Intermolecular C–H Activations of Hydrocarbons Initiated by a Tungsten Trimethylsilylallyl Complex

Thermolysis of Cp*W­(NO)­(Npt)­(η3-CH2CHCHSiMe3) [Cp* = η5-C5Me5; Npt = CH2CMe3] at 55 °C
leads to the loss of neopentane and the formation of the 16-electron
η2-allene intermediate Cp*W­(NO)­(η2-CH2CCHSiMe3), which activates
hydrocarbons at their methyl groups. In the case of linear alkanes,
only terminal C–H activation occurs. This selectivity persists
in the presence of an ether functionality, but not with other oxygen-containing
substrates such as aldehydes and alcohols. With these latter substrates,
the organometallic complex is oxidized to Cp*W­(O)2(Npt).
The existence of the allene intermediate has been confirmed by its
reaction with PMe3 to form the 18-electron adduct and by
its diagnostic reaction with cyclohexene. Carbonylation of Cp*W­(NO)­(Npt)­(η3-CH2CHCHSiMe3) with CO (550 psig) at
room temperature results in the clean formation of the corresponding
Cp*W­(NO)­(η1-C­(O)­Npt)­(η3-CH2CHCHSiMe3) complex, which exists as a mixture of
two interconverting isomers differing in their modes of attachment
of the (η3-CH2CHCHSiMe3) ligands
to the tungsten centers. The congeneric molybdenum complex, Cp*Mo­(NO)­(Npt)­(η3-CH2CHCHSiMe3), has also been synthesized,
and although it generates the requisite η2-allene
intermediate upon thermolysis, its preferred mode of reactivity is
coupling of the allyl and alkyl ligands. Consequently, the molybdenum
complex is inferior to the tungsten system for effecting C–H
activations. All new complexes have been characterized by conventional
spectroscopic and analytical methods, and the solid-state molecular
structures of most of them have been established by single-crystal
X-ray crystallographic analyses.