Photochemistry of Alkenes. 6. Direct Irradiation of 1,5-Hexadienes: and Sigmatropic Allyl Shifts

Article abstract of DOI:10.1021/ja00394a028

The photochemical behavior of a series of alkyl-substituted 1,5-hexadienes has been studied.Competing allylcyclopropane formation and rearrangement to an isomeric 1,5-hexadiene via competing <1,2> and <1,3> sigmatropic allyl shifts, respectively, were observed.In one case a <3,2> product (30 from 27) and in another a <3,3> product (35 from 31) was obtained, but only in low yield.Extensive cis <=> trans isomerization competed with the allyl shifts, and, in addition, other products were obtained which are characteristic of the photochemical behavior of isolated double bonds.At low conversion diene 10t underwent stereoselective rearrangement to 8t and 11t, whereas the cis isomer 10c afforded mixtures of the cis and trans isomers of 8 and 11.Attempts to induce the allyl migrations by sensitization with p-xylene or by irradiation at 254 nm afforded only cis <=> trans isomerization.It is concluded that the allyl migrations occur via an excited state involving interaction between the double bonds and requiring a conformation in which the central C3-C4 bond is in a plane orthogonal to both double bonds, as proposed previously.In contrast with the acyclic dienes, 1,5-cyclooctadiene (50) afforded products 51-53 from competing <3,2>, <1,3>, and <3,3> allyl shifts, respectively.The divergent behavior in this case apparently arises because appropriate interaction between the two double bonds can occur only in the boat conformation.