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 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.