Rhodium-phosphine complexes, reaction kinetics

Quite different types of rhodium compound can give very similar reaction rates in a system which shows a kinetic dependence on the rhodium catalyst concentration. In particular, rhodium(III) halides and rho-dium(I) phosphine complexes give almost identical reaction rates after an initial induction period. Thus, in the case of these two systems, it appears that a common species is being formed. 

The mechanisms of hydrogenation by neutral or cationic mononuclear complexes of rhodium are reasonably well understood and are discussed in detail in Chapter 2. The system is, however, sufficiently complex that new information is constantly being obtained. For example, it has recently been shown that a solvated complex is kinetically important when benzene is the solvent. This may have major implications in studies of complexes of aryl-phosphines or diphosphines, such as Ph2P(CH2)nPPh2 or diop. It should be noted that for complexes of chelating chiral diphosphines (see Chapter 4), the enantioselective step involves the least stable of two possible catalytic intermediates the complex which is stable, isolable, and present in highest concentration is not as important in determining the reaction rate and does not yield the major product. " " ... 

The important discovery by Wilkinson [1] that rhodium afforded active and selective hydroformylation catalysts under mild conditions in the presence of triphenylphosphine as a hgand triggered a lot of research on hydroformylation, especially on hgand effects and mechanistic aspects. It is commonly accepted that the mechanism for the cobalt catalyzed hydroformylation as postulated by Heck and Breslow [2] can be apphed to phosphine modified rhodium carbonyl as well. Kinetic studies of the rhodium triphenylphosphine catalyst have shown that the addition of the aUcene to the hydride rhodium complex and/or the hydride migration step is probably rate-limiting [3] (Chapter 4). In most phosphine modified systems an inverse reaction rate dependency on phosphine ligand concentration or carbon monoxide pressure is observed [4]. 

The catalysts used in hydroformylation are typically organometallic complexes. Cobalt-based catalysts dominated hydroformylation until 1970s thereafter rhodium-based catalysts were commerciahzed. Synthesized aldehydes are typical intermediates for chemical industry [5]. A typical hydroformylation catalyst is modified with a ligand, e.g., tiiphenylphoshine. In recent years, a lot of effort has been put on the ligand chemistry in order to find new ligands for tailored processes [7-9]. In the present study, phosphine-based rhodium catalysts were used for hydroformylation of 1-butene. Despite intensive research on hydroformylation in the last 50 years, both the reaction mechanisms and kinetics are not in the most cases clear. Both associative and dissociative mechanisms have been proposed [5-6]. The discrepancies in mechanistic speculations have also led to a variety of rate equations for hydroformylation processes. 

No catalyst has an infinite lifetime. The accepted view of a catalytic cycle is that it proceeds via a series of reactive species, be they transient transition state type structures or relatively more stable intermediates. Reaction of such intermediates with either excess ligand or substrate can give rise to very stable complexes that are kinetically incompetent of sustaining catalysis. The textbook example of this is triphenylphosphine modified rhodium hydroformylation, where a plot of activity versus ligand metal ratio shows the classical volcano plot whereby activity reaches a peak at a certain ratio but then falls off rapidly in the presence of excess phosphine, see Figure... 

A system kinetically very similar to the phosphine-free rhodium carbonyl catalyst is obtained with bulky phosphites (Fig 6.3). At temperatures from 50 to 80°C, and CO and H2 partial pressures ranging from 10 to 70 bar, the rate of aldehyde formation is first order in H2 and approximately minus one order in CO. The reaction rate is independent of the concentration of 1-octene at conversions below 30%. The reaction was found to be first order in rhodium concentration and insensitive to the phosphite/rhodium ratio, provided that the absolute concentration was sufficiently high to generate a hydride complex from the pentanedionate precursor (reaction 9). 

Proving the individual steps of the catalytic cycle in rhodium-catalyzed hy-droformylation is much less elaborate than in the cobalt case. Therefore, the nature of complexes involved in the catalytic cycle has been deduced mostly from the kinetics of the aldehyde formation and from spectroscopy of the reaction solutions. In situ infrared and NMR spectroscopy revealed so far only the main resting states of the catalyst being the pentacoordinate hydride- and the pen-tacoordinate acylrhodium complexes. The concentration of the postulated active intermediates in equilibrium with the resting states is obviously too low for direct observation. The main support of the involvement of 16-electron complexes is the negative effect of carbon monoxide and phosphine concentration on the rate of aldehyde formation. 

Cationic rhodium complexes catalyse the oxidative anti-Markovnikov amination of aromatic alkenes to enamines, a process that is accompanied by a simultaneous formation of 1 equiv. of ethylbenzene. Kinetic and mechanistic studies reveal that the yield and the rate of the reaction increase on increasing the styrene amine ratio. Furthermore, the type of phosphine ligand greatly influences the reaction. The formation of cationic rhodium-alkene-amine complexes has been proposed to be the first step towards the active catalytic species. ... 

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