Access to stereodefined (Z)-allylsilanes and (Z)-allylic alcohols via cobalt-catalyzed regioselective hydrosilylation of allenes

Hydrosilylation of allenes is the addition of a hydrogen atom and a silyl group to a carbon–carbon double bond of an allene molecule and represents a straightforward and atom-economical approach to prepare synthetically versatile allylsilanes and vinylsilanes. However, this reaction generally produces six possible isomeric organosilanes, and the biggest challenge in developing this reaction is to control both regioselectivity and stereoselectivity. The majorities of the developed allene hydrosilylation reactions show high selectivity towards the production of vinylsilanes or branched allylsilanes. By employing a cobalt catalyst generated from readily available and bench-stable cobalt precursor and phosphine-based ligands, here we show that this reaction proceeds under mild conditions in a regioselective and stereoselective manner, and affords synthetically challenging, but valuable linear cis-allylsilanes with excellent stereoselectivity (generally cis to trans ratios: >98:2). This cobalt-catalyzed (Z)-selective allene hydrosilylation provides a general approach to access molecules containing stereodefined (Z)-alkene units. Controlling selectivity in the hydrosilylation of allenes poses serious challenges in terms of product stereochemistry. Here the authors show that the title reaction proceeds with excellent (Z)-selectivity by use of a cobalt catalyst in the presence of phosphine-based ligands.

A llylsilanes are a type of organosilanes with an allyl group on a silicon atom, and the stereochemistry around the allylic double bond may be E (cis) or Z (trans). Allylsilanes are synthetically valuable building blocks because of their non-toxicity, high stability and versatile applications in organic synthesis and material science [1][2][3] . They have been employed as monomers for syntheses of silicon-containing polymers and undergo a variety of organic transformations [4][5][6] . As such, various methods have been developed to prepare allylsilanes and the majority of these approaches produce thermodynamically more stable (E)-allylsilanes [7][8][9] . However, the stereoselective synthesis of a wide range of (Z)-allylsilanes still remains challenging and rare [10][11][12][13][14] . Catalytic hydrosilylation of allenes (Fig. 1a) is one of the most straightforward and atom-economical approaches to synthesize (Z)-allylsilanes, provided that catalysts and reaction conditions favoring the formation of (Z)-allylsilanes can be identified.

Results
Evaluation of reaction conditions. We initiated our studies of Co-catalyzed hydrosilylation of allenes by evaluating reaction conditions for the reaction of cyclohexylallene with phenylsilane. This reaction can potentially produce six organosilanes from either 1,2-hydrosilylation ((E)-1a, (Z)-1a, (E)-1a′, and (Z)-1a′) or 2,3-hydrosilylation (1a″ and 1a‴), as depicted in Table 1. We tested this reaction with various cobalt catalysts that were generated in situ from bench-stable Co(acac) 2 and phosphine ligands. In general, these experiments were conducted with 2 mol % cobalt catalysts in THF at room temperature for 18 h. The results of these experiments are summarized in Table 1.
Here we developed a one-pot procedure to synthesize (Z)-allylic alcohols containing disubstituted or trisubstituted alkenes by combining the cobalt-catalyzed allene hydrosilylation and the subsequent oxidation of the resulting (Z)-allylsilanes with H 2 O 2 under basic conditions. A series of (Z)-allylic alcohols (5a-5h) can be prepared in good isolated yields using this one-pot procedure (Fig. 5c, see Supplementary Methods for the detailed procedure).

