Triarylphosphine catalyst

The first investigations of rhodium-catalyzed hydroformylation in room-temperature Hquid molten salts were published by Chauvin et al. in 1995 [6, 67]. The hydroformylation of 1-pentene with the neutral Rh(CO)2(acac)/triarylphosphine catalyst system was carried out as a biphasic reaction with [BMIM][Pp6] as the ionic liquid. 

The above approach of an intermolecular ortho-alkylation followed by an intermolecular Mizoroki-Heck coupling was later extended to heteroaryl iodides by Lautens [48], Using a Pd(OAc)2/triarylphosphine catalyst system, 3-iodothiophene, -benzothiophene, and -indole were transformed to the o/t/zo-alkylation/Mizoroki-Heck coupling products in good to excellent yields (Scheme 17). Unfortunately, 2-iodoheteroaryls were found to be poor substrates for the reaction. 

Dehalogenation of monochlorotoluenes can be readily effected with hydrogen and noble metal catalysts (34). Conversion of -chlorotoluene to Ncyanotoluene is accompHshed by reaction with tetraethyl ammonium cyanide and zero-valent Group (VIII) metal complexes, such as those of nickel or palladium (35). The reaction proceeds by initial oxidative addition of the aryl haHde to the zerovalent metal complex, followed by attack of cyanide ion on the metal and reductive elimination of the aryl cyanide. Methylstyrene is prepared from -chlorotoluene by a vinylation reaction using ethylene as the reagent and a catalyst derived from zinc, a triarylphosphine, and a nickel salt (36). 

Most hydroformylation investigations reported since 1960 have involved trialkyl or triarylphosphine complexes of cobalt and, more recently, of rhodium. Infrared studies of phosphine complex catalysts under reaction conditions as well as simple metal carbonyl systems have provided substantial information about the postulated mechanisms. Spectra of a cobalt 1-octene system at 250 atm pressure and 150°C (21) contained absorptions characteristic for the acyl intermediate C8H17COCo(CO)4 (2103 and 2002 cm-1) and Co2(CO)8. The amount of acyl species present under these steady-state conditions increased with a change in the CO/ H2 ratio in the order 3/1 > 1/1 > 1/3. This suggests that for this system under these conditions, hydrogenolysis of the acyl cobalt species is a rate-determining step. 

The trinuclear species Ru3(CO)i2 was less active as a catalyst unless triarylphosphines or triaryl phosphites were added, in which case the activity was greater than that of the mononuclear species. The catalytic polynuclear species was probably H4Ru4(CO)8L4. 

Finally, Cristau and coworkers have reported on a quite efficient preparation of triphenylphosphine oxide (Figure 2.13) by a similar addition-elimination reaction of red phosphorus with iodobenzene in the presence of a Lewis acid catalyst followed by oxidation of an intermediate tetraarylphosphonium salt.42 This approach holds the potential for the preparation of a variety of triarylphosphine oxides without proceeding through the normally used Grignard reagent. Of course, a variety of approaches is available for the efficient reduction of phosphine oxides and quaternary phosphonium salts to the parent phosphine, including the use of lithium aluminum hydride,43 meth-ylpolysiloxane,44 trichlorosilane,45 and hexachlorodisilane.46... 

Mixed triarylphosphines have been generated by addition-elimination reactions mediated by both palladium and nickel catalysts.45-47... 

A simple method for the in-situ preparation of Wilkinson-type catalysts consists of the addition of the appropriate amount of the triarylphosphine to the rhodium dimers, [Rh(/<-Cl)(diene) 2 or Rh(//-Cl)(cyclooctene)2]2, according to Eqs. (4) and (5). The best results are usually obtained for a rhodium/phosphine ratio of 1 2. 

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-... 

Phosphonium salts can be synthesized from unsaturated compounds by addition of a triarylphosphine and an acid in the presence of a palladium or rhodium catalyst. Transition metal catalysis turned out to be effective for the synthesis of organophosphorous compounds. 

Investigation of the derivatized triarylphosphine in PP2 demonstrated that this catalyst system was much more stable under the same reaction conditions compared to that derived from the phosphite. The rhodium leaching level was dramatically reduced (0.05 % in one case). Omission of toluene from this system allows development of a process which is nearing the rigorous retention that would be required for commercial application, whilst retaining a high rate and good selectivity to the linear aldehyde product. The refined system also compares well with commercial processes. 

Cluster or bimetallic reactions have also been proposed in addition to monometallic oxidative addition reactions. The reactions do not basically change. Reactions involving breaking of C-H bonds have been proposed. For palladium catalysed decomposition of triarylphosphines this is not the case [32], Likewise, Rh, Co, and Ru hydroformylation catalysts give aryl derivatives not involving C-H activation [33], Several rhodium complexes catalyse the exchange of aryl substituents at triarylphosphines [34] ... 

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). 

A control experiment wherein the phosphorous was replaced with a CH group proceeded with no diastereoselechvity and at a significantly reduced rate, thus providing convincing support for the proposed catalyst-directing role of the triarylphosphine group. 

Tungsten- and molybdenum-catalyzed methods involving vinylidene intermediates have been described for this transformation (Chapter 5). The use of rhodium provides some advantages in terms of catalyst turnover and selectivity. The catalyst formed in situ from a RhCl source and a fiuormated triarylphosphine promotes the cyclization of a variety of alkynols (Table 9.6). 

Quaternary phosphonium iodides are also good choices for the iodide salt catalyst component because they are highly active and, in some cases, soluble in the reaction and recovery processes. The simple quaternization of tri(n-alkyl)phosphines or triarylphosphines with n-alkyl iodides produces a wide variety of low cost phosphonium iodide salts ... 

In 1976, water-soluble triarylphosphine, TPPTS (43, Figure 15), was synthesized and used for two-phase hydroformylation system. Here the rhodium catalyst is soluble in water, whereas the substrate and the product remain in organic solvents. The catalytic performance of the rhodium-TPPTS complexes is similar to the ordinary... 

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