)-Catalyzed Alkene Hydrosilylation with Tertiary Silanes

1-octene reacted with PMDS in the presence of 3 mol % of 1 in neat at 80 °C, forming the linear hydrosilylation product in 75% yield with high selectivity. The hydrosilylation of cyclopentene also proceeded at 80 °C, albeit with a higher catalyst loading (10 mol %). Furthermore, 2-octene underwent the tandem isomerization and hydrosilyaltion with PMDS and 1,1,1,3,5,5,5-heptamethyltrisiloxane (MD′M) to form the 1-silyl products in 80% and 82% isolated yields, respectively. The utilization of strong π-acidic ligands, such as CO, presumably promotes the formation of low-spin Fe species, which renders the Fe complex feasible to effect alkene hydrosilylation. The seminal work by Chirik and co-workers suggested that the use of redox active ligand is another solution for the generation of an active hydrosilylation catalyst.24,38−40 In 2004, the Chirik group reported the selective antiMarkovnikov hydrosilylation reaction of terminal olefins with PhSiH3 and Ph2SiH2 catalyzed by a bis(imino)pyridine (also named as pyridinediimine, PDI) ligated Fe bis(dinitrogen) complex bearing the iPr groups in the 2,6-positions of the Naryl rings (iPrPDI)Fe(N2)2 (2) (Scheme 4).38 No side reactions

of the PDI ligands resulted in a marked improvement of the catalytic performance. Using the sterically less demanding complex [(MePDI)Fe(N2)2](μ2-N2) containing the Me groups in the 2,6-postions of the N-aryl ring (4), the hydrosilylation of terminal olefins with tertiary silanes including Et3SiH, (EtO)3SiH, and MD′M, occurred efficiently to form the linear alkylsilanes in high yields. Remarkably, TONs up to 25000 and turnover frequencies up to 100 000 h−1 could be achieved.24 This represents the most active catalyst among any base-metal hydrosilylation catalysts reported to date, and the activity is comparable to Karstedt’s Pt catalyst. The Fe catalyst is even superior to the Pt catalyst in the hydrosilylation of the methylcapped ally polyethers; the undesired olefin isomerization associated with the Pt-catalyzed hydrosilylation could be avoided in the reaction using the Fe catalyst system. The reports by Chirik have inspired extensive studies using Fe complexes supported by redox active ligands for alkene hydrosilylations. Although the PDI Fe dinitrogen complexes display very high activity and regioselectivity in the hydrosilylation of α-olefins, the synthetic utilities of these Fe catalysts are affected by their high sensitivity toward moisture and air. To overcome this limitation, the in situ reduction of the benchstable PDI Fe(II) dihalide complexes with external activation reagents (cocatalyst) have been studied. Thomas and coworkers have shown that various organometallic species could serve as the catalyst activators.43 Specifically, upon the activation with the Grignard reagent EtMgBr, the Et-substituted PDI Fe(II) dichloride complex (5) is active for the hydrosilylation of terminal olefins, including 1,1-disubstitued olefins, with primary, secondary, and tertiary silanes (Scheme 6). The catalyst generated in situ exhibited good functional-group tolerance and high regioselectivity for the formation of the antiMarkovnikov products. Functional groups including halide, primary amine, ester, ketone, aldimine, and nitrile were well tolerated. By comparison, the procedure using the catalyst generated in situ required higher catalyst loadings (1 mol %) relative to the catalysis using Chirik’s Fe dinitrogen complexes.

Scheme 4. (iPrPDI)Fe(N2)2-Catalyzed Alkene Hydrosilylation and Stoichiometric Reaction with PhSiH3

were observed during the hydrosilylation process. The hydrosilylations of 1,1-disubstituted and cyclic internal alkenes with PhSiH3 proceeded smoothly, although longer reaction times were required relative to the reactions of monosubstituted terminal alkenes. Trisubstituted alkenes were unreactive under the catalytic conditions. The acyclic 1,2disubstituted alkenes underwent the tandem olefin-isomerization and hydrosilylation to yield the linear alkylsilane. The 1229


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ACS Catalysis Scheme 6. (EtPDI)FeCl2/EtMgBr-Catalyzed Alkene Hydrosilylation

Scheme 8. Terpyridine Fe(II) Dihalide ComplexesCatalyzed Alkene Hydrosilylation

Preliminary mechanistic studies by Thomas revealed the formation of an Fe(I) species as the active catalyst.43 More recently, the Thomas group described a combination of (EtPDI)Fe(OTf)2 (6) and (iPr)2NEt for anti-Markovnikov alkene hydrosilylations with PhSiH3 (Scheme 7). Because both

appeared to be more efficient than that with secondary silanes Ph2SiH2 and MePhSiH2. However, these Fe complexes exhibited no activity for the hydrosilylations with tertiary silanes. The Chirik group reported the synthesis and characterization of Fe dialkyl complexes ligated by PDI (8a−d), terpy (9) and pyridine bis(oxazoline) (PyBox) (10) ligands and evaluated their catalytic activities in alkene hydrosilylations with tertiary silanes (Scheme 9).46 Studies of the electronic structures using

Scheme 7. (EtPDI)Fe(OTf)2/(iPr)2NEt-Catalyzed Alkene Hydrosilylation

Scheme 9. Iron Dialkyl Complex-Catalyzed Alkene Hydrosilylation

the Fe precursor and the tertiary amine are bench-stable, the procedure could be applied without the need for air- and moisture-free techniques and environments.44 Specifically, using 2−4 mol % of 6 and 25 mol % of (iPr)2NEt, a variety of functionalized terminal alkenes were hydrosilylated with PhSiH3 in neat at ambient temperature over 1−18 h, forming the corresponding linear products in moderate to high isolated yields. The tertiary amine (iPr)2NEt serves as the activator, and the use of the triflate anion is crucial to the in situ generation of the active catalyst because the weak binding affinity of the anion makes the Fe precursor amenable to the amine-induced activation. The reactions using hydrosilanes other than PhSiH3 were not disclosed in this work.44 The nitrogen-based tridentate ligands used for the construction of the iron hydrosilylation catalysts are not limited to the PDI type. In 2012, the Nakazawa45 and Chirik46 groups independently reported terpyridine (terpy) type Fe(II) complexes for the hydrosilylation. A series of Fe(II) dibromide complexes with terpy and its derivatives as the ligands (7) were prepared by Nakazawa and co-workers (Scheme 8).45 Their work showed that the substitution in the 6- and 5″-/6″positions in the terpy ligands was necessary to prevent forming the catalytically inactive Fe(terpy)2 species. The variant with a bulky aryl substituent in the 6-position and a small Me substituent in the 5″- or 6″-position in the terpy type ligand proved to be most productive among the terpy Fe(II) complexes investigated. Following in situ activation with NaBEt3H (3.6 equiv to the Fe precursor), the Fe complex catalyzed the alkene hydrosilylation with primary and secondary silanes at 0.05−0.1 mol % catalyst loadings. TONs up to 1530 could be achieved in the reaction with PhSiH3. The reactions were exclusively selective for the formation of linear products. The catalytic hydrosilylations with the primary silane PhSiH3

spectroscopic and computational tools revealed that all of these dialkyl complexes contain high-spin Fe(III) centers with antiferromagnetic coupling to the radical anionic ligands, highlighting the redox-active nature of the PDI, terpy, and PyBox ligands. The less sterically congested PDI Fe complexes 8a−c with Me or Et groups in the 2,6-positions, or with only one iPr group in the 2-position of the N-aryl ring, enable the addition of the tertiary hydrosiloxanes MD′M and (EtO)3SiH to α-olefins, but are inactive for the hydrosilylation with Et3SiH. The terpy Fe complex 9 is active for the hydrosilylations with Et3SiH, MD′M, but inactive for the reaction with (EtO)3SiH. Furthermore, vinylcyclohexene oxide could be selectively hydrosilylated using 9 without substrate decomposition. Note that such a substrate could not be tolerated by the PDI Fe catalysts. The product resulting from the hydrosilylation of the epoxide-bearing alkene may find applications in textile finishes, silicone-modified organic polymers and UV-release coating. Finally, when the PyBox Fe complex 10 and the very bulky PDI Fe complex 8d bearing two iPr groups in the 2,6-positions of 1230


