Microwave-Assisted Aqueous Suzuki Cross-Coupling Reactions

Abstract

The poly(ethylene glycol) ester of bromo-, iodo-, and triflate-para-substituted benzoates are smoothly cross-coupled with aryl boronic acids (Suzuki reaction) under "ligandless" palladium acetate catalysis in water. The reaction proceeds without organic cosolvent under conventional thermal conditions (70 °C, 2 h) and under microwave irradiation (75 W, 2-4 min). The polymeric support remains stable under both reaction conditions. Whereas conventional thermal conditions induced ester cleavage (up to 45%), this side reaction is suppressed when microwave conditions are employed. Aryl nonaflates give fair yields under these conditions. Non-polymer-bound aryl halides form biaryls in good to excellent yields in water/poly(ethylene glycol) mixtures under microwave irradiation (4 min, 75 W).

Controlled Microwave Heating in Modern Organic Synthesis

“…Separation of the functionalized matrix is achieved by either solvent or heat precipitation, membrane filtration, or sizeexclusion chromatography. [233] A variety of successful microwave-assisted transformations involving soluble polymers such as polyethylene glycol (PEG) have been reported since 1999, [234] and most recently by Sun and co-workers using controlled open-vessel microwave conditions. [235,236] In the example shown in Scheme 49 polyethylene glycol of molecular weight 6000 (PEG 6000) was used as a support for the synthesis of a small library of thiohydantoins.…”

Recent applications of the Suzuki–Miyaura cross-coupling reaction in organic synthesis

“…The reaction proceeds well on PEG-bound aryl iodides, bromides, triflates and non-aflates in the presence of ligandless Pd(OAc) 2 as the catalyst and without any organic co-solvent. They later used the same strategy to synthesise 1H-benzimidazoles, 1H-imidazo [4,5-b]pyridines 107 and 1H-imidazo [4,5-c]pyridines. 108 Varma 109 investigated the effect of various bases on the PEG-based coupling reaction in the presence of microwave irradiation (Scheme 47) and found KF to be the most convenient protocol.…”

Shape‐Switchable Azo‐Macrocycles

“…To further improve the yield, microwave assisted aqueous Suzuki cross coupling conditions were applied. [291] With these conditions the desired amine 71 was isolated as well, but only in 41% yield.…”