Rhodium-catalysed reactions carbonylation

A mechanistic study by Haynes et al. demonstrated that the same basic reaction cycle operates for rhodium-catalysed methanol carbonylation in both homogeneous and supported systems [59]. The catalytically active complex [Rh(CO)2l2] was supported on an ion exchange resin based on poly(4-vinylpyridine-co-styrene-co-divinylbenzene) in which the pendant pyridyl groups had been quaternised by reaction with Mel. Heterogenisation of the Rh(I) complex was achieved by reaction of the quaternised polymer with the dimer, [Rh(CO)2l]2 (Scheme 11). Infrared spectroscopy revealed i (CO) bands for the supported [Rh(CO)2l2] anions at frequencies very similar to those observed in solution spectra. The structure of the supported complex was confirmed by EXAFS measurements, which revealed a square planar geometry comparable to that found in solution and the solid state. The first X-ray crystal structures of salts of [Rh(CO)2l2]" were also reported in this study. 

The proposed mechanism for this carbonylation reaction, which occurs in the absence of water, involves basic catalytic steps similiar to the rhodium-catalysed methanol carbonylation process (see Section 2.1.2.1.1). The mechanism leads to the formation of acetyl iodide, which reacts with methyl formate to produce the mixed anhydride [133]. 

The palladium catalysed reaction -follows a rate law which is independent on the substrate concentration, but dependent on the CO pressure. In a later work, a AS =-233 J mol " "K, instead o-f -414, has been reported -for this react i on [1843, tor which it has been confirmed a zero order in substrate and first order in each metallic component and in CO pressure. The carbonyl ation of an intermediate complex forming the isocyanate is considered the rate determinin step in the palladium-catalysed reaction. In this work[183j, the oxidative addition of the nitro compound to the catalyst was considered a more likely rate determining step in the case of the rhodium-catalysed reaction. 

Carbonylation and decarbonylation reactions of alkyl complexes in catalytic cycles have been reviewed . A full account of the carbonylation and homologation of formic and other carboxylic acid esters catalysed by Ru/CO/I systems at 200 C and 150-200 atm CO/H2 has appeared. In a novel reaction, cyclobutanones are converted to disiloxycyclopentenes with hydrosilane and CO in the presence of cobalt carbonyl (reaction 4) . The oxidative addition of Mel to [Rh(CO)2l2] in aprotic solvents (MeOH, CHCI3, THF, MeOAc), the rate determining step in carbonylation of methyl acetate and methyl halides, is promoted by iodides, such as Bu jN+I", and bases (eg 1-methylimidazole) . A further kinetic study of rhodium catalysed methanol carbonylation has appeared . The carbonylation of methanol by catalysts prepared by deposition of Rh complexes on silica alumina or zeolites is comparable with the homogeneous analogue . 

Monsanto developed the rhodium-catalysed process for the carbonylation of methanol to produce acetic acid in the late sixties. It is a large-scale operation employing a rhodium/iodide catalyst converting methanol and carbon monoxide into acetic acid. At standard conditions the reaction is thermodynamically allowed,... 

The reaction of alcohols with CO can also be catalysed by palladium iodides, and various ligands or solvents. Acetic acid is prepared by the reaction of MeOH with CO in the presence of a catalyst system comprising a palladium compound, an ionic iodide compound, a sulfone solvent at conditions similar to those of the rhodium system (180 °C, 60 bar), and, in some cases, traces of a nickel-bipyridine compound were added. Sulfones or phosphine oxides play a stabilising role in preventing metal precipitation [26], Palladium(II) salts catalyse the carbonylation of methyl iodide in methanol to methyl acetate in the presence of an excess of iodide, even without amine or phosphine co-ligands platinum(II) salts are less effective [27],... 

Several C-labelled tetraphenyl arsonium salts of Rh(III) containing complex anions have been synthesized to investigate, by the N.M.R. method, the mechanism of the rhodium/iodine-catalysed industrial carbonylation of methanol used for acetic acid manufacture. A revised catalytic cycle for the reaction has been proposed (equation... 

Reaction with benzene follows a similar pathway, yielding a bicyclo[3.2.2]nonatriene structure. Vinylcarbenoids also react with pyrroles to give tropanes via a cyclopropanation-Cope rearrangement route. The direct addition of carbenes to acetylenes does not give satisfactory yields of cyclopropanes, but the rhodium carboxylate catalysed reaction of diazo compounds with acetylenes is a useful source of cyclopro-panes. Carbenoids can also attack a carbonyl oxygen atom, giving rise to a zwitterion (249). An excellent review of intramolecular carbenoid reactions has appeared. ... 

Access to a 1,4-dicarbonyl substrate has been realised in several ways. Examples include alkylation of imines with 2-alkoxy-allyl halides (equivalents of 2-halo-ketones),addition of /3-ketoester anions to nitroalkenes, followed by Nef reaction,and rhodium-catalysed carbonylation of 2-substituted acrolein acetals. The dialdehyde (as a mono-acetal) necessary for a synthesis of diethyl furan-3,4-dicarboxylate was obtained by two successive Claisen condensations between diethyl succinate and ethyl formate, as shown in the sequence below. 

At 120 °C and 1 atm, AH =18 Kcal mol and AS = -30 cal mol K have been calculated [112]. In nonpolar solvents such as decane the reaction does not proceed. The deoxygenation of the nitro group to nitroso is considered to be the rate determining step of the reaction, since the subsequent carbonylation of the nitroso compounds to give the isocyanate derivative is about ten times faster than the one of the nitro compound in the presence of [Rh(CO)2Cl]2 as catalyst [114, 115], a reaction which is accelerated by M0CI5. The mechanism of this reactions is discussed in more detail in Chapter 6 we mention here that side-on rhodium nitrosoarene complexes were proposed as intermediates. However, which really are the coordination modes of the nitrosoarene to rhodium(I) in this system is still a question to be solved. As a matter of fact, the known arylnitroso complexes of mononuclear rhodium(I) complexes have a nitrogen-o-bonded nitroso group [116], and some of them can catalyse the carbonylation of nitrosobenzene to PhNCO[117]. 

Aspects of the rhodium-catalysed hydroformylation of olefins have been reviewed. " Copper(ii) acetate catalyses the highly stereoselective solvolysis of -alkenylpentafluorosilicates to -alkenyl ethers under an atmosphere of air. Since the pentafluorosilicates can be obtained via hydrosilylation of acetylenes, the sequence represents a regio- and stereo-selective transformation of acetylenes into carbonyl precursors in moderate yield. The reaction of vinylmercurials and mercury carboxylates catalysed by palladium(ii) acetate provides a stereospecific route to enol carboxylates, which are valuable precursors of specific enolates. ... 

Palladium is a useful catalyst in several reactions which lead to keto-esters. p-Keto-esters react with allylic carbonates, with catalysis by palladium, in a decarboxylative allylation reaction. y-Keto-esters are prepared, in reasonable yield, by the palladium-catalysed regioselective oxidation of Py-unsaturated esters. y-Keto-esters are obtained in good yield by the palladium-catalysed Reformatsky reaction of ethyl bromoacetate and acid chlorides. Derivatives of y-ketopimelic acid are formed by the rhodium-carbonyl-catalysed reaction of derivatives of acrylic acid with carbon monoxide. A mild method for the conversion of propiolic esters into P-keto-esters via thiol addition has been reported (Scheme 63) it has been used in a formal synthesis of ( )-thienamycin. 

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