Asymmetric Synthesis of 3‐Allyloxindoles and 3‐Allenyloxindoles by Scandium(III)‐Catalyzed Claisen Rearrangement Reactions

Scandium‐catalyzed asymmetric Claisen rearrangement reactions of 2‐allyloxyindoles and 2‐propargyloxyindoles provide a novel approach to diverse 3‐allyloxindoles and 3‐allenyloxindoles in excellent yields (up to 99%) and enantioselectivity (up to 99% ee ) under mild reaction conditions. The scandium catalyst was derived from Sc(OTf )₃ and Pybox ligand.     

Mononuclear ruthenium(II) complexes bearing the (S,S)-Pr-pybox ligand

The complexes trans-[RuCl₂(L){(S,S)-ⁱPr-pybox}] ((S,S)-ⁱPr-pybox = 2,6-bis[4′-(S)-isopropyloxazolin-2′-yl]pyridine, L = PMe₃ (1), P(OMe)₃ (2), PPh₂(CH₂CHCH₂) (3), CNBn (5), CNCy (6) and MeCN (7)) have been synthesized by substitution of ethylene on the precursor trans-[RuCl₂(η²-C₂H₄){(S,S)-ⁱPr-pybox}]. This complex also reacts with cyclooctadiene (cod) or norbornadiene (nbd) and NaPF₆, in refluxing methanol, giving the coordination compounds [RuCl(η⁴-cod){(S,S)-ⁱPr-pybox}][PF₆] (8) and [RuCl(η⁴-nbd){(S,S)-ⁱPr-pybox}][PF₆] (9). The structures of complexes [RuCl(CO)(PPh₃)(H-pybox)][BF₄] (H-pybox = 2,6-bis(dihydrooxazolin-2′-yl)pyridine) (4), 6 and 8, have been resolved by X-ray diffraction methods. The catalytic activity of the new complexes in transfer hydrogenation of acetophenone has also been examined.     

Enantioselective synthesis of alpha-aminopropargylphosphonates

α-Aminopropargylphosphonates have been synthesized for the first time in good yields and enantiomeric excesses (up to 81% ee) by using a copper(I)-pybox complex as the catalyst.     

Asymmetric catalysis in a micro reactor—Ce, Yb and Lu catalysed enantioselective addition of trimethylsilyl cyanide to benzaldehyde

A T-shaped micro reactor was used for the optimisation of reaction conditions for the enantioselective silylcyanation of benzaldehyde catalysed by lanthanide-pybox complexes. Compared to a conventional batch procedure, higher conversion was observed within shorter reaction time. The micro reactor process involving Lu(III) afforded essentially the same enantioselectivity as the batch process (73 vs 76% ee), whereas the enantioselectivity was lower in the micro reactor for catalysts containing Yb(III) (53 compared to 72%). Ce(III) provided very low selectivity in both types of processes (1 and 11% ee, respectively). A study of the effect of additives showed that the enantioselectivity in the Yb catalysed reaction performed in the micro reactor could be increased to 66%, whereas only a minor improvement, to 78% ee, was observed in the reaction with Lu.     

Construction of optical active metallo-supramolecular polymers from enantiopure bis-pybox ligands

Two types of optical active metallo-supramolecular polymers were successfully constructed from the complexation of two enantiopure bis-pyridine-dioxazole (bis-pybox) ligands and 3d transition metal ions such as Zn²⁺ and Fe²⁺ in organic solution. The self-assembly process was investigated via UV–vis and fluorescent titration. The chiral characteristic of polymers was detected by circular dichroism. It was revealed that the phenyl-substituted polymers showed higher stability than the benzyl-substituted ones. Temperature-dependent circular dichroism spectra demonstrated the dynamic structure of this type polymer under the variation of temperature. This work highlights the opportunity in designing and constructing of the optical active metallo-supramolecular polymers from the organic privilege ligands in the field of asymmetric catalysis.     

Pybox ligand-promoted copper(I)-catalyzed three-component tandem coupling-annulation of terminal alkynes, amines and ortho-alkynylaryl aldehydes

Pybox ligand was utilized to promote a highly efficient, one-pot, copper(I)-catalyzed three-component tandem addition and cyclization of ortho-alkynylaryl aldehydes, primary amines, and terminal alkynes. Dihydroisoquinoline derivatives 2 were achieved with excellent to moderate yields. In some examples, water was found as an additive to activate the reaction with or without Pybox ligand. Compared with the reported methods, in which imine had to be pre-prepared and purified, the novel process provides a greener choice.