Dual Nickel- and Photoredox-Catalyzed Asymmetric Reductive Cross-Coupling To Access Chiral Secondary Benzylic Alcohols

Transition-metal-catalyzed asymmetric cross-coupling represents a powerful strategy for C-C bond formation and the synthesis of enantiomerically pure molecules. Here, we report a dual nickel/photoredox-catalyzed enantioselective reductive cross-coupling of aryl halides with α-bromobenzoates, readily generated from aliphatic aldehydes, to provide diverse chiral secondary benzylic alcohols that are important motifs in bioactive natural products and pharmaceuticals. This dual catalytic system features mild conditions, good functional group tolerance, broad substrate scope, excellent enantiocontrol, and avoidance of stoichiometric metal reductants, presenting great potential for late-stage functionalization of complex molecules.

First Reusable Catalyst for the Reductive Coupling Reaction of Organohalides with Aldehydes

In this study, we simulate the reductive coupling (Barbier-Grignard-type) reaction of organohalides with aldehydes using a new reusable catalyst. In this regard, bimetallic alloys of NiCo encapsulated in melamine-based dendrimers (MBD) immobilized on magnetic nanoparticles symbolized as γ-Fe2O3-MBD/NiCo were designed and synthesized. The structure and properties of the catalyst were studied by a variety of techniques such as Fourier transform infrared (FT-IR) spectroscopy, X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), transmission electron microscopy (TEM), thermogravimetric analysis (TGA), vibrating sample magnetometry (VSM), energy-dispersive spectrometry (EDS) mapping, and inductively coupled plasma (ICP). The presence of NiCo nanoalloys was confirmed by XRD and XPS analysis, TEM images, and EDS mapping. Various secondary alcohols were produced in good to high yields by reductive coupling of different types of aldehydes and organohalides in the presence of HCO2K as a nonmetallic reducing agent in aqueous media catalyzed by γ-Fe2O3-MBD/NiCo. In these reactions, the high catalytic performance of γ-Fe2O3-MBD/NiCo was achieved in comparison to monometallic counterparts due to the synergistic cooperative effect of Co and Ni in the NiCo nanoalloys. Magnetic and hydrophilic properties of the catalyst facilitate the catalyst recyclability for seven runs. The reusability of γ-Fe2O3-MBD/NiCo, use of water as an environmentally friendly solvent, ease of processing, and absence of metal additives make this process an excellent choice for the reductive coupling reaction to produce secondary alcohols from aldehydes. This is the first report on these kinds of reactions using a reusable catalyst.

Photoassisted Cobalt-Catalyzed Asymmetric Reductive Grignard-Type Addition of Aryl Iodides

Grignard addition is one of the most important methods used for syntheses of alcohol compounds and has been known for over a hundred years. However, research on asymmetric catalysis relies on the use of organometallic nucleophiles. Here, we report the first visible-light-induced cobalt-catalyzed asymmetric reductive Grignard-type addition for synthesizing chiral benzyl alcohols (>50 examples, up to 99% yield, and 99% ee). This methodology has the advantages of mild reaction conditions, good functionality tolerance, excellent enantiocontrol, the avoidance of mass metal wastes, and the use of precious metal catalysts. Kinetic realization studies suggested that migratory insertion of an aryl cobalt species into the aldehyde was the rate-determining step of the reductive addition reaction.

Design and Synthesis of Tunable Chiral 2,2'-Bipyridine Ligands: Application to the Enantioselective Nickel-Catalyzed Reductive Arylation of Aldehydes

A new class of chiral 2,2'-bipyridine ligands, SBpy, featuring minimized short-range steric hindrance and structural tunability was rationally designed and developed, and the effectiveness was demonstrated in the first highly enantioselective Ni-catalyzed addition of aryl halides to aldehydes. In comparison with known approaches using preformed aryl metallic reagents, this reaction is more step-economical and functional group tolerant. The reaction mechanism and a model of stereocontrol were proposed based on experimental and computational results.

