Electrophilic rearrangement surface

The rearrangement of light and deuterium-labelled cis- and trans-2-methyl-3-phenyloxiranes (1, 2 and 1, 2 ) was studied on ZnO, Al-O-j and WO, and in the presence of BF,. Both in the gas phase (473-673 K) and in the liquid phase (298-413 K), l-phenyl-2-propanone (3) and 2-phenylpropanal (4) were formed with high selectivities (0-90% and 11-80%, respectively). Ring-opening was found to occur by selective fission of the benzyl C-0 bond. Mechanistic studies revealed the formation of an open carbenium ion or a double-bonded surface intermediate. The acidic (electrophilic) and basic characters of the oxides determine the product distributions by affecting the relative importance of the competing mechanisms. 

Mechanistic insights as proposed by Kumar and coworkers supported the very design of the branching cascade strategy because the successful nucleophiles have utilized the dispersed electrophilic sites over the surface of common precursors 27 beautifully to provide different products via different cascade reactions (Scheme 27.4). In many cases, the reaction sequence apparently begins with the addition of nucleophiles to aldehyde or ketoester moiety and followed by further cyclization on to the chromones moiety (cascades I, III, IV, VI) before the second nucleophilic addition or rearrangement leading to diverse scaffolds. 

The surface (catalysis) Addo-basic Acid catalysis Basic catalysis Activation of alkanes and alcenes Breaking of C-C bonds in aUphatics Formation of C-C bonds in ahphatics Rearrangements of C-C bonds Reactions of isomerisation Reactions of nucleophUic substitution and addition CycUzation reactions Reactions of electrophilic substitution on aromatic rings Dehydrogenation of alcohols Isomerization of the double bonds of olefins... 

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