Single-Electron-Transfer-Induced Coupling of Arylzinc Reagents with Aryl and Alkenyl Halides

Purple Light-Promoted Coupling of Bromopyridines with Grignard Reagents via SET

Alkyl- and arylpyridines and 2,2'-bipyridines are conventionally prepared by Minisci reactions of pyridines and transition metal-catalyzed coupling reactions of halopyridines. Herein, purple light-promoted radical coupling reactions of 2- or 4-bromopyridines with Grignard reagents in Et2O or a mixture of Et2O and tetrahydrofuran in regular glassware without the need for a transition metal catalyst were disclosed for the first time. Methyl, primary and secondary alkyl, cycloalkyl, aryl, heteroaryl, pyridyl, and alkynyl Grignard reagents were compatible with the protocol. As a result, alkyl- and arylpyridines and 2,2'-bipyridines were easily prepared. Single electron transfer from the Grignard reagent to bromopyridine was stimulated by purple light. An electron extruded from the dimerization of the Grignard reagent worked as the catalyst. Light on/off experiments indicated that constant irradiation was required for product formation. Studies of radical clock substrates verified the involvement of a pyridyl radical from bromopyridine and the noninvolvement of an alkyl or aryl radical from the Grignard reagent. The available proof supports a photoinduced SRN mechanism for the new coupling reactions.

Electrochemically Driven Nickel-Catalyzed Halogenation of Unsaturated Halide and Triflate Derivatives

A robust electrochemically driven nickel-catalyzed halogen exchange of unsaturated halides and triflates (Br to Cl, I to Cl, I to Br, and OTf to Cl) is reported. A combination of NiCl2  ⋅ glyme as the precatalyst, 2,2'-bipyridine as a ligand, NMP as the solvent, and electrochemistry allowed the generation of a nickel species that promotes reductive elimination of the desired product. This paired electrochemical halogenation is compatible with a range of unsaturated halides and triflates, including heterocycles, dihaloarenes, and alkenes with good functional-group tolerance. Joint experimental and theoretical mechanistic investigations highlighted three catalytic events: i) oxidative addition of the aryl halide to a Ni(0) species to deliver a Ni(II) intermediate; ii) halide metathesis at Ni(II); iii) electrochemical oxidation of Ni(II) to Ni(III) to enable the formation of the desired aryl halide upon reductive elimination. This methodology allows the replacement of heavy halogens (I or Br) or polar atoms (O) with the corresponding lighter and more lipophilic Cl group to block undesired reactivity or modify the properties of drug and agrochemical candidates.

Nickel-Catalyzed Cross-Coupling Reaction of Aryl Methyl Sulfides with Aryl Bromides

A nickel-catalyzed cross-coupling reaction of aryl methyl sulfides with aryl bromides has been developed to access biaryls in yields of up to 86%. The reactions proceeded well using Ni(COD)2 as catalyst with the ligand BINAP (2,2'-bis(diphenylphosphanyl)-1,1'-binaphthalene) in the presence of magnesium. The method has a broad scope of substrates and is scalable. The wide availability of commercially available aryl bromides and the absence of preparation and preparation of organometallic reagents make the reaction of high application value.

Manipulation of an electron by photoirradiation in the electron-catalyzed cross-coupling reaction

An electron has recently been shown to catalyze the cross-coupling reaction of organometallic compounds with aryl halides. In terms of green and sustainable chemistry, the electron catalysis is much more desirable than the inevitably used transition metal catalysis but a high temperature of more than 100°C is required to achieve it. Here, we disclose that visible light photoirradiation accelerates the electron-catalyzed reaction of arylzinc reagents with aryl halides with the aid of a photoredox catalysis. Photoexcitation of a photoredox catalyst and an anion radical intermediate respectively affects the supply and transfer of the electron catalyst, promoting the cross-coupling reaction to proceed at room temperature. The supply of the electron catalyst by the photoredox catalysis makes the scope of aryl halides wider.

Denitrative Hydroxylation of Unactivated Nitroarenes

A one-step method for the conversion of nitroarenes into phenols under operationally simple, transition-metal-free conditions is described. This denitrative functionalization protocol provides a concise and economical alternative to conventional three-step synthetic sequences. Experimental and computational studies suggest that nitroarenes may be substituted by an electron-catalysed radical-nucleophilic substitution (S_RN 1) chain mechanism.

