Photoinduzierte Cycloadditionen von aliphatischen 2H‐Azirinen. 37. Mitteilung über Photoreaktionen

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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 (65%) 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, λmax 239 nm, ϵ 240) at −196° with light of wavelength 245 nm in a hydrocarbonglass gives rise to a pronounced maximum at 280 nm, for which an ϵ 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.