Thermolysis and Photosensitized Oxygenation of Tetrasubstituted Cyclopropenes

Article abstract of DOI:10.1021/jo991803b

Bicyclic cyclopropenes 14a, 14b, and 26 were prepared by various synthetic routes. Polymer rose Bengal (p-RB) photosensitized oxygenation of bicyclooctenes 14a,b in CDCl₃ proceeded sluggishly (variable O₂ 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 yielded both dienes (Z- and E-29, 30, and 31) and enones (E- and Z-12, 32, 34). By contrast, p-RB photosensitized oxidation of bicyclononene 26 in CDCl₃ proceeded somewhat more rapidly (O₂ uptake of ca. 1 equiv in 2.5 h) yielding enones (20, 42-45) exclusively upon GC separation. The diene products, observed in the case of 14, result from the thermolysis of the remaining unreacted cyclopropenes, while the enones are the oxygenation products. The oxygenation was slowed by radical inhibitors, but not by ¹O₂ quenchers; nor were any oxidation products observed when these cyclopropenes were reacted with triphenylphosphine ozonide, a chemical ¹O₂ source. The data indicates that a photosensitizer-initiated free radical autoxidative process is involved. Likely intermediates in this oxygenation are epoxide 27 or 37 and hydroperoxide 28 or 38, for the bicyclooctene (14) and bicyclononene (26) systems, respectively. The absence of ¹O₂ product in these cyclopropene systems, in contradistinction to their higher homologues, may be attributable to either the relatively long C_α-H_allylic distance in alkylcyclopropenes, which places the abstractable allylic hydrogen "out of reach", or their relatively high IP. 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 α-keto hydroperoxides 51/54 and hemiperketals 52/55 (the cyclic form of β-keto hydroperoxides 53/56). Phosphine reduction of these peroxides yields the corresponding alcohols 33/43 and 32/42.