Inclusion-Enhanced Optical Yield and E/Z Ratio in Enantiodifferentiating Photoisomerization of Cyclooctene Included and Sensitized by .beta.-Cyclodextrin Monobenzoate

Abstract

Cyclodextrins (CDs), a series of cyclic oligomers of Dglucopyranose, are known to accommodate a variety of organic guests in their chiral hydrophobic cavities, forming inclusion compounds. Consequently, they have been applied widely to biomimetic chemistry as well as to separation science and technology, including chiral re~ognition.'-~ In sharp contrast to the well-documented stoichiometric and catalytic thermal asymmetric syntheses utilizing the CD's chiral c a v i t~, ' ,~,~-~* little effort appears to have been devoted to the study of asymmetric photochemistry in the chiral ~a v i t y ? , '~-'~ Only a few stoichiometric asymmetric photoreactions of CD complexes have hitherto been investigated in the solid state.I3-l5 We have reported recently that the direct irradiation at 185 nm of a 1:l complex of (a-cyclooctene (1Z) with p-CD in the solid state or in water suspension leads to prompt Z-E isomerization affording apparent photostationary-state (pss) En mixtures, but that the (@-isomer (1E) obtained is almost racemic: optical purity (op) <l%.15,'6 It is then obvious that simple inclusion in CD is not sufficient to induce appreciable optical activity in the included prochiral guest during the photochemical transformation in the chiral cavity.

Supramolecular Photochirogenesis

“…Indeed, the ee of produced 4E is significantly improved from nearly zero obtained with native β -CD 65 to 24% by using 6 -O -(methyl phthaloyl) -β -CD 67b as chiral sensitizer. 130 More recently, it was reported that the photoisomerization of 4Z sensitized by 6 -O -( mmethoxybenzoyl) -β -CD 67g gives a much better ee of 46%, which is the highest ever reported for analogous CD -based sensitizers. 124,128 In contrast, both ortho and para isomers 67f and 67h afford only poor enantioselectivity under the same reaction condition, revealing that a small difference in host structure can critically affect the optical yield and further that not only the choice of suitable host but also the fi ne -tuning of sensitizer structure are equally crucial for obtaining the best result in supramolecular photochirogenesis.…”

Templating Photoreactions in Solution

“…The strategy relies on the covalent linkage of benzoyl sensitizers to the rim (O -6 position) of α -, β -, or γ -CDs. 111,112 Particularly promising results have been reported for β -CD, which presents the best cavity size match, possessing a 6 -O -(3 -methoxybenzoyl) substituent, which gives ee ' s approaching 50% (Fig. 2.20 ).…”

Catalytic Supramolecular Photochirogenesis

“…[20] Photoisomerization of (Z)-cyclooctene (1Z) mediated by sensitizer-modified CDs (Scheme 1) is the earliest example of an enantiodifferentiating supramolecular photosensitization. [21] Direct photoirradiation of a 1:1 complex of 1Z with native b-CD at 185 nm gives nearly racemic 1E in 0.5-1.5% enantiomeric excess (ee), revealing the poor photochirogenic ability of the native b-CD cavity for 1Z. [22] In contrast, the supramolecular photoisomerization of 1Z included and sensitized by a 0.1 equivalent of benzoyl-CD (4a) affords 1E in up to 11% ee.…”

“…[22] In contrast, the supramolecular photoisomerization of 1Z included and sensitized by a 0.1 equivalent of benzoyl-CD (4a) affords 1E in up to 11% ee. [21] Since then, a series of sensitizer-modified a-, band g-CD derivatives 4b-m (Figure 1) have been prepared for investigating the supramolecular photoisomerization of 1Z, and cyclooctadienes (2ZZ and 3ZZ) (Scheme 1). [23][24][25][26][27][28][29][30][31] hν + A nice trick employed in this supramolecular photochirogenic sensitization system is that the sensitizing ability is turned on only when a guest substrate is included in the host cavity.…”