Hydroformylation Catalytic cycle

The hydrocarboxylation reactions discussed above have been proposed to involve direct addition of water to the metal center prior to elimination of the product, analogous to the oxidative addition of hydrogen to a metal center at the end of a hydroformylation catalytic cycle. Another class of hydrocarboxylation reactions is more analogous to the haUde-promoted Monsanto acetic acid process, where one has a reductive elimination of an acyl halide species that is rapidly hydrolyzed with free water to generate the carboxylic acid and HX. 

The formation of isomeric aldehydes is caused by cobalt organic intermediates, which are formed by the reaction of the olefin with the cobalt carbonyl catalyst. These cobalt organic compounds isomerize rapidly into a mixture of isomer position cobalt organic compounds. The primary cobalt organic compound, carrying a terminal fixed metal atom, is thermodynamically more stable than the isomeric internal secondary cobalt organic compounds. Due to the less steric hindrance of the terminal isomers their further reaction in the catalytic cycle is favored. Therefore in the hydroformylation of an olefin the unbranched aldehyde is the main reaction product, independent of the position of the double bond in the olefinic educt ( contrathermodynamic olefin isomerization) [49]. 

An unusual enhancement of catalytic activity in a two-phase system has been reported by Fremy et al. (1998) for the hydroformylation of acrylic esters using Rh complex of TPTS as catalyst. Even though acrylic esters have reasonable solubility in water, rate enhancements in two-phase systems by a factor of 2 to 14 have been reported. It seems that water is not an inert solvent but also acts as a reactant or a co-ordinating solvent which can modify elementary steps of the catalytic cycle (Cornilis, 1997). 

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... 

Today, iridium compounds find so many varied applications in contemporary homogeneous catalysis it is difficult to recall that, until the late 1970s, rhodium was one of only two metals considered likely to serve as useful catalysts, at that time typically for hydrogenation or hydroformylation. Indeed, catalyst/solvent combinations such as [IrCl(PPh3)3]/MeOH, which were modeled directly on what was previously successful for rhodium, failed for iridium. Although iridium was still considered potentially to be useful, this was only for the demonstration of stoichiometric reactions related to proposed catalytic cycles. Iridium tends to form stronger metal-ligand bonds (e.g., Cp(CO)Rh-CO, 46 kcal mol-1 Cp(CO)Ir-CO, 57 kcal mol ), and consequently compounds which act as reactive intermediates for rhodium can sometimes be isolated in the case of iridium. 

The potential energy surface for the hydroformylation of ethylene has been mapped out for several catalytic model systems at various levels of theory. In 1997, Morokuma and co-workers [17], considering HRh(CO)2(PH3) as the unsaturated catalytic species that coordinates alkene, reported free energies for the full catalytic cycle at the ab initio MP2//RHF level. Recently, in 2001, Decker and Cundari [18] published CCSD(T)//B3LYP results for the HRh(CO)(PH3)2 catalytic complex, which would persist under high phosphine concentrations. Potential energy surfaces for both Rh-catalyzed model systems were qualitatively very similar. The catalytic cycle has no large barriers or deep thermodynamic wells to trap the... 

The catalytic cycle for the rhodium catalyzed hydroformylation has been extensively studied mainly for RhH(CO)(PPh3)3. A general proposal [2,27] that includes the steps of the reaction is shown in Scheme 2. 

Tandem procedures under hydroformylation conditions cannot only make use of the intrinsic reactivity of the aldehyde carbonyl group and its acidic a-position but they also include conversions of the metal alkyl and metal acyl systems which are intermediates in the catalytic cycle of hydroformylation. Metal alkyls can undergo -elimination leading to olefin isomerization, or couplings, respectively, insertion of unsaturated units enlarging the carbon skeleton. Similarly, metal acyls can be trapped by addition of nucleophiles or undergo insertion of unsaturated units to form synthetically useful ketones (Scheme 1). 

The catalytic cycle for hydroformylation reactions has also been established for certain homogeneous catalysts. Scheme 8.4 illustrates that for HRh(CO)2(PPh3)2, although the cycle is the same for the analogous cobalt catalyst. 

Scheme 8.4 Catalytic cycle for the hydroformylation of C=C bonds using HRh(CO)2(PPh3)2. Step 1, ligand dissociation step 2, ligand association step 3, /J-hydride transfer step 4, ligand dissociation step 5, CO insertion step 6, oxidative addition of H2 steP 2, reductive elimination step 8, ligand association...

A key issue in the hydroformylation reaction is the ratio of linear and branched product being produced (Figure 7.1). Scientifically it is an interesting question how the linearity can be influenced and maximised by influencing the kinetics and changing the ligands. The catalytic cycle for the formation of linear aldehyde is shown in Figure 7.2. The first processes for... 

Consequently a cartridge system for the hydroformylation of a variety of different alkenes using a single batch of catalyst was developed to fully exploit the potential of this approach (Scheme 1). Four different alkenes were applied in total using the same batch of catalyst for nine consecutive catalytic cycles. Conversion was driven to completeness in all cases, with selectivities remaining unchanged. Only 1.2% of rhodium and 2.4% of phosphorus was lost in nine cycles. Representative results are shown in Table 1. 

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