Journal of Organic Chemistry 2010-08-20

Solvent and temperature effects on diastereodifferentiating Paternó-Büchi reaction of chiral alkyl cyanobenzoates with diphenylethene upon direct versus charge-transfer excitation.

Kazuyuki Matsumura, Tadashi Mori, Yoshihisa Inoue

Index: J. Org. Chem. 75(16) , 5461-9, (2010)

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Abstract

In the Paternó-Büchi reaction of chiral p-cyanobenzoates (1) with 1,1-diphenylethene (2), we revealed that the excited charge-transfer (CT) complex formed upon selective excitation at the CT band is distinctly different in structure and reactivity from the conventional exciplex generated through the direct excitation of acceptor 1 which subsequently associates with donor 2. Thus, the favored diastereoface upon photocycloaddition, as well as the temperature- and solvent-dependent behavior of the product's diastereoselectivity, were highly contrasting, often opposite, to each other upon direct versus CT excitation. From the activation parameters obtained by the Eyring analyses of the diastereoselectivity, we are able to infer that the conventional exciplex is relatively flexible and susceptible to the environmental variants, whereas the CT complex is better pi-pi stacked and more rigid in the ground state and also in the excited state, leading to the significantly smaller differential activation enthalpies and entropies. More interestingly, the signs of the differential activation parameters determined for direct and CT excitation are consistently opposite to each other and the isokinetic temperatures calculated therefrom differ significantly, unambiguously revealing the distinctly different nature in structure and reactivity of these two excited-state complex species. Thus, the combined use of irradiation wavelength, temperature, and solvent provides us with a convenient, powerful tool not only for elucidating the mechanistic details of photoreaction but also for critically controlling the stereochemical outcomes of photochirogenic reaction.