Degenerate Transesterification of 3,5-Dimethylphenolate/3,5-Dimethylphenyl Esters in Weakly Polar, Aprotic Solvents. Reactions of Aggregates and Complex-Induced Proximity Effects

Article abstract of DOI:10.1021/ja00009a034

The rates of exchange of the 3,5-dimethylphenolate ion between lithium 3,5-dimethylphenolate-d₆ and a series of 3,5-dimethylphenyl esters have been determined in the weakly polar, aprotic solvents dioxolane, dimethoxyethane (DME), tetrahydrofuran (THF), and pyridine. The esters include the propionate, butyrate, methoxyacctate, β-methoxypropionate, 4-methoxybutyrate, 2-tetrahydrofuroate, 2-furoate, (N,N-dimethylamino)acetate, (methylthio)acetate, 2- and 4-pyridine-carboxylates, 2-pyridylacetate, 4-pyridylacetate, phenylacetate, andp-methoxy-,p-chloro-, and p-(trifluoromethyl)phenylacetates. The rates and kinetic orders of the reactions of 3,5-dimethylphenyl propionate in various solvents at 35°C gave the following second-order rate constants (10⁴k₂, L mol^-1 sec^-1) for the following major aggregate species: THF tetramer, 6.5; DME tetramer, 3.3 (40°C); dioxolane, 13, hexamer, 71; pyridine tetramer, 2.2, dimer, 29. For 3,5-dimethylphenyl β-methoxypropionate, the order of reactivity is dioxolane > DME > THF. These results are interpreted in terms of a preequilibrium in which a solvent on lithium in the tetramer is replaced by the ester. The rates of transesterification have been compared with the rates of hydrolysis in 30% aqueous ethanol for the above series of esters. Those esters that have a second Lewis base center proximal to the ester function show significantly increased reactivity in transesterification, which is attributed to a complex-induced proximity effect.