Tridentate Phosphine Ligands Research Articles

Pinwheel-like tridentate phosphines for controlling divergent regioselectivity in Pd-catalyzed alkoxycarbonylation of alkenes

The elaborate control of regioselectivity regulated by the involved ligands with specific structural skeleton and compositions is always demanded in alkoxycarbonylation of alkenes. Herein, a series of pinwheel-like tridentate phosphine ligands with rotatable flexibility (L3) or hard rigidity (L5–L7) were synthesized and characterized for the first time, in which the incorporated three PPh2-fragments are located at meta-positions of phenyl groups. It was proved that the tridentate phosphine of L3 with the flexible configuration favorably corresponded to the linear regioselectivity with l/b ratio of 98/2, whereas L7 with the rigid configuration absolutely did favor to the branched regioselectivity with l/b ratio of 1/99. The presence of the three PPh2-frgaments in L3 did contribute to the remarkable performance of L3-Pd(TFA)2 system in terms of activity, linear regioselectivity and stability, subjected to 10-run recycling uses in anti-Markovnikov’s methoxycarbonylation of styrene without activity loss and Pd-black precipitation. In comparison, although the optimal L7-Pd(MeCN)2Cl2 system could catalyze Markovnikov’s alkoxycarbonylation of alkenes to afford the branched esters with excellent regioselectivity in the fresh use, its stability was too poor to support the recycling experiments. L3-Pd(TFA)2 and L7-Pd(MeCN)2Cl2 catalytic systems were both applicable to a wide range of substrates in alkoxycarbonylation of alkenes to afford the target esters in the isolated yields of 41–91 %, with the well maintained divergent regioselectivity.

Reaction mechanism of decomposition of phosphine-bridged Pd(I)-Pd(I) dimers into Pd(II) monomers by molecular iodine

Mechanistic studies on decomposition of Pd(I)-Pd(I) dimers via Pd(II)-Pd(II) intermediates into Pd(II) monomers were performed by the reaction of [Pd2I2(dppm)2] (1) (dppm = bis(diphenylphosphino)methane) and [Pd2(p3)2](BF4)2 (2) (p3 = bis(diphenylphosphinoethyl)phenylphosphine) with molecular iodine, where dppm and p3 are bridging “short-bite” bidentate and “regular-bite” tridentate phosphine ligands, respectively. It was previously reported that the decomposition rate constant for the Pd(II)-Pd(II) intermediate of 1, [Pd2I4(dppm)2], to give the monomeric [PdI2(dppm)] was independent of the I2 concentration under the pseudo-first-order conditions with excess I2. On the other hand, we have found that the decomposition was retarded by an addition of I2, which reduces the nucleophilicity and donating ability of the iodido ligand by association of I2 to give the I-–I2 interaction. The activation parameters of the decomposition with a large negative ΔS‡ value similar to substitution reactions of usual four-coordinate squarer-planar palladium(II) complexes suggested the associative intramolecular substitution mechanism. The oxidation of 2 to the Pd(II)-Pd(II) intermediate, [Pd2I2(p3)2]2+, and its decomposition into the monomeric [PdI(p3)]+ by I2 were also investigated kinetically and both reactions were also retarded with an increase in the I2 concentration. From the activation parameters of the intramolecular decomposition, it was indicated that the associative character was reduced because of breakdown of the sterically restricted framework. Such different properties in solution of 1 and 2 were applied complementarily to Heck-type C–C coupling reaction and dehalogenation reaction of aryl halide, respectively.

Photo- and Temperature-Induced Reversible Structural Transformation between Dodecanuclear and Pentadecanuclear Gold(I) Sulfido Complexes.

Stimuli-responsive structural transformation has attracted much attention for its potential to mimic the behavior of biological transformations and functions. Here, two unprecedented dodecanuclear and pentadecanuclear gold(I) sulfido clusters (denoted trans-Au12 and trans-Au15, respectively) with impressive stimuli-responsive interconversion have been obtained by taking advantage of the judiciously designed tridentate phosphine ligand Ltrans as the building block. Both UV light and temperature can be applied to trigger the structural conversions between trans-Au12 and trans-Au15. In addition, NMR, high-resolution electrospray ionization mass spectrometry, and UV-vis absorption spectroscopy have been employed to monitor the transformation process and decipher the mechanism of structural conversion. This work not only provides a paradigm to investigate photo-induced cluster-to-cluster transformation based on polydentate phosphine ligands but also offers a new direction for the construction of the stimuli-responsive materials.

