Computational modelling of Pd-catalysed alkoxycarbonylation of alkenes and alkynes

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 Silico Investigation of Palladium-Catalyzed Chemoselective Monoalkoxycarbonylation of 1,3-diynes for Conjugated Enynes Synthesis

The palladium-catalyzed monoalkoxycarbonylation of 1,3-diynes provides a chemoselective method for the construction of synthetically useful conjugated enynes. Here, in silico unraveling the detailed mechanism of this reaction and the origin of chemoselectivity were conducted. It is shown that the alkoxycarbonylation reaction preferably proceeds by a NH-Pd pathway, which including three substeps: hydropalladation, CO migratory insertion and methanolysis. The effectiveness of the NH-Pd catalytic system is attributed to the alkynyl-palladium π-back-bonding interaction, C-H⋅⋅⋅π interaction in reactant moiety and d-pπ conjugation between the Pd center and alkenyl group. The hydropalladation step was identified as the rate- and chemoselectivity-determining step, and the first alkoxycarbonylation requires a much lower energy barrier in comparison with the second alkoxycarbonylation, in line with the experimental outcomes that the monoalkoxycarbonylation product was obtained in high yield. Distortion-interaction analysis indicates the more favorable monoalkoxycarbonylation (compared to double alkoxycarbonylation) is caused by steric effect.

Mechanistic Insight into Palladium/Brønsted Acid Catalyzed Methoxycarbonylation and Hydromethoxylation of Internal Alkene: A Computational Study

Density functional theory (DFT) calculations were performed to study the palladium/Brønsted acid-catalyzed methoxycarbonylation and hydromethoxylation reactions of internal alkene. The calculated results show that the pyridyl group (N atom) in bidentate phosphine ligand with built-in base (L1) plays a crucial role in controlling the selectivity. With the help of the pyridyl group, the methanolysis steps in the methoxycarbonylation reaction and the hydromethoxylation reaction become easy, and both the linear ester methyl 3,4-dimethylpentanoate (P1) and the hydromethoxylation product 2-methoxy-2,3-dimethylbutane (P2) could be obtained. In contrast, the possibility of leading to branched ester P1' was ruled out according to our calculations. The steric effect could account for the observed selectivity. In the presence of the DPEphos ligand (L2) that does not bear the pyridyl group, the methanolysis step in the methoxycarbonylation reaction becomes the rate-determining step with a high overall energy barrier. Neither linear nor branched methoxycarbonylation product could be generated. The palladium/Brønsted acid co-catalyzed hydromethoxylation also become difficult without the assistance of the pyridyl group in the presence of the L2 ligand. Instead, TsOH-catalyzed hydromethoxylation reaction could take place to generate the ether product P2.

(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.

Palladium-Catalyzed Regio- and Chemoselective Double-Alkoxycarbonylation of 1,3-Diynes: A Computational Study

The palladium-catalyzed double-alkoxycarbonylation of 1,3-diynes provides an efficient approach for the selective synthesis of 1,2,3,4-tetrasubstituted conjugated dienes. In this report, density functional theory calculations have been performed to elucidate the...