Polymer supported sulphonic acids

Using a soliddiquid two-phase system of the sodium arenesulphinite in 1,2-dimethoxyethane, or in the complete absence of a solvent, permits the use of less reactive haloalkanes [3,4], This is a particularly good method for the preparation of sulphones where the sulphinic acid salts are readily available and, in addition to the synthesis of the tolyl sulphones listed in Table 4.28, it has been used to prepare phenyl sulphones [3]. Phenyl sulphones have also been prepared in good yield using a polymer supported catalyst [5] (Table 4.29). As the system is not poisoned by iodide ions, reactive iodoalkanes can be used and there is the additional advantages in the ease of isolation of the product and the re-use of the catalyst. 

Notes Col. 1. — (i) PE-PSSA polystyrene sulphonic acid grafted onto a polyethylene matrix and containing indicated amounts of divinyl benzene 120) (ii) Ionac MC-3470 (Ionac Corp., USA) powdered mixture of PSSA ion exchanger and inert polymer compressed hot with embedded mesh of supporting material120) (iii) Cellophane PUT 600/23 sheet (British Cellophane Ltd.) washed to remove plasticizer110) (iv) Dye A C. I. Direct Blue 1 (tetrasulphonate substantive dye)110 (v) Dye B Procion Yellow HA (disulphonate reactive dye, I.C.I. Ltd.)121. ... 

A related series of 5-substituted-2-amino-oxadiazole compounds have also been prepared in a one-pot procedure using a microwave-assisted cyclisation procedure (Scheme 6.26)164. Rapid preparation of the pre-requisite ureas from the mono acyl hydrazines and various isocyanates (or the isothiocyanate) was easily achieved by simple mixing. The resulting products were then cyclo dehydrated by one of the two procedures either by the addition of polymer-supported DMAP and tosyl chloride or alternatively with an immobilised carbodiimide and catalytic sulphonic acid. Purity in most cases was excellent after only filtration through a small plug of silica but an SCX-2 cartridge (sulphonic acid functionalised - catch and release) could be used in the cases where reactions required additional purification. 

Because the thermal stability of polystyrenes and polymethacrylates is limited to 200°C, continuous use of a polymer-supported reactive species tends to be limited to significantly lower temperatures than this (see polymeric sulphonic acids). There is considerable interest in supporting, in particular, alkene oxidation catalysts on polymers and to operate reactions at temperatures above 200°C. To achieve this, novel thermo-oxidatively stable supports are required and some progress has been made in this direction. More details of specific applications will be given later, but supports based on, for example, polyacrylonitrile [50-52], polyamides [53-56], polysulphone [57, 58], polyaniline [59] and polybenzimidazole... 

A wide diversity of catalysts have been supported on polymers [14, 17, 18, 19], but perhaps the four most important groups are asymmetric catalysts, phase transfer catalysts, sulphonic acid catalysts and transition metal complex catalysts. 

Apart from the ion-pairing aspect, the view that the activity of sulphonic acid groups does not markedly depend on local structure is perhaps supported by the observation by Sakurada et al. that a change in tacticity of the resin - which is expected to affect the geometrical distance between polymer groups - did not alter the catalytic behaviour of sulphonated polystyrene resins [26]. 

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