Palladium(ii)-complexes of bi- and tri-dentate phosphine ligands: precursor for palladium–phosphorous nanoparticles and activity towards Suzuki–Miyaura coupling

Pd7P3/Pd3P0.8 NPs and their precursor complexes were investigated for Suzuki–Miyaura coupling, which involves both homogeneous and heterogeneous catalytic processes.

Late 3d Metal-Catalyzed (Cross-) Dimerization of Terminal and Internal Alkynes.

This review will outline the recent advances in chemo-, regio-, and stereoselective (cross-) dimerization of terminal alkynes to generate 1,3-enynes using different types of iron and cobalt catalysts with altering oxidation states of the active species. In general, the used ligands have a crucial effect on the stereoselectivity of the reaction; e.g., bidentate phosphine ligands in cobalt catalysts can generate the E-configured head-to-head dimerization product, while tridentate phosphine ligands can generate either the Z-configured head-to-head dimerization product or the branched head-to-tail isomer. Furthermore, the hydroalkynylation of silyl-substituted acetylenes as donors to internal alkynes as acceptors will be discussed using cobalt and nickel catalysts.

A Chatt-Type Catalyst with One Coordination Site for Dinitrogen Reduction to Ammonia.

With [Mo(N2)(P2 MePP2 Ph)] the first Chatt‐type complex with one coordination site catalytically converting N2 to ammonia is presented. Employing SmI2 as reductant and H2O as proton source 26 equivalents of ammonia are generated. Analogous Mo0‐N2 complexes supported by a combination of bi‐ and tridentate phosphine ligands are catalytically inactive under the same conditions. These findings are interpreted by analyzing structural and spectroscopic features of the employed systems, leading to the conclusion that the catalytic activity of the title complex is due to the strong activation of N2 and the unique topology of the pentadentate tetrapodal (pentaPod) ligand P2 MePP2 Ph. The analogous hydrazido(2‐) complex [Mo(NNH2)(P2 MePP2 Ph)](BArF)2 is generated by protonation with HBArF in ether and characterized by NMR and vibrational spectroscopy. Importantly, it is shown to be catalytically active as well. Along with the fact that the structure of the title complex precludes dimerization this demonstrates that the corresponding catalytic cycle follows a mononuclear pathway. The implications of a PCET mechanism on this reactive scheme are considered.

Reductive cyanation of organic chlorides using CO2 and NH3 via Triphos\u2013Ni(I) species

Cyano-containing compounds constitute important pharmaceuticals, agrochemicals and organic materials. Traditional cyanation methods often rely on the use of toxic metal cyanides which have serious disposal, storage and transportation issues. Therefore, there is an increasing need to develop general and efficient catalytic methods for cyanide-free production of nitriles. Here we report the reductive cyanation of organic chlorides using CO2/NH3 as the electrophilic CN source. The use of tridentate phosphine ligand Triphos allows for the nickel-catalyzed cyanation of a broad array of aryl and aliphatic chlorides to produce the desired nitrile products in good yields, and with excellent functional group tolerance. Cheap and bench-stable urea was also shown as suitable CN source, suggesting promising application potential. Mechanistic studies imply that Triphos-Ni(I) species are responsible for the reductive C-C coupling approach involving isocyanate intermediates. This method expands the application potential of reductive cyanation in the synthesis of functionalized nitrile compounds under cyanide-free conditions, which is valuable for safe synthesis of (isotope-labeled) drugs.

