Chiral Template-Directed Regio-, Diastereo-, and Enantioselective Photodimerization of an Anthracene Derivative Assisted by Complementary Amidinium–Carboxylate Salt Bridge Formation

A series of optically active amidine dimers composed of m-terphenyl backbones joined by a variety of linkers, such as achiral and chiral p-phenylene and chiral amide linkers, were synthesized and used as templates for the regio- (head-to-tail (HT) or head-to-head (HH)), diastereo- (anti or syn), and enantioselective [4 + 4] photocyclodimerization of an achiral m-terphenyl-based carboxylic acid monomer bearing a prochiral 2-substituted anthracene at one end (1) through complementary amidinium–carboxylate salt bridges. The amidine dimers linked by p-phenylene linkages almost exclusively afforded the chiral syn-HT and anti-HH dimers at 25 °C, while those joined by amide linkers produced all four dimers. The p-phenylene-linked templates tended to enhance the syn-HT-photodimer formation at high temperatures with no significant changes in the product enantiomeric excess (ee), while the anti-HH-photodimer formation remarkably increased with the decreasing temperature accompanied by a significant enhancement of t...