MECHANISMS FOR THE UNCATALYZED AND HYDROGEN-ION CATALYZED-REACTIONS OF A SIMPLE QUINONE METHIDE WITH SOLVENT AND HALIDE-IONS

The quinone methide 4-[bis(trifluoromethyl)methylene]cyclohexa-2,5-dienone (1) has been prepared in situ from the solvolysis of 4-MeOArC(CF3)2OTs in 50:50 (v/v) trifluoroethanol/water, and its reactions with solvent and halide ions have been studied. Rate constants for the uncatalyzed and hydrogen ion catalyzed reactions of solvent, Cl-, Br-, and I- with 1 to form the respective phenols 4-HOArC(CF3)2Y, equilibrium constants for the addition of HI and HBr, and solvent deuterium isotope effects are reported. The equilibrium constant for the addition of HI to 1 is 17-fold larger than for the addition of HBr. The reactions of 1 have been compared with the reactions of the O-methylated analogue 4-MeOArC(CF3)2+. Methylation of the carbonyl oxygen causes a approximately 1O(19)-fold increase in K(eq) for the addition of Br- to the electrophile and a large decrease in the selectivity for the addition of solvent and halide ions. Rate constants for the capture of the highly unstable O-methylated carbocation by nucleophilic reagents do not obey the N+ equation, which requires that carbocation selectivity be independent of electrophile reactivity. The solvent deuterium isotope effects on the uncatalyzed addition of water and bromide ion are (k(o))HOH/(k(o))DOD = 2.0 and (k(Br))HOH/(kBr)DOD = 1.0, respectively. The difference in the isotope effects on these two reactions may be explained by considering the changes in the interaction of solvent with the water and bromide ion nucleophiles that occur on moving to the reaction transition states. The solvent deuterium isotope effects on the hydrogen ion catalyzed addition of water and bromide ion are k(H)/k(D) = 0.68 and k(HBr)/k(DBr) = 1.0, respectively. The difference in the isotope effects on these reactions and several other experimental observations are consistent with a stepwise mechanism for the H+-catalyzed addition of solvent and a concerted mechanism for the H+-catalyzed reactions of halide ions in which protonation of the carbonyl oxygen and carbon-bromine bond formation occur in a single step.