Regioselectivity of Addition to the Azavinylidene Ligand in Tp ' W(CO)(eta(2)-HC CH)(N=CHMe): Electrophilic Addition versus Oxidation and Radical Coupling

摘要

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 alpha-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 alpha-nitrogen lone pair, maintaining a W-II 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 electrophlle addition at the beta-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 beta-carbon concomitantly oxidizes the metal center to W-IV and raises the CO stretching frequency to 2109 cm(-1) versus 1946 cm(-1) for the W-II imine complex This reactivity is also seen with [CPh2(C6H4OMe)][BF4]. This variable reactivity demonstrates the flexibility of the 1-azavinylidene ligand when it is paired with an adjacent alkyne ligand.