Phosphine-modified rhodium catalysts

Oxidative addition to complex 1 is the slowest and rate-determining step in the reaction scheme and also it is a singular step, involving the conversion of the catalyst resting state to a more reactive 2. An obvious way to obtain a faster catalyst is the substitution of carbonyl ligands in 1 by electron-donating phosphines, as organometallic chemistry tells us this variation never fails. Indeed, several variants that are indeed fester are known [11], but none of them has found application. 

As bidentates seemed to give more stable complexes, and because trans configurations seemed to be favoured by some of the monodentates, Thomas and Suss-Fink investigated the use of ligands that are bidentate and can coordinate in a trans fashion [11,16], This has led to the development of new diphosphines based on condensation reactions of 2-diphenylphos-phinobenzoic acid with aminoalcohols or diols, reminiscent of the wide bite angle ligands used by Trost for the asymmetric allylic alkylation (Chapter 13.2). The best 

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The search for catalyst systems which could effect the 0x0 reaction under milder conditions and produce higher yields of the desired aldehyde resulted in processes utilizing rhodium. Oxo capacity built since the mid-1970s, both in the United States and elsewhere, has largely employed tertiary phosphine-modified rhodium catalysts. For example, over 50% of the world s butyraldehyde (qv) is produced by the LP Oxo process, technology Hcensed by Union Carbide Corporation and Davy Process Technology. 

Figure 6.6. Phosphine modified rhodium catalyst for AcOH synthesis...

The hydroformylation of conjugated dienes with unmodified cobalt catalysts is slow, since the insertion reaction of the diene generates an tj3-cobalt complex by hydride addition at a terminal carbon (equation 10).5 The stable -cobalt complex does not undergo facile CO insertion. Low yields of a mixture of n- and iso-valeraldehyde are obtained. The use of phosphine-modified rhodium catalysts gives a complex mixture of Cs monoaldehydes (58%) and C6 dialdehydes (42%). A mixture of mono- and di-aldehydes are also obtained from 1,3- and 1,4-cyclohexadienes with a modified rhodium catalyst (equation ll).29 The 3-cyclohexenecarbaldehyde, an intermediate in the hydrocarbonylation of both 1,3- and 1,4-cyclo-hexadiene, is converted in 73% yield, to the same mixture of dialdehydes (cis.trans = 35 65) as is produced from either diene. 

Phosphine-modified rhodium catalysts hydroformylate alkynes to saturated aldehydes.1 The reaction most likely proceeds by a rapid hydrogenation to yield the alkene, followed by hydroformylation. 

Hydroformylation of 2,6-dimethyl-6-hepten-2-ol produces hydroxycitronellal (equation 12).22 Subjecting allyl alcohol to hydroformylation reaction conditions with HCo(CO>4 yields only propanal, isomerization taking place more rapidly than hydroformylation.2 Phosphine-modified rhodium catalysts will convert allyl alcohol to butane-1,4-diol under mild conditions in the presence of excess phosphine, however (equation 13).5 30 31 When isomerization is blocked, hydroformylation proceeds normally (equation 14). An elegant synthesis of the Prelog-Djerassi lactone has been accomplished starting with the hydroformylation of an allylic alcohol (equation IS).32... 

Nitro groups are tolerated in hydroformylation as are amines, provided they are protected. The reaction of o-nitrostyrene gives an intermediate for the synthesis of 3-methylindole (equation 28).38 With a phosphine-modified rhodium catalyst, the reaction is regioselective, placing the formyl group in the a-posi-tion. 

The hydroformylation mechanism for phosphine-modified rhodium catalysts follows with minor modifications the Heck-Breslow cycle. HRh(CO)(TPP)3 [11] is believed to be the precursor of the active hydroformylation species. First synthesized by Vaska in 1963 [98] and structurally characterized in the same year [99], Wilkinson introduced this phosphine-stabilized rhodium catalyst to hydroformylation five years later [100]. As one of life s ironies, Vaska even compared HRh(CO)(TPP)3 in detail with HCo(CO)4 as an example of structurally related hy-drido complexes [98]. Unfortunately he did not draw the conclusion that the rhodium complex should be used in the oxo reaction. According to Wilkinson, two possible pathways are imaginable the associative and the dissociative mechanisms. Preceding the catalytic cycle are several equilibria which generate the key intermediate HRh(CO)2(TPP)2 (Scheme 4 L = ligand). 

