ENOLBORATION .5. AN EXAMINATION OF THE EFFECTS OF AMINE, SOLVENT, AND OTHER REACTION PARAMETERS ON THE STEREOSELECTIVE ENOLBORATION OF KETONES WITH VARIOUS CHX(2)BX REAGENTS - AN OPTIMIZED PROCEDURE TO ACHIEVE THE STEREOSELECTIVE SYNTHESIS OF E-ENOL BORINATES FROM REPRESENTATIVE KETONES USING CHX(2)BCL/ET(3)N

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 B-11 NMR study of the interaction of representative tertiary amines of variable steric requirements with dicyclohexylchloroborane, ChX2BCl, suggests that the smaller amines coordinate strongly with Chx2BCl, while the more bulky amines do not. These amines have also been examined for the enolboration of diethyl ketone with ChX2BCI, 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 N,N-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 Chx2BX 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 ChX2BCI/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 in nonpolar solvents and in dilute medium, whereas the corresponding Z enolates are more favored in polar solvents and in relatively concentrated medium. The other reaction parameters, such as the enolization and the aldolization temperatures, and the order and the rate of addition of the various substrates, have essentially no influence on the stereochemistry. However, the aldolization at -78-degrees-C for 2 h without allowing the reaction mixture to warm to room temperature improves the anti aldol selectivity. An understanding of these various effects in controlling the stereochemistry of the enolboration and the achievement of the selective synthesis of E enol borinates from representative ketones using Chx2-BCl/Et3N under optimized reaction conditions are emphasized in this exploratory study.