Recent Advances in the Development of Chiral Gold Complexes for Catalytic Asymmetric Catalysis

Asymmetric gold catalysis has been rapidly developed in the past ten years. Breakthroughs have been made by rational design and meticulous selection of chiral ligands. This review summarizes newly developed gold-catalyzed enantioselective organic transformations and recent progress in ligand design (since 2016), organized according to different types of chiral ligands, including bisphosphine ligands, monophosphine ligands, phosphite-derived ligands, and N-heterocyclic carbene ligands for asymmetric gold(I) catalysis as well as heterocyclic carbene ligands and oxazoline ligands for asymmetric gold(III) catalysis.     

超分子双膦配体的构筑及应用研究进展

超分子双膦配体是一类新兴起的基于非共价键作用构筑的双膦配体,近年来引起人们的重视.与传统的共价键连接的双膦配体相比,利用非共价相互作用的可逆性和选择性,超分子双膦配体具有合成简便,组合灵活,易于合成超分子配体库,并利用组合化学的方法对催化体系进行优化和筛选等优点.详细综述了近几年发展的基于氢键、配位键、主客体作用和静电作用等弱相互作用的超分子双膦配体,重点讨论了它们的构建方法以及在不对称催化反应中的应用,并对其发展前景进行了展望.     

C1 and C2-symmetric carbohydrate phosphorus ligands in asymmetric catalysis

Carbohydrates are increasingly used as starting materials for the synthesis of enantiopure ligands. They contain a considerable number of stereocenters, and compounds with all possible configurational combinations are readily available. This tutorial review focuses on ligands obtained by the introduction of phosphorus functionalities (mainly phosphinite, phosphite or phosphine) into a carbohydrate framework. They are classified according to their structural features. In this review, ligands are organised as C1 ligands with a pyranoside or furanoside structure, and C2 ligands. Particular attention is paid to water soluble ligands prepared from carbohydrates. General methods for the preparation of the ligands are presented in order to show how the backbones can be obtained from simple carbohydrates. The catalytic results obtained in commonly studied processes are presented in tables in order to facilitate the comparison between the ligands. The advantages and limitations of the use of ligands based on carbohydrates are discussed.     

Kinetics study of the binding of multivalent ligands on size-selected gold nanoparticles

The effect of ligand multivalency and nanoparticle size on the binding kinetics of thiol ligands on gold nanoparticles is investigated by exchanging monovalently bound pyrene on gold nanoparticles against flexible mono- and multivalent thiol ligands. Variable-sized gold nanoparticles of 2.2 ± 0.4, 3.2 ± 0.7, and 4.4 ± 0.9 nm diameter are used as substrates. The particles are coated by thiol functionalized pyrene ligands and the binding kinetics of the thiol ligands is studied by time-resolved fluorescence spectroscopy. The effect of multivalency on the binding kinetics is evaluated by comparing the rate constants of ligands of different valency. This comparison reveals that the multivalent ligands are exchanging substantially more rapidly than the monovalent ones. A particle size dependence of the rate constants is also observed, which is used to derive structural information on the binding of the mono- and multivalent ligands to the nanoparticle surface.     

Interplay and Competition Between Two Different Types of Redox-Active Ligands in Cobalt Complexes: How to Allocate the Electrons?

The field of molecular transition metal complexes with redox-active ligands is dominated by compounds with one or two units of the same redox-active ligand; complexes in which different redox-active ligands are bound to the same metal are uncommon. This work reports the first molecular coordination compounds in which redox-active bisguanidine or urea azine (biguanidine) ligands as well as oxolene ligands are bound to the same cobalt atom. The combination of two different redox-active ligands leads to mono- as well as unprecedented dinuclear cobalt complexes, being multiple (four or six) center redox systems with intriguing electronic structures, all exhibiting radical ligands. By changing the redox potential of the ligands through derivatisation, the electronic structure of the complexes could be altered in a rational way.     