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Scheme 11. (PONN)FeX2/NaBEt3H-Catalyzed Alkene Hydrosilylation

the N-aryl ring were used, no hydrosilylation activity was observed in the reactions with all three tertiary silanes, MD′M, (EtO)3SiH, and Et3SiH. Chirik and co-workers showed that the catalyst activation with an external cocatalyst is not necessary for the Fe dialkyl complexes.46 However, a comparison between the dinitrogen adduct (iPrPDI)Fe(N2)2 (2) and the dialkyl species (iPrPDI)Fe(CH2SiMe3)2 (8d) containing the same PDI ligand reveals different catalytic activities: the former is effective for the hydrosilylations with (EtO)3SiH and MD′M (vide supra), while the latter is inactive for the hydrosilylations with (EtO)3SiH or MD′M. These results indicate the iron precatalysts can offer different catalytic properties, even though they contain the same ancillary ligands. Most recently, Nakazawa and co-workers developed a number of Fe(II) dihalide complexes 11a−d supported by iminobipyridine (BPI) ligands (Scheme 10). Such complexes, Scheme 10. (RBIPAr)FeBr2/NaBEt3H-Catalyzed Alkene Hydrosilylation

PDI Fe complexes due to the substitution of the π-accepting imino subunit for the more electron-donating phosphino subunit. Upon activation with NaBHEt3, the anti-Markovnikov hydrosilylation of 1-alkenes with primary, secondary, and tertiary hydrosilanes could be obtained using the (PONN)Fe complexes with different steric properties. Note that there is no single catalyst that enables alkene hydrosilylations with all 1°, 2°, and 3° silanes. Whereas the least sterically hindered (PONN)Fe complexes (12g, 12h) are effective for the hydrosilylation with secondary and tertiary silanes, the relatively bulky Fe complexes (12a and 12c) with larger substituents at the P atom and in the ortho-positions of the N-aryl ring are effective for the silylation with the primary silane. More importantly, various functionalized terminal alkenes, such as those containing amide, ester and ketone groups, undergo the chemoselective hydrosilylation of the terminal C−C double bond, rather than the hydrosilylation of the carbonyl groups. For example, the reaction of 5-hexene-2-one with Ph2SiH2 gave the alkene hydrosilylation product in 86% isolated yield using 2 mol % of 12h, while the ketone moiety remained intact. The high chemoselectivity has been tentatively attributed to the electron-rich property of the Fe center, which leads to a favorable coordination of the C−C double bond over the C−O double bond.48 Although the (PONN)FeX2/NaBEt3H system exhibits high chemoselectivity and excellent functional-group tolerance, it is much less efficient than Chirik’s PDI Fe dinitrogen system in the hydrosilylation of terminal alkenes. Huang and co-workers found that the PONN ligands are prone to degradation via P−O bond cleavage, which likely has an adverse effect on the catalytic efficiency. Very recently, this laboratory prepared more robust phosphine-iminopyridine (PCNN) ligands containing a “CH2” linker between the P atom and the pyridyl backbone (Scheme 12), and examined the catalytic activities of the corresponding PCNN Fe(II) dichloride complexes (13) for alkene hydrosilylations. 49 Using only 0.02 mol % of (tBuPCNNiPr)FeCl2 13 (51 ppm Fe metal) and 0.08 mol % of NaBEt3H as the activator, the reaction of 1-octene with PhSiH3 in neat gave the linear hydrosilylation product in 79% yield (3950 turnovers). The data indicated that complex (tBuPCNNiPr)FeCl2 is much more productive than (tBuPONNiPr)FeCl2 bearing the O-linker since the hydrosilylation using the latter required 1 mol % of the precatalyst. Note that the reactions with the secondary silane (Ph2SiH2) and tertiary

upon activation with NaBEt3H, are highly active for alkene hydrosilylation reactions with PhSiH3, Ph2SiH2, PhMeSiH2, or Ph2MeSiH.47 For example, in the presence of 0.008 mol % of 11b, 1-octene (2 equiv) was hydrosilylated with Ph2SiH2 in neat at room temperature to form the hydrosilylation product Ph2(octyl)SiH in 96% yield and the double hydrosilylation product Ph2(octyl)2Si in 1.2% yield with a TON of 12250. The (RBIPAr)FeBr2/NaBEt3H-catalyzed hydrosilylation of 1-octene with PhSiH3 produced not only the monoalkylated and dialkylated silanes but also the trialkylated silane. However, the authors demonstrated that the selective formation of Ph(octyl)SiH2 and Ph(octyl)2SiH could be achieved by changing the reaction temperatures, catalyst loadings or adding organic solvent. Furthermore, the cyclic internal alkene, cyclohexene (10 equiv) reacted with PhSiH3 using 0.1 mol % of 11b and 0.4 mol % of NaBEt3H to give PhCySiH2 as the major product (80% yield). In 2013, Huang and co-workers prepared and characterized a series of Fe(II) dihalide complexes of phosphinite-iminopyridine (PONN) ligands (12). The steric properties of these complexes can be easily modified by tuning the substituents at the P atom and in 2,6-positions of the N-aryl ring (Scheme 11).48 Studies of their electronic properties established a more electron-rich Fe center in complexes 12 compared to that in the 1231


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ACS Catalysis Scheme 12. (tBuPCNNiPr)FeCl2-Catalyzed Hydrosilylation of 1-Octene with PhSiH3

More recently, the same group reported an improvement of this catalyst system by replacing the air-sensitive catalyst precursors, (COT)2Fe and (MPDE)2Fe, with a stable, easy-tohandle iron source Fe(OPv)2 (Pv = pivaloyl) (Scheme 14).51 Scheme 14. Fe(OPv)2/CNAd-Catalyzed Hydrosilylation of Styrene and Its Derivatives with Hydrosiloxanes

silane ((EtO)3SiH or MD′M) using the (PCNN)Fe catalysts formed decent amounts of the dehydrogenative silylation products as the side-products. Similar to their PONN analogues, the PCNN Fe catalysts tolerate a wide array of functionalized alkenes. Nagashima and co-workers reported that a simple combination of (COT)2Fe (14, COT = 1,3,5,7-cyclotetraene) and adamantyl isocyanide (CNAd) is effective for the antiMarkovnikov hydrosilylation of styrene and its derivatives (Scheme 13).50 After screening a variety of mono-, di-, and

This new Fe(OPv)2/CNAd system is effective for the hydrosilylation of styrene derivatives and allyl ethers with high anti-Markovnikov selectivity, but not for the general aliphatic alkenes. Note that high TONs (up to 9700) can be obtained in the hydrosilylation of styrene with (Me3SiO)Me2SiH (PMDS) using 0.01 mol % of the Fe catalyst at 50 °C over 24 h. An external activator is not required in this case because PMDS can activate the pivaloyl Fe(II) precursor. Following earlier studies on cobalt complexes of iminopyridine-oxazoline (IPO) ligands for asymmetric hydroboration of 1,1-disubstituted vinylarenes,52,53 the Lu group disclosed that the Fe analogous, (IPO)FeCl2 (16), upon activation with NaBEt3H, is enantioselective for the anti-Markovnikov hydrosilylation of 1,1-disubstituted vinylarenes with Ph2 SiH 2 (Scheme 15).54 The reactions with 1,1-disubstituted aliphatic

