Eite Drent

Efficient palladium catalysts for the copolymerization of carbon monoxide with olefins to produce perfectly alternating polyketones

Abstract A class of highly efficient homogeneous palladium catalyst systems has been developed for the production of perfectly alternating copolymers of carbon monoxide with ethylene. Mixtures of carbon monoxide, ethylene and propene are converted into the corresponding alternating carbon monoxide/olefin terpolymers in which C 3 units randomly replace ethylene units along the chain. The essential features of the new catalyst systems are that they are formed by the combination of an equimolar quantity of a suitable bidentate phosphine ligand with a palladium(II) species in which the counter anions are weakly coordinating. For a series of diphenylphosphinoalkanes of general formula Ph 2 P(CH 2 ) m PPh 2 the most efficient catalyst system for the production of high-molecular-weight polyketones is that with m = 3. High rates with conversions of more than one million molecules of carbon monoxide and ethylene per palladium center are obtained. In methanol, the majority of the polymer chains produced are polyketo-esters of general formula H(CH 2 CH 2 CO) n OMe; analyses of methanol-soluble oligomer fractions shows that diesters MeOCO(CH 2 CH 2 CO) n OMe and diketones H(CH 2 CH 2 CO) n CH 2 CH 3 are also formed. Two interlinked catalytic cycles are invoked to account for the formation of polyketones with keto-ester, diester and diketone end groups.

Homogeneously catalysed isomerisation of allylic alcohols to carbonyl compounds

Abstract Isomerisation of allylic alcohols forms an elegant shortcut to carbonyl compounds in a completely atom-economical process that offers several useful applications in natural-product synthesis and in bulk chemical processes. This review focuses on the heart of isomerisation catalysis: the catalyst. Combinations of transition metals (from Group 4 to 10), ligands and reaction conditions are compared with respect to yield, turnovers, rate and selectivity. A selected number of clever solutions to synthetic problems are highlighted, such as the synthesis of enols and enolates, chiral carbonyl compounds and silyl substituted ketones. Furthermore, a general overview of the mechanisms proposed for the isomerisation of allylic alcohols is given while some catalyst systems are singled out to discuss mechanistic research.

Efficient palladium catalysts for the carbonylation of alkynes

Abstract A class of highly efficient homogeneous palladium catalysts has been developed for the carbonylation of alkynes. One application of interest is the selective production of methyl methacrylate by methoxycarbonylation of propyne. The essential feature of the new catalyst systems is that they are formed by the combination of a ligand containing a 2-pyridylphosphine moiety with a palladium(II) species and a proton source containing weakly coordinating anions. High turn-over numbers of more than 40000 mol (mol Pd) −1 h −1 and selectivities towards methyl methacrylate of up to 99.95% can be obtained under mild conditions. It is suggested that the 2-pyridylphosphine ligand plays an essential role both as a chelating PN ligand in the selectivity-determining step and as a monocoordinated ligand in the rate-determining step of the catalytic cycle.

Homogeneous catalysis by cationic palladium complexes. Precision catalysis in the carbonylation of alkynes

Abstract A class of highly efficient homogeneous palladium cationic catalysts has been developed for the carbonylation of alkynes. An interesting application is

The oxo-synthesis catalyzed by cationic palladium complexes, selectivity control by neutral ligand and anion

Abstract Catalyst systems consisting of a palladium(II) diphosphine complex with weakly or non-coordinating counterions are efficient catalysts for the hydrocarbonylation of both aliphatic and functionalized olefins. Moreover, variations of ligand, anion and/or solvent can be used to steer the reaction towards alcohols, aldehydes, ketones or oligoketones. Non-coordinating anions and arylphosphine ligands produce primarily (oligo)ketones; increasing ligand basicity or anion coordination strength shifts selectivity towards aldehydes and alcohols. For the mechanisms of the aldehyde-producing step, we propose heterolytic dihydrogen cleavage, assisted by the anion. At high electrophilicity of the palladium center, selective ketone formation is observed. The reactions described here constitute the first examples of selective formation of ketones by hydrocarbonylation of higher olefins.

Mechanism of alkyne alkoxycarbonylation at a Pd catalyst with P,N hemilabile ligands: a density functional study.

