title: 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 re/si 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 re/si complexes revealed that the product's ee is critically controlled not only by the relative abundance of the re/si 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.
authors: Department of Applied Chemistry, Osaka University, 2-1 Yamadaoka, Suita 565-0871, Japan.
authors: Yuko
authors: Y
authors: Kawanami
authors: Hiroaki
authors: H
authors: Umehara
authors: Jun-ichi
authors: J
authors: Mizoguchi
authors: Masaki
authors: M
authors: Nishijima
authors: Gaku
authors: G
authors: Fukuhara
authors: Cheng
authors: C
authors: Yang
authors: Tadashi
authors: T
authors: Mori
authors: Yoshihisa
authors: Y
authors: Inoue
mesh_terms: D000873:Anthracenes / Q000737:chemistry*; D002264:Carboxylic Acids / Q000737:chemistry*; D003500:Cyclization; D019281:Dimerization; D006860:Hydrogen Bonding; D055668:Photochemical Processes; D011789:Quantum Theory; D013237:Stereoisomerism; D013816:Thermodynamics
chemical_list: D000873:Anthracenes; D002264:Carboxylic Acids; C107996:anthracene-1-carboxylic acid; C034020:anthracene