Phosphonium tosylates, ionic liquids

While it is clear that acidic chloroaluminate ionic liquids give very good results, their use is often not practicable and alternatives are desirable. Excellent stereoselectivity was obtained with phosphonium tosylate ionic liquids, [R3PR ][OTs], in the reaction between isoprene and methyl acrylate affording essentially exclusively the 1,4-isomcr.1 081 However, with... 

Preparation of phosphonium salts as ionic liquids [PiBu3Et][tosylate] Gytec Technology Gorp., USA 2001 24... 

In this context, the use of ionic liquids with halogen-free anions may become more and more popular. In 1998, Andersen et al. published a paper describing the use of some phosphonium tosylates (all with melting points >70 °C) in the rhodium-catalyzed hydroformylation of 1-hexene [13]. More recently, in our laboratories, we found that ionic liquids with halogen-free anions and with much lower melting points could be synthesized and used as solvents in transition metal catalysis. [BMIM][n-CgHi7S04] (mp = 35 °C), for example, could be used as catalyst solvent in the rhodium-catalyzed hydroformylation of 1-octene [14]. 

Hydroformylation. Using RhjCOAc) in phosphonium tosylates for hydro-formylation of 1-alkenes at 120°, the catalyst recovery is facilitated. Thus, on cooling of the reaction mixture the rhodium acetate is completely retained in the solid phase and the liquid products are obtained by simple decantation. Variation of substituents at the phosphorus atom of ionic solvents [e.g., (PhjPEt) OTs vs. (BUjPEO OTs ] has remarkable effects on the ratio of alkanals and 2-methylalkanals. 

It is finally worth noting that phosphonium tosylates [31], and more recently pyridinium-based ionic liquids [32], have also been used as solvents for the Diels-Alder reactions of isoprene with methyl acrylate, acrylic acids, but-3-en-2-one and acrylonitrile (Scheme 5.1-6). 

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... 

The catalytic activity of phosphonium salts in the synthesis of cyclic carbonates (e.g., propylene carbonate from propylene oxide and carbon dioxide) was found to be greatly enhanced by their immobilization onto silica that itself had no catalytic effect. The discipline of ionic liquids keeps developing dynamically. Phosphonium ionic liquids have received much less attention than ammonium salts in the past, but the situation is changing. Triphenylalkyl-, tetraalkyl- and functionalized-phospho-nium tosylates (Fig. 11) that exhibit melting points mostly above 100 °C were introduced and characterized. 

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