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 (A) mediated by a hydrogen-bonding template l-prolinol (P) to accelerate the formation of chiral anti-head-to-head and achiral syn-head-to-head 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 A_m·P_n complexes (m, n = 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 A with homochiral P 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.