Phase Selectively Soluble Polystyrene-Supported Organocatalysts
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Alkane phase selectively soluble poly(4-alkylstyrene) supports have been developed. 4-Methyl-, 4-tert-butyl, 4-dodecyl-, and 4-octadecylstyrene were copolymerized with 5-10 mol % of 4-chloromethylstyrene to afford co- and terpolymers containing chloromethyl pendant groups so that a fluorescent dye can be attached. By varying the structure and the length of the alkyl groups, derivatives of these polymers with covalently coupled fluorescent dansyl groups as catalyst surrogates show a significant increase in phase selective solubility in thermomorphic and latent biphasic systems. The advantage of alkyl-substituted polystyrenes is that they are phase-selectively soluble which means that a polymer-bound catalyst can be separated from products in a biphasic separation that avoids a solvent-intensive precipitation process. Coupling of a 4-dimethylaminopyridine (DMAP) analog, Cinchona alkaloid derivative or phosphine-ligated metal catalyst to the poly(4-alkylstyrene) supports was used to prepare alkylated-polystyrene-bound catalysts. The recycling of these polymer-supported catalysts was affected using a biphasic liquid/liquid separation step after a monophasic reaction. Alkyl-substituted soluble polystyrene supports are found to be highly phase selectively soluble in heptane phase so that organo- and transition metal catalysts can be separated from products by thermomorphic or latent biphasic separations with minimum loss of the catalyst in the polar phase, which was monitored by fluorescence spectroscopy or by inductively coupled plasma mass spectrometry (ICP-MS).