Rhodium complexes homogeneous 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)... 

Similar multinuclear processes have been noted for homogeneous hydrogenation by complexes of metals other than rhodium. For example, successive reactions of an alkene with two [HCo(CN)5] anions give the corresponding alkyl complex anion and alkane. Similarly,... 

Encapsulated rhodium complexes were prepared from Rh-exchanged NaY zeolite by complexation with (S)-prolinamide or M-tert-butyl-(S)-prolinamide [73,74]. Although these catalysts showed higher specific activity than their homogeneous counterparts in non-enantioselective hydrogenations, the hydrogenation of prochiral substrates, such as methyl (Z)-acetamidocinnamate [73] or ( )-2-methyl-2-pentenoic acid [74], led to low... 

A number of catalysts are known to effect homogeneous hydrogenation of aromatic hydrocarbons, e.g., some oxidized rhodium complexes (/, p. 238), some rhodium 7r-complexes with phenyl carboxylates (/, p. 283), some Ziegler systems (/, p. 363), and Co2(CO)8 (/, p. 173). However, the catalysts in the first three systems are not well characterized, and the carbonyl systems require fairly severe hydroformylation conditions, although they are reasonably selective, possibly via radical pathways (Section II, C). 

The synthesis of cationic rhodium complexes constitutes another important contribution of the late 1960s. The preparation of cationic complexes of formula [Rh(diene)(PR3)2]+ was reported by several laboratories in the period 1968-1970 [17, 18]. Osborn and coworkers made the important discovery that these complexes, when treated with molecular hydrogen, yield [RhH2(PR3)2(S)2]+ (S = sol-vent). These rhodium(III) complexes function as homogeneous hydrogenation catalysts under mild conditions for the reduction of alkenes, dienes, alkynes, and ketones [17, 19]. Related complexes with chiral diphosphines have been very important in modern enantioselective catalytic hydrogenations (see Section 1.1.6). 

Several heteroaromatic compounds can be hydrogenated by [Rh(COD) (PPh3)2]+ species. Thus, this cationic complex has been reported to be a catalyst precursor for the homogeneous hydrogenation of heteroaromatic compounds such as quinoline [32] or benzothiophene [33]. Detailed mechanistic cycles have been proposed by Sanchez-Delgado and coworkers. The mechanism of hydrogenation of benzothiophene by the cationic rhodium(III) complex, [Rh(C5Me5) (MeCN)3]2+, has been elucidated by Fish and coworkers [34]. 

Following Wilkinson s discovery of [RhCl(PPh3)3] as an homogeneous hydrogenation catalyst for unhindered alkenes [14b, 35], and the development of methods to prepare chiral phosphines by Mislow [36] and Horner [37], Knowles [38] and Horner [15, 39] each showed that, with the use of optically active tertiary phosphines as ligands in complexes of rhodium, the enantioselective asymmetric hydrogenation of prochiral C=C double bonds is possible (Scheme 1.8). 

A crucial achievement significantly stimulated the development of the investigation in the field of homogeneous enantioselective catalysis. The Knowles group established a method for the industrial synthesis of I-DOPA, a drug used for the treatment of Parkinson s disease. The key step of the process is the enantiomeric hydrogenation of a prochiral enamide, and this reaction is efficiently catalyzed by the air-stable rhodium complex [Rh(COD)((PP)-CAMP)2]BF4 (Scheme 1.12). 

A similar phenomenon was observed in a homogeneous rhodium complex catalyzed hydrogenation (equation 56)6. 

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