Chloride-Oxide Exchange Catalysis

With both the Fu and the Denmark catalysts it can be assumed that catalysis is effected by formation of a highly electrophilic silicon cation D from tetrachlorosi-lane and the nucleophilic catalyst C, i.e. by attack of the pyridine N-oxide or of the phosphoramide O-atom on silicon, followed by ionization (Scheme 13.38). The latter cation can then activate the epoxide toward nucleophilic attack by the chloride ion. Exchange of the product silane for another molecule of tetrachlorosilane completes the catalytic cycle [75],... 

TTomogeneous catalysis has made considerable progress in the past decade. Even so there is still uncertainty as to the detailed mechanism of many catalytic reactions. This is particularly true of Pd (II) catalysis. The advances in organic synthesis by Pd (II) catalytic chemistry have been impressive a recent review (I) of two volumes had few references before 1963. However the advances have mainly been in synthetic chemistry, and relatively little effort has been expended on studies of mechanism in this chemistry. To fill this gap we have undertaken a systematic study of the basic mechanisms of Pd (II) catalysis. Some choice of the many catalytic reactions available for study had to be made. For several reasons the reactions chosen for initial study were olefin oxidation and exchange reactions of vinylic and allylic esters and chlorides. Work has concentrated on the PdCl2-NaCl or LiCl systems in... 

Shrigadi, N. B., A. B. Shinde, and S. D. Samant. 2003. Study of catalytic activity of free and KlO-supported iron oxyhydroxides and oxides in the Friedel-Crafts benzylation reaction using benzyl chloride/alcohol to understand their role in the catalysis by the Fe-exchanged/impregnated K10 catalysts. Appl. Catal. A 252 23-25. 

The labeling of furan (Table XII) is similar to thiophene, since the most active of the borohydride-reduced catalysts is iridium. However, self-activated iridium oxide is the most active of all for general deuteration, whereas nickel chloride is the most efficient and selective for the a. position, only slight polymerization being observed. Acid catalysis should be used with caution for exchange labeling, since furan readily polymerizes above 30° in the presence of hydrochloric acid and chlorides. 

The rates of reaction of hypophosphorous acid with iodine bromine ", chlorine ", iodine chlorides , iodate , selenious and tel-lurous acids, silver nitrate , cupric chloride and mercuric chloride" (all forming phosphorous acid or phosphites) have been measured, and the results of the earlier work summarized clearly" . All the data are consistent with the hypothesis that there is prior transformation to some reactive form (I). This form (I) does not discriminate very effectively between different oxidants and thus the oxidation steps are presumed to have rates close to the diffusion-controlled limit. The rates of formation of I deduced in these studies are close enough to the rates of deuterium and tritium exchange for the residual difference to represent an isotope effect. Mitchell wrote the formula H5PO3 for I. Others have supposed it to be a tautomer e.g. HPO(OH)2. Both the isotopic exchange results and the oxidation studies require that its formation and decomposition be subject to acid catalysis. For the general mechanism... 

Several studies have used palladium catalysis in the arylation of benzoxazoles. A palladium catalyst with a phosphine ligand allows their reaction with aryl mesylates without the requirement for acid or copper additives. In the reaction with arene-sulfonyl chloride, palladium is used in combination with copper. A plausible mechanism involves initial cupration of the benzoxazole followed by copper—palladium exchange and oxidative addition of the sulfonyl chloride to palladium to give (84). This intermediate may lose sulfur dioxide to give an aryl palladium species, which, on reductive elimination, yields 2-arylbenzoxazole. The arylation of benzoxazoles and benzthiazoles with aryl boronic acids is also catalysed by a combination of palladium... 

Effects of cationic (cetylpyridinium chloride, CPC) and anionic (SDS) micelles on the rate of reaction of chromium(VI) oxidation of formaldehyde have been studied in the presence and absence of picolinic acid. Cationic micelles (CPC) inhibit whereas anionic micelles (SDS) catalyze the reaction rates that could be attributed to electrostatic interactions between reactants (cationic metal ions and catalyst H+) and ionic head groups of ionic micelles. Experimentally determined kinetic data on these metaUomicellar-mediated reactions have been explained by different kinetic models such as pseudophase ion-exchange (PIE) model, Monger s enzyme-kinetic-type model, and Piszkiewicz s cooperativity model (Chapter 3). The rate of oxidation of proline by vanadium(V) with water acting as nucleophile is catalyzed by aqueous micelles. Effects of anionic micelles (SDS) on the rate of A-bromobenzamide-catalyzed oxidation of ethanol, propanol, and n-butanol in acidic medium reveal the presence of premicellar catalysis that has been rationalized in light of the positive cooperativity model. ... 

These reactions complement recently developed palladium(0)amination reactions [146,147,148] and related procedures using a copper(I) [149] - or ni-ckel(O) [151] - catalysis. As indicated above, the mild reaction conditions are compatible with a range of functional groups. Functionalized arylmagnesium chlorides such as 309 prepared by an I/Mg-exchange readily undergo addition reactions to aryl oxazolines. The addition-elimination of 309 to the -methoxy aryloxa-zoline followed by an ortHo-lithiation and substitution with ethylene oxide leads to a polyfunctionalized aromatic intermediate 310 for alkaloid synthesis (Scheme 4.68) [165]. 

---