Asymmetric Photoreactions within Zeolites: Role of Confinement and Alkali Metal Ions † J. SIVAGURU, ARUNKUMAR NATARAJAN, LAKSHMI S. KAANUMALLE, J. SHAILAJA, SUNDARARAJAN UPPILI, ABRAHAM JOY, AND V. RAMAMURTHY* Department of Chemistry, Tulane University, New Orleans, Louisiana 70118 Received November 21, 2002 ABSTRACT 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. Introduction Following the communication by Hammond and Cole in 1965 on the use of optically active sensitizers in the photosensitized isomerization of cis-diphenylcyclopro- pane, 1 several groups have performed enantio- and dias- tereoselective phototransformations, both in solution and the solid state. 2 Despite considerable efforts, the enan- tiomeric excesses (ee’s) obtained under ambient condi- tions in solution continue to be less than 50%. The best results in solution have been obtained via the chiral auxiliary methodology, yielding in select examples dia- stereomeric excesses (de’s) close to 100%. 3 Asymmetric photochemistry in the crystalline state is based on the “chance” crystallization of achiral molecules in chiral space groups. 4 Because of the limited probability of this happening, there are relatively few examples of asym- metric induction during photolysis of achiral molecules in the crystalline state. 5 A more general methodology known as the “ionic chiral auxiliary approach” introduced by Scheffer facilitates the crystallization of achiral mol- ecules in chiral space groups. 6 On the basis of this strategy, Scheffer and co-workers have provided a number of examples that yield photoproducts in very high ee (or de) in the crystalline state. Recognizing the problem of crystallizing achiral mol- ecules in chiral space groups, several researchers have explored chiral hosts as the reaction medium. The earliest such report is that of Natta on the photopolymerization of 1,3-dienes included in the channels of optically active perhydrotriphenylene, 7 and the most successful ones are those of Toda using chiral diol hosts. 8 While crystalline and host -guest assemblies have been extremely useful to conducting enantioselective photoreactions, their gen- eral applicability has been limited. Not all molecules crystallize either alone or in the presence of organic host systems. The reactivity of molecules in the crystalline state and in solid host -guest assemblies is controlled by the details of molecular packing. Currently, molecular pack- ing, and consequently the chemical reactivity in the crystalline state, cannot be reliably predicted. 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. Our approach has been to employ readily available and inexpensive zeolites as media to bring about asymmetric induction in photochemical reactions. The internal struc- ture of the faujasite class of zeolites is characterized by a three-dimensional network of supercages (ca. 13 Å in diameter) interconnected by tetrahedrally disposed win- dows (ca. 8 Å diameter, Figure 1). 9 Due to the difference in charge between the [SiO 4 ] 4- and [AlO 4 ] 5- tetrahedra which form the framework of the zeolite, the net charge of an aluminum containing zeolite is negative and hence must be balanced by a cation. Charge compensating cations are located at three different sites within the zeolite framework, of which only two (types II and III) are expected to be readily accessible to the adsorbed organic molecule. The Yzeolite used in this study has only type I and II sites occupied. The results of our studies within zeolites in the context of asymmetric photochemistry are highlighted in this Account. 10 The Chiral Inductor Method To provide the asymmetric environment lacking in zeolites during the reaction, a chiral source must be introduced. For this purpose, in the approach we refer to as the chiral inductor method (CIM), the nonchiral interior surface of the zeolite is rendered “locally chiral” by adsorption of a † Dedicated to Professor N. J. Turro on the occasion of his 65th birthday. *Corresponding author. J. Sivaguru, A. Natarajan, and L. S. Kaanumalle, having completed their M.Sc. at IIT Madras, IIT Bombay and IIT Madras respectively, are currently carrying out research on asymmetric photochemistry at Tulane toward their Ph.D. dissertations. J. Shailaja (M.Sc., University of Madras), S. Uppili (M.Sc., IIT Bombay), and A. Joy (M.Sc, University of Hyderabad), having obtained Ph. D. degrees from Tulane, are currently carrying out postdoctoral work in the laboratories of D. Gin (University of Colorado), D. Allara (Pennsylvania State University), and G. Schuster (Georgia Tech), respectively. V. Ramamurthy (M.Sc., IIT Madras, Ph.D, University of Hawaii) had his postdoctoral training in the laboratories of P. de Mayo (University of Western Ontario) and N. J. Turro (Columbia University). Prior to joining Tulane in 1994, he was on the faculty at the Organic Chemistry Department, Indian Institute of Science, Bangalore (1978-87), and Central Research, The Du Pont Company (1987-1994). His main research interests have been on understanding the excited- state behavior of organic molecules in confined media (crystals, micelles, cyclodextrins, and zeolites) and exploring upper excited-state chemistry. His accomplishments in these areas have been highlighted in four previous Accounts. Acc. Chem. Res. 2003, 36, 509-521 10.1021/ar020269i CCC: $25.00  2003 American Chemical Society VOL. 36, NO. 7, 2003 / ACCOUNTS OF CHEM ICAL RESEARCH 509 Published on Web 06/06/2003