Divergent Ring‐Opening Functionalization and Reductive Coupling of Pyridines via Zirconium Alkylidene Complexes

Abstract

Abstract The transformation of N‐heterocycles such as pyridines is of fundamental and practical importance, yet direct cleavage of their aromatic C─N bonds remains challenging due to strong electronic delocalization and intrinsic stability. Here we report an unprecedented nonreductive cleavage of the aromatic C─N bond in pyridines via direct ring‐opening metathesis with PCP‐ligated (PCP = 2,6‐( t Bu 2 PCH 2 ) 2 ‐C 6 H 3 ) zirconium alkylidene complexes under mild conditions. Experimental and computational studies reveal that these transformations proceed via highly reactive [Zr]═C species instead of alkylidyne pathways, forming acyclic Zr‐imido‐triene complexes that undergo imido/oxo exchange with acetone. This enables a concise conversion of pyridines into conjugated iminotrienes that otherwise require multistep syntheses. The ancillary ligand dictates divergent pathways: alkyl‐, alkoxide‐, or aryloxide‐containing alkylidenes selectively promote ring‐opening functionalization, whereas the chloride analogue triggers rare reductive homocoupling of pyridines to bipyridine dianions. This ligand‐controlled reactivity arises from steric and electronic effects of the chloride variant, facilitating C─H activation and C─C bond formation. These findings demonstrate a novel approach to aromatic C─N bond scission and dual functionalization at both termini of the opened pyridine rings, along with a reductive homocoupling pathway to bipyridine dianions, providing new opportunities for catalytic transformations of aromatic N‐heterocycles.