Asymmetric Cyanosilylation Of Aldehydes Research Articles

Asymmetric Cyanosilylation of Aldehydes by a Lewis Acid/Base Synergistic Catalyst of Chiral Metal Clusters.

Metal clusters with well-defined crystal structures are extremely useful for studying the synergistic catalytic effects and associated catalytic mechanisms. In this study, two pairs of chiral lanthanide-transition metal clusters (R)/(S)-Co3Ln2 (Ln = Tb or Dy) were synthesized using Schiff-base ligands [(R)- or (S)-H3L] with multiple Lewis base sites (O sites). The as-prepared (R)/(S)-Co3Ln2 chiral metal clusters exhibited good catalytic functionality in the asymmetric synthesis of chiral cyanohydrins, with high conversions of up to 99% and medium-to-high enantiomeric excess values of up to 78%. The catalysis process followed a mechanism in which the bifunctional metal clusters of (R)/(S)-Co3Ln2, containing Lewis acid sites and Lewis base sites, simultaneously activated the aldehydes and trimethylsilyl cyanide, respectively. Consequently, synergistic catalysis was realized. The enantioselectivity of the different aldehydes and stereochemical configuration of the resulting products are attributed to the formation of a steric chiral pocket via the external chiral ligands on the clusters. In addition, heterogeneous asymmetric cyanosilylation using (R)/(S)-Co3Ln2 chiral metal clusters achieved high chemoselectivity and regioselectivity under mild conditions.

Development of Asymmetric Reactions Catalyzed by Ruthenium Complexes with Two Kinds of Ligands

Abstract Two types of chiral Ru(II) complexes, each with two kinds of ligands, have been designed and utilized as catalysts for several asymmetric reactions under appropriate conditions. The first type, the diphosphine/diamine–Ru(II)-type complexes, were found to catalyze the hydrogenation of a variety of simple and functionalized ketones as well as imino compounds with high activity and enantioselectivity. The double asymmetric hydrogenation of α,β-unsaturated ketones into the chiral saturated alcohols was achieved by using the dual catalyst system, which reversibly forms two catalytic species. The asymmetric isomerization of primary allylic alcohols into the optically active aldehydes with almost perfect enantioselectivity was realized with this type of catalyst. The second type, the amino acid/diphosphine–Ru(II)-type complexes combined with Li compounds, exhibited excellent catalyst performance in the asymmetric cyanosilylation of aldehydes as well as simple and functionalized ketones. The isolated Ru·Li bimetallic complexes were suitable for the asymmetric hydrocyanation of aldehydes. This combined system was applicable to the asymmetric conjugate hydrocyanation of α,β-unsaturated ketones and carboxylic acid derivatives, and the Strecker-type reaction of the π-isoelectronic N-alkoxycarbonyl aldimines. This account describes the concept underlying the design of these catalysts, and the catalyst performance in the asymmetric reactions.

Chiral Metal–Organic Framework as a Platform for Cooperative Catalysis in Asymmetric Cyanosilylation of Aldehydes

In this work, we demonstrate cooperative asymmetric catalysis by a metal–organic framework (MOF) as exemplified in the context of catalyzing cyanation of aldehydes with a VO(salen)-MOF, which after oxidation affords remarkably increased stereoselectivity (up to >99% ee) compared to the homogeneous VO(salen) counterpart as a result of the pairs of VO(salen) units in close proximity within its open channels. The cooperative asymmetric catalysis has been evidenced by the significantly decreased stereoselectivity and activity when one VO(salen) in such pairs of VO(salen) units is replaced with one Cu(salen), which results in blocking the VO–VO synergistic pathway while prompting unimolecular activation of substrates. The heterogeneous nature of VO(salen)-MOF has been verified by the fact that it can be easily recycled and reused without significant loss of catalytic activity and enantioselectivity, and its practical utility as asymmetric cyanation catalysist has been illustrated in the gram-scale synthesis of...

Asymmetric Cyanosilylation of Aldehydes by a Helical Polyaminoacid

Asymmetric Cyanosilylation of Aldehydes by a Helical Polyaminoacid

Asymmetric Cyanation with the Chiral Ru-Li Combined Catalysts

The combined systems of phenylglycinate/­BINAP/Ru(II) complex and Li compounds have been found to act as highly reactive and enantioselective catalysts for cyanosilylation and hydrocyanation of aldehydes, α-keto esters, α,β-unsaturated ketones, and N-protected aldimines. In this account, we describe the concept of catalyst design, the catalytic efficiency of the novel combined systems in the asymmetric cyanation, and the proposed reaction pathway based on the spectral analysis data. 1 Introduction 2 Cyanosilylation of Aldehydes and Ketones Catalyzed by LiCl 3 Asymmetric Cyanosilylation of Aldehydes and α-Keto ­Esters 3.1 Design of Chiral Ru–Li Combined Catalyst Systems 3.2 Cyanosilylation of Aldehydes 3.3 Cyanosilylation of α-Keto Esters 4 Asymmetric Hydrocyanation of Aldehydes, α,β-Unsaturated Ketones, and Aldimines 4.1 Hydrocyanation of Aldehydes 4.2 Conjugate Cyanation of α,β-Unsaturated Ketones 4.3 Strecker-Type Reaction 5 Mechanistic Considerations for Cyanosilylation of Aldehydes and Ketones 5.1 Achiral Reaction Catalyzed by LiCl 5.2 Asymmetric Reaction with the Chiral Ru–Li Combined Catalyst 6 Conclusion

Fine Modification of Salen Ligands – Effects on the Salen–Ti‐Catalyzed Asymmetric Cyanosilylation of Aldehydes

AbstractNew bifunctional N‐oxide salen–TiIV complexes and a pyrrolidine salen–TiIV complex in combination with achiral N‐oxide were developed and applied to the asymmetric addition of trimethylsilyl cyanide to aldehydes. Notably, both enantiomers of trimethylsilyl ethers of cyanohydrins could be easily prepared by modifying the catalysts employed in this reaction, which are based on the same chiral diamine collar derived from L‐tartaric acid. The products were obtained generally with moderate to good enantiomeric excesses and excellent yields by using relatively low catalyst loadings and only 1.05 equivalents of trimethylsilylcyanide (TMSCN).

ChemInform Abstract: Design of a New Bifunctional Asymmetric Catalyst from Carbohydrates: Application to Catalytic Asymmetric Cyanosilylation of Aldehydes and Acetophenone.

Abstract A new active Lewis acid–Lewis base bifunctional asymmetric catalyst 7 was developed using carbohydrate as a scaffold and this catalyst was applied to the asymmetric cyanosilylation of aldehydes and acetophenone. The β-Ph group at the 6-position of 7 was found to be important for a high asymmetric induction (up to 80% ee) by bringing the Al and the phosphine oxide at optimum positions for a dual activation of carbonyl compounds and TMSCN.

Asymmetric Cyanosilylation of Aldehydes by Chiral Ti‐TADDOL Complex.

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Asymmetric Cyanosilylation of Aldehydes by Chiral Ti-TADDOL Complex

A variety of chiral cyanohydrins were readily prepared from aldehydes with TMSCN under the influence of chiral titanium complex formed in situ from <TEX>$Ti(O^iPr)_4$</TEX>/TADDOL(I) in the presence of <TEX>$Ph_3PO$</TEX> as additive. The double activation method produces trimethylsilylethers in excellent yield (95%) with moderate enantiomeric excess.