Interconversion of Nitrenes, Azirenes, and Diradicals: Rearrangement of 3-Isoquinolylnitrene to o-Cyanophenylketenimine and 1-Cyanoisoindole

Photolysis of tetrazolo[1,5-b]isoquinoline/3-azidoisoquinoline 22T/22A generates 3-isoquinolylnitrene 23, which has been characterized together with a diradical species (25) by Ar matrix ESR spectroscopy. Photolysis at λ > 300 nm generates azirene 24, characterized by IR spectroscopy, whereas further broad-band UV photolysis destroys the azirene to produce o-cyanophenylketenimine 17. The use of ¹⁵N-labeled tetrazole/azide 22T′/22A′ demonstrates rapid equilibration of two regioisomeric ¹⁵N-labeled azirenes 24′ and 24′′ prior to formation of 17. Flash vacuum thermolysis (FVT) of 22T/22A affords 1-cyano-2H-isoindole 27 in quantitative yield. FVT of ¹⁵N-labeled tetrazole/azide 22T′/22A′ causes scrambling of ¹⁵N label in the 1-cyano-2H-isoindole product. It is concluded that the interconversion of azirenes 24 takes place via the unobserved diazacycloheptatetraene/diazacycloheptatrienylidene 32/33, and that the rearrangement of azirene to ketenimine 17 and 1-cyanoisoindole 27 takes place via reversion to nitrene 23 followed by ring opening to diradical 25.