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The 4-oxocyclohexa-2,5-dienylidenes are an interesting class of highly electrophilic carbenes. We investigated the reactivity of the 2,3,5,6-tetrafluorinated and -tetrachlorinated derivatives 1b and 1c with small molecules under conditions of matrix isolation. The reactions with molecular oxygen and with carbon monoxide produce the expected carbonyl O-oxides and ketenes, respectively. As a result of the extreme electrophilicity of 1b and c both carbenes insert with no or very small activation barriers into H2 or the CH bonds of hydrocarbons. Obviously, spin restrictions for these formally spin-forbidden reactions do not result in substantial thermal activation barriers.

The EPR spectrum of triplet 4-oxo-2,3,5,6-tetrafluorocyclohexa-2,5-dienylidene 1 was recorded in solid argon at 15 K. Carbene 1 reacts with acetylene under the conditions of matrix isolation yielding triplet vinylmethylene 4, which was characterized by its IR, UV-vis, and EPR spectrum. Carbene 4 is photolabile and is converted to spiro compound 5 on irradiation with lambda > 515 nm. The reaction of triplet carbene 1 with acetylene to produce triplet carbene 4 is predicted to be exothermic by 55 kcal mol(-1) at the B3LYP/6-31G(d,p) level of theory. The cis isomer is calculated to be only 0.4 kcal mol(-1) less stable than trans-4 at this level of theory. According to our calculations, singlet carbene S-4 is not a minimum on the C(8)F(4)H(2)O potential energy surface; however, at the T-4 geometry, the lowest lying singlet state is predicted to be 20.7 kcal mol(-1) higher in energy. The subsequent photochemical cyclization of T-4 yielding spiro compound 5 is exothermic by 10.3 kcal mol(-1) relative to T-4 and by 31.1 kcal mol(-1) relative to S-4. 4-Ethinyl-2,3,5,6-tetrafluorocyclohexa-2,5-dienone 9, the C-H insertion product of 1 and acetylene, was not observed experimentally, although it is favored energetically by 4.3 kcal mol(-1) over 5.