Phenoxides onium

In contrast to the neutral property of onium bromides, onium phenoxides and carboxylates work as base catalysts by considering the general basicity of the phe-noxide and carboxylate anions (Figure 14.4). Based on this property, chiral onium phenoxides and related compounds have been employed both as Lewis and Bron-sted base catalysts for homogeneous reachons [6]. 

Onium Phenoxides and Related Compounds as Bronsted Base Catalysts... 

A slow non-competing liquid/liquid reaction with no catalyst present gave only 78 % O-alkylation. Thus the active site of the lipophilic phosphonium ion catalysts appears to be aprotic, just as in analogous phase transfer catalyzed alkylations with soluble quaternary ammonium salts 60), Regen 78) argued that the onium ion sites of both the 17% and the 52% RS tri-n-butylphosphonium ion catalysts 1 are hydrated, on the basis of measurements of water contents of the resins in chloride form. Mon-tanari has reported measurements that showed only 3.0-3.8 mols of water per chloride ion in similar 25 % RS catalysts 74). He argued that such small hydration levels do not constitute an aqueous environment for the displacement reactions. No measurements of the water content of catalysts containing phenoxide or 2-naphthoxide ions have been reported. 

In the reaction of activated aryl halides with phenoxide and thiophenoxide in chloro- or o-dichloro-benzene at reflux, pyridinium salts (48 R, R = —-CH2CH2CHMeCH2CH2— R = Me, Bu", n-C6Hi3 R = CH2CHEtBu, CH2BU1) proved to be superior catalysts to simpler onium salts, like TBAB, due mainly to their greater thermal stability.162... 

Based on the design of chiral quaternary onium bromides and chlorides as chiral phase-transfer catalysts, chiral quaternary onium fluorides [5] and phenoxides [6] have been developed as chiral base catalysts for homogeneous reactions. 

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