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 n(2)-azaallenium complexes cis- [(terpy)OsCl(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(n(2)(C,N)-R[Ar]C=N=CHR)]PF6 display hindered rotation about the C-C-ispo bond, because pi-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.