Microwave Chemistry Highlights
A short summary of articles published within the last month on microwave-assisted organic synthesis (MAOS)
Phosphine Gold(I)-Catalyzed Hydroamination of AlkenesChi-Ming Che and co-workers from the University of Hong Kong have reported on the intra- and intermolecular hydroamination of unactivated alkenes (Org. Lett. 2006, 8, ASAP. ). In a catalyst screening the phosphine gold(I) catalyst combinations (PCy3)AuCl/AgOTf and (PPh3)AuCl/AgOTf, respectively, showed the best catalytic activity. By applying microwave heating not only were reaction times reduced (e.g. 30 h → 30 min) but also the amount of catalyst, compared to conventional heating.
A new protocol for the Pd-catalyzed fluorination of C-H bonds was developed by the group of Melanie S. Sanford at the University of Michigan (J. Am. Chem. Soc. 2006, 128, 7134. ). This reaction proceeds via a Pd(OAc)2-catalyzed C-H activation/oxidative fluorination step where an electrophilic fluorinating reagent (1 or 2) is necessary for successful aromatic and benzylic C-F bond formations. Compared to thermal heating, higher yields could be achieved with microwave irradiation due to shorter reaction times.
Parallel Synthesis of 2,4-Disubstitued 5-Aminoimidazoles
Comparison studies of thermal vs. microwave heating in the synthesis of 2,4-disubstituted 5-aminoimidazoles 3 have been conducted by Yulin Lam and co-workers from National University of Singapore (J. Comb. Chem. 2006, 8, ASAP. ). Higher yields and considerable rate enhancements (up to ca. 130 times for the overall reaction) could be achieved by applying microwave heating for all three synthetic steps. The synthesis of 5-aminoimidazole scaffolds 3 was performed in parallel employing a multivessel rotor system, making the protocol amenable for high-throughput synthesis.
One-Pot Ugi Four-Component Reaction and Intramolecular O-Alkylation
The synthesis of 3,4-dihydro-3-oxo-2H-1,4-benzoxazines 2 was disclosed by the group of Wei-Min Dai at Zhejiang University, China (Tetrahedron 2006, 62, 6774. ). Highly functionalized benzoxazine derivatives 2 were obtained via a one-pot Ugi four-component reaction of 2-aminophenols, aromatic aldehydes, α-bromoalkanoic acids and isocyanides and subsequent base catalyzed O-alkylation of 1. By additional post-modification of scaffolds 3 through CuI-catalyzed intramolecular amidations, a novel class of heterocyclic derivatives could be achieved.