(Dihalogenmethyl)palladium(II)-Komplexe aus Palladium(O)-Vorstufen des Dibenzylidenacetons: Synthese, Strukturchemie und Reaktivität ag]

Important Role of NH-Carbazole in Aryl Amination Reactions Catalyzed by 2-Aminobiphenyl Palladacycles

2-Aminobiphenyl palladacycles are among the most successful precatalysts for Pd-catalyzed cross-coupling reactions, including aryl amination. However, the role of NH-carbazole, a byproduct of precatalyst activation, remains poorly understood. Herein, the mechanism of the aryl amination reactions catalyzed by a cationic 2-aminobiphenyl palladacycle supported by a terphenyl phosphine ligand, PCyp₂Ar^Xyl2 (Cyp = cyclopentyl; Ar^Xyl2 = 2,6-bis(2,6-dimethylphenyl)phenyl), P1, has been thoroughly investigated. Combining computational and experimental studies, we found that the Pd(II) oxidative addition intermediate reacts with NH-carbazole in the presence of the base (NaO ^t Bu) to yield a stable aryl carbazolyl Pd(II) complex. This species functions as the catalyst resting state, providing the amount of monoligated LPd(0) species required for catalysis and minimizing Pd decomposition. In the case of a reaction with aniline, an equilibrium between the carbazolyl complex and the on-cycle anilido analogue is established, which allows for a fast reaction at room temperature. In contrast, heating is required in a reaction with alkylamines, whose deprotonation involves coordination to the Pd center. A microkinetic model was built combining computational and experimental data to validate the mechanistic proposals. In conclusion, our study shows that despite the rate reduction observed in some reactions by the formation of the aryl carbazolyl Pd(II) complex, this species reduces catalyst decomposition and could be considered an alternative precatalyst in cross-coupling reactions.

A Highly Active Ylide-Functionalized Phosphine for Palladium-Catalyzed Aminations of Aryl Chlorides

Ylide-functionalized phosphine ligands (YPhos) were rationally designed to fit the requirements of Buchwald-Hartwig aminations at room temperature. This ligand class combines a strong electron-donating ability comparable to NHC ligands with high steric demand similar to biaryl phosphines. The active Pd species are stabilized by agostic C-H⋅⋅⋅Pd rather than by Pd-arene interactions. The practical advantage of YPhos ligands arises from their easy and scalable synthesis from widely available, inexpensive starting materials. Benchmark studies showed that YPhos-Pd complexes are superior to the best-known phosphine ligands in room-temperature aminations of aryl chlorides. The utility of the catalysts was demonstrated by the synthesis of various arylamines in high yields within short reaction times.

Enantioselective Construction of Tertiary Boronic Esters by Conjunctive Cross-Coupling

Catalytic enantioselective conjunctive cross-coupling has been developed to construct tertiary alkylboronic esters. These reactions occur with good yield and enantioselectivity for a range of substrates. Mechanistic experiments reveal aspects of the catalytic cycle that allow hindered substrates to react without significant complicating side reactions.

Steric and Electronic Influences of Buchwald-Type Alkyl-JohnPhos Ligands

The electron-donating and steric properties of Buchwald-type ligands ([1,1'-biphenyl-2-yl]dialkylphosphine; R-JohnPhos, where R = Me, Et, (i)Pr, Cy, (t)Bu) were determined. The π-acidity and σ-donating properties of the R-JohnPhos ligands were quantified using a Cotton-Kraihanzel analysis of the Cr(0)(CO)5(R-JohnPhos) complexes. Somewhat surprisingly, the σ-donating abilities of the R-JohnPhos ligands follow the trend (t)Bu-JohnPhos < Et-JohnPhos < (i)Pr-JohnPhos < Cy-JohnPhos ≪ Me-JohnPhos. This ordering is proposed to arise from competition between the intrinsic electron-donating ability of the R groups (Me < Et < (i)Pr ≈ Cy < (t)Bu) and steric interactions (front and back strain) that decrease the electron-donating ability of the phosphine. X-ray crystallographic data of 22 metal complexes (general forms: trans-Cr(0)(CO)4(PR3)2, Pd(0)(PR3)2(η(2)-dba), and trans-Pd(II)(Cl)2(PR3)2) were also analyzed to help explain the electronic trends measured for the R-JohnPhos ligands. The R-JohnPhos ligands are exceptionally sensitive to back strain in comparison to typical phosphines, and the strong σ-donating ability of the Methyl-JohnPhos ligand is attributed to its ability to avoid both front strain and back strain. Consequently, the -PMe2 moiety allows for very short phosphorus-metal bond distances. Because of the sterically dominating o-biphenyl and close phosphorus-metal bond distances, MeJPhos maintains a large overall steric profile that is actually larger than that of CyJPhos as measured by percent buried volume (%V(bur)). Overall, the -PMe2 moiety is a powerful way to incorporate strong σ-donation into "designer" phosphines while retaining other advantageous structural and reactivity properties.

