A STUDY OF THE COMPETITION BETWEEN THE DI-PI-METHANE AND THE AZADI-PI-METHANE PROCESSES IN 2-VINYL-BETA,GAMMA-UNSATURATED OXIME DERIVATIVES - THE NOVEL AZADI-PI-METHANE REACTIVITY OF GAMMA-UNSATURATED OXIMES

A study aimed at detecting intramolecular competition between the di-pi-methane (DPM) rear-rangement and the azadi-pi-methane (ADPM) process has been carried out. The results show that direct or acetophenone-sensitized irradiation of 2-(2,2-diphenylvinyl)-2-methyl-4,4-diphenyl-3- butenal oxime acetate 3 and the corresponding oxime trifluoroacetate 11 undergo only the DPM process to yield cyclopropanes. Similar DPM rearrangement was observed for 2-methyl-4,4-diphenyl-2-vinyl-3-butenonitrile 22. These are examples of triplet DPM reactivity in acyclic substrates where the central carbon has only one electron-withdrawing group. There is only one case of such reactivity in the aryl di-pi-methane process. However, 2-methyl-4,4-diphenyl-2-vinyl-2-vinyl-3-butenal oxime acetate 26 and the corresponding trifluoroacetate 15 undergo the ADPM rearrangement exclusively on direct or acetophenone-sensitized irradiation. The selectivity observed is interpreted as being dependent on the relative stabilities of the 1,4-bridged biradical intermediates. Based on previous failures to observe the ADPM reactions of beta,gamma-unsaturated oximes, an attempt to suppress the ADPM reactivity in compounds with a substitution pattern such as that present in 15 and 26 was made using the parent oxime 21. Surprisingly this compound reacts efficiently by the ADPM process and affords cyclopropane 30. This is the first example of ADPM reactivity of an acyclic beta,gamma-unsaturated oxime. Ah the rearrangements described are stereoselective. This reaction was extended to other oximes 45, 47a and 47b and also to the oxime ether 36. The photoreaction of 47b shows that the ADPM rearrangement of oximes can be extended to ketone derivatives. Previous studies have shown that such reactivity is uncommon. beta,gamma-Unsaturated oximes are usually considered to be photochemically inert but these results have shown that changes in substitution can promote efficient ADPM reactivity.