Direct Synthesis of Ketones from Methyl Esters by Nickel-Catalyzed Suzuki-Miyaura Coupling.

The direct conversion of alkyl esters to ketones has been hindered by the sluggish reactivity of the starting materials and the susceptibility of the product towards subsequent nucleophilic attack. We have now achieved a cross-coupling approach to this transformation using nickel, a bulky N-heterocyclic carbene ligand, and alkyl organoboron coupling partners. 65 alkyl ketones bearing diverse functional groups and heterocyclic scaffolds have been synthesized with this method. Catalyst-controlled chemoselectivity is observed for C(acyl)-O bond activation of multi-functional substrates bearing other bonds prone to cleavage by Ni, including aryl ether, aryl fluoride, and N-Ph amide functional groups. Density functional theory calculations provide mechanistic support for a Ni0 /NiII catalytic cycle and demonstrate how stabilizing non-covalent interactions between the bulky catalyst and substrate are critical for the reaction's success.

Exhaustive Reduction of Esters Enabled by Nickel Catalysis.

We report a one-step procedure to directly reduce unactivated aryl esters into their corresponding tolyl derivatives. This is achieved by an organosilane-mediated ester hydrosilylation reaction and subsequent Ni/NHC-catalyzed hydrogenolysis. The resulting conditions provide a direct and efficient alternative to multi-step procedures for this transformation that often require the use of hazardous metal hydrides. Applications in the synthesis of -CD3-containing products, derivatization of bioactive molecules, and chemoselective reduction in the presence of other C-O bonds are demonstrated.

Nickel-Catalyzed Domino Heck-Type Reactions Using Methyl Esters as Cross-Coupling Electrophiles.

While esters are frequently used as traditional electrophiles in substitution chemistry, their application in cross-coupling chemistry is still in its infancy. This work demonstrates that methyl esters can be used as coupling electrophiles in Ni-catalyzed Heck-type reactions through the challenging cleavage of the C(acyl)-O bond under relatively mild reaction conditions at either 80 or 100 °C. With the σ-NiII intermediate generated from the insertion of acyl NiII species into the tethered C=C bond, carbonyl-retentive products were formed by domino Heck/Suzuki-Miyaura coupling and Heck/reduction pathways when organoboron and mild hydride nucleophiles are used.

Brønsted Acid Enabled Nickel-Catalyzed Hydroalkenylation of Aldehydes with Styrene and its Derivatives.

A Brønsted acid enabled nickel-catalyzed hydroalkenylation of aldehydes and styrene derivatives has been developed. The Brønsted acid acts as a proton shuttle to transfer a proton from the alkene to the aldehyde, thereby leading to an economical and byproduct-free coupling. A series of synthetically useful allylic alcohols were obtained through one-step reactions from readily available styrene derivatives and aliphatic aldehydes in up to 88 % yield and with high linear selectivity.

Iron-Catalyzed Regioselective Transfer Hydrogenative Couplings of Unactivated Aldehydes with Simple Alkenes.

An FeBr3 -catalyzed reductive coupling of various aldehydes with alkenes that proceeds through a direct hydride transfer pathway has been developed. With (i) PrOH as the hydrogen donor under mild conditions, previously challenging coupling reactions of unactivated alkyl and aryl aldehydes with simple alkenes, such as styrene derivatives and α-olefins, proceeded smoothly to furnish a diverse range of functionalized alcohols with complete linear regioselectivity.

Ni-catalyzed arylation of alkynes with organoboronic acids and aldehydes to access stereodefined allylic alcohols.

A new, efficient and practical method for the three-component arylative coupling of aldehydes, alkynes and arylboronic acids has been developed through nickel catalysis. This transformation provides diverse Z-selective tetrasubstituted allylic alcohols without the use of any aggressive oragnometallic nucleophiles or reductants. Moreover, benzylalcohols are viable coupling partners via oxidation state manipulation and arylative coupling in one single catalytic cycle. This reaction features a direct and flexible approach for the preparation of stereodefined arylated allylic alcohols with broad substrate scope under mild conditions. The utility of this protocol is demonstrated through the synthesis of diverse biologically active molecular derivatives.

Regio- and chemoselective hydroamination of unactivated alkenes with anthranils via NiH-catalysis.

A NiH-catalyzed polarity-reversed hydroamination of ß,γ-, γ,δ- and δ,ε-unsaturated alkenes with electrophilic anthranils was developed. This reaction proceeds in a highly regio- and chemoselective manner to afford γ, δ and ε-arylamines bearing a carbonyl or alcohol functionality with 100% atom efficiency. Preliminary mechanistic studies indicate that the chemoselectivity is controlled by the base and the alcohol product is derived from the base-catalyzed hydrosilylation of the CO bond.

Cross-coupling reactions with esters, aldehydes, and alcohols.

Cross-coupling reactions to form biaryls and π bond addition reactions to prepare substituted carbonyls or alcohols represent two of the most frequently performed families of chemical reactions. Recent progress in catalysis has uncovered substantial overlap between these two seemingly distinct topics. In particular, esters, aldehydes, and alcohols have been shown to act as carbon-based coupling partners in a range of Ni- and Pd-catalyzed reactions to prepare amides, ketones, substituted alcohols, alkanes, and more. These reactions provide promising alternatives to commonly used stoichiometric or multi-step reaction sequences. In this feature article, a selection of these transformations will be discussed with an emphasis on the key mechanistic steps that allow these non-traditional substrates to be incorporated into cross-coupling-like catalytic cycles.