Syntheses of fluorous quaternary ammonium salts and their application as phase transfer catalysts for halide substitution reactions in extremely nonpolar fluorous solvents
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Perfluoromethyldecalin solutions of the fluorous alkyl halides Rf8(CH2)mX (m=2, 3; X=Cl, I) are inert toward aqueous NaCl, KI, KCN, and NaOAc. However, substitution occurs at 100 C in the presence of 10 mol % of the fluorous ammonium salts (Rf8(CH2)2)(Rf8(CH2)5)3N+ I- (1) or (Rf8(CH2)3)4N+ Br- (2) (10 mol %), which are fully or partially soluble in perfluoromethyldecalin under these conditions. Stoichiometric reactions of (a) 1 and Rf8(CH2)3Br, and (b) 2 and Rf8(CH2)2I are conducted in perfluoromethyldecalin at 100 C, and yield the same Rf8(CH2)mI/Rf8(CH2)mBr equilibrium ratio (60-65:40-35). This shows that ionic displacements can take place in extremely nonpolar fluorous phases, and suggests a classical phase transfer mechanism for the catalyzed reactions. Interestingly, the non-fluorous ammonium salt mixture CH3(CH3(CH2)m)3N+ Cl- (3, Aliquat 336; m=2:1 7/9) also catalyzes halide substitutions, but under triphasic conditions with 3 suspended between the lower fluorous and upper aqueous layers. NMR experiments establish very low solubilities in both phases, suggesting interfacial catalysis. 2009 Elsevier Ltd. All rights reserved.