Tandem hydrogenation and condensation of fluorinated α,β-unsaturated ketones with primary amines, catalyzed by nickel

A simple homogeneous catalytic system based on nickel phosphine complexes has been developed for the transfer hydrogenation and condensation of α,β-unsaturated ketones to yield saturated ones and saturated imines using primary amines as hydrogen donors.

Heterometallic complexes with cube-type [MTi3N4] cores containing Group 10 metals in a variety of oxidation states

The metalloligand [{Ti(η⁵-C₅Me₅)(μ-NH)}₃(μ₃-N)] is capable of stabilizing Group 10 metals in the oxidation states 0,iioriv.

The role of solvent in organometallic chemistry — Oxidative addition with dichloromethane or chloroform

Dimethylplatinum(II) complexes [PtMe2(NN)], with NN = diimine ligand, can react with dichloromethane or chloroform solvent to give the corresponding organoplatinum(IV) complexes [PtClMe2(CH2Cl)(NN)] or [PtClMe2(CHCl2)(NN)], respectively. The products can exist in isomeric forms, corresponding to products of cis or trans oxidative addition. The structures of three dichloromethane adducts and one chloroform adduct are reported.

pi-Acidic alkene ligand effects in Pd-catalysed cross-coupling processes: exploiting the interaction of dibenzylidene acetone (dba) and related ligands with Pd(0) and Pd(II)

pi-Acidic alkene (olefin) ligands positively influence Pd-catalysed cross-coupling processes, interacting with both palladium(0) and palladium(ii) species, in some cases stabilising key catalytic intermediates. Rates of oxidative addition and reductive elimination are both affected. In certain cases, beta-hydrogen elimination can be slowed down by pi-acidic alkenes, which opens up new reaction pathways (e.g. interception of sigma-alkylpalladium(ii) species by appropriate nucleophiles). pi-Acidic alkene ligands can act independently or in a synergistic fashion with another two-electron donor ligand (e.g. amine, phosphine or N-heterocyclic carbene). The purpose of this perspective article is to highlight the impressive results that can be obtained using pi-acidic alkene ligands, with a particular focus on dibenzylidene acetone (dba) derivatives. Other types of alkene ligands, e.g. macrocyclic alkenes, are also discussed.

Palladium-catalyzed addition of arylboronic acids to aldehydes

[reaction: see text] Arylboronic acids react with aldehydes in the presence of a base and a catalytic amount of a palladium(0) complex with chloroform, affording the corresponding secondary alcohols in good yields. General palladium complexes have no catalytic activity without chloroform. Chloroform is essential for this reaction, and palladium complex that was prepared from Pd(PPh3)4 with CHCl3 showed good catalytic acitivity as well.

Changes in Coordination of Sterically Demanding Hybrid Imidazolylphosphine Ligands on Pd(0) and Pd(II)

