Hydroformylation functionalized phosphines

Table 1.4 Examples of functionalized phosphines for biphasic aqueous hydroformylations.

Rhodium complexes generated from the polyethylene glycol)-functionalized phosphine 9 (n = 1, x = 0, R = Me, Bu), which should behave as a nonionic surfactant and be able to induce micelle formation, have been used as catalysts in the hydroformylation of 1-dodecene in an aqueous/organic two-phase system [31]. The conversion of 1-dodecene was 80% and the n/iso ratio 60 40, with no carryover of the rhodium catalyst into the organic phase. The Rh/9 (n = 1, x = 0, R = Me, Bu) catalyst remained active after one recycle step [31],... 

As mentioned earlier, in the Ruhrchemie-Rhone Poulenc process for propene hydroformylation the pH of the aqueous phase is kept between 5 and 6. This seems to be an optimum in order to avoid acid- and base-catalyzed side reactions of aldehydes and degradation of TPPTS. Nevertheless, it has been observed in this [93] and in many other cases [38,94-96,104,128,131] that the [RhH(CO)(P)3] (P = water-soluble phosphine) catalysts work more actively at higher pH. This is unusual for a reaction in which (seemingly) no charged species are involved. For example, in 1-octene hydroformylation with [ RhCl(COD) 2] + TPPTS catalyst in a biphasic medium the rates increased by two- to five-fold when the pH was changed from 7 to 10 [93,96]. In the same detailed kinetic studies [93,96] it was also established that the rate of 1-octene hydroformylation was a significantly different function of reaction parameters such as catalyst concentration, CO and hydrogen pressure at pH 7 than at pH 10. 

Rhodium complexes of the phosphine-functionalized carbosilane dendrimers are active for the hydroformylation of alkenes. The influence of the flexibility of the dendritic backbone on the catalytic performance was characterized by comparing dendritic ligands 84a-84c (conditions toluene, 80°C, 20 bar CO/H2) 49). 

The group of Van Leeuwen has reported the synthesis of a series of functionalized diphenylphosphines using carbosilane dendrimers as supports. These were applied as ligands for palladium-catalyzed allylic substitution and amination, as well as for rhodium-catalyzed hydroformylation reactions [20,21,44,45]. Carbosilane dendrimers containing two and three carbon atoms between the silicon branching points were used as models in order to investigate the effect of compactness and flexibility of the dendritic ligands on the catalytic performance of their metal complexes. Peripherally phosphine-functionalized carbosilane dendrimers (with both monodentate... 

An example of a large scale application of this concept is the Ruhrchemie/ Rhone Poulenc process for the hydroformylation of propylene to n-butanal, which employs a water-soluble rhodium(I) complex of trisulfonated triphenyl-phosphine (tppts) as the catalyst [103]. The same complex also functions as the catalyst in the Rhone Poulenc process for the manufacture of the vitamin A intermediate, geranylacetone, via reaction of myrcene with methyl acetoacetate in an aqueous biphasic system (Fig. 1.35) [104]. 

As far as rates are concerned, the catalytic activity varies in a nonlinear fashion as a function of phosphine concentration. In Figure 6 the dependence of rate and selectivity for hydroformylation of propene with HRh(CO)(TPP)3 on the TPP concentration is depicted [138]. The selectivity of the reaction remains constant above an L/Rh ratio of 5 1, at a point where the activity reaches a maximum. Further increase in ligand concentration leads to lower rates. 

The first calculation of the complete hydroformylation cycle with Rh-phosphine catalysts (substrate = ethylene, model ligand = PH3) was published in 1997 [3]. The QM methods used are HF and MP2, respectively (cf. Section 3.1.2.1). Hybrid DFT methods such as B3LYP [4], however, are more appropriate in terms of both accuracy and efficiency [5, 6] (cf. Section 3.1.2.1). Therefore, the same model system was recalculated [7] on the level B3LYP functional/DZVP basis set [8]/quasi-relativistic pseudopotentials on rhodium [9]. Since homologous Ir catalysts are interesting alternatives from an economic point of view [10], calculations with the central metal Ir were also made. This comparative treatment is supported by the experimental assumption of a common mechanism [11], which equals the Heck-Breslow mechanism of the cobalt-catalyzed reaction [12],... 

The first water-soluble system specifically designed to combine both functions of a ligand and a surfactant in one molecule and applied in transition-metal-cata-lyzed conversions of highly water-insoluble substrates in micellar systems is the zwitterionic tenside trisulfoalkylated tris(2-pyridyl)phosphine, 2 (n = 0, 3, 5, 7, 9, 11) [4, 14, 72, 74]. Turnover frequencies (TOF) up to 340 h-1 were achieved in the micellar hydroformylation of 1-tetradecene to pentadecanals, according to Eq. (1), using Rh/2 catalysts at 125 °C, by fine tuning of the hydrophilic/lipophilic properties of the tenside system 2 [4, 14]. In sharp contrast, Rh/TPPTS catalysts gave only traces of pentadecanals under the same biphasic conditions. 

In order to rationalize the effect whereby the activity in the Rh/2-catalyzed hydroformylation of 1-tetradecene goes through a maximum as a function of the tail length of the surfactant 2, the model of simplified spherical (Hartley) ionic micelle [9a-c] (Figure 1) was proposed [14, 15], The core of the micelle is probably composed of the hydrophobic tail of the tenside phosphine 2 where 1-tetradecene is solubilized (Figure 1, stippled part). 

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