Synthesis and application of fluorous-tagged phosphonium salts
thesisposted on 15.12.2014, 10:35 by Kathleen Margaret Weber
This thesis details the synthesis of a range of fluorous-tagged phosphonium salts in order to investigate their applications as phase transfer catalysts (PTC). Fluorous-tagged phosphonium salts have been prepared by the quatemisation of the fluorous tertiary phosphines, P(CH2CH2C6Fi3)3, P-CeHiCeFn and P-CerCrCfkCgFn, with benzyl bromide (BnBr), butyl bromide (BuBr), lH,lH,2H,2H-perfluorooctyl triflate (C6F13CH2CH20S02CF3) and butyl triflate (n-C4H9OSOCF3) in clean and high yielding reactions. All of the fluorous phosphonium salts were evaluated in potassium picrate extraction experiments in order to define their abilities to transfer picrate from an aqueous phase into an organic phase (benzotrifluoride). All of the salts showed good extraction of the picrate anion into the organic phase (68-100%) demonstrating their potential for phase transfer catalysis. The fluorous-tagged phosphonium salts were tested in a model halide exchange reaction under phase transfer conditions. Both liquid-liquid and solid-liquid systems were tested, but the phosphonium salts performed better under liquid-liquid conditions. Recovery of the fluorous-tagged salts for subsequent reactions was attempted using fluorous reverse phase silica gel (FRPSG). Limited success was achieved due to the decomposition of the aromatic phosphonium salts during the phase transfer catalysed reaction. The stability of the 1st generation of aromatic phosphonium salts was therefore examined under liquid-liquid (BTF/water) and solid-liquid (KI/BTF) phase transfer conditions. It was determined that the decomposition gave mainly the phosphine oxide and the stability of the phosphonium salts, that contained fluorous ponytails directly attached to the aromatic ring, was restricted under these conditions. A series of 2nd generation salts containing the P-CeFUCFbCICgFn unit were synthesised and showed good preliminary stability results under liquid-liquid phase transfer conditions.