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 f 8 (CH 2 ) m X ( 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 ( R f 8 (CH 2 ) 2 )(R f 8 (CH 2 ) 5 ) 3 N + I − ( 1 ) or ( R f 8 (CH 2 ) 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 f 8 (CH 2 ) 3 Br, and (b) 2 and R f 8 (CH 2 ) 2 I are conducted in perfluoromethyldecalin at 100 °C, and yield the same R f 8 (CH 2 ) m I/ R f 8 (CH 2 ) 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 CH 3 (CH 3 (CH 2 ) m ) 3 N + 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.