Chiral counter-anion strategy

More recently Toste and coworkers reported the use of the chiral counter-anion strategy with a functionalized BNP (4.29), to carry out some asymmetric hydroxycyliza-tion reactions with gold (1) catalysts (see Chapter 3, Section 3.3.3) (Scheme 4.3). The authors showed that the chiral counter-anion strategy has a profound effect on the enantioselectivity and generates products in 90-99% enantiomeric excess. These results hold promise for a golden future in this field. ... 

Asymmetric hydoxycyclization reaction using the chiral counter-anion BNP (4.29) strategy. 

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... 

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