Xiaoyong
Du
,
Wenjun
Hou
,
Yanlu
Zhang
and
Zheng
Huang
*
State Key Laboratory of Organometallic Chemistry, Shanghai Institute of Organic Chemistry, 345 Lingling Road, Shanghai, 200032, China. E-mail: huangzh@sioc.ac.cn; Fax: +86 5492 5533; Tel: +86 5492 5522
First published on 28th April 2017
A phosphine-iminopyridine (PCNN) cobalt-catalyzed Z-selective hydrosilylation of terminal alkynes with Ph2SiH2 has been developed for the synthesis of (Z)-β-vinylsilanes with high regio- and stereoselectivity and wide functional group tolerance. Furthermore, the Co-catalyzed hydrosilylations of unsymmetrical arylalkyl disubstituted internal alkynes afford syn-addition products with unique regioselectivity: the silyl group is added to the alkyl-substituted carbon, instead of the aryl-substituted carbon. The (Z)-β-vinylsilane products are further applied to Pd-catalyzed Hiyama–Denmark cross-couplings for stereoselective synthesis of (Z)-disubstituted alkenes.
Over the past decade, the low abundance, high cost, and the environmental concerns associated with the noble metals have spurred extensive studies toward developing earth-abundant base-metal alternatives.7 So far, Fe,8 Co,9 and Ni10 complexes have been proven effective for alkyne hydrosilylation, but only bis(imino)pyridine iron complexes gave high (Z)-β-selectivity in the hydrosilylations of two terminal alkyne substrates.8c,d Very recently, several groups reported that cobalt complexes could catalyze the hydrosilylation of terminal alkynes with high α- or (E)-β-selectivity. In 2014, Deng developed a carbene-ligated Co(I) complex and applied it to the hydrosilylation of alkynes with high (E)-β-selectivities (Scheme 1b).11 Thomas described a combination of the (MesPDI)CoCl2 complex of bis(imino)pyridine and NaOtBu for (E)-β-selective hydrosilylation with 1-hexyne with PhSiH3 (Scheme 1b).12 In 2016, Lu13 and our group14 independently reported Co complexes of iminopyridine-oxazoline (IPO) or Pybox ligands for the α-selective hydrosilylation of terminal aryl alkynes with Ph2SiH2 (Scheme 1c). However, the use of cobalt complexes for the hydrosilylation of terminal alkynes with (Z)-β-selectivity is rare.15 During the submission of this manuscript, Ge reported a bis(imino)pyridine cobalt-catalyzed Z-selective hydrosilylation of terminal alkynes with PhSiH3.16 Herein, we describe a highly Z-selective hydrosilylation of terminal alkynes with Ph2SiH2 using a (PCNN)CoCl2/NaBEt3H catalyst system (Scheme 1a), which has been previously developed by our group for the Markovnikov hydrosilylation of terminal alkenes.17 Moreover, the (PCNN)Co catalyst effects the hydrosilylation of internal alkynes, and intriguingly, the reactions of arylalkyl disubstituted alkynes produce vinylsilanes containing the silyl unit at the alkyl-substituted olefinic carbon, rather than the aryl-substituted one. The (Z)-β-vinylsilane products can undergo Pd-catalyzed Hiyama–Denmark cross-couplings to form (Z)-disubstituted alkenes.
