Secondary Alkyl Halides in Transition-Metal-Catalyzed Cross-Coupling Reactions

Nickel-Catalyzed Regioselective Hydroalkylation and Hydroarylation of Alkenyl Boronic Esters

Metal hydride catalyzed hydrocarbonation reactions of alkenes are an efficient approach to construct new carbon-carbon bonds from readily available alkenes. However, the regioselectivity of hydrocarbonation remains challenging to be controlled. In nickel hydride (NiH) catalyzed hydrocarbonation, linear selectivity is most often obtained because of the relative stability of the linear Ni-alkyl intermediate over its branched counterpart. Herein, we show that the boronic pinacol ester (Bpin) group directs a Ni-catalyzed hydrocarbonation to occur at its adjacent carbon center, resulting in formal branch selectivity. Both alkyl and aryl halides can be used as electrophiles in this hydrocarbonation, providing access to a wide range of secondary alkyl Bpin derivatives, which are valuable building blocks in synthetic chemistry. The utility of the method is demonstrated by the late-stage functionalization of natural products and drug molecules, the synthesis of an anticancer agent, and iterative syntheses.

Deoxygenative Borylation of Secondary and Tertiary Alcohols

Two different approaches for the deoxygenative radical borylation of secondary and tertiary alcohols are presented. These transformations either proceed through a metal‐free silyl‐radical‐mediated pathway or utilize visible‐light photoredox catalysis. Readily available xanthates or methyl oxalates are used as radical precursors. The reactions show broad substrate scope and high functional‐group tolerance, and are conducted under mild and practical conditions.

Ni-catalyzed deaminative cross-electrophile coupling of Katritzky salts with halides via C─N bond activation

The reductive cross-coupling of sp³-hybridized carbon centers represents great synthetic values and insurmountable challenges. In this work, we report a nickel-catalyzed deaminative cross-electrophile coupling reaction to construct C(sp)─C(sp³), C(sp²)─C(sp³), and C(sp³)─C(sp³) bonds. A wide range of coupling partners including aryl iodides, bromoalkynes, or alkyl bromides are stitched with alkylpyridinium salts that derived from the corresponding primary amines. The advantages of this methodology are showcased in the two-step synthesis of the key lactonic moiety of (+)-compactin and (+)-mevinolin. The one-pot procedure without isolation of alkylpyridinium tetrafluoroborate salt is also proven to be successful. This cross-coupling strategy of two electrophiles provides a highly valuable vista for the convenient installation of alkyl substituents and late functionalizations of sp³ carbons.

Exploiting the trifluoroethyl group as a precatalyst ligand in nickel-catalyzed Suzuki-type alkylations

We report herein the exploitment of the partially fluorinated trifluoroethyl as precatalyst ligands in nickel-catalyzed Suzuki-type alkylation and fluoroalkylation coupling reactions. Compared with the [L n NiII(aryl)(X)] precatalysts, the unique characters of bis-trifluoroethyl ligands imparted precatalyst [(bipy)Ni(CH2CF3)2] with bench-top stability, good solubilities in organic media and interesting catalytic activities. Preliminary mechanistic studies reveal that an eliminative extrusion of a vinylidene difluoride (VDF, CH2[double bond, length as m-dash]CF2) mask from [(bipy)Ni(CH2CF3)2] is a critical step for the initiation of a catalytic reaction.

Catalysis with Palladium Complexes Photoexcited by Visible Light

Palladium catalysis induced by visible light is an emerging field of catalysis. In contrast to classical reactions catalyzed by Pd complexes in the ground state, which mostly proceed through two-electron redox processes, the mechanisms of these new methods based on photoexcited Pd complexes usually operate through transfer of a single electron. Such processes lead to putative hybrid Pd/radical species, which exhibit both radical and classical Pd-type reactivity. This Minireview highlights the recent progress in this rapidly growing area.

