Abstract
A mild diastereoselective and enantioselective cobalt-catalyzed hydrosilylation reaction of achiral cyclopropenes has been developed. In this protocol, various substituted cyclopropenes and arylsilanes were transformed, in the presence of readily available chiral cobalt complex, into silylcyclopropanes with high selectivities.
Highlights
A s three-membered carbocycle is a molecular backbone that frequently appears in natural products and related biologically active compounds,1 the enantioselective preparation of polysubstituted cyclopropanes has extensively been investigated over the last few decades.2 In addition, the inherent ring strain of three-membered rings promotes a series of easy transformations such as ring opening,3 ring expansion,4 cycloaddition,5 rearrangement6 toward the formation of more complex carbon skeletons
NA, not analyzed; ND, no detection of the desired product. bEntries 1, 3, 4, and 6: full conversion; entry 2: 10% conversion; entries 7 and 8: no conversion. cDetermined by GC analysis of hydrolyzed aliquots. dDetermined by chiral HPLC. e5 mol % cobalt complex (L4·CoCl2) and 10 mol % NaBHEt3 were used in the reaction
Since the racemic L1·CoCl2 complex was the best combination for a diastereoselective hydrosilylation reaction, a series of cobalt complexes possessing similar structures but with different chiral backbones were prepared and submitted to the hydrosilylation reaction on our model substrate 1a
Summary
20:1 ND 10:1 20:1 7.3:1 20:1 ND ND enantiomeric ratio, erd NA NA 86:14 96:04 96:04 96:04 NA NA aThe reactions were run on a 0.10 mmol scale, [Co] (10 mol %), ligand or additive (20 mol %) in toluene (0.1 M) and the reaction mixture was stirred at the indicated temperature for 5 h. The best combination was found when the chiral cobalt complex L4· CoCl2 was engaged in our model reaction and afforded the corresponding product 2a with high diastereoselectivity and enantioselectivity (dr = 20:1, er = 96:04), demonstrating that the nature of the chiral ligand significantly influences the diastereoselectivity of the reaction (Table 1, entry 4) To further confirm this steric effect of the chiral ligand on the diastereoselectivity of the hydrosilylation reaction, a more sterically hindered chiral complex (L5·CoCl2) was synthesized and tested under our standard conditions. A mild diastereoselective and enantioselective cobalt-catalyzed hydrosilylation reaction of achiral cyclopropenes has been developed In this protocol, various substituted cyclopropenes and arylsilanes were transformed, in the presence of readily available chiral cobalt complex, into silylcyclopropanes with high selectivities.
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