Rhodium-Catalyzed Oxidative Cycloaddition of N-tert-Butoxycarbonylhydrazones with Alkynes for the Synthesis of Functionalized Pyrroles via C(sp(3))-H Bond Functionalization

摘要

A rhodium(III)-catalyzed cycloaddition of N-tert-butoxycarbonylhydrazones with internal alkynes was developed. The reaction features a regioselective -imino alkyl C(sp(3))-H bond functionalization resulting in selective formation of highly functionalized NH-free pyrroles. Our studies showed that utilizing the N-tert-butoxycarbonyl (N-Boc) as the oxidizing directing group is critical for achieving the observed pyrrole formation versus the isoquinoline formation. To account for the pyrrole formation, we hypothesized that a prior tautomerization of the N-Boc-hydrazones to enamines should occur, followed by regioselective C(sp(2))-H cleavage to form a putative five-membered rhodacycle. Subsequent coupling of the rhodacycle with the alkynes would afford the pyrrole products.