LASER FLASH-PHOTOLYSIS STUDIES ON 4-OXOCYCLOHEXA-2,5-DIENYLIDENES

Reactions of 4-oxocyclohexa-2,5-dienylidene (1) and two derivatives have been investigated using laser flash photolysis techniques. Photolysis of 4-diazocyclohexa-2,5-dien-1-one (4) in 1,1,2-trichlorotrifluoroethane allows the indirect detection of the corresponding carbene 1. This carbene reacts with triplet oxygen at close to the diffusion-controlled rate, forming the carbonyl 0-oxide (lambda(max) = 470 nm). Reaction of triplet 1 with moderate concentrations of cyclohexane leads to formation of phenoxyl radical (lambda(max) = 395 nm). At high concentrations or in neat cyclohexane the predominant path appears to be carbene insertion into the C-H bond. Other reactions examined include 0-H insertion in methanol and the indirect formation of ylides from acetone, acetonitrile, and pyridine. The reactivity pattern that emerges indicates that carbene 1 and its derivatives, 2 and 3, have triplet ground states and that the singlet-triplet energy gap is moderately large. Ylide formation is only observed when the reactants are present in low concentrations and involve a two-step reaction from the triplet carbene; no ylides are observed in neat acetone or acetonitrile suggesting that for the singlet carbene insertion into C-H bonds is favored over direct ylide formation.