The elusive benzocyclobutenylidene: A combined computational and experimental attempt

Ab initio and density functional theory calculations predict that benzocycl obutenylidene (1) has a singlet ground state in contrast to the parent phen ylcarbene and many other simply substituted arylcarbenes. Calculations also predict that 1 should lie in a relatively deep potential well, while its t ripler state is 14.5 kcal mol(-1) higher in energy. However,:attempts to ob serve 1 directly by photolysis of two different nitrogenous precursors were not successful. Irradiation of diazobenzocyclobutene (7) (lambda > 534 nm or lambda > 300 nm) or azibenzocycllobutene (10) (lambda > 328 nm) in Ar ma trixes at 10 K leads to the formation of the strained cycloalkyne 7-methyle necyclohepta-3,5-dien-1-yne (3). C-13-Labeled 3 was also prepared in a simi lar manner. There is very good agreement between experimental IR spectra an d computationally derived harmonic vibrational frequencies for 3 and [C-13] -3 and excellent agreement between observed and calculated isotopic shifts. Prolonged short-wavelength irradiation converts 3 into benzocyclobutadiene (5). Phenylacetylene (6) and benzocyclo- butadiene dimer (11) were identif ied as products arising from flash vacuum pyrolysis of diazirine 10 at 500 degreesC.