Cross- versus Homo-Photocyclodimerization of Anthracene and 2‑Anthracenecarboxylic Acid Mediated by a Chiral Hydrogen-Bonding Template. Factors Controlling the Cross-/Homo-Selectivity and Enantioselectivity

Abstract

Competitive cross-/homo-photocyclodimerization of anthracene (AN) and 2-anthracenecarboxylic acid (AC) mediated by a chiral hydrogen-bonding template (TKS) was investigated under various conditions. The cross-photocyclodimerization was favored by a factor of 4–5 at all temperatures and wavelengths examined to afford the AC-AN cross-dimer in 80–84% yield even at AN/AC = 1 and in 98% yield at AN/AC = 10. The enantiomeric excesses (ee’s) obtained were 27–47% for the homo-dimers and 21–24% for the cross-dimer. The absolute configuration of the cross-dimer was determined by comparing the experimental and theoretical circular dichroism spectra and further correlated with the <i>re/si</i> enantiotopic-face selectivity upon AC-TKS complexation in the ground state. Detailed analyses of the complexation behavior and the fluorescence lifetime and cyclodimerization rate of excited <i>re/si</i> complexes revealed that the product’s ee is critically controlled not only by the relative abundance of the <i>re/si</i> complexes in the ground and excited states but also by their relative photocyclodimerization rate. Crucially, the ground-state thermodynamics and the excited-state kinetics are not synergistic but offsetting in enantiotopic-face selectivity, and the latter overwhelms the former to give the homo- and cross-dimers in modest ee’s. Finally, some practical strategies for enhancing the enantioselectivity in chiral template-mediated photochirogenesis have been proposed