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

The alkyne complexes [(PNP)Ti(η2-HCCH)(CH2 tBu)] (2) and [(PNP)Ti(η2-HCCMe)(CH2 tBu)] (3) have been prepared by treatment of [(PNP)TiCHtBu(OTf)] (1) with the Grignard reagents H2CCHMgCl and MeHCCHMgBr, respectively. Complex 3 can be also prepared using the Grignard H2CC(Me)MgBr and 1. The 2-butyne complex [(PNP)Ti(η2-MeCCMe)(CH2 tBu)] (4) can be similarly prepared from 1 and MeHCC(Me)MgBr. Complexes 2 and 3 have been characterized with a battery of multidimensional and multinuclear (1H, 13C, and 31P) NMR spectroscopic experiments, including selectively 31P decoupled 1H{31P}, 1H–31P HMBC, 1H–31P HOESY, and 31P EXSY. Variable-temperature 1H and 31P{1H} NMR spectroscopy reveals that the acetylene ligand in 2 exhibits a rotational barrier of 11 kcal mol–1, and such a process has been corroborated by theoretical studies. Formation of the titanium alkyne ligand in complexes 2 and 3 proceeds via the vinyl intermediate [(PNP)TiCHtBu(CHCHR)] followed by a concerted, metal-mediated β-hydrogen abstraction step that has been computed to have a barrier of 20–22 kcal mol–1. The geometry and rotational mechanism of the alkyne ligand in 2 are presented and compared with those of the ethylene derivative [(PNP)Ti(η2-H2CCH2)(CH2 tBu)] (5), which does not display rotation of the bound ethylene under the same conditions.