Bulky Phosphine Ligands Research Articles

Synthesis and characterization of new rhodium and iridium complexes with trianisylphosphine, PAn 3, and evaluation of their catalytic behavior in the homogeneous hydrogenation of cinnamaldehyde

A new family of rhodium and iridium compounds with the bulky tris( ortho-methoxyphenyl) phosphine (PAn 3) was synthesized and characterized by NMR methods. The X-ray crystal structures of RhCl(PAn 3)(COD) ( 1) and Ir[(PBz 3)(PAn 3)(COD)]PF 6 ( 4) have been determined. A stabilizing agostic interaction has been crystallographically observed in both compounds, due to the steric hindrance of the ortho-substituted phosphine ligand and its presence has been associated to the fluxional behavior shown by the complexes on the NMR timescale in solution. Iridium complexes containing PBz 3 and/or PAn 3 have been evaluated as catalyst precursors for the hydrogenation of trans-cinnamaldehyde (CNA), and their activities have also been compared to those of other iridium complexes containing bulky phosphine ligands, such as PTol 3 (tris- ortho-tolyl-phosphine). The catalytic experiments show that irrespectively of the phosphine combination, all of the evaluated catalysts prevalently hydrogenate the C C moiety. However, the product selectivity can be tuned by changing either the substrate/catalyst ratio or the phosphine ligand at the metal center. The catalyst with PAn 3 proved to be more efficient and also gave higher yields of the enol product, indicating that stereoelectronic effects are responsible for the changes in selectivity.

Palladium-Catalyzed α-Arylation of Oxindoles

A catalyst generated from Pd(dba)2 and the bulky electron-rich phosphine ligand 2-(dicyclohexylphosphino)-2',4', 6'-tri-i-propyl-1-1'-biphenyl is effective for the alpha-arylation of oxindoles. Generation of the potassium-enolates of a range of oxindoles allows coupling with aryl chlorides, bromides, and triflates. Significant variation of the substitution pattern on both the oxindole and aryl halide is possible.

Catalytic Nucleophilic Ring Closure ofπ-Allylpalladium Intermediates

The authors describe the development of a palladium-catalyzed intermolecular ­cycloaddition that involves a nucleophliic ring closure to the central carbon of a π-allylpalladium intermediate. Spiro[2,4]heptanes are obtained in good yields and moderate diastereoselectivities using small phosphite ligands. If bulky phosphine ligands are used, a selective [4+2] cycloaddition occurs, leading to exo-methylene cyclohexane products.

Unprecedented coordination chemistry of a chloro(phosphine)gold(I) complex: [(Ad2BnP)2Au][AuCl2

The reaction of (tht)AuCl (tht=tetrahydrothiophene) with the bulky phosphine ligand Ad2BnP yields [(Ad2BnP)2Au][AuCl2] (1, Ad=1-adamantyl, Bn=benzyl) as the first example of a 1:1 chloro(phosphine)gold(I) complex having an ionic structure both in solution and solid state. The crystal structure of 1 revealed that the Au(I) atom is linearly coordinated by the phosphorus atoms and effectively shielded from the surroundings by the bulky substituents of the phosphine ligand. For comparison, the analogous complex (Ad2nBuP)AuCl (2, nBu=n-butyl) has been synthesized and analysed by single crystal X-ray diffraction. In its crystal, it has a common rod-like structure known from R3PAuCl complexes with bulky phosphine ligands.

