Laser flash photolysis study of methyl derivatives of phenyl azide

Laser flash photolysis (Nd:YAG, 35 ps, 266 nm, 10 mJ; KrF excimer, 349 nm, 12 ns, 70 mJ; XeCl excimer, 308 nm, 17 ns, 50 mJ) of methyl substituted phe nyl azides produces the corresponding singlet nitrenes. The singlet nitrene s were detected directly, and their decay was analyzed to yield the observe d rate constant k(OBS). The observed rate constant is the sum of k(R) + k(I SC) where k(R) is the absolute rate constant of rearrangement to an azirine and k(ISC) is the absolute rate constant of intersystem crossing. Values o f k(OBS) were measured as a function of temperature and dissected assuming that k(ISC) is temperature-independent, It was shown that k(ISC) is indepen dent of temperature for 2,6-dimethyl- and 2,3,6-trimethylphenyl singlet nit renes. The barriers to rearrangement of singlet phenylnitrene, p-methylphen ylnitrene, o-methylphenylnitrene, 2,6-dimethylphenylnitrene, and 2,4,6-trim ethylphenylnitrene in hydrocarbon solvents are found to be 5.6, 5.8, 5.3, 7 .0, and 7.3 +/- 0.4 kcal/mol, respectively. The data demonstrate that two o rtho methyl substituents retard cyclization but that a single ortho or para substituent does not. It is concluded that the rate of retardation provide d by the methyl groups has a steric origin, as predicted by Karney and Bord en (J. Am. Chem. Sec. 1997, 119, 3347). These authors predict that ortho me thylation raises the barrier to cyclization by 1-2 kcal/mol, in excellent a greement with our results. Our results are also consistent with the work of Sundberg et al. (J. Am. Chem. Sec. 1972, 94, 513), which demonstrated that singlet 2-methylphenylnitrene cyclizes away from the ortho methyl substitu ent. Methyl substitution is found to increase the absolute rate constant of intersystem crossing.