Catalytic and Stoichiometric Cumulene Formation within Dimeric Group 2 Acetylides

A series of β-diketiminate-supported magnesium and calcium acetylide complexes have been synthesized by σ-bond metathesis of magnesium n-butyl or magnesium and calcium amido precursors and a range of terminal acetylenes. The dimeric complexes have been characterized by NMR spectroscopy and X-ray diffraction analysis. The homoleptic bis­(amido) and dialkyl complexes [M­{X­(SiMe3)2}2(THF)2] (M = Ca, Sr; X = N, CH) have been assessed for the atom-efficient, catalytic head-to-head dimerization of donor-functionalized terminal alkynes into butatrienes and aryl-/silyl-substituted terminal acetylenes into 1,3-enynes. Deuterium labeling studies of the catalytic reactions are suggested to imply that triene formation requires concerted proton delivery and rearrangement via an adjacent methylene group at a bimetallic alkaline-earth species.