Achiral Zeolites as Reaction Media for Chiral Photochemistry

Abstract

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%.

Enantioselective synthesis of heterocyclic compounds using photochemical reactions

“…They are used as catalysts in many domains of chemistry, also in chemical industry. Zeolites have also been used as host structure for photochemical reactions [148][149][150]. The photochemical electrocyclization of pyridone derivatives such as 79 have been performed in super cages of MY zeolites (Scheme 19) [151].…”

Enhanced thermal and photo-stability of a para-substituted dicumyl ketone intercalated in a layered double hydroxide

“…Supramolecular chemistry provides, however, several other strategies to control photoreactions through confinement of a substrate within an organizing medium, better known as host-guest chemistry ( Ramamurthy and Gupta, 2015 ; Ramamurthy and Sivaguru, 2016 ). Examples of hosts which have been investigated include inorganic porous and layered materials [zeolites, clays, layered double hydroxides (LDHs)] ( Ogawa and Kuroda, 1995 ; Scaiano and García, 1999 ; Shichi and Takagi, 2000 ; Ramamurthy, 2019 ), metallo-cages (Pd nanocage) ( Karthikeyan and Ramamurthy, 2007 ; Ramamurthy and Gupta, 2015 ; Ramamurthy and Sivaguru, 2016 ), and organic molecular containers (cyclodextrins, cucurbiturils, calixarenes, octa-acid cavitand) ( Karthikeyan and Ramamurthy, 2007 ; Ramamurthy, 2015 ; Ramamurthy and Gupta, 2015 ; Ramamurthy and Sivaguru, 2016 ; Pattabiraman et al, 2018 ). Among inorganic hosts, the interlayer region of LDHs provides an expandable two-dimensional (2D) reaction field for spatially controlled photochemical transformations ( Ogawa and Kuroda, 1995 ; Newman and Jones, 1998 ).…”

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

“…Supramolecular photochirogenesis, executing photoreaction in a chiral supramolecular environment, has recently made great strides to provide some promising inventions to overcome or circumvent the obstacles hindering efficient photochirogenesis, i.e., the short lifetime, high reactivity, and weakly interacting nature of the electronically excited molecules . Various chiral hosts, such as chirally modified zeolites, − chiral synthetic templates, biomacromolecules, cyclodextrins (CDs), and polymers, have hitherto been exploited as the chirality sources to provide the resulting photoproducts with good-to-excellent enantioselectivities. Of these chiral hosts, inherently chiral, readily available, and easily modifiable CDs are particularly attractive and frequently employed in supramolecular photochirogeneses.…”