Divergent Synthesis of Vinyl-, Benzyl-, and Borylsilanes: Aryl to Alkyl 1,5-Palladium Migration/Coupling Sequences

Organosilicon compounds have been extensively utilized both in industry and academia. Studies on the syntheses of diverse organosilanes is highly appealing. Through-space metal/hydrogen shifts allow functionalization of C−H bonds at a remote site, which are otherwise difficult to achieve. However, until now, an aryl to alkyl 1,5-palladium migration process seems to have not been presented. Reported herein is the remote olefination, arylation, and borylation of a methyl group on silicon to access diverse vinyl-, benzyl-, and borylsilanes, constituting a unique C(sp³)−H transformation based on a 1,5-palladium migration process.     

Cross‐Coupling between Hydrazine and Aryl Halides with Hydroxide Base at Low Loadings of Palladium by Rate‐Determining Deprotonation of Bound Hydrazine

Reported here is the Pd‐catalyzed C–N coupling of hydrazine with (hetero)aryl chlorides and bromides to form aryl hydrazines with catalyst loadings as low as 100 ppm of Pd and KOH as base. Mechanistic studies revealed two catalyst resting states: an arylpalladium(II) hydroxide and arylpalladium(II) chloride. These compounds are present in two interconnected catalytic cycles and react with hydrazine and base or hydrazine alone to give the product. The selectivity of the hydroxide complex with hydrazine to form aryl over diaryl hydrazine was lower than that of the chloride complex, as well as the catalytic reaction. In contrast, the selectivity of the chloride complex closely matched that of the catalytic reaction, indicating that the aryl hydrazine is derived from this complex. Kinetic studies showed that the coupling process occurs by rate‐limiting deprotonation of a hydrazine‐bound arylpalladium(II) chloride complex to give an arylpalladium(II) hydrazido complex.     

Palladium/Copper Dual Catalysis for the Cross‐Coupling of Aryl(trialkyl)silanes with Aryl Bromides

Whereas aryl(trialkyl)silanes are considered to be ideal organometallic reagents for cross‐coupling reactions owing to their stability, low toxicity, solubility, and easy accessibility, they are generally inert under typical cross‐coupling conditions. Disclosed herein is a palladium/copper catalytic system that enables the cross‐coupling of trimethyl, triethyl, tert‐butyldimethyl, and triisopropyl aryl silanes with aryl bromides. This process is applicable to the sequential C?H and C?Si bond arylation of thiophenes and the synthesis of poly(thiophene–fluorene)s.     

Palladium Catalysis Enables Benzylation of α,α‐Difluoroketone Enolates

A palladium‐catalyzed decarboxylative benzylation reaction of α,α‐difluoroketone enolates is reported, in which the key C(α)?C(sp³) bond is generated by reductive elimination from a palladium intermediate. The transformation provides convergent access to α‐benzyl‐α,α‐difluoroketone‐based products, and should be useful for accessing biological probes.     

Direct Access to α,α‐Difluoroacylated Arenes by Palladium‐Catalyzed Carbonylation of (Hetero)Aryl Boronic Acid Derivatives

A palladium‐catalyzed carbonylative coupling of (hetero)aryl boronates or boronic acid salts with carbon monoxide and α‐bromo‐α,α‐difluoroamides and bromo‐α,α‐difluoroesters is described herein. The method is useful for the synthesis of a diverse selection of (hetero)aryl α,α‐difluoro‐β‐ketoamides and α,α‐difluoro‐β‐ketoesters, which are useful building blocks for the generation of functionalized difluoroacylated and difluoroalkyl arenes. The method could be further extended to a one‐pot protocol for the formation of difluoroacetophenones.     

Nickel‐Catalyzed Electrochemical Reductive Relay Cross‐Coupling of Alkyl Halides to Aryl Halides

A highly regioselective Ni‐catalyzed electrochemical reductive relay cross‐coupling between an aryl halide and an alkyl halide has been developed in an undivided cell. Various functional groups are tolerated under these mild reaction conditions, which provides an alternative approach for the synthesis of 1,1‐diarylalkanes.     

Barbier–Negishi Coupling of Secondary Alkyl Bromides with Aryl and Alkenyl Triflates and Nonaflates

A mild and practical Barbier–Negishi coupling of secondary alkyl bromides with aryl and alkenyl triflates and nonaflates has been developed. This challenging reaction was enabled by the use of a very bulky imidazole‐based phosphine ligand, which resulted in good yields as well as good chemo‐ and site selectivities for a broad range of substrates at room temperature and under non‐aqueous conditions. This reaction was extended to primary alkyl bromides by using an analogous pyrazole‐based ligand.     

