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

Acid‐Catalysed Formation of Imidazoles from 2 H ‐Azirines or Vinylazides and Nitriles The reaction of 2 H ‐azirines with nitriles in the presence of boron trifluoride etherate to yield the corresponding imidazoles is described. 2,3‐Diphenyl‐2 H ‐azirine ( 10 ) gives 2‐substituted 4,5‐diphenyl imidazoles in moderate bis good yields (see Table 1 ). The reaction of 10 with 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. 2 H ‐Azirine 10 and ethyl cyanoacetate yield the expected imidazole 18 (30%) but also 2‐cyanomethyl‐4,5‐diphenyloxazole ( 20 ; 7%) (see Scheme 4 ). The yield of imidazole formation mainly depends on the substituents in position 2 of the 2 H ‐azirines (see Scheme 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 2 H ‐azirines. These observations are in agreement with the occurrence of 1‐azaallyl cations formed by ring opening of the 2 H ‐azirines linked to the Lewis acid (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‐2 H ‐azirine ( 24 ) gives 2,4‐diphenylimidazole ( 33 ; Scheme 6 ) in the presence of benzonitrile and boron trifluoride etherate, the azide 28 yields only acetanilide (86%). In the presence of triethyloxonium tetrafluoroborate 2 H ‐azirines and benzonitrile react to yield the corresponding 1‐ethylimidazoles (see Scheme 9 ). This again demonstrates that 1‐azaallyl cations must be intermediates which react with the nitrile presumably in a Ritter type reaction. 13 C‐NMR. spectra of 2 H ‐azirines are also reported (Table 2) .