Supraphos: A supramolecular strategy to prepare bidentate ligands

Herein, we report a new strategy for the preparation of chelating bidentate ligands, which involves the mixing of two mondentate ligands functionalized with complementary binding sites. The assembly process is based on selective metal-ligand interactions employing phosphite zinc(II) porphyrins 1-6 and the nitrogen-containing phosphorus ligands b-i (Scheme 1). Only 14 monodentate ligands were utilized to generate a library of 48 palladium catalysts based on supraphos-type bidentate ligands. The characterization of rhodium complexes based on representative Supramolecular bidentate ligands and the comparison of their performance in the hydroformylation of styrene will be presented. The current library of catalysts was tested in the asymmetric palladium-catalyzed alkylation of rac-1,3-diphenyl-2-propenyl acetate, which resulted in a large variety in the observed enantioselectivity for the different catalysts. Importantly, small variations in the supraphos building blocks, lead to large differences in the enantioselectivity imposed by the catalyst, the most selective catalyst producing 97% ee.     

Supramolecular approaches to generate libraries of chelating bidentate ligands for homogeneous catalysis

The process of catalyst discovery and development relying on combinatorial methods has suffered so far from the difficult access to structurally diverse and large libraries of ligands, in particular the structurally more complex class of bidentate ligands. A completely new approach to streamline the difficult ligand synthesis process is to use structurally less complex monodentate ligands that self-assemble in the coordination sphere of a metal center through noncovalent attractive ligand-ligand interactions to generate bidentate, chelating ligands. When complementary attractive ligand-ligand interactions are employed, it is even possible to generate libraries of defined chelate-ligand catalysts by simply mixing two different monomeric ligands. This Minireview summarizes the first approaches and results in this new field of combinatorial homogeneous catalysis.     

Copper(I)-2-(2′-pyridyl)benzimidazole catalyzed N-arylation of indoles

CuI-2-(2′-pyridyl)benzimidazole catalyst system can serve efficiently to promote N-arylation of various indoles to afford the N-arylated indoles. The bidentate ligand, 2-(2′-pyridyl)benzimidazole was proved superior to monodentate nitrogen-based ligands and well-known bidentate ligands such as 2,2′-bipyridyl and 1,10-phenanthroline.     

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.     

Iridium-Catalyzed Domino Hydroformylation/Hydrogenation of Olefins to Alcohols: Synergy of Two Ligands

A novel one-pot iridium-catalyzed domino hydroxymethylation of olefins, which relies on using two different ligands at the same time, is reported. DFT computation reveals different activities for the individual hydroformylation and hydrogenation steps in the presence of mono- and bidentate ligands. Whereas bidentate ligands have higher hydrogenation activity, monodentate ligands show higher hydroformylation activity. Accordingly, a catalyst system is introduced that uses dual ligands in the whole domino process. Control experiments show that the overall selectivity is kinetically controlled. Both computation and experiment explain the function of the two optimized ligands during the domino process.     

Supramolecular bidentate phosphorus ligands based on bis-zinc(II) and bis-tin(IV) porphyrin building blocks

Selective metal-ligand interactions have been used to prepare supramolecular bidentate ligands by mixing monodentate ligands with a suitable template. For these assemblies pyridine phosphorus ligands and a zinc(II) porphyrin dimer were used. In the rhodium-catalysed hydroformylation of 1-octene and styrene improved selectivities have been obtained for some of the assembled bidentate ligand systems. In the palladium catalysed asymmetric allylic alkylation similar effects were observed; the enantioselectivity increased by using a bisporphyrin template. The preparation of supramolecular catalyst systems was also explored using tin-oxygen interactions. Dihydroxotin(IV) porphyrin and carboxylic phosphorus ligands assemble into supramolecular ligands and the phosphorus donor atom coordinates to transition metals. The stronger oxygen-tin bond, compared to pyridine-zinc does not result in a better performance of the catalyst.     

