Transmission of polar effects. Part 21. Alkaline hydrolysis of the 2′- and 4′-substituted 2-methoxycarbonyldiphenylacetylenes and (Z)-2′- and -4′-substituted 2-methoxycarbonylstilbenes and the ionisation and esterification, with diazodiphenylmethane of the acids of the latter series

Abstract

The rate coefficients for the alkaline hydrolysis of a series of 2′- and 4′-substituted 2-methoxy-carbonyldiphenylacetylenes and (Z)-2-methoxycarbonylstilbenes have been determined in 70%(v/v) dimethyl sulphoxide–water at both 30.0 and 50.0 °C and at 24.0, 30.0, 40.0, and 50.0 °C, respectively. The pK_a values of (Z)-2′- and -4′-substituted 2-carboxystilbenes have been measured in 80%(w/w) 2-methoxyethanol-water at 25 °C. The rate coefficients for the esterification of the corresponding acids with diazodiphenylmethane have been measured at 30.0 °C. For the diphenyl-acetylene system, normal substituent effects are observed from both the 2′- and 4′-position. A trans-mission coefficient for polar effects of ca. 0.2 has been calculated for both positions. The evidence, including the carbonyl stretching frequencies of the esters, indicates an s-trans conformer for the 2′-substituted 2-methoxycarbonyldiphenylacetylenes. For the (Z)-stilbene system, normal substituent effects are observed from both the 2′- and 4′-position. Transmission coefficients for polar effects, with regard to both the 2′- or 4′-position and the reaction studied, vary between 0.2 and 0.7. This appears to arise from the conformers available to the (Z)-2′- and -4′-substituted 2-carboxystilbenes.