Synthesis and Characterization of Chiral Iridium Complexes Bearing Carbohydrate Functionalized Pyridincarboxamide Ligands and Their Application as Catalysts in the Asymmetric Transfer Hydrogenation of ?-Ketoacids in Water


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[Cp*IrLnCl] complexes [L1 = (methyl-beta-D-gluco-pyranosid-2-yl)picolinamide, 1; L2 = (methyl-3,4,6-tri-O-acetyl-beta- D-glucopyranosid-2-yl)picolinamide, 2; L3 = (2,3,4,6-tetra-O-acetyl-beta-D-glucopyranosid-1-yl)picolinamide, 3] have been synthe-sized and completely characterized in solution, by 1D-and 2D-NMR spectroscopy, and in the solid state, by X-ray single crystal diffractometry. Despite the chirality of the Ln-moiety and metal, a single diastereoisomer is observed for L1 (1) and L2 (2) having a (R)-iridium configuration: the pyranose moiety is oriented in a way to minimize the interactions of the axial protons, vicinal to the amide moiety, and Cp*, with the OMe-group pointing toward the Cp*-ligand and away from Ir-Cl. Such a diastereoisomer is also favored by the establishment of an O-H center dot center dot center dot Cl-Ir hydrogen bond (2.356 angstrom) and by the minimization of the steric repulsion between one acetyl moiety of L2 and Cp* and picolinamide ligands in 1 and 2, respectively. DFT calculations computed a stabilization by more than 5.9 and 3.1 kcal/mol of this diastereoisomer with respect to other possible ones. Two interconverting diastereoisomers with different chirality at iridium are instead observed in solution for complex 3 in which-CH2OAc [3a, 63%, (R)] and -OAc [3b, 37%, (S)] moieties, respectively, are oriented toward Cp* and N-arm of picolinamide ligands. Consistently, DFT calculations indicate that 3a and 3b have a comparable stability (OLE = 1.2 kcal/mol). Complexes 1-3 catalyze the asymmetric transfer hydrogenation of RC(O)C(O)OH to RCH(OH)C(O)OH [R = Ph (PGA), CH2Ph (PPA), CH2(4-OH)C6H4 (HPPA)], using both HCOOH and H3PO3 as hydrogen donor, in water at pH 7 (by phosphate buffer), with excellent chemoselectivity and efficiency (conversion >99%) and moderate to good enantioselectivity (30- 70% ee). Utilizing catalyst 3 instead of 2, bearing the pseudoenantiomeric L3 of L2 ligand, causes a reduction of the percentage of the major enantiomer (R) with PGA and an inversion of stereoselectivity from (R) to (S) with PPA and HPPA substrates.
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