A new series of dinuclear Au(i) complexes linked by diethynylpyridine groups

A series of novel digold complexes incorporating ethynyl pyridine derivatives as a spacer unit, [(R(3)P)Au(C[triple bond]C)X(C[triple bond]C)Au(PR(3))] (R = Ph, X = 2,5-pyridine (1); R = Cy (cyclohexane), X = 2,5-pyridine (2); R = Ph, X = 2,6-pyridine (3); R = Ph, X = 2,5'-bipyridine (4); R = Ph, X = 2,6'-bipyridine (5)), has been synthesised. All the complexes have been characterised spectroscopically and the structures determined by single-crystal X-ray crystallography. The central (C[triple bond]C)(X)(C[triple bond]C) unit is essentially linear for complexes 1, 2 and 4 and kinked for complexes 3 and 5, but only in 1, with the shortest spacer group and the less bulky phosphine ligand, is there evidence of d(10)...d(10) Au...Au interactions (Au-Au 3.351(2) A). The solution UV/visible absorption and emission spectra for all the complexes are similar to those of the free ligands suggesting that the spectra are dominated by pi-pi* ligand-centred transitions and this is confirmed by DFT calculations.

Highly active iridium(i) complexes for catalytic hydrogen isotope exchange

Practically convenient methods have been developed for the preparation of new iridium complexes, possessing bulky N-heterocyclic carbene and phosphine ligands; these routinely handled complexes are highly active catalysts within directed hydrogen isotope exchange processes.

Hydrogen Activation by Unsaturated Mixed‐Metal Cluster Complexes: New Directions

There has been a renewed interest in the chemistry of hydrogen as a result of the ever-increasing global demands for energy. Recent studies have revealed new electronically unsaturated polynuclear metal complexes containing bulky ligands that exhibit a variety of reactions with hydrogen, including facile addition and elimination under mild conditions. Materials and molecules that can reversibly absorb large quantities of hydrogen are very attractive for hydrogen storage and hydrogenation catalysis. This Minireview summarizes recent studies of reactions of hydrogen with unsaturated mixed-metal cluster complexes containing platinum and bulky phosphine ligands. Some related studies on bimetallic cooperativity and the synthesis of trimetallic nanoparticles on mesoporous supports that exhibit high activity and selectivity for catalytic hydrogenations are also discussed.

Palladium-Catalyzed Suzuki−Miyaura Cross-Couplings of Aryl Tosylates with Potassium Aryltrifluoroborates

Pd-catalyzed Suzuki-Miyaura cross-coupling reaction of aryl tosylates with potassium aryl trifluoroborate in the presence of bulky and electron-rich phosphine ligand is described. In addition, a useful chemoselective coupling of an aryl chloride in the presence of a tosyloxy group was demonstrated.

Insights into Amine Binding to Biaryl Phosphine Palladium Oxidative Addition Complexes and Reductive Elimination from Biaryl Phosphine Arylpalladium Amido Complexes via Density Functional Theory

We present results on the binding of a variety amines to monoligated oxidative addition complexes of the type L1Pd(Ar)Cl, where L is 2-dicyclohexylphosphino-2',6'-dimethoxybiphenyl (SPhos, 1) or 2-dicyclohexylphosphino-2',4',6'-tri-ispropylbiphenyl (XPhos, 2). The binding of an amine to oxidative addition complexes composed of 1 and 2 is more complex than with smaller ligands as intermediate Pd(II) complexes with bulky biaryl phosphine ligands disfavor amine binding to favorable conformations of oxidative addition complexes. Additionally, thermodynamic and kinetic parameters for reductive elimination from complexes of the type L1Pd(amido)Ph (where amido = EtNH, Me2N, PhNH) are discussed. From this data, we suggest a possible mechanism for (biaryl phosphine) Pd-catalyzed amination reactions that is more intricate than previously thought.

Reductive Elimination of Ether from T-Shaped, Monomeric Arylpalladium Alkoxides

Isolated truth: The synthesis, structures, and reductive elimination chemistry of arylpalladium(II) phenoxide and alkoxide complexes with a single bulky phosphine ligand are reported. These complexes are true intermediates in palladium-catalyzed etherification of aryl halides. They undergo reductive elimination of the alkyl aryl ether directly from the isolated complex (see scheme), and the rates of these reductive eliminations are slower than those from related arylpalladium amido complexes. Ad=adamantyl.

