Cinchonium salts

Asymmetric alkylation of a-substituted benzyl cyanides has been achieved using A-benzylquininium and cinchonium salts using procedures analogous to those described in 6.2.2 [19, 20], Analogous reactions catalysed by l-alkyl-3-hydroxy-quinuclidinium salts are less effective than those catalysed by quininium salts [20], whereas 1 -methyl-1 -diphenylmethyleneimino-2(5)-hydroxymethylpyrroli-dinium iodide is a more effective catalyst (>70% ee) [18]. 

Stereoselective aziridation of electron-deficient alkenes by A-acyl arylhydroxyl-amines (Scheme 12.11) is facilitated by cinchonium salts [5]. Optimum selectivity... 

Phase-transfer catalysed oxidation of ketones with dioxygen under basic conditions in the presence of triethyl phosphite and a cinchonium salt produces a-hydroxy-ketones (Schemes 12.14 and 12.15, Table 12.9) in good overall yield (-95%) and with a high enantiomeric excess [>70% ee using N-(4-trifluoromethyIbenzyl)cincho-nium bromide] [29], Lower asymmetric induction is observed with ephedrinium salts, polymer-supported salts and, surprisingly, by cinchonidinium salts. 

Reports on arylphosphonates include the synthesis and resolution of new racemic phosphonic acid methyl (213) and dimethyl esters (214) via cinchonium salts (Scheme 56). The absolute configuration of the phosphonic ester products was also established. 

The advantages of PTC reactions are moderate reaction conditions, practically no formation of by-products, a simple work-up procedure (the organic product is exclusively found in the organic phase), and the use of inexpensive solvents without a need for anhydrous reaction conditions. PTC reactions have been widely adopted, including in industrial processes, for substitution, displacement, condensation, oxidation and reduction, as well as polymerization reactions. The application of chiral ammonium salts such as A-(9-anthracenylmethyl)cinchonium and -cinchonidinium salts as PT catalysts even allows enantioselective alkylation reactions with ee values up to 80-90% see reference [883] for a review. Crown ethers, cryptands, and polyethylene glycol (PEG) dialkyl ethers have also been used as PT catalysts, particularly for solid-liquid PTC reactions cf. Eqs. (5-127) to (5-130) in Section 5.5.4. 

Molecular complexation with cinchonium quaternary salts was later applied for the successful resolution of other, nonsymmetrical biaryl derivatives - 2-amino-2 -hydroxybinaphthyl 20 [41] and isoquinoline 21 [42]. 

---