Rhodium/1 complexes

Interestingly, the rhodium complex, reminiscent of cisplatin, displays some differences of toxicity, depending on the mode of its administration. No T/C values are included because no strictly comparable data are available but the general trend is conveyed. 

The complex m r-[RhCl3(NH3)3j has been reported to have good activity, the best of the rhodium—amine complexes, but it is very insoluble [2]. Rhodium complexes are of interest because of the demonstration of their antibacterial action [7] (see Chapter 9) in structures of type [RhCl2(py)4] , which show, however, only marginal antitumour activity. Conversely, the most antitumour active rhodium species in general show few antibacterial 

The dehydrogenation of alcohols is thermodynamically more favorable when the hydrogen released is consumed for olefin hydrogenation, as was observed for soluble rhodium complexes as catalysts [13,14]. 

Hydrogen transfer from sec-alcohols to alkenes, affording ketones and alkanes, is catalyzed by [MCI (CgH 2 P 3 where M = Rh and Ir 

Copper(II) ions effect photocatalytic H evolution from ethanol solutions [16]. 

MoO Cacac) and VOCOPr ) has indicated the participation of dioxygen via a free-radical chain process involving ketyl radicals [17,18]. 

The crystal structure of the sulfur-bridged complex [Fe(fj -C5H4S)2]Rh(f -C5Me5)-(PMe3) is illustrated in Fig. 7-41 [135]. 

The rhodium atom assumes a (distorted) tetrahedral coordination. The cyclo-pentadienyl rings of the ferrocene unit are nearly eclipsed and nearly parallel (tilting angle 0.7°). 

In complex 3, both the rhodium atoms have octahedral coordination, whereas in 4 one rhodium atom has square pyramidal geometry and the other octahedral geometry. 

From the electrochemical viewpoint, the complexes display a first oxidation process based on the ferrocene unit(s) followed by a second one-electron oxidation centered on the dirhodium moiety [137, 138]. Both the processes have features of chemical reversibility. The relevant redox potentials are summarized in Table 7-26, also in comparison with the corresponding metallated molecules containing tri-phenylphosphine instead of the ferrocenylphosphines. 

The fact that the triphenylphosphino substituted analogues are more easily oxidized (by about 0.2 V) suggests that the introduction of the ferrocene molecules withdraws significant electron density from the central dirhodium fragment. 

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

Metal Complex. Complexation gas chromatography was first introduced by V. Schurig in 1980 (118) and employs transition metals (eg, nickel, cobalt, manganese or rhodium) complexed with chiral terpenoid ketoenolate ligands such as 3-ttifluoroacetyl-lR-camphorate (6),... 

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). 

Catalytic Asymmetric Hydroboration. The hydroboration of olefins with catecholborane (an achiral hydroborating agent) is cataly2ed by cationic rhodium complexes with enantiomericaHy pure phosphines, eg, [Rh(cod)2]BE4BINAP, where cod is 1,5-cyclooctadiene and BINAP is... 

Mechanism ofLP Oxo Rea.ction. The LP Oxo reaction proceeds through a number of rhodium complex equilibria analogous to those ia the Heck-Breslow mechanism described for the ligand-free cobalt process (see Fig. 1). 

Rhodium complexes with oxygen ligands, not nearly as numerous as those with amine and phosphine complexes, do, however, exist. A variety of compounds are known, iucluding [Rh(ox)3] [18307-26-1], [Rh(acac)3] [14284-92-5], the hexaaqua ion [Rh(OH2)3] [16920-31 -3], and Schiff base complexes. Soluble rhodium sulfate, Rh2(804 )3-a H2 0, exists iu a yellow form [15274-75-6], which probably coutaius [Rh(H20)3], and a red form [15274-78-9], which contains coordinated sulfate (125). The stmcture of the soluble nitrate [Rh(N03)3 2H20 [10139-58-9] is also complex (126). Another... 

