Arylcarbenes and Arylnitrenes

Having presented the simple, open-chain unsaturated carbenes and nitrenes, we can now discuss the more complicated aromatic systems. 

The mechanisms of the rearrangements and interconversions of arylcarbenes and arylnitrenes have been reviewed in considerable detail.101314 It is the purpose of this section to summarize the most important findings and to update the previous reviews. 

Crow et a/.201,204 found that also 3- and 4-pyridylcarbenes are converted to phenylnitrene. A 13C-labeling study of 4-pyridylcarbene by Thetaz 

Wentrup, C. Mayor, and R. Gleiter, Helv. Chim. Acta 55, 2628 (1972) erratum, 3066. 

Although the gross features of the rearrangements are known, the exact nature of the intermediates has been the subject of much discussion. They are presented above as azacycloheptatrienylidenes (e.g., 161) in analogy with the 

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Before discussing recent studies on the ring expansion reactions of phenylcarbene and phenylnitrene, we will describe some of the earlier experimental and theoretical work on these molecules. Our purpose here is to give a brief overview, in order to provide a context for the discussion of more recent results. For detailed descriptions of the chemistry of arylcarbenes and arylnitrenes, we refer the reader to the many excellent reviews in this area.1,2... 

Similar arguments to those outlined above have been put forward to account for the interconversion of arylcarbenes and arylnitrenes " . ... 

Conclusion the ring expansion of arylcarbenes and arylnitrenes can involve discrete bicycloheptatriene intermediates though not necessarily so. Whether they do or not depends on the stabilities of these intermediates. Bicycloheptatriene instability is no obstacle to ring expansion. The ring expansion of acenaphthyl-carbene > may be cited as a further example. 

As shown in Table 3, triplet lb is computed to be 25-26 kcal/mol lower in enthalpy than triplet lc.77 Table 3 also shows that radicals 8b and 8c, formed by adding a hydrogen atom to lb and lc, respectively, differ in enthalpy by only 1-3 kcal/mol. Therefore, the large enthalpy difference between 3lb and 3lc is not due to a difference between the abilities of the phenyl and pyridyl groups to stabilize an unpaired tt electron. Instead it must reflect an intrinsic enthalpy difference between arylnitrenes and arylcarbenes. Table 3 also shows that aniline (9b) and fl-picoline (9c) are also predicted to have very similar enthalpies, thus providing further evidence that the large enthalpy difference between lb and lc is, indeed, due to the fact that lb is a nitrene, while lc is a carbene. 

Gamer, G.V., Mobbs, D.B., Suschitzky, H., and MiUership, J.S., Syntheses of heterocyclic compounds. Part 24. Cyclisation studies with ort/zo-substituted arylcarbene arylnitrene precursors, J. Chem. Soc. (C), 3693, 1971. 

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