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 L 2 BCl (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 L 2 BCl reagents, provide a compelling theoretical explanation for the stereochemical outcome of such reactions.