Thermolysis and photosensitized oxygenation of tetrasubstituted cyclopropenes

Bicyclic cyclopropenes 14a, 14b, and 26 were prepared by various synthetic routes. Polymer rose Bengal (p-RB) photosensitized oxygenation of bicyclooc tenes 14a,b in CDCl3 proceeded sluggishly (variable O-2 uptake of ca. 0.35- 0.75 equiv in 8 h) and was accompanied by sensitizer bleaching. Preparative gas chromatography of the complex product mixtures from 14a and 14b yielde d both dienes (Z- and E-29, 30, and 31) and enones (E- and Z-12, 32, 34). B y contrast, p-RB photosensitized oxidation of bicyclononene 26 in CDCl3 pro ceeded somewhat more rapidly (O-2 uptake of ca. I equiv in 2.5 h) yielding enones (20, 42-45) exclusively upon GC separation. The diene products, obse rved in the case of 14, result from the thermolysis of the remaining unreac ted cyclopropenes, while the enones are the oxygenation products. The oxyge nation was slowed by radical inhibitors, but not by O-1(2) quenchers; nor w ere any oxidation products observed when these cyclopropenes were reacted w ith triphenylphosphine ozonide, a chemical O-1(2) source. The data indicate s that a photosensitizer-initiated free radical autoxidative process is inv olved. Likely intermediates in this oxygenation are epoxide 27 or 37 and hy droperoxide 28 or 38, for the bicyclooctene (14) and bicyclononene (26) sys tems, respectively. The absence of O-1(2) product in these cyclopropene sys tems, in contradistinction to their higher homologues, may be attributable to either the relatively long C-alpha-H-allylic distance in alkylcycloprope nes, which places the abstractable allylic hydrogen "out of reach", or thei r relatively high IF. Either, or both, of these factors may have slowed the rate of the singlet oxygenation of the cyclopropenes to a point where free radical processes compete favorably. In the course of this study,. we also explored the singlet oxygenation (DABCO inhibited) of enones 12a,b and 20. These generated, respectively, a mixture of peroxides identified as alpha- keto hydroperoxides 51/54 and hemiperketals 52/55 (the cyclic form of beta- keto hydroperoxides 53/58). Phosphine reduction of these peroxides yields t he corresponding alcohols 33/43 and 32/42.