Zeolites charge-transfer complex with oxygen

Takeya, H., Kuriyama, Y. and Kojima, M. (1998). Photooxygenation of stilbenes in zeolite by excitation of their contact charge transfer complexes with oxygen. Tetrahedron Lett. 39, 5967-5970... 

Kojima and co-workers also examined the photo-oxidation of stilbenes in zeolite Y [150]. The presence of O2 in the stilbene-NaY sample led to a red-shifting of the tail end of the diffuse reflectance spectrum and was assigned to the charge-transfer complex with O2. Upon excitation at 313/366 nm, both cis- and /ra .v-stilbene undergo photo-oxygenation to form benzaldehyde. Phenanthrene, the expected product from the ]2 -f 4] cycloaddition reaction of excited m-stilbene, was also formed. The details of the mechanism leading to benzaldehyde and phenanthrene... 

In 1993, Blatter and Frei [34] extended the Aronovitch and Mazur [28] photo-oxidation into zeolitic media, which resulted in several distinctive advantages as described below. Irradiation in the visible region (633 nm) of zeolite NaY loaded with 2,3-dimethyl-2-butene, 16, and oxygen resulted in formation of allylic hydroperoxide, 17, and a small amount of acetone. The reaction was followed by in situ Fourier-transform infrared (FTlR) spectroscopy and the products were identified by comparison to authentic samples. The allylic hydroperoxide was stable at - 50°C but decomposed when the zeolite sample was warmed to 20°C [35]. In order to rationalize these observations, it was suggested that absorption of light by an alkene/Oi charge-transfer complex resulted in electron transfer to give an alkene radical cation-superoxide ion pair which collapses... 

The experimental facts whose interpretation demonstrates the presence of the oxo-binuclear structures are the followings (1) The AE carbonate formation in the zeolites [58]. The MeCOsMe carbonates are fixed in the zeolite framework and do not move so that AE carbonates can appear when a MeOxMe binuclear cluster exists already in the zeolite. The immobility of the AE carbonates was confirmed from comparison of electron densities at the critical Me- O bond points with the oxygen atoms of the carbonates or of the framework [54] (2) The CO oxidation processes under thermal activation, for which the mechanism with a charge transfer stabilized CO-O2 complex cannot be applied. The present interpretation arrives to a reasonable barrier values around of 15 kcal/mol [53, 54] (3) The formation of two different hydroxyl groups at 3643 and 3533 cm after addition of dried HCl (or HCN) to totally dehydrated MgY [58]. The first band can be interpreted due to the barrierless transformation of MeOMe to ClMe-0(H)-Me upon HCl addition. The... 

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