EVIDENCE FOR THE NONCONCERTED ADDITION OF DIFLUOROVINYLIDENE TO ACETYLENES

Difluorovinylidene (Ib) is an extremely electrophilic and reactive sin glet carbene that can be generated in argon matrices in high yields. T he thermal reaction of Ib with difluoroacetylene (2b) at 35-40K result s in the formation of singlet allenylcarbene 4b as the primary product , which on subsequent irradiation with visible light (lambda > 420 nm) rearranges to methylene-cyclopropene 3b. The new compounds were ident ified by IR spectroscopy in combination with density functional theory (DFT) calculations at the B3LYP/G-311G(d,p) level of theory These res ults clearly demonstrate the non-concerted formation of 3b, Under simi lar conditions the thermal reaction of vinylidene Ib with acetylene (2 c) directly leads to methylenecyclopropene 3c as the main product. Sin ce-according to the DFT calculations-allenyl-carbene Ic is not a minim um, the formation of 3c most likely is a concerted, although highly as ynchronous reaction, Visible irradiation (lambda > 329 nm) of 3c produ ces enyne 11, which on CV irradiation gives 1,1-difluorobutatriene (10 ).