J. Shailaja

Singlet Oxygen Mediated Oxidation of Olefins within Zeolites: Selectivity and Complexities

Abstract Thiazine dyes such as thionin, methylene blue and methylene green have been cation exchanged within monovalent cation exchanged Y zeolites. Depending on the water content, the dye molecules exist as either monomers (‘dry’) or dimers (‘wet’). The monomeric dye, upon excitation with visible light, generates singlet oxygen, which has been utilized to oxidize alkenes to hydroperoxides. In the case of trisubstituted alkenes, hydroperoxidation within zeolites occurs with a certain amount of regioselectivity. The oxidation within zeolites is accompanied by photodecomposition of the dye and the product hydroperoxides and acid catalyzed rearrangement of the alkenes. In order to understand the observed selectivity, ab initio and DFT calculations on model systems have been performed. The calculations confirm fairly strong cation-alkene binding as well as additional geometric and orbital distortions. Computed activation energies for hydrogen abstraction suggest a significant rate retardation due to metal coordination. At both the MP2 and B3LYP levels, formation of the tertiary hydroperoxide by hydrogen abstraction from the methyl group (4-position) of 2-methyl 2-butene is calculated to be favored by a small margin. Between the gem-dimethyl units, abstraction from the syn methyl group is favored slightly compared to the anti counterpart. These predictions are not compatible with the observed regioselectivities. Further experimental and theoretical studies are underway to understand the observed regioselective oxidation within zeolites.

Cyclodextrin mediated solvent-free enantioselective photocyclization of N-alkyl pyridones ☆

Irradiation of N-methyl pyridone and N-ethyl pyridone included in β-cyclodextrin yields the photocylized product, chiral 2-azabicyclo[2.2.0]-hex-5-en-3-ones, in ∼60% enantiomeric excess. The inclusion complex is readily made by mechanically mixing the host β-cyclodextrin and the guest pyridone.

Cation controlled singlet oxygen mediated oxidation of olefins within zeolites

Oxidation of trialkyl olefins has been performed within zeolites employing thionin as the singlet oxygen sensitizer. Unusual selectivity in favor of secondary hydroperoxides is observed within zeolites. In light of the fact that in solution such a selectivity is never observed the selectivity reported here is of great significance. Based on computational results the observed selectivity is attributed to conformational control of the reactant olefins by cations present in zeolites. Cation-π interaction seems to control the behavior of the olefins towards singlet oxygen 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.

Controlling chemistry with cations: photochemistry within zeolites

The alkali ions present in the supercages of zeolites X and Y interact with included guest molecules through quadrupolar (cation-pi), and dipolar (cation-carbonyl) interactions. The presence of such interactions can be inferred through solid-state NMR spectra of the guest molecules. Alkali ions, as illustrated in this article, can be exploited to control the photochemical and photophysical behaviors of the guest molecules. For example, molecules that rarely phosphoresce can be induced to do so within heavy cation-exchanged zeolites. The nature (electronic configuration) of the lowest triplet state of carbonyl compounds can be altered with the help of light alkali metal ions. This state switch (n pi*-pi pi*) helps to bring out reactivity that normally remains dormant. Selectivity obtained during the singlet oxygen oxidation of olefins within zeolites illustrates the remarkable control that can be exerted on photoreactions with the help of a confined medium that also has active sites. The reaction cavities of zeolites, like enzymes, are not only well-defined and confined, but also have active sites that closely guide the reactant molecule from start to finish. The examples provided here illustrate that zeolites are far more useful than simple shape-selective catalysts.

Zeolite-coated quartz fibers as media for photochemical and photophysical studies

Zeolite-coated optical fibers are useful as media to carry out asymmetric photochemical reactions and for sensing polyaromatic compounds.