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

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

Suzuki-Type Couplings

A recent publication by Nicholas J. Turner and co-workers from the University of Edinburgh (J. Org. Chem. 2004, 69, 6920. ) describes the microwave-assisted palladium-catalyzed cross-coupling of β-chloroalkylidene/arylidene malonates ("vinyl chlorides") with boronic acids in a Suzuki-type fashion. The Suzuki arylation reaction was found to proceed with a wide range of arylboronic acids including electron-rich, electron-deficient and sterically demanding systems. For most reactions the presence of 1 mol% of the air-stable palladium catalyst [(t-Bu)2P(OH)]2PdCl2 provided a high yield of the desired β-aryl/alkylarylidene malonates. The catalyst was also found active to promote standard Suzuki reactions of aryl chlorides under microwave conditions.

α-Arylation of Esters Using Reformatsky Reagents

Mark G. Moloney and co-workers from Oxford University (Tetrahedron Lett. 2004, 45, 7395. ) have reported the palladium-catalyzed α-arylation of esters and amides with organozinc reagents in a one-pot fashion. The required Reformatsky reagents were readily prepared by microwave irradiation of the corresponding bromoacetate or bromoacetamide with zinc in THF for 5 min at 100 °C. Addition of this Reformatsky reagent to the coupling partner, an aryl bromide and the relevant catalyst/ligand in THF followed by further irradiation for 5-10 min at 120 °C provided the arylacetic esters or amides in good yields.

Ladder Polymers via Suzuki and Stille Couplings

The synthesis of fully conjugated semiconducting para-phenylene ladder polymers via microwave-assisted palladium-mediated "double" Suzuki and Stille-type reactions has been demonstrated by Ullrich Scherf and co-workers from Germany (Adv. Funct. Mater. 2004, 14, 352. ) . Compared to the classical thermal protocols, reaction times were reduced by orders of magnitude and the molecular weight distributions of the polymers were changed significantly comparing microwave and thermal runs.

Homolytic Aromatic Substitution/Horner-Wadsworth-Emmons Olefinations 

An efficient one-pot sequence comprising a homolytic aromatic substitution followed by an ionic Horner-Wadsworth-Emmons olefination for the production of a small library of α,β-unsaturated oxindoles was disclosed by Armido Studer and co-workers from The Philipps-Universität Marburg, Germany (Org. Lett. 2004, 6, 3477. ) . Suitable TEMPO-derived alkoxyamine precursors were exposed to microwave irradiation DMF for 2 min to generate an oxindole intermediate via a radical reaction pathway (intramolecular homolytic aromatic substitution). After addition of KOt-Bu base and a suitable aromatic aldehyde the mixture was exposed again to microwave irradiation at 180 °C for 6 min to provide the α,β-unsaturated oxindoles in moderate to high overall yields.

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