Diphenylcarbenes hydrogen abstraction

Triplet diphenylcarbene, detected spectroscopically on excimer laser flash photolysis of diphenyldiazomethane, readily abstracts hydrogen from cyclohexane to give the CHPh2 radical.Analogous hydrogen abstraction also occurs from alcohols but at low temperatures and in a polycrystalline matrix in contrast to reaction in solution in which O-H bond insertion is preferred. In a similar fashion, irradiation of diphenyldiazomethane in polycrystalline (S)-butan-2-ol at — gave amongst other products an enantiomerically pure... 

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. 

Unlike phenyldiazomethane which is converted on irradiation into cyclohepta-1,2,4,6-tetraene, the carbenes (52) generated by photodecomposition of the naphthyldiazomethanes (53) undergo rearrangement to the benzobicyclo[4,1.0]hepta-2,4,6-trienes (54). Hydrogen abstraction by triplet diphenylcarbene is observed on photodecomposition of diphenyldiazomethane in cyclohexane, whereas singlet-derived solvent insertion reactions compete with hydrogen... 

Observation of emission and absorption in the NMR benzylic proton signal from an irradiated solution of diphenyldiazomethane in toluene was the first example showing the importance of the CIDNP technique. Triplet diphenylcarbene generated by photolysis of diphenyldiazomethane in toluene abstracts a hydrogen atom to generate the triplet radical pair, which either recombines to give 31, or diffuses apart, ultimately to produce dimers of each fragment (32, 33, Scheme 9.7,... 

Triplet bis(trifluoromethyl)methylene also reacts with oxygen to yield excited hexafluoroacetone.44 Diphenylcarbene does not insert into C—H bonds but does abstract hydrogen atoms from suitably reactive hydrocarbons. Coupling of the radicals so formed can yield some of the same products which would arise from direct insertion. 

Japanese chemists2 have also reported cases in which the addition of the copper chelate modifies the reactions of carbenes. Thus diphenylcarbene (thermal decomposition of diphenyldiazomethane, 1) is converted mainly into benzophenone azine (2) when generated in an aprotic medium and into 1,1,2,2-tetraphenylethane (3) when generated in a protic medium. The second reaction is considered to proceed by abstraction of hydrogen from the solvent to form benzhydryl radicals which then... 

The stabilization of the benzhydryl ylid is expected to be quite high since the related fluorenylid (2) can be isolated as a salt free solid. The high yield of 32 was attributed to hydrogen atom abstraction from the cyclohexene by the diphenylcarbene and subsequent coupling of di-phenylmethyl radicals 46>. 

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