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

The ketonisation of two enols of carboxylic acids, 2,2-bis(2,4,6-trimethylphenyl)ethene-l,l-diol 2a and 2,2-bis(pentamethylphenyl)ethene-l,l-diol 2b have been studied at 25 degrees C in 1:1 acetonitrile-water. The stability of these compounds arises from the inhibition of the beta-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)(2)C=C(OH)(2) 2a and 8.63 for (PMP)(2)C=C(OH)(2) 2b. A pH-rate profile has been determined over the full pH range for both species. General acid catalysis was detected, yielding Bronsted a values of 0.35 and 0.34 for 2a and 2b, respectively. Similarly, Bronsted alpha' 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 Mes(2)C=C(OH)(OCH3) lies closer to the enediol, Mes(2)C=C(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-beta as protonation occurs.