On the mechanism of the [4+2] cycloaddition of 1O2 with 1,3-dienes: Identifying the putative biradical/dipolar intermediate by employing the gem-diphenylcyclopropylcarbinyl radical clock as a mechanistic probe

Abstract

The mechanism of the [4+2] cycloaddition reaction of singlet oxygen (1O2) with simple acyclic dienes, i.e. (E)/(Z)-1 and (E,E)/(Z,E)-2, was investigated by employing the gem-diphenylcyclopropylcarbinyl radical clock as a hypersensitive mechanistic probe. In both cases, benzophenone, a decomposition product of the rearranged endoperoxide intermediates 11 and 12, was identified as the major product. The formation of this product is consistent with the intervention of an open biradical or dipolar intermediate having a lifetime greater than 2 psec. Our attempts to further trap any endoperoxide intermediate revealed a new mode of reactivity of these dienic systems with 3O2, that mainly affords the corresponding 1,2-dioxolane derivatives (Å)-13 and (Å, Å)-14, respectively, through a photoinduced electron transfer mechanism, co-sensitized by methylene blue (MB) and thiourea. The structures of (E)-13 and (E,E)-14 were supported by experimental and theoretical NMR studies.