Mechanism rhodium catalysis

M(CxC) matrix, 32 290-291, 311-313 Measurements, interpretation of, in experimental catalysis, 2 251 Mechanism see also specific types cobalt catalysis, 32 342-349 dehydrocyclization, 29 279-283 rhodium catalysis, 32 369-375 ruthenium catalysis, 32 381-387 space, 32 280... 

The first term represents the classic unicyclic rhodium catalysis, while the second indicates a hydride attack on an acyl species. These spectroscopic and kinetic results strongly suggested the presence of bimetallic catalytic binuclear elimination as the origin of synergism of both metals rather than cluster catalysis. This detailed evidence for such a catalytic mechanism, and its implications for selectivity and nonlinear catalytic activity illustrate the important mechanistic knowledge that can be revealed by this powerful in situ spectroscopic technique. 

In both catalytic cycles none of the complexes exceed 18 electrons. In order to account for the effect of phosphine on the nib ratio in rhodium catalysis (vide infra), an associative mechanism has been proposed in which alkene coordinates directly to the 18-electron HRh(CO)2L2 complex. Since this gives a 20-electron complex, this mechanism is not particularly attractive. A number of the intermediates in the rhodium catalytic cycle have been verified by various spectroscopic techniques.13,14... 

Reactions of soluble metal complexes, whose mechanisms of catalysis appear to be reasonably well known, can serve as a guide to the main reaction paths followed on heterogeneous catalysts. Mononuclear complexes catalyze syn addition of H2 to alkynes to yield initially only cis isomers, as in equation (25). 5 More recently, Muetterties and coworkers showed that the dinuclear rhodium hydride complex ( yi-H)Rh[P(OPr )3]2 2 (38) converts alkynes to trans isomers as initial products (equation 26). The alkyne addition compound (39) was isolated its structure shows the vinyl group bonded to one rhodium atom by a a-bond and to the other by a ir-bond, while the substituents on the vinyl group are trans to one another. This structure resembles ones hypothesized earlier to explain the formation of trans isomers and alkanes. Hydrogenations of alkynes which are catalyzed by the dinuclear rhodium hydride are much slower than the hydrogenation of an alkene catalyzed by the dinuclear tetrahydride (40), which is formed rapidly from (38) in the presence of H2 (equation 11). ... 

The area of catalytic activation of most unreactive C-Cl bonds has flourished tremendously over the last decade. Because of its considerable practical importance the field keeps growing at an impressive pace. Numerous novel techniques have been developed for the synthesis of various functionalized aromatic compounds from the corresponding chloroarenes. A series of new palladium, nickel, and rhodium catalysts have been synthesized for C-Cl activation and much information has been accrued on the mechanism of catalysis with these complexes. 

Chloroformamide and iodoformamide compounds, by direct interaction between a palladium carbamoyl complex and CI2 or I2 have been recently reported by us [12b, 13]. As far as the mechanism of catalysis under more drastic temperature conditions is concerned, in which our materials lose rhodium ions, the aniline synthesis could still occur through the above mechanism carried out by Rh " " ions eluted in solution. 

Rhodium. Just as two-electron reduction of rutheniumfra) chloride yields a catalytically active species (see above), so also does reduction of rhodium(m) chloride. In the presence of alcohol and base RhCl(CO)(PR3)a can be produced, which catalyses hydrogen transfer from an alcohol (as alkoxide) to olefins. The mechanism of catalysis by these rhodium(i) species RhX(CO)(PR3)2 is thought to be the same as that for the more widely studied (see below) IrX(CO)(PR3)a. ... 

