Rearrangements of 4‑Quinolylcarbene, 3‑Quinolylcarbene, and 2‑Quinolylcarbene to 1-Naphthylnitrene and Cyanoindenes by Falling Solid Flash Vacuum Pyrolysis

The relationship between 4-quinolylcarbene 17, 3-quinolylcarbene 21, 2-quinolylcarbene 25, and 1-naphthylnitrene 35 has been explored experimentally and computationally. The diazomethylquinolines generated from (5-tetrazolyl)­quinolines or 1,2,3-triazolo­[1,5-a]­quinoline by conventional flash vacuum pyrolysis (FVP) were observed by IR spectroscopy. The carbenes were generated by falling solid flash vacuum pyrolysis (FS-FVP). 4-Quinolylcarbene 17 was found to rearrange to 3-quinolylcarbene 21 and then to 2-quinolylcarbene 25, and finally via 1-naphthylnitrene 35 to 1-cyanoindene 36, which then isomerizes to 3- and 2-cyanoindenes 12 and 13. The thermal rearrangement of 2-quinolylcarbene to 1-naphthylnitrene was verified by ESR spectroscopy. The reaction mechanism has been elucidated with the help of calculations of the structures and energies of the quinolylcarbenes and 1-naphthylnitrene and the intervening aza-benzobicyclo[4.1.0]­heptatrienes, aza-benzocycloheptatetraenes, and aza-benzocycloheptatrienylidenes and the transition states connecting them at the B3LYP/6-31G* level. The nonobserved 1,2-hydrogen shifts in aza-benzocycloheptatetraenes/aza-benzocycloheptatrienylidenes are found to have very high activation barriers.