KINETICS OF HYDROLYSIS OF BENZYLIDENEACETYLACETONE

The hydrolysis of the title compound occurs by the four-step mechanism typical for the hydrolysis of olefins PhCH=CXY where X and/or Y are electron-withdrawing groups. These steps are nucleophilic attack by OH- or water on the olefin to form PhCH(OH)C(COMe)(2) (T-OH(-)); carbon protonation of T-OH(-) to form phCH(OH)CH(COMe)(2) (T-OH(0)); oxygen deprotonation of T-OH(0) to form PhCH(O-)CH(COMe)(2) (T-O(-)); and collapse of T-O(-) to PhCH=O and acetylacetonate anion. In strongly basic solution two kinetic processes were observed. To a good approximation the fast one can be assigned to reversible OH- addition to the substrate (k(1)(OH), k(-1)(H2O)) to form T-OH(-) and the slow one to irreversible conversion of To, to final products with rate-limiting carbon protonation of TOH by water (k(2)(H2O)). This latter interpretation is at variance with one offered earlier by Calmon and Calmon. In acidic solution only one kinetic process was observed; it corresponds to rate-limiting water addition to the substrate (k(1)(H2O)). Rate constants for hydronium ion catalyzed reversion of T-OH(-) to reactants (k(1)(H2O)) and for carbon protonation of T-OH(-) by H3O+ (K-2(H)), acetic acid, and DABCOH(-) (k(2)(BH)) were also obtained from pH-jump experiments on T-CH(-). Approximate intrinsic rate constants for water and OH- addition to benzylideneacetylacetone were calculated from k(2)(H2O) and k(1)(OH), respectively, and compared with those for the corresponding reactions of benzylidenemalondialdehyde and benzylidene Meldrum's acid as well as other olefins. This comparison indicates that the intrinsic reactivity of benzylideneacetylacetone toward OH- and water is ''normal,'' and sheds more light on its ''abnormal'' intrinsic reactivity toward secondary alicyclic amines reported earlier.