Cobalt-catalyzed regioselective stereoconvergent Markovnikov 1,2-hydrosilylation of conjugated dienes† †Electronic supplementary information (ESI) available. See DOI: 10.1039/c7sc04002d

The first transition metal-catalyzed stereoconvergent Markovnikov 1,2-hydrosilylation of (E/Z)-dienes was effectively achieved with excellent E-selectivities using a cobalt catalyst.


Introduction
Allylsilanes are synthetically valuable building blocks due to their non-toxicity, high stability and versatile applications in organic synthesis and material science. 1 Among the methods for allylsilane synthesis, 2 transition metal-catalyzed hydrosilylation of conjugated dienes is the most straightforward approach to prepare these synthetically valuable allylsilanes. 3 Recently, the hydrosilylation of alkenes 4 and alkynes 5 has been extensively studied with cobalt catalysts, 6 particularly due to their higher abundance and lower toxicity compared to platinum catalysts for hydrosilylation reactions. 7 More importantly, recent studies indicate that cobalt catalysts can offer a precise control of regio-and stereoselectivities. 5 However, highly selective cobalt catalysts for the hydrosilylation of conjugated dienes still remain rare and are under development. 3c,8 Conjugated dienes can undergo 1,4-and 1,2-hydrosilylation and the selectivity is dependent on the catalyst employed. The majority of the catalysts based on Fe, Co, and Ni show high selectivity towards 1,4-hydrosilylation (Scheme 1A). 3 For example, Hilt reported a highly selective cobalt catalyst for the 1,4-hydrosilylation of isoprene in the presence of P(n-Bu) 3 , 3c and Ritter reported a well-dened Fe(0) complex ligated by 2-iminopyridines for the 1,4-hydrosilylation of 1,3-dienes. 3d In contrast, the 1,2-hydrosilylation of conjugated dienes has been barely studied, and two catalysts based on Pt and Co have been used to catalyze this 1,2-hydrosilylation. 8,9 In addition, these two catalysts show high selectivity towards anti-Markovnikov hydrosilylation (Scheme 1B). However, transition metal catalysts for Markovnikov 1,2-hydrosilylation of conjugated dienes, especially for stereoconvergent Markovnikov 1,2-hydrosilylation of E/Z-dienes, still remain unknown. Driven by our research interest in developing base metal catalysts for transformations of unsaturated organic molecules, 4f,5f,10 herein we report the highly selective cobalt-catalyzed stereoconvergent Markovnikov 1,2-hydrosilylation of a wide range of functionalized conjugated (E/Z)-dienes (Scheme 1C). In addition, we also identied a cobalt catalyst that selectively catalyzes the hydrosilylation of the (E)-isomer of an (E/Z)-diene with the (Z)-isomer unreacted. This discovery would represent a convenient protocol to purify (Z)-dienes from (E/Z)-isomeric dienes, which are generally more accessible than stereodened (Z)-or (E)-dienes.

Results and discussion
We chose the reaction of (E)-1-phenyl-1,3-butadiene with PhSiH 3 to evaluate the reaction conditions for this Co-catalyzed hydrosilylation of conjugated dienes. We tested this reaction with cobalt catalysts generated from Co(acac) 2 and various nitrogen-and phosphine-based ligands. The selected examples are summarized in Table 1. In general, these reactions were conducted with 3 mol% of Co(acac) 2 and 3 mol% ligand at 50 C for 3 h.
Cobalt catalysts generated from the combination of Co(acac) 2 and nitrogen-based ligands, such as Ph PDI, TF ADPI or PyBox, did catalyze the 1,2-hydrosilylation of (E)-1-phenyl-1,3butadiene to full conversions, but these reactions produced a mixture of products 1a and 3a with low selectivities (entries 1-3 in Table 1). 8 The reactions conducted with a combination of Co(acac) 2 and bisphosphine ligands, such as dppm, dppe, or dppbz, proceeded to low to modest conversions, but with complete selectivities (>99%) for Markovnikov 1,2-hydrosilylation (entries 4-6 in Table 1). In particular, the reactions catalyzed by the combination of Co(acac) 2 and binap or xantphos proceeded to high or full conversions with excellent selectivities to branched allylsilane 1a (entries 7 and 8 in Table  1). As dienes are thermally less stable and can undergo polymerization, we tested this hydrosilylation at lower temperatures. The reaction conducted with 1 mol% of Co(acac) 2 and 1 mol% of xantphos at room temperature proceeded to full conversion and afforded the desired allylsilane 1a in an increased yield with an excellent Markovnikov selectivity (entry 9 in Table 1).
Under the identied conditions (entry 9 in Table 1), we studied the scope of conjugated trans-dienes for this reaction. These results are summarized in Table 2. In general, a wide range of conjugated trans-dienes reacted smoothly with PhSiH 3 in the presence of 1 mol% of Co(acac) 2 and xantphos at room temperature, affording the corresponding (E)-allylsilanes (1a-1n in Table 2) in high yields (64-92%) with excellent regioselectivities (b/l ¼ >99 : 1). The scope of these trans-dienes encompassed aryl-substituted (1a-1h in Table 2), alkylsubstituted (1i-1k in Table 2), and multiple-substituted dienes (1a-1h in Table 2). The GC-MS analysis of the crude mixtures of these reactions revealed that organosilane products from either 1,4-hydrosilylation or anti-Markovnikov 1,2-hydrosilylation of these trans-dienes were not formed during the Co-catalyzed Markovnikov 1,2-hydrosilylation. In addition, we also tested Table 1 Evaluation of the conditions for the Co-catalyzed hydrosilylation of 1-phenyl-1,3-butadiene a

