Zinc Stabilized Azo-anion Radical in Dehydrogenative Synthesis of N‑Heterocycles. An Exclusively Ligand Centered Redox Controlled Approach

Herein
we report an exclusively ligand-centered redox controlled
approach for the dehydrogenation of a variety of N-heterocycles using
a Zn­(II)-stabilized azo-anion radical complex as the catalyst. A simple,
easy-to-prepare, and bench-stable Zn­(II)-complex (1b)
featuring the tridentate arylazo pincer, 2-((4-chlorophenyl)­diazenyl)-1,10-phenanthroline,
in the presence of zinc-dust, undergoes reduction to form the azo-anion
radical species [1b]− which efficiently
dehydrogenates various saturated N-heterocycles such as 1,2,3,4-tetrahydro-2-methylquinoline,
1,2,3,4-tetrahydro-isoquinoline, indoline, 2-phenyl-2,3-dihydro-1H-benzoimidazole, 2,3-dihydro-2-phenylquinazolin-4­(1H)-one, and 1,2,3,4-tetrahydro-2-phenylquinazolines, among
others, under air. The catalyst has further been found to be compatible
with the cascade synthesis of these N-heterocycles via dehydrogenative
coupling of alcohols with other suitable coupling partners under air.
Mechanistic investigation reveals that the dehydrogenation reactions
proceed via a one-electron hydrogen atom transfer (HAT) pathway where
the zinc-stabilized azo-anion radical ligand abstracts the hydrogen
atom from the organic substrate(s), and the whole catalytic cycle
proceeds via the exclusive involvement of the ligand-centered redox
events where the zinc acts only as the template.