Nucleophilic Addition to Olefins. 5. Reaction of 1,1-Dinitro-2,2-diphenylethylene with Water and Hydroxide Ion in 50percent Me2SO-50percent Water. Complete Kinetic Analysis of Hydrolytic Cleavage of the C=C Double Bond in Acidic and Basic Solution

Article abstract of DOI:10.1021/ja00406a030

Hydrolysis of 1,1-dinitro-2,2-diphenylethylene (2) to form benzophenone and dinitromethane (or its anion) was studied in 50percent Me2SO-50percent H2O and also in 50percent Me2SO-50percent D2O.Experiments were conducted over a pH range from ca.1 to ca.16.The data can be interpreted by Scheme I.Solvent isotope effects, observation of general acid and general base catalysis, and structure-reactivity relationships were used to assign rate-limiting steps under various conditions and to probe into the mechanistic details of the varios steps.The major conclusions are the following. (1) The equilibrium constants for OH^- and water addition to 2 to form T_OH- (Scheme I) are of comparable magnitude to those for the corresponding reactions of benzylidene Meldrum's acid (1), but the rate constants are much lower for 2 than for 1.This indicates a higher intrinsic kinetic barrier for the nitro-activated olefin and fits into a previously reported pattern, according to which activating substituents that are most effective in delocalizing negative charge lead to the highest kinetic barrier. (2) Carbon protonation of T_OH- follows an Eigen curve similar to that for 1,1-dinitroethane anion (Figure 7) but which is displaced upward by nearly 1 log unit.This indicates a higher intrinsic protonation rate because of smaller charge delocalization in T_OH- owing to an enhanced steric hindrance to coplanarity of the nitro groups in T_OH-. (3) Intramolecular proton transfer from the OH group to the carbanionic site in T_OH- (k_i in Scheme I) is insignificant, which is in contrast to the behavior of the addition complex between 2 and morpholine. (4) The base-catalyzed breakdown of T_OH⁰ into benzophenone and dinitromethane anion occures by rate-limiting oxygen deprotonation (k₃^B, k₃^OH in Scheme I), which implies that k₄ for CH(NO2)2^- departure from T_OH^O- is >>2*10⁹ s^-1, a remarkably high rate for a carbanionic leaving group.The water-catalyzed breakdown of T_OH⁰ proceeds by a different mechanism, which is most likely concerted, with a transition state as 8 or possibly 9. (5) The acid-catalyzed breakdown of T_OH^2- occurs by rate-limiting carbon protonation (k₆^BH in Scheme I),, but the water-catalyzed breakdown follows a different mechanism.Various possibilities are discussed, and a slight preference is given to a preassociation mechanism (Scheme III).