A new protocol for synthesizing diarylmethanes using a benzyltitanium reagent as a nucleophile

The first palladium-catalyzed cross-coupling of various substituted benzyltitaniums with aryl triflates is presented for the synthesis of diarylmethanes in yields of up to 94% through highly selective C–O bond functionalization. The benzyltitaniums act as nucleophiles to realize the C(sp2)–C(sp3) cross-coupling with high efficiency in short reaction times. The reactions proceed at 60°C and show excellent functional groups tolerance.

Organozinc-mediated direct cross-coupling under microwave irradiation

We report a direct cross-coupling reaction between (het)aryl pivalates/tosylates and di(het)arylzinc species in 2-methyltetrahydrofuran/ N-methyl pyrrolidone (1:1), which occurs via C–O bond cleavage under microwave irradiation. The reaction takes place smoothly in short reaction times without the addition of any catalyst or ligand. The reaction is suitable for a broad scope of substrates and exhibits good functional group compatibility, utilizes a simple work-up procedure, and gives the desired products in high purity.

Cascade Approach to Highly Functionalized Biaryls by a Nucleophilic Aromatic Substitution with Arylhydroxylamines

A transition-metal free synthesis of highly functionalized 2-hydroxy-2'-amino-1,1'-biaryls from N-arylhydroxylamines has been developed. This operationally simple and readily scalable approach relies on a cascade of reactions that initially generates transient N, O-diarylhydroxylamines, via direct O-arylation, which then undergo rapid [3,3]-sigmatropic rearrangement and subsequent rearomatization to form NOBIN-type products. These structurally diverse functionalized biaryls are obtained under mild conditions in good to excellent isolated yields.

Electron-Catalyzed Cross-Coupling of Arylboron Compounds with Aryl Iodides

Arylboroxines in combination with zinc chloride and potassium tert-butoxide were found to undergo the electron-catalyzed cross-coupling with aryl iodides to give the corresponding biaryls without the aid of transition-metal catalysis.

Electron-Catalyzed Coupling of Magnesium Amides with Aryl Iodides

An electron was found to catalyze the coupling of magnesium diarylamides with aryl iodides giving triarylamines through a radical-anion intermediate. The transformation requires no transition metal catalysts or additives, and a wide array of products are formed in good-to-excellent yields.

A Zinc Catalyzed C(sp3 )-C(sp2 ) Suzuki-Miyaura Cross-Coupling Reaction Mediated by Aryl-Zincates

The Suzuki-Miyaura (SM) reaction is one of the most important methods for C-C bond formation in chemical synthesis. In this communication, we show for the first time that the low toxicity, inexpensive element zinc is able to catalyze SM reactions. The cross-coupling of benzyl bromides with aryl borates is catalyzed by ZnBr2 , in a process that is free from added ligand, and is compatible with a range of functionalized benzyl bromides and arylboronic acid pinacol esters. Initial mechanistic investigations indicate that the selective in situ formation of triaryl zincates is crucial to promote selective cross-coupling reactivity, which is facilitated by employing an arylborate of optimal nucleophilicity.

Organozinc-Mediated Direct C-C Bond Formation via C-N Bond Cleavage of Ammonium Salts

We report a direct cross-coupling reaction between diarylzinc (Ar₂ Zn) and aryltrimethylammonium salts (ArNMe₃⁺ ⋅^- OTf) in the presence of LiCl, via C-N bond cleavage. The reaction takes place smoothly upon heating in THF without any external catalyst, enabling an efficient and chemoselective formation of biaryl products. Mechanistic studies indicate that the reaction proceeds through a single electron transfer route.

Single electron transfer-induced coupling of alkynylzinc reagents with aryl and alkenyl iodides

An alkynyl–aryl coupling without the aid of transition metal catalysis has been achieved for the first time.

The transition-metal-catalyst-free oxidative homocoupling of organomanganese reagents prepared by the insertion of magnesium into organic halides in the presence of MnCl2·2LiCl

Organomanganese reagents were prepared by the insertion of magnesium into aryl halides in the presence of MnCl2·2LiCl. These organomanganese reagents smoothly undergo 1,2-addition, acylation, and Pd-catalyzed cross-coupling with various electrophiles. Especially, the oxidative homocoupling of organomanganese reagents was completed in one pot without an additional transition-metal catalyst.

SET-induced biaryl cross-coupling: an S(RN)1 reaction

The SET-induced biaryl cross-coupling reaction is established as the first example of a Grignard S_RN1 reaction. The reaction is examined within the mechanistic framework of dissociative electron transfer in the presence of a Lewis acid. DFT calculations show that the reaction proceeds through a radical intermediate in the form of an Mg ion-radical cage, which eludes detection in trapping experiments by reacting quickly to form an MgPh₂ radical anion intermediate. A new mechanism is proposed.