Catalysts containing phosphine ligands

In most cases the catalysts of homogeneous hydrogenation contain a metal ion from the platinum group and a certain number of tertiary phosphine ligands. Several papers describe such systems, a compilation of which is found in Table 3.2. Hydrogenation catalysts with no phosphine 

In addition to the catalysts listed in Table 2, several rhodium complexes of the various diphosphines prepared by acylation of bis(2-diphenylphosphinoethyl)amine were used for the hydrogenation of unsaturated acids as well as for that of pyruvic acid, allyl alcohol and flavin mononucleotide [59,60], Reactions were run in 0.1 M phosphate buffer (pH = 7.0) at 25 °C under 2.5 bar H2 pressure. Initial rates were in the range of 1.6-200 mol H2/molRh.h. 

Despite their catalytic (preparative) efficiency similar colloidal systems will be only occasionally included into the present description of aqueous organometallic catalysis although it should be kept in mind that in aqueous systems they can be formed easily. Catalysis by colloids is a fast growing, important field in its own right, and special interest is turned recently to nanosized colloidal catalysts [62-64], This, however, is outside the scope of this book. 

In most aqueous/organic biphasic systems, the catalyst resides in the aqueous phase and the substrates and products are dissolved in (or constitute) the organic phase. In a few cases a reverse setup was applied i.e. the catalyst was dissolved in the organic phase and the substrates and products in the aqueous one. This way, in one of the earliest attempts of liquid-liquid biphasic catalysis an aqueous solution of butane-diol was hydrogenated with a [RhCl(PPh3)3] catalyst dissolved in benzene [22], 

Although this arrangement obviates the need for modifications of organometallic catalysts in order to make them water soluble, the number of interesting water soluble substrates is rather limited. Nevertheless a few such efforts are worth mentioning. 

---

In this current chapter, we will focus on how transition metal catalysts containing phosphine ligands have been computationally treated and which is the effect of these ligands on their catalytic activity. Particularly, computational smdies concerning one of the most important transformations in organometaUic chemistry will be reviewed The palladium-catalyzed C-C Cross-Coupling reactions. 

In this section catalysts containing monophosphines for aqueous-phase catalysis will be presented, the influence of ligand variation on catalyst activity being emphasized. Syntheses of hydrophilic monodentate phosphine ligands will be discussed very briefly. Reference to catalysts containing other ligands (see Sections 3.2.2 to 3.2.6) is made only where appropriate. 

The few studies made with catalytic precursors containing phosphine ligands have revealed that the presence of such ligands in the clusters generally results in slower catalytic rates and sometimes in ligand degradation via P-C or C-H bond-activation processes. Current data do not, therefore, warrant recommendation of the use of phosphine-substituted clusters as catalyst precursors for hydrogenation reactions. 

Several factors are needed to realize the tandem process. First, the catalyst should not be poisioned by the high concentration of amine. Tlie use of bidentate ligands helps to retard catalyst decomposition by this route. Second, the catalyst must be sufficiently electron rich to catalyze hydrogenation of the relatively electron-rich enamine, while not being too electron rich to prevent hydroformylation. These properties have been achieved with a number of systems, many of which have been outlined in a review on tandem processes initiated by hydroformylation. Some of these catalysts lack phosphine ligands, while others contain PPhj or more modern bisphosphine or bisphosphite ligands shown by the examples in the equations above. 

As seven-coordinate tungsten(II) compounds containing phosphine ligands have been shown by Bencze [29-32] and others [33, 37] to be efficient catalysts in a number of important catalytic reactions, we have turned our attention towards the synthesis of more soluble and versatile derivatives containing nitrile ligands [38, 39]. Nitrile ligands are more labile than phosphine and may result in enhanced activity. 

Modifying the catalysts by phosphine ligands is a very effective way to achieve high n/iso ratios in the commercially most important hydroformylation of aliphatic olefins (see Table 1). For more details on phosphorus-containing catalyst modifiers, see the section Catalyst Modifiers in Hydroformylation. 

---