Scheme 13. (COT)2Fe/CNAd- and (MPDE)2/CNAdCatalyzed Hydrosilylation of Styrene and Its Derivatives with Hydrosiloxanes

Scheme 15. (iPrIPOiPr)FeCl2-Catalyzed Enantioselective Hydrosilylation of 1,1-Disubstituted Aryl Alkenes with Ph2SiH2

tridentate N-, P-, and S-containing ligands, the authors identified that the π-acidic isocyanides, which are isoelectronic with CO, were the suitable ligands. Among the isocyanides investigated in this work, the most sterically hindered CNAd gave the hydrosilylation product in best yield; no dehydrogenative silylation product (vinylsilane) was formed when the reaction was carried out at 50 °C or room temperature. The optimal ratio of (COT)2Fe: CNAd was found to be 1:2 for the hydrosilylation of styrene. Furthermore, the stoichiometric reaction of (COT)2Fe and CNAd resulted in the formation of (η4-COT)2Fe(CNAd) and (η4-COT)Fe(CNAd)3. On the basis of these data, the authors proposed (η4-COT)Fe(CNAd)2L (L= alkene, η2-(H−SiR3), or a vacant coordination site) as a primary intermediate. In addition to (COT)2Fe, Nagashima identified the open ferrocene Fe(II) species, (MPDE)2Fe (15, MPDE = η5-3-methylpentadienyl), as a useful precursor for the hydrosilylation of styrene (Scheme 13). Note that while these Fe catalyst systems are effective for the hydrosilylations of styrene derivatives, the hydrosilylations of aliphatic terminal alkenes were accompanied by side reactions, the dehydrogenative silylation and alkene hydrogenation reactions. For example, the reaction of 1-octene with PMDS using (COT)2Fe/CNAd gave a mixture of 1,1,3,3,3-pentamethyl-1octyldisiloxane and three side-products, vinylsilane, allylsilane, and n-octane in a ratio of 13:9:38:40.

alkenes, however, gave low enantioselectivities. Many functional groups, including acetal-protected aldehyde and ketone, amino, and imine, were demonstrated to be compatible with the catalyst system.54 This method represents a rare example of the Fe-catalyzed asymmetric alkene hydrosilylation. While numerous examples have shown that the tridentate ligands are suitable for the Fe-catalyzed hydrosilylation of terminal alkenes, Ritter and co-workers demonstrated that bidentate iminopyridine ligands are good choice for Fecatalyzed 1,4-hydrosilylation of 1,3-dienes (Scheme 16).55 In this study, an Fe(II) bis(aryl) complex 17 was employed as the precatalyst, which undergoes facile reductive elimination upon the reaction with 2 equiv of iminopyridine to generate a fivecoordinate, formal Fe(0) complex 19 ligated by two iminopyridines and one pyridine. Given the redox-active nature of the iminopyridine ligand, this Fe complex is best described as an Fe(II) species containing two radical, anionic ligands.41,42 Complex 19 proved to be active for the hydrosilylation of 1,3dienes. Kinetic profiles revealed that the optimal ratio of metal/ iminopyridine is 1:1 and an excess amount of the iminopyridine 1232


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ACS Catalysis Scheme 16. 1,4-Hydrosilylation of 1,3-Dienes Using Fe(II) Complexes of Imino-pyridine Ligands

Scheme 17. Co2(CO)8-Catalyzed Hydrosilylation of Terminal Alkenes and (Vinyl)polystyrene

hydrosilylation.58 Styrene and several functionalized alkenes, including allyl di(triethoxysilyl)amine and 1-vinyl-o-carborane, could be hydrosilylated with (EtO) 3 SiH to give the corresponding anti-Markovnikov products in moderate to high yields. In addition, Darling and co-workers applied this Co catalyst to the synthesis of chlorosilyl-functionalized polystyrene via the hydrosilylation of the residual vinyl groups in (vinyl)polystyrene.59 The resulting polymers could be used as the polymer-supported organosilicon protecting groups in solid-phase synthesis. The report by Chalk and Harrod have stimulated many studies on the catalyst development and mechanistic studies.60,61 In 1993, Grant and Brookhart reported that an electrophilic Co(III) alkyl complex [Cp*(P(OMe)3CoCH2CH2−μ-H)]+[BArF4]− (22, ArF = 3,5-(CF3)2C6H3) is effective for the anti-Markovnikov hydrosilylation of 1hexene with Et3SiH (Scheme 18).62 More importantly, they

ligand would retard the hydrosilylation. The results suggest that an Fe species bearing only one iminopyridine ligand is most likely the intermediate in the catalytic cycle. Evaluation of various iminopyridines established that the Fe complexes of ligands 20 and 21 with N-alkyl substituents are most effective for the 1,4-hydrosilylations of 2-substituted 1,3-dienes with tertiary silanes, furnishing (E)-allyl silanes in good yields with high regio- and stereoselectivity. The silyl group is added to the less sterically congested terminal carbon in the dienes presumably due to the steric effect. A wide range of functional groups, including esters, epoxides, ethers, and amines, were demonstrated to be compatible with the catalyst.

Scheme 18. [Cp*(P(OMe)3)Co(CH2CH3)][BArF4]Catalyzed Hydrosilylation of 1-Hexene with Et3SiH

3. COBALT-CATALYZED ALKENE HYDROSILYLATION The concentration of the cobalt metal in the earth’s crust (20 ppm) is much lower than that of Fe (41 000 ppm), but it is far more abundant than any platinum group metals.56 Over the last five decades, a number of Co-based alkene hydrosilylation catalysts have been developed. Recently, Sun and Deng published an in-depth review on the Co-catalyzed hydrosilylation of alkenes and alkynes;23 the reader is directed to this publication for a detailed coverage. Note that several new studies of the Co-catalyzed alkene hydrosilylations have been reported after the publication of Deng’s review. The present review will focus more on the very recent advances in the Cocatalyzed alkene hydrosilylations. The first Co-catalyzed alkene hydrosilylation reaction was reported by Chalk and Harrod in 1965.57 The low-valent Co(0) complex Co2(CO)8 was found to be an efficient catalyst for the anti-Markovnikov hydrosilylation of aliphatic terminal alkenes with tertiary silanes including Cl2PhSiH, (MeO)3SiH, and Et3SiH. In comparison with the reactions catalyzed by its iron analogue, Fe(CO)5 (Scheme 2), the hydrosilylation with Co2(CO)8 proceeded under milder conditions (0−60 °C) and in a more selective manner (Scheme 17). The side reactions, dehydrogenative silylation and hydrogenation that often occurred in the catalysis using Fe(CO)5, were not observed in the Co-catalyzed process.32,33 However, Co2(CO)8 could effect the isomerization of 1-alkenes to form inactive internal alkenes, and thus, an excess of alkenes (3 equiv relative to silane) was required. Later, Kalinin and co-workers broadened the substrate scope of the Co2(CO)8-catalyzed

provided direct evidence for a silyl migration insertion pathway accounting for the Co-catalyzed alkene hydrosilylation process, which contrasts to the Chalk-Harrod mechanism involving a hydride migration insertion pathway. The authors were able to detect and characterize the catalyst resting state, a β-silylalkyl Co(III) species [Cp*(P(OMe)3CoCH(Bu)CH(SiEt3)-μ-H)+ (23) using low-temperature NMR techniques. In 2013, Deng and co-workers synthesized a series of silyldonor-functionalized N-heterocyclic carbene (NHC) Co(II) complexes (25) via a Co-mediated C−H activation and silylation route.63 Both of the NHC and the silyl groups are strong σ-donating ligands. These Co(II) complexes exhibited very high activity in the hydrosilylation of 1-octene with the primary silane PhSiH3 (Scheme 19). In the presence of 0.005 mol % 25a, the reaction of 1-octene with PhSiH3 formed the linear product n-octylsilane in 70% yield after 24 h, corresponding to a TON of 14 000. Small amounts of products resulting from Markovnikov hydrosilylation, olefin isomerization, and olefin hydrogenation were detected as the sideproducts. The silyl-functionalized NHC-Co(II) complexes is 1233


