ABSTRACT
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 ß-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.
ABSTRACT
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,ß-disubstituted enamides.
ABSTRACT
The copper(I) alkoxide-catalyzed release of a silicon-based cuprate reagent from a silicon-boron pronucleophile is applied to the addition across carbon-carbon triple bonds. Commercially available CuBrâ Me2S was found to be a general precatalyst that secures high regiocontrol for both aryl- and alkyl-substituted terminal as well as internal alkynes. The solvent greatly influences the regioisomeric ratio, favoring the linear regioisomer with terminal acceptors. This facile protocol even allows for the transformation of internal acceptors with remarkable levels of regio- and diastereocontrol.