Entry | Cat. | Silane | Solvent | 3a/[Si] | Yield [%] | |
---|---|---|---|---|---|---|
4a | 5a | |||||
a Reaction conditions: 3a (0.72–0.9 mmol) and silane (0.6 mol) in THF (1 mL) or under neat conditions at 25 °C. The yields were determined by 1H NMR spectroscopy. b Without precat. c Without NaHBEt3. | ||||||
1 | 1a | Ph2SiH2 | THF | 1.2 | 0 | 0 |
2 | 1b | Ph2SiH2 | THF | 1.2 | 0 | 0 |
3 | 2a | Ph2SiH2 | THF | 1.2 | 13 | 18 |
4 | 2b | Ph2SiH2 | THF | 1.2 | 85 | 3 |
5 | 2c | Ph2SiH2 | THF | 1.2 | 83 | 3 |
6 | 2b | Ph2SiH2 | None | 1.5 | 95 | 1 |
7b | 2b | Ph2SiH2 | None | 1.5 | 0 | 0 |
8c | 2b | Ph2SiH2 | None | 1.5 | 0 | 0 |
9 | 2b | (EtO)3SiH | None | 1.5 | <5 | <5 |
10 | 2b | MD′M | None | 1.5 | 0 | 0 |
11 | 2b | Et3SiH | None | 1.5 | 0 | 0 |
12 | 2b | PhMe2SiH | None | 1.5 | 0 | 0 |
13 | 2b | PhSiH3 | None | 1.5 | 50 | 45 |
Further optimizations were conducted to improve the selectivity for cis-vinylsilane. The best Z-selectivity was obtained when the reaction occurred under solvent-free conditions using a 1.5:1 ratio of 3a:Ph2SiH2 in the presence of 1 mol% of 2b and 2 mol% of NaHBEt3; the run at 25 °C for 24 h produced (Z)-vinylsilane 4a in 95% yield with the Z/E stereoselectivity of 95:1 (entry 6). A control experiment without the precatalyst 2b, but with NaBEt3H, gave no hydrosilylation product (entry 7), and another control without NaHBEt3 indicated that the addition of the activator was necessary for the catalysis (entry 8). Finally, we explored the catalytic performance of this Co catalyst for hydrosilylations with primary and tertiary hydrosilanes. Tertiary silanes (EtO)3SiH, MD′M (1,1,1,3,5,5,5-heptamethyltrisiloxane), Et3SiH and PhMe2SiH were unreactive under the reaction conditions (entries 9–12). Although the reaction with PhSiH3 gave the anti-Markovnikov products in high yield (95% totally), the stereoselectivity was low (Z/E = ∼1:1) (entry 13).
To evaluate the scope of the Co-catalyzed alkyne hydrosilylation, the reactions with a variety of functionalized terminal alkynes and a number of internal alkynes were carried out using 2b as the precatalyst. The results are summarized in Table 2. All reactions with Ph2SiH2 proceeded to completion within 24 h at room temperature under solvent-free conditions. For terminal alkyne substrates, most reactions were highly selective for the formation of the (Z)-β-vinylsilane products (Z/E > 95:5). Linear terminal alkynes gave the corresponding products 4a–d in moderate-to-high isolated yield with excellent selectivity. 3-Cyclohexyl-1-propyne 3e and 3-cyclopentyl-1-propyne 3f underwent hydrosilylations in useful yields and high selectivities. The catalyst is compatible with terminal alkynes bearing various functional groups, including chloride (4g), nitrile (4h), tertiary amine (4i, 4j), amide (4k) and even ester (4o). Aliphatic alkynes containing protecting groups, such as silylether (4l, 4m) and tosylate (4n, 88%), afforded the desired (Z)-β-vinylsilanes in high yields and high selectivities. Aryl terminal alkynes are unreactive under the catalytic conditions, as demonstrated by the full recovery of the alkyne in the reaction of phenylacetylene with Ph2SiH2.
