Value of zeolites in asymmetric induction during photocyclization of pyridones, cyclohexadienones and naphthalenones

Two strategies, namely chiral inductor and chiral auxiliary approaches, have been examined within zeolites with the aim of achieving asymmetric induction during the photocyclization of cyclohexadienone, naphthalenone and pyridone derivatives. Within zeolites, enantioselectivity as high as 55% and diastereoselectivity as high as 88% have been obtained. The observed stereoselectivities are significant given the fact that these reactions gave very little stereoselectivities in isotropic solution media. The results obtained on the photocyclization of dienones, naphthalenones and N-alkyl pyridones within zeolites compliment our earlier investigations on the photocyclization of tropolone derivatives, the geometric isomerization of 1,2-diphenylcyclopropanes and 2,3-diphenyl-1-benzoyl cyclopropanes, and the Norrish type II reaction of a-oxoamides, phenyl adamantyl ketones, phenyl norbornyl ketones and phenyl cyclohexyl ketones. With the help of these examples, we have established the importance of zeolite and its charge compensating cations in effecting asymmetric induction in photochemical reactions

Facial selective photoreduction of steroids: role of zeolites

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Asymmetric photoreactions within zeolites: role of confinement and alkali metal ions

In this Account strategies using zeolites as media to achieve chiral induction are presented. Diastereomeric excesses as high as 90% and enantiomeric excesses up to 78% have been obtained with selected systems within zeolites. The same systems show no asymmetric induction in solution. Chiral induction is dependent on the alkali ions present in the zeolites. Alkali ions control not only the extent of asymmetric induction but often the isomer being enhanced. Results of ab initio computations have allowed us to gain an insight into the observed selectivity within zeolites

Achieving Enantio and Diastereoselectivities in Photoreactions Through the Use of a Confined Space

The efforts of chemists during the past few decades have advanced the field of thermal asymmetric synthesis to a great extent [1]. Complex molecules can now be synthesized as single enantiomers. Unfortunately, asymmetric photochemical reactions have not enjoyed the same level of success [2]. In the past, chiral solvents, chiral auxiliaries, circularly polarized light, and chiral sensitizers have been utilized to conduct enantioselective photoreactions. The highest chiral induction achieved by any of these approaches at ambient temperature and pressure has been ~30% (2–10% e.e. is common in photochemical reactions under the above conditions). Crystalline state and solid host-guest assemblies have, on the other hand, provided the most encouraging results [3]. Two approaches have been used to achieve chiral induction in the crystalline state. In one, by the Weizmann Institute Group, the achiral reactant is crystallized into a chiral space group [4]. The limited chances of such crystallization of organic molecules renders this approach less general. In the second approach, due to Scheffer and co-workers [5], an ionic chiral auxiliary is used to effect a chiral environment. This limits the approach to molecules with carboxylic acid groups that form crystallizable salts with chiral amines or vice versa. Yet another successful approach due to Toda and co-workers [6] has made use of organic hosts that contain chiral centers (e.g., deoxycholic acid, cyclodextrin, 1,6-bis (o-chlorophenyl)-1,6-diphenyl-2,4-diyne-1,6-diol,). The success of this approach is limited to guests that can form solid solutions with the host without disturbing the hosts macro-structure. The reactivity of molecules in the crystalline state and in solid host-guest assemblies is controlled by the details of molecular packing. Currently, molecular packing and consequently the chemical reactivity in the crystalline state, can not be reliably predicted [7]. Therefore even after successfully crystallizing a molecule in a chiral space group or complexing a molecule with a chiral host or a chiral auxiliary, there is no guarantee that the guest will react in the crystalline state. Hence, even though crystalline and host-guest assemblies have been very useful in conducting enantioselective photoreactions, their general applicability thus far has been limited

Heavy-cation-induced phosphorescence of alkanones and azoalkanes in zeolites as hosts: induced S<SUB>1</SUB> (nπ<SUP>∗</SUP>) to T<SUB>1</SUB> (nπ<SUP>∗</SUP>) intersystem crossing and S<SUB>0</SUB> to T<SUB>1</SUB> (nπ<SUP>∗</SUP>) absorption

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Alkali Ion-Controlled Excited-State Ordering of Acetophenones Included in Zeolites: Emission, Solid-State NMR, and Computational Studies

The nature of the lowest triplet excited state of acetophenones included in zeolites has been inferred through steady-state and time-resolved emission spectra. Acetophenone shows cation-dependent state switching. Within NaLiY and NaY zeolites, the emitting state is identified to have$\pi\pi^*$ character, whereas in NaRbY and NaCsY, two emissions characteristic of n$\pi^*$ and $\pi\pi^*$were observed. In contrast, 4¢-methoxyacetophenone does not show cation-dependent state switching; in all alkali cation-exchanged zeolites, the lowest triplet is identified to have $\pi\pi^*$character. The results are attributed to a specific cation-acetophenone interaction. Static, MAS, and CP-MAS spectra of 13C-enriched acetophenone included in MY zeolites confirm the presence of such an interaction. The data reveals that the extent of interaction, as reflected by the molecular mobility,depends on the cation. Small cations such as Li+ and Na+ interact strongly whereas large cations such as Rb+ and Cs+ interact weakly with acetophenone. Consistent with these trends, small cations are found to switch the lowest triplet to $\pi\pi^*$ character, whereas the large cations leave the n$\pi^*$ and $\pi\pi^*$ triplet states of acetophenone close to each other. Computational studies provide strong support for these interpretations. B3LYP/6-31G* calculations were carried out on acetophenone and 4¢-methoxyacetophenone as well as their Li+ and Na+ complexes. Geometries with cations bound to the carbonyl, phenyl, and methoxy groups were examined. The most-stable structures involve a cation-carbonyl interaction, which stabilizes the n orbital and, in turn, destabilizes the n$\pi^*$ triplet state. Excited-state energetics were quantified using TDDFT/6-31+G* calculations. Consistent with experimental observations, acetophenone and 4¢-methoxyacetophenone are predicted to have nð* and $\pi\pi^*$ as their lowest triplet states, respectively. Complexation with $Li^+$ or $Na^+$ ispredicted to lead to a $\pi\pi^*$ triplet as the lowest excited state for both compounds. The present study, combining steady-state and time-resolved emission spectra, solid state NMR, and computations, demonstrates the occurrence of cation-dependent state switching in acetophenones and offers an internally consistent explanationof the effect in terms of specific cation-carbonyl interaction