The fascinating carbene‐nitrene rearrangement can be observed directly by ESR spectroscopy. In the thermal decomposition of the carbene precursor 4 (via 3) with subsequent matrix isolation, the product gave the same ESR signal as phenylnitrene 2 from phenylazide 1! In the aza‐analogous system, the seven‐membered carbene 5 is probably formed as intermediate of the rearrangemet.

Flash vacuum thermolysis (FVT) of phenyl azide 29 as well as precursors of 2-pyridylcarbene 34 and 4-pyridylcarbene 25 affords phenylnitrene 30 (labeled or unlabeled), as revealed by matrix isolation electron spin resonance spectroscopy. FVT of 1-(13)C-phenyl azide 29 affords 1-cyanocyclopentadiene (cpCN) 32, which is exclusively labeled on the CN carbon, thus demonstrating direct ring contraction in phenylnitrene 30 without the intervention of cycloperambulation and 1,3-H shifts. However, the cpCN obtained by rearrangement of pyridyl-2-((13)C-carbene) 34 carries (13)C label on all carbon atoms, including the CN carbon. Calculations at the B3LYP/6-31G* level and in part at the CASSCF/6-31G* and CASPT2/cc-pVDZ//CASSCF(8,8)/cc-pVDZ levels support a new mechanism whereby 2-pyridylcarbene rearranges in part via 1-azacyclohepta-1,2,4,6-tetraene 36 to phenylnitrene, which then undergoes direct ring contraction to cpCN. Another portion of 2-pyridylcarbene undergoes ring expansion to 4-azacyclohepta-1,2,4,6-tetraene 42, which then by trans-annular cyclization affords 6-azabicyclo[3.2.0]cyclohepta-1,3,5-triene 43. Further rearrangement of 43 via the spiroazirine 44 and biradical/vinylnitrene 45 affords cpCN with the label on the CN group. An analogous mechanisms accounts for the labeling pattern in fulvenallene 60 formed by ring contraction of 1-(13)C-phenylcarbene 59 in the FVT of 1-(13)C-phenyldiazomethane 58.

Flash vacuum pyrolysis (FVP) of 1-(5-(13)C-5-tetrazolyl)isoquinoline 18 generates 1-((13)C-diazomethyl)isoquinoline 19 and 1-isoquinolyl-((13)C-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 (13)C label distribution in the cyanoindenes was determined by (13)C 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 (13)C 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.

XI SO. ( 2 0 m L ) was condensed at -190°C into a pressure flask containing l.Og (6.8 mmol) of 1 and I.0g (3.4mmol) of 3. The colorless solution was allowed to warm up in the dark to room temperature and stirred for 18 h. The moderately soluble precipitate that formed was filtered off and the soI\rnt was removed from the filtrate in vacuum. The residue was recrystallized from SO?, affording 1.6 g (79%) of 4 as pale yellow crystals. m.p = I15 'C (decomposition at 125'C).-IR (nujol): v=2175, 700, 575,498,464, and 395 cm-'.-Raman: v=2175,683, 658,498,460,430, and 206 cm '[9] Freshly prepared 2 (1.56 g, 8.7 mmol), which had been dried in a vacuum d t -30°C. was heated rapidly to the melting point (-3°C) and then added under a stream of nitrogen to 3 (1.29 g, 4.3 mmol) in a pressure ilask at -190°C. In the evacuated flask, 20 m L of SOz was condensed and the stirred solution was then slowly warmed in a cold bath ( -50°C') in the dark over 4 h to room temperature. After two hours of stirring. the sparingly soluble solid was filtered off and the solvent removed from the filtrate. Orange crystals of 5 were obtained by recrystallizing the residue from SO?. Yield: 1.9 g (67%). m.p.=92'C (decomposition dl IlO"C)-lR (nujol): v=2165, 705, 692, 685, 465, and 395 cm ~ : v=2165, 678, 506, 438,420, 401, 223, 189, 180, 127, 93, and 78 cm . '. Raman

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.