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alkene substrates (styrene derivatives and allyl ethers, cf. Scheme 14).51 In the same paper, Nagashima and co-workers showed that the replacement of Fe(OPv)2 with Co(OPv)2 led to a catalyst system with an improved substrate scope relative to the Fe. Terminal aliphatic alkenes and 1,1-disubstituted αmethylstyrenes were hydrosilylated with hydrosiloxanes by the Co catalyst to give the anti-Markovnikov products in moderate to high yields (Scheme 21). The reactions typically occurred

Scheme 19. Silyl NHC Cobalt(II) Complex-Catalyzed Hydrosilylation of 1-Octene with PhSiH3

Scheme 21. Co(OPv)2/CNAd-Catalyzed Hydrosilylation of 1,1-Disubstituted Aryl Alkenes and Alkyl Alkenes

more active than any other Co hydrosilylation catalysts previously reported in the hydrosilylations using PhSiH3. The reaction with secondary silanes showed reduced activities. The hydrosilylations with tertiary silanes as the reagents using complexes 24 were not disclosed. In 2015, Holland and co-workers developed β-diketiminatesupported Co(I)-arene complexes and applied them in the antiMarkovnikov hydrosilylation of alkenes with PhSiH3 and (EtO)3SiH (Scheme 20).64 Regardless of the nature of the Scheme 20. β-Diketiminate-Support Co(I) Arene Complex for Catalytic Hydrosilylation of Terminal and Internal Alkenes

with 1 mol % of the catalyst, but in the case of the hydrosilylation of α-methylstyrene, the catalysis with only 0.05 mol % of Co(OPv)2 gave 1885 turnovers. However, the hydrosilylation of the monosubstituted styrene and its derivatives resulted in the concomitant dehydrogenative silylation reaction. This Co catalyst is also effective for the tandem olefin-isomerization and hydrosilylation. In the presence of 1 mol % of Co(OPv)2 and 3 mol % of CNAd, 2octene reacted with PMDS at 80 °C to form n-octylsilane in 94% isolated yield. It was revealed that the hydrosiloxane used as the hydrosilylation reagent could also serve as the catalyst activator. When using the preactivated precursors, the hydrosilylation reaction proceeded at lower reaction temperature and needed shorter reaction time in comparison with the run without the preactivation. For instance, the hydrosilylation of α-methylstyrene with polymethylhydrosiloxane Me 2 HSi(OSiMe2)27OSiHMe2 catalyzed by the Co(OPv)2/CNAd/ (EtO)3SiH system, in which (EtO)3SiH acted as the catalyst activator, formed the modified silicone fluid in 99% isolated yield at 50 °C for 24 h. As a comparison, the reaction with Co(OPv)2/CNAd, but without (EtO)3SiH (i.e., no preactivation), required a higher temperature (80 °C) and gave a lower yield of the hydrosilylation product (77%).51 Despite their high activity and excellent anti-Markovnikov regioselectivity,24,38,40 the instability of Chirik’s PDI Fe bis(dinitrogen) complexes poses a potential challenge in industrial application. To overcome this limitation, the same group recently developed several bench-stable Co(II) bis(carboxylate) complexes with N-alkyl imine-substituted PDI ligands (28) for the alkene hydrosilylation (Scheme 22).66 Two important features of this catalyst system are noteworthy. First, similar to the observations by Nagashima, the Co(II) carboxylate complexes can be activated by the tertiary silane substrate to form the active hydrosilylation catalysts. Second, the N-alkyl substituted PDI Co complex promotes the hydrosilylation while the N-aryl substituted variant is selective

hydrosilane reagents employed, the less sterically hindered catalyst generally exhibited higher activity than the bulky catalyst, and the catalyst with N-mesityl substituents (26) was determined to be most effective among the diketiminate catalysts investigated. A wide range of substrates, including the functionalized alkenes bearing silyl ether, halide, ester, tertiary amine and amide substituents could be hydrosilylated at temperatures in the range from 25 to 60 °C, forming the corresponding linear hydrosilylation products in good to high isolated yields. Furthermore, the hydrosilylations of internal alkenes resulted in the formation of linear alkylsilanes via the tandem isomerization and hydrosilylation sequence. Previous study by this group showed that the β-diketiminate Co(I) toluene complex 27 is an active olefin isomerization catalyst.65 A combination of 27 (2−4 mol %) and 26 (0.5−2 mol %) provided an effective system for the conversion of internal hexenes into n-hexylsilane with PhSiH3 or (EtO)3SiH as the hydrosilylation reagent. As shown above, the Fe catalyst generated from Fe(OPv)2 and CNAd is active for the hydrosilylation of very limited 1234


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ACS Catalysis Scheme 22. (TFAPDI)Co(2-EH)2-Catalyzed Hydrosilylation of Alkenes with Tertiary Silanes

MD′M). The reaction proceeded at room temperature to produce the anti-Markovnikov addition products in good to high isolated yields. Importantly, mechanistic studies revealed a silyl Co(III) hydride species as the key intermediate. Addition of Ph2SiH2 to 30 resulted in the in situ generation of (DIPPCCC)Co(SiHPh2)(H)(N2) (31), which was characterized by 1H and 29Si NMR spectroscopies. The stoichiometric reaction of 31 with 1-octene formed the hydrosilylation product n-octylsilane and regenerated 30. Here the use of bis(carbene)-based pincer ligand is believed to promote the formation of a low-valent, electron-rich Co complex, which is amenable to the oxidative addition of the H−Si bond to the metal center. The Huang group reported that the phosphine-iminopyridine (PCNN) Fe(II) complexes, upon activation with NaBEt3H, are regioselective for the anti-Markovnikov hydrosilylation of 1-alkenes.49 In sharp contrast, the Co(II) complexes bearing the same type of ligands provided high Markovnikov selectivity in the hydrosilylation of aliphatic alkenes with PhSiH3 (Scheme 24). 49 Thus, a simple

for the dehydrogenative silylation. In a separate paper,10 Chirik and co-workers showed that the dehydrogenative silylation of aliphatic 1-alkenes occurred favorably over the hydrosilylation when a N-mesityl imine-substituted complex (MesPDI)CoCH3 (Mes = 2,4,6-(CH3)3-C6H2) (29) was used as the precatalyst (vide infra). By contrast, this work using the sterically unencumbered, N-alkyl-substituted PDI ligand led to high selectivity for the hydrosilylation. The authors attributed the selectivity for the saturated alkylsilanes to the use of lesshindered ligands, which would facilitate the bimolecular reaction between the cobalt alkyl species and the silane relative to the unimolecular β-hydrogen elimination. In addition, the catalyst is effective for the hydrosilylations of various functionalized alkenes including the industrially relevant allyl glycidyl ether. The cross-linking between polydimethylsiloxanes bearing terminal vinyl groups and polymethylhydrosiloxanes using this Co catalyst has also been demonstrated. The reaction occurred at 80 °C in 5 min to yield colorless gel products when 1−2 ppm of Co was used (based on Co metal, wt/wt). In 2016, a novel bis(carbene)-based pincer Co(I) dinitrogen complex ( D I P P CCC)CoN 2 (30) ( D I P P CCC = bis(diisopropylphenyl-imidazol-2-ylidene)-phenyl) has been reported by the Fout group (Scheme 23).67 Remarkably, the bis(carbene) complex enables the chemoselective hydrosilylation of terminal alkenes containing highly reactive functional groups such as formyl, ketone, hydroxyl, and nitrile functionalities. The standard conditions employed 5 mol % of the Co catalyst and secondary and tertiary silanes as the hydrosilylation reagents (e.g., Ph2SiH2, PhMe2SiH, and

