Cobalt-Catalyzed Benzylzincation of Alkynes

Iron-Catalyzed Vinylzincation of Terminal Alkynes

Organozinc reagents are among the most commonly used organometallic reagents in modern synthetic chemistry, and multifunctionalized organozinc reagents can be synthesized from structurally simple, readily available ones by means of alkyne carbozincation. However, this method suffers from poor tolerance for terminal alkynes, and transformation of the newly introduced organic groups is difficult, which limits its applications. Herein, we report a method for vinylzincation of terminal alkynes catalyzed by newly developed iron catalysts bearing 1,10-phenanthroline-imine ligands. This method provides efficient access to novel organozinc reagents with a diverse array of structures and functional groups from readily available vinylzinc reagents and terminal alkynes. The method features excellent functional group tolerance (tolerated functional groups include amino, amide, cyano, ester, hydroxyl, sulfonyl, acetal, phosphono, pyridyl), a good substrate scope (suitable terminal alkynes include aryl, alkenyl, and alkyl acetylenes bearing various functional groups), and high chemoselectivity, regioselectivity, and stereoselectivity. The method could significantly improve the synthetic efficiency of various important bioactive molecules, including vitamin A. Mechanistic studies indicate that the new iron-1,10-phenanthroline-imine catalysts developed in this study have an extremely crowded reaction pocket, which promotes efficient transfer of the vinyl group to the alkynes, disfavors substitution reactions between the zinc reagent and the terminal C-H bond of the alkynes, and prevents the further reactions of the products. Our findings show that iron catalysts can be superior to other metal catalysts in terms of activity, chemoselectivity, regioselectivity, and stereoselectivity when suitable ligands are used.

Recent advances in the stereoselective synthesis of acyclic all-carbon tetrasubstituted alkenes

Alkenes bearing four carbon-based groups are ubiquitous motifs in chemical sciences due to their various applications from medicinal to materials chemistry, and as chemical platforms for the synthesis of complex, chiral molecules. As such, tremendous research efforts are currently ongoing in order to develop general procedures for the challenging stereoselective synthesis of all-carbon tetrasubstituted alkenes, especially for acyclic structures. Since classical approaches to carbon-carbon double bonds are not suitable for the high steric demand around tetrasubstituted alkenes, a variety of unique approaches to access these privileged functional groups have been developed in recent years. This review article highlights the most significant developments in the field from 2007 to 2020, with an emphasis on the mechanisms and remaining limitations of these contemporary methods. Specifically, recent advances in internal alkyne carbofunctionalizations, in multicomponent couplings or other cross-couplings from nucleophilic or electrophilic alkenyl partners, and in the development of miscellaneous methods, are discussed.

Syntheses of 7-Substituted Anthra[2,3- b]thiophene Derivatives and Naphtho[2,3- b:6,7- b']dithiophene

7-R-Anthra[2,3- b]thiophene derivatives (1, R = H, Me, i-Pr, or MeO) are prepared in three steps (in average overall yield >50%) starting from ( E)-4-RC₆H₄CH₂(HOCH₂)C═CI(CH₂OH). The latter are commercial or readily prepared from 2-butyne-1,4-diol and ArCH₂Cl (both costing <1 cent/mmol) at 10 g scales. These allow for the selective formation of (otherwise unattainable) higher solubility 7-derivatives. Similar methods allow for the preparation of naphtho[2,3- b:6,7- b']dithiophene 2 using equally low-cost starting materials.

Preparation of Organozinc Reagents via Catalyst Controlled Three-Component Coupling between Alkyne, Iodoarene, and Bis(iodozincio)methane

Three-component coupling between an alkyne, iodoarene, and bis(iodozincio)methane yields allylic zinc with a tetrasubstituted alkene moiety in the presence of a nickel catalyst. The reaction proceeds via aryl nickelation of the alkyne and subsequent cross-coupling with bis(iodozincio)methane. Meanwhile, the same combination in the presence of a palladium and cobalt catalyst gives tetrasubstituted alkenylzinc. The reaction proceeds via a palladium-catalyzed cross-coupling of iodoarene with bis(iodozincio)methane followed by a cobalt-catalyzed benzylzincation of alkyne.

Cobalt-Catalyzed C(sp2)-C(sp3) Cross-Coupling Reactions of Diarylmanganese Reagents with Secondary Alkyl Iodides

A cobalt-catalyzed cross-coupling of diarylmanganese reagents with secondary alkyl iodides using the THF-soluble salt CoCl₂·2LiCl, which leads to the cross-coupling products in up to 92% yield, is reported. High diastereoselectivities can be reached in these cross-couplings (dr up to 99:1). Remarkably, rearrangement of secondary alkyl iodides to unbranched products was not observed in these C-C forming reactions.

Nickel-Catalyzed Stereoselective Dicarbofunctionalization of Alkynes

A nickel-catalyzed three-component reaction involving terminal alkynes, boronic acids, and alkyl halides is presented herein. Trisubstituted alkenes can be obtained in a highly regio- and stereocontrolled manner by the simultaneous addition of both aryl and alkyl groups across the triple bond in a radical-mediated process. The reaction, devoid of air- and moisture-sensitive organometallic reagents and catalysts, is operationally simple and offers a broad scope and functional-group tolerance.

Recent advances in transition-metal-catalyzed intermolecular carbomagnesiation and carbozincation

Carbomagnesiation and carbozincation reactions are efficient and direct routes to prepare complex and stereodefined organomagnesium and organozinc reagents. However, carbon-carbon unsaturated bonds are generally unreactive toward organomagnesium and organozinc reagents. Thus, transition metals were employed to accomplish the carbometalation involving wide varieties of substrates and reagents. Recent advances of transition-metal-catalyzed carbomagnesiation and carbozincation reactions are reviewed in this article. The contents are separated into five sections: carbomagnesiation and carbozincation of (1) alkynes bearing an electron-withdrawing group; (2) alkynes bearing a directing group; (3) strained cyclopropenes; (4) unactivated alkynes or alkenes; and (5) substrates that have two carbon-carbon unsaturated bonds (allenes, dienes, enynes, or diynes).

Rhodium-catalyzed arylzincation of terminal allenes providing allylzinc reagents and its application to versatile three-component coupling reaction

Rhodium-catalyzed regioselective arylzincation of terminal allenes affords synthetically useful functionalized allylzinc reagents. The allylzinc reagents react with a variety of electrophiles such as acetonitrile, offering a more versatile three-component coupling reaction.