Rhodium species

The hydrosilylation of acetophenone by diphenylsilane in CH2CI2 at rt was used as a test reaction to compare the selectivity obtained with the carbene ligands (Scheme 36). The reactions were performed in the presence of a sUght excess of AgBp4 (1.2% mol). In these conditions, the N-mesityl-substituted catalyst 57c (1% mol) gave the highest selectivity (65% ee). The in situ formation of square-planar cationic rhodium species 58 as active catalysts appears to be crucial since the same reaction performed without silver salt gave both poor yield (53%) and enantioselectivity (13%). 

Subsequently, cationic rhodium catalysts are also found to be effective for the regio- and stereoselective hydrosilation of alkynes in aqueous media. Recently, Oshima et al. reported a rhodium-catalyzed hydrosilylation of alkynes in an aqueous micellar system. A combination of [RhCl(nbd)]2 and bis-(diphenylphosphi no)propanc (dppp) were shown to be effective for the ( >selective hydrosilation in the presence of sodium dodecylsulfate (SDS), an anionic surfactant, in water.86 An anionic surfactant is essential for this ( )-selective hydrosilation, possibly because anionic micelles are helpful for the formation of a cationic rhodium species via dissociation of the Rh-Cl bond. For example, Triton X-100, a neutral surfactant, gave nonstereoselective hydrosilation whereas methyltrioctylammonium chloride, a cationic surfactant, resulted in none of the hydrosilation products. It was also found that the selectivity can be switched from E to Z in the presence of sodium iodide (Eq. 4.47). 

Oshima et al. explored a cationic rhodium-catalyzed intramolecular [4+2] annulation of l,3-dien-8-ynes in water in the presence of sodium dodecyl sulfate (SDS), an anionic surfactant.132 When the substrate l,3-dien-8-yne was a terminal alkyne, the reaction provided an inter-molecular [2+2+2] product (Eq. 4.68). In water, a reactive cationic rhodium species was formed by the dissociation of the Rh-Cl bond in the presence of SDS. The SDS forms negatively charged micelles, which would concentrate the cationic rhodium species (Scheme 4.15). 

In a similar way as described for the hydroformylation, the rhodium-catalyzed silaformylation can also be used in a domino process. The elementary step is the formation of an alkenyl-rhodium species by insertion of an alkyne into a Rh-Si bond (silylrhodation), which provides the trigger for a carbocyclization, followed by an insertion of CO. Thus, when Matsuda and coworkers [216] treated a solution of the 1,6-enyne 6/2-87 in benzene with the dimethylphenylsilane under CO pressure (36 kg cm"2) in the presence of catalytic amounts of Rh4(CO)12, the cyclopentane derivative 6/2-88 was obtained in 85 % yield. The procedure is not restricted to the formation of carbocycles rather, heterocycles can also be synthesized using 1,6-enynes as 6/2-89 and 6/2-90 with a heteroatom in the tether (Scheme 6/2.19). Interestingly, 6/2-91 did not lead to the domino product neither could 1,7-enynes be used as substrates, while the Thorpe-Ingold effect (geminal substitution) seems important in achieving good yields. 

Other companies (e.g., Hoechst) have developed a slightly different process in which the water content is low in order to save CO feedstock. In the absence of water it turned out that the catalyst precipitates. Clearly, at low water concentrations the reduction of rhodium(III) back to rhodium(I) is much slower, but the formation of the trivalent rhodium species is reduced in the first place, because the HI content decreases with the water concentration. The water content is kept low by adding part of the methanol in the form of methyl acetate. Indeed, the shift reaction is now suppressed. Stabilization of the rhodium species and lowering of the HI content can be achieved by the addition of iodide salts. High reaction rates and low catalyst usage can be achieved at low reactor water concentration by the introduction of tertiary phosphine oxide additives.8 The kinetics of the title reaction with respect to [MeOH] change if H20 is used as a solvent instead of AcOH.9 Kinetic data for the Rh-catalyzed carbonylation of methanol have been critically analyzed. The discrepancy between the reaction rate constants is due to ignoring the effect of vapor-liquid equilibrium of the iodide promoter.10... 

The existence of two different rhodium species co-existing on the silica support can be used as an advantage by controlling their relative amount. Under standard hydroformylation conditions, the cationic species and the neutral hydride complex are both present in significant amounts. Hence hydroformylation and hydrogenation will both proceed under a CO/H2 atmosphere. Indeed a clean one-pot reaction of 1-octene to 1-nonanol was performed, using the supported catalyst for a hydroformylation-hydrogenation cascade reaction. 98 % of the 1-octene was converted in the... 

Scheme 7.8. Equilibria between different rhodium species present during hydroformylation in an ionic liquid...

The reaction of DMG and CIRh(PPh3)3 leads to the formation of several rhodium compounds, six of which were isolated and characterized. The compounds arise from oxidative insertion of the rhodium species into a ... 

The initial step is an oxidative addition of RhCI(PPh3)3 to a C-0 bond of the ester moiety and produces rhodium-carbon and rhodium-oxygen bonds. Adjacent rhodium species can undergo further reaction with the formation of anhydride linkages. This anhydride formation may occur between adjacent pairs of reactants, between pairs in the same chain, or between pairs that are present in different chains. All of these reactions are observed, and in however the last reaction is the one of interest here since this leads to cross-linking and char formation. Rhodium is present in both the chary material and in the soluble fractions. From the reaction pathway in order for rhodium elimination to occur, two rhodium-inserted... 

