ChemInform Abstract: Catalytic Enantiodifferentiating Hydrogenation with Commercial Nickel Powders Chirally Modified by Tartaric Acid and Sodium Bromide.

Abstract

Optically active compounds are the key elements of natural systems such as living organisms. Therefore, the production of such compounds is of prime importance, especially in the fields of pharmaceuticals, agrochemicals, and so on. Furthermore, novel and emerging approaches to supramolecular chemistry, nanoscience, biomimetics, and sensing also require optically active molecules of high enantiopurity. Several methods, such as optical resolution and catalytic and enzymatic asymmetric syntheses, have been developed thus far to obtain enantiomeric compounds. Of these approaches, heterogeneous chiral catalysis is one of the most promising techniques for the large-scale production of enantiopure compounds, featuring facile preparation, simple separation, and easy recovery and reuse of the catalyst, as well as the timeand cost-saving, environmentally benign methods. Practically, there are two most effective heterogeneous catalytic systems for enantiodifferentiating hydrogenation: (1) cinchona alkaloid-modified metallic platinum and palladium catalysts and (2) tartaric acid (TA)-modified nickel catalysts. Both catalysts give high enantioselectivities for specific prochiral substrates. The platinum-based chiral catalysts hydrogenate activated ketones, such as a-ketoesters, ketopantolactones, pyrrolidinetriones, aketoacetals, a-ketoethers, a-diketones, and other related compounds, in 95–98% ee, whereas the modified palladium catalysts reduce a,b-unsaturated carboxylic acids and alkene derivatives in 90–94% ee. In contrast, the TA-modified nickel catalysts prepared in the presence of NaBr reduce b-ketoesters and 2-alkanones and give the corresponding alcohols in up to 98 and 85% ee, respectively. In the preparation of TA-modified nickel catalyst, activated metallic nickel powders, such as Raney nickel, reduced nickel (prepared by the reduction of nickel oxide), supported nickel, and activated commercial nickel powder, 34] are commonly used. The commercially available nickel powder is usually activated through treatment with hydrogen stream at an elevated temperature, which is followed by the chiral modification by immersing the activated nickel powder into an aqueous solution containing TA and NaBr. The hydrogen pretreatment is not necessary if the commercial nickel powder is used as a nickel base, because the modification solution (containing TA and NaBr) is adjusted to pH 3.2 and thus it can remove the oxidized material from the nickel surface. Thus, the TA-NaBr solution plays dual roles of cleaning and modifying the nickel surface to afford a smooth surface structure appropriate for the enantiodifferentiation by removing defects. This risk-free surface activation without using the hydrogen pretreatment is an important step toward the large-scale production and application of chirally modified nickel catalysts in industry. Herein, to establish the protocol for preparing highly efficient TA/NaBr-modified nickel catalysts without preactivation, The chirally modified nickel catalysts for the enantiodifferentiating hydrogenation of b-ketoesters are prepared conventionally by immersing hydrogen-activated metallic nickel into an aqueous solution of enantiopure tartaric acid, in which the preactivation of nickel is essential. Herein, we revealed that even commercially available nickel powders without any pretreatment can catalyze the enantiodifferentiating hydrogenation of b-ketoesters to give the corresponding b-hydroxyesters in quantitative yield and high enantioselectivity (up to 91%) under optimized conditions. The immediate use of commercially available nickel powders and the reproducible high chemical and optical yields not only expand the scope of heterogeneous asymmetric catalysis but also pave the way for the practical application and industrial use of chirally modified nickel catalysts.