ENOLBORATION .6. DICYCLOHEXYLIODOBORANE, A VERSATILE REAGENT FOR THE STEREOSELECTIVE SYNTHESIS OF EITHER Z-ENOLATES OR E-ENOLATES FROM REPRESENTATIVE ESTERS

A smooth, rapid, quantitative, and highly stereoselective synthesis of either Z or E enolates from representative esters has been achieved w ith dicyclohexyliodoborane, Chx(2)BI, in the presence of a suitable te rtiary amine, such as triethylamine or N,N-diisopropylethylamine. A sy stematic investigation of the enolboration of ethyl propionate and eth yl phenylacetate, as model esters, by the various Chx(2)BX and BX-9-BB N reagents (X = OMs, I, and Br) established Chx(2)BI as the preferred reagent in terms of yield and selectivity. Further study of representa tive esters (RCH(2)-COOR') with Chx(2)BI established that the steric r equirements of both the alkyl group (R) at the ct-position and the alk oxy group (OR') play a significant role in controlling the enolate geo metry. The steric requirements of the amine (R''N-3) also contribute c onsiderably to the stereoselectivity of the reaction, The present stud y provides a simple procedure for the synthesis of Z or E enol borinat es from representative esters (RCH(2)COOR') using the combined stereod irecting effects of the alkyl (R) and the alkoxy (OR') groups. These e nol borinates are highly reactive with aldehydes at temperatures as lo w as -78 degrees C and are exceptionally stereoselective even at 0 deg rees C, In this exploratory study, the synthesis of stereoselective en olates from representative esters (RCH(2)COOR') using Chx(2)BI/R''N-3 is discussed, with special emphasis on the effects of the steric requi rements of R and OR' in controlling the enolate geometry.