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Microwave Chemistry Highlights
A short summary of articles published within the last month on microwave-assisted organic synthesis (MAOS)

Thursday, August 15, 2004
C. Oliver Kappe
Karl-Franzens-University Graz

Suzuki Cross Couplings for Oligothiophene Synthesis

A recent publication by G. Barbarella and co-workers from Bologna, Italy (J. Org. Chem. 2004, 69, 4821. ) reports the rapid preparation of poorly soluble unsubstituted and modified α-quinque and sexithiophenes by the extensive use of microwave-assisted Suzuki and Sonogashira cross couplings, in addition to one-pot borylation/Suzuki sequences. Several of the prepared oligothiophenes show liquid-crystalline properties.

Sonogashira Cross Couplings in Supramolecular Chemistry

Timothy M. Swager and co-workers from the MIT have employed (J. Am. Chem. Soc. 2004, 126, 8638. ) a microwave-assisted double Sonogashira-Hagihara coupling for the synthesis of rotaxanated conjugated sensory polymers. Microwave irradiation of a suitable diiodo rotaxane with a corresponding aryl diacetylene under standard microwave Sonogashira coupling conditions at 115 C for 50 min provided the poly(p-phenylene ethynylene)s which displayed interesting macromolecular photophysical properties. Using the microwave approach the reaction time for the synthesis of the rotaxanated conjugated polymer was reduced from two days to less than one hour.

Palladium-Catalyzed Amination Reactions

Niels Skjaerbaek and coworkers from LEO Pharma, Denmark have disclosed (J. Org. Chem. 2004, 69, 4936. ) an efficient protocol for the synthesis of aryl aminobenzophenone p38 MAP kinase inhibitors using microwave-assisted Pd-catalyzed aminations. Several different strategies involving halide, triflate and tosylate leaving groups were investigated, in addition to an alternative amination mode (method B). Amination of an electronically diverse array of aryl halides with a variety of anilines was realized in good to excellent yields for most cases, without the necessity to work under an inert atmosphere.

Diels-Alder Cycloaddition to Single-Wall Carbon Nanotubes

The microwave-assisted sidewall functionalization of single-wall carbon nanotubes (SWNT) via Diels-Alder cycloaddition with o-quinodimethane has been demonstrated by Fernando Langa and co-workers from Spain (Chem. Commun. 2004, 1734. ). The required o-quinodimethane was generated in situ from 4,5-benzo-1,2-oxathiin-2-oxide (sultin) by refluxing in 1,2-dichlorobenzene under open vessel microwave irradiation conditions. In the presence of ester functionalized SWNTs cycloadditions takes place within 45 min. Conventional refluxing in 1,2-dichlorobenzene for 3 days leads only to a low degree of conversion.

Reproduced by permission of The Royal Society of Chemistry

C. O. Kappe, Org. Chem. Highlights 2004, August 15.