Counteranions chiral

Scheme 11.24 Chiral counteranion effect on the gold catalyzed hydroamination/cyclization of aminoallenes [118],...

Scheme 5.19 Chiral counteranion strategy for Pd-catalyzed asymmetric allylic C—H bond functionalization reported by Gong.

Besides the metal complexes based on Ni and Pd that were previously proved suitable for the asymmetric hydrovinylation reactions, Ru and Co complexes have also been employed in this type of transformation. The one sole example of Ru-catalyzed enantioselective hydrovinylation was reported by the List group in 2012. In their initial studies, they envisioned that Ni complexes with chiral counteranions might enable the desired reaction. However, it was found that these species either were unreactive or did not furnish enantioselective control in the hydrovinylation reactions. In this context, Jiang and List focused their attention on Ru catalysis (Table 9.6). Systematic evaluation on the effects of Ru complexes ligated by different phosphine ligands as well as various chiral phosphate anions introduced by the corresponding silver salts demonstrated that the combination of Ru complex Ru-3 and additive Ag-1 was the optimal catalyst, leading to acceptable results (entry 8). 

In 2008, List s group developed an epoxidation using asymmetric counteranion-directed catalysis (ACDC) [177]. In this work, the epoxidation of 1,2-disubstituted enals (33) and (3,(3-disubstituted, a, 3-unsaturated aldehydes (115) was explored. Instead of using a chiral amine (e.g. Jprgensen-Hayashi s catalyst), an achiral amine and a chiral counteranion (a phosphoric acid derived from BINOL), were employed. 

Whereas the [3-carboxyl sulfides were isolated in modest ee, the selenides were synthesized with up to 84% ee, which indicated that the chiral counteranion of catalyst 35 served as a more effective element of stereocontrol in the nucleophilic opening of seleniranium rather than thiiranium ions. 

Mukherjee S, List B (2007) Chiral Counteranions in Asymmetric Transition-Metal Catalysis Highly Enantioselective PdlBr nsted Acid-Catalyzed Direct a-Allylation of Aldehydes. J... 

Li CQ, Wang C, Villa-Marcos B, Xiao JL (2008) Chiral Counteranion-Aided Asymmetric Hydrogenation of Acyclic Imines. J Am Chem Soc 130 14450... 

However, 0,0-acetals present a particular challenge, as their synthesis does not involve an iminium ion. Controlling the enantioselective addition to an oxocarbenium ion, the commonly invoked intermediate in the synthesis of acetals, is an important issue in asymmetric Brpnsted acid catalysis (Scheme 3). In contrast to the reactions involving the related iminium ion that features a strong hydrogen bond with its chiral counteranion, there are only scarce examples of asymmetric additions of nucleophiles to oxocarbenium ions [13-15]. 

Scheme 8 Suggested indiscriminate binding of an oxocarbenium ion with its chiral counteranion as compared to a well-defined iminium cation-anion pair...

Lan, Y.-B., Zhao, H., Liu, Z.-M., Liu, G.-G., Tao, J.-C., Wang, X.-W. (2011). Chiral counteranion synergistic organocatalysis under high temperature efficient construction of optically pure spiro[cyclohexanone-oxindole] backbone. Organic Letters, 13,4866-4869. 

Scheme 26.12 Chiral counteranions in organocatalytic SNl-type reactions.

Amino catalysts are one of the most dominant organocatalysts in the activation and transformation of aldehydes or ketones through enamine or iminium intermediates. Initially, the presence of an acid co-catalyst is crucial for the catalytic activation. Later it was reaUzed that the introduction of a chiral counteranion to the catalytic system enables the reactions proceeding through cationic intermediates to be conducted in a highly enantioselective manner, and thus stereochemical control could be effectively induced by the chiral counteranion. This strategy has been defined as asymmetric counterion-directed catalysis (ACDC) [25]. 

In 2007, Melchiorre and coworkers [27] reported a dual catalyst system based on the combination of chiral primary amine and N-protected amino acid. The combinational chiral counteranion led to the first example of highly enantioselective Friedel-Crafts alkylation of indoles 80 with simple a,P-unsaturated ketones 79 (Scheme 43.16). The N-protected amino acid had a beneficial effect on the... 

Li C, Wang C, Villa-Marcos B, Xiao J. Chiral counteranion-aided asymmetric hydrogenation of acyclic imines. J. Am. Chem. Soc. 2008 130(44) 14450-14451. 

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