Enantioselective Palladium‐Catalyzed Oxidative β,β‐Fluoroarylation of α,β‐Unsaturated Carbonyl Derivatives

The site‐selective palladium‐catalyzed three‐component coupling of deactivated alkenes, arylboronic acids, and N‐fluorobenzenesulfonimide is disclosed herein. The developed methodology establishes a general, modular, and step‐economical approach to the stereoselective β‐fluorination of α,β‐unsaturated systems.     

Highly Stereoselective Synthesis of 1,3‐Dienes through an Aryl to Vinyl 1,4‐Palladium Migration/Heck Sequence

An efficient aryl to vinyl 1,4‐palladium migration/Heck sequence was developed for the stereoselective synthesis of 1,3‐dienes. High stereoselectivity was observed not only for 1,3‐dienes bearing two similar aryl groups at terminal positions, but also for those with configurations shown to be unfavorable with previous methods.     

Palladium‐Catalyzed Carbonylations of Aryl Bromides using Paraformaldehyde: Synthesis of Aldehydes and Esters

Carbonylation reactions represent useful tools for organic synthesis. However, the necessity to use gaseous carbon monoxide is a disadvantage for most organic chemists. To solve this problem, novel protocols have been developed for conducting palladium‐catalyzed reductive carbonylations of aryl bromides and alkoxycarbonylations using paraformaldehyde as an external CO source (CO gas free). Hence, aromatic aldehydes and esters were synthesized in moderate to good yields.     

Pd‐Catalyzed Carbonylative α‐Arylation of Aryl Bromides: Scope and Mechanistic Studies

Reaction conditions for the three‐component synthesis of aryl 1,3‐diketones are reported applying the palladium‐catalyzed carbonylative α‐arylation of ketones with aryl bromides. The optimal conditions were found by using a catalytic system derived from [Pd(dba)₂] (dba=dibenzylideneacetone) as the palladium source and 1,3‐bis(diphenylphosphino)propane (DPPP) as the bidentate ligand. These transformations were run in the two‐chamber reactor, COware, applying only 1.5 equivalents of carbon monoxide generated from the CO‐releasing compound, 9‐methylfluorene‐9‐carbonyl chloride (COgen). The methodology proved adaptable to a wide variety of aryl and heteroaryl bromides leading to a diverse range of aryl 1,3‐diketones. A mechanistic investigation of this transformation relying on ³¹P and ¹³C NMR spectroscopy was undertaken to determine the possible catalytic pathway. Our results revealed that the combination of [Pd(dba)₂] and DPPP was only reactive towards 4‐bromoanisole in the presence of the sodium enolate of propiophenone suggesting that a [Pd(dppp)(enolate)] anion was initially generated before the oxidative‐addition step. Subsequent CO insertion into an [Pd(Ar)(dppp)(enolate)] species provided the 1,3‐diketone. These results indicate that a catalytic cycle, different from the classical carbonylation mechanism proposed by Heck, is operating. To investigate the effect of the dba ligand, the Pd⁰ precursor, [Pd(η³‐1‐PhC₃H₄)(η⁵‐C₅H₅)], was examined. In the presence of DPPP, and in contrast to [Pd(dba)₂], its oxidative addition with 4‐bromoanisole occurred smoothly providing the [PdBr(Ar)(dppp)] complex. After treatment with CO, the acyl complex [Pd(CO)Br(Ar)(dppp)] was generated, however, its treatment with the sodium enolate led exclusively to the acylated enol in high yield. Nevertheless, the carbonylative α‐arylation of 4‐bromoanisole with either catalytic or stoichiometric [Pd(η³‐1‐PhC₃H₄)(η⁵‐C₅H₅)] over a short reaction time, led to the 1,3‐diketone product. Because none of the acylated enol was detected, this implied that a similar mechanistic pathway is operating as that observed for the same transformation with [Pd(dba)₂] as the Pd source.     

Synthesis,Structure, and Reactivity of 5‐(Aryl)dibenzothiophenium Triflates

A synthetic protocol for the preparation of 5‐(aryl)dibenzothiophenium salts starting from inexpensive dibenzothiophene S‐oxide and simple arenes is reported. The scope of the method regarding the nature of the arene is evaluated, intermediates along the reaction sequence have been trapped, and side‐reactions identified. In addition, the X‐ray structures of a complete set of these salts are reported and their reactivities studied. Specifically, chemoselective Suzuki coupling is observed at the dibenzothiophenium in the presence of iodides.     

