104901-63-5Relevant articles and documents
Supramolecular enantiodifferentiating photocyclodimerization of 2-anthracenecarboxylate mediated by capped γ-cyclodextrins: Critical control of enantioselectivity by cap rigidity
Yang, Cheng,Mori, Tadashi,Inoue, Yoshihisa
, p. 5786 - 5794 (2008/12/21)
(Graph Presented) A series of γ-cyclodextrins (CDs) modified with capping and noncapping aromatic group(s) were synthesized to mediate the enantiodifferentiating [4 + 4] photocyclodimerization of 2-anthracenecarboxylic acid (AC). The complexation behavior of these γ-CDs with AC was studied by circular dichroism, UV-vis, and NMR spectroscopy to reveal the formation of stable 1:2 host-guest complexes in all cases. The capped γ-CD with a biphenyl group bridging the A and D glucose units was shown to confine the included AC molecules most strictly among the capped and noncapped γ-CDs examined. Photocyclodimerization of AC mediated by capped γ-CDs considerably improved the yield and enantiomeric excess (ee) of the head-to-head photodimer 3. The ee and the absolute configuration of syn-head-to-tail photodimer 2 critically depended on the rigidity of capping. Thus, the flexibly capped and rim-substituted γ-CDs afforded 2 in moderate ee's of around 40%, whereas γ-CD with a rigid biphenyl cap gave the antipodal 2 in -58% ee. Interestingly, the ee of 2 mediated by flexibly capped γ-CDs was highly sensitive to the temperature variation as a consequence of large differential entropy changes in the enantiodifferentiation process. In contrast, the entropy effect does not appear to play a significant role in the photocyclodimerization of AC with rigidly capped γ-CDs. The differential enthalpy and entropy changes obtained for the enantiodifferentiating photocyclodimerization mediated by native and most of the modified γ-CDs gave an excellent enthalpy-entropy compensation plot with an exception of the biphenyl-capped γ-CD, indicating the operation of significantly different enantiodifferentiation mechanism within the rigidly capped cyclodextrin cavity.