Rhodium complex hydrogenation

Butadiene Hydrogenation. Rhodium complexes of the type Rh(diene)(dppe)+, where dppe = 1,2-bis(diphenylphosphino)ethane, are catalyst precursors for overall 1,2 and 1,4 addition of hydrogen to 1,3-butadienes. In these reactions the distribution of terminal and internal olefin products is kinetically regulated by the reaction pathways of a common RhH(R)(dppe)+ intermediate (13). Under homogeneous reaction conditions, the thermodynamically more stable internal olefin products (1,4-addition) are favored over the synthetically more useful terminal olefin products (1,2 addition). However, significant increases in the yield of 1,2 addition products can be achieved by intercalation of the catalyst precursor in hectorite. (14)... 

Asymmetric Hydrogenation. Rhodium complexes of the type Rh(diene)(diphos )+, where diphos is a chiral bidentate diphosphine ligand, are catalyst precursors for the asymmetric hydrogenation of certain prochiral olefins (15). Asymmetric hydrogenation of a-acylaminoacrylates, for example, affords chiral amino acid derivatives, some of which have medicinal utility such as L-DOPA. 

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

An early attempt to hydroformylate butenediol using a cobalt carbonyl catalyst gave tetrahydro-2-furanmethanol (95), presumably by aHybc rearrangement to 3-butene-l,2-diol before hydroformylation. Later, hydroformylation of butenediol diacetate with a rhodium complex as catalyst gave the acetate of 3-formyl-3-buten-l-ol (96). Hydrogenation in such a system gave 2-methyl-1,4-butanediol (97). 

The Wilkinson hydrogenation cycle shown in Figure 3 (16) was worked out in experiments that included isolation and identification of individual rhodium complexes, measurements of equiUbria of individual steps, deterrnination of rates of individual steps under conditions of stoichiometric reaction with certain reactants missing so that the catalytic cycle could not occur, and deterrnination of rates of the overall catalytic reaction. The cycle demonstrates some generally important points about catalysis the predominant species present in the reacting solution and the only ones that are easily observable by spectroscopic methods, eg, RhCl[P(CgH 2]3> 6 5)312 (olefin), and RhCl2[P(CgH )2]4, are outside the cycle, possibly in virtual equiUbrium with... 

The strategy of the catalyst development was to use a rhodium complex similar to those of the Wilkinson hydrogenation but containing bulky chiral ligands in an attempt to direct the stereochemistry of the catalytic reaction to favor the desired L isomer of the product (17). Active and stereoselective catalysts have been found and used in commercial practice, although there is now a more economical route to L-dopa than through hydrogenation of the prochiral precursor. 

The influence of the concentration of hydrogen in [BMIM][PFg] and [BMIM][BF4] on the asymmetric hydrogenation of a-acetamidocinnamic acid catalyzed by rhodium complexes bearing a chiral ligand has been investigated. FFydrogen was found to be four times more soluble in the [BFJ -based salt than in the [PFg] -based one. 

The rhodium complexes are excellent catalysts for hydrogenation of NBR. At low temperature and pressure, high catalyst concentrations are used to obtain a better rate of reactions. Due to higher selectivity of the reaction, pressure and temperature can be increased to very high values. Consequently the rhodium concentration can be greatly reduced, which leads to high turnover rates. The only practical drawback of Rh complex is its high cost. This has initiated the development of techniques for catalyst removal and recovery (see Section VU), as well as alternate catalyst systems based on cheaper noble metals, such as ruthenium or palladium (see Sections IV.A and B). 

Ethane, (K)-l-cyclohexyl-L2-bis(diphenylphosphino)-rhodium complexes asymmetric hydrogenation, 6,253 Ethane, tetracyano-metal complexes, 2,263 Ethane, tetrakis(aminomethyl)-metal complexes, 2, 56 Ethane, tris[l, 1, l-(trisaminomethyl)]-complexes structure, 1,26... 

---