The regio-and stereoselective addition of germanium and zinc across the CC triple bond of nitrogen-, sulfur-, oxygen-, and phosphorous-substituted terminal and internal alkynes is achieved by reaction with a combination of R 3 GeH and Et 2 Zn. Diagnostic experiments support a radical-chain mechanism and the -zincated vinylgermanes that show exceptional stability are characterized by NMR spectroscopy and X-ray crystallography. The unique feature of this new radical germylzincation reaction is that the C(sp 2)-Zn bond formed remains available for subsequent in situ Cu(I)-or Pd(0)-mediated CC or C-heteroatom bond formation with retention of the double bond geometry. These protocols offer a modular access to elaborated tri-and tetrasubstituted vinylgermanes decorated with heteroatom substituents to germanium that are useful for the preparation of stereodefined alkenes. Additional data and discussion, experimental details, NMR spectra for new compounds, and X-ray crystal structures (PDF) Crystallographic information files (CIF)
The silylzincation of terminal ynamides is achieved through a radical-chain process involving (Me3Si)3SiH and R2Zn. A potentially competing polar mechanism is excluded on the basis of diagnostic control experiments. The unique feature of this addition across the C≡C bond is its trans selectivity. One-pot electrophilic substitution of the C(sp2)-Zn bond by Cu(I)-mediated C-C bond formation and subsequent manipulation of the C(sp2)-Si bond provides a modular access to Z-α,β-disubstituted enamides.
Zinc reagents (Me2PhSi)2Zn and [(Me3Si)3Si]2Zn undergo highly regio- and stereoselective addition across the carbon-carbon triple bond of nitrogen-, sulfur-, oxygen-, and phosphorus-substituted terminal alkynes in the absence of copper or any other catalyst. Both reagents yield exclusively β-isomers, and the stereoselectivity is determined by the silyl group: Me2PhSi for cis or (Me3Si)3Si for trans. These stereodivergent silylzincation protocols offer an efficient access to heteroatom-substituted vinylsilanes with either double bond geometry, including trisubstituted vinylsilanes by one-pot electrophilic substitution of the intermediate C(sp(2))-Zn bond by copper(I)-mediated carbon-carbon bond formation.
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