Pressure control of enantiodifferentiating polar addition of 1,1-diphenylpropene sensitized by chiral naphthalenecarboxylates

Abstract

Effects of pressure on the enantiodifferentiating methanol addition to 1,1-diphenylpropene (1) sensitized by chiral naphthalenedicarboxylates (3 and 4) were investigated over 0.1–400 MPa. The logarithm of enantiomeric excess (ee) of photoadduct, i.e. 1,1-diphenyl-2-methoxypropane (2), was a linear function of both pressure (P) and temperature (T); further, the product chirality was switched by P in some cases. From the slope of P − ln(k_R/k_S) plot, the differential activation volume (ΔΔV^‡) was determined for the first time for bimolecular asymmetric photoreactions. The ΔΔV^‡ values obtained are mostly larger than those obtained for relevant unimolecular photoreactions, and are a critical function of the nature of the chiral auxiliary and solvent, indicating conformation changes of the intervening diastereomeric exciplex or transition state in different solvents. Indeed, fluorescence spectral examinations of the sensitizer and exciplex under high pressure revealed the existence of exciplexes of variable energy and structure, which may rationalize the different ΔΔV^‡ and product ee obtained. A three-dimensional diagram, correlating the ee with P and T, was constructed from the pressure dependence data at different T, from which we may propose an idea of the multidimensional control of asymmetric reaction by the combined use of the entropy-related enviromental factors.