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

Saturday, December 15, 2007
Doris Dallinger
Karl-Franzens-University Graz

Synthesis of Polysubstituted Aromatic Nitriles

The group of Mark Lautens from the University of Toronto has reported on the Pd-catalyzed C-H functionalization reaction for the synthesis of highly substituted benzonitriles (J. Am. Chem. Soc. 2007, 129, 15372. ). Bicyclic products 1 were obtained in one step by an intramolecular alkylation/cyanation sequence. Both electron-withdrawing and -donating o-substituents are tolerated. Tricyclic benzonitriles can be obtained via a double o-alkylation/cyanation sequence. Biaryl compounds 2 were synthesized via an intermolecular o-arylation/cyanation sequence employing electron-deficient aryl bromides. The reaction conditions regarding ligand, cyanation reagent and solvent had to be changed and when aryl iodides lacking an o-blocking group were employed, only complex mixtures of products were received. Important for both strategies is norbornene which is able to initiate the competitive C-H functionalization pathway, so that C-C and C-CN bond formation can occur sequentially.

Cu Nanoparticle-Catalyzed Aryl-Sulfur Bond Formation

Brindaban Ranu and co-workers from the Indian Association for the Cultivation of Science in Kolkata have developed the ligand-free coupling of aryl iodides with thiophenols and alkanethiols under Cu nanoparticle (4-6 nm) catalysis (Adv. Synth. Catal. 2007, 349, 2690. ). The arylsulfide synthesis proceeded in good to excellent yields and high purities. Compared to conventional heating at 120 C, the reaction times could be accelerated from 12-15 h to only 5-7 min. Other Cu sources such as metallic Cu, CuI or Cu powder lead to lower product yields, indicating the key role of Cu nanoparticles in this C-S coupling reaction.

Intramolecular Cycloaddition of Azomethin Ylides

In course of the preparation of a polycyclic pyrrolidine library, Mark Kurth and co-workers from the University of California, Davis, have disclosed an intramolecular azomethin ylide 1,3-dipolar cycloaddition as key transformation (Org. Lett. 2007, 9, 5055. ). Fused pyrrolidine scaffolds 4 were obtained in high regio- and stereoselectivity via condensation of benzimidazole-carbaldehydes 1 with a secondary amino ester 2 to form the S-shaped ylide 3 and the subsequent intramolecular 1,3-dipolar cycloaddition of 3. Importantly, the nature of the secondary amine plays a crucial role in the outcome of the reaction (R1 = H, R2 = Ph: 10%; R1 = H, R2 = Me: 59%).

D. Dallinger, Org. Chem. Highlights 2007, December 15.