A detailed mechanism for alkyne alkoxycarbonylation mediated by a palladium catalyst has been characterised at the B3PW91-D3/PCM level of density functional theory (including bulk solvation and dispersion corrections). This transformation, investigated via the methoxycarbonylation of propyne, involves a uniquely dual role for the P,N hemilabile ligand acting co-catalytically as both an in situ base and proton relay coupled with a Pd(0) centre, allowing for surmountable barriers (highest ΔG(≠) of 22.9 kcal mol(-1) for alcoholysis). This proton-shuffle between methanol and coordinated propyne accounts for experimental requirements (high acid concentration) and reproduces observed regioselectivities as a function of ligand structure. A simple ligand modification is proposed, which is predicted to improve catalytic turnover by three orders of magnitude.

A kinetic study of the reduction of divalent copper-exchanged faujasite with butadiene and ammonia

Abstract ESR and phosphorescence spectroscopic studies have shown that in the presence of butadiene or ammonia, divalent copper-exchanged zeolites X and Y are reduced to the monovalent form. Kinetic measurements show that in all these cases the rate of reduction is second order in the divalent-copper concentration. Furthermore, the rate of reduction is found to be markedly dependent on the dehydration temperature adopted prior to reduction. A mechanism involving formation of Bronsted and Lewis acid sites is proposed to explain these observations.

Two reactions of allylic alcohols catalysed by ruthenium cyclopentadienyl complexes with didentate phosphine ligands: isomerisation and ether formation

The activity of chloro(η5-cyclopentadienyl)(didentate phosphine)ruthenium(II) complexes in the catalysis of two types of reaction with allylic alcohols is described. The isomerisation of 3-buten-2-ol to butanone (MEK) and allyl alcohol to propanal proceeds at high rates. A trend in catalytic activity is observed upon variation of the carbon-chain length in the didentate phosphine ligand: dppm<dppe<dppp<dppb. Complexes with rigid didentate phosphine ligands like cis-dppv and dppph show no activity. The second type of reaction constitutes the first example of a ruthenium-catalysed ether formation directly from allylic alcohols. Homo-coupled ethers like di-allyl ether (DAE) are easily formed as well as ethers from heterocoupling of allyl alcohol with aromatic and aliphatic alcohols. In fact, the ruthenium complexes achieve much higher turnover frequencies and turnover numbers than reported before in palladium-catalysed ether formation. Complexes with dppe and dppp in the presence of a conjugated diene switch from isomerisation to ether formation, but the new complex [RuClCp(o-MeO-dppe)] (Cp=η5-cyclopentadienyl) (3) has proven to be very active in ether formation, even in the absence of a diene. The mechanisms of the reactions have been studied by using both deuterium-labelled substrates and 31P nuclear magnetic resonance (NMR).

Palladium catalysed copolymerisation of ethene with alkylacrylates: polar comonomer built into the linear polymer chain.

Copolymerisation of ethene and alkylacrylates is catalysed by palladium modified with di(2-methoxyphenyl)phosphinobenzene-2-sulfonic acid (DOPPBS); a linear polymer is produced in which acrylate units are incorporated into the polyethylene backbone.

Ruthenium-Catalyzed Isomerization of Allylic Alcohols: Oxidation State Determines Resistance Against Diene Inhibition

The novel complex mer-[RuCl3(dmso)(phen)] (1) has been prepared and characterized by X-ray diffraction. The ruthenium center is in a distorted octahedral environment with the three chloride ions coordinated in a mer-fashion and an S-bonded dmso ligand trans to one of the phen nitrogen atoms. Electrochemical experiments show two reversible waves in acetonitrile solution, corresponding to the couples RuIII/II (0.11 V) and Ru IV/III (1.73 V). The analogous Ru II complex cis,cis-[RuCl2(dmso)2(phen)] (2) shows one reversible wave at 1.08 V, corresponding to the Ru III/II couple. Both complexes exhibit high catalytic activity in the isomerization of 3-buten-2-ol to butanone. The initial turnover frequency (TOF) for 1 in diglyme/water at 130 °C is 295 h −1 with a firstorder kini of 0.6 h −1 , while 2 reaches an initial TOF of 260 h −1 and a kini of 0.5 h −1 . A cumulative turnover number (TON) of 1025 has been obtained with 1 as a catalyst precursor. The activity of 1 and 2 has been compared to that of in situ mixtures with both Ru III and Ru II precursors. All Ru II complexes are deactivated before 100% conversion has been attained.