Photochirogenesis: Multidimensional control of asymmetric photochemistry

Photochirogensis, or photochemical induction of molecular chirality, is an attractive alternative to thermal or enzymatic asymmetric synthesis. Using the inherent advantage that the photochemical reaction is driven by light absorption, the effect of temperature on optical yield was investigated over a wide range. Unexpectedly, the stereochemistry of photoproduct was frequently inverted at a critical temperature (T₀), above which the optical yield increased with increasing temperature. The Eyring treatment of the relative rate constant for the production of each enantiomer revealed that the unusual temperature dependency originates from the non-zero differential entropy of activation for the enantiodifferentiating process. In this case, the enthalpy term dominates at lower temperatures, while the entropy term becomes more important above T₀, switching the product chirality. The absolute configuration of photoproduct obtained at temperatures lower than to was correlated to that of the chiral sensitizer, except for those containing very bulky chiral auxiliaries, and the stereochemical outcomes are discussed on the basis of the molecular model examinations. Interestingly, similar switching behaviour was induced by varying the pressure from 0.1 to 400 MPa. The pressure effect was investigated at different temperatures to construct three-dimensional diagrams that correlate the optical yield with temperature and pressure as mutually independent factors. The combined use of temperature and pressure provides us with a convenient, powerful tool for controlling the product chirality and optical yield in asymmetric photochemistry.