9‐Fluorenylphosphines for the Pd‐Catalyzed Sonogashira, Suzuki, and Buchwald–Hartwig Coupling Reactions in Organic Solvents and Water

The lithiation/alkylation of fluorene leads to various 9-alkyl-fluorenes (alkyl=Me, Et, iPr, -Pr, -C18H25) in>95% yields, for which lithiation and reaction with R2PCl (R=Cy, iPr, tBu) generates 9-alkyl, 9-PR2-fluorenes which constitute electron-rich and bulky phosphine ligands. The in-situ-formed palladium-phosphine complexes ([Na2PdCl4], phosphonium salt, base, substrates) were tested in the Sonogashira, Suzuki, and Buchwald-Hartwig reactions of aryl chlorides and aryl bromides in organic solvents. The Sonogashira coupling of aryl chlorides at 100-120 degrees C leads to>90% yields with 1 mol% of Pd catalyst. The Suzuki coupling of aryl chlorides typically requires 0.05 mol% of Pd catalyst at 100 degrees C in dioxane for quantitative product formation. To carry out "green" cross-coupling reactions in water, 9-ethylfluorenyldicyclohexylphosphine was reacted in sulphuric acid to generate the respective 2-sulfonated phosphonium salt. The Suzuki coupling of activated aryl chlorides by using this water-soluble catalyst requires only 0.01 mol% of Pd catalyst, while a wide range of aryl chlorides can be quantitatively converted into the respective coupling products by using 0.1-0.5 mol% of catalyst in pure water at 100 degrees C. Difficult substrate combinations, such as naphthylboronic acid or 3-pyridylboronic acid and aryl chlorides are coupled at 100 degrees C by using 0.1-0.5 mol% of catalyst in pure water to obtain the respective N-heterocycles in quantitative yields. The copper-free aqueous Sonogashira coupling of aryl bromides generates the respective tolane derivatives in>95% yield.

Suzuki–Miyaura coupling reaction of aryl chlorides using di(2,6-dimethylmorpholino)phenylphosphine as ligand

Suzuki–Miyaura coupling was achieved on a variety of aryl chlorides by using di(2,6-dimethylmorpholino)phenylphosphine (L1) as a bulky electron-rich monoaryl phosphine ligand. We report the couplings of various chlorobenzenes and heteroaryl chlorides.

Preparation of a palladium dimer with a cobalt-containing bulky phosphine ligand: Its application in palladium catalyzed Suzuki reactions

A palladium dimer with a cobalt-containing phosphine ligand, {(μ-PPh 2CH 2PPh 2)Co 2(CO) 4(μ,η-( tBu) 2PC CC 6H 4-κC 1)Pd(μ-Cl)} 2 ( 3), was prepared from the reaction of its monomer precursor, (μ-PPh 2CH 2PPh 2)Co 2(CO) 4(μ,η-( tBu) 2PC CC 6H 4-κC 1)Pd(μ-OAc) ( 2), with LiCl. The crystal structure of 3, determined by X-ray diffraction methods, revealed a doubly chloride-bridged palladium dimeric conformation. Suzuki coupling reactions of bromobenzene with phenylboronic acid were carried out catalytically using these two novel palladium complexes 2 and 3 as catalyst precursors. Factors such as the molar ratio of substrate/catalyst, reaction temperature, base and solvent that might affect the catalytic efficiencies were investigated. As a general rule, the performance is much better by employing 3 than 2 as the catalyst precursor.

