Geometrical photoisomerization of (Z)-cyclooctene sensitized by aromatic phosphate, phosphonate, phosphinate, phosphine oxide and chiral phosphoryl esters

Abstract

Aromatic phosphate, phosphonate, phosphinate and phosphine oxide have been used as effective sensitizers for the geometrical photoisomerization of (Z)-cyclooctene (1Z) to its highly strained (E)-isomer (1E). Photosensitizations with phosphate and phosphonate gave moderate photostationary-state E/Z ratios of 0.14–0.17. A comparative study of the fluorescence quenching experiments and kinetic analysis of the product yield demonstrate that the photosensitized isomerization proceeds through a mixed mechanism that involves both singlet and triplet excited states. Triphenylphosphine oxide is not fluorescent and gave a low photostationary-state E/Z ratio of 0.05. Using (–)-menthyl or (–)-bornyl phosphate, phosphonate and phosphinate as chiral sensitizers, the enantiodifferentiating photoisomerization of 1Z was performed to afford optically active 1E in enantiomeric excesses ⩽5%.