Use of chirally modified zeolites and crystals in photochemical asymmetric synthesis

Optimizing Photochirogenic Performance by Solvent-Driven Conformational Fixation in Enantiodifferentiating Photoisomerization of (Z)-Cyclooctene Mediated by Sensitizing β-Cyclodextrin Hosts

Catalytic enantiodifferentiating photoisomerization of cyclooctene (1Z) included and sensitized by regioisomeric 6-O-(o-, m-, and p-methoxybenzoyl)-β-cyclodextrins (CDs) was performed under a variety of solvent conditions for higher enantioselectivities. The enantiomeric excess (ee) of chiral (E)-isomer (1E) produced was a critical function of all the internal and external factors examined, in particular, the sensitizer structure and the solvent conditions, to afford (R)-1E in record-high ee's of up to 67% upon sensitization with the meta-substituted β-CD host in water and salt solutions but neither in 50% aqueous ethanol nor with the ortho- and para-substituted hosts. The mechanistic origin of the sudden ee enhancement achieved under the specific conditions is discussed on the basis of the circular dichroism spectral behaviors upon substrate inclusion and the compensatory enthalpy-entropy relationship of the activation parameters for the enantiodifferentiating photoisomerization.

Achiral Zeolites as Reaction Media for Chiral Photochemistry

Obtaining enantiomerically-enriched photoproducts from achiral reactants has been a long-sought goal. The various methods developed to achieve chiral induction in photoproducts during the last fifty years still suffer from a lack of predictability, generality, and simplicity. With the current emphasis on green chemistry, obtaining enantiomerically enriched products via photochemistry is a likely viable alternative for the future. Of the various approaches developed during the last three decades, the one pioneered in the author's laboratory involved the use of commercially-available and inexpensive achiral zeolites as the media. This approach does not use any solvent for the reaction. Examples from these studies are highlighted in this article. Since no chiral zeolites were available, when the work was initiated in the author's laboratory, commercially-available zeolites X and Y were modified with chiral inductors so that the reaction space becomes chiral. The results obtained established the value of chirally-modified, commercial zeolites as media for achieving chiral induction in photochemical reactions. A recent report of the synthesis of a chiral zeolite is likely to stimulate zeolite-based chiral photochemistry in synthesizing enantiomerically-pure organic molecules. The availability of chiral zeolites in future is likely to energize research in this area. Our earlier observations on this topic, we believe, would be valuable for progress of the field. Keeping this in mind, I have summarized the work carried out in our laboratory on chiral photochemistry on chirally-modified zeolites. This review does not include examples where high chiral induction has been obtained via a strategy that examines molecules appended with chiral auxiliary within achiral and chirally-modified zeolites. The latter approach yields products with diastereomeric excess >80%.

Photocycloaddition reaction of atropisomeric maleimides: mechanism and selectivity

We report a density functional study on the mechanism of the [2+2] photocyclization of atropisomeric maleimides.

Supramolecular photochirogenesis

Supramolecular photochirogenesis is a rapidly growing interdisciplinary area of science at the boundary of photochemistry, asymmetric synthesis and supramolecular chemistry. The major advantage of supramolecular photochirogenesis over the conventional molecular one is entropic in origin, being achieved by preorganizing substrate(s) in the ground state and manipulating subsequent photochemical transformation by weak but non-transient interactions in chiral supramolecular media. The chirality transfer often becomes more efficient through the cooperative non-covalent interactions and the confinement by host in both ground and excited states. Thus, all of the ground- and excited-state events, including complexation stoichiometry and affinity, chiroptical properties, photophysical behaviour and photochemical reactivity, jointly play pivotal roles in supramolecular photochirogenesis. This may appear to cause complication but in reality expands the range of manipulable factors and available experimental/theoretical tools for elucidating the mechanism and controlling photochirogenic processes both thermodynamically and kinetically, from which some new concepts/methodologies unique to supramolecular photochemistry, such as non-sensitizing catalytic photochirogenesis and wavelength-controlled photochirogenesis, have already been developed. In this review, we will discuss the recent progress and future perspective of supramolecular photochirogenesis.

Helical mesoporous silica as an inorganic heterogeneous chiral trigger for asymmetric autocatalysis with amplification of enantiomeric excess

P and M-Helical mesoporous silica was found to act as a chiral inorganic trigger for asymmetric autocatalysis to afford (S) and (R)-pyrimidyl alkanol with up to >99.5% ee, respectively.

Synthesis and structural studies of a new class of quaternary ammonium salts, which are derivatives of cage adamanzane type aminal 1, 3, 6, 8-tetraazatricyclo[4.3.1.13,8]undecane (TATU)

Background

Novel mono N-alkyl quaternary ammonium salts (3a-f) were prepared using the Menschutkin reaction from the cage adamanzane type aminal 1,3,6,8-tetraazatricyclo[4.3.1.1^3,8]undecane (TATU) and alkyl iodides, such as methyl, ethyl, propyl, butyl, pentyl and hexyl iodide (2a-f), in dry acetonitrile at room temperature.