Discussion
(Z)-allylsilanes are thermodynamically less stable than (E)-allylsilanes and are susceptible to Z/E-isomerization to form the thermodynamically more stable (E)-allylsilanes in the presence of a Co-H species 40 Fig. 2 suggest that the cobalt catalyst generated from Co(acac) 2 and binap does not catalyze the Z/E-isomerization of these Z-allylsilanes. Indeed, the isolated (Z)-allylsilane (Z)-1l does not undergo Z/E-isomerization in the presence of 2 mol % Co(acac) 2 /binap and 1 equivalent of PhSiH 3 (Fig. 6a). However, (Z)-allylsilane (Z)-1l was isomerized to (E)-1l at room temperature in 24 h in the presence of 1 mol% Co(acac) 2 /xantphos and 1 equivalent of PhSiH 3 (Fig. 6a). We also tested these isomerization reactions in the absence of 1 equivalent of PhSiH 3 and comparable results were obtained (Fig. 6b). Due to this Z/E-isomerization, the reaction in Fig. 2b afforded a mixture of (Z)-1b and (E)-1b with a ratio of 63:37 when the reaction time was extended to 48 h (Fig. 6e). Interestingly, the Z/E-isomerization of the isolated (Z)-3a did not occur in the presence of Co(acac) 2 /xantphos catalyst (Fig. 6c, d). We attributed the lack of Z/E-isomerization for the trisubstituted allylsilane (Z)-3a to the increased steric hindrance around the C=C bond, compared to the disubstituted allylsilane (Z)-1l.
To understand the mechanism of this cobalt-catalyzed allene hydrosilylation, we conducted a deuterium-labeling reaction between buta-2,3-dien-2-ylbenzene and PhSiD 3 . This reaction afforded the corresponding (Z)-allylsilane with a deuterium atom located trans to the phenyl group (Fig. 7a). In addition, a mercury test suggests the homogeneous nature of this catalytic hydrosilylation reaction (Fig. 7b). To provide insight into the cobalt intermediate for this allene hydrosilylation, we tested the reaction of Co(acac) 2 and PhSiH 3 in the presence of various bisphosphine ligands and found that the reaction using 2 equivalents of dppbz ligand generated a well-defined Co I -H complex (dppbz) 2 CoH (6) in 70% isolated yield (Fig. 7c) 53 . Complex 6 was active for allene hydrosilylation with regio-and stereoselectivity matching those of the corresponding reaction catalyzed by Co(acac) 2 and dppbz ligand (see Supplementary Table 1 for the detailed evaluation of hydrosilylation with complex 6).
On the basis of the result of the deuterium-labeling experiment (Fig. 7a), the generation of a catalytically active Co(I)-H intermediate (Fig. 7c), and the precedent of the cobalt-catalyzed hydrosilylation of alkenes 27,28 , we propose a hydrometalation pathway with a Co(I) hydride intermediate for this Co-catalyzed stereoselective hydrosilylation of allenes (Fig. 7d). In such a mechanism, migratory insertion of the allene substrate into a Co (I)-H species forms an η 1 -bound allylcobalt intermediate (I). The steric repulsion between the R L of the allyl group and the ligand of the cobalt catalyst makes the formation of η 3 -bound allylcobalt intermediate (II) unfavorable. Subsequent σ-bond metathesis 54 between the η 1 -bound allylcobalt intermediate (I) and hydrosilane produces the (Z)-allylsilane product and regenerates the Co (I)-H species. Minimizing the steric interaction between the Co (I)-H species and the substituents on the allene substrate accounts for the observed (Z)-selectivity, which suggests that reducing the steric difference between two substituents of 1,1-disubstituted allenes will decrease the Z/E-selectivity. Indeed, the hydrosilylation of 1-cyclopropyl-1-phenylallene produces a mixture of (Z/ E)-allylsilanes with a Z/E-ratio of 63:37 (Fig. 7e). Such steric interaction has been proposed in a Rh-catalyzed stereoselective hydroformylation of terminal allenes 55 .
In summary, we have developed a highly regio-and stereoselective allene hydrosilylation catalyzed by cobalt complexes generated from bench-stable Co(acac) 2 and binap or xantphos ligand. A wide range of monosubstituted and disubstituted terminal allenes reacted to afford the corresponding linear (Z)allylsilanes in high yields with excellent stereoselectivities. This cobalt-catalyzed allene hydrosilylation coupled with sequential oxidation of the resulting (Z)-allylsilane provided a practical onepot approach to prepare synthetically challenging (Z)-allylic alcohols. Further studies to develop cobalt-catalyzed selective hydrofunctionalization of other types of multiply unsaturated molecules are on going.

Methods
General procedure for cobalt-catalyzed allene hydrosilylation. In an argonfilled dry box, Co(acac) 2 , phosphine ligand, THF(1 mL) and a magnetic stirring bar were added to a 4-mL screw-capped vial and the mixture was stirred for 5 min. Then terminal allenes (0.500 mmol) and silane (1.1 eq, 0.550 mmol) were added. The vial was sealed with a cap containing a PTFE septum and removed from the dry box. The reaction mixture was stirred at room temperature for 18 h (Fig. 3) or 3 h (Fig. 4) and the resulting solution was concentrated in vacuum. The crude product was purified by column chromatography on silica gel with a mixture of ethyl acetate and hexane as eluent.   Fig. 7 Reaction mechanism. a A deuterium-labeling reaction. b Mercury test. c A cobalt hydride complex 6 prepared by reduction of Co(acac) 2 with PhSiH 3 . d proposed catalytic cycle for cobalt-catalyzed stereoselective 1,2-hydrosilylation of allenes. e hydrosilylation of 1-cyclopropyl-1-phenylallene, an allene containing a radical clock NATURE COMMUNICATIONS | DOI: 10.1038/s41467-017-02382-7 ARTICLE Data availability. The authors declare that all the data supporting the findings of this study are available within the article and Supplementary Information files, and also are available from the corresponding author upon reasonable request.