a Reaction conditions: Ph2SiH2 (0.6 mmol) and alkyne (0.9 mmol) at 25 °C. Yield of isolated products. Z/E ratios of isolated products were determined by 1H NMR spectroscopy. b With 1.2 mmol 3h. A 84:16 Z-β/α ratio was determined by 1H NMR spectroscopy. c With 2 mol% 2b and 4 mol% NaBEt3H. d Regioselectivities determined by 1H NMR spectroscopy. |
---|
Internal alkynes underwent the hydrosilylation reactions under the catalytic conditions to form the products in good-to-high yields. The hydrosilylation of symmetric dialkyl-substituted alkynes (3p, 3q) occurred to form the syn-addition products in high yields (4p, 4q). Next, we turned our attention to the more challenging unsymmetrical internal alkynes. The unsymmetrical dialkyl-substituted alkynes with different steric properties, such as 1-tert-butyl-1-propyne (3r) and 4-methyl-pent-2-yne (3s), were suitable for the selective hydrosilylation, producing the syn-addition products 4r and 4s with the incorporation of the silyl group into the less sterically demanding sp-hybridised carbon. The reaction of arylalkyl disubstituted alkynes (3t–v) gave the products in 91–94% yields (4t–v). Noteworthily, the processes exhibited good regioselectivities, furnishing tri-substituted olefins with the Ph2HSi unit selectively added to the alkyl-substituted olefinic carbon, which is in contrast to previous reports on Co-catalyzed hydrosilylations of arylalkyl disubstituted alkynes that yielded vinylsilanes with the Si atom adjacent to the aryl group.9e,11,13
Due to their high stability, non-toxicity, and ease of handling, vinylsilanes have become valuable intermediates for Pd-catalyzed Hiyama–Denmark cross-couplings.18 However, the silane reagents used in the coupling reactions often require heteroatom substituent(s) (e.g., O or F) to make the Si atom feasible toward activation with a base or fluoride source.19 To this end, it is of interest to develop conditions for cross-couplings of our (Z)-vinylsilane products bearing the Ph2HSi moiety.20 After a screening of various mono- and bisphosphine ligands (see the ESI†), we identified the Pd catalyst generated from Pd(OAc)2 (10 mol%) and 1,4-bis(diphenylphosphino)butane (dppb, 12 mol%) was effective for stereoselective couplings of (Z)-vinylsilane (4) with aryl iodides using TBAF as the activator (2 equiv.). Most reactions occurred at 30 °C to form the cis-1,2-disubstituted alkenes in high yields and high stereoselectivities (Table 3). The electrophiles 6 bearing both electron-donating and -withdrawing groups were suitable coupling partners. Functionalized groups, such as ester (7a), ketone (7b), tertiary amine (7d) and nitrile (7j) could be tolerated. The reaction of 6c containing a nitro group provided the coupling product 7c in high yield, albeit with relatively low Z-selectivity (75:25). Aryl iodide 6i with a meta-MeO group gave the product with somewhat decreased selectivity (7i, 88:12) compared to the reaction with 4-iodoanisole (7h, 97:3). Aryl bromide is suitable for the coupling reaction as demonstrated by the formation of the desired product 7h in useful yield (62%) with 4-bromoanisole as the reagent. Heteroaromatic iodide, such as 2-iodothiophene and 2-acetyl-5-iodothiophene, gave the corresponding products 7k and 7l in 66 and 64% isolated yields, respectively. Functional Z-vinylsilanes such as 4g and 4o reacted with 4-iodo-1,1′-biphenyl under standard conditions and gave the corresponding Z-alkenes 7m and 7n in excellent yields and selectivities.
A mechanism accounting for the formation of (Z)-β-vinylsilane is presented in Scheme 2. On the basis of the precedents of Co-catalyzed hydrosilylation processes,11,16,17,21 we propose that the catalytic cycle involves a low-valent Co(I) silyl intermediate. The insertion of the coordinated terminal alkyne into the Co–Si bond forms a vinyl Co intermediate, (Z)-l-silyl-l-alken-2-yl-cobalt (C). Due to the significant steric repulsion between the bulky silyl group and the metal moiety, complex C undergoes isomerization to form a sterically less demanding complex F, (E)-l-silyl-l-alken-2-yl-cobalt, through the Ojima–Crabtree type rearrangement.4h,5gE then reacts with Ph2SiH2 to form (Z)-vinylsilane and regenerate the Co(I) silyl species A.
Scheme 2 Proposed mechanism involving the Ojima–Crabtree type rearrangement for Co-catalyzed (Z)-selective hydrosilylation of terminal alkynes. |
Footnote |
† Electronic supplementary information (ESI) available. See DOI: 10.1039/c7qo00250e |
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