Pressure and Temperature Control of Product Chirality in Asymmetric Photochemistry. Enantiodifferentiating Photoisomerization of Cyclooctene Sensitized by Chiral Benzenepolycarboxylates

Abstract

Pressure effects upon asymmetric photosensitization have been investigated for the first time in the enantiodifferentiating Z−E photoisomerization of cyclooctene (1), sensitized by chiral aromatic esters (2−7). The product's enantiomeric excess (ee) and E/Z ratio were critical functions of the applied pressure, exhibiting an unprecedented switching of the product chirality. Depending upon the chiral sensitizer employed, the differential activation volume (ΔΔV⧧) varies widely from −3.7 to +5.6 cm3 mol-1, which is unexpectedly large for an enantiodifferentiation in the excited state. However, the ΔΔV⧧ values obtained do not correlate with the differential activation enthalpy (ΔΔH⧧) or entropy (ΔΔS⧧) obtained from temperature-dependence studies, indicating that pressure and temperature function as independent perturbants for the photoenantiodifferentiation process. Further investigations on the pressure dependence of ee at low temperatures enable us to construct the first three-dimensional diagram that correlates the product's ee with pressure and temperature changes. The combined effects of temperature and pressure provide us with a versatile tool for the multidimensional control of asymmetric photochemical reactions, in which we can switch and/or enhance the product chirality at more readily accessible temperatures and pressures, without using antipodal sensitizers.