Zum photochemischen Verhalten von Sydnonen und 1,3,4‐Oxadiazolin‐2‐onen. 56. Mitteilung über Photoreaktionen

On the Photochemistry of Sydnones and 1,2,3,4‐Oxadiazolin‐2‐ones The photochemical behaviour of several 3,4‐disubstituted sydnones ( cf. Scheme 5 ) dioxane solution was investigated. The pure sydnones give, as was already reported [2–6], 2,4,5‐trisubstituted 1,2,3‐triazoles 2 ( cf. Scheme 1 ) in 25–30% yield. In the presence of dipolarophiles ( cf. Scheme 1 ) pyrazole derivatives 3 or 4 are formed which can be taken as a proof for the formation of nitril‐imines 5 as primary products in sydnone photochemistry. Since irradiation of 2‐[ 15 N]‐3, 4‐diphenyl‐sydnone (2‐[ 15 N]‐ 15 ) in dioxane leads to the formation of 1,3‐[ 15 N]‐2,4,5‐triphenyl‐1,2,3‐triazole (1,3‐[ 15 N] 22 ; cf. Scheme 10 ), nitril‐imine formation must be induced by the creation of a bond between N(2) und C(4) in the excited sydnones ( cf. Scheme 22 ). The irradiation of sydnones in dioxane solution in the presence of carboxylic acids yields N ′‐acylhydrazides in 50–70% ( cf. Scheme 14 ), the formation of which can be explained by addition of the acids to the nitril‐imines and rearrangement of the primarily formed anhydride monohydrazones ( Scheme 15 ). By analogy, the formation of 1‐benzoyl‐2‐( t ‐butyl)‐4‐phenyl‐1,2‐diazetidin‐3‐one ( 14 ; Scheme 4 ) during the photolysis of 3‐( t ‐butyl)‐4‐phenylsydnone ( 12 ) in benzene solution ( cf. [5b]) may also be explained: Sydnone 12 undergoes two different photoreactions leading by loss of carbon dioxide to the corresponding nitril‐imine 5 (R=t‐C 4 H 9 , R′ C6H5) and by loss of isobutylene to a 1,2,3‐oxadiazolin‐5‐one of type 48 or 49 which isomerizes to yield diazophenylacetic acid (51; cf. Scheme 17). Reaction of both products (5 and 51) results in the formation of the N′‐acylhydrazide 52 ( Scheme 18) which may cyclize after loss of nitrogen to yield the diazetidinone 14 in a carbene type reaction. The triazoles 2 are formed photochemically from the corresponding 1,2‐bisazo‐ethylenes 64 ( Scheme 26) which arise from a direct ‘head‐to‐head’ dimerization of the nitril‐imines 5. This type of reaction seems to be common to all nitrilium betaines (Schema 30). The photolysis of the 2,4‐disubstituted 1,3,4‐oxadiazolines 76 does not lead to nitril‐imines 5 ( cf. [11]). On the contrary, loss of carbon monoxide induces the formation of azoketones 77 ( cf Scheme 31), which may be photo‐reduced to yield hydrazides or may undergo cleavage of the N, acyl bond to form derivatives of1,2‐diketones ( cf. Scheme 32).