2-Pyridylnitrene and 3-Pyridazylcarbene and Their Relationship via Ring-Expansion, Ring-Opening, Ring-Contraction, and Fragmentation

Abstract

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.

Chemical Activation in Azide and Nitrene Chemistry: Methyl Azide, Phenyl Azide, Naphthyl Azides, Pyridyl Azides, Benzotriazoles, and Triazolopyridines

“…Depending on the route and the method of calculation (B3LYP and CASPT2), CA will be present to the extent of at least 52 kcal mol À1 in the initially formed 2-cyano-2H-pyrrole 33, or ,74 kcal mol À1 in the 2-cyanopyrrole 31 itself (Scheme 9). [50] Not surprisingly therefore, the principal reaction product is again a mixture of 2-and 3-cyanopyrroles, even under the mildest possible conditions (400-5008C/10 À3 hPa) (Scheme 9). [50,54] 6-Nitrotetrazolo[1, 5-…”

“…[50] Not surprisingly therefore, the principal reaction product is again a mixture of 2-and 3-cyanopyrroles, even under the mildest possible conditions (400-5008C/10 À3 hPa) (Scheme 9). [50,54] 6-Nitrotetrazolo[1, 5-…”

“…4 and Scheme 5). [50] FVT of 2-(5-tetrazolyl)pyrimidine 34 also affords a mixture of 2-and 3-cyanopyrroles (Scheme 6). Similarly, the tetrazoles 35 and 36 afford mixtures of three principal dimethylcyanopyrroles 37]39 (Scheme 6).…”

“…Three different routes to cyanopyrroles from 2-pyridylnitrene 53 have been identified, [50] whereby the overall lowest energy path proceeds via ring opening to the biradical/vinylnitrene 56 (Scheme 9). Depending on the route and the method of calculation (B3LYP and CASPT2), CA will be present to the extent of at least 52 kcal mol À1 in the initially formed 2-cyano-2H-pyrrole 33, or ,74 kcal mol À1 in the 2-cyanopyrrole 31 itself (Scheme 9).…”

“…Sigmatropic shifts of H and CN interconverting the cyanopyrroles (energies in kcal mol À1 at the B3LYP/6-31G* level). [50] Cyanopyrroles from tetrazole precursors of pyrimidinylcarbenes. [45] 4-nitro-2,5-dimethylisoxazole in N 2 flow at 4008C (0.1 hPa) afforded 1-cyano-1-nitroacetone in quantitative yield, i.e.…”