Group 6 metal carbonyl complexes of a bulky phosphine: The crystal structures of tris(trimethylsilyl)phosphine-M(0)pentacarbonyl, M = chromium, molybdenum, and tungsten

The crystal structures of M(P{Si(CH3)3}3)(CO)5, M = Cr (1), Mo (2), and W (3), have been determined. Crystal data for 1, trigonal crystal system, space group = P31, a, b = 9.2118(6) A, c = 22.416(3) A, V = 1647.3(2) A3, Z = 3; for 2, trigonal crystal system, space group = P32, a, b = 9.3394(3) A, c = 22.7375(12) A, V = 1717.56(12) A3, Z = 3; trigonal crystal system, space group = P32, a, b = 9.3147(5) A, c = 22.6955(16) A, V = 1705.33(18) A3, Z = 3. All three structures show distorted octahedral coordination environments around the metal center and exhibit especially long M–P bond distances, illustrating the unique steric and electronic properties of this bulky phosphine ligand. The 31P{1H} NMR spectra of the compounds are also reported.

Palladium-Mediated Intramolecular Aryl Amination on Furanose Derivatives: An Expedient Approach to the Synthesis of Chiral Benzoxazocine Derivatives and Tricyclic Nucleosides

Pd-catalyzed intramolecular arylamination on sugar derivatives has been accomplished by using bulky biaryl phosphine ligands. An application of this methodology on a variety of D-glucose-derived substrates, 2a-f, led to the synthesis of highly functionalized cis-fused tricyclic oxazocines, 3a-e. The products could subsequently be transformed to the optically active benzoxazocine derivative 4 and tricyclic nucleoside 6. This is the first example of the synthesis of eight-membered rings via intramolecular cycloamination of furanose derivatives, which provides a very useful method for the catalytic synthesis of medium-ring heterocycles.

Syntheses and characterizations of methylpalladium complexes bearing a biphenyl-based bulky phosphine ligand: Weak interactions suggested by NBO and QTAIM analyses

A methylpalladium chloride complex bearing a biphenyl-based bulky phosphine ligand, t Bu 2P(biphenyl-2-yl) ( 1), namely [Pd( 1)(Me)Cl] 2 (= 8), was synthesized by the reaction of (cod)Pd(Me)Cl with 1. A following reaction of 8 with AgOTf gave the corresponding triflate complex, Pd( 1)(Me)OTf (= 9). These complexes were fully characterized by NMR spectroscopy and structurally characterized by X-ray crystallographic study. In the solid state of 8, biphenyl-based phosphine ligand 1 played a role of monodentate phosphine ligand with a possible weak η 1-coordination from the phenyl ring at 2′-position of 1. On the other hand, phosphine ligand 1 in 9 showed a bidentate coordination mode consist of σ-donation of the phosphine and η 2-coordination of the phenyl ring at 1′- and 2′-positions. Natural bond orbital (NBO) and quantum theory of atoms in molecules (QTAIM) calculations also supported the existence of these weak interactions.

Bis(azido) compounds of Pd and Pt with bulky phosphine ligands (dppn=1,8-bis(diphenylphosphino)naphthalene, dppf=1,1′-bis(diphenylphosphino)ferrocene, 1-dpn=1-diphenylphosphino-naphthalene): Preparation, structures, and reactivity toward isocyanides

Pd-bis(azido) compounds [Pd(dppn)(N 3) 2] and [Pd(dppf)(N 3) 2], which contain bulky chelating bis(phosphine) ligands (dppn=1,8-bis(diphenylphosphino)naphthalene, dppf=1,1′-bis(diphenylphosphino)ferrocene), were prepared from the corresponding chlorides and NaN 3. We also prepared the Pt-bis(azido) compound [Pt(1-dpn)(SMe 2)(N 3) 2] containing a bulky monodentate phosphine (1-dpn=1-diphenylphosphino-naphthalene). All these compounds underwent [2+3] cycloaddition with isocyanides (R–NC, R=cyclohexyl, tert-butyl, 2,6-dimethylphenyl) to convert azido ligands to five-membered, C-coordinated tetrazolate rings. In addition, we observed the [Pd(dppn)Cl 2]-mediated C–C coupling of PhC CH to generate the η 2-PhC C–C CPh ligand. All compounds have been structurally characterized by X-ray diffraction.

