Enantio- and Regioconvergent Nickel-Catalyzed C(sp3 )-C(sp3 ) Cross-Coupling of Allylic Electrophiles Steered by a Silyl Group

Metal-Catalyzed Enantioconvergent Transformations

Enantioconvergent catalysis has expanded asymmetric synthesis to new methodologies able to convert racemic compounds into a single enantiomer. This review covers recent advances in transition-metal-catalyzed transformations, such as radical-based cross-coupling of racemic alkyl electrophiles with nucleophiles or racemic alkylmetals with electrophiles and reductive cross-coupling of two electrophiles mainly under Ni/bis(oxazoline) catalysis. C-H functionalization of racemic electrophiles or nucleophiles can be performed in an enantioconvergent manner. Hydroalkylation of alkenes, allenes, and acetylenes is an alternative to cross-coupling reactions. Hydrogen autotransfer has been applied to amination of racemic alcohols and C-C bond forming reactions (Guerbet reaction). Other metal-catalyzed reactions involve addition of racemic allylic systems to carbonyl compounds, propargylation of alcohols and phenols, amination of racemic 3-bromooxindoles, allenylation of carbonyl compounds with racemic allenolates or propargyl bromides, and hydroxylation of racemic 1,3-dicarbonyl compounds.

Enantio- and Regioconvergent Synthesis of γ-Stereogenic Vinyl Germanes and Their Use as Masked Vinyl Halides

A nickel-catalyzed enantio- and regioconvergent alkylation of regioisomeric mixtures of racemic germylated allylic electrophiles with alkyl nucleophiles is reported. Key to success is a newly developed hept-4-yl-substituted Pybox ligand that enables accessing various chiral γ-germyl α-alkyl allylic building blocks in excellent yields and enantioselectivities. The reason for the regioconvergence is the steering effect of the bulky germyl group. The resulting vinyl germanes can be easily halodegermylated without racemization of the allylic stereocenter to afford synthetically valuable γ-stereogenic vinyl halides.

Asymmetric construction of allylicstereogenic carbon center featuring atrifluoromethyl group via enantioselective reductive fluoroalkylation

Emerging as a powerful tool for lead optimization in pharmaceutical research and development, to develop the facile, general protocols that allows the incorporation of fluorine-containing motif in drug candidates has accumulated enormous research interest in recent years. Among these important motifs, the incorporation of strategic motif CF₃ on aliphatic chain especially with the concomitant construction of trifluoromethylated alkanes bearing a CF₃-substituted stereogenic carbon, is of paramount importance. Herein, we disclose an asymmetric nickel-catalyzed reductive trifluoroalkylation of alkenyl halides for enantioselective syntheses of diverse α-trifluoromethylated allylic alkanes, offering a general protocol to access the trifluoromethyl analogue to chiral α-methylated allylic alkanes, one of the most prevalent key components among natural products and pharmaceuticals. Utilities of the method including the application of the asymmetric trifluoroalkylation on multiple biologically active complex molecules, derivatization of transformable alkenyl functionality were demonstrated, providing a facile method in the diversity-oriented syntheses of CF₃-containing chiral drugs and bioactive-molecules.

Enantio- and Regioconvergent Nickel-Catalyzed Allylic Substitution of Racemic α- or γ-Silylated Allylic Bromides with Benzylzinc Reagents

An enantio- and regioconvergent nickel-catalyzed benzylation of racemic silylated allylic electrophiles with benzylzinc nucleophiles is reported. The key feature of this method is that the homocoupling pathways of both the nucleophile and the electrophile are minimized. A diverse set of electronically modified benzylzinc reagents was tolerated. The vinylsilane products with allylic stereocenters were formed in moderate to high yields with high enantioselectivities. The regioconvergence is the result of the steering effect of the silyl group.

DFT insight into asymmetric alkyl-alkyl bond formation via nickel-catalysed enantioconvergent reductive coupling of racemic electrophiles with olefins

A DFT study has been conducted to understand the asymmetric alkyl–alkyl bond formation through nickel-catalysed reductive coupling of racemic alkyl bromide with olefin in the presence of hydrosilane and K₃PO₄. The key findings of the study include: (i) under the reductive experimental conditions, the Ni(ii) precursor is easily activated/reduced to Ni(0) species which can serve as an active species to start a Ni(0)/Ni(ii) catalytic cycle. (ii) Alternatively, the reaction may proceed via a Ni(i)/Ni(ii)/Ni(iii) catalytic cycle starting with a Ni(i) species such as Ni(i)–Br. The generation of a Ni(i) active species via comproportionation of Ni(ii) and Ni(0) species is highly unlikely, because the necessary Ni(0) species is strongly stabilized by olefin. Alternatively, a cage effect enabled generation of a Ni(i) active catalyst from the Ni(ii) species involved in the Ni(0)/Ni(ii) cycle was proposed to be a viable mechanism. (iii) In both catalytic cycles, K₃PO₄ greatly facilitates the hydrosilane hydride transfer for reducing olefin to an alkyl coupling partner. The reduction proceeds by converting a Ni–Br bond to a Ni–H bond via hydrosilane hydride transfer to a Ni–alkyl bond via olefin insertion. On the basis of two catalytic cycles, the origins for enantioconvergence and enantioselectivity control were discussed.

The enantioconvergent alkyl–alkyl coupling involves two competitive catalytic cycles with nickel(0) and nickel(i) active catalysts, respectively. K₃PO₄ plays a crucial role to enable the hydride transfer from hydrosilane to nickel–bromine species.

Solvent-free synthesis of enantioenriched β-silyl nitroalkanes under organocatalytic conditions

An enantioselective 1,4-conjugate addition of nitromethane to β-silyl α,β-unsaturated carbonyl compounds catalyzed by bifunctional squaramide catalysts has been developed. This methodology offers both enantiomers of β-silyl nitroalkanes in good to excellent yields (up to 92%) and enantioselectivities (up to 97.5% ee) under solvent-free conditions at room temperature. Control experiments reveal that the presence of a β-silyl group in the enones is crucial for high reactivity under the optimized reaction conditions.

Bifunctional squaramide catalyzed asymmetric synthesis of chiral α-mercaptosilanes

Bifunctional squaramide-catalyzed nucleophilic addition of thiophenols to easily available β-silyl α,β-unsaturated carbonyl compounds has been successfully developed. A structurally diverse set of chiral α-mercaptosilanes was efficiently prepared in good to excellent yields with acceptable enantioselectivities. The reaction features mild reaction conditions, a broad substrate scope, and easy scale-up.

Enantio- and Regioconvergent Nickel-Catalyzed C(sp3 )-C(sp3 ) Cross-Coupling of Allylic Electrophiles Steered by a Silyl Group

A two‐step sequence for the enantio‐ and diastereoselective synthesis of exclusively alkyl‐substituted acyclic allylic systems with a stereocenter in the allylic position is reported. The asymmetric induction and the site selectivity are controlled in an enantio‐ and regioconvergent nickel‐catalyzed C(sp³)−C(sp³) cross‐coupling of regioisomeric mixtures of racemic α‐/γ‐silylated allylic halides and primary alkylzinc reagents. The silyl group steers the allylic displacement towards the formation of the vinylsilane regioisomer, and the resulting C(sp²)−Si bond serves as a linchpin for the installation of various C(sp³) substituents in a subsequent step.