Privileged chiral catalysts

While many of the observed events of the MBH reaction could be included in this scheme, the mechanism failed in some critical cases [47]. First, the mechanism did not provide any clue as to why stereocontrol is so difficult in MBH reactions. Privileged nucleophilic chiral catalysts [48], which in the past have usually allowed good results in related asymmetric transformations, afforded only modest asymmetric induction. This fact was surprising, and pointed to lack of understanding of the basic factors governing the selectivity of the reaction. Other obser-... 

Yoon TP, Jacobsen EN (2003) Privileged chiral catalysts. Science 299 1691-1693... 

Many axially chiral compounds are Privileged Chiral Ligands . It means that it is highly probable that their application will lead to high enantioselectivity and/or activity of the resulting chiral catalysts. It is no wonder that many efforts have been made in order to attach axially chiral substiments, for example easily available... 

The advent of chiral Brpnsted base catalysis began with the recognition that the Cinchona alkaloids serve as excellent catalysts [7-12] and privileged structures... 

Figure 6.3 Stereoselective, chiral thiourea derivatives of achiral benchmark thiourea organocatalyst N,N -bis [3,5-(trifluoromethyl)phenyl]thiourea 9 stereoselective hydrogen-bonding thiourea organocatalysts incorporating the privileged 3,5-bis(trifluoromethylphenyl)thiourea moiety. The (thio)urea catalyst structure is the leitmotif for the chapter organization.

In 2003, Takemoto and co-workers introduced the first tertiary amrne-function-ahzed thiourea catalyst [129]. This new type of stereoselective thiourea catalyst incorporating both (R,R)-l,2-diaminocyclohexane as the chiral scaffold and the privileged 3,5-bis(trifluoromethyl)phenyl thiourea motif for strong hydrogen-bonding substrate binding, marked the introduction of the concept of bifunctional-... 

One of the most significant discoveries in asymmetric catalysis in the last 15 years was the importance of chiral metal salen catalysts. Chiral salen complexes exhibit high enantioselectivities in several mechanistically distinct reactions. As a result, they have been classified as privileged structures . ... 

The enantioselective hydrogenation of olefins is the best-studied reaction with the most industrial applications [6, 11 a, 12 a]. Over the most recent decades, a few privileged substitution patterns have evolved that almost guarantee high ee values (olefins 17, 18) and the state-of-the-art is summarized in Table 2. With few exceptions, Rh and Ru complexes of a limited number of chiral diphosphine families are the preferred catalysts but, in any case, the optimal complex (metal, ligand, anion, etc.) has to be determined for each substrate. 

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. 

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