Tertiary-poly-amine ligands as stabilisers of transition metal complexes with uncommon oxidation states

Abstract

Tertiary-amine ligands are known to be poorer [sgrave] donor ligands than the corresponding primary- or secondary-amine ligands. They are known to shift the redox potentials of given couples to the anodic direction relative to the corresponding complexes with primary- or secondary-amine ligands. A review of data in the literature and of recent results on nickel complexes with tetra-aza-macrocyclic ligands and copper complexes with open chain polyamine ligands suggests that the major source for these effects is the poorer solvation of the complexes with the tertiary-amine complexes due to the lack of hydrogen bonding between the complexes and the solvent, or the counter ions. Thus the stabilisation of low valent transition metal complexes by tertiary-amine ligands is due to thermodynamic reasons. On the other hand, tertiary-amine-macrocyclic ligands stabilise high valent complexes because the route to the formation of imine groups is kinetically inhibited in these complexes.     

Biotin ligands labeled with daunomycin as an electrochemical probe for avidin and biotin interaction

Electroactive biotin ligands were prepared by the reaction of daunomycin with biotinylating reagents with a different spacer. These biotin ligands exhibited similar electrochemical properties to those of daunomycin, but the adsorptivity of the ligands on the electrode increased with increasing length of the spacer. The electrode response of these ligands decreased when specifically bound with avidin. This made it possible to detect electroinactive avidin indirectly. Biotin was detected by observing the competitive reaction between biotin and the ligands for the limited binding sites of avidin. The binding strength of the labeled biotins with avidin was compared with that of unlabeled biotin by using an enzyme assay.     

Novel benzoferrocenyl chiral ligands: Synthesis and evaluation of their suitability for asymmetric catalysis

Four benzoferrocenyl phosphorus chiral ligands were conveniently prepared in good overall yields. These ligands were found to be stable in solid form and in solution. Two of the four ligands were resolved by chiral HPLC. Unlike a reported bis(phosphino-η⁵-indenyl)iron(II) complex, in which the indenyl ligands undergo ring flipping through an η¹-intermediate, these two ligands were found to be configurationally stable in solution and in solid state. The suitability of these ligands for enantioselective catalysis was assessed in studies on allylic alkylation reactions. When the two less sterically hindered ligands were used, excellent chemical yields were obtained, but the other two more sterically hindered ones gave lower yields. When the two enantiopure ligands were used, enantioselectivity of up to 51% ee was observed. These findings suggest that benzoferrocene derivatives may be used as chiral ligands for asymmetric catalysis.     

Synthesis and Characterization of Some Mixed Ligand-Selenito and Tellurito Chromium(III) Complexes

New mixed ligand Cr(III) complexes were prepared where diamine or oxalato ligands are coordinated together with either tellurito, selenito, or hydrogenselenito ions to form nine octahedral complexes. The complexes were characterized by chemical analyses, IR and UV-visible spectra, magnetic, and conductivity measurements. The tellurito and selenito ligands act as monodentate ligands, coupled with the bidentate diamine ligands. On the other hand, they act as bidentate chelate ligands when coordinated together with the oxalate ligand. However, hydrogenselenite ion act as a monodentate ligand coupled with the oxalate ligand. IR spectra indicated that the inorganic ligands are coordinated to the Cr(III) ion through their oxygen atoms. One of the bulky diamine molecules, 1,2-pn or 1,3-pn, was freed from the coordination sphere of Cr(III) on the addition of the bulky inorganic anions and was replaced by two water molecules.     

The Search for Enhanced σ-Donor Ligands to Stabilize Boron-Boron Multiple Bonds

Boron-boron multiple bonds, such as those found in diborenes and diborynes, are typically stabilized by σ-donor ligands that furnish electron density to these otherwise electron-deficient species. These compounds are not only of fundamental importance in the study of chemical bonding, but can also activate small molecules in a chemistry reminiscent of that carried out by transition metals. In the pursuit of designing new and improved σ-donor ligands to stabilize diborenes and diborynes suitable to activate small molecules, we performed density functional calculations to evaluate the Lewis basicity of a series of σ-donor ligands. For this evaluation, we analysed the interaction between the boranes and the σ-donor ligands in model systems L→BX₃ (X=F and Me) using energy decomposition analyses. We found that electronic bond energies of the L→BX₃ adducts correlate well with the ionization energies of the ligands and that ligands with high or medium basicity stabilize diborynes better than ligands with low basicity. We also learnt that beryllium-based ligands are promising since they are able to stabilize L→B≡B←L diborynes without significantly reducing the triple bond character of the B≡B bond.     