Supramolecular chiral phosphorous ligands based on a [2]pseudorotaxane complex for asymmetric hydrogenation

A new type of supramolecular chiral phosphorus-based ligands was prepared from easily available monodentate ligands through complexation between dibenzylammonium salt and dibenzo[24]crown-8 macrocycle. Rhodium complexes with these supramolecular ligands were prepared, and the supramolecular bidentate ligand-containing catalyst has demonstrated better catalytic activity for all substrates, and higher enantioselectivity except for the ortho-substituted substrates than those obtained from the parent monodentate ligand in the asymmetric hydrogenation of α-dehydroamino acid esters.     

Pd-catalyzed amidation of aryl chlorides using monodentate biaryl phosphine ligands: a kinetic, computational, and synthetic investigation

We present results on the amidation of aryl halides and sulfonates utilizing a monodentate biaryl phosphine-Pd catalyst. Our results are in accord with a previous report that suggests that the formation of kappa(2)-amidate complexes is deleterious to the effectiveness of a catalyst for this transformation and that their formation can be prevented by the use of appropriate bidentate ligands. We now provide data that suggest that the use of certain monodentate ligands can also prevent the formation of the kappa(2)-amidate complexes and thereby generate more stable catalysts for the amination of aryl chlorides. Furthermore, computational studies shed light on the importance of the key feature(s) of the biaryl phosphines (a methyl group ortho to the phosphorus center) that enable the coupling to occur. The use of ligands that possess a methyl group ortho to the phosphorus center allows a variety of aryl and heteroaryl chlorides with various amides to be coupled in high yield.     

Combinatorial Approach to Chiral Tris‐ligated Carbophilic Platinum Complexes: Application to Asymmetric Catalysis

A straightforward methodology for the synthesis of libraries of chiral tris‐ligated cationic platinum complexes and their in situ evaluation as asymmetric carbophilic catalysts in a model domino hydroarylation/cyclization reaction of a 1,6‐enyne was developed. A catalyst‐generation process based on a combination of a monodentate and a bidentate phosphorus ligand allowed the formation of 108 chiral complexes. One‐pot screening of the stereoinduction obtained with this library in a test domino addition/cyclization reaction validated this approach and stressed the key role played by the monodentate ligand partner in obtaining high enantioselectivities. In the case of two challenging substrate/nucleophile combinations, the combinatorial approach resulted in a significant gain in enantioselectivity.     

Rigid bis-zinc(II) salphen building blocks for the formation of template-assisted bidentate ligands and their application in catalysis

The template-induced formation of chelating bidentate ligands by the selective self-assembly of two monodentate pyridyl phosphorus ligands on a rigid bis-zinc(II) salphen template with two identical binding sites was studied. Using UV-vis, NMR-spectroscopy and X-ray analysis the formed structures were unambiguously proven. The application of these templated bidentate ligands in transition metal catalysis showed, in most cases, typical bidentate character. Compared to previous work based on a more flexible bis-zinc(II) porphyrin template, the current catalytic data suggest that the rigidity of the template is not an important factor for the improvement of the regio- and enantioselectivity under the applied reaction conditions.     

Organometallic chemistry of amidate complexes. accelerating effect of bidentate ligands on the reductive elimination of N-aryl amidates from palladium(II)

We report a series of arylpalladium complexes of acetamidate, sulfonamidate, and deprotonated oxazolidinone ligands that undergo reductive elimination with rates and yields that depend on the binding mode of the ancillary and amidate ligands. Complexes of the acetamidate ligands containing the bidentate phosphines DPPF and Xantphos as ancillary ligands undergo reductive elimination. The rate and yield were higher from the complex ligated by Xantphos, which contains a larger bite angle. In contrast, the analogous amidate complex containing a single sterically hindered monodentate ligand and a kappa2-bound amidate ligand does not undergo reductive elimination. This trend of faster reductive elimination from complexes containing bidentate ancillary ligands than from a complex with a single monodentate ancillary ligand is unusual and is consistent with an effect of the denticity of the ancillary ligand on the binding mode of the amidate. Complexes of sulfonamidate ligands underwent reductive elimination faster than complexes of acetamidates, and reductive elimination occurred from complexes containing both bidentate and monodentate ancillary ligands. Like reductive elimination from the acetamidate complexes, reductive eliminations from the sulfonamidate complexes were faster when the complexes possessed bidentate Xantphos and kappa1-sulfonamidate ligands.     