Scheme 24. (iPrPCNNMe)CoCl2-Catalyzed Markovnikov Hydrosilylation of Alkenes with PhSiH3

replacement of the metal center from Fe to Co led to an opposite regioselectivity in the hydrosilylaltion of 1-alkene. Among the PCNN Co complexes investigated, the least bulky precatalyst (iPrPCNNMe)CoCl2 (32c) with the iPr substituent at the P atom and the Me substituent in the 2,6-positions of the N-aryl group proved to be most effective for the Markovnikov hydrosilylation of 1-alkenes. A wide variety of functionalized aliphatic 1-alkenes bearing halide, ether, ester, acetal, and amide functional groups were hydrosilylated chemoselectively with PhSiH3 by 32c (0.05−2 mol %) to give the Markovnikov products in good isolated yields. Most of the substrates gave >99:1 branched/linear selectivities. Different from the (PCNN)FeCl2/NaBEt3H system, there is no need for an external reductant, such as NaBEt3H, to activate the PCNN Co(II) dichloride precursor. The stoichiometric experiments revealed that the silane reagent could act as the activator in the hydrosilylation reaction with PhSiH3. The reaction of the dichloride complex (iPrPCNNMe)CoCl2 (32c) with PhSiH3 in C6D6 at 60 °C generated a diamagnetic Co(I) species that was identified as (iPrPCNNMe)CoCl (33). This Co(I) monochloride is probably further converted to a Co silyl species, which is catalytically relevant to the hydrosilylation process. Additional experimental and computational studies are

Scheme 23. (DIPPCCC)CoN2-Catalyzed Alkene Hydrosilylation

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proposed that the two large SiMe3 groups promote the dissociation of PPh3 in 34c to form a coordinatively unsaturated, catalytically active species, whereas complexes 34a and 34b need NaBPh4 for the abstraction of PPh3 to generate the active catalyst.77 Using a bidentate NHC-pyridine supported Ni(II) allyl complex (35) as the precatalyst, the Valerga group recently reported the Markovnikov hydrosilylation of styrene with PhSiH3 (Scheme 25b).78 In the absence of a cocatalyst, the hydrosilylation of styrene with PhSiH3 and the cationic complex 35 (1 mol %) yielded PhCH(Me)SiH2Ph in moderate to good yields at ambient temperature. Preliminary mechanistic investigations revealed the formation of a Ni(II) hydride intermediate in the catalytic cycle. In contrast to the Markovnikov selectivities obtained in the reactions using the indenyl and NHC-pyridine Ni complexes, Gevorgyan and co-workers observed the anti-Markovnikov hydrosilylation of styrene with Ph2SiH2 (Scheme 25c).79 With 2 m o l % o f N i B r 2 ( P P h 3 ) 2 , t h e l in ea r p r o d u c t PhCH2CH2SiHPh2 was obtained in 87% isolated yield at 80 °C over 6 h. The resulting hydrosilylation product PhCH 2 CH 2 SiHPh 2 could be further converted into a dihydrobenzosilole through [Ir(OMe) (COD)]2/dtbpy-catalyzed intramolecular dehydrogenative cyclization. In 2012, Lipschutz and Tilley reported the synthesis and characterization of a novel strictly two-coordinate bis(amino) Ni(II) complex (36) and explored its reactivity toward a variety of small molecules (Scheme 26).80 Using 36 as the catalyst (2

needed to obtain a full understanding of the reaction mechanism and thus to provide insight into the origin of the regiochemistry.

4. NICKEL-CATALYZED ALKENE HYDROSILYLATION As a first-row, group 10 transition metal, nickel has long been considered as an inexpensive and abundant alternative to platinum.68 In fact, nickel salts and its complexes have garnered attention for alkene hydrosilylation since the 1950s.69−73 Early studies of some ill-defined Ni-based complexes, such as Raney nickel69 and nickel carbonyl,70 showed the potential application of Ni catalysts in the hydrosilylation reaction. However, these systems suffered from limited substrate scope, low yield, harsh reaction conditions, and undesired side reactions.69−73 A number of well-defined Ni complexes supported by various types of ligands have been developed more recently, and these new Ni catalysts have exhibited enhanced activity and high selectivity in the alkene hydrosilylations. A couple of Ni catalyst systems offered Markovnikov selectivities in the hydrosilylation of styrene and its derivatives. The indenyl Ni(II) complexes (R-Ind)Ni(PPh3)Cl (R-Ind =1Me-indenyl, 34a; 1-SiMe3-indenyl, 34b; 1,3-(SiMe3)2-indenyl, 34c) described by Zargarian and co-workers in a series of papers showed high efficiency and Markovnikov regioselectivity in the reaction of styrene with PhSiH3 (Scheme 25a).74−77 Scheme 25. Ni-Based Catalysts for Hydrosilylation of Styrene

Scheme 26. Bis(amino) Ni(II) Complex-Catalyzed Hydrosilylation of 1-Octene with Ph2SiH2

mol %), the simple aliphatic alkene, 1-octene was hydrosilylated with Ph2SiH2 at ambient temperature to form the antiMarkovnikov product in >95% yield. The substrate scope regarding to the olefins and silanes has not been disclosed yet. In 2015, Shimada, Nakajima, and co-workers reported that a salicylaldiminato Ni(II) methyl complex (37) is effective for the anti-Markovnikov hydrosilylation of both alkyl- and arylsubstituted terminal alkenes (Scheme 27).81 The hydrosilylations with secondary silanes (Et2SiH2, Ph2SiH2) in the presence of 0.5 mol % of 37 occurred in CH3CN at room temperature within 1 h to give the linear alkylsilanes in low to high yields. However, tertiary and primary silanes were not applicable. The dimethylamino and phenylthio functionalities in the alkene substrates were compatible with the catalyst. The internal alkene, 2-octene, was hydrosilylated successfully at 50 °C to give n-octylsilane in 85% yield via the isomerization and hydrosilylation sequence. The regioselective 1,4-hydrosilylation of isoprene with Et2SiH2 was also realized, forming allylsilane with the addition of the silyl group to the less-hindered terminal carbon in isoprene. The combination of 1H and 29Si NMR spectroscopies provided direct evidence in support of the formation of a Ni(II) silyl species, which was generated upon the reaction of 37 with Ph2SiH2. Such a silyl species proved to

Using NaBPh4 (10 mol %) as an activator/cocatalyst, all three catalysts 34a−c (1 mol %) exhibited good catalytic activities, producing the Markovnikov hydrosilylation product PhCH(Me)SiH2 Ph in high yield and >99:1 branched/linear selectivity. When the reaction was carried out in the absence of NaBPh4, the most bulky complex 34c with the 1,3-(SiMe3)2indenyl ligand gave PhCH(Me)SiH2Ph in a yield similar to that obtained in the reaction with NaBPh4. In contrast, complexes 34a and 34b gave much lower yields in the absence of NaBPh4 compared to the reactions with NaBPh4.77 The authors 1236


