Privileged catalysts

Cinchona alkaloids are readily available natural chiral compounds and have a long history to be utilized as organocatalysts in asymmetric catalysis [3, 4]. They are multifunctional, tunable, and more importantly, they could promote a diversity of reactions through different catalytic mechanisms, which make them privileged catalysts in organocatalysis. In this chapter, the applications of cinchona alkaloids and their derivatives for asymmetric cydoaddition reactions after 2000, especially for the construction of a variety of five- and six-membered cyclic compounds, are discussed. 

The first enantioselective intermolecular C—H bond insertion which could be of practical synthetic application was reported in 1997 by Davies and Hansen (Scheme 1.2). In the presence of a variety of relatively unreactive cycloalkane solvents 2 (compared with C—H bonds at the allylic and ben lic positions, as well as C—H bonds a to a heteroatom), the Rh complex Rh2(S-DOSP)4, a privileged catalyst derived from L-proline, was found capable to catalyze the decomposition of aryldiazoacetates 1, inducing the functionalization of cycloalkanes 3 (Scheme 1.2, eqn (1)). Circumventing chemoselectivity and... 

Another class of reaction for which chiral tertiary amines are privileged catalysts is the Morita-Baylis-Hillman type (477, 478). One of the first applications of Cinchona alkaloids to mediate an asymmetric Morita-Baylis-Hillman reaction in a natural product synthesis was reported by Hatakeyama et al. in 2001 (479). Using a stoichiometric amount of (3-isocupreidine (568), a stereoselective addition of hexafluoroisopropyl acrylate (569) to aldehyde 570 could be carried out in good yield and with excellent selectivity (99% ee) (Scheme 119). The chiral p-hydroxy ester 571 was converted further into the epoxide 572, a known intermediate in the synthesis of epopromycin B (573). Epopromycin B (573) is a plant cell wall... 

Li H, Chen YG, Deng L (2011) Cinchona Alkaloids. In Zhou QL (ed) Privileged Catalysts and Ligands in Asymmetric Catalysis. Wiley-VCH, Weinheim, p 361... 

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. 

Chiral base catalysis is one of the most versatile and broadly applicable types of catalysis. In particular, the potential of tertiary amines to act both as a base and as a nucleophilic catalyst makes chiral tertiary amines like Cinchona alkaloids a privileged catalyst structure in modem synthesis chemistry. In addition, the field of achiral phosphine and carbene catalysis has proven its potential in numerous applications in the past and it is probably only a matter of time until chiral phosphines and carbenes will also be used routinely for other presently demanding natural product total synthesis (Table 7). 

Group 10 Nickel, Palladium, and Platinum. In homogeneous catalysis, group 10 metals have become privileged catalysts in many organic... 

Aims Design and utilize "privileged" catalysts that show multiple modes of reactivity... 

The catalytically active metal species must have a vacant coordination site, i.e. NVE = 16 at most or even 14, in order to allow substrate molecules to coordinate. Sometimes, weak ligands or solvent ligands can be present and are easily displaced by substrate molecules. Bulky phosphines such as triphenylphosphine are easily dissociatively displaced and thus constitute a reservoir of vacant coordination sites that can be filled or emptied at will. Noble metals (2" and 3 lines of transition metals of groups 8, 9 and 10) easily forming 16-electron species are privileged catalysts. 

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