Microwave Chemistry Highlights
A recent publication by Ji-Chang Xiao and Jeanīne M. Shreeve from the University of Idaho (J. Fluorine Chem. 2005, 126, 473. ) describes the rapid electrophilic fluorination of 1,3-dicarbonyl compounds using SelectfluorŪ as fluorinating agent. By employing 1 equivalent of Selectfluor under neutral reaction conditions the 2-monofluorinated products were obtained. Treatment of 1,3-dicarbonyls with 3 equivalents of Selectfluor in the presence of tetrabutylammonium hydroxide (TBAH) as base, difluorination can be achieved in a single reaction step, also resulting in high yields.
Palladium-Catalyzed Intramolecular Cyclization
Mark Lautens and co-workers from Toronto University (J. Am. Chem. Soc. 2005, 127, 72, ) have reported on palladium-catalyzed intramolecular cross-coupling reactions between aryl iodides and allyl moieties. Various five- to seven membered carbo- and heterocycles were synthesized applying microwave irradiation at 160 °C for only 1 minute. Compared to conventional thermal heating the yields could be improved by up to 40 %.
The manganese (IV) dioxide-catalyzed synthesis of quinoxalines has been demonstrated by So Yeon Kim and co-workers from Seoul National University (Chem. Commun. 2005, 1321. ). The quinoxalines were obtained from a variety of α-hydroxyketones by trapping with aromatic or aliphatic 1,2-diamines under solventless microwave conditions for 1 minute at 70 °C. By applying the same protocol, also benzimidazoles from aldehydes and 1,2-diaminobenzene were prepared.
The synthesis of pyruvate-based enol ethers and enamines via the Petasis olefination has been demonstrated by Timothy Gallagher and co-workers from the University of Bristol (Tetrahedron Lett. 2005, 46, 297, ). The olefination of unsymmetrical oxalates is highly regioselective by employing the Petasis reagent and is dramatically improved when microwave heating is applied. Shorter reaction times and higher yields are achieved by irradiating in toluene/THF for 0.5-3 hours at 120-150 °C.