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A series of fluorinated aryl azides and fluorinated azidobenzoates were studied by laser flash photolysis techniques. Using the pyridine ylide probe method it was possible to determine whether a singlet nitrene or ring-expanded ketenimine ylide is the trappable intermediate that is generated at ambient temperature. It was determined that two fluorine substituents, ortho and ortho' substituted relative to the azide group, are required to retard ring expansion and allow bimolecular capture of the singlet nitrene. LFP of ortho, ortho' difluorinated aryl azides in methanol produces the ground triplet state of the nitrene. The results are consistent with chemical analysis of reaction mixtures. The implications of this data for the design of photoaffinity labeling reagents are discussed.

Several fluorinated aryl azides (2,6 and 2,4-difluorophenyl azide, 2,4,6-triflurorophenyl azide, and methyl 4-azido-2,3,5,6-tetrafluorobenzoate) were studied by laser flash photolysis techniques. In each case, the azides from singlet nitrenes which can be intercepted with pyridine to form ylides. This allowed determination of the rate constant for ring expansion of the singlet nitrene to the dehydroazepine and measurement of the absolute rate constants for reaction of the singlet nitrenes with typical quenchers.

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