The interpretation of the formation of the Ci3-lactone requires a sequence of mechanistical pathways which are unknown so far in rhodium-catalysis. Two proposals for the mechanism were given in Equation 12. The mechanism of path B is similar to that shown for palladium catalysis. A rhodium Cg-carboxylate complex is formed which under further incorporation of butadiene could yield the lactone. In the mechanism of path A three molecules of butadiene react with the starting rhodium compound forming a C- 2 Chain, which is bound to the rhodium by two n -ally1 systems and one olefinic double bond. Carbon dioxide inserts into one of the rhodium allyl bonds thus forming a C- 3-carboxyl ate complex, which yields the new C-13-lactone. 

Although the mechanism in rhodium catalysis is stiU unclear, it is believed that the reaction initiates through an oxidative cyclization of a diene with a low-valent rhodium species to produce a rhodacycle, which undergoes p-hydride elimination to form the common intermediate for both exo- and endo-observed products (Scheme 7.40) [59]. 

There is more to tire Wilkinson hydrogenation mechanism tlian tire cycle itself a number of species in tire cycle are drained away by reaction to fomi species outside tire cycle. Thus, for example, PPh (Ph is phenyl) drains rhodium from tire cycle and tlius it inliibits tire catalytic reaction (slows it down). However, PPh plays anotlier, essential role—it is part of tire catalytically active species and, as an electron-donor ligand, it affects tire reactivities of tire intemiediates in tire cycle in such a way tliat tliey react rapidly and lead to catalysis. Thus, tliere is a tradeoff tliat implies an optimum ratio of PPh to Rli. 

CO oxidation, an important step in automotive exhaust catalysis, is relatively simple and has been the subject of numerous fundamental studies. The reaction is catalyzed by noble metals such as platinum, palladium, rhodium, iridium, and even by gold, provided the gold particles are very small. We will assume that the oxidation on such catalysts proceeds through a mechanism in which adsorbed CO, O and CO2 are equilibrated with the gas phase, i.e. that we can use the quasi-equilibrium approximation. 

Laine and co-workers have studied the mechanism involved in rhodium-catalysed benzaldehyde hydrogenation, using [Rh6(CO)i6] as catalyst precursor. Following kinetic arguments, the authors proposed cluster catalysis with a limiting step corresponding to the break of metal-metal bond and/or isomerisation of the cluster formation [22]. 

This review has highlighted the key contributions of modern surface science to the understanding of the kinetics and mechanism of nitrogen oxide reduction catalysis. As discussed above, the conversion of NO has been taken as the standard to represent other NOx, and CO has typically been used as the reducing agent in these studies. The bulk of the work has been carried out on rhodium and palladium surfaces, the most common transition metals used in three-way catalytic converters. 

The combined information gathered from kinetic studies,184 in situ high-pressure NMR experiments,184,185,195 and the isolation of intermediates related to catalysis, leads to a common mechanism for all the hydrogenolysis reactions of (102)-(104) and other thiophenes catalyzed by triphos- or SULPHOS-rhodium complexes in conjuction with strong Bronsted bases. This mechanism (Scheme 41) involves the usual steps of C—S insertion, hydrogenation of the C—S inserted thiophene to the corresponding thiolate, and base-assisted reductive elimination of the thiol to complete the cycle.184 185 195-198... 

A key feature of the mechanism of Wilkinson s catalyst is that catalysis begins with reaction of the solvated catalyst, RhCl(PPh3)2S (S=solvent), and H2 to form a solvated dihydride Rh(H)2Cl(PPh3)2S [1], In a subsequent step the alkene binds to the catalyst and then is transformed into product via migratory insertion and reductive elimination steps. Schrock and Osborn investigated solvated cationic complexes [M(PR3)2S2]+ (M=Rh, Ir and S= solvent) that are closely related to Wilkinson s catalyst. Similarly to Wilkinson s catalyst, the mechanistic sequence proposed by Schrock and Osborn features initial reaction of the catalyst with H2 followed by reaction of the dihydride with alkene for the case of monophosphine-ligated rhodium and iridium catalysts [12-17]. Such mechanisms commonly are characterized... 

Keywords Carbonylation Catalysis Rhodium Iridium Mechanism... 

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