Ru-Gang Xie

A simple copper salt catalysed the coupling of imidazole with arylboronic acids in protic solventDedicated to Professor Xie Ming-Gui on the occasion of his 65th birthday.

In the presence of a catalytic amount of a simple copper salt, the coupling of imidazole with arylboronic acids was performed in methanol to give corresponding N-arylimidazoles in almost quantitative yields; this coupling reaction could also be performed in aqueous solutions to give N-arylimidazoles in excellent yields.

Synthesis and selective anion recognition of imidazolium cyclophanes

Cyclophanes based on imidazolium or benzimidazolium groups were synthesized as anion recognition motifs by quaternarization of the bridged imidazole or benzimidazole compounds with dibromides in acetonitrile under reflux with high dilution. X-Ray analysis showed that C- H· · ·Br 2 hydrogen bonds connected the hydrogen atoms of the cationic imidazolium rings, the m-xylene ring and the spacer of the macrocycle with bromide anions. 1 H NMR study in DMSO-d6 showed that the H-2 of imidazolium rings and the proton of the benzene ring were shifted downfield upon addition of Br 2 , suggesting the formation of the C- H· · ·Br 2 hydrogen bonds between the cyclophane and bromide anion in solution. UV spectroscopic titration in acetonitrile at 258C showed 1:1 complexes between the cyclophanes and halide anions, and the binding constants (Ka) and Gibbs free energy changes (2DG8) were calculated according to the modified Benesi - Hildebrand equation. Cyclophane 1·2PF6 exhibits selective recognition for F 2 ,C l 2 ,B r 2 and I 2 in acetonitrile. The binding constant of 1·2PF6 with Cl 2 is 4.06£10 4 M 21 , 2, 5 and 2000 times those of 1·2PF6 with Br 2 ,F 2 and I 2 , respectively. Binding experiments indicate that the electrostatic interactions, hydrogen bonding and preorganization of the binding sites of the hosts play essential roles in the anion recognition by imidazolium cyclophanes. q 2002 Elsevier Science Ltd. All rights reserved.

Hydrolytic metalloenzyme models: Enantioselective hydrolysis of long chain α-amino acid esters by chiral metallomicelles composed of lipophilic l-histidinol

Abstract Chiral metallomicellar supramolecular systems composed of lipophilic l -histidinol have been investigated for their catalytic activities and enantioselectivities in the hydrolysis of N-protected α-amino acid esters. Kinetic evidence suggests that the mechanism of the deacylation promoted by ligand 4a with a hydroxyl group and 2a lacking a hydroxyl group is different. The apparent p K a of Zn 2+ - 4a is 6.8 in the micelllar aggregate. The highest enantioselectivity in hydrolysis of R ( S )-C 12 -Leu-PNP ( k S / k R =3.01) was obtained with 4a -Zn 2+ as catalyst.

Design, synthesis and structure of new chiral squaric acid monoaminoalcohols and diaminoalcohols and their use as catalysts in asymmetric reduction of ketones and diketones

Abstract Many chiral squaric acid aminoalcohols and C2-symmetric diaminoalcohols have been synthesized and their in situ formed chiral boron heterocycles have been used as catalysts for the enantioselective reduction of prochiral ketones and diketones by borane to give alcohols with up to 99% enantiomeric excess and 99% yield. The effects of solvent, catalyst–substrate ratio and temperature were also investigated.

Chiral monoaminoalcohols and diaminoalcohols of squaric acid: new catalysts for the asymmetric reduction of ketones by borane

Two series of new chiral ligands, squaric acid aminoalcohols and C2-symmetric squaric acid diaminoalcohols have been synthesized. The chiral oxazaborolidines formed in situ from these ligands have been used in the enantioselective borane reduction of prochiral ketones and diketones to afford alcohol products with up to 99% enantiomeric excesses. The structures of the ligands have an obvious effect on the ee of the resulting alcohols.

Enantioselective recognitions of chiral molecular tweezers containing imidazoliums for amino acids

Two kinds of novel chiral molecular tweezers containing imidazoliums were synthesized from L-alanine, L-phenylalanine, and L-glutamic acid. They are constructed by the chiral imidazolium pincers and two different spacers which are 1,3-bis (bromomethyl)benzene and 2,6-bis(bromomethyl)pyridine, respectively. The enantioselective recognition of L- and D-amino acid derivatives by these molecular tweezers was investigated by UV spectroscopic titration experiments and good enantioselectivities were obtained, which are highly sensitive to whether the spacer has the binding site and the pincers has the other aromatic rings besides imidazolium ring. The host molecular 3b.2PF6- showed remarkable enantioselectivity for N-Boc protected histidine methyl ester, affording K(L)/K(D) of 5.10.

Chiral squaric prolinols: a new type of ligand for the asymmetric reduction of prochiral ketones by borane

Abstract A series of chiral bifunctional squaric prolinol ligands, having N, S substituents at C(3) of the squaric ring were synthesized and applied to the asymmetric borane reduction of prochiral ketones via an in situ formed chiral boron heterocycle, affording secondary alcohols with high yields and excellent enantiomeric excesses (up to 99%). The crystal structure of 5a was obtained and the mechanism of the catalytic asymmetric reduction is also discussed.

Novel chiral imidazole cyclophane receptors: synthesis and enantioselective recognition for amino acid derivatives.

Novel chiral imidazole cyclophane receptors were synthesized by highly selective N-alkylation of the imadazolyl 1N-position of the bridged histidine diester 2 with the dibromide in the presence of NaH; these receptors exhibit good chiral recognition toward the enantiomers of L- and D-amino acid derivatives (up to KD/KL = 3.52, delta delta G0 = -3.11 kJ mol-1) in CHCl3 at 25.0 degrees C.

Synthesis of New Cheral Macrocyclic Tetraoxo Polyamines Containing Pyridine Ring and Functional Arms

Abstract Seven new chiral macrocyclic tetraoxo polyamines containing pyridine ring and functional arms derived from L-histidine, L-alanine, L-leucine and L-phenylalanine, respectively, have been synthesized and characterized by MS, 1H NMR and elemental analysis.

Enantioselective hydrolysis of long chain α-amino acid esters by chiral sulfur-containing macrocyclic metallomicelles

A novel chiral lipophilic sulfur-containing macrocyclic ligand 5 with bis-pendant alcohols in the proximity of the coordination center has been synthesized. Its metal ion complexes have been investigated as catalysts for the enantioselective hydrolysis of long chain α-amino acid esters in aqueous comicellar solution with Brij35. Large rate accelerations (up to 220 times) and moderate enantioselectivities (up to 4.85 (kS/kR)) employing the macrocyclic 5-Cu2+ have been observed, whereas the acyclic 3-Cu2+ exhibits less reactivity and stereoselectivity. Taking the analogous ligand 4, lacking the hydroxy groups leads to a dramatic rate decrease, and an inversion of enantioselectivity is observed. The pKa value of the hydroxyl bound to Cu2+ is determined to be pKa=7.2 under our micellar reaction conditions.