The Mechanism of the Hydroalkoxycarbonylation of Ethene and Alkene–CO Copolymerization Catalyzed by PdII–Diphosphine Cations

Competing Mechanisms in Palladium-Catalyzed Alkoxycarbonylation of Styrene

Palladium-catalyzed carbonylation is a versatile method for the synthesis of various aldehydes, esters, lactones, or lactams. Alkoxycarbonylation of alkenes with carbon monoxide and alcohol produces either saturated or unsaturated esters as a result of two distinct catalytic cycles. The existing literature presents an inconsistent account of the procedures favoring oxidative carbonylation products. In this study, we have monitored the intermediates featured in both catalytic cycles of the methoxycarbonylation of styrene PhCH=CH2 as a model substrate, including all short-lived intermediates, using mass spectrometry. Comparing the reaction kinetics of the intermediates in both cycles in the same reaction mixture shows that the reaction proceeding via alkoxy intermediate [PdII]-OR, which gives rise to the unsaturated product PhCH=CHCO2Me, is faster. However, with an advancing reaction time, the gradually changing reaction conditions begin to favor the catalytic cycle dominated by palladium hydride [PdII]-H and alkyl intermediates, affording the saturated products PhCH2CH2CO2Me and PhCH(CO2Me)CH3 preferentially. The role of the oxidant proved to be crucial: using p-benzoquinone results in a gradual decrease of the pH during the reaction, swaying the system from oxidative conditions toward the palladium hydride cycle. By contrast, copper(II) acetate as an oxidant guards the pH within the 5-7 range and facilitates the formation of the alkoxy palladium complex [PdII]-OR, which favors the oxidative reaction producing PhCH=CHCO2Me with high selectivity. Hence, it is the oxidant, rather than the catalyst, that controls the reaction outcome by a mechanistic switch. Unraveling these principles broadens the scope for developing alkoxycarbonylation reactions and their application in organic synthesis.

(In situ) spectroscopic studies on state-of-the-art Pd(ii) catalysts in solution for the alkoxycarbonylation of alkenes

(In situ) liquid-phase spectroscopic investigations on state-of-the-art Pd catalysts modified with pyridyl-substituted diphosphine ligands for alkene alkoxycarbonylations have been performed for characterizing resting state complexes in solution.

Room-temperature Pd-catalyzed methoxycarbonylation of terminal alkynes with high branched selectivity enabled by bisphosphine-picolinamide ligand

We report the room-temperature Pd-catalyzed methoxy-carbonylation with high branched selectivity using a new class of bisphosphine-picolinamide ligands. Systematic optimization of ligand structures and reaction conditions revealed the significance of both the picolinamide and bisphosphine groups in the ligand backbone. This strategic design of ligand was leveraged to deliver various α-substituted acrylates in good to excellent yields.

Computational modelling of Pd-catalysed alkoxycarbonylation of alkenes and alkynes

This perspective highlights the computational modelling of alkene and alkyne alkoxycarbonylation at palladium catalysts. We cover studies on Pd-catalysed alkoxycarbonylation of alkenes with bidentate diphosphine ligands, which reveal a hydride pathway is operating with an intermolecular alcoholysis step, where explicit solvation is mandatory to estimate the overall barriers correctly and model alcoholysis/copolymerisation selectivities. Subsequently, we discuss Pd-catalysed alkyne alkoxycarbonylation with P,N-chelating ligands, where an in situ base mechanism is operating involving ketene-type intermediates. We also discuss catalyst poisoning due to allene and designing a potential new catalyst tolerant towards allene poisoning.

Ferrocenylmethylphosphanes and the Alpha Process for Methoxycarbonylation: The Original Story

The Lucite Alpha process is the predominant technology for the preparation of acrylics. This two-stage process involves the palladium-catalysed formation of methyl propanoate from ethene, CO, and methanol, followed by the oxidative formylation of methyl propanoate into methyl methacrylate. A range of bis-1,2-disubstituted aminomethylferrocenes has been prepared and characterised. These complexes serve as precursors to a variety of bulky ferrocenylmethyldiphosphanes that, in turn, function as ligands in the palladium-catalysed process. We describe the crystal structures of five ligand precursors and provide a rationale for their design. In situ catalyst testing on palladium complexes derived from ferrocenylphosphanes demonstrates that these are highly selective (>99.5%) catalysts for the formation of methyl propanoate from ethene, CO, and methanol and have turnover numbers exceeding 50,000. This article credits those researchers who worked on this project in the early days, who received little or no credit for their achievements and endeavours.

Palladium-catalysed methoxycarbonylation of ethene with bidentate diphosphine ligands: a density functional theory study

The mechanism and origin of selectivity of Pd-catalysed formation of methyl propionate is elucidated through DFT calculations.

Perspectives for the microbial production of methyl propionate integrated with product recovery

A new approach was studied for bio-based production of methyl propionate, a precursor of methyl methacrylate. Recombinant E. coli cells were used to perform a cascade reaction in which 2-butanol is reduced to butanone using alcohol dehydrogenase, and butanone is oxidized to methyl propionate and ethyl acetate using a Baeyer-Villiger monooxygenase (BVMO). Product was removed by in situ stripping. The conversion was in line with a model comprising product formation and stripping kinetics. The maximum conversion rates were 1.14 g-butanone/(L h), 0.11 g-ethyl acetate/(L h), and 0.09 g-methyl propionate/(L h). The enzyme regioselectivity towards methyl propionate was 43% of total ester. Starting from biomass-based production of 2-butanol, full-scale ester production with conventional product purification was calculated to be competitive with petrochemical production if the monooxygenase activity and regioselectivity are enhanced, and the costs of bio-based 2-butanol are minimized.

