Organocatalysts phase transfer catalysis

Asymmetric phase-transfer catalysis is a method that has for almost three decades proven its high utility. Although its typical application is for (non-natural) amino acid synthesis, over the years other types of applications have been reported. The unique capability of quaternary ammonium salts to form chiral ion pairs with anionic intermediates gives access to stereoselective transformations that are otherwise very difficult to conduct using metal catalysts or other organocatalysts. Thus, this catalytic principle has created its own very powerful niche within the field of asymmetric catalysis. As can be seen in Table 5 below, the privileged catalyst structures are mostly Cinchona alkaloid-based, whereas the highly potent Maruoka-type catalysts have so far not been applied routinely to complex natural product total synthesis. 

Building upon these concepts, this chapter firstly gives an insight into the modes of action of a selection of non-covalent chiral organocatalysts, employing chiral Brpnsted acid catalysis, chiral Brpnsted base catalysis, and chiral phase-transfer catalysis (PTC). Further sections of this chapter describe two separate case studies that aim to compare and contrast selected covalent and non-covalent strategies for achieving two distinct processes, acyl transfer reactions and asymmetric pericyclic processes. 

Toste and cmworkers reported an asymmetric fluorination of a-branched cyclohexanones (252) involving a eombination of chiral anion (254) phase-transfer catalysis to activate Selectfluor (253) and enamine activation cycle, using a protected amino acid (255) as organocatalyst (Scheme 67). ... 

I.3.I. Chiral Phase-Transfer Catalysis The exploration of modified cinchona alkaloid organocatalysts for asymmetric synthesis indicates that the quaternary ammonium salt derived from cinchona alkaloids is one of the best catalysts in the asymmetric Michael reaction. In 2000, Perrard and co-workers used A/-meth-ylanthracenylquininium (or quinidinium) chloride salt (Q-a or QD-a) for catalyzing the asymmetric Michael addition of dimethyl malonate to 2-pentyl-2-cyclo-penten-l-one (Scheme 9.6). ... 

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