Dirhodium-Catalyzed Transannulation of N-Sulfonyl-1,2,3-triazoles to 2,3-Dehydropiperazines.

The dirhodium(II)-catalyzed synthesis of a range of C2-substituted 2,3-dehydropiperazines using 1-mesyl-1,2,3-triazoles and ß-haloalkylcarbamates is reported. The reaction is proposed to proceed through an α-imino rhodium carbene 1,3-insertion into N-H followed by a base-mediated cyclization. C-Substituted dehydropiperazines can also be conducted directly from terminal alkynes in a three-step, one-pot operation, forming the triazole in situ. This methodology has also been expanded to afford several 2,5-disubstituted 2,3-dehydropiperazines as well as a larger 4,5,6,7-tetrahydro-1H-1,4-diazepine derivative.