The ligands bearing PEG polymers immobilize the Pd catalyst in the fluid phase created by the PEG chains, which stops the Pd from mixing into the solid phase of the reaction and becoming deactivated through aggregation. Six different ligands are prepared and the ligand with the longest PEG chain was found to offer the greatest reaction acceleration and yield for the cross-coupling. Pd(OAc)2 is used as the Pd source, and CsF and H2O are also added. The reaction temperature is maintained at 45 °C using a heat gun, and the addition of 1,5-cyclooctadiene to the reaction is found to stabilize the active monomeric Pd(0) catalyst. Interestingly, phosphine ligands bearing different polymeric chains, such as polystyrene and polyamide, do not promote the coupling reaction.
The optimized conditions are compatible with a range of substrates, including different halides (X = Cl and Br) and polyaromatic halides, as well as a range of boronic acids, providing cross-coupled products in high yields. Importantly, aryl halides that are poorly soluble in organic solvents, such as Vat Orange 3, are tolerated, highlighting the value of this mechanochemical approach. However, an elevated temperature of 80 °C is required. Additionally, powder X-ray diffraction and differential scanning calorimetry analysis suggest that the fluid PEG chains of the ligand and the solid substrates form a fluid–crystalline phase-separated structure under mechanochemical conditions.
This is a preview of subscription content, access via your institution