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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.

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3-cyano-l(3//)-isobenzofuranone, 27613-27-0; iV-[3-chloro-4methoxyphenyl]benzamide, 125593-62-6; 3-chloro-p-anisidine, 5345-54-0; l-bromo-2-(3-hydroxypropyl)-4,5-dimethoxybenzene, 125593-63-7; l-bromo-2[3-(fert-butyldimethylsilyloxy)propyl]-4,5-dimethoxybenzene, 125593-64-8; 3-(2-bromo-4,5-dimethoxyphenybpropionic acid, 52679-49-9. Supplementary Material Available: Raney nickel reduction of 4d and 5d to la, all nucleophilic substitution reactions of 2b, zinc-copper reductions of 10a and 10b, all compounds dealing with the preparation of 14c and its chemistry, the reaction of 2b with s-BuLi, and !H NMR spectra of 3c, 4d-f, 5d,e, and 12a (15 pages). Ordering information is given on any current masthead

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