Novel carborane complexes of ruthenium with tridentate phosphine ligands: Synthesis and application in Atom Transfer Radical Polymerization

Novel complexes of ruthenium with tridentate phosphine ligands: tris(diphenylphosphinomethyl)ethane and bis(2-diphenylphosphinoethyl)phenylphosphine were synthethised starting from the well-known exo-nido-ruthenacarborane [exo-5,6,10-{Ru(Ph3P)2Cl}-5,6,10-(μ-H)3-10-H-nido-7,8-C2B9H8] via ligand substitution followed by exo-nido- to closo-rearrangement in toluene. The novel compounds were characterized by NMR or EPR spectrometry, MALDI TOF mass-spectrometry and cyclic voltammetry. A single-crystal X-ray diffraction study confirmed closo-structure of the obtained complexes and the presence of η3-coordinated triphosphine ligands in it structures. The correlations between ligand structure and complex stability and reactivity were established. The possibility of application of ruthenacarboranes with tridentate phosphine ligands in Atom Transfer Radical Polymerization (ATRP) was explored. It was found out that complexes based on tris(diphenylphosphino)methane are the most suitable catalysts.

Synthesis and characterization of tridentate phosphine ligands incorporating long methylene chains and ethoxysilane groups for immobilizing molecular rhodium catalysts

The new tridentate phosphine ligands EtOSi((CH2)nPPh2)3 and O[Si((CH2)nPPh2)3]2 (n = 4, 7, 11), as well as their precursor ethoxysilanes EtOSi((CH2)mCH = CH2)3 and disiloxanes O[Si((CH2)mCH = CH2)3]2 (m = 2, 5, 9) and EtOSi((CH2)11PPh2RhClCOD)3 (COD = cyclooctadiene) have been synthesized and fully characterized. The ethoxysilane- and disiloxane-containing phosphine ligands have been immobilized on silica via covalent siloxane bonds. The immobilized linkers have been characterized by 31P and 29Si CP/MAS and HRMAS NMR spectroscopy. The covalent siloxane bonds between the linkers and the silica surface prevent translational mobility of the phosphines. Immobilized Wilkinson-type Rh hydrogenation catalysts have been obtained by ligand exchange of ClRh(PPh3)3 and ClRhpyCOD (py = pyridine) with the surface-bound ligands. The single crystal X-ray structure of ClRhpyCOD has been reported. The activities and lifetimes have been studied for the hydrogenation of 1-dodecene. The new catalysts are highly active and they can be recycled up to 15 times without major loss of activity. Within the first hours of the catalytic reaction the initially molecular complexes form Rh nanoparticles with a narrow size distribution on the surface. The nanoparticles do not leach into the supernatant solution and are not air-sensitive.

The synthesis of 3,3,3-(Ph2P)3CH-closo-3,1,2-RuC2B9H11, a novel ruthenacarborane complex with a chelate tridentate ligand

A closo-metallacarborane complex of ruthenium(ii), 3,3,3-(Ph2P)3CH-closo-3,1,2-Ru-C2B9H11, is synthesized and structurally characterized. It is the first example of the metallacarborane clusters with chelate tridentate phosphine ligands. Quantum chemical calculations were carried out to evaluate the possibility of dissociation of one of the Ru-P bonds in this molecule with formation of reactive 16-electron species. According to the cyclic voltammetry study, the complex undergoes irreversible oxidation.

Enol Ester Synthesis via Cobalt-Catalyzed Regio- and Stereoselective Addition of Carboxylic Acids to Alkynes

A cobalt-catalyzed highly regio- and stereoselective hydro-oxycarbonylation of alkynes is reported. Both terminal and internal alkynes can react with carboxylic acids to afford enol esters in high yields. The catalyst generated from Co(BF4)2, tridentate phosphine ligand L5, and zinc in situ exhibits much higher reactivity than the corresponding cobalt/diphosphine complex.

Selective formation of a two-dimensional coordination polymer based on a tridentate phospholane ligand and gold(i).

The tridentate phosphine ligand 1,3,5-tris[(E)-(4-phospholano-2,6-diethyl)styryl]benzene (1) reacts with [AuCl(tht)] (tht = tetrahydrothiophene) independent of the stoichiometry employed with selective formation of a two-dimensional coordination polymer (ratio 1 : 1) with the gold(i) cations in a trigonal-planar [3 + 1] coordination geometry. Each of the three coordinating phosphine units originates from another ligand, thus forming a polymeric structure.

Iron-Catalyzed Ortho C-H Methylation of Aromatics Bearing a Simple Carbonyl Group with Methylaluminum and Tridentate Phosphine Ligand.

