Enantiodifferentiating Photocyclodimerization of 2‑Anthracenecarboxylic Acid via Competitive Binary/Ternary Hydrogen-Bonded Complexes with 4‑Benzamidoprolinol

Circular dichroism (CD) spectral examinations at various host/guest ratios revealed that 2-anthracenecarboxylic acid (AC) forms not only 1:1 but also novel 2:1 hydrogen-bonded/π-stacked complexes with a chiral 4-benzamidoprolinol template (TKS159). The 2:1 complexation is a minor process but causes significant CD spectral changes as a consequence of the exciton coupling interaction of two AC chromophores and greatly accelerates the head-to-head photocyclodimerization to significantly affect the stereochemical outcomes

Supramolecular Photochirogenesis with a Higher-Order Complex: Highly Accelerated Exclusively Head-to-Head Photocyclodimerization of 2‑Anthracenecarboxylic Acid via 2:2 Complexation with Prolinol

An unprecedented 2:2 complex was shown to intervene in the enantiodifferentiating photocyclodimerization of 2-anthracenecarboxylic acid (<b>A</b>) mediated by a hydrogen-bonding template l-prolinol (<b>P</b>) to accelerate the formation of chiral <i>anti-head-to-head</i> and achiral <i>syn-head-to-head</i> cyclodimers in >99% combined yield with enhanced enantioselectivities of up to 72% ee for the former. The supramolecular complexation and photochirogenic behaviors, as well as the plausible structures, of intervening <b>A</b>_<i>m</i>·<b>P</b>_<i>n</i> complexes (<i>m</i>, <i>n</i> = 1 or 2) were elucidated by combined theoretical and experimental spectroscopic, photophysical, and photochemical studies. Furthermore, the photochemical chiral amplification was achieved for the first time by utilizing the preferential 2:2 complexation of <b>A</b> with homochiral <b>P</b> to give normalized product enantioselectivities higher than those of the template used. The present strategy based on the higher-order hydrogen-bonding motif, which is potentially applicable to a variety of carboxylic acids and β-aminoalcohols, is not only conceptually new and expandable to other (photo)­chirogenic and sensing systems but also may serve as a versatile tool for achieving photochemical asymmetric amplification and constructing chiral functional supramolecular architectures