Dennis J. M. Snelders

Hexacationic Dendriphos ligands in the Pd-catalyzed Suzuki-Miyaura cross-coupling reaction: scope and mechanistic studies.

The combination of Pd(2)dba(3) x CHCl(3) and hexacationic triarylphosphine-based Dendriphos ligands (1-3) leads to a highly active catalytic system in the Suzuki-Miyaura cross-coupling reaction. Under relatively mild reaction conditions, nonactivated aryl bromides and activated aryl chlorides can be coupled at a low Pd loading (0.1 mol %). The observed activity of this catalytic system, in particular in coupling reactions of aryl chlorides, is dramatically higher than that of conventional Pd catalysts employing triarylphosphine ligands. Through control and poisoning experiments, it is concluded that a homogeneous Pd(0)-Dendriphos complex is the active species in this catalytic system. Despite their triarylphosphine-based structure, Dendriphos ligands behave as very bulky phosphine ligands and lead to a preferential formation of coordinatively unsaturated and catalytically active Pd(0) species, which explains the observed high catalytic activity for these systems. The presence of six permanent cationic charges in the backbone of this class of ligands is proposed to result in a significant interligand Coulombic repulsion and plays a crucial role in their bulky behavior. In the coupling reactions of activated aryl chlorides, a positive dendritic kinetic effect was observed among the different Dendriphos generations, indicating an increased ability of the higher ligand generations to stabilize the active species due to steric effects. For aryl bromides, no dendritic effect was observed due to a shift in the rate-determining step in the catalytic cycle, from oxidative addition for aryl chlorides to transmetalation for aryl bromides.

Steric, Electronic, and Secondary Effects on the Coordination Chemistry of Ionic Phosphine Ligands and the Catalytic Behavior of Their Metal Complexes

The effects of introducing ionic functionalities in phosphine ligands on the coordination chemistry of these ligands and the catalytic behavior of the corresponding metal complexes are reviewed. The steric and electronic consequences of such functionalizations are discussed. Apart from these steric and electronic effects, the presence of charged groups often leads to additional, supramolecular interactions that occur in the second coordination sphere of the metal complex, such as intramolecular, interligand hydrogen bonding and Coulombic repulsion. These interactions can significantly alter the behavior of the phosphine ligand in question. Such effects have been observed in phosphine-metal association/dissociation equilibria, ligand substitution reactions, and stereoisomerism in phosphine-metal complexes. By drawing general conclusions, this review offers an insight into the coordination and catalytic behavior of phosphine ligands containing ionic functionalities and their corresponding metal complexes.

Ruthenium(II)-Catalyzed Hydrogen Generation from Formic Acid using Cationic, Ammoniomethyl-Substituted Triarylphosphine Ligands

The selective catalytic decomposition of formic acid into hydrogen and carbon dioxide has been achieved in water under mild conditions. For the first time, a ruthenium ion in combination with a series of oligocationic, ammoniomethyl‐substituted triarylphosphines was used for this reaction, as opposed to previously used anionic and neutral ligands. These cationic phosphines vary in size and charge and therefore have different hydrophilic, steric, and electronic properties. Excellent catalytic activities were achieved in the formic acid dehydrogenation reaction and correlations between the activity and the ligand structure were made. High turnover frequencies (TOFs) of 1950 h−1 and turnover numbers (TONs) over 10 000 were obtained through optimization of the catalytic system.

Efficient synthesis of β-chlorovinylketones from acetylene in chloroaluminate ionic liquids.

A method for the Friedel-Crafts-type insertion reaction of acetylene with acid chlorides in chloroaluminate ionic liquids is presented. The use of ionic liquids not only serves to avoid the use of carbon tetrachloride or 1,2-dichloroethane but also suppresses side reactions, notably the polymerization of acetylene, which occurs in these chlorinated solvents. Consequently, the products can be isolated using a simpler purification procedure, giving a range of aromatic and aliphatic β-chlorovinyl ketones in high yield and purity.

Structure–activity relationships of oligocationic, ammonium-functionalized triarylphosphines as ligands in the Pd-Catalyzed Suzuki–Miyaura reaction

A series of oligocationic triarylphosphine ligands, containing a varying number (from 2 to 6) of meta‐ammoniomethyl substituents, have been synthesized, characterized, and tested as ligands in the Pd‐catalyzed Suzuki–Miyaura cross‐coupling reaction. By systematic catalytic investigations and spectroscopic (NMR and UV/Vis) studies of these ligands in the palladium(0) complexes as well as of their neutral amine analogues and of triphenylphosphine, structure–activity relations have been established. The substitution of a triarylphosphine core with cationic meta‐ammoniomethyl substituents increases the steric demand (cone angle) of the phosphine ligand and decreases its σ‐donation strength. Furthermore, Coulombic inter‐ligand repulsion forces are introduced. The catalytic activity in the Suzuki–Miyaura reaction within this class of ligands is governed by the number of cationic charges in the ligand structure, rather than by their steric properties or their σ‐donating strength. With an increasing number of charges, the preference for the formation of coordinatively unsaturated phosphine–palladium species increases, which leads to a higher catalytic activity through faster catalyst activation.

A new, easily recyclable arylating agent based on a diphosphino-digold(I) complex.

The synthesis of two organogold(I) complexes, [(Au(NCN))2(dppbp)] (6) and [(Au(Phebox))2(dppbp)] (9), and their application in subsequent transmetalating reactions are described. A trinuclear organogold(I) complex, [(AuCl)3(tdpppb)] (4) is also reported, which exhibits a surprisingly high solubility in dichloromethane. It was found that 6 and 9 can cleanly transfer the anionic NCN-([C(6)H(3)(CH(2)NMe(2))2-2,6]-) or Phebox-([2,6-bis(oxazolinyl)phenyl]-) moiety to Ti(IV) and Pd(II) centers, respectively. The coproduct [(AuCl)2(dppbp)] (3, dppbp is [4-Ph(2)PC(6)H(4)]2 (1)) formed during this transmetalation reaction, precipitates almost quantitatively from the reaction mixture (toluene) and can thus be separated by simple filtration. In comparison, [AuCl(PPh3)], formed as the coproduct in the transmetalation reaction of [Au(NCN)(PPh3)] with metal salts, has a higher solubility in apolar solvents and thus is more difficult to separate from the resultant organometallic complex. Digold complex 6 has been characterized by NMR spectroscopy and crystallographic analyses. These analyses show that the two gold units are essentially independent. The formation of a dimetallic transmetalating agent based on gold(I) had no effect on its transmetalating properties.