Microwave Chemistry Highlights
Oxidative Heck Arylations
A recent publication by Mats Larhed and co-workers from Uppsala University (J. Org. Chem. 2004, 69, 5212. ) describes dioxygen-promoted regioselective oxidative Heck arylations of electron-rich olefins with arylboronic acids. Controlled microwave heating and increased oxygen pressure were used to reduce the required reaction time from 18 h (50 °C, oxygen balloon) to one hour (100 °C, 3 bar oxygen pressure). The reaction was performed both on a 1 mmol scale in a 25 mL vessel (single-mode microwave cavity) and on 10 mmol scale in a 300 mL vessel (multi-mode microwave reactor) using a continuous flow of oxygen.
Suzuki Couplings Utilizing Immobilized Palladium Catalysts
Andreas Kirschning and co-workers from Hannover University (Synlett, 2004, 1699. ) and a team from Abbott Laboratories (Y. Wang, D. R. Sauer, Org. Lett. 2004, 6, 2793, ) have reported very efficient Suzuki coupling reactions involving immobilized Palladium catalysts. The methods can be used to couple a range of aryl halides and triflates with boronic acids in solvents such as water or ethanol. The Hannover team has used a novel type of solid Pd(II)-precatalyst that can easily be prepared from 4-pyridine-aldoxime and Na2PdCl4 (catalyst 1), whereas the Abbott researchers employed polyethylene-supported FibreCat Pd catalysts (catalysts 2-4) under similar conditions.
Synthesis of Radiolabelled Levulinic Acid
The synthesis of [1-14C]levulinic acid has been demonstrated by Steen K. Johansen and co-workers from Denmark (J. Label. Compd. Radiopharm. 2004, 47, 627. ) . In all three of the synthetic steps, starting from bromo[1-14C]acetic acid, microwave heating was used in order to accelerate the reactions, allowing the total preparation to proceed in less than one hour. The labelled levulinic acid was subsequently transformed into (5Z)-4-bromo-5-(bromomethylene)-2(5H)-furanone in a bromination/oxidation sequence (not shown), a potent quorum sensing inhibitor.