A facile approach to ketones via Pd-catalyzed sequential carbonylation of olefins with formic acid

An effective palladium-catalyzed sequential carbonylation of olefins with formic acid is described.

Carbonylation of ethene catalysed by Pd(II)-phosphine complexes

This review deals with olefin carbonylation catalysed by Pd(II)-phosphine complexes in protic solvents. In particular, the results obtained in the carbonylation with ethene are reviewed. After a short description of the basic concepts relevant to this catalysis, the review treats in greater details the influence of the bite angle, skeletal rigidity, electronic and steric bulk properties of the ligand on the formation of the products, which range from high molecular weight perfectly alternating polyketones to methyl propanoate. It is shown that the steric bulk plays a major role in directing the selectivity. Particular emphasis is given to the factors governing the very active and selective catalysis to methyl propanoate, including the mechanism of the catalytic cycles with diphosphine- and monophosphine-catalysts. A brief note on the synthesis of methyl propanoate using a “Lucite” type catalyst in ionic liquids is also illustrated. A chapter is dedicated to the carbonylation of olefins in aqueous reaction media. The nonalternating CO-ethene copolymerization is also treated.

A unique palladium catalyst for efficient and selective alkoxycarbonylation of olefins with formates

Forget about CO! Carbonylations are among the most important homogeneously catalyzed reactions in the chemical industry, but typically require carbon monoxide. Instead, straightforward and efficient alkoxycarbonylations of olefins can proceed with alkyl formates in the presence of a specific palladium catalyst. Aromatic, terminal aliphatic, and internal olefins are carbonylated to give industrially important linear esters at low catalyst loadings.

Away from phosgene: reductive carbonylation of nitroarenes and oxidative carbonylation of amines, understanding the mechanism to improve performance

Recent progress, with some emphasis on the author's own work, on the understanding of the mechanism of the reductive carbonylation of nitroarenes and of the oxidative carbonylation of amines is discussed, highlighting the close connection between the two reactions and trying to unify scattered data in the literature. Isocyanates, carbamates and ureas can be obtained by either procedure, without the need for the use of the toxic and corrosive phosgene.

Evidence for Spontaneous Release of Acrylates from a Transition-Metal Complex Upon Coupling Ethene or Propene with a Carboxylic Moiety or CO2

The development of a new synthetic approach to acrylates based on the formation of alkyl esters of acrylic acids has been studied. A preformed Pd-COOMe moiety is used as a model system to investigate the insertion of an olefin into the Pd--C bond. The fast elimination of acrylate is observed. Density functional calculations support the experimental findings and allow the characterization of transition states along the reaction pathway. The first example of olefin/CO(2) coupling with facile release of ethyl acrylate is also presented.

Ester versus Polyketone Formation in the Palladium−Diphosphine Catalyzed Carbonylation of Ethene

The origin of the chemoselectivity of palladium catalysts containing bidentate phosphine ligands toward either methoxycarbonylation of ethene or the copolymerization of ethene and carbon monoxide was investigated using density functional theory based calculations. For a palladium catalyst containing the electron-donating bis(dimethylphosphino)ethane (dmpe) ligand, the rate determining step for chain propagation is shown to be the insertion of ethene into the metal-acyl bond. The high barrier for chain propagation is attributed to the low stability of the ethene intermediate, (dmpe)Pd(ethene)(C(O)CH3). For the competing methanolysis process, the most likely pathway involves the formation of (dmpe)Pd(CH3OH)(C(O)CH3) via dissociative ligand exchange, followed by a solvent mediated proton-transfer/reductive- elimination process. The overall barrier for this process is higher than the barrier for ethene insertion into the palladium-acetyl bond, in line with the experimentally observed preference of this type of catalyst toward the formation of polyketone. Electronic bite angle effects on the rates of ethene insertion and ethanoyl methanolysis were evaluated using four electronically and sterically related ligands (Me)2P(CH2)nP(Me)2 (n = 1-4). Steric effects were studied for larger tert-butyl substituted ligands using a QM/MM methodology. The results show that ethene coordination to the metal center and subsequent insertion into the palladium-ethanoyl bond are disfavored by the addition of steric bulk around the metal center. Key intermediates in the methanolysis mechanism, on the other hand, are stabilized because of electronic effects caused by increasing the bite angle of the diphosphine ligand. The combined effects explain successfully which ligands give polymer and which ones give methyl propionate as the major products of the reaction.

Structurally characterized intermediates in the stepwise insertion of CO–ethylene or CO–methyl acrylate into the metal–carbon bond of Pd(ii) complexes stabilized by (phosphinomethyl)oxazoline ligands

The initial CO-ethylene or CO-methyl acrylate insertion steps into the Pd-Me bond of methylpalladium(II) complexes with (phosphinomethyl)oxazoline ligands, leading to metallacycles, have been fully characterized, including by X-ray diffraction.