Nickamine and Analogous Nickel Pincer Catalysts for Cross-Coupling of Alkyl Halides and Hydrosilylation of Alkenes

Ligand development plays an essential role in the advance of homogeneous catalysis. Tridentate, meridionally coordinating ligands, commonly termed pincer ligands, have been established as a privileged class of ligands in catalysis because they confer high stability while maintaining electronic tenability to the resulting metal complexes. Pincer ligands containing "soft" donors such as phosphines are typically used for late transition-metal ions, which are considered "soft" acids. Driven by our interest to develop base-metal catalysis and in view of the "hard" character of base-metal ions, our group explored a pincer ligand containing only "hard" nitrogen donors. A prototypical nickel complex of this ligand, "Nickamine", turned out to be an efficient catalyst in a wide range of organic reactions. Because of its propensity to mediate single-electron redox chemistry, Nickamine is particularly suited to catalyze cross-coupling of nonactivated alkyl halides through radical pathways. These coupling partners have been challenging substrates for traditional, palladium-based catalysts because of difficult oxidative addition and nonproductive β-H elimination. The high activity of Nickamine for cross-coupling leads to high chemoselectivity and functional group tolerance, even when reactive Grignard reagents are employed as nucleophiles. The scope of the catalysis is broad and encompasses sp³-sp³, sp³-sp², and sp³-sp cross-coupling. The defined nature of Nickamine facilitated the mechanistic study of cross-coupling reactions. Experiments involving radical-probe substrates, presumed intermediates and dormant species, kinetics, and density functional theory computations revealed a bimetallic oxidative addition pathway. In this pathway, two Ni centers each provide one electron to support the two-electron activation of an alkyl halide substrate. The success of Nickamine motivated our systematic structure-activity studies aiming at improved activity in certain reactions through ligand modification. Indeed, better catalysts have been developed for cross-coupling of secondary alkyl halides as well as direct alkynylation of alkyl halides. The improvement is attributed to a more accessible Ni center in the new catalysts than in Nickamine. Surprisingly, the improvement could be obtained simply by replacing a dimethyl amino group in Nickamine with a pyrrolidino group. During the study of the catalytic cycle of Nickamine in cross-coupling reactions, we synthesized the corresponding Ni-H species. Consequently, we explored the catalytic application of Nickamine in Ni-H mediated reactions, such as hydrosilylation. To our delight, Nickamine is a chemoselective catalyst for hydrosilylation of alkenes while tolerating a reactive C=O group. An analogous Ni pincer complex was found to catalyze unusual hydrosilylation reactions using alkoxy hydrosilanes as surrogates of gaseous silanes.

Synthesis of Alkyl Halides from Aldehydes via Deformylative Halogenation

An unprecedented deformylative halogenation of aldehydes to alkyl halides is presented. Under oxidative conditions, 1,4-dihydropyridine (DHP), derived from an aldehyde, generated a C(sp3)- radical that coupled with a halogen radical that was generated from inexpensive and atom-economical halogen sources (NaBr, NaI, or HCl), to yield an alkyl halide. Because of the mild conditions, a wide range of functional groups were tolerated, and excellent site selectivity was achieved.

Nickel-Catalyzed Decarboxylative Alkenylation of Anhydrides with Vinyl Triflates or Halides

Decarboxylative cross-coupling of aliphatic acid anhydrides with vinyl triflates or halides was accomplished via nickel catalysis. This methodology works well with a broad array of substrates and features abundant functional group tolerance. Notably, our approach addresses the issue of safe and environmental installation of methyl or ethyl group into molecular scaffolds. The method possesses high chemoselectivity toward alkyl groups when aliphatic/aromatic mixed anhydrides are involved. Furthermore, diverse ketones could be modified with our strategy.

Alkyl Carbon-Carbon Bond Formation by Nickel/Photoredox Cross-Coupling

The union of photoredox and nickel catalysis has resulted in a renaissance in radical chemistry as well as in the use of nickel-catalyzed transformations, specifically for carbon-carbon bond formation. Collectively, these advances address the longstanding challenge of late-stage cross-coupling of functionalized alkyl fragments. Empowered by the notion that photocatalytically generated alkyl radicals readily undergo capture by Ni complexes, wholly new feedstocks for cross-coupling have been realized. Herein, we highlight recent developments in several types of alkyl cross-couplings that are accessible exclusively through this approach.

Harnessing Alkylpyridinium Salts as Electrophiles in Deaminative Alkyl-Alkyl Cross-Couplings

A Negishi cross-coupling of alkylpyridinium salts and alkylzinc halides has been developed. This is the first example of alkyl-alkyl bond formation via cross-coupling of an alkyl amine derivative with an unactivated alkyl group, and allows both primary and secondary alkylpyridinium salts to react with primary alkylzinc halides with high functional group tolerance. When combined with formation of the pyridinium salts from primary amines, this method enables the noncanonical transformation of NH2 groups into a wide range of alkyl substituents with broad functional group tolerance.

