Zeolites supercage

The reaction used to test these solid catalysts was the aziridination of styrene with AT-tosyliminophenyliodinane (Phi = NTos) (Scheme 10). In most cases, enantioselectivities were low or moderate (up to 60% ee). The loss of enantioselectivity on changing from ligand 11a to ligand 12 was attributed to the fact that ligand 12 is too big for the copper complex to be accommodated into the zeolite supercages. Further studies carried out with hgands 11a and 11b [62] demonstrated that the reaction is more enantioselective with the supported catalyst (82% ee with 11a and 77% ee with 11b) than in solution (54% ee with 11a and 31% ee with 11b). This trend supports the confinement effect of the zeolite structure on the stereoselectivity of the reaction. 

When the metal nanoparticles are inserted into zeolite supercages, the size of the metal particles is confined according to the size of the supercage. However, after reduction of the precursor metal ions in a stream of hydrogen, the protons replacing the metal ions in the cation exchange position also interfere with the metal particles, influencing thereby their chemisorption and catalytic properties. 

Encapsulation of [Co(bpy)3]2+ within zeolite frameworks has also been shown to have a remarkable influence on the electronic spin state of the complex.240 Distortions imparted on the tris-chelate complex by the confines of the zeolite supercage are found to be responsible for stabilizing the unusual low-spin electronic ground state.241,242 The [Co(bpy)3]3+/2+ couple has been measured for the encapsulated complex and it has been found that the complexes remain within the zeolite and do not exchange with the bulk solution.243 Electrochemistry of [Co(bpy)3]3+/2+ immobilized within a sol-gel has also been studied.244... 

Nuclear magnetic resonance provides means to study molecular dynamics in every state of matter. When going from solid state over liquids to gases, besides mole- cular reorientations, translational diffusion occurs as well. CD4 molecule inserted into a zeolite supercage provides a new specific model system for studies of rotational and translational dynamics by deuteron NMR. 

Fischer-Tropsch synthesis could be "tailored by the use of iron, cobalt and ruthenium carbonyl complexes deposited on faujasite Y-type zeolite as starting materials for the preparation of catalysts. Short chain hydrocarbons, i.e. in the C-j-Cq range are obtained. It appears that the formation and the stabilization of small metallic aggregates into the zeolite supercage are the prerequisite to induce a chain length limitation in the hydrocondensation of carbon monoxide. However, the control of this selectivity through either a definite particle size of the metal or a shape selectivity of the zeolite is still a matter of speculation. Further work is needed to solve this dilemna. 

Fischer-Tropsch catalysis, 38 332 hydroformylation activity, 38 329-330 in NaY supercages, reversible formation and isomer transformation, 38 374 phosphino polystyrene support, 38 39 reactivity, 38 317-319, 323 ship-in-bottle synthesis in NaY zeolite supercages, 38 368-370... 

For the achievement of mass balances in intrazeolite photooxygenation reaction, >80%, loading levels of 0.1-0.3 adsorbed molecules per zeolite supercage have been successfully used in the past. However, the recent observation by Pace and Clennan, that replacing the solvent hexane with peril uorohexane was very crucial for the efficiency of the reaction, allowed the zeolite medium to be used for preparative scale photooxygenation reactions (500 mg of alkene), without loss of the product selectivity or the reduction of the mass balance. 

A promising unprecedented application of the chiral enecarbamates Ic in asymmetric synthesis is based on the ship-in-the-bottle strategy, which entails the oxidation of these substrates in zeolite supercages . In this novel concept, presumably dioxetanes intervene as intermediates, as illustrated for the oxidation of the chiral enecarbamate Ic in the NaY zeolite (Scheme 6). By starting with a 50 50 mixture of the diastereomeric enecarbamates (45, 3 R)-lc and (45, 3 5 )-lc, absorbed by the NaY zeolite, its oxidation furnishes the enantiomerically enriched (ee ca 50%) S -methyldesoxybenzoin, whereas the (4R,3 R)-lc and (4R,3 S)-lc diastereomeric mixture affords preferentially (ee ca 47%) the R enantiomer however, racemic methylbenzoin is obtained when the chirality center at the C-4 position in the oxazolidinone is removed. Evidently, appreciable asymmetric induction is mediated by the optically active oxazolidinone auxiliary. 

Unquestionably, the advantage of the present methodology is that the intermediary dioxetane serves as a vehicle to place the chiral inductor (the oxazolidinone auxiliary) and the racemic substrate to be resolved (the methyldesoxybenzoin) in one and the same zeolite supercage. These represent optimal conditions for efficacious asymmetric induction, a novel application of chiral dioxetanes which merits further elaboration. 

Fig. 2.1.6.6 FTIR spectra of a) the Jacobsen ligand, b) the Jacobsen catalyst, and c) the manganese-salen complex, in zeolite supercages.

For the catalysts prepared by entrapment in the zeolite supercages, only the epoxidation of (-)-a-pinene as depicted in Scheme 2.1.6.3 and of limonene was... 

Side-Chain Alkylation. There is continued interest in the alkylation of toluene with methanol because of the potential of the process in practical application to produce styrene.430 Basic catalysts, specifically, alkali cation-exchanged zeolites, were tested in the transformation. The alkali cation acts as weak Lewis acid site, and the basic sites are the framework oxygen atoms. The base strength and catalytic activity of these materials can be significantly increased by incorporating alkali metal or alkali metal oxide clusters in the zeolite supercages. Results up to 1995 are summarized in a review.430... 

The spectrum for LaY impregnated with vanadyl naphthenate shows a characteristic band at 365 nm that loses most of its intensity after calcination, Figs. 5a, 5b. This is not surprising since Pompe et al (30), using TGA/DTA data,have shown that the oxidative decomposition of the vanadyl naphthenate is complete at 500°C. Electron paramagnetic resonance (EPR) studies have shown that vanadium (after calcination) is stabilized mainly in the form of vanadyl (V02+) cations in the zeolite supercages (29). 

A nonlinear increase of Ss upon increasing 0 is evident for all the adsorbate/adsorbent systems (Figure 3a) the curvatures reflect many-body interactions between a single xenon atom with benzene molecules within the zeolite supercage. Regressional fitting by a third-degree polynomial to the experimental values produces ... 

Irradiation of (S )-tropolone 2-methyl butyl ether in solution yields a 4-electron electrocyclization product as a 1 1 diastereomeric mixture (Sch. 8) [106]. In solution the presence of the chiral auxiliary in proximity to the reactive center has no influence on the product stereochemistry. When irradiated within NaY zeolite, however, the same molecule affords the cyclized product in 53% diastereomeric excess. The restricted space of the zeolite supercage apparently forces communication between the chiral center and the reaction site. 

---