Intermediates in the decomposition of aliphatic diazo-compounds. Part 17. Formation and reaction of diazodiphenylmethane anion radical in solution

Abstract

The one-electron reduction of diazodiphenylmethane in dimethylformamide (DMF) which gives largely diphenylmethane and benzophenone azine has previously been reported to involve generation from the diazoalkane anion radical of diphenylmethylene anion radical. The reaction has now been re-examined in CH3CN as well as DMF and the corresponding perdeuteriated solvents. It is established that at high sweep rates in both solvents one-electron reduction of Ph2CN2 is quasi-reversible, the heterogeneous charge charge-transfer rate being markedly influenced by the nature of the supporting electrolyte. Examination of the kinetics of disappearance of Ph2CN2˙– by linear sweep voltammetry and double potential step chronoamperometry shows that the reaction conforms to a first-order kinetic law. In CD3CN, however, the rate coefficient is decreased by a factor of over 20 indicating that the solvent is involved chemically in the disappearance of Ph2CN2˙–. Since a good proton donor, diethyl malonate, does not affect the rate, it is inferred that Ph2CN2˙– abstracts a hydrogen atom from the solvent. It would seem that quantum mechanical tunnelling occurs in CH3CN, but not in DMF where the kinetic hydrogen isotope effect is only about four. A product isotope effect is also observed and interpreted in terms of competing reactions of Ph2CN2˙–: (i) hydrogen atom abstraction from the solvent; (ii) attack on Ph2CN2 Or Ph2CN2˙– giving azine, a process analogous to that observed in electrochemical decomposition of 9-diazofluorene; (iii) reaction with molecular oxygen giving benzophenone. There is no evidence for unimolecular loss of nitrogen from Ph2CN2˙– to give the carbene anion radical.