Transition-Metal-Catalyzed Alkyl Heck-Type Reactions

The Heck reaction is one of the most reliable and useful strategies for the construction of C-C bonds in organic synthesis. However, in contrast to the well-established aryl Heck reaction, the analogous reaction employing alkyl electrophiles is much less developed. Significant progress in this area was recently achieved by merging radical-mediated and transition-metal-catalyzed approaches. This review summarizes the advances in alkyl Heck-type reactions from its discovery early in the 1970s up until the end of 2018.

Alkenylation of unactivated alkyl bromides through visible light photocatalysis

Two visible-light driven alkenylation reactions of unactivated alkyl bromides, which were enabled by the use of Ir(dF(CF₃)ppy)₂(dtbbpy)PF₆ as the photocatalyst and (TMS)₃SiH as the atom transfer reagent to activate the alkyl bromides, were described for the first time.

Copper-catalyzed intermolecular oxidative trifluoromethyl-arylation of styrenes with NaSO2CF3 and indoles involving C–H functionalization

A new copper-catalyzed three-component oxidative 1,2-trifluoromethylarylation of styrenes with NaSO₂CF₃ and indoles involving aryl C–H bond functionalization is described.

Nickel-catalyzed alkyl–alkyl cross-coupling reactions of non-activated secondary alkyl bromides with aldehydes as alkyl carbanion equivalents

A novel nickel-catalyzed alkyl–alkyl cross coupling of non-activated secondary alkyl bromides with aldehydes via hydrazone intermediates has been developed.

Metal-Free Radical Borylation of Alkyl and Aryl Iodides

A metal-free radical borylation of alkyl and aryl iodides with bis(catecholato)diboron (B2 cat2 ) as the boron source under mild conditions is introduced. The borylation reaction is operationally easy to conduct and shows high functional group tolerance and broad substrate scope. Radical clock experiments and density functional theory calculations provide insights into the mechanism and rate constants for C-radical borylation with B2 cat2 are disclosed.

Visible-Light-Induced Organic Photochemical Reactions through Energy-Transfer Pathways

Visible-light photocatalysis is a rapidly developing and powerful strategy to initiate organic transformations, as it closely adheres to the tenants of green and sustainable chemistry. Generally, most visible-light-induced photochemical reactions occur through single-electron transfer (SET) pathways. Recently, visible-light-induced energy-transfer (EnT) reactions have received considerable attentions from the synthetic community as this strategy provides a distinct reaction pathway, and remarkable achievements have been made in this field. In this Review, we highlight the most recent advances in visible-light-induced EnT reactions.

Asymmetric synthesis via stereospecific C-N and C-O bond activation of alkyl amine and alcohol derivatives

This perspective showcases our development of benzylic and allylic amine and alcohol derivatives as electrophiles for stereospecific, nickel-catalyzed cross-coupling reactions, as well as the prior art that inspired our efforts. The success of our effort has relied on the use of benzyl ammonium triflates as electrophiles for cross-couplings via C-N bond activation and benzylic and allylic carboxylates for cross-couplings via C-O bond activation. Our work, along with others' exciting discoveries, has demonstrated the potential of stereospecific, nickel-catalyzed cross-couplings of alkyl electrophiles in asymmetric synthesis, and enables efficient generation of both tertiary and quaternary stereocenters.

Pd-Catalyzed Alkylation of (Iso)quinolines and Arenes: 2-Acylpyridine Compounds as Alkylation Reagents

The first Pd-catalyzed alkylation of (iso)quinolines and arenes is reported. The readily available and bench-stable 2-acylpyridine compounds were used as an alkylation reagent to form the structurally versatile alkylated (iso)quinolines and arenes. The method affords a convenient pathway for the introduction of alkyl groups into organic molecules.

Rhodium-Catalyzed Generation of Anhydrous Hydrogen Iodide: An Effective Method for the Preparation of Iodoalkanes

The preparation of anhydrous hydrogen iodide directly from molecular hydrogen and iodine using a rhodium catalyst is reported for the first time. The anhydrous hydrogen iodide generated was proven to be highly active in the transformations of alkenes, phenyl aldehydes, alcohols, and cyclic ethers to the corresponding iodoalkanes. Therefore, the present methodology not only has provided convenient access to anhydrous hydrogen iodide but also offers a practical preparation method for various iodoalkanes in excellent atom economy.