Rhodium‐Catalyzed Oxidative Cycloaddition of <i>N</i>‐<i>tert</i>‐Butoxycarbonylhydrazones with Alkynes for the Synthesis of Functionalized Pyrroles <i>via</i> C(<i>sp</i>³)–H Bond Functionalization

Abstract

Abstract A rhodium(III)‐catalyzed cycloaddition of N ‐ tert ‐butoxycarbonylhydrazones with internal alkynes was developed. The reaction features a regioselective α‐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. magnified image