Enolboration. 6. Dicyclohexyliodoborane, a Versatile Reagent for theStereoselective Synthesis of Either Z or E Enolates from Representative Esters

摘要

A smooth, rapid, quantitative and highly stereoselective synthesis of either Z orE enolates from representative esters has been achieved with dicyclohexyliodo-borane, Chx2BI, in the presence of a suitable tertiary amine, such as triethylamine or N,N-diisopropylethylamine. A systematic investigation of the enolboration of ethyl propionate and ethyl phenylacetate, as model esters, by the various Chx2BX and B-X-9-BBN reagents (X = OMs, I, and Br) established Chx2BI as the preferred reagent in terms of yield and selectivity. Further study of representative esters (RCH2COOR') with Chx2BI established that both the steric requirements of the alkyl group (R) at the (alpha-position and the alkoxy group (OR') play a significant role in controlling the enolate geometry. The steric requirements of the amine (R 3N) also contribute considerably to the stereoselectivity of the reaction. The present study provides a simple procedure for the synthesis of Z or E enol borinates from representative esters (RCH2COOR') using the combined stereodirecting effects of the alkyl (R) and the alkoxy (OR') groups. These enol borinates are highly reactive with aldehydes at temperatures as low as -78 deg C and are exceptionally stereoselective even at 0 deg C. In this exploratory study, the synthesis of stereoselective enolates from representative esters (RCH2COOR') using Chx2BI/R 3N is discussed, with special emphasis on the effects of the steric requirements of R and OR'in controlling the enolate geometry.