Entry
Ligand this hydrosilylation with secondary hydrosilanes (Ph 2 SiH 2 and PhMeSiH 2 ), and these reactions proceeded smoothly to afford tertiary cinnamylsilanes (1a 0 and 1a 00 in Table 2) in high isolated yields. However, this Co-catalyzed hydrosilylation did not occur with dialkylsilane (Et 2 SiH 2 ) or tertiary hydrosilanes, such as (EtO) 3 SiH and (EtO) 2 MeSiH. Subsequently, we studied the scope of conjugated dienes containing a mixture of (E/Z)-isomeric 1,3-dienes for this hydrosilylation reaction, and the results are summarized in Table 3. Generally, a wide range of (E/Z)-dienes, with E/Z ratios given in the brackets in Table 3, underwent this Markovnikov 1,2-hydrosilylation in a stereoconvergent manner with full conversions, affording the corresponding (E)-allylsilanes (1a-1z in Table 3) in high isolated yields with high regio-and stereoselectivities (b/l ¼ >99 : 1; E/Z ¼ >99 : 1). The GC-MS analysis of the crude reaction mixtures indicated that these reactions also produced small amounts of 1,4-hydrosilylation products, and the ratios of the products from 1,2-and 1,4-hydrosilylation are listed in Table 3 and abbreviated as 1,2/1,4 ratios.
As both the Co(acac) 2 and xantphos used for this hydrosilylation reaction are bench-stable, we tested the hydrosilylation of 1-(buta-1,3-dien-1-yl)-4-methoxybenzene with PhSiH 3 on a 10 mmol scale with 1 mol% of Co(acac) 2 /xantphos weighed on the benchtop without using a dry box. This reaction proceeded to the full conversion of the diene substrate and afforded 1o in 87% isolated yield (eqn (1)).
To understand the stereoconvergence of this hydrosilylation of (E/Z)-dienes, we analyzed the reaction of (E/Z)-1-phenyl-1,3butadiene and found that the (E)-isomer was consumed at a signicantly higher rate than the (Z)-isomer. 11 In addition, the hydrosilylation of (Z)-1-phenyl-1,3-butadiene was studied, and this reaction afforded the 1,2-hydrosilylation product 1a together with a signicant amount of the 1, Scheme 2A). The results of this reaction and the reactions of the E-isomer (entry 9 in Table 1) and the mixture of the (E/Z)-isomers (entry 10 in Table 1) suggest that allylsilane 2a was formed by 1,4-hydrosilylation of the (Z)-isomer. To provide insight into the isomerization of the internal Z-alkene in the diene to the E-alkene in product 1a, we subsequently conducted a deuterium-labelling experiment using the (E/Z)-isomers and PhSiD 3 (Scheme 2B) and found that deuterium was solely incorporated into the methyl groups of 1a and 2a. This lack of deuterium incorporation onto the internal vinylic carbons suggests that this E/Z-isomerization through migratory insertion of the Z-alkene into a Co-H species followed by b-H elimination, 12 as indicated in Scheme 2C, is unlikely. Furthermore, we also tested the reaction of the (E)-isomer with PhSiD 3 and the same deuterium incorporation was observed (Scheme 2D).
Based on the results of the experiments in Scheme 2 and the precedent of Co-catalyzed hydrosilylation of alkenes, 4f,6 we proposed a hydrometalation pathway with a Co(I)-H intermediate for this Co-catalyzed Markovnikov hydrosilylation of conjugated dienes (Scheme 3). 2,1-Migratory insertion of the (E)-diene into a Co-H species forms an allylcobalt intermediate I, which turns over with PhSiH 3 to release the allylsilane product and regenerate the Co-H species.
For the hydrosilylation of (Z)-dienes, 2,1-migratory insertion of (Z)-dienes occurs to generate an allylcobalt species II, which undergoes s-p-s isomerization to form the allylcobalt intermediate III. 13 This allylcobalt species reacts with PhSiH 3 to give a 1,4hydrosilylation product. In addition, the allylcobalt species III also undergoes s-p-s isomerization to generate the allylcobalt intermediate I, and this explains the observed stereoconvergency of the (Z/E)-diene hydrosilylation. Both allylcobalt species I and III can react with PhSiH 3 to generate allylsilane products and the difference in the sterics around the Co-C bonds in these two allylcobalt species may account for the product ratio observed for the reaction listed in Scheme 2A.
Aer developing this stereoconvergent hydrosilylation reaction, we rationalized that the separation of a (Z)-diene from Z/Ediene mixtures could be achieved if we could identify a cobalt catalyst that can convert only the (E)-isomer of dienes. We tested various cobalt catalysts generated from the combination of Co(acac) 2 and bisphosphine ligands for this purpose. To our delight, we found that the cobalt complex from Co(acac) 2 /binap was active for Markovnikov 1,2-hydrosilylation of (E)-1-phenyl-1,3diene (Scheme 4A) but did not catalyze the hydrosilylation or the isomerization of (Z)-1-phenyl-1,3-diene (Scheme 4B). Then, we conducted this hydrosilylation reaction with a E/Z-mixture of 1phenyl-1,3-diene (Scheme 4C). As expected, this reaction afforded (E)-allylsilane 1a in 58% isolated yield and (Z)-1-phenyl-1,3-diene was recovered in 45% isolated yield with a Z/E ratio of 98 : 2.
We subsequently studied cobalt catalysts generated from Co(acac) 2 and chiral bisphosphine ligands in order to develop Scheme 2 Hydrosilylation of stereodefined diene using Co(acac) 2 / xantphos. Scheme 3 Proposed catalytic pathways for the Co-catalyzed stereoconvergent Markovnikov hydrosilylation of dienes.

