Sulfonic acid phosphine ligands

Fig. 9 Zwitterionic nature of the sulfonic acid phosphine ligand...

Scheme 6 Single-pot synthesis of sulfonic acid phosphine ligands...

The reaction of alcohols with CO can also be catalysed by palladium iodides, and various ligands or solvents. Acetic acid is prepared by the reaction of MeOH with CO in the presence of a catalyst system comprising a palladium compound, an ionic iodide compound, a sulfone solvent at conditions similar to those of the rhodium system (180 °C, 60 bar), and, in some cases, traces of a nickel-bipyridine compound were added. Sulfones or phosphine oxides play a stabilising role in preventing metal precipitation [26], Palladium(II) salts catalyse the carbonylation of methyl iodide in methanol to methyl acetate in the presence of an excess of iodide, even without amine or phosphine co-ligands platinum(II) salts are less effective [27],... 

The synthesis of sodium trifluoropropenylbenzenesulfonate, (Equation 12) uses the diazo compound obtained from aniline-2-sulfonic acid and isopropyl nitrite in isopropanol for the oxidative addition to palladium(0)/dba, in the absence of phosphine ligands, with NaOAc as buifer. The substrate itself contributes to palladium stabilization (Equation 16). 

The catalyst [CoH(CN)5] is soluble in water. It is selective for the reduction of olefinic double bonds in a,y9-unsaturated systems. Reduction of NO2 groups only occurs at elevated pressures. Hydrogenolysis of C-Hal bonds is observed [20]. Progress as far as water-soluble hydrogenation catalysts is concerned has also been made with Wilkinson-type catalysts by using phosphine ligands with sulfonic acid substituents [44]. 

The di-(TPPDS)239 and tri-sulfonated phosphines (116) (Scheme 8) (TPPTS)239 are also known, the latter being the most widely used water-solubilizing phosphine ligand, as of 2002. The synthesis of (116) is ostensibly similar to that of (115), although more recently it has been shown that addition of orthoboric acid to the sulfonation mixture reduces the amount of tertiary phosphine oxide side products.240 This general sulfonation procedure has been used for the preparation of various ditertiary (e.g., (117))241 242 and chiral sulfonated phosphines (e.g., (118)).241,243-245 Carboxylic acid- and carboxylate-modified phosphines (e.g., (119) and (120)) are also known.246,247... 

Similarly superior results in comparison to the application of sulfonated phosphine ligands were achieved in the Pd-catalyzed biaryl coupling of arylboronic acids in aqueous media by use of the ligand glcaPHOS 11 [16]. The phosphine was prepared in a single step by the condensation of D-glucono-1,5-lactone with p-diphenylphosphinobenzylamine. 

Several representatives of the widely studied tertiary phosphine ligands for enantioselective hydrogenation in aqueous solutions are shown as Structures 21 -39. It is seen that the most successful ligands in this field do have their water-soluble, mostly sulfonated, derivatives [7, 11]. In case of acid-sensitive compounds, solubility in water could be achieved by attaching dimethylamino substituents to the parent arylphosphines and by their further protonation or quaternization. Monophosphines, such as 21 [88], played a minor role in comparison with the chelating diphosphines. Water-soluble phosphine-phosphinite ligands (L) were obtained from trehalose and used as components of with [Rh(cod)(L)[BF4]] catalysts for the enantioselective hydrogenation ofenamides [157]. 

Strongly basic cationic phosphine ligands 75, 76 containing guanidino functions were prepared either in the reaction of 3-aminopropyldiphenylphosphine with lH-pyrazole-l-carboxamide under basic conditions in DMF [75] or by the addition of dimethylcyanamide to the amino groups of tertiary (3-aminophenyl)phosphines in acidic medium [70], These phosphines (as acetate or chloride salts) are highly soluble in water in some cases the solubility reaches that of TPPMS. Another noteworthy feature of these compounds that they are considerably less sensitive to air oxidation then the anionic (e.g. sulfonated) phosphines. 

In analogy to hydroformylation, alkenes react with SO2 and H2 to give a so-called hydrosulftnation product, sulfinic acids [116]. Cationic Pd(II) and Pt(II) complexes bearing bidentate phosphine ligands are effective catalyst precursors. A plausible mechanism for the hydrosulfination involves formation of alkyl intermediates by olefin insertion into metal hydrides, subsequent insertion of SO2, and reformation of the hydrides with the release of sulfinic acids (Scheme 7.19). However, ahphatic sulfinic acids readily undergo disproportionation to give thiosulfinic acid esters, sulfonic acids, and water at the reaction temperature. The unstable sulfinic acids can be conveniently converted into y-oxo sulfones by addition of a,-unsaturated carbonyl compounds as Michael acceptors to the reaction mixtine (Eq. 7.23) [117]. 

The effect of the catalyst and the hgands has not been discussed yet however, it plays an important role. For example, by replacing the classical phosphine ligands by a hydrosoluble sulfonated triphenylphosphine (TPPTS), Safi and Sinou carried out a reaction with butadiene monoxide and ethyl acetoacetate in a two-phase aqueous-organic medium.f The recycled water layer containing the catalyst could be reused with very high efficiency for a second reaction. As a matter of fact, the corresponding ethyl ester of the 2-acetyl-6-hydroxy-4-hexenoic acid was isolated in 80% and 87% yield (Z/E = 15 85) after the first and the second reaction, respectively. 

An aqueous two-phase hydroformylation went on stream at Ruhrchemie AG in 1984 (fourth generation) at their site in Oberhausen/Germany with an annual capacity of 100 kt/a [1]. The current capacity is 500kt/a. The Rh catalyst is immobilized in the aqueous phase. A sulfonated phosphine ligand (TPPTS, trisodium salt of 3,3, 3 Lphosphinidynetris(benzenesulfonic acid) confers the metal catalyst with high solubility in water. The catalyst is removed into the aqueous phase before distillation of the product, which avoids thermal stress. The loss of rhodium is in the range of parts per billion. 

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