Photogeneration and thermogeneration of carbonyl ylides: 1,3-dipolar cycloadditions of 2,3-diaryloxiranes to dipolarophiles

Abstract

A series of symmetrically substituted 2,3-diaryloxiranes 1 - 4 (Fig. 1) has been studied as photoprecursors for carbonyl ylides [l - 31. The stereochemistry of the adducts obtained on the interception of these 4rzn transient system with a variety of dipolarophiles provides information on the mode(s) of electrocyclic opening of the oxiranes to carbonyl ylides as well as on the mechanism of the 4n + 2 cycloaddition processes. While lb undergoes photoinduced ring opening, for example, in the expected disrotatory process, the ~rurzs counterpart, as well as 3a and 3b, behaves anomalously. The stereochemistry of the dipolarophiles in general is preserved in the cycloadducts; this is consistent with a concerted addition process. However, solvent effects, steric hindrance and possibly secondary orbital overlap factors may all play a role in determining the product distribution. The dipolar species generated photochemically from tram- and &s-2,3bis(2-naphthyl) oxiranes (3a and 3b), unlike those highest occupied molecular orbital ylides derived from the isomeric stilbene oxides, were found to add to electron-rich as well as to electron-deficient alkenes. The cycloaddition reactions of 3 were identified as ground state processes which entail the initial formation of a triplet excited state intermediate. The isomeric bis(2-naphthyl) oxiranes 3 and stilbene oxides 4 also undergo thermally induced cycloadditions. Thermal additions involving such simple unsubstituted oxiranes represent reactions heretofore unreported. In these conversions, only the frans oxiranes behave in a manner consistent with the principles of orbital topology, and we conclude that thermal equilibration of the ylides must precede cycloaddition. A comparison of the adduct ratios obtained by electron transfer sensitization with dicyanonaphthdene (DCN) using oxiranes 1 and 4 as substrates with the adduct ratios obtained on direct irradiation and thermolysis was conducted [4]. It is evident from such comparisons that regardless of the manner in which G-c bond cleavage is induced a common intermediate, presumably a ground state ylide, is produced before the ensuing interception. It is noteworthy that oxiranes 5 and 6 bearing electron-deficient substituents such as cyano and carbomethyoxy groups on the three-membered ring undergo direct and energy-transfer-induced photoisomerization. In contrast these substrates proved to be unreactive toward electron transfer photoisomerization using