Microwave Chemistry Highlights
Synthesis of Unsymmetric Sulfonamides
Shahnaz Ghassemi and Kristin Fuchs from Biotage, Virginia have reported on a general method for the synthesis of a set of unsymmetric sulfamides in combination with an alternative method for a Boc-removal strategy (Mol. Diversity 2005, 9, 295. ). The first step was a one-pot synthesis of Boc-protected sulfamides, followed by N-alkylation via Mitsunobu reaction to introduce more diversity. Silica-bonded phenylsulfonic acid (Si-TsOH) was used for Boc-removal, which was possible in 5 minutes at 100 ░C for all examples. All synthesis steps have been performed using microwave heating. For releasing the sulfamides from Si-TsOH, NH3 in MeOH was used.
Pd-Catalyzed Amination of Aryl Nonaflates
Stephen L. Buchwald and his group from the Massachusetts Institute of Technology, Cambridge have described a fast protocol for the coupling of (hetero)aryl nonaflates and amines (J. Org. Chem. 2005, 71, 430. ). An excellent functional group tolerance was observed for the synthesis of diverse arylamines using the catalyst systems Pd2dba3 with ligands 1, 2 or 3 respectively, and the soluble amine bases DBU (1,8-diazabicyclo[5.4.0]undec-7-ene) and MTBD (7-methyl-1,5,7-triazabicyclo[4.4.0]dec-5-ene).
Suzuki-Couplings with Organotrifluoroborates
In a recent publication Nicholas E. Leadbeater and co-workers from the University of Connecticut have demonstrated the synthesis of functionalized biaryls via Suzuki-coupling (Tetrahedron Lett. 2005, 47, 217. ). By using potassium organotrifluoroborates as an alternative to boronic acids and ultra low palladium loading (2.5 ppm), efficient coupling with arylhalides could be performed in 5 minutes at 150 ░C under microwave irradiation.
Synthesis of 1-Alkyl-4-Imidazolecarboxylates
By applying a “catch and release” strategy, the group of Giampaolo Giacomelli from UniversitÓ degli Studi di Sassari has synthesized a set of 1-Alkyl-4-imidazolecarboxylates (J. Comb. Chem. 2005, 7, 905. ). The resin-bound isocyanoacrylate 1 was prepared under microwave conditions in only 20 minutes, compared to 36 h of oil-bath heating. For the next step, resin 1 was reacted with a set of amines under open vessel microwave conditions to give imidazolecarboxylates 2 in excellent yields and high purities. Furthermore, the solid support could be recycled and used successfully for 3-4 cycles.