Excited-State Dynamics Achieved Ultimate Stereocontrol of Photocyclodimerization of Anthracenecarboxylates on a Glucose Scaffold

Near-perfect stereoselectivity was attained in the diastereodifferentiating [4 + 4] photocyclodimerization of 2-anthracenecarboxylates tethered to a glucose scaffold not by thermodynamically tuning the conformer equilibrium in the ground state but by kinetically controlling the conformer dynamics and reactivity in the excited state, which enabled us, after removal of the scaffold, to obtain a single enantiomer of chiral <i>anti-head-to-head</i>-cyclodimer in >99% optical and 96% chemical yield from an ensemble of four precursor conformers

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

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

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