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.     

Catalytic asymmetric transformations with fine-tunable biphenol-based monodentate ligands

A library of new fine-tunable monodentate phosphite and phosphoramidite ligands based on chiral biphenol have been designed and developed. These monodentate phosphorus ligands have exhibited excellent enantioselectivity in the Pd-catalyzed asymmetric allylic alkylation and Rh-catalyzed asymmetric hydrogenation.     

Quantitative chirality analysis of molecular subunits of bis(oxazoline)copper(II) complexes in relation to their enantioselective catalytic activity

The analysis of the quantitative chirality of molecular subunits of spirocyclic bisoxazoline copper complexes was carried out in order to identify the molecular portions most responsible for the chirality of the whole molecule, and therefore also for its catalytic enantioselectivity. It is shown that the smallest fragment that carries the information on molecular chirality contains only a portion of the bidentate bisoxazoline ligands and most atoms of the two monodentate ligands. The structural parameters that are best correlated to the chirality measures of the studied systems are the bond angles at the bridgehead spiro-carbon atom. A prediction is made for an analogous catalyst with potentially high enatioselectivity.     

Chiral monooxazolines as modular copper(I)-heterocomplex building blocks: investigations on the catalytic asymmetric cyclopropanation of alkenes

Novel chiral monodentate oxazoline ligands have been synthesized in good yields. The catalytic activity of these monodentate oxazoline/Cu catalysts was evaluated in the catalytic asymmetric cyclopropanation of styrene and α-methylstyrene, giving moderate to good enantioselectivities (up to 74% ee for the trans-cyclopropane product) and full conversions (up to 100%). In an attempt to enhance the enantioselectivities of the cyclopropanations, heterocombinations of these ligands were used. Unfortunately, with the data set that was used in this study, no improvements were observed. However, to gain an insight into the nature of the active catalyst present under these circumstances, NMR, mass spectrometric and computational studies were carried out and indicated the presence of bidentate heterocomplexes in the equilibrium mixture. Analysis of the stereoselectivities (ees and des) did not prove very useful in pin-pointing the identity of the active chiral catalyst and only afforded a very weak conclusion. In order to ascertain the importance of the π-π interactions, the monodentate oxazoline ligands 3a and 3b were synthesized and screened in these reactions, and the resulting stereoselectivities were compared to the results obtained using ligands 1a and 1b. There seemed to be very weak π-π interactions at work.     

铑催化不对称氢化2-苯基-2-丙烯醇合成手性2-苯基丙醇

通过铑催化不对称催化氢化的方法,实现了2-苯基-2-丙烯醇的不对称催化氢化,对一系列单齿及双齿磷配体进行了筛选,在温和的条件下,以较高对映体选择性的合成了手性2-苯基丙醇,最高获得了70%的ee值。     

Chelating effect as a driving force for the selective formation of heteroligated Pt(II) complexes with bidentate phosphino-chalcoether ligands

