Mechanistic Understanding of Arylation vs Alkylation of Aliphatic Csp3–H Bonds by Decatungstate–Nickel Catalysis

Bis(pinacolato)diboron-Enabled Ni-Catalyzed Reductive Arylation/Vinylation of Alkyl Electrophiles

Herein, the use of economically and environmentally friendly bis(pinacolato)diboron (B2Pin2) is described as a non-metallic reductant in mediating Ni-catalyzed C(sp3)-C(sp2) reductive cross-coupling of alkyl electrophiles with aryl/vinyl halides. This method exhibits excellent suitability for heteroaryl halides and alkyl halides/Katritzky salts. The present study is compatible with an in situ halogenation of alcohol method, allowing for selective mono-functionalization of diols and bio-relevant alcohols (e.g., carbohydrates). The use of B2Pin2 shows potential for easy scalability without introducing additional metal impurities into the products. It is observed for the first time in the realm of cross-electrophile coupling chemistry that B2Pin2 can sever as a reductant to reduce NiII to Ni0. This mechanistic insight may inspire the development of new reductive bond-forming methodologies that can otherwise be difficult to achieve with a metal reductant.

C(sp3)-C(sp3) Coupling of Cycloalkanes and Alkyl Halides via Dual Photocatalytic Hydrogen Atom Transfer and Nickel Catalysis

Functionalization of C(sp3)-H bonds represents the most straightforward and atom-economical transformation in organic synthesis. An innovative approach integrating photocatalytic hydrogen atom transfer (HAT) and transition metal catalysis has made significant progress in the coupling of α-heterosubstituted C-H bonds with alkyl halides. However, unactivated alkanes were ineffective as a result of the preponderance of byproduct formation. Herein, we demonstrate direct HAT and nickel catalysis in the coupling of cycloalkanes and benzyl bromides/primary alkyl iodides. Additionally, tetrabutylammonium decatungstate (TBADT) was recovered and recycled.

O-H bond activation of β,γ-unsaturated oximes via hydrogen atom transfer (HAT) and photoredox dual catalysis

Hydrogen atom transfer (HAT) and photoredox dual catalysis provides a unique opportunity in organic synthesis, enabling the direct activation of C/Si/S-H bonds. However, the activation of O-H bonds of β,γ-unsaturated oximes poses a challenge due to their relatively high redox potential, which exceeds the oxidizing capacity of most currently developed photocatalysts. We here demonstrate that the combination of HAT and photoredox catalysis allows the activation of O-H bond of β,γ-unsaturated oximes. The strategy effectively addresses the oxime's high redox potential and offers a universal pathway for iminoxyl radical formation. Leveraging the versatility of this approach, a diverse array of valuable heterocycles have been synthesized with the use of different radical acceptors. Mechanistic studies confirm a HAT process for the O-H bond activation.

Nickel/Photo-Cocatalyzed Three-Component Alkyl-Acylation of Aryl-Activated Alkenes

Herein, we disclose a nickel/photo-cocatalyzed three-component alkyl-acylation of aryl-substituted alkenes with aldehydes and electron-withdrawing-group-activated alkyl bromides, providing straightforward access to various ketones under mild and ligand-free conditions. The photocatalyst TBADT plays a dual role in activating the acyl C-H bond of aldehydes via hydrogen atom transfer and reducing the C-Br bond of alkyl bromides via single-electron transfer. While the terminal C-C bond is forged through polarity-matched radical-type addition, nickel is likely involved in the acylation step.

Selective C(sp3)-H arylation/alkylation of alkanes enabled by paired electrocatalysis

We report a combination of electrocatalysis and photoredox catalysis to perform selective C(sp3)-H arylation/alkylation of alkanes, in which a binary catalytic system based on earth-abundant iron and nickel is applied. Reaction selectivity between two-component C(sp3)-H arylation and three-component C(sp3)-H alkylation is tuned by modulating the applied current and light source. Importantly, an ultra-low anodic potential (~0.23 V vs. Ag/AgCl) is applied in this protocol, thus enabling compatibility with a variety of functional groups (>70 examples). The robustness of the method is further demonstrated on a preparative scale and applied to late-stage diversification of natural products and pharmaceutical derivatives.

