Molten phosphonium salts

The ruthenium complex-catalyzed hydroformylation of 1-alkene was first examined by Wilkinson s group. Ru(CO)3(PPh3)2/phosphine catalysts were found to have moderate catalytic activity [35-37]. Ru3(CO)i2 [38] and anionic hydridocluster complexes such as [NEt4][Ru3H(CO)ii] [39] have also been shown to have catalytic activity. In molten phosphonium salt, Ru3(CO)i2/2,2 -bipyridine has high catalytic activity [40]. The Ru3(CO)i2/l,10-phenanthroline catalyst in N,N-dimethylacetamide (DMAC) shows excellent activity and selectivity for u-aldehydes (Eq. 11.10) [41]. 

Catalyst-philic liquid phases can be used to promote the catalytic activity of heterogeneous catalysts, and to facilitate product-catalyst separation. A variety of different constituents of such catalyst-philic phases can be used, the most attractive being quaternary ammonium and phosphonium salts, PEGs, as well as water and other kinds of low-temperature molten salts. In each system, the catalyst-philic phase is characterized as being separate from the remainder of the reaction mixture, and the catalyst should reside within this phase. 

Reactions conducted in molten quaternary phosphonium salts require no other solvent (199). This material serves as both promoter and reaction medium. Care must be exercised in choosing the salt in such a reaction, since any decomposition could lead to products such as trialkylphosphines and alkyl halides which are expected to be deleterious to catalyst performance. Tetrabutylphosphonium bromide is reported to provide a stable catalyst medium which can be recycled (199, 200), but other related salts show evidence of thermal decomposition during catalytic reactions. Experiments in tetrabutylphosphonium acetate, for example, are found to produce large amounts of methyl and ethylene glycol acetate esters (199). 

Aryl bromides as well as aryl chlorides show increased reactivity in the Mizo-roki-Heck vinylation (eq. (3)) with various olefins when the reaction is performed in molten ammonium or phosphonium salts [28] (see also Section 3.1.6). Many common catalysts, including ligand-free Pd salts, show a substantial inerease in activity and thermal stability. 

A particularly broad potential for application in syngas reactions is shown by ruthenium carbonyl clusters. Iodide promoters seem to favor ethylene glycol (155,156) the formation of [HRu3(CO),]- and [Ru(CO)3I3]- was observed under the catalytic conditions. These species possibly have a synergistic effect on the catalytic process. Imidazole promoters have been found to increase the catalytic activity for both methanol and ethylene glycol formation (158-160). Quaternary phosphonium salt melts have been used as solvents in these cases the anion [HRu3(CO)u] was detected in the mixture (169). Cobalt iodide as cocatalyst in molten [PBu4]Br directs the catalytic synthesis toward acetic add (163). With... 

ILs can be arbitrarily defined as non-corrosive molten salts that are fluid at temperatures below 100 °G. The vast majority of these compounds are based in cations of tetraalkyl ammonium and phosphonium salts and heteroaromatics (Figure 1), typically associated with inorganic and organic anions such as BF4, PF6, N(GF3S02)2, GF3SO3, RGO2, NO3, GIO4, amino-acids, MX . 

The direct, one-step production of DMF from carbon monoxide, hydrogen, and ammonia has also been reported. A mthenium carbonyl catalyst is used, either ia a polar organic solvent (20) or ia a phosphonium molten salt medium (21). 

The first example of a Heck reaction in a molten salt stems from as early as 1996 when tetraalkylammonium and phosphonium halides were used as reaction media for the coupling between arylhalides and n-butyl acrylate. Particularly good results were achieved in trihexyl(tetradecyl)phosphonium chloride, both in terms of reactivity and ease of product isolation. 

Ionic liquids (IL) are salts melting at low temperatures, and represent a novel class of solvents with non-molecular ionic character. In contrast to a classical molten salt, which is a high-melting, highly viscous, and very corrosive medium, an ionic liquid is already liquid at temperatures below 100 °C and is of relatively low viscosity [4]. In most cases, ionic liquids consist of combinations of cations such as ammonium, phosphonium, imidazolium, or pyridinium with anions such as halides, phosphates, borates, sulfonates, or sulfates. The combination of cation and anion has a great influence on the physical properties of the resulting ionic liquid. By careful choice of cation and anion it is possible to fine tune the properties of the ionic liquid and provide a tailor-made solution for each task (Fig. 1), and this is why ionic liquids are often referred to as designer solvents or materials. 

Ionic liquids differ from classical molten salts by being liquids at room temperature, or their melting points are below 100 °C. They have an ionic structure and usually consist of an organic cation and an inorganic or organic anion. The most common cations found in ionic liquids are the tetraalkylammonium, tetraalkyl-phosphonium, N-alkylpyridinium, 1,3-dialkylimidazolium, and trialkylsulfonium moieties [11]. Some common ionic liquid cations and anions are presented in Fig. 7.1. 

The use of molten salts based on phosphonium tosylates has also been reported for Diels-Alder reactions [175]. These salts have higher melting points than most ionic liquids in common use and hence the reactions were performed in a sealed tube. The authors claim very high selectivities in the reaction of isoprene with MVK or methyl acrylate. The effect of temperature on the selectivity in phosphonium tosylates gave reduced endoxxo ratios at higher temperatures [176]. The Diels-Alder reactions of isoprene with acrylonitrile, acrylic acid and methacrylic acid in pyridinium ionic liquids ([EtPy][BF4] or [EtPy][F3CC02]) were found to give the expected cyclohexene structures [177]. The authors show that... 

Jn a potentially far reaching application for melt catalysis by the transition metals, we at Texaco have demonstrated the synthesis of a range of commodity chemicals and fuels directly from CO/H2 via the use of ruthenium-containing molten salt catalysis. Products include ethylene glycol, Ci-C4 alcohols, acetic acid, acetate esters, C2+ olefins and vicinal glycol esters. In its simplest form, this new class of melt catalyst comprises one or more ruthenium sources, e.g. ruthenium carbonyls, oxides, complexes, etc. dispersed in a low-melting (m.p. <150 C) quaternary phosphonium or ammonium salt (e.g. tetrabutylphos-phonium bromide). The key components are selected such that ... 

---