A laser flash photolysis and quantum chemical study of the fluorinated derivatives of singlet phenylnitrene

Laser flash photolysis (LFP, Nd:YAG laser, 35 ps, 266 nin, 10 mJ or KrF exc imer laser, 10 ns, 249 nm, 50 mJ) of 2-fluoro, 4-fluoro, 3,5-difluoro, 2,6- difluoro, and 2,3,4,5,6-pentafluorophenyl azides produces the corresponding singlet nitrenes. The singlet nitrenes were detected by transient absorpti on spectroscopy, and their spectra are characterized by sharp absorption ba nds with maxima in the range of 300-365 nm. The kinetics of their decay wer e analyzed as a function of temperature to yield observed decay rate consta nts, k(OBS) The observed rate constant in inert solvents is the sum of k(R) + k(ISC) where kR is the absolute rate constant of rearrangement of single t nitrene to an azirine and k(ISC) is the absolute rate constant of nitrene intersystem crossing (ISC). Values of k(R) and k(ISC) were deduced after a ssuming that k(ISC) is independent of temperature. Barriers to cyclization of 4-fluoro-, 3,5-difluoro-, 2-fluoro-, 2,6-difluoro-, and 2,3,4,5,6-pentaf luorophenylnitrene in inert solvents are 5.3 +/- 0.3, 5.5 +/- 0.3, 6.7 +/- 0.3, 8.0 +/- 1.5, and 8.8 +/- 0.4 kcal/mol, respectively. The barrier to cy clization of parent singlet phenylnitrene is 5.6 +/- 0.3 kcal/mol. All of t hese values are in good quantitative agreement with CASPT2 calculations of the relative barrier heights for the conversion of fluoro-substituted singl et aryl nitrenes to benzazirines (Karney, W. L. and Borden, W. T. J. Am. Ch em. Sec. 1997, 119, 3347), A single ortho-fluorine substituent exerts a sma ll but significant bystander effect on remote cyclization that is not steri c in origin. The influence of two ortho-fluorine substituents on the cycliz ation is pronounced. In the case of the singlet 2-fluorophenylnitrene syste m, evidence is presented that the benzazirine is an intermediate and that t he corresponding singlet nitrene and benzazirine interconvert. Ab initio ca lculations at different levels of theory on a series of benzazirines, their isomeric ketenimines, and the transition states; converting the benzazirin es to ketenimines were performed. The computational results are in good qua litative and quantitative agreement with the experimental findings.