Ring Contraction in Arylcarbenes and Arylnitrenes; Rearrangements of 1- and 3‑Isoquinolylcarbenes and 2‑Naphthylnitrene to Cyanoindenes

Flash vacuum pyrolysis (FVP) of 1-(5-13C-5-tetrazolyl)­isoquinoline 18 generates 1-(13C-diazomethyl)­isoquinoline 19 and 1-isoquinolyl-(13C-carbene) 22, which undergoes carbene–nitrene rearrangement to 2-naphthylnitrene 23. The thermally generated nitrene 23 is observed directly by matrix-isolation ESR spectroscopy, but undergoes ring contraction to a mixture of 3- and 2-cyanoindenes 26 and 27 under the FVP conditions. The 13C label distribution in the cyanoindenes was determined by 13C NMR spectroscopy and indicates the occurrence of two parallel paths of ring contraction starting from 1-isoquinolylcarbene; path a via ring expansion to 3-aza-benzo­[c]­cyclohepta-1,2,4,6-tetraene 32 bifurcating to 2-naphthylnitrene 23 and 2-aza-benzobicyclo[3.2.0]­heptatriene 39 (paths a1 and a2); and path b via ring closure of the carbene onto the ring nitrogen, yielding 1-aza-benzo­[d]­bicyclo­[4.1.0]­hepta-2,4,6-triene 34 and 3-aza-benzo­[d]­cyclohepta-2,3,5,7-tetraene 35. Product studies demand that the major path is route a1 via 2-naphthylnitrene 23, which then undergoes direct ring contraction to 1-cyanoindene; but the 13C label distribution requires that the non-nitrene route b contributes significantly. The two reaction paths are modeled at the B3LYP/6-31G* level. The initially formed carbene 22 is estimated to carry chemical activation of some 40 kcal/mol. This allows both reaction channels to proceed simultaneously under low-pressure FVP conditions. FVP of 3-(5-tetrazolyl)­isoquinoline 28 similarly generates 3-diazomethylisoquinoline 29 and 3-isoquinolylcarbene 30, which rearranges to 3- and 2-cyanoindenes 26 and 27.