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ACS Catalysis Scheme 27. (Salicylaldiminato)methyl Ni(II) ComplexCatalyzed Alkene Hydrosilylation

Scheme 29. Cationic Ni(II) Allyl Complex-Catalyzed Alkene Hydrosilylation

n-octylSiEt2H in 91% isolated yield. Small amounts of sideproducts resulting from silane redistribution and olefin isomerization were also detected. The scope of the silane substrates is relatively narrow because tertiary and primary silanes are not suitable substrates. Notably, the computational data suggested that the allyl group in 39 is a noninnocent ligand in the hydrosilylation process; the coordinated allyl group can promote the Ni-catalyzed hydrosilylation by lowering the energy barriers for the Si−H bond cleavage and the Si−C bond formation steps.83 In 2015, the Uyeda group reported a rare dinuclear Ni(I) complex (iPrNDI)Ni2(C6H6) (40) supported by a doubly reduced naphthyridine-diimine (NDI) ligand and explored its catalytic activity for the hydrosilylation reaction (Scheme 30).84

be active for the catalytic hydrosilylation of alkene with Ph2SiH2. On the basis of these results, the authors proposed the mechanism invoking a Ni(II) silyl species, rather than a Ni(II) hydride species. In 2016, the same group developed an efficient catalyst system for the hydrosilyaltion of terminal alkenes and 1,3dienes with tertiary or secondary hydrosilanes using simple Ni(acac)2 (acac = acetylacetonato, 38a) and its derivatives (38b, 38c) as the catalyst precursors (Scheme 28).82 Upon

Scheme 30. (iPrNDI)Ni2(C6H6)-Catalyzed Alkene Hydrosilylation

Scheme 28. Bis(acetylacetonato) Ni(II)/NaBEt3H-Catalyzed Hydrosilylation of Alkenes and 1,3-Dienes

activation with NaBEt3H (1 equiv relative to Ni), terminal alkenes bearing siloxyl, amino and epoxyl groups were hydrosilylated with Ph2SiH2 or (EtO)3SiH in the presence of 38a or 38b (0.5−1 mol %) in THF at ambient temperature, yielding the corresponding anti-Markovnikov additional products in 42−88% isolated yield after 1−96 h. Under similar conditions, 2-substituted 1,3-dienes underwent hydrosilylation with various tertiary and secondary silanes to form the 1,4addition products in moderate to high yields. In contrast to the selective formation of the linear allyl silanes observed in the reactions with the salicylaldiminato Ni complex81 and Ritter’s iminopyridine Fe complex,55 these Ni catalysts gave moderate to good regioselectivities in favor of the branched products (linear/branched = 22/78−2:98, see Scheme 28). More recently, the Shimada group reported that an arenesupported cationic Ni(II) allyl complex (39) is effective for the hydrosilylation of 1-octene with Et2SiH2 (Scheme 29).83 The reaction using 0.5 mol % of 39 at room temperature in 9 h gave

The NDI ligand is capable of stabilizing the electron-rich Ni(I)−Ni(I) bond because the π-system allows the electron equivalents to be stored in ligand. At 5 mol % loading of 40, 1octene was hydrosilylated with Ph2SiH2 to form the antiMarkovnikov product in 77% yield. The substitution of Ph2SiH2 with Et2SiH2 led to a lower yield of the hydrosilylation product (35%). Enones underwent the 1,4-hydrosilylation to form the silyl enol ether in high yield with a 6:1 E/Z selectivity (Scheme 30). The stoichiometric experiment revealed that 40 reacted rapidly and reversibly with the secondary silanes (Ph2SiH2 or Et2SiH2) in C6D6 to form a stable silane adduct (iPrNDI)Ni2(R2SiH2) (41). This silane complex 41a was directed also observed by the 1H NMR spectroscopy in the catalytic reaction of 1-octene with Ph2SiH2, indicating that the silane adduct is the catalyst resting state. In 2015, Hu and co-workers found that the Ni(II) methoxy complex of an anionic bis(amino)amide pincer (42) is highly efficient for the anti-Markovnikov hydrosilylation of terminal 1237


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catalyzed disproportionation of alkoxysilane to form hydrosilane, which reacts with alkene through Ni-catalyzed antiMarkovnikov hydrosilylation to form alkylsilane. Although several Ni catalysts have exhibited good catalytic activity and high regioselectivity in the hydrosilylation reactions, most of the above-mentioned Ni catalysts utilized primary or secondary silanes as the reagents. There are only a few examples of Ni-catalyzed hydrosilylations using the more commercially relevant tertiary hydrosilanes. In 2016, the Chirik group reported a Ni complex of a bidentate diimine ligand for the anti-Markovnikov alkene hydrosilylation with tertiary hydroalkoxysilanes or hydrosiloxanes as the reagents (Scheme 33).87 The combination of a readily available α-diimine ligand

alkenes with Ph2SiH2 (Scheme 31). The catalytic processes gave very high TONs (up to 10 000) and turnover frequencies Scheme 31. (MeN2N)Ni(OMe)-Catalyzed Alkene Hydrosilylation

Scheme 33. Selective Hydrosilylation of Alkenes with Hydrosiloxanes using Ni(II) Bis(carboxylates) and αDiimine Ligand (up to 83 000 h−1).85 Cyclic alkenes and various functionalized terminal alkenes containing epoxide, halide, ether, ester, amino, and amido groups underwent chemoselective hydrosilylation with Ph2SiH2 using 1−5 mol % of 42 at ambient temperature in THF, furnishing the linear alkylsilanes in moderate to high isolated yields. When dimethylacetamide (DMA) was used as the solvent, the chemoselective hydrosilylation of terminal alkenes containing the challenging aldehyde and ketone functional groups with Ph2SiH2 could be realized (Scheme 31). Furthermore, acyclic internal alkenes reacted with Ph2SiH2 in the presence of 10 mol % of 42 to form linear alkylsilane through the isomerization−hydrosilylation reaction. Further studies by Hu led to the discovery of a new bis(oxazoline)-based pincer Ni(II) complex [iPr2-(S,S)-Bopa]NiCl (43) for the hydrosilyation of alkenes (Scheme 32).86 In

(iPrDI, 44, 1 mol %), air-stable Ni(2-EH)2 (2-EH = 2ethylhexanoate, 1 mol %), and (EtO)3SiH (18 mol %) generated an active Ni catalyst, which effected the hydrosilylations of 1-octene with various tertiary silanes at ambient temperature to give the anti-Markovnikov addition products in high isolated yields and high regioselectivity. The reaction is scalable as demonstrated by the hydrosilylation of 1-octene with (EtO)3SiH on a 10 g scale using 0.1 mol % of the catalyst. This catalyst system was also applied to the industrially relevant silicone cross-linking, the addition of a polymeric hydridosilane to a polymeric vinyl silane. Using 100 ppm of Ni (based on Ni metal, wt/wt), the cross-linking occurred at 80 °C to give colorless gel product after 2 h. The good catalytic performance of the Ni catalyst generated in situ promoted the authors to synthesize and characterize the active nickel complex. Treatment of Ni(2-EH) 2 with (EtO)3SiH and iPrDI formed a diamagnetic hydride-bridged dinuclear species [(iPrDI)NiH]2 (45) containing two formally Ni(I) centers. Studies of the electronic structure of 45 by DFT calculations established one-electron reduced diimine ligands and the Ni(II) centers. This dimeric species was determined to be an off-cycle catalyst resting state. The rate law established by kinetic studies indicated that the Ni hydride dimer undergoes dissociation to form a monomeric nickel hydride (iPrDI)NiH as the catalytically active intermediate. Furthermore, the reaction of 1-octene with the deuterated silane ((EtO)3SiD) catalyzed by 45 gave the alkylsilane product with deuterium in all positions of the alkyl chain. The results of deuterium-labeling and kinetic studies are consistent with a hydride migration pathway, in which alkene insertion into the

