Singlet-versus triplet-sensitized enantiodifferentiating photoisomerization of cyclooctene: remarkable effects of spin multiplicity upon optical yield

Abstract

Photosensitized enantiodifferentiating geometrical isomerizations of (Z)-cyclooctene (1Z), producing the chiral (E)-isomer (1E), were performed in triplet, as well as singlet, states in order to clarify the effects of spin multiplicity upon the product's enantiomeric excess (ee) or optical purity (op). In sharp contrast to the moderate and highly temperature-dependent ee/op values, up to 10%, of 1E produced in the singlet sensitization with (–)-menthyl benzene(di)carboxylates, much lower and temperature-independent ee/op values around 1%, were obtained by using the structurally related chiral aromatic ethers as formal ‘triplet’ sensitizers. The ee/op values obtained in the ‘triplet’ sensitizations decreased significantly with decreasing substrate concentration, approaching nearly zero at extremely low 1Z concentrations, while the photostationary E/Z ratio, (E/Z)_pss, increased dramatically from 0.013 to 0.22 in the same range of concentration. Both concentration dependences of ee/op and (E/Z)_pss were shown to originate from the concurrent operation of the singlet sensitization at higher concentrations as evidenced by fluorescence quenching experiments. The authentic triplet sensitization was demonstrated to be totally ineffective as a method of asymmetric induction at least for the present system, probably owing to the lack of formation of a structurally well defined triplet exciplex.