Screening of Various Catalysts for the Asymmetric Cyclopropanation of Structurally Typical Silyl Enol Ethers—Scope and Limitations of Evans's Bisoxazoline Copper Catalyst

Abstract

AbstractThe scope and limitations of asymmetric syntheses of 2‐siloxycyclopropanecarboxylates 3 were examined by combination of structurally typical silyl enol ethers and methyl diazoacetate with various chiral catalysts. It was found that the Schiff base complex 5ṁCu(OAc)2 and the bisoxazoline complex 6ṁCuOTf gave the highest stereoselectivities (e.g., 72% ee for cis‐3d with 5ṁCu(OAc)2 as catalyst or 76% and 73% ee for cis‐ and trans‐3a, respectively, with 6‐CuOTf as catalyst). High enantioselectivities or diastereoselectivities were obtained. Other catalysts based on copper, rhodium or ruthenium complexes afforded significantly lower values. In further investigations, Evans's bisoxazoline complex 6‐CuOTf proved to be limited to 1,1‐disubstituted silyl enol ethers (e.g. 1a and 1e) for high enantiomeric excesses (up to > 95%, poor diastereoselectivities) and to 1,2‐substituted enol ethers (e.g. 1e) for high diastereoselectivities (cis:trans < 3:97, eetrans = 49%).