Regime Switch in the Dual-Catalyzed Coupling of Alkyl Silicates with Aryl Bromides

Metallaphotoredox catalyzed cross-coupling of an arylbromide (Ar-Br) with an alkyl bis(catecholato)silicate (R-Si⊖ ) has been analyzed in depth using a continuum of analytical techniques (EPR, fluorine NMR, electrochemistry, photophysics) and modeling (micro-kinetics and DFT calculations). These studies converged on the impact of four control parameters consisting in the initial concentrations of the iridium photocatalyst ([Ir]0 ), nickel precatalyst ([Ni]0 ) and silicate ([R-Si⊖ ]0 ) as well as light intensity I0 for an efficient reaction between Ar-Br and R-Si⊖ . More precisely, two regimes were found to be possibly at play. The first one relies on an equimolar consumption of Ar-Br with R-Si⊖ smoothly leading to Ar-R, with no side-product from R-Si⊖ and a second one in which R-Si⊖ is simultaneously coupled to Ar-Br and degraded to R-H. This integrative approach could serve as a case study for the investigation of other metallaphotoredox catalysis manifolds of synthetic significance.

The mechanism of visible light-induced C-C cross-coupling by Csp3-H bond activation

Csp3-C cross-coupling by activating Csp3-H bonds is a dream reaction for the chemical community, and visible light-induced transition metal-catalysis under mild reaction conditions is considered a powerful tool to achieve it. Advancement of this research area is still in its infancy because of the chemical and technical complexity of this catalysis. Mechanistic studies illuminating the operative reaction pathways can rationalize the increasing amount of experimental catalysis data and provide the knowledge allowing faster and rational advances in the field. This goal requires complementary experimental and theoretical mechanistic studies, as each of them is unfit to clarify the operative mechanisms alone. In this tutorial review we summarize representative experimental and computational mechanistic studies, highlighting weaknesses, strengths, and synergies between the two approaches.

Asymmetric imino-acylation of alkenes enabled by HAT-photo/nickel cocatalysis

By merging nickel-mediated facially selective aza-Heck cyclization and radical acyl C-H activation promoted by tetrabutylammonium decatungstate (TBADT) as a hydrogen atom transfer (HAT) photocatalyst, we accomplish an asymmetric imino-acylation of oxime ester-tethered alkenes with readily available aldehydes as the acyl source, enabling the synthesis of highly enantioenriched pyrrolines bearing an acyl-substituted stereogenic center under mild conditions. Preliminary mechanistic studies support a Ni(i)/Ni(ii)/Ni(iii) catalytic sequence involving the intramolecular migratory insertion of a tethered olefinic unit into the Ni(iii)-N bond as the enantiodiscriminating step.

Three-component carboacylation of alkenes via cooperative nickelaphotoredox catalysis

Various commercially available acyl chlorides, aldehydes, and alkanes were exploited for versatile three-component 1,2-carboacylations of alkenes to forge two vicinal C–C bonds through the cooperative action of nickel and sodium decatungstate catalysis. A wealth of ketones with high levels of structural complexity was rapidly obtained via direct functionalization of C(sp²)/C(sp³)–H bonds in a modular manner. Furthermore, a regioselective late-stage modification of natural products showcased the practical utility of the strategy, generally featuring high resource economy and ample substrate scope.

Various commercially available acyl chlorides, aldehydes, and alkanes were exploited for versatile three-component 1,2-carboacylations of alkenes to forge two vicinal C–C bonds through the cooperative action of nickel and sodium decatungstate catalysis.

Nickel Mediated Enantioselective Photoredox Allylation of Aldehydes with Visible Light

Here we report a practical, highly enantioselective photoredox allylation of aldehydes mediated by chiral nickel complexes with commercially available allyl acetate as the allylating agent. The methodology allows the clean stereoselective allylation of aldehydes in good to excellent yields and up to 93% e.e. using a catalytic amount of NiCl 2 (glyme) in the presence of the chiral aminoindanol-derived bis(oxazoline) as the chiral ligand. The photoredox system is constituted by the organic dye 3DPAFIPN and a Hantzsch's ester as the sacrificial reductant. The reaction proceeds under visible light irradiation (blue LEDs, 456 nm) at 8-12 °C with excellent stereoselectivities. Compared to other published procedures, no metal reductants (such as Zn or Mn), additives (e.g. CuI) or air-sensitive Ni(COD) 2 are necessary for this reaction. Accurate DFT calculations and photophysical experiments have clarified the mechanistic picture of this stereoselective allylation reaction showing a key role played by Hantzsch's ester for the turnover of the catalyst.