Low-coordinate organometallic complexes are important in structure and catalysis, and hemilability or secondary interactions such as hydrogen bonding enabled by hybrid ligands are receiving increasing attention. To study the factors controlling these phenomena, three new imidazol-2-ylphosphine ligands, L, were made. In these ligands, the bulk around P and the hindrance at the basic and potentially coordinating imidazole N-3 were varied. Remarkably, L(2)Pd(0) complexes 3a-c were shown to be two-coordinate, 12-electron species, despite the availability of imidazole N-3 to enter into eta(2)-P,N chelation. In oxidative additions of C-X bonds to the Pd(0) complexes, reaction rates and products could be controlled by the nature of the C and X groups and the R groups on the phosphine. Most significantly, whereas 4c-PhI and 4c-MeOTf from 3c are normal trans-bis(phosphine)Pd(R)(X) species, 5a-PhI, 5a-PhBr, and 5b-PhI from 3a and 3b were shown by X-ray diffraction to be a monomeric species with a single eta(2)-P,N-chelating phosphine. From 3a and methyl triflate, an ionic complex [6a-Me](+)[OTf](-) with one chelating and one nonchelating phosphine was formed, with temperature-dependent windshield-wiper exchange of the two, showing hemilability. Thus, large phosphine substituents (R = tert-butyl rather than isopropyl) favor chelation. The chelate Pd-imidazole N-3 bond is longer when the heterocyclic nitrogen is hindered by an adjacent tert-butyl group at C-4 (comparing 5a-PhI and 5b-PhI). Finally, whereas in [8b-Ph](+)[OTf](-) from 5b-PhI and isopropylamine, the amine coordinates without chelate opening or hydrogen bonding, in [10c-Me](+)[OTf](-) made from 4c-MeOTf and isopropylamine, the amine is not only coordinated at N but also donates a hydrogen bond to each phosphine imidazol-2-yl substituent.

π-Conjugated N-heterocyclic compounds: correlation of computational and electrochemical data

Electrochemical reduction potentials of a broad selection of nitrogen-containing molecules suitable as bridging (dipodal and tripodal) ligands in coordination and organometallic chemistry are reported and compared with results of semiempirical calculations. Trends of electrode potentials observed experimentally agree with respective calculated data, deviations can be explained by invoking peculiarities of the involved molecular orbitals and ligand-electrode surface interactions.

Unprecedented eight-palladium(i) crown-cycle with metal–metal unsupported bonds

Tri(N-pyrrolyl)phosphine reacted with the sigma/pi complex [Pd(mu-Cl)(COD-MeO)]2 to give the octa-cycle [Pd(mu-Cl)[P(pyrl)3]]8 containing four Pd(I)-Pd(I) unbridged bonds.

Oxidative addition of the imidazolium cation to zerovalent Ni, Pd, and Pt: a combined density functional and experimental study

Oxidative addition of different imidazolium cations to zerovalent group 10 metals, to afford heterocyclic carbene complexes, has been investigated by both density functional theory (DFT) and experimental studies. The theoretical analysis shows that addition of imidazoliums to Pt(0) and Ni(0) is more exothermic than to Pd(0), and Ni(0) is predicted to react with a much lower barrier than either Pt(0) or Pd(0). Strongly basic supporting ligands on the metal, as well as cis-chelating ligands, increase the exothermicity of the reaction and also lower the activation barrier. The addition of 2-H imidazoliums is easier and more exothermic than addition of 2-alkylimidazoliums, and a halo-imidazolium is expected to further lower the barrier to oxidative addition and increase the exothermicity. The DFT results show that all three of the metals should be able to oxidatively add imidazolium cations under appropriate conditions. Experimental studies confirmed that oxidative addition is possible, and a number of Pt- and Pd-carbene complexes were prepared via oxidative addition of imidazolium salts to M(0) precursors. Most significantly, oxidative addition of 2-H azolium salts was found to readily occur, and the reaction of 1,3-dimethylimidazolium tetrafluoroborate with Pt(PPh(3))(2) and Pt(PCy(3))(2) affords [PtH(dmiy)(PPh(3))(2)]BF(4) (10) and [PtH(dmiy)(PCy(3))(2)]BF(4) (11), while reaction between 3,4-dimethylthiazolium tetrafluoroborate and Pt(PCy(3))(2) yields [PtH(dmty)(PCy(3))(2)]BF(4) (12) (dmiy = 1,3-dimethylimidazolin-2-ylidene, dmty = 3,4-dimethylthiazolin-2-ylidene). Addition of 2-iodo-1,3,4,5-tetramethylimidazolium tetrafluoroborate to Pt(PPh(3))(4) or Pd(dcype)(dba) yields [PtI(tmiy)(PPh(3))(2)]BF(4) (9) and [PdI(tmiy)(dcype)]BF(4) (14), respectively (tmiy = 1,3,4,5-tetramethylimidazolin-2-ylidene, dcype = 1,3-bis(dicyclohexylphosphino)ethane)). X-ray crystal structures are reported for complexes 9 and 11 (cis and trans). These studies clearly show for the first time that oxidative addition of imidazolium and thiazolium cations is possible, and the results are discussed in terms of the ramifications for catalysis in imidazolium-based ionic liquids with both carbene-based and non-carbene-based complexes.

