Chiral bismuth

Aldehydes, ketones, and acetals react with allyltrimethylsilane in the presence of a catalytic amount of BiX3 (X = C1, Br, OTf) to give homoallyl alcohols or homoallyl alkyl ethers (Equation (52)).91-93 The BiX3-catalyzed allylation of aldehydes and sequential intramolecular etherification of the resulting homoallylic silyl ethers are involved in the stereoselective synthesis of polysubstituted tetrahydropyrans (Equation (53)).94,95 Similarly, these Lewis acids catalyze the cyanation of aldehydes and ketones with cyanotrimethylsilane. When a chiral bismuth(m) catalyst is used in the cyanation, cyanohydrines are obtained in up to 72% ee (Equation (54)). a-Aminonitriles are prepared directly from aldehydes, amines, and cyanotrimethysilane by the BiCl3-catalyzed Strecker-type reaction. 

The allylation and cyanation of aldehydes and ketones are mediated by BiCl3 and BiBr3 [174, 175]. When a chiral bismuth(lll) catalyst is used for cyanation, cyanohydrins are obtained in up to 72% ee (Scheme 14.85) [175]. The Bi(OTf)3-promoted intramolecular Sakurai cyclization of homoallylic alcohols is involved as a key step in the stereoselective synthesis of polysubstituted tetrahydropyrans (Scheme 14.86) [176]. In the presence of the BiCl3-xMl binary catalyst, allyltrimethylsilane [177] and silyl enolates [178] are acylated to give aUyl ketones and /l-dikelories, respectively. 

Ogawa, C. Azoulay, S. Kobayashi, S. (2005) Bismuth triflate-chiral bipyridine complex catalyzed asymmetric ring opening reactions of meso-epoxide in water., Heterocycles, 66 201-206. 

Abstract Several bismuth-catalyzed synthetic reactions, which proceed well in aqueous media, are discussed. Due to increasing demand of water as a solvent in organic synthesis, catalysts that can be used in aqueous media are becoming more and more important. Although bismuth Lewis acids are not very stable in water, it has been revealed that they can be stabilized by basic ligands. Chiral amine and related basic ligands combined with bismuth Lewis acids are particularly useful in asymmetric catalysis in aqueous media. On the other hand, bismuth hydroxide is stable and works as an efficient catalyst for carbon-carbon bond-forming reactions in water. 

Bismuth triflate has been reported by Dubac as an efficient catalyst for the Mukaiyama aldol reaction with silyl enol ethers [27] and was recently used with a chiral ligand, as reported by Kobayashi in an elegant hydroxymethylation reaction... 

Aldimines react with allyl bromide in the presence of metallic bismuth and tetrabutylammonium bromide in acetonitrile to give homoallyl amines (Equation (42)).77 When a chiral imine derived from benzaldehyde and (S)-valine methyl ester is used as the substrate, the allylation with allylic bismuth(m) species takes place smoothly by the assistance of a Lewis acid such as BF3-OEt2 and A1C13 to afford a chiral homoallylic amine with high diastereoselec-tivity (Equation (43)).78... 

Ferrocene analogs that possess a heteroatom in place of one carbon atom have been known for some time. The most common of these heteroferrocenes are the azaferrocenes and the phosphaferrocenes, though complexes having snlfur, boron, arsenic, antimony, bismuth, and nickel atoms are known. Review articles that are either comprehensive (in the case of phosphaferrocenes), or cover aspects of this chemistry (in the case of azaferrocenes), are available space restrictions for this review do not permit complete coverage of these areas. Instead, recent developments in the area of planar chiral heteroferrocenes, especially as it relates to asymmetric catalyst design, will be the primary focus here. 

Stereoselective Diels-Alder reactions have been reported in several cases. Enantioselective Diels-Alder reactions of l-phenoxycarbonyl-l,2-dihydropyridine with 1-alkylated acryloyl-pyrazolidin-3-ones using a chiral cationic palla-dium-phophinooxazolidine catalyst afforded chiral isoquinuclidines with excellent enantioselectivity <2005TL5677>. Bismuth(lll) chloride-mediated diasteroselective intramolecular [4-f2] cycloaddition reactions of A-allyl derivatives of pyrazole aldehydes led to fused sulfur-containing pyrazole heterocycles <2003SC3063>. A highly diastereoselective intramolecular hetero-Diels-Alder approach toward tetracyclic pyrazoles from 5-(3-methyl-2-butenylthio)-3-methyl-l-phenyl-4-pyrazolecarboxaldehyde has been reported <1997SL1155>. 

Recently, unsymmetrical bismuthonium salts bearing four different aryl ligands have been prepared, and the chirality at bismuth has been investigated by H-NMR [990M5668]. The activation energy of isomerization at the bismuth center is strongly dependent on the nucleophilicity of the counter anions as well as the polarity of the solvents employed. This is rationalized by a pseudorotation mechanism via pentacoordinate species at the transition state. 

The first example of a highly enantioselective bismuth-catalyzed aldol reaction was reported by Kobayashi and co-workers (196) in 2005 (Scheme 51) the Bi(OTf)3/chiral pyridine complex gave the aldol adducts of silyl enolates 149 and formaldehyde in moderate yields and good to excellent enantioselectivities (up to 95% ee). 

The group of Miyaura [78, 89] applied arylbismuth compounds [88] in these metal-catalyzed reactions. The application of this compound type in such met2il-catalyzed reactions is stiU rare. In fact, as has already been mentioned, Miyaura s group reported the use of chiral phosphane/dicationic palladium(II) complexes (based on (S,S)-Chiraphos and (S,S)-Dipamp) in the 1,4-addition of triaryl-bismuth reagents to cyclic and acyclic enones in aqueous methanol at low temperature (Scheme 5.32). Enantioselectivities of up to 95% ee were achieved for cyclic enones. The use of Cu(BF4)2 to help the reoxidation of Pd(0) to the catalyticaUy active Pd(II) species was an advantage, as well as the efficiency of the aryl transfer, allowing utilization of 2-2.8 of the three phenyl groups on the bismuth atom. Several years ago, Yamamoto, Nishikata, and Miyaura [68c] published a personal account of... 

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