Alkaline Hydrolysis of Nonphenolic β-0-4 Model Diastereomers: Substituent Effect on the Leaving Phenoxide

Abstract

Nonphenolic β-0-4 lignin model diastereomers [erythro or threo 1-(3,4-dimethoxyphenyl)-2-(4-substituted-2-methoxyphenoxy)-propan-1-oll were synthesized and the alkaline hydrolysis rates. and activation parameters for some compounds, determined. These rates and activation parameters were compared to rates determined earlier for similar models with 2-carbon side chains. Unfortunately, the three derivative with an electron-withdrawing formyl substituent could not be synthesized. The unsubstituted and 4'-Me erythro isomers hydrolyzed about one-third faster than the respective threo analogue. The unsubstituted and 4'-Me 3-carbon side chain models, both erythro and threo, hydrolyzed slightly faster than the 2-carbon analogues, while the erythro 4'-CHO diastereomer hydrolyzed approximately 8 times faster than the formyl analogue with a 2-carbon side chain. Both unsubstituted diastereomers had similar activation parameters as the unsubstituted 2-carbon side chain model. The erythro 4'-CHO diastereomer had a similar ΔH * as the 2-carbon side chain analogue. but the ΔS * was much smaller for the 3- than the 2-carbon side chain model. Explanations for the hydrolysis rate differences are proposed.