Asymmetric catalysis has received considerable attention over the past few decades, and its contribution toward organic synthesis has become increasingly significant. The recent exceptional advances in this area attest to a range of conceptual breakthroughs in chemical sciences in general, and to the practical benefits of organic synthesis, not only in laboratories but also in industry. Asymmetric catalysis has tremendous applications in many fields such as pharmaceuticals, agrochemicals, pheromones, fungicides, and animal health products. Asymmetric catalysis is classified by catalyst type into organic small molecule catalysis, coordination complex catalysis and bio enzyme catalysis. In recent decades, chiral coordination complex catalysis has attracted much attention. In 1966, Ryoji Noyori discovered the first example of asymmetric catalysis using a structurally well-defined chiral transition metal complex. Shortly thereafter, chiral phosphorus ligands were also introduced into the asymmetric hydrogenation reaction. The catalytic activity of a chiral coordination complex originates from the metal and the asymmetry of the metal-catalyzed process is induced by the organic ligands attached to that metal. These organic ancillaries, which may be viewed as a chiral scaffolding, control the binding of reactants and their subsequent reaction paths through steric and electronic interactions.
Application
Asymmetric catalytic synthesis generally refers to the use of rationally designed chiral coordination complexes (catalyst amount) as chiral templates to control the enantiomeric aspects of the reactants and selectively convert many pre-chiral substrates into products of a specific configuration, achieving chiral amplification and chiral multiplication. The main applications of coordination complexes in asymmetric catalysis are listed below:
- Acts as a catalyst for asymmetric hydrogenation reactions
Most chiral cyclic structures such as chiral cyclic amines, ethers, alkanes, etc., are commonly found in numerous bioactive natural products and important pharmaceuticals. Among the various synthetic methods toward the preparation of such valuable molecules, asymmetric hydrogenation of cyclic substrates bearing unsaturated bonds is one of the most powerful, economical, and environmentally benign procedures. An increasing number of complexes are showing different catalytic activities. For example, Pizzano and co-workers [1] performed an asymmetric reduction of N-aryl imines derived from acetophenones by using Ru complexes bearing both a pybox (2,6-bis(oxazoline)pyridine) and a monodentate phosphite ligand has been described. The catalysts show good activity with a diverse range of substrates. Zhou and co-workers [2] developed chiral iridium catalysts, which showed excellent enantioselectivities for the hydrogenation of simple ketones.
- Acts as a catalyst for asymmetric Michael addition reactions
Michael addition (otherwise known as the Michael reaction) is defined as the addition of a stabilized carbon nucleophile to an α, β-unsaturated compound bearing an electron-withdrawing group (e.g., a nitro or carbonyl group), and provides an efficient method for the construction of C-C bonds in organic synthesis. Notably, the asymmetric Michael addition provides a rapid access to versatile important chiral building blocks and intermediates for the synthesis of bioactive agrochemicals and pharmaceutical compounds. In recent years, chiral transition metal complexes with high activity have been widely used in asymmetric Michael addition reactions. For example, Kang and co-workers [3] developed a highly enantioselective Michael addition of pyrazolones with α, β-unsaturated 2-acyl imidazoles. In the presence of 1 mol % of chiral-at-metal Rh (III) complex, the corresponding adducts were obtained in good yields (85%-96%) with excellent enantioselectivities (up to >99%). Zhang and co-workers [4] also developed an asymmetric Michael addition of β, γ-unsaturated α-keto esters with diphenylphosphine using the P-stereogenic PCP pincer-Pd complex as a chiral catalyst. The corresponding hydrophosphination products were obtained in good yields (up to 94%) and with moderate to good enantioselectivities (up to 93% ee) under the optimum reaction conditions.
- Acts as a catalyst for asymmetric allylic alkylation reactions
The catalytic asymmetric allylic alkylation reaction is a well-established and important reaction that has been widely used in fundamental transformations and a variety of chemical bonds formations. The first catalyst that could be used to catalyze allylic alkylation reactions was palladium complexes. It can successfully catalyze various nucleophilic reactions of carbon, nitrogen, oxygen, and sulfur with high yields and good stereoselectivity. Meanwhile In addition to palladium complexes, non-precious transition metal complexes of the third period have likewise attracted extensive interest from scientists. For example, Baeza and co-workers [5] disclosed the application of the copper (II) triflate-tert-butyl-bisoxazoline complex as catalyst for the asymmetric allylic alkylation of β-keto esters employing allylic alcohols as alkylating agents. By using this new protocol different keto esters were successfully alkylated, generating two consecutive all carbon stereogenic centers.
- Acts as a catalyst for other reactions
Coordination complexes were also reported as catalysts in other asymmetric reactions such as reductions, hydrosilylations, hydroaminations, polymerizations, Mukaiyama aldol reactions and many others.
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References
- Pizzano, A.; et al. Highly enantioselective hydrogenation of N-Aryl imines derived from acetophenones by using Ru-Pybox complexes under hydrogenation or transfer hydrogenation conditions in isopropanol. Chem. Eur. J. 2015, 21: 549-553.
- Zhou, Q. L.; et al. An additional coordination group leads to extremely efficient chiral iridium catalysts for asymmetric hydrogenation of ketones. Angew. Chem. Int. Ed. 2011, 50: 7329-7332.
- Kang, Q.; et al. Chiral-at-metal Rh (III) complex-catalyzed michael addition of pyrazolones with α, β-unsaturated 2‑acyl imidazoles. Org. Lett. 2018, 20: 1312-1315.
- Zhang, W. B.; et al. Asymmetric Michael addition of diphenylphosphine to β, γ-unsaturated α-keto esters catalyzed by a P-stereogenic pincer-Pd complex. Tetrahedron. 2015, 71: 6832-6839.
- Baeza, A.; et al. Copper-catalyzed asymmetric allylic alkylation of β-keto esters with allylic alcohols. Adv. Synth. Catal. 2017, 359: 1-8.
