Sulfonium salts phase transfer catalysts

The first metal(salen) complex investigated as an asymmetric phase-transfer catalyst was methionine-derived sulfonium salt 29. It was anticipated that the sulfonium salt... 

Two-phase systems, usually consisting of a strongly alkaline aqueous rfiase and methylene chloride, are also effective and convenient. Trialkylsulfonium salts with methylsulfate counterions can be used alone under these conditions, whereas those with halide counterions generally benefit from added phase transfer catalysts (equation 2). Dodecyldimethylsulfonium chloride was found to be an effective phase transfer reagent. A sulfonium salt covalently attached to a polymeric resin was found most effective when a phase transfer agent was also employed. ... 

Workers from Hoffmann-La Roche have also devised a linear synthesis of racemic LTA4 methyl ester during the course of a synthesis of LTE4. They too employed a sulfonium salt (Scheme 3.22) in the epoxide-forming step to obtain a dienediyne epoxide. This step was made reasonably efficient by the use of a two-phase reaction system with a phase-transfer catalyst. Their sequence is also amenable to reasonable scale-up as they describe the preparation of 18 g of the epoxide mixture in a single reaction. A unique aspect of this synthesis is... 

Recent work has indicated that solvents may even behave as phase transfer catalysts under certain conditions [50]. Generation of sulfur ylides by reaction of a sulfonium salt with a solid base was found to proceed via deprotonation of the solvent (Figure 5.21). It is believed that other weakly acidic solvents, such as rer -butanol and dimethylsulfoxide, behave in a similar manner, although it is not known whether these will function if they are present in just catalytic amounts. 

Ruthenium catalysts have also been applied as effective catalysts. The selective oxidation of 12 could be achieved by using terpyridine-derived ruthenium complexes, with catalyst loadings below 1 mol% and the addition of a phase transfer catalyst (PTC an ammonium, phosphonium, or sulfonium salt) in biphasic aqueous systems, but also in methanol without PTC, and without the need for adding... 

The formation of sulfonium salt is a slow and reversible reaction. The formation of the ylide is rapid and reversible [182], The protic conditions keep the ylide at low concentration. The reversibility avoids ring cleavage of the cychc catalyst and at the same time rapid ylide (re-)generation enables epoxidation [182]. Additives (e.g. Bu NI, Nal, Bu NHSO, catechol) have been used to activate the alkylhahde in order to inaease the rate of the formation of sulfonium salt. It is supposed that the additives can also work as phase transfer catalysts in the reaction, supporting the extraction of the hydroxide anion into the organic phase and/or the deprotonation of the sulfonium salt [184,194]. 

CH acidities of sulfonium and phosphonium salts are in the range accessible for the PTC so they can be deprotonated in liquid-liquid and liquid-solid two-phase systems. Since in the generated ylides the charges are internally neutralized the PT catalysts suppl3dng lipophilic cations are not necessary for transfer of the reacting species into the organic phase. Nevertheless in many cases of the reactions of sulfonium and phosphonium ylides, PT catalyst was used and recommended. Thus in two-phase systems with or without catalyst, an efficient synthesis of oxiranes via reactions of sulfonium ylides with aldehydes is executed (eqs. 66 and 67). 

---