Family of Well-Defined Chiral-at-Cobalt(III) Complexes as Metal-Templated Hydrogen-Bond-Donor Catalysts: Effect of Chirality at the Metal Center on the Stereochemical Outcome of the Reaction

Abstract

A family of well-defined Λand Δ-diastereomeric octahedral cationic chiral-at-cobalt complexes were obtained by a simple two-step reaction of (R,R)-1,2-diaminocyclohexane, (R,R)-1,2-diphenylethylenediamine, or (S)-2-(aminomethyl)pyrrolidine and substituted salicylaldehydes with a cobalt(III) salt. It was observed for the first time that the use of an excess of cobalt(III) salt provides both the enantiopure Λ and Δ forms of the corresponding cobalt(III) complexes 1 and 2 in a ratio of diastereomers ranging from 1:1.6 to >20:1 (Λ/Δ) and in 31−95% combined yields. The obtained complexes were robust, air-and bench-stable, soluble in most of organic solvents, and insoluble in water. Through variation of the substituents in the phenyl ring of the salicylaldehyde moiety, it was shown that both steric and electronic effects of substituents have a significant impact on the formation of Λ and Δ isomers. Next, the efficacies of the enantiopure metal-templated complexes 1−3 were investigated in three benchmark asymmetric reactions in order to compare their catalytic activity. The chiral cobalt(III) complexes 1−3 were tested as enantioselective hydrogen-bond-donor catalysts in such important reactions as the Michael addition of the O'Donnell substrate to methyl acrylate, epoxidation of chalcone, and trimethylsilylcyanation of benzaldehyde. It was clearly demonstrated that the chirality at the cobalt center has an impact on the stereochemical outcome of the reactions. In particular, the Λ(R,R)-1 and Δ(R,R)-1 complexes acted as "pseudoenantiomeric" catalysts in the epoxidation and trimethylsilylcyanoation reactions, providing both enantiomers of the products with up to 57% enantiomeric excess.Article pubs.acs.org/IC

Post-Modification of Octahedral Chiral-at-Metal Cobalt(III) Complexes by Suzuki–Miyaura Cross-Coupling and Evaluation of Their Catalytic Activity

“…Recently, we introduced the well-defined stereochemically inert octahedral chiral-at-metal Co(III) complexes based on Schiff bases of the enantiopure 1,2-cyclohexanediamine (CHD) or 1,2-diphenylethylenediamine (dPhED) and salicylaldehydes as efficient metal-templated hydrogen bond donor (HBD) catalysts for various asymmetric transformations . On the other hand, the Gladysz and Meggers groups independently have developed chiral-at-metal cobalt(III)- and iridium(III)-templated HBDs, which have efficiently catalyzed a broad range of asymmetric reactions. , In these complexes a metal center mainly serves as an activator of the coordinated ligands (NH 2 groups) increasing their HBD ability, as well as a source of chirality (metal-centered chirality). , The configurations are designated according to the mutual arrangement of tridentate ligands around the cobalt ion: Λ (a left-handed propeller) and Δ (a right-handed propeller) configurations (see Figure ).…”

Rhodium(III) Werner Complexes with 1,2‐Diphenylethylenediamine Ligands: Syntheses, Structures, and Applications as Chiral Hydrogen Bond Donor Catalysts and Agents for Enantiomer Purity Determinations

“…Such data allow the dominance of C 3 face NH/chloride interactions, as well as other trends, to be systematically or less subjectively expressed. Of course, hydrogen bonding has been noted in a variety of other cobalt(III) adducts of nitrogen donor ligands, [4g,27] but is often only treated in passing. Recently, Larionov, who has developed additional families of cobalt(III) hydrogen bond donor catalysts, [4] has initiated more detailed analyses [4g] …”

Mechanochemical Synthesis of Fluorinated Imines

“…The high reactivity of imines results from the presence of the multifunctional C = N bond in their structure, which is able to undergo a wide spectrum of chemical transformations, including reduction, condensation, cyclisation, cycloaddition, nucleophilic addition as well as multicomponent reactions, leading to the formation of various biologically and chemically relevant products. As ligands readily complexing the metals of the d and f blocks, imines are also valuable reagents in coordination chemistry [ 1 , 2 , 3 , 4 , 5 ]. The diversity of compounds containing the azomethine unit shows that the synthetic potential of imines both as substrates and intermediates is indeed great.…”