Thus, two quite different forms of rhodium catalyst precursor can give the same rhodium species under the carbonylation reaction conditions. 

This catalytic cycle, generating acetyl iodide from methyl iodide, has been demonstrated by carbonylation of anhydrous methyl iodide at 80°C and CO partial pressure of 3 atm using [(C6H5)4As][Rh(CO)2X2] as catalysts. After several hours reaction, acetyl iodide can be identified by NMR and infrared techniques. However, under anhydrous conditions some catalyst deactivation occurs, apparently by halogen abstraction from the acetyl iodide, giving rhodium species such as frans-[Rh(CO)2I4] and [Rh(CO)I4] . Such dehalogenation reactions are common with d8 and d10 species, particularly in reactions with species containing weak... 

The [Rh(CO)2I2] ion is clearly an important species in systems derived from several different catalyst precursors fortuitously, it is a relatively nucleophilic rhodium species. Thus it reacts with methyl iodide at room temperature, whereas the related uncharged species, [Rh(CO)2Cl]2, is unreactive toward methyl iodide at low temperatures. This difference between neutral and charged species is also evidenced markedly in the relative reactivities of [RhL2(CO)X] and [RhL(CO)X2] toward methyl iodide, where a difference of five orders of magnitude has been observed (19). 

Formation of the methyl-rhodium complex is analogous to the formation of CH3-C(C0)4 from CH30H2 arid Co(C0K as proposed by Wender. The difference here is that the nature of the active rhodium species is not known. Under the present conditions,homologation does not occur because CO is not present however, addition of the methyl-rhodium species to benzaldehyde must occur as shown in (19), metal adds to the oxygen. The product in (19) is then subject to acid catalyzed etherification to obtain the methyl ether. 

Unfortunately, because of the exceptional number of interrelated equilibria between various rhodium clusters and Rh(CO) it seems unlikely that it will be possible to identify which rhodium species is responsible for the hydrogenation reactions. 

Rhodium species in oxidation states I and III are involved in the process. Rhodium-catalyzed hydrogenations generally involve oxidative addition reactions, followed by the reverse process of reductive elimination in the final step. Another common elimination process is the so-called (l-elimination, which accounts for the frequent side reaction of isomerization of alkenes, according to Eq. (1) ... 

Using a chiral diphosphine Rh(I) complex as the catalyst, high catalytic activity and a good branched/linear ratio can be achieved, but in most cases the ee values remain moderate.122 The enantioselectivity of Rh(I)-catalyzed reactions depends on the amount of added chiral ligand because of the much higher catalytic activity of ligand-free rhodium species. Usually 4 6 equivalents... 

Alkynes react with the bulky germanium hydride (MejSdjGeH to selectively yield (Z)-alkenes (Equation (105)).67 The hydrogermylation of alkynols or alkynes can be catalyzed by a rhodium complex (Equation (106), Table 18) and some of the intermediates were identified (Scheme 16).132 Similar rhodium species react with alkynes to yield alkenyl complexes,133 and other transition metal complexes have been employed as hydrogermylation catalysts including those containing palladium.134,135... 

In the case of BDPP with a bite angle of 90°, the high-pressure NMR and high-pressure IR studies showed the structures of the hydrido dicarbonyl diphosphine resting state as an axial-equatorial BPT. Similar behavior was observed for the furanoside diphosphines. Dinuclear rhodium species in equilibrium with the mononuclear pentacoordinate rhodium hydride carbonyl diphosphines have been found for these ligands. The position of this equilibrium depends on the hydrogen concentration and the ligands. The rate... 

These authors propose as the mechanism for this reaction a reversible oxidative addition of the aryl-phosphido fragments to a low valent rhodium species. A facile aryl exchange has been described for complexes Pd(PPh3)2(C6H4CH3)I. The authors [35] suggest a pathway involving oxidative additions and reductive eliminations. The mechanism outlined below, however, can also explain the results of these two studies. 

The hydrogenation reaction is carried out with a substituted cinnamic acid. The acetamido group is of particular importance because it functions as a secondary complexation function in addition to the alkene functionality. In the first step the alkene co-ordinates to the cationic rhodium species (containing an enantiopure phosphine DIPAMP in Figures 4.4 and 4.5 with the chirality at phosphorus carrying three different substituents, Ph, o-An, CH2) for which there are several diasteromeric structures due to ... 

More sterically demanding ligands will favour the formation of species containing a low number of ligands L and therefore more CO ligands. A high proportion of CO ligands also leads to electron poor rhodium species and thus to enhanced dissociation of CO. For phosphites this effect has been clearly observed [40] as will be discussed in section 8.4. 

During hydroformylation the IR spectra of the rhodium species (diphosphine)Rh(CO)2H for ligands 26, 18, and 30 do not change. The respective ee ae equilibria are not influenced, and no other carbonyl complexes are observed thus these are the catalyst s resting state. The coupling constants in the NMR spectra reflect the equilibria between ae and ee species. 

While the l b ratio increases with the %-vaIuc. no electronic effect of the ligands on the selectivity for linear aldehyde is observed. The selectivities for linear aldehyde are all between 92 and 93%. The increase in l b ratio with decreasing phosphine basicity can be attributed completely to an increased tendency of the branched alkyl rhodium species to form 2-octene instead of... 

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