Phosphine Ligands in the Palladium‐Catalysed Methoxycarbonylation of Ethene: Insights into the Catalytic Cycle through an HP NMR Spectroscopic Study

Abstract

Novel cis-1,2-bis(di-tert-butyl-phosphinomethyl) carbocyclic ligands 6-9 have been prepared and the corresponding palladium complexes [Pd(O(3)SCH(3))(L-L)][O(3)SCH(3)] (L-L=diphosphine) 32-35 synthesised and characterised by NMR spectroscopy and X-ray diffraction. These diphosphine ligands give very active catalysts for the palladium-catalysed methoxycarbonylation of ethene. The activity varies with the size of the carbocyclic backbone, ligands 7 and 9, containing four- and six-membered ring backbones giving more active systems. The acid used as co-catalyst has a strong influence on the activity, with excess trifluoroacetic acid affording the highest conversion, whereas excess methyl sulfonic acid inhibits the catalytic system. An in operando NMR spectroscopic mechanistic study has established the catalytic cycle and resting state of the catalyst under operating reaction conditions. Although the catalysis follows the hydride pathway, the resting state is shown to be the hydride precursor complex [Pd(O(3)SCH(3))(L-L)][O(3)SCH(3)], which demonstrates that an isolable/detectable hydride complex is not a prerequisite for this mechanism.

Palladium complexes of (benzoimidazol-2-ylmethyl)amine ligands as catalysts for methoxycarbonylation of olefins

“…We did not observe any effect on the catalytic activity of 4 with change in reaction temperature as was evident in comparable conversions of 85% and 86% at 60 o C and 90 o C respectively. The effect of catalyst concentration was also investigated by varying substrate/catalyst ratio from 100 to 400 at fixed substrate concentration (Table 2, entries 9, [13][14]. Increasing the substrate/catalyst ratio from 100 to 400 led to a remarkable decrease in activity from 86% to 55% accordance with the previous reports of Vavasori and co-workers [55].…”

“…For instance, under low partial pressures of carbon monoxide and moderate temperatures, some of these catalyst systems show high regioselectivity (>90%) towards branched esters [13][14][15][16][17][18][19][20][21][22][23][24][25].…”

Interplay of bite angle and cone angle effects. A comparison between o-C6H4(CH2PR2)(PR′2) and o-C6H4(CH2PR2)(CH2PR′2) as ligands for Pd-catalysed ethene hydromethoxycarbonylation

“…8,9 It is clear from Fig. 1 and 2 that the z-coordinates of the backbone phenylene group render the CH 2 protons and the t Bu substituents inequivalent in both 1a and 2a.…”

Highly efficient reduction of tertiary phosphine oxides and sulfides with amine-assisted aluminum hydrides under mild conditions

“…Currently reduction of phosphine oxides [13][14][15][16], versus substitution reactions of carbon nucleophiles with P-Cl derivatives or of carbon electrophiles with alkali metal phosphides [7][8][9], and phosphination of alkenes, alkynes, or aromatic halids [10][11][12], is the major method of synthesis of phosphines. Among many reducing agents silanes and aluminum hydrides remain the reagents of choice for reduction of phosphine oxides [17][18][19][20][21][22][23][24][25].…”