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

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posted on 2016-01-11, 00:00 authored by Ross J. Beattie, Peter S. White, Joseph L. Templeton
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 CO stretching frequency to 2109 cm–1 versus 1946 cm–1 for the WII 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.

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