Recent developments in the non-cyclopentadienyl organometallic and related chemistry of scandium

Up to the early to mid 1990s the organometallic chemistry of scandium was dominated by cyclopentadienyl derivatives. This present article highlights advances in the synthesis and reactivity of non-cyclopentadienyl organometallic and related compounds of scandium. These include: compounds containing arene and other eta(x)-CxRx ligands; compounds with macrocyclic and fac-L3 ligands; compounds with polydentate ligands that incorporate amide donors; compounds with bidentate, monoanionic N,N' donor ligands; and compounds with iminophenolate, bis(phenoxide) and some other anionic O-donor ligands.     

Halide-induced supramolecular ligand rearrangement

A novel reaction involving the halide-induced rearrangement of ligands within supramolecular Rh(I) complexes containing hemilabile ligands is presented. Three analogous bis- and trishemilabile ligands have been synthesized to construct bi- and trimetallic Rh(I) macrocyclic complexes. An intentionally added halide source results in the formal rotation of only one hemilabile ligand along the axis that is perpendicular to the plane defined by the aryl backbone of the hemilabile ligands. X-ray structures, as determined by X-ray crystallography, of key intermediates and products are presented.     

Amplification of bifunctional ligands for calmodulin from a dynamic combinatorial library

A well known strategy to prepare high affinity ligands for a biological receptor is to link together low affinity ligands. DCC (dynamic combinatorial chemistry) was used to select bifunctional protein ligands with high affinity relative to the corresponding monofunctional ligands. Thiol to disulfide linkage generated a small dynamic library of bifunctional ligands in the presence of calmodulin, a protein with two independently mobile domains. The binding constant of the bifunctional ligand (disulfide) most amplified by the presence of calmodulin is nearly two orders of magnitude higher than that of the corresponding monofunctional ligand (thiol).     

New insight in the role of modifying ligands in the sol-gel processing of metal alkoxide precursors: A possibility to approach new classes of materials

This paper summarizes recent literature data and presents new experimental data on the mechanisms of chemical modification, hydrolysis and polycondensation of the alkoxides and demonstrates possibilities to approach new classes of materials, exploiting these mechanisms. Low reactivity of silicon alkoxides is improved by either basic catalysis exploiting an S_N2 mechanism or acidic catalysis facilitating a proton-assisted S_N1 mechanism as well as by modification with chelating ligands. Metal alkoxides are much stronger Lewis bases compared to silicon alkoxides and the acidity of water is strong enough to achieve their rapid hydrolysis via proton-assisted S_N1 pathway even in the absence of additional catalysts. Introduction of the modifying chelating ligands is leading generally to increased charge distribution in the precursor molecules. Modifying chelating ligands are also appreciably smaller than the alkoxide ligands they replace. The modification with chelating ligands is thus facilitating the kinetics of hydrolysis and polycondensation. The size and shape of the primary particles formed in sol-gel treatment of metal alkoxides are defined not by kinetic factors in their hydrolysis and polycondensation but by the interactions on the phase boundary, which is in its turn directed by the ligand properties. The products of the fast hydrolysis and condensation sequence consist of micelles templated by self-assembly of ligands (mainly oxo-species). This concept provides explanations for commonly observed material properties and allows for the development of new strategies for the preparation of materials. We discuss the formation of inverted micelles, obtained by the appropriate choice of solvents, which allows for the formation of hollow spheres. The modifying β-diketonate ligands act as the surfactant and form an interface between the hollow sphere and the solvent. Retention of ligands inside the gel particles is possible only if ligands possessing both chelating and bridging properties are applied. Application of such ligands, for example, diethanolamine, permits to prepare new transition metal oxide based microporous membranes.     

