A Synthetic Cycle for Heteroarene Synthesis by Nitride Insertion

[cis-terpyOsNCl2]PF6 inserts nitrogen into indenes to afford isoquinolines. The insertion proceeds through an azaallenium intermediate, with mechanistic and computational studies suggesting that azaallenium formation occurs via a stepwise aziridination followed by electrocyclic ring opening. Examination of a substrate diverted from aromatization resulted in inner-sphere ligand rearrangement to afford an azaallyl chloride complex; kinetic studies suggest this species forms via an analogous azaallenium intermediate. Studies on base-promoted release of the isoquinoline suggest that the azaallyl chloride is not an obligate intermediate for aromatization and the parent azaallenium can directly eliminate to give a neutral dichloroosmium(II)-isoquinoline intermediate. The heteroarene can subsequently be thermally liberated to afford an osmium(II) acetonitrile complex, which can be oxidized to regenerate the nitride over two steps. First, in the presence of ammonia, oxidation affords an osmium(III) amidine, with subsequent oxidation in the presence of toluene sulfonic acid ultimately regenerating the nitride. The mechanistic foundation set by this synthetic cycle opens the door to the further development of nitrogen insertion heteroarene syntheses promoted by late transition metal nitrides.