The halide-induced ligand rearrangement reaction (HILR) has been employed to provide selective and exclusive in situ formation of heteroligated Rh(I), Pd(II), and Pt(II) complexes with bidentate phosphino-chalcoether ligands. To gain insights on the nature of this unique reaction, we explored this process via the stepwise addition of bidentate phosphino-chalcoether (P, X; X = S or Se) and relevant monodentate phosphine ligands with a Pt(II) metal precursor. The corresponding monoligated complexes were obtained in quantitative yields by reacting 1 equiv of a P, X bidentate ligand with Pt(II) and were fully characterized via single crystal X-ray diffraction studies and heteronuclear ((31)P, (77)Se, and (195)Pt) NMR spectroscopy in solution. These species were further reacted with a second equivalent of either a bidentate ligand or the monodentate ethyl diphenylphosphine ligand, resulting in the clean formation of the heteroligated species or, in the case of the monodentate ligand with an electron-withdrawing bidentate ligand, a mixture of products. On the basis of competitive exchange reactions between these heteroligated, homoligated, and monoligated complexes, we conclude that ligand chelation plays a crucial role in the Pt(II) HILR. The in situ preferable formation of the stable monoligated complex allows for ligand sorting to occur in these systems. In all cases where the heteroligated product results, the driving force to these species is ligand chelation.     

Hydrogen bonding makes a difference in the rhodium-catalyzed enantioselective hydrogenation using monodentate phosphoramidites

A new generation of monodentate phosphoramidite ligands bearing a primary amine moiety was found to display comparable or better efficiency than bisphosphines in the Rh-catalyzed asymmetric hydrogenation of challenging substrates, such as (Z)-methyl alpha-acetoxyacrylate or (E)-beta-aryl itaconate derivatives, affording the corresponding hydrogenation products with excellent enantioselectivities (up to >99% ee). The presence of intermolecular hydrogen bonding (HB) between two monodentate ligands in the catalyst was found to be critical for excellent catalyst performance. This finding provides a basis for design and development of further catalyst systems using this type of monodentate phosphoramidite ligands.     

Pyrazine as a building block for molecular architectures with PtII

A series of pyrazine (pz) complexes containing cis-(NH(3))(2)Pt(II), (tmeda)Pt(II) (tmeda = N,N,N',N'-tetramethylethylenediamine), and trans-(NH(3))(2)Pt(II) entities have been prepared and characterized by X-ray crystallography and/or 1H NMR spectroscopy. In these compounds, the pz ligands act as monodentate (1-3) or bidentate bridging ligands (4-7). Three variants of the latter case are described: a dinuclear complex [Pt(II)]2 (4b), a cyclic tetranuclear [Pt(II)](4) complex (5), and a trinuclear mixed-metal complex [Pt2Ag] (7). Mono- and bidentate binding modes are readily differentiated by 1H NMR spectroscopy, and the assignment of pz protons in the case of monodentate coordination is aided by the observation of (195)Pt satellites. Formation of the open molecular box cis-[{(NH3)2Pt(pz)}4](NO3)8.3.67H2O (5) from cis-(NH3)2Pt(II) and pz follows expectations of the "molecular library approach" for the generation of a cyclic tetramer.     

On the stability of metal–aminoacid complexes in water based on water–ligand exchange reactions and electronic properties: Detailed study on iron–glycine hexacoordinated complexes

Thermodynamic stability of metal–aminoacid complexes in water is discussed in terms of the Gibbs free energy of water–ligand exchange processes, and the electronic stabilizing factors thoroughly investigated by means of 1‐electron and 2‐electron density properties. Hexacoordinated complexes formed between iron cations and glycine molecules acting as monodentate or bidentate ligands have been chosen as targets for the current study. Results agree with experimental findings, and complexes formed with bidentate ligands are found to be more stable than those formed with monodentate ones. The larger the number of the coordinated glycine molecules the more stable is the complex. Fe(III) complexes are more stable than Fe(II) ones, but differences are small and the Fe³⁺/Fe²⁺ exchange process appears to be energetically feasible for these complexes. Formation of the second glycine–iron interaction involving the amino nitrogen in the bidentate ligands is enthalpycally unfavorable but takes place due to the large entropy rise of the process. The larger stability of Fe(III) complexes is due however to the balance between energetic and solvation terms, which is favorable to these complexes. Electron density properties account satisfactorily for the electronic energy changes along the complex formation in terms of ligand–metal electron transfer and covalent bond orders. © 2010 Wiley Periodicals, Inc. J Comput Chem 2010