Sundararajan Uppili

The influence of chiral auxiliaries is enhanced within zeolites

Zeolites significantly enhance the influence of chiral auxiliaries during photochemical reactions. The generality of this phenomenon has been tested with three independent examples. Chiral auxiliaries that lead to 1:1 mixtures of diastereomers in solution give products in up to 80% diastereomeric excess in the presence of faujasite type zeolites.

Controlling the reactive state through cation binding: photochemistry of enones within zeolites

The nature of the lowest triplet state of enones is altered by the cations present within Y zeolites. Alkali metal ions, such as Li+, are predicted to interact with the carbonyl unit of enones in a collinear fashion and significantly lower both the p-type n and π-2 orbitals. Excited state energies, estimated at the CIS(D)/6-31+G* level, show that the lowest triplet is n-π* in character for the enones, but switch to π–π* on coordination with Li+. Observed product distribution within zeolite is consistent with this theoretical prediction.

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.

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.