Stannaborate transition metal chemistry: ligand properties, reactivity, and density functional theory calculations of platinum and palladium complexes

Three stannaborate complexes of platinum(II) and a novel stannoborate palladium(II) derivative have been prepared in excellent yield. The tin transition metal bond is formed through nucleophilic substitution and the resulting complexes [Bu3MeN] [trans-[(Et3P)2Pt(SnB11H11)H]] (6), [trans-[(Et3P)2Pt(SnB11H11)(CNtBu)]] (7), [Bu3MeN]2[trans-[(Et3P)2Pt(SnB11H11)2-(CNtBu)]] (8), and [Bu3MeN][(dppe)-Pd(SnB11H11)Me] (12) (dppe = 1,2-bis-(diphenylphosphanyl)ethane) were characterized by NMR spectroscopy and elemental analysis. In the cases of the zwitterion 7, the pentacoordinated complex 9, the palladium salt 12 and [(triphos)Pt(SnB11H11)] (10) (triphos = 1,1,1-tris(diphenylphosphanylmethyl)ethane), their solid-state structures are determined by X-ray crystal structure analyses. The trans influence of the [SnB11H11] ligand is evaluated from the results of the IR spectroscopy and X-ray crystallographic structures of complexes 6, 7, and 12. The dipole moment of the zwitterion 7 is calculated by density functional theory (DFT) methods. The alignment of the dipole moments of the polar molecules 7 and 12 in the solid state is discussed.

Palladium/P,O-Ligand-Catalyzed Suzuki Cross-Coupling Reactions of Arylboronic Acids and Aryl Chlorides. Isolation and Structural Characterization of (P,O)-Pd(dba) Complex

The phenyl backbone-derived P,O-ligands 1 and 2 were investigated for their utility as ligands in palladium/ligand-catalyzed Suzuki reactions. The 2-(2'-dicyclohexylphosphinophenyl)-2-methyl-1,3-dioxolane (ligand 1) in combination with Pd(dba)(2) affords an efficient catalyst for general Suzuki reactions of a wide variety of arylboronic acids and aryl chlorides, bromides, and iodides to afford the desired biaryl products in high isolated yields. Arylboronic acids and aryl chlorides containing electron-poor, electron-rich, and ortho substituents participate effectively. In contrast, the structurally related ligand 2-(2'-dicyclohexylphosphinophenyl)-1,3-dioxolane (ligand 2) was found to be less efficient under similar conditions. The reaction of ligand 1 with Pd(dba)(2) affords the complex LPd(dba) (14, L = 1). The NMR spectroscopic and X-ray crystallographic data of complex 14 establish that ligand 1 functions as a P,O-chelating ligand in complex 14. The reaction of ligand 2 (2 equiv) with Pd(dba)(2) and excess 4-(t)()Bu-C(6)H(4)Br or the ligand displacement reaction of {Pd[P(o-tolyl)(3)](4-(t)()Bu-C(6)H(4))(&mgr;-Br)}(2) with ligand 2 affords the bis-phosphine complex L(2)Pd(4-(t)()Bu-C(6)H(4))(Br) (13, L = 2). The NMR spectroscopic data of complex 13 establish that ligand 2 in complex 13 functions as a nonchelating ligand. Thus, the higher efficiency of ligand 1 over ligand 2 in Pd/L-catalyzed Suzuki arylation of aryl chlorides can be ascribed to the ability of ligand 1 to generate and stabilize mono-phosphine P,O-chelating Pd/L intermediates, which appear to be most suitable for Suzuki arylation reactions involving certain substrates and conditions.