Regioselectivity of Addition to the Azavinylidene Ligand in Tp′W(CO)(η2-HC≡CH)(N═CHMe): Electrophilic Addition versus Oxidation and Radical Coupling

The Tp′W(CO)(HCCH)(N═CHMe) (Tp′ = hydridotris(3,5-dimethylpyrazolyl)borate) molecule contains an electron-rich 1-azavinylidene ligand where the preferred site for electrophile addition is the α-nitrogen. As is observed in other cases for electrophile addition to a 1-azavinylidene with a cis-alkyne ligand, this fragment will add small electrophiles such as H+, Me+, and Et+ to the α-nitrogen lone pair, maintaining a WII metal center and forming the coordinated imine products [Tp′W(CO)(HCCH)(NH═CHMe)][BF4], [Tp′W(CO)(HCCH)(N(Me)═CHMe)][OTf], and [Tp′W(CO)(HCCH)(N(Et)═CHMe)][OTf]. In contrast, when a one-electron oxidant is employed, net electrophile addition at the β-carbon can be achieved. Changing the nature of the electrophile to [CPh3][BF4] leads to a rapid one-electron oxidation and selective radical combination to form the imido product [Tp′W(CO)(HCCH)(NCH(CH3)CPh3)][BF4]. This net addition of an electrophile at the 1-azavinylidene β-carbon concomitantly oxidizes the metal center to WIV and raises the ...