Nickel-Catalyzed Intermolecular Asymmetric Addition of Aryl Iodides across Aldehydes

Enantioenriched alcohols comprise much of the framework of organic molecules. Here, we first report that chiral nickel complexes can catalyze the intermolecular enantioselective addition of aryl iodides across aldehydes to provide diverse optically active secondary alcohols using zinc metal as the reducing agent. This method shows a broad substrate scope under mild reaction conditions and precludes the traditional strategy through the pre-generation of organometallic reagents. Mechanistic studies indicate that an in situ formed arylnickel, instead of an arylzinc, adds efficiently to aldehydes, forming a new C-C bond and a chiral nickel alkoxide that may be turned over by zinc powder.

Catalytic Aldehyde and Alcohol Arylation Reactions Facilitated by a 1,5-Diaza-3,7-diphosphacyclooctane Ligand

We report a catalytic method to access secondary alcohols by the coupling of aryl iodides. Either aldehydes or alcohols can be used as reaction partners, making the transformation reductive or redox-neutral, respectively. The reaction is mediated by a Ni catalyst and a 1,5-diaza-3,7-diphosphacyclooctane. This P₂N₂ ligand, which has previously been unrecognized in cross-coupling and related reactions, was found to avoid deleterious aryl halide reduction pathways that dominate with more traditional phosphines and NHCs. An interrupted carbonyl-Heck type mechanism is proposed to be operative, with a key 1,2-insertion step forging the new C-C bond and forming a nickel alkoxide that may be turned over by an alcohol reductant. The same catalyst was also found to enable synthesis of ketone products from either aldehydes or alcohols, demonstrating control over the oxidation state of both the starting materials and products.

Nickel-Catalyzed Reductive Allylation of Aldehydes with Allyl Acetates

Carbonyl allylation reactions constitute an important step in the formation of carbon–carbon reactions, and involve various related reactions that chiefly use allylmetal reagents. This report presents a nickel-catalyzed carbonyl allylation reaction using allyl acetate, which produces homoallyl alcohols in moderate to good yields, as an efficient methodology under reductive coupling conditions.

Cross-coupling reactions with esters, aldehydes, and alcohols

This feature article describes how diverse oxygen-containing functional groups such as esters, aldehydes, and alcohols can participate in cross-coupling reactions to prepare amides, ketones, alcohols, and beyond.

Halogen–Lithium Exchange of Sensitive (Hetero)aromatic Halides under Barbier Conditions in a Continuous Flow Set-Up

A halogen–lithium exchange reaction of (hetero)aromatic halides performed in the presence of various electrophiles such as aldehydes, ketones, Weinreb amides, and imines using BuLi as exchange reagent and a commercially available flow set-up is reported. The organolithiums generated in situ were instantaneously trapped with various electrophiles (Barbier conditions) resulting in the formation of polyfunctional (hetero)arenes. This method enables the functionalization of (hetero)arenes containing highly sensitive functional groups such as esters­, which are not tolerated in batch conditions.

Nickel-Catalyzed Asymmetric Addition of Aromatic Halides to Ketones: Highly Enantioselective Synthesis of Chiral 2,3-Dihydrobenzofurans Containing a Tertiary Alcohol

A highly enantioselective and straightforward synthetic procedure to chiral 3-hydroxy-2,3-dihydrobenzofurans has been developed by nickel/bisoxazoline-catalyzed intramolecular asymmetric addition of aryl halides to unactivated ketones, giving 2,3-dihydrobenzofurans with a chiral tertiary alcohol at the C-3 position in good yields and excellent enantioselectivities (up to 92% yield and 98% ee). The gram-scale reaction also proceeded smoothly without a loss of yield and enantioselectivity.

Nickel-catalyzed intramolecular desymmetrization addition of aryl halides to 1,3-diketones

A nickel-catalyzed intramolecular addition of aryl halides to 1,3-diketones was first developed to afford a polycyclic framework with two tetrasubstituted carbons in excellent diastereoselectivity. Moderate enantioselectivities were also achieved.