A kinetic study of the reaction of diazoalkanes with triphenylphosphine: structure and reactivity in a biphilic process

Abstract

Second-order rate coefficients and activation parameters are reported for the reaction in solution of triphenylphosphine with a series of diazoalkanes (Ar₂CN₂) having two aromatic groups attached to the diazo-carbon atom, 9-diazofluorene (DAF), 5-diazo-5H-dibenzo[a,d]cycloheptene (DBSE), its 10,11-dihydro analogue (DBSA), and diazodiphenylmethane (DDM). Although the product phosphazine Ar₂CNNPPh₃ arises formally by nucleophilic attack of phosphorus on the terminal nitrogen atom of the diazoalkane, the pattern of reactivity approximates to that for proton transfer from acetic acid to the diazoalkane under similar conditions, highlighting the biphilic nature of the reaction. The results can be rationalised in terms of qualitative FMO theory taking into account both HOMO(Ph₃P)–LUMO (Ar₂CN₂) and HOMO(Ar₂CN₂)–LUMO (Ph₃P) interactions. This is lent qualified support by MNDO calculations on Ar₂CN₂ based on molecular parameters from the X-ray crystal structure of DAF itself and of the ketones corresponding to DBSE, DBSA, and DDM.

The reactivity of DDM and the effect of 4,4′-disubstitution by CH₃O–, CH₃–, and Cl–, which gives rise to a U-shaped Hammett ρσ correlation, is not easily interpreted within the FMO framework. Conformational changes which affect the interacting orbitals as the reactants are transformed into transition states offer an alternative interpretation.