Abstract
The chiral structure of antibiotic vancomycin (Van) was exploited as an innovative coordination sphere for the preparation of an IrCp* based hybrid catalysts. We found that Van is able to coordinate iridium (Ir(III)) and the complexation was demonstrated by several analytical techniques such as MALDI-TOF, UV, Circular dichroism (CD), Raman IR, and NMR. The hybrid system so obtained was employed in the Asymmetric Transfer Hydrogenation (ATH) of cyclic imines allowing to obtain a valuable 61% e.e. (R) in the asymmetric reduction of quinaldine 2. The catalytic system exhibited a saturation kinetics with a calculated efficiency of Kcat/KM = 0.688 h−1mM−1.
Highlights
The chiral structure of antibiotic vancomycin (Van) was exploited as an innovative coordination sphere for the preparation of an IrCp* based hybrid catalysts
Vancomycin (Van) is a front-line glycopeptide antibiotic produced by Streptomyces orientalis and active against Gram positive infections
Its copper binding site involves N-terminal imino nitrogen, two consecutive nitrogen atoms in the peptide chain, and one oxygen atom from asparagine amide group. This ability to form a stable complex with transition metals, the atropoisomerism induced by the restricted rotation around the aryl-aryl bonds, and the macrocyclic chiral basket-like structure all make Van an interesting, yet unexplored, ligand for asymmetric catalysis
Summary
Reaction conditions: substrate concentration 16 mM, 4 mol% Van, 1 mol% [IrCp*Cl2 ]2 , buffer, HCOONa 3 M, 18 h and at 25 ◦ C. (a) substrate 16 mM, 8 mol% Van, 1 mol% [IrCp*Cl2 ]2 , buffer, HCOONa 3 M, 18 h and at 25 ◦ C. An inversion of configuration was observed in the ATH of 3-methylbenzo[d]isothiazole 1,1-dioxide 3 going from basic pH (42% e.e. for (R) enantiomer in phosphate buffer 0.1 M pH 8, Table 1, Entry 1, Sub 3) to acid one (30% e.e. for (S) enantiomer in MES buffer 1.2 M pH 5, Table 1, Entry 6, Sub 3) along with a significant difference in the reaction rates (from 92% conversion to 20%) This behavior could be explained in terms of both different electronic and steric properties of the imine substrates depending on the buffer solution, the pH and the ionic strength [47,48,49], in interaction with the different stabilized conformation of the [Ir(Cp*)(Van)Cl] complex [32,50]. The reproducibility of the in situ complexation mode was confirmed by the results obtained in independent experiments giving us a reasonable prospect for the possibility to improve the versatility of this system and the possibility to apply it to different types of enantioselective reactions, eventually changing the metal center to rhodium or ruthenium
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