Water-soluble catalyst precursors

One of the more popular techniques for producing dispersed-phase catalysts involves the use of water- or oil-soluble catalyst precursors. Small amounts of the water-soluble catalyst precursor are added to the coal-vehicle feed and are subsequently converted to a highly dispersed, insoluble catalytic phase. The reaction... 

More recently, Taqui Khan and co-workers (70) introduced the potentially tetradentate ethylenediaminetetraacetate ligand in the ruthenium coordination sphere in order to obtain an efficient water-soluble catalyst precursor. Indeed, starting from the ruthenium(III) aquo EDTA species [Ru(EDTA)(H20)] , carbonylation gives the paramagnetic carbonyl complex [Ru(EDTA)(CO)] which is able to induce the heterolytic activation of dihydrogen (Scheme 3). The hydroformylation of hex-l-ene performed at 50 bar (CO/H2= 1/1) and 130°C in a 80/20 ethanol-water solvent... 

Polymerization via Suzuki coupling in aqueous solution was first reported by Novak et al. [Eq. (11)] [236]. A water-soluble catalyst precursor, [Pd(TPPMS)3] (TPPMS = 3-Ph2PC6H4SO3Na) was employed. The resulting polymer is water soluble, and has a weight average molecular weight on the order of 5 x lO" g mol . ... 

Of particular significance to achieve environmentally benign systems would be the use of water as a solvent, but this is also challenging in view of the lack of solubility of the alkanes and, commonly, also of the metal catalysts. The approach followed in the author s group often involves the use of hydrosoluble ligands at appropriate metal centers, which can lead to the formation of water-soluble catalyst precursors. Examples are indicated in the following sections. 

Suitably functionalized azoderivatives of jS-diketones (ADB or arylhydrazones of j8-diketones, AHBD) (Scheme 2.2b) are also convenient tiydrosoluble species toward water-soluble catalyst precursors in this field and of particular interest are those bearing an acid substituent (carboxylic or sulfonic group) [21], which can operate without requiring the common presence of an added acid promoter. The acidic moiety conceivably has two main roles provides water solubility and acts as the acid promoter. Hence, the complexes [Cu(ADB)(H20)(Me2NCH0)] (Scheme 2.3b) and [ Cu(/u-ADB)(MeOH) 2][ADB = p-COOH substituted (2-hydroxy-phenylhydrazone)pentane-2,4-dione] appear to behave as dual-role catalyst precursors, in acid-free medium, combining, in each molecule, an active copper center and an acid site (TONs and yields up to 163 and 14%, respectively, are achieved for the model oxidation of cyclohexane, in NCMe/aqueous H2O2, at 50 °C) [21],... 

Sulfide catalysts have been dispersed directly on the coal surface. Very high dispersion on the catalyst may allow direct interactions between the catalyst and solid coal. The first application of this approach utilized molten chloride as the starting material. Later, oil- and water-soluble iron precursors were impregnated or ion exchanged onto the coal surface through the interaction with oxygen functional groups (56 -60). 

This screening concept was also applied to liquid/liquid systems. As a test reaction, the isomerization of allylic alcohols to carbonyls with water-soluble catalysts in a biphasic heptane/water system was chosen [109,113]. The catalysts (metal precursor, Rh, Ru, Pd, Ni ligands, sulfonated phosphane or disphosphane ligands) were injected the liquid carrier 2 (water). The substrates (different allylic alcohols) were injected into liquid carrier 1 (heptane) ... 

Since the use of alcohol in the sol-gel process can lead to aggregation of the colloidal metal, Au/SiC>2 catalysts have been prepared without alcoholic solution, using tetramethyl orthosilicate (TMOS) as a water-soluble silicic precursor and colloidal gold by reducing aqueous HAuCLi with magnesium citrate.114 However, the size of the gold particles was not reported, but reduction by citrate ion does not usually produce small particles. 

