An ab initio study of the enolboration of 3-pentanone mediated by boron monochlorides L2BCl

Using ab initio methods at the HF/6-31G∗∗ level, we have studied the mechanism of the enolboration of 3-pentanone mediated by boron monochlorides L2BCl (L=H, methyl and isopropyl) and trimethyl amine. The size of the L group has been found to have a decisive influence on the configuration (E or Z) of the formed boron enolate. Thus, whereas our calculations predict that dimethylboron chloride yields the Z enolate with high stereoselectivity, diisopropylboron chloride is found to yield predominantly the E enolate. The difference in behavior is due mainly to steric features related to the conformational bias of the respective ketone–boron chloride complexes formed reversibly in the first step of the process. These findings, which are in good agreement with experimental results in aldol reactions with L2BCl reagents, provide a compelling theoretical explanation for the stereochemical outcome of such reactions.