Alkenes rhodium hydrogenation

Rhodium and cobalt participate in several reactions that are of value in organic syntheses. Rhodium and cobalt are active catalysts for the reaction of alkenes with hydrogen and carbon monoxide to give aldehydes, known as hydroformylation,281... 

In the hydrogenation of alkenes, rhodium-, ruthenium- and iridium-phosphine catalysts are typically used [2-4]. Rhodium-phosphine complexes, such as Wilkinson s catalyst, are effective for obtaining alkanes under atmospheric pres-... 

The catalyst used is typically platinum, palladium, rhodium, or ruthenium, or sometimes an appropriate derivative. Precise details of the reaction remain vague, but we believe the catalyst surface binds to both the substrate, e.g. an alkene, and hydrogen, weakening or breaking the jr bond of the alkene and the a bond of hydrogen. Sequential addition of hydrogen atoms to the alkene carbons then occurs and generates the alkane, which is then released from the surface. 

Abstract The applications of hybrid DFT/molecular mechanics (DFT/MM) methods to the study of reactions catalyzed by transition metal complexes are reviewed. Special attention is given to the processes that have been studied in more detail, such as olefin polymerization, rhodium hydrogenation of alkenes, osmium dihydroxylation of alkenes and hydroformylation by rhodium catalysts. DFT/MM methods are shown, by comparison with experiment and with full quantum mechanics calculations, to allow a reasonably accurate computational study of experimentally relevant problems which otherwise would be out of reach for theoretical chemistry. 

Detailed studies of the hydrogenation of cinnamic acid derivatives (Fig. 6.22) by Halpern [53] and Brown [54] have revealed that the most stable intermediate of the two alkene adducts is not the one that leads to the major observed enantiomeric product, see Fig. 6.24. This means that the least stable intermediate alkene-rhodium complex reacts faster in the subsequent reaction step involving... 

To complete the alkene hydrogenation reaction sequence, the first hydrogen transfer must be followed by a second, which results in the reductive elimination of the alkane product. This proceeds through a three-centered transition state. The catalytic cycle is shown in Fig. 22-1 but the process is actually more complicated since the equilibria are dependent on phosphine, alkene, rhodium concentrations, temperature, and pressure. 

The introduction of rhodium has allowed the development of processes which operate under much milder conditions and lower pressures, are highly selective, and avoid loss of alkene by hydrogenation. Although the catalyst is active at moderate temperature, plants are usually operated at 120°C to give a high n/iso (linear/ branched) ratio. The key to selectivity is the use of triphenylphosphine in large excess which leads to >95% straight chain anti-Markovnikov product. The process is used for the hydroformylation of propene to n-butyraldehyde, allyl alcohol to butanediol, and maleic anhydride to 1,4-butanediol, tetrahydrofuran, and y-butyrolactone. 

The rhodium-catalyzed hydrogenation of alkenes (see Hydrogenation Isomerization of Alkenes) has also... 

The cationic complex (VII, X = Cl) forms a symmetrical ( Pnmr i.r.) complex with diphenylacetylene, which is very labile. The complexes [Rh2Cl2(/Lt-CO)(/Lc-acet)(dppm)2], where acet is hexafluoro-2-butyne or acetylenedicarboxylic acid dimethyl ester, are also symmetrical, and the acet ligand is coplanar with the rhodium atoms. The C C bond length is increased to 1.32 A, and so the complex is truly described as a cis-dimetallated alkene. Comparison with the reactions with CO suggests that the activation of an alkyne (or alkene) to hydrogenation may occur when the alkyne is initially coordinated to the catalyst at a terminal position, e.g., (XII), with subsequent oxidative addition and hydrogen transfer steps. A complex of the type (XII) may instead be only a minor but active component of a solution of which the alkyne-bridged complex is the major but less active component. ... 

The elimination reaction, given in Equation 7e, is accompanied by a formal reduction in the oxidation state of the rhodium from (III) to (I). The stereochemistry of this reaction has also attracted substantial interest. If /8-hydrogens are absent, the alkyl halide is eliminated with retention of configuration at the carbon. If /8-hydrogens are present, /8-hydride elimination occurs as shown in Equation 7f, giving an alkene and hydrogen halide. Investigations into the /8-hydride elimination have shown... 

Scheme 6.14. A representation of the reduction of an alkene with hydrogen (H2) and chlorotris(triphenylphosphine)rhodium (ClRh[(C6H5)3P]3).

When steric hindrance inhibits hydrogenation on one face of a double bond, addition will take place exclusively to the less hindered face. This principle has been used to develop enantioselective or so-called asymmetric hydrogenation. The process employs homogeneous (soluble) catalysts, consisting of a metal, such as rhodium, and an enantiopure chiral phosphine ligand, which binds to the metal. A typical example is the Rh complex of the diphosphine (R,R)-DIPAMP (margin). After coordination of the alkene double bond and a molecule of H2 to rhodium, hydrogenation occurs via syn addition, just as in the case of insoluble metal catalysts. 

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

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