Subsequently, water-soluble catalysts have been developed for use in aqueous biphasic systems. One such catalyst precursor is RuHCl(TPPTS)2(L)2 (where TPPTS = triphenylphosphine trisulfonate and L = aniline or a similar base). 

Besides this work with rather lipophilic catalyst precursors, two water-soluble analogues have been investigated [Rh(tos)(cod)(H2O)] (30 tos = tosylate) and the similar compound [Rh(tos)(nbd)(H20)] (31) [148]. The authors have verified that water is still the best reaction medium for polymerization of phenylacetylene compared to toluene, THF and neat monomer, thus indicating a possible involvement of water in the formation of the active catalyst (830 TO in water). With catalyst precursor 30, low polydispersities and a cis content of 90% are found, whereas catalyst precursor 31 affords a larger polydispersity but a higher cis content (100%). Interestingly, these catalysts proved to be tolerant to air since similar polymerization results are obtained with tap water without exclusion of air. 

As far as is known, the only industrial application of the water-soluble catalyst for the hydroformylation of 5 -functionalized alkenes has been developed by Kura-ray [17]. In this process, 7-octen-l-al is hydroformylated into nonane-1,9-dial, a precursor of nonene-l,9-diol, by using a rhodium catalyst and the monosulfonated tri-phenylphosphine as water-soluble ligand in a 1 1 sulfolane/water system. At the completion of reaction, the aldehydes are extracted from the reaction mixture with a primary alcohol or a mixture of primary alcohol and saturated aliphatic hydrocarbon (cf. Section 6.9). 

CO in a mixed solvent of H2O/DMF (1/1 or 1/2, v/v), and even in water alone, depending on the solubility of the substrate (Eq. 6.31)7 The palladium(II) complexes Pd(OAc)2, K2PdCl4, PdCl2(PPh3)2, and Pd(NH3)4Cl2 are used as the precursors of the catalyst, using either K2CO3 or NaOAc as the base. lodoxyarenes can be carbonylated in water alone due to their solubility in the solvent. Recent work has been done on the use of water-soluble catalysts. Under the appropriate pressure and temperature conditions, aryl mercaptans (thiophenols) can also be carbonylated in aqueous media with cobalt carbonyl as the catalyst. " ... 

The Heck reaction is compatible with water, and water-soluble catalysts have successfully been employed (entry 10). Alkali metal salts (NaHCOj, K2CO3, and KOAc) are effective bases in the smooth reactions of acrylic acid with o-, m-, or p-iodobenzoic acid or p-iodophenol in which water-soluble salts are formed and very high yields are encountered (entry 11). Activated heteroaryl chlorides are good arylpalladium precursors (entry 12) while nonactivated aryl chlorides have to date been considered to be less useful in the Heck reaction. In entry 13, the recent protocol devised by Littke and Fu for arylation with nonactivated chlorobenzenes is shown. ... 

Hydroaminomethylations with ammonia are difficult to develop because the product amine is more nucleophilic than ammonia and preferentially condenses with the aldehyde to form the precursor to the final amine product. Thus, hydroaminomethylations with ammonia tend to form dialkyl- and trialkylamines. To overcome this problem in selectivity, hydroaminomethylations of ammonia have been conducted with a water-soluble catalyst in a biphasic system. Under these conditions, the reaction occurs preferentially with ammonia in the aqueous phase, because it is more soluble than the alkylamine product, and the reactions favor formation of the primary amine. The use of a water-soluble version... 

Because of its volatility, the cobalt catalyst codistills with the product aldehyde necessitating a separate catalyst separation step known as decobalting. This is typically done by contacting the product stream with an aqueous carboxyhc acid, eg, acetic acid, subsequently separating the aqueous cobalt carboxylate, and returning the cobalt to the process as active catalyst precursor (2). Alternatively, the aldehyde product stream may be decobalted by contacting it with aqueous caustic soda which converts the catalyst into the water-soluble Co(CO). This stream is decanted from the product, acidified, and recycled as active HCo(CO)4. 

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