Nitrene phenyl substituent effects

Phenyl Substituent Effects on Methylene, Nitrenium Ion, and Nitrene... 

A phenyl substituent reduces the value of AEst from 36 kcal mol" in NH to 18 kcal mol" in PhN. This reduction in AEst is between three and four times larger than that caused by the phenyl group in PhCH. Understanding the reason for the much larger phenyl substituent effect on AEst in the nitrene than in the carbene requires knowing which of the two components of A in NH is more stabilized by the phenyl group in PhN. 

This property is relatively rare in the very large number of reactions for which substituent effects were evaluated quantitatively106. It seems to be common, however, for all dediazoniations of arenediazonium ions and of related compounds, e.g. of substituted phenyl azides forming nitrenes, as well as for additions of carbenes to alkenes. 

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. 

Intersystem crossing rate constants of ortho- and meto-substituted singlet phenyl nitrenes are presented in Table 3. Mono- and di-ort/io-fluorine substituents have no influence on ISC rate constants. No effect with meta, metfl-difluoro substitution is observed either. Pentafluoro substitution has no efffect on kisc in pentane although a modest acceleration is observed in the more polar solvent methylene chloride. 

As with phenyl azides, meta substituents have no effect on the rate, whereas para substituents accelerate it. Hence the two systems must have a common primary process, which is the formation of a nitrene, viz. 

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