Thermal Activation of Hydrocarbon C−H Bonds Initiated by a Tungsten Allyl Complex

Gentle thermolysis of the allyl complex, Cp*W(NO)(CH<sub>2</sub>CMe<sub>3</sub>)(η<sup>3</sup>-H<sub>2</sub>CCHCMe<sub>2</sub>) (<b>1</b>), at 50 °C in neat hydrocarbon solutions results in the loss of neopentane and the generation of transient intermediates that subsequently activate solvent C−H bonds. Thus, thermal reactions of <b>1</b> with tetramethylsilane, mesitylene, and benzene effect single C−H activations and lead to the exclusive formation of Cp*W(NO)(CH<sub>2</sub>SiMe<sub>3</sub>)(η<sup>3</sup>-H<sub>2</sub>CCHCMe<sub>2</sub>) (<b>2</b>), Cp*W(NO)(CH<sub>2</sub>C<sub>6</sub>H<sub>3</sub>-3,5-Me<sub>2</sub>)(η<sup>3</sup>-H<sub>2</sub>CCHCMe<sub>2</sub>) (<b>3</b>), and Cp*W(NO)(C<sub>6</sub>H<sub>5</sub>)(η<sup>3</sup>-H<sub>2</sub>CCHCMe<sub>2</sub>) (<b>4</b>), respectively. The products of reactions of <b>1</b> with other methyl-substituted arenes indicate an inherent preference of the system for the activation of stronger arene sp<sup>2</sup> C−H bonds. For example, C−H bond activation of <i>p</i>-xylene leads to the formation of Cp*W(NO)(CH<sub>2</sub>C<sub>6</sub>H<sub>4</sub>-4-Me)(η<sup>3</sup>-H<sub>2</sub>CCHCMe<sub>2</sub>) (<b>5</b>) (26%) and Cp*W(NO)(C<sub>6</sub>H<sub>3</sub>-2,5-Me<sub>2</sub>)(η<sup>3</sup>-H<sub>2</sub>CCHCMe<sub>2</sub>) (<b>6</b>) (74%). Mechanistic and labeling studies indicate that the transient C−H-activating intermediates are the allene complex, Cp*W(NO)(η<sup>2</sup>-H<sub>2</sub>CCCMe<sub>2</sub>) (<b>A</b>), and the η<sup>2</sup>-diene complex, Cp*W(NO)(η<sup>2</sup>-H<sub>2</sub>CCHC(Me)CH<sub>2</sub>) (<b>B</b>). Intermediates <b>A</b> and <b>B</b> react with cyclohexene to form Cp*W(NO)(η<sup>3</sup>-CH<sub>2</sub>C(2-cyclohexenyl)CMe<sub>2</sub>)(H) (<b>18</b>) and Cp*W(NO)(η<sup>3</sup>-CH<sub>2</sub>CHC)(Me)CH<sub>2</sub>C<sub>β</sub>H(C<sub>4</sub>H<sub>8</sub>)C<sub>α</sub>H (<b>19</b>), respectively, and intermediate <b>A</b> can be isolated as its PMe<sub>3</sub> adduct, Cp*W(NO)(PMe<sub>3</sub>)(η<sup>2</sup>-H<sub>2</sub>CCCMe<sub>2</sub>) (<b>20</b>). Interestingly, thermal reaction of <b>1</b> with 2,3-dimethylbut-2-ene results in the formation of a species that undergoes η<sup>3</sup> → η<sup>1</sup> isomerization of the dimethylallyl ligand following the initial C−H bond-activating step to yield Cp*W(NO)(η<sup>3</sup>-CMe<sub>2</sub>CMeCH<sub>2</sub>)(η<sup>1</sup>-CH<sub>2</sub>CHCMe<sub>2</sub>) (<b>21</b>). Thermolyses of <b>1</b> in alkane solvents afford allyl hydride complexes resulting from three successive C−H bond-activation reactions. For instance, <b>1</b> in cyclohexane converts to Cp*W(NO)(η<sup>3</sup>-C<sub>6</sub>H<sub>9</sub>)(H) (<b>22</b>) with dimethylpropylcyclohexane being formed as a byproduct, and in methylcyclohexane it forms the two isomeric complexes, Cp*W(NO)(η<sup>3</sup>-C<sub>7</sub>H<sub>11</sub>)(H) (<b>23a</b>,<b>b</b>). All new complexes have been characterized by conventional spectroscopic methods, and the solid-state molecular structures of <b>2</b>, <b>3</b>, <b>4</b>, <b>18</b>, <b>19</b>,<b> 20</b>, and <b>21</b> have been established by X-ray crystallographic analyses.