Alkaline hydrolysis of nonphenolic β-O-4 lignin model dimers: Further studies of the substituent effect on the leaving phenoxide

Reaction rates and activation parameters for the alkaline hydrolysis of nonphenolic β-O-4 lignin models [2-(4-substituted phenoxy)-1-(3,4-dimethoxyphenyl)-1-ethanol] were compared to 2-methoxylated B-ring analogues run earlier. The Hammett plot with the best fit used σ- values to give a ρ of 1.9 ± 0.2. while the 2-methoxyl-substituted compounds had a ρ of 2.3 ± 0.3. Compounds with a methoxyl group at the ortho position of the leaving phenoxide ion hydrolyzed slightly faster than the non-methoxylated analogues, indicating that the 2-methoxy group is a weak electron-withdrawer. The strong electron-withdrawing 4-CHO and 4-CF3 substituents gave significantly different activation parameters than the unsubstituted compound. Experiments with 18O-labeled water suggested that our earlier proposed SNAr mechanism for the alkaline hydrolysis of strong electron-withdrawing compounds is not correct. It appears that all nonphenolic β-O-4 dimers are hydrolyzed by a neighboring group mechanism.