Azirines phenylnitrene

Our calculations on the ring expansion of the lowest singlet state of phenylnitrene ( A2-lb) to azacycloheptatetraene (3b) predict a two-step mechanism that is analogous to that for the rearrangement of la to 3a and which involves the bicyclic azirine intermediate 2b.61 The CASPT2 energetics are depicted in Fig. 5, and the CASSCF optimized geometries of the stationary points are shown in Fig. 6. 

Earlier, Dunkin and Thomson had observed that matrix-isolated triplet 10a did not undergo photochemical ring expansion.83 However, Morawietz and Sander have recently provided evidence for photochemical conversion of 310a and 310b to the corresponding fluorinated azirines (Scheme 19).48d This represents a rare instance where an azabicyclo[4.1.0]heptatriene, the putative intermediate in the ring expansion of a phenylnitrene, has actually been observed. 

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. ... 

As with phenylcarbene (35) (c/. Scheme 5), the ring expansion of phenylnitrene (75) could occur directly on the singlet potential energy surface or via a bicycHc intermediate, in this case azirine 92. Such azirines were first detected in the matrix photolysis of 1- and 2-azidonaphthalene. For example, photolysis of 93 generated a photoproduct with an IR absorption at 1730 cm, which in turn gave a cyclic ketenimine [v(C=C=N) at 1926 cm ] on further photolysis. It was proposed that the intermediate with the 1730 cm IRband was the tricyclic azirine 94, which subsequently rearranged to 95. At the time of these experiments, however, reliable computations of IR transitions were not available, so a definitive identification of the azirines was scarcely possible. Nevertheless, a clear parallel was found here with the behavior of aryl carbenes, which give detectable cyclopropenes in the naphthalene series (c/. 44 and 45) but not with the monocyclic carbenes (see the brief discussion of this point in Section 14.4). 

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