| Abstract
| - The EPR spectrum of triplet 4-oxo-2,3,5,6-tetrafluorocyclohexa-2,5-dienylidene 1 was recorded insolid argon at 15 K. Carbene 1 reacts with acetylene under the conditions of matrix isolation yieldingtriplet vinylmethylene 4, which was characterized by its IR, UV−vis, and EPR spectrum. Carbene4 is photolabile and is converted to spiro compound 5 on irradiation with λ> 515 nm. The reactionof triplet carbene 1 with acetylene to produce triplet carbene 4 is predicted to be exothermic by 55kcal mol-1 at the B3LYP/6-31G(d,p) level of theory. The cis isomer is calculated to be only 0.4 kcalmol-1 less stable than trans-4 at this level of theory. According to our calculations, singlet carbeneS-4 is not a minimum on the C8F4H2O potential energy surface; however, at the T-4 geometry, thelowest lying singlet state is predicted to be 20.7 kcal mol-1 higher in energy. The subsequentphotochemical cyclization of T-4 yielding spiro compound 5 is exothermic by 10.3 kcal mol-1 relativeto 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 isfavored energetically by 4.3 kcal mol-1 over 5.
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