Rhodium-TPPMS complex

Water soluble Rh/tppts and Rh/tppms complexes dissolved in nonaqueous media such as the ionic liquids, l-ethyl-3-methylimidazolium or l-n-butyl-3-methylimidazolium salt have also been used as catalysts in the hydroformylation of 1-pentene employing a two phase system.15,16 The yields obtained were 16-33% (TOF=59-103 h 1) without any leaching of the rhodium from the ionic liquid to the aldehydes/feedstock phase. Rh/PPh3 catalysts exhibited higher rates (TOF=333 h 1) for the same biphasic reaction albeit with leaching of rhodium due to the uncharged nature of the catalytic system.15... 

Water-soluble rhodium(I) complexes with TPPTS, TPPMS, and PTA ligands, such as [RhCl(TPPTS)3], are capable of hydrogenating aldehydes, although their catalytic activity is inferior to the ruthenium complexes discussed above [116]. In sharp contrast to the ruthenium(II)-based catalysts, in reactions of unsaturated al-... 

Several water-soluble ruthenium complexes, with P = TPPMS, TPPTS, or PTA ligands (cf. Section 2.2.3.2), catalyze the selective reduction of crotonaldehyde, 3-methyl-2-butenal (prenal), and trans-cinnamaldehyde to the corresponding unsaturated alcohols (Scheme 2) [33—36]. Chemical yields are often close to quantitative in reasonable times and the selectivity toward the aUyhc alcohol is very high (> 95%). The selectivity of the reactions is critically influenced by the pH of the aqueous phase [11] as well as by the H2 pressure [37]. The hydrogenation of propionaldehyde, catalyzed by Ru(II)/TPPTS complexes, was dramatically accelerated by the addition of inorganic salts [38], too. In sharp contrast to the Ru(II)-based catalysts, in hydrogenation of unsaturated aldehydes rhodium(I) complexes preferentially promote the reaction of the C=C double bond, although with incomplete selectivity [33, 39]. 

The use of water-soluble ligands was referred to previously for both ruthenium and rhodium complexes. As in the case of ruthenium complexes, the use of an aqueous biphasic system leads to a clear enhancement of selectivity towards the unsaturated alcohol [34]. Among the series of systems tested, the most convenient catalysts were obtained from mixtures of OsCl3 3H20 with TPPMS (or better still TPPTS) as they are easily prepared and provide reasonable activities and modest selectivities. As with their ruthenium and rhodium analogues, the main advantage is the ease of catalyst recycling with no loss of activity or selectivity. However, the ruthenium-based catalysts are far superior. 

It is to be mentioned that water-soluble phosphine complexes of rhodium(I), such as [RhCl(TPPMS)3], [RhCl(TPPTS)3], [RhCl(PTA)3], either preformed, or prepared in situ, catalyze the hydrogenation of unsaturated aldehydes at the C=C bond [187, 204, 205]. As an example, at 80 °C and 20 bar H2, in 0.3-3 h cinnamaldehyde and crotonaldehyde were hydrogenated to the corresponding saturated aldehydes with 93 % and 90 % conversion, accompanied with 95.7 % and 95 % selectivity, respectively. Using a water/toluene mixture as reaction medium allowed recycling of the catalyst in the aqueous phase with no loss of activity. 

Unfortunately, for all these reasons the conclusions cannot be applied quantitatively for description of the pH effects in the RCH-RP process. There are gross differences between the parameters of the measurements in [97] and those of the industrial process (temperature, partial pressure of H2, absence or presence of CO), furthermore the industrial catalyst is preformed from rhodium acetate rather than chloride. Although there is no big difference in the steric bulk of TPPTS and TPPMS [98], at least not on the basis of their respective Tolman cone angles, noticable differences in the thermodynamic stability of their complexes may still arise from the slight alterations in steric and electronic parameters of these two ligands being unequally sulfonated. Nevertheless, the laws of thermodynamics should be obeyed and equilibria like (4.2) should contribute to the pH-effects in the industrial process, too. 

Virtually quantitative conversions were observed in the hydroformylation of 1-tetradecene with rhodium complexes generated from the lithium salt of tppms or the lithium (sodium) salts of 21 (Table 2 R=Ph n=3,4) and 22 (Table 2) in methanol as solvent.127,334 Catalyst recycling involved evaporation of methanol and addition of water to form a two phase system, separation of the aqueous phase, evaporation to dryness and addition of MeOH. 

Reduction of unsaturated substrates can also be performed by hydrogen transfer, usually from formate, catalyzed by rhodium and ruthenium complexes. Joo and co-workers have shown that RuCl2(tppms)2 [44] and RuCl2(PTA)4 [45,46] transforms aromatic as well as a, -unsaturated aldehydes to the corresponding aromatic or unsaturated alcohols, with a selectivity up to 98% in the latter case,... 

However, it can also be argued that the TPP simply enhances the solubility in the organic phase of the mixed rhodium complexes that are formed, just as rhodium complexes with TPPDS or TPPMS instead of TPPTS do. In addition, it has to be considered that the promoter ligand TPP will stay in the crude aldehyde mixture after phase separation and will have to be separated by a distillation step. 

A second alternative for the separation of hydroformylation products from a rhodium [8] or cobalt [9] catalyst is to perform the catalytic reaction in a polar solvent using complexes of monosulfonated trialkyl- or triarylphosphines (e.g., TPPMS). Addition of both water and an apolar solvent such as cyclohexane gives a biphasic system. After separation of the apolar layer, the added apolar solvent must be stripped from the products. In order to form a homogeneous system with new substrate alkene, the polar catalytic phase must be freed from water, e.g., by azeotropic or extractive distillation. Clearly, these extra co-distillation steps are energy-consuming. 

Efficient separation of catalyst from the reaction medium can be achieved by extraction of polar rhodium complexes of TPPMS with water [8]. However, many catalytic processes for the production of fine chemicals require the use of modified tailor-made catalysts. The introduction of amphiphilic substituents on phosphines opens the way to easy separation of ligands that induce high selectivity and/or activity in catalytic reactions simply by extraction with acidic or basic water. 

It is to be mentioned that water-soluble phosphine complexes of rhodium(I), such as [RhCl(TPPMS)3], [RhCl(TPPTS)3], [RhCl(PTA)3],... 

In contrast to the case of the water soluble [RhClP3] complexes (P = PTA, TPPMS or TPPTS) which did not promote the reduction of C=0 function in aldehydes or ketones in biphasic systems, [RhCl(PPh3)3] was found an active catalyst for reduction of ketones with aqueous HCOONa (Scheme 3.32). The reaction was aided by phase transfer catalysis using Aliquat-336 and required a large excess of PPh3 to prevent reduction of rhodium into inactive metal. Substrates like acetophenone, butyrophenone, cyclohexanone and dibenzyl-ketone were reduced to the corresponding secondary carbinols with turnover frequencies of 10-40 h 1 [251]. 

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