Oxyfunctionalization of Alkenes by Dye-Sensitized Intrazeolite Photooxygenation

Current Organic Synthesis, 2005, 2, 281-299 281 Oxyfunctionalization of Alkenes by Dye-Sensitized Intrazeolite Photooxygenation Manolis Stratakis* Department of Chemistry, University of Crete, 71409 Iraklion, Greece Abstract: This review focuses on the recent achievements towards the selective formation of allylic 1 hydroperoxides by the reaction of singlet molecular oxygen ( O2) with alkenes adsorbed in the cavities of zeolite Na-Y. The product distribution by zeolite confinement is often dramatically different compared to the photooxygenation reaction in a homogeneous medium. Cation – π interactions and cation – singlet oxygen interactions, in the rate-limiting transition states, are most likely responsible for the changes in product selectivity within Na-Y. Keywords: Singlet oxygen, allylic hydroperoxides, regioselectivity, diastereoselectivity, zeolite Na-Y, cation – π interactions. STRUCTURAL FEATURES OF FAUJASITES The faujasite framework consists of two main cages, the supercage, and the sodalites (Fig. 1). The supercage results Zeolites are crystalline aluminosilicates whose primary from the assembly of the smaller sodalite cages. The access 4- 5- structure is formed by SiO4 and AlO4 tetrahedra sharing to the supercages occurs by four 12-membered ring the edges [1]. Their tertiary structure forms strictly uniform “windows”, of approximately 7-8 Å in diameter. channels and cavities of molecular dimensions that are repeated along the zeolite lattice. Due to the lower valence of The “windows” are tetrahedrally distributed around the centre of the supercages, which are approximately 13 Å in the aluminum relative to silicon, the excess negative charge diameter. The charge-compensating cations in faujasites (one per Al atom) is balanced by alkali metal cations, occupy three different positions as shown in Fig. 1, namely mainly Na+. The cations are placed in the interior of cavities and can be easily exchanged. The cation-exchange process Type I-III [3]. Only cations at sites II and III can interact to leads to new type of materials having interesting properties the hosted organic compounds. Depending on the size of the [2], and it is expected in the near future that the metal- cation, the supercages in the alkali metal exchanged Y supported zeolites will find significant applications in zeolites (M-Y) have enough net volume to host relatively applied chemistry. large organic molecules. For example, Na-Y can adsorb molecules even of the size of a steroid. On the other hand, An important class of the zeolite family are the for type X faujasites, the free volume of the supercages is faujasites, known as zeolites X and Y, which have the more limited due to the extra compensating cations, and typical composition for the unit cell as follows: finds less applications in organic chemistry. Type Na-X Na86(AlO2)86(SiO2)106 · x H2O The performance of organic reactions in organized media, Type Na-Y Na (AlO ) (SiO ) · x H O e.g. by zeolite confinement [4-6], and the use of zeolites as 56 2 56 2 136 2 selective and “green” catalysts [7] for organic transformations has been popularized in recent years. The main advantage of the zeolites to be tested as media or catalysts for carrying out organic reactions, is the so called “shape selectivity” [8]. The shape selectivity can be categorized into 3 types: (i) Reactant selectivity; from a mixture of reactants only the molecules with the appropriate shape can be adsorbed into the cavities. (ii) Product selectivity; only those products, produced from the intrazeolite reaction, having the appropriate shape can diffuse out of the pores, and (iii) Transition state shape selectivity; due to the zeolite confinement, the relative stability of transition states for the possible reaction pathways can be significantly altered. Fig. (1). Structure of the faujasite supercage assembled by the sodalite cages (the arrows indicate the positions of the cations). PHOTOOXYGENATION REACTIONS IN ORGANIZED MEDIA *Address correspondence to these authors at the Department of Chemistry, University of Crete, 71409 Iraklion, Greece; E-mail: The partial exchange of the Na+ cations within the zeolite [email protected] Na-Y supercages by dye molecules that have the structure of 1570-1794/05 $50.00+.00 © 2005 Bentham Science Publishers Ltd. 282 Current Organic Synthesis, 2005, Vol. 2, No. 2 Manolis Stratakis organic cations (e.g. methylene blue, thionin, etc.) [9] For the achievement of mass balances in intrazeolite triggered the interest to examine the dye-sensitized photooxygenation reaction >80%, loading levels of 0.1-0.3 photooxygenation of organic compounds within the confined adsorbed molecules per zeolite supercage have been environment of the faujasite Y cavities. The reaction of successfully