Conclusions
In summary, we have developed the rst Co-catalyzed Markovnikov 1,2-hydrosilylation of conjugated dienes with a catalyst generated from Co(acac) 2 and xantphos. A broad scope of transdienes underwent this Markovnikov hydrosilylation to afford (E)allylsilanes in high isolated yields and with excellent regioselectivities (b/l ¼ >99 : 1). In addition, (E/Z)-isomeric 1,3-dienes reacted in a stereoconvergent manner to form (E)-allylsilanes with good to excellent regioselectivity (ratios of 1,2/1,4-hydrosilylation up to 99 : 1). This stereoconvergence resulted from a s-p-s isomerization of the allylcobalt intermediate. In particular, we also identied a cobalt catalyst, Co(acac) 2 /binap, for selectively converting the (E)-isomer of a mixture of (E/Z)-isomers, and this allows the separation of (Z)-dienes from a mixture of (E/Z)-dienes.

Experimental details
General procedures for stereoconvergent hydrosilylation of (E/ Z)-dienes In an Ar-lled glovebox, a mixture of Co(acac) 2 (4.0 mmol) and xantphos (4.0 mmol) in THF (1 mL) was added into a 4 mL screwcapped vial containing a magnetic stirring bar. The resulting mixture was stirred for 2 min prior to adding phenylsilane (0.500 mmol) and (E/Z)-1,3-dienes (0.400 mmol) successively. The vial was removed from the glove box, and the mixture was stirred at room temperature for 6 hours. Aer that, the crude reaction mixture was concentrated under vacuum and the residue was puried by ash column chromatography using a mixture of ethyl acetate and hexane as an eluent. The conditions for the ash chromatography and the data for the characterization of the products are listed in the ESI. †

Conflicts of interest
The authors declare no conict of interest.