Laser flash photolysis phenylnitrene

Laser flash photolysis (266 nm) of phenyl azide in pentane at 233 K produces a transient absorption spectrum with two sharp bands with maxima at 335 and 352 nm (Fig. 11.4). Spectrum 1 was measured, point by point, 2 ns after the laser pulse. In later work, the spectrum of 33s was reinvestigated and an additional very weak, long wavelength absorption band at 540 nm was observed (Spectrum 2). The transient spectrum of Figure 11.4 was assigned to singlet phenylnitrene in its lowest open-shell electronic configuration ( 2). 

The key intermediate of Scheme 1 is singlet phenylnitrene the only intermediate which in 1992 had not been detected directly or chemically intercepted in the parent system. In 1997 our group" " and the Wirz group, simultaneously reported that laser flash photolysis of phenyl azide or phenyl isocyanate 5 produces a previously undetected transient with = 350 nm and a lifetime of ns at ambient temperature. 

Laser flash photolysis studies were performed on 2-cyano (8c), 4-cyano (8e) and 2,6-dicyanophenyl (8f) azide. The results are given in Table 6. In pentane the barrier to cyclization of orr/io-cyanophenylnitrene is the same, within experimental error, to that of parent phenylnitrene. The barrier to cyclization of 2,6-dicyanophenylnitrene (8f) is about 1 kcahmol larger than that of parent phenylnitrene ( PN). Variation of solvent has only a small effect on the kinetics. 

Schnapp, K. A., and Platz, M. S. (1993) A laser flash photolysis study of di-, tri- and tetrafluori-nated phenylnitrenes Implications for photoaffinity labeling. Bioconjugate Chem. 4,178— 183. 

Perhaps their most significant result is that the opposite trends in changes in the barrier heights for the two steps with increasing steric bulk infers that the intermediate azirine might be observable if a sufficiently bulky substituted phenylnitrene could be produced. Just such a molecule is 2,4,6-tri-t-butylphenylnitrene. Computations indicate a barrier of 4.1 kcal mol for the first step and 6.3 kcal moC for the second step. Laser flash photolysis detected the azirine intermediate 27 having a lifetime of 62 ns and a barrier to ring open to the azepine 28 of 7.4 0.2 kcal moL. ... 

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