Triarylphosphine ligands

This particular system is efficient and very specific under mild conditions (room temperature, 1 atm.) for the reduction of olefins. Moreover, it is very sensitive to steric hindrance, as might be expected with a fairly crowded catalyst. Increasing the bulk of the ligands (triarylphosphines) reduces the activity of the catalyst. Internal olefins are reduced much more slowly than terminal ones. Moreover, the system does not catalyze the reduction of most common organic groups (keto, cyano, nitro, aryl) and is therefore very chemoselective. 

A cr-aryl-Pd bond is formed by the transfer of an aryl group even from arylphosphines to Pd and alkene insertion takes placefl 17-119], This reaction is slow and it is not a serious problem when triarylphosphine is used as a ligand. The cinnamate 149 is obtained by the reaction of PhsP with acrylate in the presence of Pd(OAc)2 in AcOH. 

Thereafter, however, P-chirogenic phosphine ligands were the subject of less investigation since the synthesis of highly enantiomerically enriched P-stereo-genic phosphines often proves difficult. Another reluctance Hes in the fact that this class of phosphines, especially diaryl- and triarylphosphines, is conforma-tionally unstable and gradually racemize at high temperature [57,58]. In contrast, optically active trialkylphosphines are known to be optically stable even at considerably elevated temperature. 

Other species tested in situ are chelating bis-imidazolium structures with phe-nylene spacers [48], and bidentate C-P ligands [49] that achieved, in the case of triarylphosphine-imidazolium salts, very good yields for a wide variety of substituted aryl bromides (Scheme 6.11) [50]. 

In a serial mode (Fig. 36.1), one experimental step (in catalysis research this is usually the preparation of the ligand or the catalyst) is repeated n times before moving on to the next step. The only difference with traditional research is that the complete experiment (synfhesis/testing/analysis) is carried out for a set of catalysts rather than for an individual species. For example, a library of ligands from the same class can be assembled via traditional organic synthesis prior to its testing in catalysis. (A library of compounds is a rather large collection of different compounds with some common features and usually the same function, for example triarylphosphines or imidazolidinones.) Ideally, the compounds in the library can be structurally varied in at least two positions to ere-... 

The Feringa/Minnaard/de Vries group has further extended the scope of this cinnamate hydrogenation (Table 36.4) [50]. In all cases, a pronounced effect of the added triarylphosphine was found usually, the best results were obtained with a combination of ligands 10 g or lOh in combination with ri-ortho-lolyl-... 

In this scheme L stands for triarylphosphines and S for solvent (ethanol, toluene). The alkene is simply ethene. The first step in this sequence is the dissociation of one ligand L which is replaced by a solvent molecule. Several... 

In 1986 a new process came on stream employing a two-phase system with rhodium in a water phase and the substrate and the product in an organic phase. For propene this process is the most attractive one at present. The catalyst used is a rhodium complex with a sulphonated triarylphosphine, which is highly water-soluble (in the order of 1 kg of the ligand "dissolves" in 1 kg of water). The ligand, tppts (Figure 8.6), forms complexes with rhodium that are most likely very similar to the ordinary triphenylphosphine complexes (i.e. RhH(CO)(PPh3)3). 

In a first set of experiments all ligands were evaluated independently [23]. Regioselectivities were in the typical order for monodentate triarylphosphine/ rhodium catalysts of75 25 to 81 19 [24]. Next, all possible ten ligand combinations... 

The measurements of the effect of the ligand-rhodium ratio on the rate of hydrogenation listed in Table III show that for the p-dimethyl-aminophenyl phosphines the optimum ligand-rhodium ratio is not 2, as with the triarylphosphines studied by Wilkinson and co-workers (3). With the p-dimethylamino substituted tertiary phosphines L2 and L3 the... 

The effect of varying the triorganophosphine ligands in the palladium-catalyzed carbonylations has been looked at briefly. In general, triarylphos-phines are much better than triorgano phosphites and trialkylphosphines. Changes in the substituents in the triarylphosphine have only minor effects upon reaction rates (//). 

---