Syntheses of fluorous quaternary ammonium salts and their application as phase transfer catalysts for halide substitution reactions in extremely nonpolar fluorous solvents

Perfluoromethyldecalin solutions of the fluorous alkyl halides R-f8(CH2)(m)X (m=2,3: X=Cl, 1) are inert toward aqueous NaCl, KI, KCN, and NaOAc. However, substitution occurs at 100 degrees C in the presence of 10 mol % of the fluorous ammonium salts (R-f8(CH2)(2))(R-f8(CH2)(5))(3)N+ I- (1) or (R-f8(CH2)(3))(4)N+ Br- (2) (10 mol %), which are fully or partially soluble in perfluoromethyldecalin under these conditions. Stoichiometric reactions of(a) 1 and R-f8(CH2)(3)Br, and (b) 2 and R-f8(CH2)(2)I are conducted in perfluoromethyldecalin at 100 degrees C, and yield the same R-f8(CH2)(m)I/R-f8(CH2)(m)Br 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))(3)N+ Cl- (3, Aliquat (R) 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. (C) 2009 Elsevier Ltd. All rights reserved.