Distannoxanes, fluorous

Otera has reported that fluorous distannoxanes such as 23, which dissociate to give Lewis acidic species, catalyze transesterifications in or-ganic/fluorous solvent mixtures [8,9]. Although 23 was insoluble in toluene at room temperature, it dissolved at reflux and efficiently promoted the transformation in reaction D of Scheme 4, as well as others. The catalyst precipitated upon cooling, but a fluorous solvent extraction was utilized for recovery (100%). Another thermomorphic fluorous Lewis acid catalyst was developed by Mikami [11]. He found that the ytterbium tris(sulfonamide) 24 could be used for Friedel-Crafts acylations imder homogeneous conditions in CICH2CH2CI at 80 °C, and precipitated upon cooHng to -20 °C (reaction E, Scheme 4). 

For a practical transesterification reaction, next to high yields, an equimolar ratio of the reactants is desirable. The catalyst should be neutral and easily separable, and no special technology for alcohol removal should be needed [11]. These requirements are fulfilled by the fluorous tagged distannoxane catalyst 6. Transesterification yields are quantitative and the FC-72 solution (FC-72 and FC-75 are commercially available mixtures of perfluoroalkanes) of the catalyst can be reused. 

The basic idea of fluorous technology stems from the facile separation of organic products and fluorous catalysts, staying in the organic and fluorous phases, respectively, owing to incompatibility of the two reaction media.The catalysts can be easily recovered from the fluorous phase, or preferably the fluorous solution can be used repeatedly without isolating the catalysts. It is reasonable to assume that such unique solubility characteristics might be reflected in the equilibration as well. Based on this simple expectation, fluorous biphasic (trans)esteriflcation has been pursued by use of fluorous distannoxane catalysts. 

Upon confirmation of the highly fluorophilic character of fluorous distannoxanes, these compounds were utilized as catalysts for fluorous biphasic fransesferification. First the simplest system, where FC-72 was the sole solvent, was employed. The reactants, in a 1 1 ratio, were heated in the presence of the catalyst at 150 °C in FC-72 in a pressure bottle for 16 hours and the reaction mixture was washed with toluene. GLC analysis of the toluene layer exhibited a single peak assignable to the desired ester... 

Toward ideal (trans) esterification by use of fluorous distannoxane catalysts. Accounts of chemical research, 37, 288-296,0001-4842. 

Such distannoxanes have been found to be highly active and selective catalysts for (trans)esterification reactions of methyl esters or acids with a twofold excess of alcohol and 0.5 mol% of distannoxane, either solventless or in toluene.A similar 100% yield in esters could be observed with a stoichiometric amount of alcohol in fluorous solvent that permitted recycling of the catalyst without loss of its performances. " The proposed mechanism consists in the activation of the alcohol via Sn-X (Y) ligands exchange to Sn-OR ones (Scheme 21.13b), while the ester is coordinated through the ketonic o>ygen atom to an adjacent tin centre (Scheme 21.13c). A similar dinuclear activation was reported for the melt transesterification of a variety of polyesters and polyacetates in the presence of such distannoxane catalysts. 

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