Sterically hindered enols of carboxylic acids and esters. The ketonisation reactions of 2,2-bis(2,4,6-trimethylphenyl)ethene-1,1-diol and 2,2-bis(pentamethylphenyl)ethene-1,1-diol

Abstract

The ketonisation of two enols of carboxylic acids, 2,2-bis(2,4,6-trimethylphenyl)ethene-1,1-diol 2a and 2,2-bis(pentamethylphenyl)ethene-1,1-diol 2b have been studied at 25 °C in 1∶1 acetonitrile–water. The stability of these compounds arises from the inhibition of the β-carbon site to protonation, and this effect is now quantified. A pK_a of 8.18 is reported for ionisation of the first hydroxyl group of (Mes)₂CC(OH)₂ 2a and 8.63 for (PMP)₂CC(OH)₂ 2b. A pH–rate profile has been determined over the full pH range for both species. General acid catalysis was detected, yielding Brønsted α values of 0.35 and 0.34 for 2a and 2b, respectively. Similarly, Brønsted α′ values of 0.48 and 0.45 were calculated for general acid catalysis of both enolate mono-anions. Studies of the ketonisation of the enols of the corresponding methyl esters have permitted comparative studies on the reactivity of enols of carboxylic acids, esters and ketene acetals. The reactivity of Mes₂CC(OH)(OCH₃) lies closer to the enediol, Mes₂CC(OH)₂ 2a than to the ketene acetal, suggesting that in this case, one hydroxy group is almost as efficient as two such groups in the stabilisation of the positive charge developing at C_β as protonation occurs.