For phosphine-modified rhodium catalysts, namely HRh(CO)(TPP)3, the rate is dependent on the different parameters, as follows ... 

Scheme 6. Deactivation of phosphine-modified rhodium catalysts in the UCC process (above) and the RCH/RP process (below). Ort/io-metallation can be excluded for both...

Table 9.8 gives economic data about Oxo synthesis on propylene for processes using cobalt tetracarbonyl hydride phosphine-modified cobalt and phosphine-modified rhodium catalysts. 

The concept of TRPTC provides a reasonable explanation for the satisfactory catalytic reactivity of Rh/nonionic phosphine complexes in the case of the two-phase hydroformylation of higher olefins. At a temperature lower than the cloud point, a nonionic phosphine-modified rhodium catalyst would remain in the aqueous phase since the partition of the catalyst between water and a nonpolar aprotic organic solvent strongly favors the aqueous phase. On heating to a temperature higher than the cloud point, however, the catalyst loses its hydrate shell, transfers into the organic phase and then catalyzes the transformation of alkenes to aide-... 

Unmodified cobalt catalyst Phosphine modified cobalt catalyst Phosphine modified rhodium catalyst... 

Rhone-Poulenc and Ruhrchemie (now Hoechst) developed a process in the 1980s based on a water soluble rhodium catalyst modified with triphenylphosphine sulphonate ligand that can produce normal to iso ratios as high as 20. Previous phosphine modified rhodium catalysts were oil soluble. 

The procedure will now be explained for the example of oxo synthesis. Conjugated dienes are converted into mono- and dialdehydes by phosphine-modified rhodium catalysts [12]. The target quantity, in diis case the extent of dialdehyde formation, depends mainly on the three reaction parameters temperature (A), cocatalyst ratio (5), and total pressure (Q. A 2 factorial design was carried out (Table 13-8). The evaluation of the test results by the Yates scheme is shown in Table 13-9. 

Typical composition of product distribution obtained with tertiary phosphine-modified rhodium catalysts can be seen in Table 4. 

Trialkylphosphine-modified cobalt catalysts and unmodified rhodium catalysts show similar olefin isomerization activity, but the most common tertiary phosphine-modified rhodium catalysts are generally ineffective in double-bond isomerization (77-79). On the other hand, phosphite-modified rhodium catalysts were found to be highly effective at alkene isomerization (80-83). 

Some rhodium catalysts for hydroformylation of epoxides have been claimed by Union Carbide and Shell (Scheme 6.106) [17]. By the use of phosphites (e.g., Alkanox 240, BIPHEPHOS) or phosphine-modified rhodium catalysts, the selectivity was enhanced and the reaction temperature could be lowered. Noteworthy, occasionally a higher partial pressure of Hg in the syngas mixture with a total pressure of 90 bar was employed. 

Also, pyridine or substituted pyridines as solvents, for example, y-picoline (4-methyl pyridine), enhanced the catalytic activity of unmodified or phosphine-modified rhodium catalysts [23]. Yields of >90% of HOCH2CHO were achieved within 4 h at 70 °C. Above 100 °C, the reaction was complicated by the formation of straight-chain polyols. Interestingly, the addition of protonic acids enhanced the rate, and almost perfect chemoselectivity was noted (only 1.9% methanol). 

Several studies of phosphine-modified rhodium catalysts indicate that three phosphorus atoms are coordinated to the rhodium at the instant that... 

These materials are prepared by the covalent attachment of ionic hquids to the support surface or by simple deposition of the ionic liquid phases containing catalytically active species on the surface of the support (usually silica-based or polymeric materials including membranes). In various cases, the procedure involves the simple dissolution of a sulfonated phosphine-modified rhodium catalyst into a supported ionic liquid, while the alkene constitutes the organic phase. This method reduces the amount of ionic liquid and allows for a facUe and efficient separation of products from catalyst. In comparison to traditional biphasic systems, higher catalytic activity and lower metal leaching can be obtained by appropriately tuning the experimental conditions [35—41]. 

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