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.

Template effects and supramolecular control of photoreactions in solution

“…15) forms 1 : 1 complexes with 15, affording mixtures of photodimers that are enriched in the anti HT products, with ee's of 10-30% (up to 40% at À50 1C). [158][159][160][161] Similarly, the chiral shield 8a developed by Bach blocks one of the two pro-chiral faces of a bound pyridone molecule to direct its ensuing [4+4] photocycloaddition to cyclopentadiene. The reaction is characterized by high conversion to a 2 : 3 mixture of the endo and exo products, both of which are obtained with high ee's (Fig.…”

Effects of Temperature and Host Concentration on the Supramolecular Enantiodifferentiating [4 + 4] Photodimerization of 2-Anthracenecarboxylate through Triplet-Triplet Annihilation Catalyzed by Pt-Modified Cyclodextrins

“…Supramolecular photochirogenesis provides a promising solution to such problems, as “confined space” for the prochiral substrate(s) was created before photoexcitation through the much longer and more intimate supramolecular interactions at both ground and excited states. A variety of chiral hosts, such as cyclodextrin derivatives [2,3,4,5,6,7,8,9,10,11,12,13,14,15], chiral hydrogen-bonding templates [16,17,18,19], chiral macrocyclic molecules [20,21] and biological macromolecules [22,23], have been developed and utilized as chiral sources for efficient photochirogenic control. However, an excess amount of chiral host is demanded in most supramolecular photochirogenesis for the purpose of inhibiting the undesired racemic photoproduct resulting from the photoreaction of the uncomplexed photosubstrates in the bulk solution [24].…”

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

“…A variety of chiral hydrogen-bonding templates, including Kemp’s triacid derivatives, α,α,α′,α′-tetraaryl-2,2-dimethyl-1,3-dioxolane-4,5-dimethanol derivatives (TADDOLs), N -pyridyl­isophthalamides with a benzophenone moiety, 2-methoxy­benzamide derivatives of 4-aminoprolinol, binaphthyl­(thio)­ureas, and trans -1,2-diaminocyclohexane-based bis­(thiourea)­s have hitherto been proposed and examined as mediators for enantiodifferentiating photocyclization, [2 + 2] and [3 + 2] photocycloadditions, and [4 + 4] photocyclodimerization reactions, all of which take advantage of the stoichiometric 1:1 template–substrate complexation. Thus, Bach et al developed chiral lactam templates derived from the Kemp’s triacid and applied them to various enantiodifferentiating photocyclization and [2 + 2] photocycloaddition reactions to achieve high enantiomeric excesses (ee’s) of up to 99% .…”