UNUSUALLY LONG LIFETIMES OF THE SINGLET NITRENES DERIVED FROM 4-AZIDO-2,3,5,6-TETRAFLUOROBENZAMIDES

Laser flash photolysis (308 nm, 20 ns, 150 mJ) of 4-azido-2,3,5,6-tetr afluorobenzamides, esters, and thioesters generates singlet nitrenes w hich can be intercepted with pyridine to produce strongly absorbing yl ides. It was possible to resolve the rate of formation of the ylides a s a function of pyridine concentration. This has led to direct measure ments of the absolute rate constants of (a) the reaction of the nitren e with pyridine, (b) ring expansion of the nitrene to a ketenimine, an d (c) singlet to triplet nitrene intersystem crossing.