Syntheses, Structural Determinations of [Ru(ROCS2)2(PPh3)2]+/0 Pairs, and Kinetic Analyses of Thermal Reactions Involving Transient trans-[Ru(iPrOCS2)2(PPh3)2] Species (ROCS2- = Ethyl- or Isopropyldithiocarbonate and PPh3 = Triphenylphosphine)

Controlled-potential electrochemical oxidation of cis-[Ru(ROCS2)2(PPh3)2] (R = Et, iPr) yielded corresponding Ru(III) complexes, and the crystal structures of cis-[Ru(ROCS2)2(PPh3)2] and trans-[Ru(ROCS2)2(PPh3)2](PF6) were determined. Both pairs of complexes exhibited almost identical coordination structures. The Ru-P distances in trans-[Ru(III)(ROCS2)2(PPh3)2](PF6) [2.436(3)-2.443(3) A] were significantly longer than those in cis-[Ru(II)(ROCS2)2(PPh3)2] [2.306(1)-2.315(2) A]: the smaller ionic radius of Ru(III) than that of Ru(II) stabilizes the trans conformation for the Ru(III) complex due to the steric requirement of bulky phosphine ligands while mutual trans influence by the phosphine ligands induces significant elongation of the Ru(III)-P bonds. Cyclic voltammograms of the cis-[Ru(ROCS2)2(PPh3)2] and trans-[Ru(ROCS2)2(PPh3)2]+ complexes in dichloromethane solution exhibited typical dual redox signals corresponding to the cis-[Ru(ROCS2)2(PPh3)2](+/0) (ca. +0.15 and +0.10 V vs ferrocenium/ferrocene couple for R = Et and iPr, respectively) and to trans-[Ru(ROCS2)2(PPh3)2](+/0) (-0.05 and -0.15 V vs ferrocenium/ferrocene for R = Et and iPr, respectively) couples. Analyses on the basis of the Nicholson and Shain's method revealed that the thermal disappearance rate of transient trans-[Ru(ROCS2)2(PPh3)2] was dependent on the concentration of PPh3 in the bulk: the rate constant for the intramolecular isomerization reaction of trans-[Ru(iPrOCS2)2(PPh3)2] was determined as 0.338 +/- 0.004 s(-1) at 298.3 K (deltaH* = 41.8 +/- 1.5 kJ mol(-1) and deltaS* = -114 +/- 7 J mol(-1) K(-1)), while the dissociation rate constant of coordinated PPh3 from the trans-[Ru(iPrOCS2)2(PPh3)2] species was estimated as 0.113 +/- 0.008 s(-1) at 298.3 K (deltaH* = 97.6 +/- 0.8 kJ mol(-1) and deltaS* = 64 +/- 3 J mol(-1) K(-1)), by monitoring the EC reaction (electrode reaction followed by chemical processes) at different concentrations of PPh3 in the bulk. It was found that the trans to cis isomerization reaction takes place via the partial dissociation of iPrOCS2(-) from Ru(II), contrary to the previous claim that it takes place by the twist mechanism.

(Z)‐Selective Cross‐Dimerization of Arylacetylenes with Silylacetylenes Catalyzed by Vinylideneruthenium Complexes.

AbstractFor Abstract see ChemInform Abstract in Full Text.

Rhodium-Catalyzed Addition of Alkynes to Activated Ketones and Aldehydes

[reaction: see text] The rhodium-catalyzed addition of alkynes to 1,2-diketones, 1,2-ketoesters, and aldehydes provides a method for the synthesis of tertiary alkynyl alcohols under mild conditions. The reaction tolerates many functional groups (such as carboxylic acids) that are incompatible with other methods. The alkyne addition reaction proceeds best using bulky phosphine ligands such as 2-(di-tert-butylphosphino)biphenyl. This method fills a void in the more common zinc-catalyzed processes, which give poor yields with enolizable 1,2-dicarbonyl substrates.