Template-induced formation of heterobidentate ligands and their application in the asymmetric hydroformylation of styrene

We report the template-induced formation of chelating heterobidentate ligands by the selective self-assembly of two different monodentate ligands on a rigid bis-zinc(II)-salphen template with two identical binding sites; these templated heterobidentate ligands induce much higher enantioselectivities (up to 72% ee) in the rhodium-catalyzed asymmetric hydroformylation of styrene than any of the corresponding homobidentate ligands or non-templated mixed ligand combinations (up to 13% ee).     

Solid-phase synthesis of chiral N-acylethylenediamines and their use as ligands for the asymmetric addition of alkylzinc and alkenylzinc reagents to aldehydes

A solid-phase procedure has been developed for the synthesis of chiral N-acylethylenediamine ligands. The ligands are obtained in good yield and purity, without the need for chromatography or other purification methods. Several new and previously reported ligands were prepared using this procedure. These compounds were examined as catalysts for the enantioselective addition of alkylzinc reagents to aldehydes. In all cases, the crude ligands from the solid-phase syntheses catalyzed the reactions with similar yields and stereoselectivities when compared to reactions using ligands that had been purified by standard methods. Preliminary studies were also performed with ligands 3a and 3f as catalysts for the addition of alkenylzinc reagents to aldehydes to give chiral allylic alcohols. Ligand 3f was found to catalyze this addition reaction in up to 76% ee.     

Novel rhodium catalyst for asymmetric hydroformylation of styrene: Study of electronic and steric effects of phosphorus seven-membered ring ligands

The asymmetric hydroformylation of styrene catalysed by rhodium complexes modified by monodentate phosphepine ligands has been investigated. The effects of a systematic variation of the phosphorus substituent on catalytic activity and selectivity are reported as well as the outcome of a combinatorial approach for the formation of mixed ligands complexes with several commercially available ligands. Effective catalytic systems have been devised for the reaction that provide for full conversion within 24 h and regioselectivity up to 96% in the branched aldehyde and enantioselectivities up to 48%. The use of bidentate ligands built up by binaphthyl-supported seven-membered phosphacyclic units does not allow for further improvement in the reaction outcome.     

Reversible regulation of protein binding affinity by a DNA machine

We report a DNA machine that can reversibly regulate target binding affinity on the basis of distance-dependent bivalent binding. It is a tweezer-like DNA machine that can tune the spatial distance between two ligands to construct or destroy the bivalent binding. The DNA machine can strongly bind to the target protein when the ligands are placed at an appropriate distance but releases the target when the bivalent binding is disrupted by enlargement of the distance between the ligands. This "capture-release" cycle could be repeatedly driven by single-stranded DNA without changing the ligands and target protein.     

含光学活性联二萘酚基团可溶性高分子手性配体的合成及其不对称加成反应

不对称催化是有机化学研究的前沿领域和发展方向．近10年来,手性配体的研究得到了长足的发展,大量的小分子手性配体被报道,其中许多手性配体展示了优异的催化性能．但是由于昂贵的手性配体（金属催化剂）分离、回收和再利用的问题,从而限制了小分子催化剂的应用．因此,解决手性金属催化剂的分离与回收问题是不对称催化研究领域的一个热点问题．为了解决这一关键问题,国内外科学家从负载的角度出发已经成功地发展了众多的研究方法,无机负载法、交联高分子负载法、可溶性高分子负载法、树状大分子负载法等．许多负载的手性金属催化剂已经表现出优异的催化活性和高的对映选择活性．     

Synthesis and properties of nanosilicon stabilized by butyl and perfluorobutyl ligands

Nanocrystalline silicon stabilized by butyl and perfluorobutyl ligands that form improper surface states of silicon nanocrystals were synthesized. The presence of perfluorobutyl ligands on the surface of silicon nanocrystals was proved by IR spectroscopy. Nanocrystals with perfluorobutyl ligands form aggregates, which decreases the efficiency of photoluminescence. The nanocrystals with butyl ligands have smaller size but their photoluminescence can be clearly recorded.