Surface-Catalyzed Rearrangements

In a recent study the IT-catalyzed rearrangement of allyl (3,5-di-terr-butylphenyl) ether was used as selective probe for the outer surface activity.44 The best passivation results were obtained by treatment of the Beta crystals with tetraethyl orthosilicate (TEOS). Slow hydrolysis of TEOS by traces of water present in the pores of the zeolite provides a thin, porous layer of amorphous silica and leads to complete outer surface passivation on 1 p,m crystals. 

The rearrangement of allyl 0-ketoesters absorbed on an alumina surface at 60°C is one of the modifications, other modifications include a ruthenium complex catalyzed rearrangement. ... 

Acid catalysis by titanium silicate molecular sieves another area characterized by recent major progress. Whereas only two categories of acid-catalyzed reactions (the Beckmann rearrangement and MTBE synthesis) were included in the review by Notari in 1996 (33), the list has grown significantly since then. In view of the presence of weak Lewis acid sites on the surfaces of these catalysts, they can be used for reactions that require such weak acidity. 

As previously mentioned, Davis (8) has shown that in model dehydrocyclization reactions with a dual function catalyst and an n-octane feedstock, isomerization of the hydrocarbon to 2-and 3-methylheptane is faster than the dehydrocyclization reaction. Although competitive isomerization of an alkane feedstock is commonly observed in model studies using monofunctional (Pt) catalysts, some of the alkanes produced can be rationalized as products of the hydrogenolysis of substituted cyclopentanes, which in turn can be formed on platinum surfaces via free radical-like mechanisms. However, the 2- and 3-methylheptane isomers (out of a total of 18 possible C8Hi8 isomers) observed with dual function catalysts are those expected from the rearrangement of n-octane via carbocation intermediates. Such acid-catalyzed isomerizations are widely acknowledged to occur via a protonated cyclopropane structure (25, 28), in this case one derived from the 2-octyl cation, which can then be the precursor... 

Rearrangement reactions catalyzed by the clay surface were observed for par-athion (an organophosphate pesticide) when it was adsorbed on montmorillonite or kaolinite in the absence of a liquid phase. The rate of rearrangement reactions increased with the polarization of the hydration water of the exchangeable cation (Mingelgrin and Saltzman 1977). Table 14.1 summarizes a series of reactions catalyzed by clay surfaces, as reported in the literature. 

In these sections of our chapter, we emphasize research advances in the area of surface acidity of specific solids that have occurred during the period from 1970 to the fall of 1976. As stated earlier, the class of solids with which we are chiefly concerned are metal oxides that catalyze skeletal rearrangements of hydrocarbons via carbonium ion intermediates. However, we have included reviews of silica gel and alumina, which are relatively inactive, because the properties of these solids form a useful frame of reference. The initial sections (Sections III.A-III.D) deal predominantly with amorphous catalysts the final sections (Sections III.E and III.F), with crystalline catalysts. 

Association of dimers, strongly catalyzed by Mg2+, produces a cyclical structure (Figure lc) that can contain four monomers only. The tetramer has C4 rotational symmetry and resembles a bowl, in which the bottom is formed by the horizontal parts of the bodies. This part is important for penetration into hpid bilayers, and it is likely that structural rearrangements required for tetramerization expose the surface regions favorable to interaction with hpid bilayers. In addition, this part represents the intracellular mouth of the channel, with a large (30 A) central hole in its center. 

Surprisingly, gold can also catalyze skeletal rearrangements of hydrocarbons for instance, the isomerization of 2,2-dimethylbutane to n-hexane has been achieved by Schmid with the aid of AU55 clusters on titanium dioxide [4c, 6] and the aromatization of the dispirocycle 1 to tetrahydronaphthalene 2 was achieved by de Meijere et al. in a reactor with gold surface at 100 °C in a few seconds (Scheme 1) [7]. 

Thus, the band positions of the irradiated semiconductor are key thermodynamic variables for in the control of the observed redox chemistry resulting from photo-catalyzed single-electron transfer (charge trapping) across the semiconductor-electrolyte interface. Whether oxygenation, rearrangement, isomerization, or other consequences follow this initial electron transfer seems to be controlled by surface effects on the relative rates of reaction. 

Davis SM, Zaera F, Somorjai GA (1982) Surface structure and temperature dependence of light-alkane skeletal rearrangement reactions catalyzed over platinum single-crystal surfaces. J Am Chem Soc 104 7453... 

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