Evidence for vinyl nitrene intermediates in the thermal rearrangement of 2H-azirines into indoles

Fe(II)-Catalyzed Amination of Aromatic C−H Bonds via Ring Opening of 2H-Azirines: Synthesis of 2,3-Disubstituted Indoles

“…However, their use as the vinyl nitrene precursor is often limited by their narrow substrate scope as they are generally prepared by the condensation of methyl azidoacetate and aromatic aldehydes 5c. Thermal rearrangement of 2-aryl-2 H -azirines via vinyl nitrene intermediates6a provides an efficient route to indoles, although it remains in scattered use in indole syntheses (Scheme 1). 6 This rearrangement could be catalyzed by Pd(PhCN) 2 Cl 2 7a or Rh 2 [OC(O)CF 3 ] 4 7b.…”

“…Thermal rearrangement of 2-aryl-2 H -azirines via vinyl nitrene intermediates6a provides an efficient route to indoles, although it remains in scattered use in indole syntheses (Scheme 1). 6 This rearrangement could be catalyzed by Pd(PhCN) 2 Cl 2 7a or Rh 2 [OC(O)CF 3 ] 4 7b. The catalytic variant of the rearrangement has also received little attention from the chemistry community and only two reports have been published to date 7.…”

“…2 H -Azirines can be readily prepared from ketones using modified Neber rearrangement processes via hydrazones in three steps developed by Padwa7b,10 or oximes in two steps developed by Taber6b (Scheme 2). Azirine 1a 6b,7 was chosen as the model substrate to study the rearrangement of 2-aryl- 2H -azirines to indoles.…”

Experimental and Theoretical Characterization of the Aromatization, Epimerization, and Fragmentation Reactions of Bi‐2H‐azirin‐2‐yls Prepared from 1,4‐Diazidobuta‐1,3‐dienes

“…Thus, the thermal load of optically active 2H-azirnes, for example, during distillation, should be avoided to exclude undesirable racemization, which occurs most probably via vinyl nitrenes as short-lived intermediates. [22] Quantum chemical calculations: To investigate the mechanism of the twofold elimination reaction, by starting from biazirinyl rac-22 and leading to the alkyne 24 and two molecules of acetonitrile, quantum chemical calculations were performed. At first, DFT-calculations by using the (U)B3LYP/6-31 + G(d) method as implemented in the GAUSSIAN 09 program package [23] were performed to localize the appropriate structures on the energy hypersurface.…”

Visible‐Light Sensitization of Vinyl Azides by Transition‐Metal Photocatalysis

“…We assigned this intermediate as azirine 4 based upon analogy to the photochemical production of azirines by UV irradiation of β-styrenyl azides. 18 The identity of azirine 4 was confirmed by adding 1 equiv of cyclopentadiene to a photosensitized reaction of 1 , which produced the hetero-Diels–Alder cycloadduct expected from interception of the highly reactive C=N moiety in 71% isolated yield and as a single diastereomer. Notably, this trapping experiment, too, is markedly more efficient under photocatalytic conditions; the same experiment conducted by direct UV photolysis at 254 nm produced only 40% of the trapped hetero-Diels–Alder cycloadduct along with numerous inseparable decomposition products.…”