Sydnone Photochemistry: Formation of Nitrenes

Photolysis of 3-phenyl- and 3-(3-pyridyl)-sydnones 9 in Ar matrices is known to furnish Earl's bicyclic lactone (bicyclo[2.1.0]oxadiazolone) 10 followed by the elimination of CO₂ and the formation of carbodiimides 6. Laser photolysis of 3-arylsydnones at 266 nm in an MTHF matrix at 5 K generates triplet arylnitrenes 8, which are characterized by their zero-field splitting parameters determined from their electron spin resonance (ESR) spectra. The potential routes from nitrile imines 3, diazirines 4, imidoylnitrenes 5, and carbodiimides 6 to arylnitrenes 8 are investigated computationally at the M06-2X/6-311+G(d,p) level. Imidoylnitrenes 5 are the key intermediates in the matrix photolyses of sydnones as well as tetrazoles, whereby the triplet states of 5 dissociate to a nitrene (8) and a nitrile, and the open-shell singlet state rearranges to a carbodiimide 6; these two processes have nearly the same activation energies of 20-22 kcal/mol.