Microwave Chemistry Highlights
Synthesis of N-Shifted Buflavine Analogues
The group of Erik Van der Eycken at the University of Leuven has employed microwave-enhanced Suzuki-Miyaura cross-coupling and ring-closing metathesis (RCM) as the key steps in the 6-step synthesis of so far unknown N-shifted buflavine analogues (Org. Lett. 2005, 7, 2723. ). Particularly the generation of the rigid, medium-sized ring system by RCM was enhanced by employing microwave heating for 5 minutes at 150 °C using 3 mol % of Grubbs-2 catalyst.
Synthesis of N-Aryl Azacycloalkanes
Rajender S. Varma and Yuhong Ju from the U.S. Environmental Protection Agency have reported (Org. Lett. 2005, 7, 2409. ) a double N-alkylation by alkyl dihalides of aniline derivatives for the synthesis of N-aryl azacycloalkanes. By applying microwave irradiation at 120 °C for 20 minutes it was possible to significantly improve the yields, compared to conventional heating, which is related to the elimination of side reactions. This “green chemistry” approach uses very mild basic aqueous conditions which tolerates not only a variety of functional groups but also simplifies the isolation of the either solid or liquid products by the occurring phase separation.
Exploring SmBr2-, SmI2- and YbI2-Mediated Reactions
Göran Hilmersson and co-workers from Göteborg University (Chem. Eur. J. 2005, 11, 3279. ) have investigated the use of lanthanide(II) halides (LnX2 = SmBr2, SmI2, YbI2) in microwave-assisted reduction- and reductive coupling reactions without the use of a co-solvent for a variety of functional groups such as α,β-unsaturated esters, aldehydes, ketones, imines and alkyl halides. A strong influence on the direction of the reactions was found to be the redox potential of the LnX2 reagent, i.e. SmBr2 induced mainly the pinacol-coupling of ketones (reaction a) or the intramolecular reductive coupling, respectively (reaction b) while the weaker YbI2 afforded mainly the reduction products (1, Reaction b)
A Continuous Flow Capillary Microreactor for Microwave-Assisted Organic Synthesis
In a recent publication Michael G. Organ and Eamon Comer from York University (J. Am. Chem. Soc. 2005, 127, 8160. ) have described the development of a capillary-based flow system for performing microscale organic synthesis under microwave irradiation. Several metal catalyst-employing reactions like Suzuki or RCM but also nucleophilic aromatic and Wittig reactions respectively showed excellent conversions with a sample irradiation time of approximately 4 minutes. By using this microreactor device the reactants can be coinjected from separate syringes, mix and react without showing problems of poor kinetics due to laminar flow.