Microwave Chemistry Highlights
A short summary of articles published within the last month on microwave-assisted organic synthesis (MAOS)
Microwave-Assisted Suzuki Couplings with Simultaneous Cooling
Suzuki-couplings using a microencapsulated palladium catalyst (Pd EnCat) under microwave heating were investigated by the group of Steven V. Ley at the University of Cambridge (Chem. Eur. J. 2006, 12, 4407. ). For those compounds giving low purities under conventional microwave conditions A, higher purities could be achieved by performing the reactions at 50 W (reaction temperature did not exceed 76 °C) with simultaneous cooling (compressed air, B) by preventing otherwise occurring thermal decompositions. Even higher purities were obtained by conducting the reactions via a flow-through approach again in combination with cooling (C) due to shorter reaction times (ca. 1 min) and therefore diminished side reactions.
Microwave-Assisted Cu(II)-Mediated Cross-Couplings with Simultaneous Cooling
Erik Van der Eycken and co-workers from University of Leuven performed Cu(II)-mediated scaffold decorations on the pyrazinone core (Org. Lett. 2006, 8, 1863. ). N-Arylations utilizing arylboronic acids applying the Chan-Lam protocol were successfully conducted under microwave heating (300 W) with intensive simultaneous cooling at 0 °C (Coolmate). Due to prevented product decomposition, excellent yields could be achieved compared to reactions at rt or elevated temperatures, respectively. High yields could be obtained by using a mixture of Et3N/pyridine (1:2) as opposed to Et3N alone: 93% compared to 69% for R = 3-CF3.
A re-evaluation of the Newman-Kwart rearrangement under microwave conditions was carried out by Jonathan D. Moseley and co-workers from AstraZeneca, Bristol (Tetrahedron 2006, 62, 4685. ). Comparisons of results obtained by microwave and thermal heating showed no significant difference which excluded any microwave effects. Additionally, a solvent rate effect has been confirmed under both microwave and thermal heating, demonstrating the stabilization of the polar transition state intermediate 1 by polar solvents like formic acid (78% compared to 23% conv. with NMP after 30 min at 140 °C).
Synthesis of N-Substituted Tetrahydroquinolinediones
The group of Shujiang Tu at the Xuzhou Normal University in China has reported on the one-pot four component reaction for the synthesis of tetrahydroquinoline-diones (Bioorg. Med. Chem. Lett. 2006, 16, 2925. ). By reacting aromatic aldehydes 1, dimedone 2, meldrumīs acid 3 and amines 4 under microwave conditions a new series of N-functionalized quinolone products 5 was obtained. Higher yields and shorter reaction times compared to conventional heating could be accomplished.