Cleavage of Conjugated Alkenes by Cationic Osmium Nitrides: Scope of the Reaction and Dynamics of the Azaallenium Products

The cationic osmium(VI) nitride complex cis-[(terpy)OsNCl2]PF6 (1; terpy = 2,2‘:6‘,2‘ ‘-terpyridine) reacts with a variety of aryl-substituted alkenes and acyclic and cyclic dienes
to form the η2-azaallenium complexes cis-[(terpy)OsCl2(RR‘CNCHR‘ ‘)]PF6 (2) by net
nitrogen atom insertion into a CC double bond. More electron-rich alkenes are more
reactive, and electron-poor alkenes such as cinnamate esters do not react. Isomeric trans-[(terpy)OsNCl2]PF6 (3), as well as the tris(pyrazolyl)methane complex [(Tpm)OsNCl2]PF6
(4), give analogous products, although the scope of the reaction is restricted to dienes with
these nitrides. The aryl-substituted azaallenium complexes cis-[(terpy)OsCl2(η2(C,N)-R[Ar]CNCHR‘ ‘)]PF6 display hindered rotation about the C−Cipso bond, because π-stacking
interactions favor a close approach of the aryl group to the terpyridine ligand. The osmium
migrates slowly between the two CN bonds of the azaallenium ligands, leading to both
regio- and stereoisomerizations. These two processes can be observed separately in the case
of cis-[(terpy)OsCl2([p-Me2NC6H4]CHNCHCH3)]PF6, with regioisomerization preceding
stereoisomerization. The isomerizations appear to take place by an “allene-roll” mechanism,
as judged by the similar rates observed for cyclic and acyclic azaallenium complexes.