Redox-Neutral [4 + 2] Annulation of N-Methoxybenzamides with Alkynes Enabled by an Osmium(II)/HOAc Catalytic System.

By making use of a direct C-H activation strategy, an efficient osmium(II)-catalyzed redox-neutral [4 + 2] annulation of N-methoxybenzamides with alkynes has been accomplished. Computational and experimental studies revealed that such transformation leading to the synthesis of the isoquinolone core might follow an Os(II)-Os(IV)-Os(II) catalytic pathway, in which an unusual HOAc-assisted oxidative addition of osmium(II) into the N-O bond to generate the osmium(IV) species was involved as one of the key transition states. Further exploration of divergent C-H activation reaction modes enabled by the osmium(II) catalyst has also been exemplified for one-pot assembly of other either linear or cyclic products.