Tolylnitrene, azirine cyclization

A single o- or p-methyl substituent has no influence on the rate of cyclization of the singlet tolylnitrene to the favored azirine. ° The methyl group has no bystander effect on benzazirine formation. Cyclization of 2,6-dimethylphenyl or 2,4,6-tri-methylphenylnitrenes necessarily proceeds toward a carbon bearing a substituent. A steric effect raises the barrier to cyclization by 1.5-2.0 kcal/mol, in excellent agreement with the predictions of Karney and Borden. The steric effect extends the lifetime of 2,6-dimethylphenylnitrene at ambient temperature to 13 ns in Freon-113 and of 2,4,6-trimethylphenylnitrene to 8 ns, in the same solvent (Table 11.4). ° ... 

A recent kinetic study demonstrated that a single ortho (69a) or para (69d) methyl substituent has no influence on the rate of cyclization of the singlet tolylnitrene to the azirine (Table 5.11). Spin localization effects are not observed, as they are with cysmo and phenyl substitution. 

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See also in sourсe #XX -- [ ]

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