Sydnone Photochemistry: Formation of Nitrenes

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