Substituent effects in the reaction of triphenylphosphine with diazodiphenylmethane: the interpretation of a U-shaped Hammett correlation in an elementary biphilic reaction

The reaction between diazodiarylmethanes and triphenylphosphine has previously been shown to be an elementary biphilic reaction in which each reactant behaves simultaneously as both electrophile and nucleophile. Frontier molecular orbital considerations provide a basis for relating structure and reactivity. Diazodiphenylmethane, however, is an order of magnitude more reactive than expected, and substituents in the phenyl groups of the diazoalkane enhance the reaction rate, whether they are electron-withdrawing or electron-releasing, giving rise to a U-shaped Hammett correlation. Evidence is now presented that the substituent effect cannot arise from disturbance of simple energy matching of the frontier molecular orbitals of the two reactants, but rather results from conformational changes in the diazo-compound on passing from the reactant to the transition state. This evidence comprises (i) the linearity of Hammett plots for the effects of substituents in triphenylphosphine on the reactivity towards diazodiphenylmethane, and (ii) a comparison of the reactivity of diazodiphenylmethanes bearing two substituents of opposite polar effect when these are present in the same and in different aromatic groups.