Results

The structures of these new quaternary ammonium salts were established using various spectral and electrospray ionization mass spectrometry (ESI-MS) analyses. Compound (3b) was also analyzed using X-ray crystallography.

Conclusion

It was noted that alkyl chain length did not significantly affect the reaction because all employed alkyl iodide electrophiles reacted in a similar fashion with the aminal 1 to produce the corresponding mono N-quaternary ammonium salts, which were characterized by spectroscopic and analytical techniques.

pH-Controlled Supramolecular Enantiodifferentiating Photocyclodimerization of 2-Anthracenecarboxylate with Capped ?-Cyclodextrins

γ-Cyclodextrins capped with p-cresolbisbenzimidazole at the primary rim were synthesized and used as chiral hosts for mediating the enantiodifferentiating [4 + 4] photocyclodimerization of 2-anthracenecarboxylate. The use of 6A,6E-capped γ-cyclodextrin led to the formation of the anti-head-to-head photodimer in –5% enantiomeric excesses (ee) at pH 11 but in 28% ee at pH 6 with accompanying switching of product chirality, for which a pH-responsive conformational change of the capping moiety is likely to be responsible.

Glossary of terms used in photochemistry, 3rd edition (IUPAC Recommendations 2006)

Abstract: The second edition of the Glossary of Terms Used in Photochemistry [Pure Appl. Chem.68, 2223-2286 (1996); <http://www.iupac.org/publications/pac/1996/pdf/6812x2223.pdf>] has been both corrected and updated. Terms have been added related to molecular anisotropy, the use of polarized radiation, nonlinear optical phenomena, and the emerging field of computation of excited species. Some changes have been introduced in this "Glossary" regarding the terms related to radiation energy to make this collection fully compatible with internationally agreed-upon terms. Many links are included to various Web pages listing quantities relevant to the work of photochemists and scientists using photochemical tools.

Chiral Layered Zincophosphate [d-Co(en)3]Zn3(H0.5PO4)2(HPO4)2 Assembled about d-Co(en)33+ Complex Cations

A new chiral layered zincophosphate [d-Co(en)3]Zn3(H(0.5)PO4)2(HPO4)2, designated ZnPO-CJ16, has been hydrothermally synthesized by using the optically pure chiral metal complex d-Co(en)3I3 as the template. It contains 4.6-net sheets which array in a helical fashion with an ABCDEF stacking sequence along the [001] direction. The chiral d-Co(en)3(3+) complex cations reside in the interlayer regions. Interestingly, there exist symmetrical O...H...O H-bonds between inorganic sheets, which results in a pseudo-three-dimensional open-framework structure stabilized by strong H-bonds. The crystal data are as follows: ZnPO-CJ16, [d-Co(en)3]Zn3(H(0.5)PO4)2(HPO4)2; M = 818.26; hexagonal; P6(5)22 (No. 179); a = 8.5832(12) A; c = 52.610(11) A; U = 3356.6(9) A(3); T = 293(2) K; Z = 6; R1 = 0.0415 (I > 2sigma(I)); wR2 = 0.1383 (all data); Flack parameter, 0.04(4).

Pressure and Temperature-Controlled Enantiodifferentiating [4+4] Photocyclodimerization of 2-Anthracenecarboxylate Mediated by Secondary Face- and Skeleton-Modified γ-Cyclodextrins

A series of secondary-face-substituted and skeleton-modified gamma-cyclodextrins (gamma-CDs) were prepared as chiral hosts for enantiodifferentiating [4+4] photocyclodimerization reactions of 2-anthracenecarboxylic acid (AC). These gamma-CD derivatives form stable ternary complexes with ACs, with altroside-bearing gamma-CDs undergoing induced-fit conformational changes upon complexation, and the photocyclodimerization of AC was, thus, dramatically accelerated. The enantiomeric excess (ee) of anti-head-to-head cyclodimer 3 was greatly enhanced in general with altroside-bearing gamma-CDs 7-9. Although mono-altro-gamma-CD 9 and 3A-azido-3A-deoxy-altro-gamma-CD 7 gave 2 in ee's smaller than those obtained with native gamma-CD, 3A-amino-3A-deoxy-altro-gamma-CD 8 yielded 2 in much higher ee's, which is likely to be ascribed to the combined effects of the less-symmetric cavity and the electrostatic interactions. The influence of temperature and high pressure on the supramolecular photochirogenic reaction has been investigated in depth. An ee as high as 71% was obtained for cyclodimer 2 in the photocyclodimerization of AC mediated by 8 at 210 MPa and -21.5 degrees C.

