Enantiodifferentiating anti-Markovnikov photoaddition of alcohols to 1,1-diphenylalkenes sensitized by chiral naphthalenecarboxylates

The photosensitized enantiodifferentiating polar additions of alcohols ((ROH)-O-2; R-2 = Me, Et, Ii-Pr, i-Pr, t-Bu) to 1,1-diphenylalkenes 1-3 (Ph2C=CHR1; R-1 = Me, Et, i-Pr) were performed over a range of temperatures in the presence of chiral 1-, 2-, 1,4-, 1,8-, 2,3-, and 2,6-naphthalene(di)carboxylate (7-12) photosensitizers, giving the chiral anti-Markovnikov adduct (4a) with optimized enantiomeric excesses (ee's) of up to 33%. An unusual switching of product chirality was induced simply by changing the irradiation temperature, leading to antipodal products at different temperatures, often affording higher ee's at higher temperatures. The differential activation parameters for the enantiodifferentiation process, which were determined from an Eyring treatment of the temperature-dependent ee values, clearly demonstrate that the unusual temperature-switching behavior of the product chirality is entropic in origin. Factors controlling the product's ee were extensively surveyed, and the steric and/or electronic structures of sensitizer, substrate, and alcohol, the solvent polarity, the alcohol concentration, and the irradiation temperature were all shown to play a crucial role. The detailed reaction mechanism and excited states involved and the origin of enantiodifferentiation, as well as the reaction kinetics and energetics, were fully elucidated for the first time from the fluorescence quenching and lifetime measurement of both sensitizer and exciplex in the presence/absence of added alcohol. We have also developed a new strategy to overcome the normally accepted tradeoff between the chemical and optical yields in this typical radical ion-mediated photoaddition.