Phenylnitrene pyridylcarbene interconversions

Another example of the reversibility of the ring expansion — one of special relevance to this chapter — is found in the interconversion of the isomeric pyridylcarbenes, and their conversion to phenylnitrene (Scheme 4).10,11... 

However, from elegant labelling studies by Wentrup and coworkers on the gas-phase thermal interconversion of phenylnitrene and 2-pyridylcarbene, it became evident that an alternative and equally viable intermediate in these nitrene ring expansions is the azacycloheptatrienylidene (216) or its tautomer, the cumulated system (217) (81AHC(28)23l). 

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