Carbenes phenylcarbene rearrangement

The rearrangement of phenylcarbene (59) to 1,2,4,6-cycloheptatetraene (60) has been extensively studied (Scheme 12).158 Recently, Warmuth has also reported the generation of carbene 59 within a HC.159 Encapsulated allene 60 was protected from dimerization, so even the H 2-D ROESY NMR spectrum of 60 HC could be measured. 

For photochemically generated (2-biphenylcarbonyl)phenylcarbene, several competing intramolecular reactions are observed, namely cyclopropanation of an aromatic nucleus (followed by norcaradiene to cycloheptatriene tautomerization), Wolff rearrangement, C-H insertion, and a carbene-to-carbene rearrangement (see Houben-Weyl, Vol. E19b, pl282). 

Phenylcarbene (12). Aromatic carbenes have received considerable attention as they undergo fascinating intramolecular rearrangements. Recently, theoretical investigations by... 

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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