Iron-catalyzed C-H functionalization of aromatics has attracted widespread attention from chemists in recent years, while the requirement of an elaborate directing group on the substrate has so far hampered the use of simple aromatic carbonyl compounds such as benzoic acid and ketones, much reducing its synthetic utility. We describe here a combination of a mildly reactive methylaluminum reagent and a new tridentate phosphine ligand for metal catalysis, 4-(bis(2-(diphenylphosphanyl)phenyl)phosphanyl)-N,N-dimethylaniline (Me2N-TP), that allows us to convert an ortho C-H bond to a C-CH3 bond in aromatics and heteroaromatics bearing simple carbonyl groups under mild oxidative conditions. The reaction is powerful enough to methylate all four ortho C-H bonds in benzophenone. The reaction tolerates a variety of functional groups, such as boronic ester, halide, sulfide, heterocycles, and enolizable ketones.

ChemInform Abstract: A Mild and Base‐Free Protocol for the Ruthenium‐Catalyzed Hydrogenation of Aliphatic and Aromatic Nitriles with Tridentate Phosphine Ligands.

AbstractThe optimal catalyst combination RUC/TDP is evaluated and applied to various aliphatic and aromatic nitriles under conditions A), mostly at 50 °C.

ChemInform Abstract: Direct Hydrogenation of Biobased Carboxylic Acids Mediated by a Nitrogen‐Centered Tridentate Phosphine Ligand.

AbstractNine biogenic acids are hydrogenated under formation of alcohols, lactones or esters in high yields.

A Mild and Base‐Free Protocol for the Ruthenium‐Catalyzed Hydrogenation of Aliphatic and Aromatic Nitriles with Tridentate Phosphine Ligands

AbstractA novel protocol for the general hydrogenation of nitriles in the absence of basic additives is described. The system is based on the combination of [Ru(cod)(methylallyl)2] (cod=cyclooctadiene) and L2. A variety of aromatic and aliphatic nitriles is hydrogenated under mild conditions (50 °C and 15 bar H2) with this system. Kinetic studies revealed higher activity in the case of aromatic nitriles compared with aliphatic ones.

ChemInform Abstract: Hydrogenation of Carboxylic Acids with a Homogeneous Cobalt Catalyst.

AbstractVarious carboxylic acids and esters are reduced to the corresponding alcohols using a combination of Co(BF4)2·6H2O and a tridentate phosphine ligand as homogeneous catalyst.

Direct Hydrogenation of Biobased Carboxylic Acids Mediated by a Nitrogen-centered Tridentate Phosphine Ligand.

A novel nitrogen-centered tridentate ligand was identified from a series of multidentate ligands and applied for the direct hydrogenation of 9 biogenic acids into alcohols, lactones and esters with high yields. Comparison of substrates and ruthenium precursors suggested that the Ru(II) hydride cationic species was more active to transform acids than the corresponding lactone or esters.

Hydrogenation of carboxylic acids with a homogeneous cobalt catalyst.

The reduction of esters and carboxylic acids to alcohols is a highly relevant conversion for the pharmaceutical and fine-chemical industries and for biomass conversion. It is commonly performed using stoichiometric reagents, and the catalytic hydrogenation of the acids previously required precious metals. Here we report the homogeneously catalyzed hydrogenation of carboxylic acids to alcohols using earth-abundant cobalt. This system, which pairs Co(BF4)2·6H2O with a tridentate phosphine ligand, can reduce a wide range of esters and carboxylic acids under relatively mild conditions (100°C, 80 bar H2) and reaches turnover numbers of up to 8000.

Ru(II)-Triphos Catalyzed Amination of Alcohols with Ammonia via Ionic Species

An active and selective system for the amination of primary alcohols to primary amines with ammonia based on ruthenium and triphos as the tridentate phosphine ligand was developed. On the basis of detailed mechanistic studies, we propose that the active catalyst is, unlike the previously reported systems on this reaction, a cationic ruthenium complex. The experimental findings are supported by detailed density functional theory (DFT) calculations on the catalytic cycle. Because of the cationic nature of the active catalyst, strong anion and solvent effects were observed in the catalytic amination reaction when using the ruthenium triphos complexes. Therefore, a higher activity could be achieved when the nonpolar solvent toluene is used in this amination instead of tetrahydrofuran. Our findings can help to develop and optimize the system systematically for an application to relevant target molecules.