On the choice of optimal conformation in linear free-energy relationships. Reactivity of 2-[(carboxymethyl)sulfanyl]-4-oxo-4-arylbutanoic acids with diphenyldiazomethane

Abstract

Rate constants for the esterification of eleven 2-[(carboxymethyl)sulfanyl]-4-oxo-4-arylbutanoic acids with diphenyldiazomethane in ethanol at 30 degrees C were determined, and correlated with substituent constants using classical Hammett and related methods. Statistically valid results for the para-substituted compounds were obtained by the Swain-Lupton approach. The compounds studied had significant conformational mobility due to seven rotatable bonds in their backbone. Going beyond the classical Hammett approach, we established a relatively fast procedure to find the optimal conformations that can be used in linear free-energy relationships, combining molecular dynamics with semiempirical calculations, and calculations using a higher level of theory (DFT and MP2). Fair correlations were observed with frontier orbitals, allowing inclusion of ortho-substituted derivatives and clarifying artifact-like data, as perceived by the Hammett-type approach.