Sydnone Photochemistry: Formation of Nitrenes.

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