Scheme 32. [iPr2-(S,S)-Bopa]NiCl/NaOtBu-Catalyzed Alkene Hydrosilylation

the presence of 43 (2.5−5 mol %) and NaOtBu (5−10 mol %), various functionalized terminal alkenes bearing ether, epoxide, silyl, halide, amine, and acetal groups were hydrosilylated with hydroalkoxysilanes, Me 2 (MeO)SiH, Me(EtO) 2 SiH, and (MeO)3SiH, producing alkyl hydrosilanes in moderate to excellent isolated yields. It should be noted that the hydroalkoxysilanes used in this work actually served as the surrogates of volatile Me2SiH2, MeSiH3, and SiH4, which would be difficult to access by other means. The alkylsilane products resulting from the first hydrosilylation with MeSiH3 and SiH4 can react with alkene again to form the secondary or tertiary silanes (i.e., double hydrosilylation). Preliminary mechanistic studies suggested that the transformation proceed by base1238


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ACS Catalysis

selective alkene dehydrogenative silylations, there have been only a few examples of base-metal-catalyzed selective dehydrogenative silylations. In 1993, Murai and co-workers reported that Fe(CO)5 is an effective catalyst for the dehydrogenative silylation reaction vinylarenes (Scheme 35).36 Specifically, the dehydrogenative

Ni−H bond is reversible and fast relative to the C−Si bond formation. Finally, the propensity of the Ni(II) alkyl complex toward facile chain walking allows for efficient isomerizationhydrosilylation of internal olefin. With 45 (0.5 mol %) as the catalyst, 4-octene was converted to n-octylsilane in 92% yield, establishing a tandem process consisting of the rapid Nicatalyzed olefin isomerization and the subsequent regioselective anti-Markovnikov hydrosilylation of the terminal olefin. Most recently, the Hu group reported a heterogeneous Ni catalyst system for the alkene hydrosilylations using tertiary hydrosilanes (Scheme 34).88 The active catalytic species, Ni

Scheme 35. Fe(CO)5-Catalyzed Dehydrogenative Silylation of Styrenes

Scheme 34. Selective Hydrosilylation of Terminal and Internal Alkenes with Tertiary Hydrosilanes Using Nickel Nanoparticle Catalyst

silylation reaction of styrene and its derivatives with Et3SiH occurred in benzene at 40−80 °C using 1 mol % of Fe(CO)5, producing the corresponding (E)-β-triethylsilylstyrenes in 66− 89% yield after 24−72 h. The catalyst system exhibited high stereoselectivity, and no (Z)-vinylsilanes products resulted. Excess styrenes (3 equiv relative to silane) were used in these reactions because the alkenes also served as the hydrogen acceptors. In 2000, Marciniec and co-workers reported that the nickel analogue of Karstedt’s catalyst, Ni2{[(CH2CH)SiMe2]2O}3 (46) is effective for the dehydrogenative silylation of terminal alkenes with tertiary hydrosilanes (Scheme 36).95 The reactions Scheme 36. Ni2{[(CH2CH)-SiMe2]2O}3-Catalyzed Dehydrogenative Silylation of Alkenes

nanoparticle, was obtained from the in situ activation of Ni(OtBu)2·nKCl (n ≈ 1.4) with hydrosilane, which was supported by the mercury poisoning test and TEM (Transmission Electron Microscopy) measurements. In the presence of 1 mol % of Ni(OtBu)2·nKCl, a wide range of functionalized terminal alkenes containing epoxide, halide, ester, amine, acetal groups were hydrosilylated with tertiary silanes at ambient temperature, giving the corresponding anti-Markovnikov products in 44−95% isolated yield after 4 h. Functionalized acyclic internal alkenes were also hydrosilylated smoothly when the catalyst loadings were increased to 2−5 mol %, yielding the 1-silyl products in moderate to excellent isolated yields (69−97%) via the olefin-isomerization and hydrosilylation sequence. Remarkably, the TBS-protected cis-9-octadecen-1-ol (oleyl alcohol) could be hydrosilylated with 2 equiv of (EtO)3SiH, furnishing the terminally silylated product in 45% isolated yield. In this case, the Ni catalyst enables the migration of C−C double bond over 8 carbons, producing the terminal olefin for the anti-Markovnikov hydrosilylation.

with PhMe2SiH or (EtO)3SiH in the presence of 0.1 mol % of 46 occurred in neat alkene and silane at 120 °C, furnishing the corresponding vinylsilanes in 23−96% GC yield, together with 4−42% of the hydrosiylation products. The trans vinylsilanes were formed selectively over the cis isomers with >10:1 E/Z ratios. The alkene scope is relatively narrow because only styrene and silyl alkenes proved to be suitable substrates. The stoichiometric reaction of 46 with (EtO)3SiH resulted in the formation of a disilyl nickel complex, which was proposed as a key catalytic intermediate. The reaction likely occurs through the insertion of alkene into the Ni−Si bond of the disilyl species, followed by β-hydrogen elimination to form vinylsilane. In 2012, Nakazawa and co-workers reported that CpFe(CO)2Me (47) effected the dehydrogenative silylation of 1,3divinyldisiloxane (Scheme 37).8 In the presence of 4−10 mol % of CpFe(CO)2Me, 1,3-divinyldisiloxane underwent the dehydrogenative silylation with various tertiary silanes (PMDS, MD′M, Ph2MeSiH) at 80 °C in toluene over 24−48 h, yielding the corresponding E-vinylsilanes products in 44−98% isolated yields. In this transformation, one C−C double group in this divinyl substrate is dehydrogenatively silylated and the other is hydrogenated. Deuterium-labeling studies were conducted using Ph2MeSiD as the hydrosilylation reagent. Under the

5. BASE-METAL-CATALYZED ALKENE DEHYDROGENATIVE SILYLATION The dehydrogenative silylation reaction, where the C−Si bond formation occurs but the unsaturation of the alkene substrate is retained, has long been viewed as an undesired side reaction in the alkene hydrosilylation process. However, the dehydrogenative silylation reaction is an attractive method for the preparation of vinylsilanes and allylsilanes, which are valuable synthetic intermediates. For example, such unsaturated silicon compounds have found application as coupling reagents in the Hiyama−Denmark cross-coupling reaction.89−92 While some precious metal catalysts, such as several iridium catalysts of bipyridine-type ligands,93,94 have been documented for the 1239


