Alkenes Wilkinson complex

The difference between this catalytic system and Wilkinson s catalyst lies in the sequence of the oxidative addition and the alkene complexation. As mentioned above, for the cationic catalysts the intermediate alkene (enamide) complex has been spectroscopically observed. Subsequently oxidative addition of H2 and insertion of the alkene occurs, followed by reductive elimination of the hydrogenation product. 

SCHEME 2. Hydrogenation of alkenes catalyzed by the Wilkinson complex. 

Pereira and Srebnik added tetrachloromethane to alkenes 430 catalyzed by the Wilkinson complex and Schwartz reagent 431 mediated by pinacolborane [473]. The mechanism of the reaction leading to 432 was proposed not to involve radicals, which was supported by failure of inhibition by galvinoxyl or BHT. 

In an intramolecular Mizoroki-Heck-type alkenylation of alkenes, Wilkinson s catalyst [RhCl(PPh3)3] (84) proved superior than other rhodium compounds (Scheme 10.29) [56]. Notably, the selectivities of the cyclization of various 2-bromo-1,6-dienes were found to be improved with Wilkinson s catalyst when compared with those observed with palladium complexes. Thereby, the corresponding l,2-bis(methylene)cyclopentanes, which themselves are valuable substrates for further cycloaddition reactions, such as 1,3-diene 86, could be isolated in high yields. 

Over the last two decades, Wilkinson complex and related phosphine complexes of rhodium(I) have been used in numerous reactions for synthetic purposes, such as in the hydrosilylation of styrene and vinylcyclo-propene to yield ring-opening products of vinylamines. The [ (dippe)Rh 2(/u.-H)2] complex [where dippe = l,2-bis(diisopropylphosphino)ethane] is active in the hydrosilylation of olefins by diphenylsilane (4). Rhodium complexes were extremely favorable catalysts for dehydrogenative silylation of alkenes and divinyldiorganosilanes (4,13). 

A well-understood catalytic cycle is tliat of the Wilkinson alkene hydrogenation (figure C2.7.2) [2]. Like most catalytic cycles, tliat shown in figure C2.7.2 is complex, involving intennediate species in tire cycle (inside tire dashed line) and otlier species outside tire cycle and in dead-end patlis. Knowledge of all but a small number of catalytic cycles is only fragmentary because of tire complexity and because, if tire catalyst is active, tire cycle turns over rapidly and tire concentrations of tire intennediates are minute thus, tliese intennediates are often not even... 

C2.7.6.1 WILKINSON HYDROGENATION OF ALKENES CATALYSED BY A RHODIUM COMPLEX... 

In 1971, a short communication was published [54] by Kumada and co-workers reporting the formation of di- and polysilanes from dihydrosilanes by the action of a platinum complex. Also the Wilkinson catalyst (Ph3P)3RhCl promotes hydrosilation. If no alkenes are present, formation of chain silanes occurs. A thorough analysis of the product distribution shows a high preference for polymers (without a catalyst, disproportionation reactions of the silanes prevail). Cross experiments indicate the formation of a silylene complex as intermediate and in solution, free silylenes could also be trapped by Et3SiH [55, 56],... 

The hydrogenation activity of the isolated hydrides 3 and 6 towards cyclooctene or 1-octene was much lower than the Wilkinson s complex, [RhCKPPhj) ], under the same conditions [2] furthermore, isomerisation of the terminal to internal alkenes competed with the hydrogenation reaction. The reduced activity may be related to the high stability of the Rh(III) hydrides, while displacement of a coordinated NHC by alkene may lead to decomposition and Rh metal formation. 

Most low-valence metal complexes are generally deactivated by air and sometimes also by water. Carbon monoxide, hydrogen cyanide, and PH3 frequently act as poisons for these catalysts. Poisoning by strongly co-ordinating molecules occurs by formation of catalytically inert complexes. An example is the poisoning of Wilkinson s catalyst for alkene hydrogenation ... 

In some cases an alternative sequence involving addition of hydrogen at rhodium prior to complexation of the alkene may operate.11 The phosphine ligands serve both to provide a stable soluble complex and to adjust the reactivity at the metal center. The a-bonded intermediates have been observed for Wilkinson s catalyst12 and for several other related catalysts.13 For example, a partially hydrogenated structure has been isolated from methyl a-acetamidocinnamate.14... 

The mechanism of alkene hydrogenation catalyzed by the neutral rhodium complex RhCl(PPh3)3 (Wilkinson s catalyst) has been characterized in detail by Halpern [36-38]. The hydrogen oxidative addition step involves initial dissociation of PPI13, which enhances the rate of hydrogen activation by a factor... 

Another important use for Wilkinson s catalyst is in the production of materials that are optically active (by what is known as enantioselective hydrogenation). When the phosphine ligand is a chiral molecule and the alkene is one that can complex to the metal to form a structure that has R or S chirality, the two possible complexes will represent two different energy states. One will be more reactive than the other, so hydrogenation will lead to a product that contains predominantly only one of the diastereomers. 

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