A theoretical and experimental investigation of Z, E stereoisomerism in some simple lignin p-quinone methides

Abstract

On the basis of 3-21G ab initio, AM1 semiempirical and molecular mechanics calculations, the structures of the E and Z stereoisomers of the simplest lignin model p-quinone methides, 2-methoxy-4-ethylidene-2,5-cyclohexadiene-1-one ( 1) and 2-methoxy-4-(2-propenylidene)-2,5-cyclohexadiene-1-one ( 2), are presented. 1H NMR spectra of the reaction mixtures of both 1 and 2 demonstrate the predominance of the Z isomer over the E isomer, a result in accordance with the ab initio calculations. The molecular mechanics and AM1 calculations predict a negligible energy difference between E and Z isomers, but yield structure parameters similar to those obtained ab initio. The results indicate that the predominance of Z stereoisomer is governed mainly by electronic, not steric interactions, and that the reaction leading to quinone methides from the corresponding phenols may be thermodynamically controlled.