Allenyl(vinyl)methane Photochemistry. Photochemistry of Methyl 4,4-Dimethyl-2,5,6-heptatrienoate Derivatives

Both the direct and sensitized photolyses of [2-(1,2-propadienyl)-substituted alkylidene]malonates gave mainly intramolecular [2+2] cycloadducts. On the other hand, the direct irradiation of methyl (E)-4,4-dimethyl-2,5,6-heptatrienoate (3a) competitively underwent E–Z geometric isomerization and intramolecular [2+2] cycloaddition, though prolonged irradiation led to [2+2] cycloadducts as main products. For methyl (E)-4,4,7-trimethyl-2,5,6-octatrienoate (3b), the intramolecular [2+2] cycloaddition occurred under both direct and sensitized irradiation and the E–Z geometric isomerization scarcely occurred. In these allenyl(vinyl)methanes under direct irradiation, the 1,2-propadienyl-substituted esters gave a 1,2-propadienylcyclopropane or (2-ethenylidenecyclopropyl)acetate as a minor product. When 3b was irradiated in the presence of hydrogen selenide in toluene, cyclopentenes were formed. The formation of the cyclopentenes indicated that the biradical intermediate is a cyclopentane-1,3-diyl radical. By MO calculations of the model compounds for the chromophores of the esters, it is suggested that the preferential initial bond for the 1,2-butadienyl- or 3-methyl-1,2-butadienyl-substituted esters is between the C6 (allene center carbon) and C2 (α-carbon), while 1,2-propadienyl-substituted esters can also be bonded between the C5 and C3 (β-carbon).