Chiral ions phosphate anions

A new catalyst salt (20) that consists of an achiral ammonium ion and a chiral phosphate anion and which catalyses highly enantioselective transfer hydrogenations of ,/J-unsaturated aldehydes to the corresponding saturated derivatives has been developed. The underlying principle, namely asymmetric counteranion-directed catalysis, is claimed to be a new strategy for highly enantioselective synthesis.357... 

The same catalyst (93) was applied with success to the first organocatalytic multi component asymmetric Biginelli reaction providing medicinally relevant chiral 3,4-dihydropyrimidini-2-(l//)-ones (94) (Scheme 29)7 Chiral acid-catalyzed inverse electron-demanding aza Diels-Alder reaction of aldimines (95) with electron-rich alkenes (Scheme 30). A new catalyst salt (96) that consist of an achiral ammonium ion and chiral phosphate anion has been developed that catalyzes highly... 

Bringing together the concept of aminocatalysis and the activation mode of chiral phosphoric acids. List and co-workers introduced the concept of asymmetric counter anion directed catalysis (ACDC) and they applied this idea to the asymmetric reduction of enals 47 (Scheme 23) [ 135]. The catalytic species is formed by an achiral ammonium ion 60 and a chiral phosphate anion 59 derived from 3,3 -bis(2,4,6-triisopropylphenyl)-1,1 -binaphthyl-2,2 -diyl hydrogen phosphate 9 (TRIP). 

Scheme 3 Oxocarbenium and iminium ions bound to chiral phosphate anions...

The discrimination of the face of the chiral carbenium ion is determined by the hindrance of the flanking groups. In the case of chiral Bronsted acids, the chiral counter ion, formed after effective protonation or partial donation of the proton, surrounds the created cationic intermediate. One face of the intermediate is effectively covered by the chiral counter ion and the nucleophile reacts with the less covered face. However, in many SNl-type transformations mediated by Lewis acids, a transition state in which a couple of protons interact with the phosphates is often invoked. This double interaction seems cmcial and only a partial success in the use of nucleophiles was achieved. Nevertheless, a series of successful S l-type reactions were discovered and are highUghted in this section. The chiral ion pairs between phosphate anions and iminiums [64] or metal ions were developed [65]. This powerful strategy is called asymmetric counteranion-directed catalysis (ACDC) and it has been applied successfully to several innovative transformations. The generation of carbenium ion in situ from alcohol, with the use of acids able to form a tight chiral ion pair with the phosphate anion, can be used in S il-type... 

Asymmetric Counteranion-Directed Catalysis (ACDC) Instead of forming a chiral iminium ion that induces stereoinduction, the ACDC concept is based on the stereochemical communication between an achiral imine and a chiral phosphate anion. To incorporate both moieties into one substance, an organic salt consisting of a chiral phosphate anion and an achiral ammonium counterpart is used. ... 

A chiral phosphate anion combined with a silver cation has been demonstrated as a powerful ion pair catalyst (113) for the aza-Mannich addition of oxazolones (110) to Af-tosyl aldimines (111). A series of qua-ternaiy a,)S-diamino acid derivatives (112) has been obtained in high yields (up to 95%), and with excellent diastereo- (up to 25 1 dr) and enantioselectivities (up to 99% ee) (Scheme 38)." ... 

The Z)3-symmetric TRISPHAT anion 88, tris(tetrachlorobenzenediolato)phosphate, is chiral and configurationally stable. It can be resolved by association with chiral ammonium salts such as cinchonidine <1997AGE608, 2004JOC8521>. TRISPHAT 88 displays high selectivity for cinchonidine and does not associate with the related diasteromer cinchonine <1998TL4825>. The selective ion pairing behavior of TRISPHAT has been exploited in... 

Chiral anion phase-transfer catalysis has been extended to the asymmetric synthesis of allylic fluorides (Scheme 7.10) [20]. The key with this chemistry was judicious selection of the electrophilic fluorine source. Selectfluor displays poor solubility in nonpolar solvents such as cyclohexane however, combining this F source with a chiral phosphate salt generated afforded a soluble ion pair that promoted the chemistry. A wide range of... 

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