Intersystem crossing singlet phenyl nitrene

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. 

First we assume that intersystem crossing and ring expansion of singlet phenyl nitrene are both irreversible processes in the presence of 1.0 M diethylamine. The ratio of 3f/-azepine 2 to azobenzene 3 formed on photolysis of phenyl azide is then related to the ratio of rate constants of ring expansion (fcEXP) to intersystem crossing (fclsc). 

On the basis of product studies, it is clear that irradiation of the naphthyl azides leads to loss of nitrogen with the likely consequent formation of nitrenes. Just as for phenyl azide, the initially formed singlet nitrenes may intersystem cross to the triplet and then dimerize to azo compounds. Clearly in the case of 2-naphthyl azide, but not 1-naphthyl azide, a closed-shell ground-state intermediate that can be trapped with diethylamine can be generated. The intermediate was formulated as the azirine on the basis of product studies [57]. Low temperature absorption spectroscopy and time-resolved laser flash photolysis experiments to be described later support the formation of azirines and provide an explanation for the different reactivity observed between the 1- and 2-substituted azides. 

Upon photolysis of phenyl azide at cryogenic temperatures (10-77 K) the singlet nitrene produced lacks the energy to undergo isomerization. Under these conditions the singlet nitrene preferentially relaxes to triplet phenyl nitrene by intersystem crossing. This process most likely has no barrier to surmount and therefore is favored at very low temperature. 

Schrock and Schuster [46] have demonstrated that the quantum yield to loss of nitrogen from triplet phenyl azide is rather low. The fact that a strong transient spectrum of the brominated triplet phenyl nitrene is observed following laser flash photolysis thereby implies that the heavy atom effect has catalyzed intersystem crossing from the singlet to the triplet state of the nitrene rather than of the azide excited state. 

The extensive mechanistic work on phenyl azides of the last years has registered further additions. The examination has been extended to 4-acedamidophenyl azide, which decomposes from the singlet and gives the nitrene that then intersystem crosses. The final product is, as usual, the azo compound (see Scheme 12)." " ... 

Phenyl azide has been claimed" to undergo expansion to 49 in 10% yield on photolysis in methanol, a very weak base. This reaction is much more facile when the azide bears on an ortho substituent containing a carbonyl group. In this case, stabilization of the singlet nitrene (and suppression of intersystem crossing to triplet) can be envisaged by formation of an intermediate anthranil. These are known to give azepines on irradiation in nucleophilic solvents ... 

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