A new homogeneous process for hydroformylation of olefins using a water-soluble catalyst has been developed (40). The catalyst is based on a rhodium complex and utilizes a water-soluble phosphine such as tri(M-sulfophenyl)phosphine. The use of an aqueous phase simplifies the separation of the catalyst and products (see Oxo process). 

Separation Processes. Separation of the catalyst from the products is a significant expense the process flow diagram and the processing cost are often dominated by the separations. Many soluble catalysts are expensive, eg, rhodium complexes, and must be recovered and recycled with high efficiency. The most common separation devices are distiUation columns extraction is also appHed. 

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. 

Metha.no Ca.rbonyla.tion, An important industrial process cataly2ed by rhodium complexes in solution is methanol carbonylation to give acetic acid. 

Polymer-supported catalysts incorporating organometaUic complexes also behave in much the same way as their soluble analogues (28). Extensive research has been done in attempts to develop supported rhodium complex catalysts for olefin hydroformylation and methanol carbonylation, but the effort has not been commercially successful. The difficulty is that the polymer-supported catalysts are not sufftciendy stable the valuable metal is continuously leached into the product stream (28). Consequendy, the soHd catalysts fail to eliminate the problems of corrosion and catalyst recovery and recycle that are characteristic of solution catalysis. 

The use of a catalyst such as cadmium oxide increases the yield of dibasic acids to about 51% of theoretical. The composition of the mixed acids is about 75% C-11 and 25% C-12 dibasic acids (73). Reaction of undecylenic acid with carbon monoxide using a triphenylphosphine—rhodium complex as catalyst gives 11-formylundecanoic acid, which, upon reaction with oxygen in the presence of Co(II) salts, gives 1,12-dodecanedioic acid in 70% yield (74). 

There are currentiy no commercial producers of C-19 dicarboxyhc acids. During the 1970s BASF and Union Camp Corporation offered developmental products, but they were never commercialized (78). The Northern Regional Research Laboratory (NRRL) carried out extensive studies on preparing C-19 dicarboxyhc acids via hydroformylation using both cobalt catalyst and rhodium complexes as catalysts (78). In addition, the NRRL developed a simplified method to prepare 9-(10)-carboxystearic acid in high yields using a palladium catalyst (79). 

C-19 dicarboxyhc acid can be made from oleic acid or derivatives and carbon monoxide by hydroformylation, hydrocarboxylation, or carbonylation. In hydroformylation, ie, the Oxo reaction or Roelen reaction, the catalyst is usually cobalt carbonyl or a rhodium complex (see Oxo process). When using a cobalt catalyst a mixture of isomeric C-19 compounds results due to isomerization of the double bond prior to carbon monoxide addition (80). 

Hydroformylation, or the 0X0 process, is the reaction of olefins with CO and H9 to make aldehydes, which may subsequently be converted to higher alcohols. The catalyst base is cobalt naph-thenate, which transforms to cobalt hydrocarbonyl in place. A rhodium complex that is more stable and mnctions at a lower temperature is also used. 

Reduction of the A" -double bond with the rhodium complex is a very slow reaction, but it has been accomplished in 17)S-hydroxyandrost-4-en-3-one (140)d The product, 4a, 5a-d2-androstan-17j3-ol-3-one (141), is a further example of the preferential a-side deuteration in homogeneous solution as contrasted with the )S-face attack with heterogeneous catalysts. [For a more convenient preparation of compound (141) see section V-C.]... 

Tautomeric rearrangements of transition-metal complexes with azole ligands are relatively scarce. The fluxional behavior of the rhodium complex 43 with a neutral 3,5-dimethylpyrazole was explained as the result of rapid processes of metallotropy and prototropy occurring simultaneously (Scheme 24) [74JOM(C)51],... 

Metal basicity and cooperative effects in the reactions of dinuclear pyrazolato rhodium complexes 98PAC779. 

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