Zur Imidazolbildung aus 2H‐Azirinen bzw. Vinylaziden und Nitrilen unter Säurekatalyse

Acid‐Catalysed Formation of Imidazoles from 2H‐Azirines or Vinylazides and Nitriles The reaction of 2H‐azirines with nitriles in the presence of boron trifluoride etherate to yield the corresponding imidazoles is described. 2,3‐Diphenyl‐2H‐azirine (10) gives 2‐substituted 4,5‐diphenyl imidazoles in moderate bis good yields (seeTable 1). The reaction of10with acrylonitrile only leads to the formation of 4,5‐diphenyl‐2‐vinylimidazole (17). No products resulting from an addition to the C,C double bond are observed. 2H‐Azirine10and ethyl cyanoacetate yield the expected imidazole18(30%) but also 2‐cyanomethyl‐4,5‐diphenyloxazole (20; 7%) (seeScheme 4). The yield of imidazole formation mainly depends on the substituents in position 2 of the 2H‐azirines (seeScheme 6), a change of the substitutents in position 3 having only little influence. The best yields are observed with a phenyl group at C(2) of the 2H‐azirines. These observations are in agreement with the occurrence of 1‐azaallyl cations formed by ring opening of the 2H‐azirines linked to theLewisacid (boron trifluoride). Similar results are obtained with the corresponding vinyl azides with the exception of 1‐azido‐1‐phenylethylene (28). Whereas the corresponding 3‐phenyl‐2H‐azirine (24) gives 2,4‐diphenylimidazole (33;Scheme 6) in the presence of benzonitrile and boron trifluoride etherate, the azide28yields only acetanilide (86%). In the presence of triethyloxonium tetrafluoroborate 2H‐azirines and benzonitrile react to yield the corresponding 1‐ethylimidazoles (seeScheme 9). This again demonstrates that 1‐azaallyl cations must be intermediates which react with the nitrile presumably in aRittertype reaction.13C‐NMR. spectra of 2H‐azirines are also reported(Table 2).