An ab initio study of the enolboration of 3-pentanone mediated by boron monochlorides L 2BCl

• Published in: Tetrahedron Vol. 57; no. 29; pp. 6239 - 6247
• Main Authors: Murga, J, Falomir, E, Carda, M, Marco, J.A
• Format: Journal Article
• Language: English
• Published: Elsevier Ltd 2001
• Subjects: ab initio calculations; aldol reaction; boron enolates; dialkyl boron monochloride; stereoselectivity; dialkyl boron monochloride; aldol reaction; ab initio calculations; stereoselectivity; boron enolates
• ISSN: 0040-4020; 1464-5416
• DOI: 10.1016/S0040-4020(01)00603-2

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 2BCl (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 2BCl reagents, provide a compelling theoretical explanation for the stereochemical outcome of such reactions. The formation of boron enolates from 3-pentanone and dialkyl boron monochlorides L 2BCl in the presence of trimethyl amine has been studied at the theoretical level by means of ab initio methods. The steric size of the boron ligands L (L≠H) has been found to play a decissive role in the E/ Z stereoselectivity of the enolboration: the larger the L group is the higher is the proportion of E enolate formed.