Oxidation of TPPTS

A Rh(TPPTS) catalyst generated by this protocol may form with TPPTSO a colloidal dispersion, which has to be considered in the interpretation of the catalysis results [167]. The oxidation of TPPTS is slower than that of PPhj, which accounts for the aqueous two-phase hydroformylation protocol. 

For monosulfonation of PPh3 the reaction mixture can be heated for a limited time [1-3] while multiple sulfonation is achieved hy letting the solution stand at room temperature for a few days [4-10], In this simplest way of the preparation several problems may arise. Under the harsh conditions of sulfonation there is always some oxidation of the phosphine into phosphine oxide and phosphine sulfides are formed, too. Furthermore, selective preparation of TPPMS (1) or TPPDS (2) requires optimum reaction temperature and time and is best achieved by constantly monitoring the reaction by NMR [10] or HPLC [7]. Even then, the product can be contaminated with unreacted starting material. However, 1 can be freed of both the non-sulfonated and the multiply sulfonated contaminants by simple methods, and in the preparation of TPPTS (3) contamination with PPh3, 1 or 2 is usually not the case. Direct sulfonation with fuming sulfuric add was also used for the preparation of the chelating diphosphines 34-38, 51, 52. 

In a different approach [11] to access pure products, the use of strong oleum (65% SO3) for sulfonation of PPh3 resulted in quantitative formation of TPPTS oxide. This was converted to the ethyl suhbester through the reaction of an intermediate silver sulfonate salt (isolated) with iodoethane. Reduction with SiHCls in toluene/THF afforded tris(3-ethylsulfonatophenyl)phosphine which was finally converted to pure 3 with NaBr in wet acetone. In four steps the overall yield was 40% (for PPhs) which compares fairly with other procedures to obtain pure TPPTS. Since phosphine oxides are readily available from easily formed quaternary phosphonium salts this method potentially allows preparation of a variety of sulfonated phosphines (e.g. (CH3)P(C6H4-3-S03Na)2). 

Even in an excess of ligands capable of stabilizing low oxidation state transition metal ions in aqueous systems, one may often observe the reduction of the central ion of a catalyst complex to the metallic state. In many cases this leads to a loss of catalytic activity, however, in certain systems an active and selective catalyst mixture is formed. Such is the case when a solution of RhCU in water methanol = 1 1 is refluxed in the presence of three equivalents of TPPTS. Evaporation to dryness gives a brown solid which is an active catalyst for the hydrogenation of a wide range of olefins in aqueous solution or in two-phase reaction systems. This solid contains a mixture of Rh(I)-phosphine complexes, TPPTS oxide and colloidal rhodium. Patin and co-workers developed a preparative scale method for biphasic hydrogenation of olefins [61], some of the substrates and products are shown on Scheme 3.3. The reaction is strongly influenced by steric effects. 

Method A. In a 50-mL, two-necked flask, a solution of 260 mg (1.0 mmol) of RI1CI3 3H20 in 20 mL of water is stirred for about 3 h after addition of 5.87 g (10 mmol) of tppts dissolved in 10 mL of water. After completion, the resultant solution which contains tppts, the corresponding oxide tppots, and small amounts of the binuclear complex [(/z-Cl)Rh(tppts)2]2 are separated by column chromatography on Sephadex G-15. The red fraction is collected and the solvent is removed in vacuo (10-2 torr). Yield 1.46 g (73%). 

PdCl2 was purchased from Johnson Matthey GmbH (Karlsruhe, Germany), t TPPTS is prepared according to the procedure of Hoechst AG Werk Ruhrcbemie11 with a purity of 99.3% (TPPTS-oxide 0.7%). A similar synthesis is described in this volume and includes the synthesis of TPPTS. The checkers initially had problems with a sample of TPPTS which had been in solution for longer than 3 months and contained 21% TPPTS-oxide. When fresh TPPTS was used, the reaction worked very well and afforded 376.2 mg (95%) yield. 

Other aqueous biphasic organometallic reactions include fat-chemical processes, such as the Ru-catalyzed oxidation of fatty alcohols to the corresponding aldehydes or acids [174, 175, 244 g]. Oxidation reactions of water-soluble ligands in aqueous biphasic reactions (especially TPPTS) have been investigated by Larpent, Patin and co-workers [176]. Recent examples of other aqueous biphasic reactions are compiled in Table 3. 

Following initial studies in 1974 the preparation of TPPTS was carried out via the sulfonation of TPP with oleum (i.e., concentrated sulfuric acid containing 20% by weight of S03) at 40 °C in one day. After hydrolysis and neutralization by NaOH, an aqueous solution of sodium sulfate and a mixture of different P compounds - consisting mainly of TPPTS and the corresponding P-oxide ( TPPOTS ) as key chemical species - were obtained (Scheme 1) [20],... 

Beletskaya and co-workers have shown that the reaction is possible in neat water as solvent. Thus, aryl iodides have been carbonylated with various palladium salts lacking phosphine ligands as depicted in Eq. (4) [7]. Although this reaction is not a truely biphasic process the results are remarkable regarding catalyst efficiency. Thus, a maximum turnover number (TON) of 100000 was described (R = p-COOH, quantitative yield after 6 days). Quite different is the performance of a water-soluble palladium phosphine catalyst described by Kalck et al. [8]. The hy-drocarboxylation of the less activated bromobenzene with either Pd(TPPTS)3 or a mixture of Pd(OAc)2 and TPPTS proceeds only sluggishly (turnover frequency TOF < 10 h 1). In order to prevent decomposition of palladium an excess of phosphine has to be used. At least 15 equiv. of ligand is necessary to prevent formation of metallic palladium. Because of rapid oxidation of the ligand the re-use of the water phase is not possible. 

The other large group of water-soluble phosphines contains ionic substituents in the aryl or alkyl groups attached to phosphorus. Mono-, di-, and trisulfonated triphenylphosphine (TPPMS, TPPDS, and TPPTS, respectively), as well as several sulfonated diphosphines can be prepared by direct sulfona-tion with fuming sulfuric acid (52,59,60). Care should be taken to avoid oxidation of the phosphorus, this can be best achieved by using an anhydrous ortho-boric acid-sulfuric acid mixture (60). Sodium or potassium salts of TPPMS can... 

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