PROTON-TRANSFER FROM CARBON ACIDS TO CARBANIONS .2. REACTION OF PHENYLNITROMETHANE WITH CARBANIONS, ENOLATE, AND NITRONATE IONS IN 90-PERCENT ME(2)SO-10-PERCENT WATER - CARBON TO CARBON OR CARBON TO OXYGEN PROTON-TRANSFER - TEST OF THE MARCUS EQUATION

The kinetics of reactions of phenylnitromethane with the anions of 9-cyanofluorene, 4-nitrophenylacetonitrile, 1,3-indandione, Meldrum's acid, acetylacetone, and nitromethane (the ''buffers'') were measured by the stopped-flow technique. With the 1,3-indandionyl and 9-cyanofluorenyl anion, two kinetic processes [tau(1)(-1) and tau(2)(-1)] could be observed, while with the other buffer anions the tau(1)(-1) process was too fast to be measured and only the slow reaction [tau(2)(-1)] was accessible. The fast process refers to the reaction of the aci-form of phenylnitromethane with the buffer anion, which in the case of the 1,3-indandione buffer appears to proceed predominantly by an oxygen to oxygen proton transfer. The slow process corresponds to the deprotonation of the carbon of phenylnitromethane by the buffer anions. With the 1,3-indandione, Meldrum's acid, and nitromethane buffers, there is strong evidence that this deprotonation occurs mainly by carbon to carbon proton transfer, while with acetylacetone the reaction represents carbon to oxygen proton transfer. These conclusions are based on estimates of the intrinsic rate constants or intrinsic barriers of the various processes. Using identity barriers determined by the recently proposed ''9-cyanofluorene method'' (Bernasconi, C. F.; Ni, J. X. J. Am. Chem. Sec. 1993, 115, 5060), we have calculated the intrinsic barriers for the reactions of PhCH(2)NO(2) with the anions of 9-cyanofluorene and 4-nitrophenylacetonitrile based on the Marcus equation. There is good agreement between the calculated and the experimental intrinsic barriers.