Photoinduzierte Cycloadditionen von aliphatischen 2H-Azirinen

Abstract

The purely aliphatic 2,3-dipropyl-2H-azirine (1) reacts on irradiation with a mercury high-pressure lamp through a Vycor filter with methyl trifluoroacetate or acetone to form 3-oxazolines 3a,b (6.5%) resp. 4 (14%) (Scheme 1). 9-Azabicyclo[6.1.0]non-1(9)-ene (5) on irradiation in the presence of the dipolarophiles methyl trifluoroacetate, methyl difluoroacetate, 1,1,1-trifluoro-propanone and acetone behaves in a similar way, whereby the corresponding bicyclic 3-oxazolines 7-10 result in yields of 60-20% (Scheme 2). By analogy with the photochemical behaviour of 3-aryl-2H-azirines it is assumed that nitrile-ylides 2 resp. 6 represent intermediates. In fact irradiation of 2,3-dipropyl-2H-azirine (1, lambda (max) 239 nm, epsilon = 240) at -196° with light of wavelength 245 nm in a hydrocarbonglass gives rise to a pronounced maximum at 280 nm, for which an epsilon of >15000 can be estimated. The quantum yield for the formation of nitrile-methylide 2 is 0,8. Irradiation of the dipole 2 at -196° or warming to -150° causes the maximum at 280 nm to disappear.