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ACS Catalysis

Scheme 39. (MesPDI)CoCH3-Catalyzed Dehydrogenative Silylation of Alkenes

Scheme 37. CpFe(CO)2Me-Catalyzed Dehydrogenative Silylation of 1,3-Divinyldisiloxane

standard catalytic conditions, the α-deuterium product was isolated in 53% yield. The date suggested that the two hydrogen atoms involved in the hydrogenation process originate from different sources: one from the hydrosilane and the other from the β-carbon in the other vinyl group resulting from β-H elimination. The in situ NMR experiments revealed an induction period of this catalysis and established an Fe silyl species as the key intermediate. In 2014, Marciniec and co-workers reported that a series of Fe(0) carbonyl complexes ligated by various multivinylsilicon ligands effect the dehydrogenative silylation of vinylsiloxane with hydroxiloxane.9 Among the Fe(0) carbonyl complexes investigated in this study, Fe{[(CH2CH)SiMe2]2O}(CO)3 (48) proved to be the most efficient one (Scheme 38). The

generated in situ from the more readily accessible dichloride complex (MesPDI)CoCl2 and NaBEt3H (2 equiv relative Co). The reactions required the addition of 2 equiv of alkene substrate relative to silane, one of which served as the sacrificial hydrogen acceptor. Addition of an external hydrogen acceptor (10 equiv), such as cyclohexene, cyclooctene, 1,5-cyclooctadiene, and norbornene, had little impact on the product distribution. The dehydrogenative silylation of acyclic internal alkenes such as cis- and tran-4-octene produced the terminal functionalized allylsilanes via the Co-mediated alkene-isomerization and dehydrogenative silylation processes, which was described as a remote functionalization of C−H bonds. Thorough mechanistic studies have been conducted to provide insight into the Co-catalyzed dehydrogenative silylation event. The reaction of (MesPDI)CoCH3 (29) and silane formed a Co silyl complex and methane. Performing a 2:1 mixture of neat 1-octene and (Me3SiO)2SiMeD in the presence of 0.5 mol % of 29 yielded 1-d1-octane along with the allylsilane. However, the reaction of trans-4-octene and (Me3SiO)2SiMeD in the presence of 1 mol % of 29 resulted in isotope scrambling in the allylsliane product. The data suggested a reversible insertion of the alkene into the Co−H bond. A primary deuterium kinetic isotope effect was determined by two parallel catalytic reactions a n d t h e i nt e rn a l co m p e t i t i o n ex p e ri m e n t us i n g (Me3SiO)2SiMeD and (Me3SiO)2SiMeH as the reagents. The data are consistent with a rate-limiting step involving the reaction of the Co alkyl complex and silane to form the Co silyl and the corresponding alkane. On the basis of these observations, the authors proposed a mechanism with the silyl and hydride Co(I) complexes as the intermediates (Scheme 40). 2,1-Insertion of alkene into the Co−Si bond in the silyl complex A, followed by β-H elimination, results in the formation of allysilane and the hydride species C. The selective formation of allylsilanes rather than vinylsilane was attributed to the preferential β-H elimination away from the sterically bulky tertiary silane substituent. The Co hydride C then reacts with alkene to form the alkyl complex D, which is converted to the silyl complex (A) upon the reaction with silane, accompanied by the formation of alkane.

Scheme 38. Fe{[(CH2CH)SiMe2]2O}(CO)3-Catalyzed Dehydrogenative Silylation of Heptamethylvinyltrisiloxane

reaction of vinylheptamethyltrisiloxane (3 equiv) with MD′M in the presence of 1 mol % of 48 in toluene at 80 °C in 10 min afforded the unsaturated product in 100% yield. Such transformation also occurred at room temperature under UV irradiation, reaching high conversion after 30 min. The stoichiometric reaction between 48 and MD′M at −40 °C revealed the formation of three Fe−H species, among which a six-coordinated silyl hydrido Fe(II) tricarbonyl (49) containing a η2-bonded 1,3-divinyltetramethyldisiloxane was proposed as the key intermediate. In addition, the Fe carbonyl complexes of multivinylsilicon ligands served as air- and moisture-stable catalysts for curing of poly(vinyl)siloxanes with polyhydrosiloxanes at temperatures in the range from 120 to 170 °C. As mentioned above, Chirik and co-workers found that the aryl-substituted PDI Co methyl complex (MesPDI)CoCH3 (29), could catalyze the dehydrogenative silylation with a variety of aliphatic terminal olefins using primary, secondary and tertiary hydrosilanes. The reactions are selective for formation of allylsilanes relative to vinylsilanes (Scheme 39).10 Similar catalytic performance could be obtained by using the catalysts 1240


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ACS Catalysis Scheme 40. Proposed Mechanism for (MesPDI)CoCH3Catalyzed Dehydrogenative Silylation of Alkenes

bond without olefin isomerization. The regio- and/or enantioselective hydrosilylation of 1,2-disubstituted olefins is a reaction worthy of being pursued. In addition, the development of catalyst systems for more challenging tri- and tetrasubstituted olefins would be desirable. Finally, the development of base-metal hydrosilylation catalysts other than Fe, Co, and Ni metals would be a welcome addition to this field. Of particular interest is the development of manganese hydrosilylation catalyst because Mn is ca. 50 times more abundant than Co in the earth’s crust and has the advantages of being nontoxic and environmentally benign. Note that Mn complexes are known for hydrosilylations of carbonyl compounds;97 however, the Mn catalysts for the alkene hydrosilylation process have remained unexploited.98

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AUTHOR INFORMATION

Corresponding Author

6. CONCLUSIONS AND FUTURE OUTLOOK During the past two decades, in particular, the period 2012− 2016, enormous progress has been achieved in the field of alkene hydrosilylations utilizing the abundant and inexpensive Fe, Co, and Ni complexes. Although many of these base-metal catalysts exhibit high anti-Markovnikov regioselectivities, the activities vary, dependent on the nature of the silane and alkene substrates. The Fe catalysts in general are more efficient than the Co and Ni catalysts in the alkene hydrosilylations with commercially relevant tertiary alkoxylsilanes or siloxysilanes. Particularly, the activity and anti-Markovnikov selectivity demonstrated by Chirik’s bis(imino)pyridine Fe dinitrogen complex [(MePDI)Fe(N2)2](μ2-N2) can be compared with the classical Pt catalysts.24 One concern in this iron system is the high sensitivity of the Fe dinitrogen precatalyst toward oxygen and moisture. Many other Fe,37,46,48−50 Co,49,62−64 and Ni80,81,83−85,87 precatalysts covered in this review, are also unstable to air and moisture. While several bench-stable Fe and Co precatalysts and different catalyst activation strategies have been developed by Thomas,44 Chirik,66 and Nagashima,50,51 the catalytically active species appeared to be air- and moisturesensitive and thus should be manipulated under inert atmosphere using dried reagents and/or solvents. The development of stable and easy-to-handle base-metal catalysts with high efficiency and improved substrate scope will be a focus of future studies. To evaluate their viability for industrial applications, the newly developed catalysts should be applied to large-scale synthesis of commercially relevant silicone. The period of the last 5 years has also seen development of functional-group-tolerant base-metal hydrosilylation catalysts and dehydrogenative silylation catalysts for the formation of synthetically valuable organosilanes. In addition to alkene hydrosilylations with anti-Markovnikov selectivity, Co catalysts have been developed for highly selective Markovnikov hydrosilylation of aliphatic terminal alkenes.49 By developing new base-metal catalysts with suitable chiral ligands, further applications of the Markovnikov hydrosilylations would include the construction of optically pure secondary alkylsilanes, which can be further converted to synthetically versatile secondary alcohols and other valuable products. When acyclic internal olefins were used as the substrates, the known base-metal complexes exclusively effected rapid olefin isomerization and anti-Markovnikov hydrosilylation of the terminal olefin to form linear alkylsilanes selectively.37,38,51,64,81,85,87,88,96 A long-standing problem in the base-metal-catalyzed hydrosilylations is the selective addition of the H−Si bond to an internal C−C double

*E-mail: [email protected]. ORCID

Zheng Huang: 0000-0001-7524-098X Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Financial support was provided by the MOST of China (2016YFA0202900, 2015CB856600) and the National Natural Science Foundation of China (21422209, 21432011, 21421091, 21272255).

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