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 a
bsorption, the effect of temperature on optical yield was investigated over
a wide range. Unexpectedly, the stereochemistry of photoproduct was freque
ntly inverted at a critical temperature (T-0), above which the optical yiel
d increased with increasing temperature. The Eyring treatment of the relati
ve rate constant for the production of each enantiomer revealed that the un
usual temperature dependency originates from the non-zero differential entr
opy of activation for the enantiodifferentiating process. In this case, the
enthalpy term dominates at lower temperatures, while the entropy term beco
mes more important above T-0, switching the product chirality. The absolute
configuration of photoproduct obtained at temperatures lower than T-0 was
correlated to that of the chiral sensitizer, except for those containing ve
ry bulky chiral auxiliaries, and the stereochemical outcomes are discussed
on the basis of the molecular model examinations. Interestingly, similar sw
itching 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 temperatu
re and pressure as mutually independent factors. The combined use of temper
ature and pressure provides us with a convenient, powerful tool for control
ling the product chirality and optical yield in asymmetric photochemistry.