Intermediates in the decomposition of aliphatic diazo-compounds. Part 13. Mechanistic studies on the reaction of diaryldiazomethanes with singlet molecular oxygen

Abstract

The reaction of 9-diazofluorene, diazodiphenylmethane, and p-substituted diazodiphenylmethanes with singlet (¹Δ_g) molecular oxygen to give mainly the corresponding ketone, has been studied in the solvents acetonitrile, chloroform, and methanol. In most cases singlet oxygen was generated photochemically using Methylene Blue as sensitiser, but the thermal decomposition of triphenyl phosphite ozonide was used for some experiments. Quantitative studies of the Methylene Blue-sensitised photo-oxidation of 9-diazofluorene, diazodiphenylmethane, and 9,10-dimethylanthracene (standard) at concentrations below 10^–3M established that the diazo-compounds decomposed only by reaction with singlet oxygen. The low relative reactivity of diazodiphenylmethane (ca. 12) compared with 9-diazofluorene, the insensitivity of the rate constant to solvent polarity, and the low value of the Hammett ρ for the reactions of a series of para-substituted diazodiphenylmethanes all suggest that the charge separation in the reactant is increased very little on passing to the transition stare. It is argued that the process is one of concerted cycloaddition yielding a heterocyclic intermediate (2). This species then decomposes with loss of N₂O (detected mass spectrometricaliy) to give the ketone. By labelling 9-diazofluorene specifically at the terminal nitrogen with ¹⁵N and examining the exhaust gases from the photo-oxidation, it is shown that 1,3-cycloaddition takes place rather than the 1,2-mode of addition reported for the isoelectronic acceptor diphenylketen. The relevance of the results to the mechanism of direct photo-oxidation of diazoalkanes is discussed.