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.
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 CuBr⋅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.
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.
Aryl-and hetaryl-substituted acetylenes undergo regio-and stereoselective silylzincation by reaction with [(Me 3 Si) 3 Si] 2 Zn in the presence of Et 2 Zn (10-110 mol%) as additive. The distinctive feature of this addition across the CÀ C triple bond is its trans stereoselectivity. The radical nature of the silylzincation process is supported by diagnostic experiments and DFT calculations, which also corroborate the role played by steric effects to obtain that stereoselectivity. The procedure can be combined in one-pot with the copper(I)-mediated electrophilic substitution of the C(sp 2 )À Zn bond, with retention of the double bond geometry. This makes it valuable for the synthesis of stereodefined di-and trisubstituted vinylsilanes.
Sulfur-, oxygen-, and phosphorus-substituted terminal alkynes undergo regio-and stereoselective silylzincation by reaction with (Me 2 PhSi) 2 Zn, (Me 3 Si) 3 SiH/Et 2 Zn or [(Me 3 Si) 3 Si] 2 Zn/Et 2 Zn. The addition across the CC triple bond always occurs with β-regioselectivity but the stereoselectivity is tunable: (Me 2 PhSi) 2 Zn for cis and (Me 3 Si) 3 SiH/Et 2 Zn or [(Me 3 Si) 3 Si] 2 Zn/Et 2 Zn for trans. The procedures making use of the zinc reagents (Me 2 PhSi) 2 Zn and [(Me 3 Si) 3 Si] 2 Zn can be combined in one-pot with a subsequent stereoretentive copper(I)mediated electrophilic substitution of the intermediate C(sp 2)-Zn bond. These stereodivergent protocols offer a regio-and stereoselective access to trisubstituted vinylsilanes decorated with sulfur-, oxygen-, and phosphorus substituents with either double-bond geometry.
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