Enolboration. 5. An examination of the effects of amine, solvent, and other reaction parameters on the stereoselective enolboration of ketones with various Chx2BX reagents. An optimized procedure to achieve the stereoselective synthesis of E enol borinates from representative ketones using Chx2BCl/Et3N

Abstract

Abstract : The effects of amine, solvent, concentration, temperature and other reaction parameters in controlling the enolate geometry have been systematically investigated in the present study. A IIB NMR study of the interaction of representative tertiary amines of variable steric requirements with dicyclohexylchloroborane, Ch(x)2BCl, suggests that the smaller amines coordinate strongly with Ch(x)2BCl, while the more bulky amines do not. These amines have also been examined for the enolboration of diethyl ketone with Ch(x)2BCl, in order to understand the effect of the steric requirements of the amine on the enolate geometry. While the smaller amines favor formation of E enol borinate, the more hindered amines favor formation of the isomeric Z enol borinate. Triethylamine and NN-diisopropylethylamine, the best amines selected in terms of yield and selectivity, have also been compared for the enolboration of two model ketones, diethyl ketone and propiophenone, using various Ch(x)2BX reagents (X = Cl, Br, I, OMs and OTf) to understand their effect with different ketones and reagents. Detailed studies for the enolboration of diethyl ketone with Ch(x) 2BCl/Et3N, with the hope of understanding the various effects on the enolate geometry and of improving the E enolate selectivity, suggest that formation of the E enolates are highly favored