Chiral Linkers Research Articles

Synthesis and structure of macrocyclic bis(hydroxynaphthoic amide)s connected by an achiral or chiral diamine.

Macrocyclic bis(hydroxynaphthoic amide)s 6, connected by an achiral or chiral diamine, were synthesized by the tandem Claisen rearrangement. CD spectra, X-ray crystallographic analyses, and variable-temperature NMR measurements of the chiral bis(hydroxynaphthoic amide)s revealed that the two hydroxynaphthalene rings in these macrocycles adopt a twisted conformation both in solution and in the crystalline state because of the steric hindrance between the two hydroxynaphthalene rings and that the chirality of the twisted conformation is generated by that of the chiral linker. Theoretical calculations revealed that the chiral linker works effectively to favor energetically one conformer of the diastereomers, although a flipping process was possible and can be observed to occur on the NMR time scale in variable-temperature experiments.

Application of an ephedrine chiral linker in a solid-phase, 'asymmetric catch-release' approach to gamma-butyrolactones.

A Sm(II)-mediated, asymmetric, intermolecular ketyl-olefin addition employing alpha,beta-unsaturated esters linked to resin through an ephedrine 'chiral link' has been applied in a direct 'asymmetric catch-release' approach to gamma-butyrolactones.

Chiral recognition by hemicarcerand-like host in aqueous solution

The synthesis and binding properties of a hydrophilic hemicarcerand-like host with three chiral linker groups and one enlarged opening, that is partially blocked by two appending coumarin moieties, are reported. The complexation behavior towards three achiral and eight chiral guests were investigated. For those guests, in which binding was observed, guest exchange was slow on the NMR time scale and allowed the observation of free and complexed guest signals. At 22°C, binding affinities ranged from 1.4 to 370 mol −1. Guest binding is driven by enthalpy. In binding studies with racemic guests, the highest diastereomeric excess of complexation (de=20%) was observed for racemic 3-methylcyclohex-1-ene. The much lower de for racemic 3-methylpent-1-ene suggests that guest rigidity is important to achieve selectivity.

The first supramolecular assemblies comprised of dimetal units and chiral dicarboxylates

The first molecules containing metal–metal bonded Mo 2 4+ units linked by chiral dicarboxylates have been prepared and characterized by X-ray crystallography and 1H NMR spectroscopy; the compounds [Mo 2(DAniF) 3] 2(dicarboxylate), 1 for l-tartrate and 2 for d-aspartate (DAniF= N, N ′-di- p-anisylformamidinate) represent a first step toward the synthesis of more complex polygonal structures assembled from M 2 units and chiral linkers.

(4 S)- p-Hydroxybenzyl-1,3-oxazolidin-2-one as a solid-supported chiral auxiliary in asymmetric 1,3-dipolar cycloadditions

Evans' chiral auxiliary was grafted onto both Merrifield and Wang resins and, after functionalisation, they were used as chiral dipolarophiles in a 1,3-dipolar cycloaddition involving both a nitrile oxide and a nitrone. The cycloadducts were cleaved and analysed by chiral HPLC: the recovered supported chiral oxazolidinone was functionalised and reused in further cycloadditions. The stereochemical results as well as the possibility of recycling the chiral linker supports the applicability of solid-supported chiral auxiliaries.

Preparation and structure–chiroptical relationships of tartaric acid-based layer-block chiral dendrimers

Two optically active, diastereoisomeric, first generation layer-block dendrimers 1 and 2 have been prepared by a convergent synthetic procedure. These chiral, layer-block dendrimers utilize 4-tert-butylphenoxy moieties as the surface groups and phloroglucinol as the branching junctures. Two different chiral units, which are derivatives of (D)- and (L)-tartaric acid, serve as the chiral linkers between the surface group and the branching juncture, or between two branching junctures. The first layer-block dendrimer 1 has an outer chiral layer made up of six (L)-tartrate derived units and an inner chiral layer of three (D)-tartrate derived units. The second layer-block dendrimer 2 has an outer shell consisting of three (D)- and three (L)-chiral units and an inner shell of three (D)-chiral units. The molar rotation of these structurally flexible, low generation dendritic compounds is proportional to the number of chiral tartrate units in excess, with the chiroptical effect of a (D)-tartrate derived chiral unit cancelling that of an (L)-tartrate derived unit on a 1∶1 basis. Circular dichroism studies, however, reveal that this cancellation effect is more effective when both the (D)- and (L)-chirons are situated within the same layer.

Synthesis and chiroptical properties of layerblock dendrimers

Abstract

Preparation and Structural Characterization of an Enantiomerically Pure, C2-Symmetric, Single-Component Ziegler−Natta α-Olefin Polymerization Catalyst

A new linked bis(cyclopentadienyl) ligand, {(C5H3-2-SiMe3-4-CMe3)2Si(OC10H6C10H6O)} (BnBpH2), has been designed to coordinate to transition metals to afford a single enantiomeric C2-symmetric ansa-metallocene. The syntheses of its dipotassium salt and (BnBp)YCl(THF) are described. Steric interactions between the 3- and 3‘-methine positions of the 1,1‘-binaphth-2,2‘-diolate rings of the chiral linker with the α-trimethylsilyl substituents on the cyclopentadienyl rings force enantioselective metalation of this ligand. Thus, coordination to yttrium occurs in an entirely diastereoselective manner: the ligand prepared from (R)-(+)-1,1‘-bi-2-naphthol directs formation of the (S)-yttrocene, (R,S)-(BnBp)YCl(THF), while that from (S)-(−)-1,1‘-bi-2-naphthol directs formation of the (R)-yttrocene, (S,R)-(BnBp)YCl(THF). Removal of coordinated tetrahydrofuran allows the preparation of (BnBp)YCH(SiMe3)2. Treatment of rac-(BnBp)YCH(SiMe3)2 with H2 yields a kinetic mixture of both heterochiral and homochiral dimers (e.g. (R,S)-(BnBp)Y(μ2-H)2-(S,R)-(BnBp) and (R,S)-(BnBp)Y(μ2-H)2-(R,S)-Y(BnBp), respectively). Over several hours this mixture undergoes conversion to the pure homochiral dimers. As anticipated, hydrogenolysis of enantiopure (BnBp)YCH(SiMe3)2 (e.g. R,S-(BnBp)YCH(SiMe3)2) affords directly only enantiopure homochiral dimer (e.g. (R,S)-(BnBp)Y(μ2-H)2-(R,S)-Y(BnBp)). The hydride, generated in situ from (R,S)-(BnBp)YCH(SiMe3)2, polymerizes 1-pentene to highly isotactic poly-1-pentene (Mn = 119 000, PDI = 1.44, mmmm >95%). The much higher solubility of the enantiopure homochiral dimers was used to grow crystals of the racemate at the interface of layered solutions of the pure optical isomers. rac-(BnBp)Y(μ2-H)2(BnBp) crystallizes in the centrosymmetric space group P1̄ (#2) with a = 11.925(4) Å, b = 19.461(8) Å, c = 23.814(10) Å and α = 82.80(3)°, β = 84.64(4)°, γ = 81.30(3)°, with a volume of 5404(4) Å3 and Z = 2.