Diphenylcarbene, reaction with oxygen

An intriguing possibility, which appears close to realization, is that the excited state reagent may be generated by methods other than irradiation. For example, diphenylcarbene reacts with oxygen (in the dark) to yield ultimately the benzophenone triplet.26 The excited state thus prepared should be fully capable of participating in the cycloaddition reaction. 

The hydrogen abstraction reaction and the reaction with oxygen presumably proceed through a triplet state, known to be the ground state of diphenylcarbene, but the alcohol and azine more probably derive from an initially formed singlet. 

A two-electron oxidation of allenes (1) (A = S, NMe) has been found to yield carbenes (2) (Scheme 1) dimerization and reaction with (2,2,6,6-tetraamethyl-piperidin-l-yl)oxyl (TEMPO) support a triplet ground state as predicted by computations. Triplet diphenylcarbenes bearing bulky substituents at the para positions have been generated and studied in rigid matrixes at low temperatures by electron spin resonance (ESR) and UV-Vis spectroscopy as well as by laser flash photolysis (LFP) in solution at room temperature. Their reactivity upon LFP was shown to be dominated by dimerization unless triplet carbene quenchers (such as oxygen and cyclohexadiene) were present. 

Suggestive evidence for the protonation of diphenylcarbene was uncovered in 1963.10 Photolysis of diphenyldiazomethane in a methanolic solution of lithium azide produced benzhydryl methyl ether and benzhydryl azide in virtually the same ratio as that obtained by solvolysis of benzhydryl chloride. These results pointed to the diphenylcarbenium ion as an intermediate in the reaction of diphenylcarbene with methanol (Scheme 3). However, many researchers preferred to explain the O-H insertion reactions of diarylcarbenes in terms of electrophilic attack at oxygen (ylide mechanism),11 until the intervention of car-bocations was demonstrated by time-resolved spectroscopy (see Section III).12... 

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