Para substituents phenylnitrene

Two para-substituents, phenyl and cyano depress and retard the rate of cyclization significantly (Table 11.2)." p-Phenyl and p-cyano are both radical stabilizing substituents. These conjugative substituents reduce the spin density on the carbon ortho to the nitrene nitrogen. The reduced spin density at carbons ortho to the nitrogen lowers the rate at which the 1,3-biradical cychzes. The effect with p-cyano and p-biphenyl singlet phenylnitrene is quite dramatic. The lifetimes of these singlet nitrenes at ambient temperature are 8 and 15 ns, respectively, and the activation barriers to cychzation are 7.2 and 6.8 kcal/mol, respectively. 

Analysis of the data of Table 1 verifies the same finding that the shift of the near-UV absorption band of singlet arylnitrenes correlates with the shift of the intense near-UV absorption band of triplet nitrenes. Furthermore, the ort/io-substituents influence the absorption spectra of singlet and triplet phenylnitrenes more significantly than do para-substituents. 

It was impossible to study the effect of strong Ji-donor substituents on the rate of cyclization, because with para-methoxy and dimethylamino substituents the phenylnitrene underwent intersystem crossing to the triplet faster than cyclization at all temperatures. ... 

Placement of fluorine substituents at both ortho positions (70d) raises the barrier to cyclization by about 3 kcal/mol relative to the unsubstituted system (Table 5.13). The lifetimes of singlet phenylnitrene ( 49) and 4-fluorophenylnitrene (70b) are about 1 ns or less at 298 K. The lifetime of 3,5-difluorophenylnitrene (70c) is about 3 ns at 298 K, but that of 2,6-difluorophenylnitrene (70d) is 260 ns, in CCI4. Because a para-fluoro group fails to exert an electronic influence on the cyclization process, it is tempting to attribute the effect of two or /io-fluorine substituents on the singlet nitrene lifetime to a simple steric effect. 

Albini, A., Bettinetti, G., and Minoli, G., Photodecomposition of some para-substituted 2-pyra-zolylphenyl azides, substituents affect the phenylnitrene S-T gap more than the barrier to ring expansion,/. Am. Chem. Soc., 121, 3104, 1999. 

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