Unprecedented Noncatalyzed anti-Carbozincation of Diethyl Acetylenedicarboxylate through Alkylzinc Group Radical Transfer

Trans-Concerted Addition to Alkynes: the case of Ynamide Silylzincation

An original concerted antarafacial mechanism for the addition of diorganosilyl-zinc reagents across the C-C triple bond of ynamides is computationally investigated using DFT calculations. This concerted mechanism, leading to a trans-product in only one step, results in the formation of a Si-C and a Zn-C σ-bond on opposite sides of the π-system. We demonstrate that the mechanism going through a η2-vinyl intermediate and the proposal of a radical chain pathway are energetically unsustainable. The retained concerted antarafacial pathway is tested on experimental selectivities: the regioselectivity, in favor of the silyl β-addition in ynamide, and stereoselectivity, which is cis- with (Me2PhSi)2Zn but trans- with [(Me3Si)3Si]2Zn, are well reproduced by DFT calculations. The regio- and stereoselectivity are discussed using the activation strain model and a chemical bonding analysis.

Reductive anti-Dizincation of Arylacetylenes

Arylacetylenes undergo anti-1,2-dizincation to afford trans-1,2-dizincioalkenes. The process employs sodium dispersion as a reducing agent and zinc chloride TMEDA complex as a reduction-resistant zinc electrophile. This reductive anti-dizincation contrasts with the conventional additive syn-dimetalation like silylzincation. The resulting dizincated alkenes undergo the cross-coupling to yield multi-substituted alkenes stereoselectively.

Diethylzinc-Mediated Radical 1,2-Addition of Alkenes and Alkynes

A novel diethylzinc-mediated radical 1,2-addition of perfluoroalkyl iodides to unactivated alkenes and alkynes is presented, which demonstrates a novel way to generate an ethyl difluoroacetate radical. This method is highly efficient and gives full conversions of the substrates, high yields of the products, and negligible byproducts and requires no column chromatography purifications. The mild conditions enable this protocol to exhibit excellent functional group compatibility.

Regio- and Stereoselective Anti-Carbozincation of Alkynyl Ethers Using ZnBr2 toward (Z)-β-Zincated Enol Ether Synthesis

(Z)-β-Aryloxyalkenylzincs are synthesized stereoselectively via anti-carbozincation among alkynyl ethers, silyl ketene acetals, and ZnBr₂. X-ray analysis revealed the structure of the zinc species is a mononuclear two-coordinate dialkenylzinc that is transformed into functionalized enol ethers as a single isomer in the reaction of various electrophiles.

Nickel-catalyzed alkyl-zincation and carboxylation of diynes

A nickel-catalyzed carbo-carboxylation of diynes with ZnR₂ is realized with the help of ester functionality demonstrating excellent chemo-, regio- and stereoselectivity.

Radical zinc-atom-transfer-based carbozincation of haloalkynes with dialkylzincs

The formation of alkylidenezinc carbenoids by 1,4-addition/carbozincation of dialkylzincs or alkyl iodides based on zinc atom radical transfer, in the presence of dimethylzinc with β-(propargyloxy)enoates having pendant iodo- and bromoalkynes, is disclosed. Formation of the carbenoid intermediate is fully stereoselective at −30 °C and arises from a formal anti-selective carbozincation reaction. Upon warming, the zinc carbenoid is stereochemically labile and isomerizes to its more stable form.

Aminomethylation of Michael acceptors: complementary radical and polar approaches mediated by dialkylzincs

Phtalimidomethyl iodide and substituted maleimidomethyl iodide were used as radical precursors in dialkylzinc-mediated radical addition to diethyl fumarate. The reactions led stereoselectively to functionalized pyrrolizidines. The radical mechanism was supported by spin-trapping experiments and rationalized by theoretical calculations. Radical additions, on the one hand, and carbozincation followed by transmetalation with copper(I), on the other, were shown to be complementary methods to achieve the formal aminomethylation of activated unsaturated compounds.