Reaction of 4-oxo-2,3,5,6-tetrafluorocyclohexa-2,5-dienylidene with acetylene: A carbene to carbene reaction

The EPR spectrum of triplet 4-oxo-2,3,5,6-tetrafluorocyclohexa-2,5-dienylid ene 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 gimel > 515 nm. The reaction of triplet carbene 1 with acetylene to produce tripl et 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 kca l mol(-1) less stable than trans-4 at this level of theory. According to ou r calculations, singlet carbene S-4 is not a minimum on the C8F4H2O potenti al energy surface; however, at the T-4 geometry, the lowest lying singlet s tate 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 p roduct of 1 and acetylene, was not observed experimentally, although it is favored energetically by 4.3 kcal mol(-1) over 5.