Enantio- and diastereodifferentiating cis,trans-photoisomerization of 2beta,3beta-diphenylcyclopropane-1alpha-carboxylic acid derivatives in organized media

Enantioselective Photochemical Reactions Enabled by Triplet Energy Transfer

For molecules with a singlet ground state, the population of triplet states is mainly possible (a) by direct excitation and subsequent intersystem crossing or (b) by energy transfer from an appropriate sensitizer. The latter scenario enables a catalytic photochemical reaction in which the sensitizer adopts the role of a catalyst undergoing several cycles of photon absorption and subsequent energy transfer to the substrate. If the product molecule of a triplet-sensitized process is chiral, this process can proceed enantioselectively upon judicious choice of a chiral triplet sensitizer. An enantioselective reaction can also occur in a dual catalytic approach in which, apart from an achiral sensitizer, a second chiral catalyst activates the substrate toward sensitization. Although the idea of enantioselective photochemical reactions via triplet intermediates has been pursued for more than 50 years, notable selectivities exceeding 90% enantiomeric excess (ee) have only been realized in the past decade. This review attempts to provide a comprehensive survey on the various photochemical reactions which were rendered enantioselective by triplet sensitization.

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%.

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.

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.

Control of enantioselectivity in the photochemical conversion of alpha-oxoamides into beta-lactam derivatives

[reaction: see text]. Several approaches to asymmetric induction in the alpha-oxoamide (1) to beta-lactam (2) photorearrangement are described. Best results are obtained via irradiation of ionic and covalent chiral auxiliary-containing reactants in the crystalline state and in the interior supercages of zeolites.

2000 Alfred Bader Award LectureIn the footsteps of Pasteur: asymmetric induction in the photochemistry of crystalline ammonium carboxylate salts

This review describes the development of a new, synthetically useful method of asymmetric synthesis in organic photochemistry. Similar in many ways to the Pasteur procedure for resolving racemic carboxylic acids and organic amines, the method relies on the use of crystalline organic salts in which the enantioselectivity of a photochemical reaction of an achiral organic ion (for example, a carboxylate anion) is governed in the solid state by the presence of an optically pure counterion (for example, an optically active ammonium ion). Such optically pure counterions are termed ionic chiral auxiliaries. Salts containing ionic chiral auxiliaries are required to crystallize in chiral space groups, which provide the asymmetric environment necessary for chiral induction. Using this methodology, we have obtained near-quantitative optical yields in a wide variety of photochemical reactions.Key words: photochemistry, solid state, chiral auxiliaries, asymmetric synthesis, crystal structure–reactivity relationships.