Cross- versus Homo-Photocyclodimerization of Anthracene and 2-Anthracenecarboxylic Acid Mediated by a Chiral Hydrogen-Bonding Template. Factors Controlling the Cross-/Homo-Selectivity and Enantioselec

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• 作者：Yuko Kawanami ; Hiroaki Umehara ; Jun-ichi Mizoguchi ; Masaki Nishijima ; Gaku Fukuhara ; Cheng Yang ; Tadashi Mori ; Yoshihisa Inoue
• 刊名：The Journal of Organic Chemistry
• 出版年：2013
• 出版时间：April 5, 2013
• 年：2013
• 卷：78
• 期：7
• 页码：3073-3085
• 全文大小：571K
• 年卷期：v.78,no.7(April 5, 2013)
• ISSN：1520-6904

文摘

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鈥? at all temperatures and wavelengths examined to afford the AC-AN cross-dimer in 80鈥?4% yield even at AN/AC = 1 and in 98% yield at AN/AC = 10. The enantiomeric excesses (ee鈥檚) obtained were 27鈥?7% for the homo-dimers and 21鈥?4% 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鈥檚 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鈥檚. Finally, some practical strategies for enhancing the enantioselectivity in chiral template-mediated photochirogenesis have been proposed.