Structures, Mechanisms, and Results in Bifunctional Catalysis and Related Species Involving Proton Transfer

Isomerization of Functionalized Olefins Using the Dinuclear Catalyst [PdI(μ-Br)(PtBu3)]2: A Mechanistic Study

In a combined experimental and computational study, the isomerization activity of the dinuclear palladium(I) complex [Pd^I(μ‐Br)(P^tBu₃)]₂ towards allyl arenes, esters, amides, ethers, and alcohols has been investigated. The calculated energy profiles for catalyst activation for two alternative dinuclear and mononuclear catalytic cycles, and for catalyst deactivation are in good agreement with the experimental results. Comparison of experimentally observed E/Z ratios at incomplete conversion with calculated kinetic selectivities revealed that a substantial amount of product must form via the dinuclear pathway, in which the isomerization is promoted cooperatively by two palladium centers. The dissociation barrier towards mononuclear Pd species is relatively high, and once the catalyst enters the energetically more favorable mononuclear pathway, only a low barrier has to be overcome towards irreversible deactivation.

Optimizing ligand structure for low-loading and fast catalysis for alkynyl-alcohol and -amine cyclization

The catalytic performance was evaluated for a series of [Ru(Cp/Cp*)(P^R₂N^R′₂)(MeCN)]PF₆ complexes, in which the steric and electronic properties of the primary coordination sphere were varied (R = Ph, t-Bu, Bn; and Cp vs. Cp*).

Enhanced Catalytic Activity of Iridium(III) Complexes by Facile Modification of C,N-Bidentate Chelating Pyridylideneamide Ligands

A set of aryl-substituted pyridylideneamide (PYA) ligands with variable donor properties owing to a pronounced zwitterionic and a neutral diene-type resonance structure were used as electronically flexible ligands at a pentamethylcyclopentadienyl (Cp*) iridium center. The straightforward synthesis of this type of ligand allows for an easy incorporation of donor substituents such as methoxy groups in different positions of the phenyl ring of the C,N-bidentate chelating PYA. These modifications considerably enhance the catalytic activity of the coordinated iridium center toward the catalytic aerobic transfer hydrogenation of carbonyls and imines as well as the hydrosilylation of phenylacetylene. Moreover, these PYA iridium complexes catalyze the base-free transfer hydrogenation of aldehydes, and to a lesser extent also of ketones. Under standard transfer hydrogenation conditions including base, aldehydes are rapidly oxidized to carboxylic acids rather than reduced to the corresponding alcohol, as is observed under base-free conditions.

Hydrophenoxylation of internal alkynes catalysed with a heterobimetallic Cu-NHC/Au-NHC system

A dual catalytic process based on a heterobimetallic system, consisting of Cu-NHC and Au-NHC complexes, was shown to be highly active and selective for the hydrophenoxylation of internal alkynes.

Transition metal complexes of the pyridylphosphine ligand o-C6H4(CH2PPy2)2

The synthesis and coordination behaviour of the pyridylphosphine ligand o-C₆H₄(CH₂PPy₂)₂ (Py = 2-pyridyl) are reported. The phosphine selenide was synthesised and the ¹J_PSe value of 738 Hz indicates the phosphorus atoms have a similar basicity to PPh₃. The ligand reacts with platinum(ii) and palladium(ii) complexes to give simple diphosphine complexes of the type [MX₂(PP)] (M = Pt, X = Cl, I, Me, Et; M = Pd, X = Cl, Me). When the ligand is reacted with chloromethyl(hexa-1,5-diene)platinum the [PtClMe(PP)] complex results, from which a series of unsymmetrical platinum complexes of the type [PtMeL(PP)]⁺ (L = PPh₃, PTA, SEt₂ and pyridine) can be made. This enabled the comparison of the cis and trans influences of a range of ligands. The following cis influence series was compiled based on ³¹P NMR data of these complexes: Py ≈ Cl > SEt₂ > PTA > PPh₃. Reaction of [PtClMe(PP)] with NaCH(SO₂CF₃)₂ and carbon monoxide slowly formed an acyl complex, where the CO had inserted in the Pt-Me bond. Attempts to achieve P,P,N chelation, through abstracting the chloride ligand in [PtClMe(PP)], were unsuccessful. When the ligand reacted with platinum(0), palladium(0) and silver(i) complexes the bis-chelated complexes [M(PP)₂] (M = Pt, Pd) and [Ag(PP)₂]⁺ were formed respectively. Reaction of the ligand with [Ir(COD)(μ-Cl)]₂ formed [IrCl(PP)(COD)]. When the chloride ligand was abstracted, the pyridyl nitrogens were able to interact with the iridium centre facilitating the isomerisation of the 1,2,5,6-η⁴-COD ligand to a 1-κ-4,5,6-η³-C₈H₁₂ ligand. The X-ray crystal structure of [Ir(1-κ-4,5,6-η³-C₈H₁₂)(PPN)]BPh₄ confirmed the P,P,N chelation mode of the ligand. In solution, this complex displayed hemilabile behaviour, with the pyridyl nitrogens exchanging at a rate faster than the NMR time scale at room temperature.

Stereoselective Gold(I)-Catalyzed Intermolecular Hydroalkoxlation of Alkynes

We report the use of cationic gold complexes [Au(NHC)(CH3CN)][BF4] and [{Au(NHC)}2(μ-OH)][BF4] (NHC = N-heterocyclic carbene) as highly active catalysts in the solvent-free hydroalkoxylation of internal alkynes using primary and secondary alcohols. Using this simple protocol, a broad range of (Z)-vinyl ethers were obtained in excellent yields and high stereoselectivities. The methodology allows for the use of catalyst loadings as low as 200 ppm for the addition of primary alcohols to internal alkynes (TON = 35 000, TOF = 2188 h-1).

(1-Methyl-1H-imidazol-3-ium-2-yl)(phenyl)phosphinate monohydrate

The title compound, C₁₀H₁₁N₂O₂P·H₂O, contains a tetra­coordinate penta­valent P atom. The phosphinate group plays a predominant role in the cohesion of the crystal structure by forming chains along the b axis via inter­molecular C—H⋯O hydrogen bonds. These chains are connected by O—H⋯O and N—H⋯O hydrogen bonding involving the lattice water.