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.

Tetrazolo[1,5-a]quinazoline (9) is converted to 2-azidoquinazoline (10) on sublimation at 200 degrees C and above, and the azide-tetrazole equilibrium is governed by entropy. 2-Quinazolylnitrenes 11 and 27 and/or their ring expansion products 14 and 29 can undergo type I (ylidic) and type II (diradicaloid) ring opening. Argon matrix photolysis of 9/10 affords 2-quinazolylnitrene (11), which has been characterized by ESR, UV, and IR spectroscopy. A minor amount of a second nitrene, formed by rearrangement or ring opening, is also observed. A diradical (19) is formed rapidly by type II ring opening and characterized by ESR spectroscopy; it decays thermally at 15 K with a half-life of ca. 47 min, in agreement with its calculated facile intersystem crossing (19T --> 19OSS) followed by facile cyclization/rearrangement to 1-cyanoindazole (21) (calculated activation barrier 1-2 kcal/mol) and N-cyanoanthranilonitrile (22). 21and 22 are the isolated end products of photolysis. 21 is also the end product of flash vacuum thermolysis. An excellent linear correlation between the zero-field splitting parameter D (cm(-1)) and the spin density rho on the nitrene N calculated at the B3LYP/EPRIII level is reported (R2 = 0.993 for over 100 nitrenes). Matrix photolysis of 3-phenyltetrazolo[1,5-a]quinazoline (25) affords the benzotriazacycloheptatetraene 29, which can be photochemically interconverted with the type I ring opening product 2-isocyano-alpha-diazo-alpha-phenyltoluene (33) as determined by IR and UV spectroscopy. The corresponding carbene 37, obtained by photolysis of 33, was detected by matrix ESR spectroscopy.

Photolysis of triazolo [1,5-b]pyridazine 8 isolated in Ar matrix generates diazomethylpyridazines 9Z and 9E and diazopentenynes 11Z and 11E as detected by IR spectroscopy. ESR spectroscopy detected the 3-pydidazylcarbene 10 as well as pent-2-en-3-yn-1-ylidene 12 formed by loss of one and two molecules of N 2 , respectively. Further photolysis caused rearrangement of the carbenes to 1,2-pentadien-4-yne 13 and 3-ethynylcyclopropene 14. Flash vacuum thermolysis (FVT) of 8 at 400-500°C with Ar matrix isolation of the products yielded 13, 14, and 1,4-pentadiyne 15. At higher temperatures, glutacononitriles 27Z and 27E were formed as well together with minor amounts of 2-and 3-cyanopyrroles 28 and 29. Tetrazolo[1,5-a]pyridine/2-azidopyridine 22T/22A yields 2-pyridylnitrene 19 as well as the novel open-chain cyanodienylnitrene 23 and the ring-expanded 1,3-diazacyclohepta-1,2,4,6-tetraene 21 on short wavelength photolysis. Nitrenes 19 and 23 were detected by ESR spectroscopy, and cumulene 21 by IR and UV spectroscopy. FVT of 22T/22A also affords 2-pyridylnitrene 19 and diazacycloheptatetraene 21, as well as glutacononitriles 27Z,E and 2-and 3-cyanopyrroles 28 and 29. Photolysis of 21 above 300 nm yields the novel spiroazirene 25, identified by its matrix IR spectrum. The reaction pathways connecting the four carbenes (10Z,E and 12Z,E) and three nitrenes (19, 23EZ, and 23ZZ) in their open-shell singlet and triplet states are elucidated with the aid of theoretical calculations at DFT, CASSCF, and CASPT2 levels. Three possible mechanisms of ring-contraction in arylnitrenes are identified: (i) via ring-opening to dienylnitrenes, (ii) concerted ring-contraction, and (iii) via spiroazirenes 25, whereby (i) is the energetically most favorable.

Flash vacuum thermolysis (FVT) of 9-azidophenanthrene 8, 6-(5-tetrazolyl)phenanthridine 18, and [1,2,3]triazolo[1,5-f]phenanthridine 19 yields 9-cyanofluorene 12 as the principal product and 4-cyanofluorene as a minor product. In all cases, when the product is condensed at or below 77 K, the seven-membered ring ketenimine 24 is detectable by IR spectroscopy (1932 cm(-1)) up to 200 K. Photolysis of Ar matrix isolated 8 at lambda = 308 or 313 nm generates at first the azirine 26, rapidly followed by the ylidic cumulene 27. The latter reverts to azirine 26 at lambda > 405 nm, and the azirine reverts to the ylidic cumulene at 313 nm. Nitrene 9 is observed by ESR spectroscopy following FVT of either azide 8, tetrazole 18, or triazole 19 with Ar matrix isolation of the products. Nitrene 9 and carbene 21 are observed by ESR spectroscopy in the Ar matrix photolyses of azide 8 and triazole 19, respectively.

Photolysis of 3-pyridyldiazomethane in an Ar matrix at 7-10 K gives 3-pyridylcarbene. Further photolysis causes ring opening to nitrile ylide 26 (formonitrile pent-2-en-4-ynylide) as the major reaction together with a minor amount of ring expansion to 1-azacyclohepta-1,3,4,6-tetraene, 27. Matrix photolysis of 3-azidopyridine leads to ring opening to formonitrile N-cyanovinylmethylide, 33.