used in the past. However, the recent singlet oxygen with organic compounds has attracted a observation by Clennan and Pace [19], that replacing the significant attention in the scientific community, not only solvent hexane with perfluorohexane was very crucial for the because of their biological role [10], but also due to their efficiency of the reaction, allowed the zeolite medium to be mechanistic interest and the valuable synthetic applications used for preparative scale photooxygenation reactions (500 as well [11]. The study of singlet oxygen reactions with mgr of alkene), without loss of the product selectivity or the alkenes using alternative supramolecular systems as reduction of the mass balance. The fluorophobicity of microreactors [12], such as pentasil zeolites [13], nafion alkenes in perfluorohexane allows the facile migration of the membranes [14] and surfactant vesicles [15], has also reactant molecules into the zeolite cavities. In hexane, on the attracted a considerable attention recently, but will not be other hand, the affinity of a simple alkene such as 2-methyl- presented in the present review article. 2-heptene for entering into the interior of Na-Y decreases by For the success of the dye-sensitized singlet oxygen ~50% relative to the fluorinated solvent. In the same study, 1O lifetime reactions within zeolite Na-Y, the loading level of the dye the authors extrapolated the upper limit for the 2 within Na-Y to be 7.5 µs. Based on this value, it was and the water content [16] in the interior of zeolite are estimated that singlet oxygen can migrate through 5,000 crucial. At increased loading levels, the dye tend to dimerize 3 1 supercages, before it deactivates to O2 or react with the and do not emit [17], thus, inefficiently producing O2 upon excitation by energy transfer to the triplet molecular oxygen. adsorbed substrate. At an approximate loading level of 1 thionin cation per 100 zeolite supercages, irradiation under a constant flow of COMPLEXITIES OF THE INTRAZEOLITE oxygen gas produces singlet oxygen efficiently, leading to PHOTOOXYGENATION REACTIONS rapid oxidation of tri- and tetrasubstituted alkenes to form ene allylic hydroperoxides [18] as shown for the It is well-known that faujasites (Na-Y or Na-X) contain photooxygenation of tetramethylethylene (1) in Scheme 1. small concentration of both Brönsted [20] and Lewis acid sites [21]. The acidic sites may cause significant problems, H C CH 3 3 Thionin/Na-Y H2C CH3 such as the decomposition or rearrangement of product and OOH reactants, giving often poor reaction mass balances. Acid- H CCH hv/O 3 3 2 H3CCH3 sensitive alkenes, such as terpenes, upon adsorption within 1 Na-Y undergo isomerization and skeletal rearrangements. Scheme 1. Ene hydroperoxidation of tetramethylethylene within Ramamurthy and co-workers have reported such problems in thionin-supported Na-Y. the intrazeolite photooxygenation of limonene [16]. In addition, tertiary allylic hydroperoxides, formed in the 23 Na - Y 45 9 67 8 monoterpene Scheme 2. Transformation of monoterpenes to p-cymene within Na-Y under thermal conditions. Oxyfunctionalization of Alkenes by Dye-Sensitized Intrazeolite Photooxygenation Current Organic Synthesis, 2005, Vol. 2, No. 2 283 photooxygenation of trisubstituted alkenes do not persist benzene binding enthalpy is roughly equivalent to the + within the Na-Y cages [18]. The unwanted side-reactions and binding enthalpy between Na and H2O. The adsorption of complexities, attributable to acid catalysis can be minimized alkenes or arenes into the interior of the zeolites is mainly or completely suppressed by treatment of the zeolite with driven by their quadrupolar interaction with the alkali metal bases such as pyridine or triethylamine for a few minutes cations present in the cages. Haw and co-workers [26] have prior to the photooxygenation reaction. presented experimental evidence for the Li+ - benzene interaction within zeolite LiZSM-5 using solid state NMR. We studied [22] the fate of several monoterpenes within Cations not only play a significant role in the adsorption thionin-supported Na-Y under thermal conditions, and found efficiency of the porous materials, but they have also that the intrazeolite transformations of the adsorbed proposed [27] to play a major role in intrazeolite reactions molecules are most probably consonant with an electron by affecting the relative energies of the possible transition transfer pathway subordinated to the presence of the acidic states through substrate binding [28]. To have a deep sites. For example, upon loading of several monoterpenes knowledge of the factors affecting the product distribution in within thionin-supported Na-Y (monocyclic, bicyclic or intrazeolite photooxygenation reactions, it was necessary

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