Photophysics on surfaces: determination of absolute fluorescence quantum yields from reflectance spectra

A method for the calculation of absolute fluorescence quantum yields for dyes attached to solid particles based on reflectance measurements is reported. The same procedure allows calculation of true reflectance spectra (free of fluorescence) for highly fluorescent materials as well. Samples ofcresyl violet were immobilized by adsorption on microgranular cellulose in the concentration range 4.5 x 10(-9) to 3.8 x 10(-6) mol g(-1). Diffuse and total reflectance spectra were recorded with and without insertion of an optical absorption filter between the output of the integrating sphere of a reflectance spectrometer and the photodetector in order to block fluorescence partially. From these data, the relative emission spectrum of the dye, the filter transmission spectrum, and the detector sensitivity, true reflectances and absolute fluorescence quantum yields were recovered. Observed fluorescence quantum yields, affected by dye aggregation and inner filter effects, were concentration and wavelength dependent, ranging approximately between 0.1 and 0.6. The analysis of remission function spectra showed that dye aggregation is negligible up to a concentration of 1.41 x 10(-7) mol g(-1). Fluorescence data were corrected for reemission and reabsorption using a suitable model [Lagorio, M. G.; Dicelio, L. E.; Litter, M. I.; San Roman, E. J. Chem. Soc., Faraday Trans. 1998, 94, 419]. Application of this model to samples showing no aggregation yielded a wavelength-independent true fluorescence quantum yield of 0.60 +/- 0.05, similar to values found in solution. The usage of cresyl violet as a reference for the evaluation of fluorescence quantum yields for weakly fluorescing samples in the solid phase is discussed.

Bovine serum albumin-mediated enantiodifferentiating photocyclodimerization of 2-anthracenecarboxylate

Enantiodifferentiating photocyclodimerization of 2-anthracenecarboxyalate (AC) was performed at 25 degrees C in aqueous buffer solution (pH 7) in the presence of bovine-serum albumin (BSA) to afford four [4 + 4] cyclodimers, i.e., anti- and syn-head-to-tail (HT) (1 and 2) and anti- and syn-head-to-head (HH) dimers (3 and 4), of which only 2 and 3 are chiral. We found that (1) BSA possesses four sets of binding sites for AC of different affinities, stoichiometries, and chiral environment for photoreaction, which bind 1, 3, 2, and 3 AC molecules with binding constants of 5.3 x 107, 1.3 x 105, 1.4 x 104, and 3.0 x 103 M-1, respectively, (2) the regioselectivity of photodimerization is switched from HT to HH by adding BSA (the HH/HT ratio varies from 0.28 to 4.3), (3) BSA-mediated photodimerization of AC affords optically active products 2 and 3 of up to 29% and 41% ee, respectively. It is emphasized that the selective excitation of bound substrate, utilizing the spectral shift upon complexation with BSA, is not a prerequisite for efficient photochirogenesis using biomolecules.

Light-induced geometric isomerization of 1,2-diphenylcyclopropanes included within Y zeolites: role of cation-guest binding

Through a systematic study of several diphenylcyclopropane derivatives, we have inferred that the cations present within a zeolite control the excited-state chemistry of these systems. In the parent 1,2-diphenylcylopropane, the cation binds to the two phenyl rings in a sandwich-type arrangement, and such a mode of binding prevents cis-to-trans isomerization. Once an ester or amide group is introduced into the system (derivatives of 2beta,3beta-diphenylcyclopropane-1alpha-carboxylic acid), the cation binds to the carbonyl group present in these chromophores and such a binding has no influence on the cis-trans isomerization process. Cation-reactant structures computed at density functional theory level have been very valuable in rationalizing the observed photochemical behavior of diphenylcyclopropane derivatives included in zeolites. While the parent system, 1,2-diphenylcylopropane, has been extensively investigated in the context of chiral induction in solution, owing to its failure to isomerize from cis to trans, the same could not be investigated in zeolites. However, esters of 2beta,3beta-diphenylcyclopropane-1alpha-carboxylic acid could be studied within zeolites in the context of chiral induction. Chiral induction as high 20% ee and 55% de has been obtained with selected systems. These numbers, although low, are much higher than what has been obtained in solution with the same system or with the parent system by other investigators (maximum approximately 10% ee).

Enhanced enantio- and diastereoselectivity via confinement and cation binding: yang photocyclization of 2-benzoyladamantane derivatives within zeolites

Irradiation of 2-benzoyladamantane derivatives in zeolites yields the endo-cyclobutanols as the only photoproduct via a gamma-hydrogen abstraction process. The cyclobutanols readily undergo retro-aldol reaction to give delta-ketoesters. The enantiomeric excess (ee) in the endo-cyclobutanols is measured by monitoring the ee in the ketoesters. Whereas in solution the ee in the product ketoester is zero, within achiral NaY zeolite, in the presence of a chiral inductor such as pseudoephedrine, ee's up to 28% have been obtained. The influence of zeolite on several chiral esters of 2-benzoyladamantane-2-carboxylic acids has also been examined. Whereas in solution the diastereomeric excess is <15%, in zeolite the delta-ketoesters are obtained in 79% de (best examples). Ab initio computations suggest that enhancement of chiral induction within zeolites is likely to be due to cation complexation with the reactant ketone. Alkali ion-organic interaction, a powerful tool, is waiting to be fully exploited in photochemical and thermal reactions. In this context zeolites could be a useful medium as one could view them as a reservoir of "naked" alkali ions that are only partially coordinated to the zeolite walls.