Photochemistry of cyclohepta-1,3-diene and cyclohepta-1,3,5-triene. Photochemical formation and chemical reactivity of the strained trans-isomer

Abstract

The direct and sensitized photolyses of cis,cis-cyclohepta-1,3-diene and cis,cis,cis-cyclohepta-1,3,5-triene have been studied in detail in search of the trans-isomer. Low-temperature irradiations and trapping experiments with acidic methanol have revealed that the primary process for the excited singlet and triplet states of cycloheptadiene is not direct disrotatory cyclization but geometrical isomerization to the highly strained cis,trans-isomer, which is detected spectroscopically, while no intermediacy of any transient species can be detected spectroscopically or trapped chemically in the photolyses of cycloheptatriene. The cis,trans-cycloheptadiene generated photochemically cyclizes thermally to bicyclo[3.2.0]hept-6-ene in neutral media or gives adducts, 3-methoxycycloheptene and 2-methoxynorcarane, in acidic methanol. The kinetic and thermodynamic parameters for the photochemical and the subsequent thermal processes are discussed.