Cobalt (III) Werner Complexes As Asymmetric Catalysts: A Bifunctional Approach
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Date
2021-06-23
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Abstract
Alfred Werner’s seminal work at the close of the 19th century was a stereochemical primer in the field of coordination chemistry. One limitation of these complexes, solubility, was modified more than 100 years later to usher in the neocomplexes of Gladysz, et al. This chiral hydrogen bond-donor catalyst has demonstrated in many embodiments to be an effective and versatile agent to carry out asymmetric Michael-type reactions.
When coordinated with three diphenylethylene diamine (dpen) ligands, this homoleptic cobalt cation exists in two easily separable diastereomeric forms (Λ and Δ), and is most commonly paired with two chloride anions and a lipophilic, non-coordinating BArf– anion (B(3,5-C6H3(CF3)2)4–). Chloride anions can be easily metathesized in biphasic conditions with chiral anions. Though these anions give only modest benefit, the significance of the anion is observed in the data. In some cases, synergistic “matched” ion pairs compared to their diastereomeric “mismatched” pair give differences in enantioselectivities of greater than 20%.
The role of the anion was later exploited through the intentional incorporation of a Brønsted base. These anions give significant improvement in rates relative to external bases of equal pKa and show comparable enantioselectivity to stronger external bases. These catalysts have also been shown to work effectively in solvent-free conditions with only minor detriment to stereoselectivity of the catalyzed reaction, but with reaction times rapidly accelerated.
Finally, libraries of novel ligands for bifunctional catalysis have been synthesized, where the synthesis of a small family of chiral secondary triamines is disclosed herein. These advances demonstrate a new bifunctional approach to catalysis with cobalt(III) Werner complexes.
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cobalt, 1,2-diamines, Werner complexes, hydrogen bonding, metal stereocenter, enantioselective catalysis