Photosensitized oxygenation of twisted 1,3-dienes: Abnormally higher reactivity of vinyl hydrogen rather than allyl hydrogen toward singlet oxygen

As one of the novel examples to investigate the characteristic reactivity o f significantly twisted 1,3-dienes, the photosensitized oxygenation of two types of significantly twisted 1,3-dienes, cis-beta-ionol derivative 2 and acyclic derivative Il, is investigated. Photosensitized oxygenation of 2a-f and 11a-e revealed that the vinyl hydrogen Ha was more reactive than the a llyl hydrogen Hb. Thus, phenyl derivative 2e and tert-butyl-substituted com pound lid selectively produced allenes 3e and 13d in 67% and 75% yield resu lting from the ene reaction of the vinyl hydrogen Ha rather than allyl alco hols 4e and 14d resulting from the allyl hydrogen abstraction in 24% and 8% yield, respectively. Furthermore, in the case of methyl-substituted compou nd 11b, the extent of the inherent reactivity of the vinyl hydrogen Ha was similar to that of the allyl hydrogen He. On the basis of X-ray analysis an d NIM and MO calculations, the discovered abnormally higher reactivity of t he vinyl hydrogen would be rationalized by considering mainly the large sig ma*-pi orbital interaction between the vinyl C-H bond and another double bo nd in significantly twisted 1,3-dienes resulting from calculations of HOMO electron densities.