Theoretical Study on Z/E Selectivity in the Oximation of a-Haloacetophenones

Abstract

A theoretical study on the selectivity in oximation reaction of α-haloacetophenones was carried out. The calculations were performed by molecular mechanics (MM+) and semiempirical (AM1 and PM3) methods. A conformational search considering the torsional angles was made using the HyperChem program and the lowest energy conformations of (Z)- and (E)-α-haloacetophenone oximes were subject to optimization by AM1 and PM3 methods. The (Z)-stereoisomers were found as the most stable structures by these different methods. Thus, the stereoselectivity, observed experimentally for (Z)-oxime formation in the oximation of α-haloacetophenones arises from the more stability of (Z)-stereoisomers. According to this data the (Z)-stereoisomer prefers the lower energy state when compared to (E)-stereoisomer, indicated that the (Z)-stereoisomer should be preferentially obtained from oximation of α-haloacetophenones, in good agreement with experimental results.