Abstraction of a Vinylic Hydrogen to Form Alkynes.
Multinuclear and Multidimensional NMR Spectroscopy and Computational
Studies Elucidating Structural Solution Behavior of Acetylene and
Propyne Complexes of Titanium
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Abstract
The
alkyne complexes [(PNP)Ti(η<sup>2</sup>-HCCH)(CH<sub>2</sub><sup>t</sup>Bu)] (<b>2</b>) and [(PNP)Ti(η<sup>2</sup>-HCCMe)(CH<sub>2</sub><sup>t</sup>Bu)] (<b>3</b>) have been prepared by treatment of [(PNP)TiCH<sup>t</sup>Bu(OTf)] (<b>1</b>) with the Grignard reagents H<sub>2</sub>CCHMgCl and MeHCCHMgBr, respectively. Complex <b>3</b> can be also prepared using the Grignard H<sub>2</sub>CC(Me)MgBr
and <b>1</b>. The 2-butyne complex [(PNP)Ti(η<sup>2</sup>-MeCCMe)(CH<sub>2</sub><sup>t</sup>Bu)] (<b>4</b>)
can be similarly prepared from <b>1</b> and MeHCC(Me)MgBr.
Complexes <b>2</b> and <b>3</b> have been characterized
with a battery of multidimensional and multinuclear (<sup>1</sup>H, <sup>13</sup>C, and <sup>31</sup>P) NMR spectroscopic experiments, including
selectively <sup>31</sup>P decoupled <sup>1</sup>H{<sup>31</sup>P}, <sup>1</sup>H–<sup>31</sup>P HMBC, <sup>1</sup>H–<sup>31</sup>P HOESY, and <sup>31</sup>P EXSY. Variable-temperature <sup>1</sup>H and <sup>31</sup>P{<sup>1</sup>H} NMR spectroscopy reveals that
the acetylene ligand in <b>2</b> exhibits a rotational barrier
of 11 kcal mol<sup>–1</sup>, and such a process has been corroborated
by theoretical studies. Formation of the titanium alkyne ligand in
complexes <b>2</b> and <b>3</b> proceeds via the vinyl
intermediate [(PNP)TiCH<sup>t</sup>Bu(CHCHR)] followed
by a concerted, metal-mediated β-hydrogen abstraction step that
has been computed to have a barrier of 20–22 kcal mol<sup>–1</sup>. The geometry and rotational mechanism of the alkyne ligand in <b>2</b> are presented and compared with those of the ethylene derivative
[(PNP)Ti(η<sup>2</sup>-H<sub>2</sub>CCH<sub>2</sub>)(CH<sub>2</sub><sup>t</sup>Bu)] (<b>5</b>), which does not display
rotation of the bound ethylene under the same conditions
