PHOTO-CHEMICAL TRANSFORMATIONS OF SMALL RING COMPOUNDS .109. PHOTO-CHEMICAL MIGRATORY APTITUDES IN THE DI-PI-METHANE REARRANGEMENT OF 5,5-DIARYL-SUBSTITUTED 2,5-DIHYDROFURANS

Irradiation of 3, 3-dimethyl-4, 5, 5-triaryl-substituted 2, 5-dihydrofurans in benzene results in a novel rearrangement to give 3-penten-l-one derivatives. Triplet sensitization leads to the formation of a 2-oxabicyclo[2.1.0]pentane intermediate which is extremely sensitive to heat and oxygen. Thermolysis of the labile 2-oxabicyclopentane results in central bond scission followed by a subsequent fragmentation to give 3-penten-l-one derivatives. The initially generated diradical intermediate can be readily trapped with oxygen to give a trioxabicyclo[2.2.1]heptane ring. The mechanism involved in these reactions consists of a di-zr-methane rearrangement. Competitive migratory aptitude studies indicate that the phenyl group migrates in slight preference to p-anisyl, p-cyanophenyl, and p-tolyl. The products of migration are basically independent of the multiplicity of the excited state. The results obtained indicate that migratory aptitudes in the excited state can be controlled by the reluctance of phenyl groups to remain behind rather than from an inherent tendency of the substituted aryl group to migrate. © 1979, American Chemical Society. All rights reserved.