Phenylnitrene, Phenylcarbene, and Pyridylcarbenes. Rearrangements to Cyanocyclopentadiene and Fulvenallene

Abstract

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.

Photolysis and Pyrolysis of Phenyltetrazoles: Formation of Phenylcarbodiimide, N‐Phenylnitrile Imine, Phenylnitrene, Indazole, and Fulvenallene

“…1929, 1488, and 886 cm –1 ) . Under our pyrolysis conditions, the yields of rearrangement of fulvenallene to ethynylcyclopentadienes 19 are very low,, and in fact, no bands ascribable to 19 are observable in Figures and S9.…”

Flash‐Vakuumpyrolyse – Techniken und Reaktionen

“…[57,58] Fulvenallen 19 wird als Umlagerungsprodukt von Phenylcarben erhalten, wenngleich nicht in einer direkten Reaktion. [57,58] Fulvenallen 19 wird als Umlagerungsprodukt von Phenylcarben erhalten, wenngleich nicht in einer direkten Reaktion.…”

“…600 8 8C [Gl. [57,58] Tr iplett-Phenylnitren dimerisiert zu Azobenzol und abstrahiert Wasserstoff von anderen Molekülen mit Bildung von Anilin. [59] Die Pyrolysechemie von Fulvenallen ist in Bezug auf Verbrennungsprozesse und PA H-Wachstum von aktuellem Interesse.…”

“…(10)]. [57,58] Interessanterweise wurde Cyancyclopentadien auch aus dem Spiroaziren 23 erhalten, das seinerseits durch Photolyse und Thermolyse von 6-Azidofulven (22)h ergestellt worden war. [62] Cyancyclopentadien (21)w urde von Crow und Wentrup [63] und nochmals von Hedaya et al [64] als ein Umlagerungsprodukt von Singulett-Phenylnitren erhalten.…”

Gold(III) Triggered Transformations of 22‐Methyl‐m‐benziporphyrin Involving an Effective Contraction of Benzene to Cyclopentadiene

“…For instance, the close proximity of C−H or C−C bonds of p ‐benziporphyrin 1 , or m ‐benziporphyrin 2 , to the metal ion enforces an unusual coordination geometry and eventually prompts unique reactivity . In fact, the contraction of benzene is part of an exclusive group of reactions in which the cleavage of aromatic structure is of fundamental importance . Thus, the reported metal‐initiated contractions of p ‐benziporphyrin 1 and m‐ benziporphyrin 2 to 21‐carbaporphyrin complexes provide the illustrative examples of such structural transformations (Scheme ) …”