One‐Pot Tandem Photoredox and Cross‐Coupling Catalysis with a Single Palladium Carbodicarbene Complex

The combination of conventional transition‐metal‐catalyzed coupling (2 e^? process) and photoredox catalysis (1 e^? process) has emerged as a powerful approach to catalyze difficult cross‐coupling reactions under mild reaction conditions. Reported is a palladium carbodicarbene (CDC) complex that mediates both a Suzuki–Miyaura coupling and photoredox catalysis for C?N bond formation upon visible‐light irradiation. These two catalytic pathways can be combined to promote both conventional transition‐metal‐catalyzed coupling and photoredox catalysis to mediate C?H arylation under ambient conditions with a single catalyst in an efficient one‐pot process.     

Synthesis of Aryldiazoacetates through Palladium(0)‐Catalyzed Deacylative Cross‐Coupling of Aryl Iodides with Acyldiazoacetates

Palladium(0)‐catalyzed deacylative cross‐coupling of aryl iodides and acyldiazocarbonyl compounds can be achieved at room temperature under mild reaction conditions. The coupling reaction represents a highly efficient and general method for the synthesis of aryldiazocarbonyl compounds, which have found wide and increasing applications as precursors for generating donor/acceptor‐substituted metallocarbenes.     

Pd‐PEPPSI‐IHeptCl: A General‐Purpose,Highly Reactive Catalyst for the Selective Coupling of Secondary Alkyl Organozincs

Dichloro[1,3‐bis(2,6‐di‐4‐heptylphenyl)imidazol‐2‐ylidene](3‐chloropyridyl)palladium(II) (Pd‐PEPPSI‐IHept^Cl), a new, very bulky yet flexible Pd–N‐heterocyclic carbene (NHC) complex has been evaluated in the cross‐coupling of secondary alkylzinc reactants with a wide variety of oxidative addition partners in high yields and excellent selectivity. The desired, direct reductive elimination branched products were obtained with no sign of migratory insertion across electron‐rich and electron‐poor aromatics and all forms of heteroaromatics (five and six membered). Impressively, there is no impact of substituents at the site of reductive elimination (i.e., ortho or even di‐ortho), which has not yet been demonstrated by another catalyst system to date.     

Trifluoromethylthiolation of Aryl Iodides and Bromides Enabled by a Bench‐Stable and Easy‐To‐Recover Dinuclear Palladium(I) Catalyst

While palladium catalysis is ubiquitous in modern chemical research, the recovery of the active transition‐metal complex under routine laboratory applications is frequently challenging. Described herein is the concept of alternative cross‐coupling cycles with a more robust (air‐, moisture‐, and thermally‐stable) dinuclear Pd^I complex, thus avoiding the handling of sensitive Pd⁰ species or ligands. Highly efficient C? SCF₃ coupling of a range of aryl iodides and bromides was achieved, and the recovery of the Pd^I complex was accomplished via simple open‐atmosphere column chromatography. Kinetic and computational data support the feasibility of dinuclear Pd^I catalysis. A novel SCF₃‐bridged Pd^I dimer was isolated, characterized by X‐ray crystallography, and verified to be a competent catalytic intermediate.     

Intramolecular Aryl Migration of Diaryliodonium Salts: Access to ortho‐Iodo Diaryl Ethers

By using vicinal trifluoromethanesulfonate‐substituted diaryliodonium salts, a novel approach was developed for the synthesis of ortho‐iodo diaryl ethers by intramolecular aryl migration. The reaction conditions are mild with a broad substrate scope. Mechanistic insight suggests a sulfonyl‐directed nucleophilic aromatic substitution pathway. Additionally, the product ortho‐iodo diaryl ethers serve as versatile synthons as demonstrated with several coupling reactions. Furthermore, a useful thyroxine analogue of the 3‐iodo‐l ‐thyronine (3‐T₁) derivative was synthesized by this aryl migration procedure.     

Palladium(0)‐Catalyzed Iminohalogenation of Alkenes: Synthesis of 2‐Halomethyl Dihydropyrroles and Mechanistic Insights into the Alkyl Halide Bond Formation

Although the advances on carbon halide reductive elimination have been made, the alkyl bromide and chloride analogues remain a challenge. Here, a palladium(0)‐catalyzed iminohalogenation of γ,δ‐unsaturated oxime esters is described, and the use of electron‐poor phosphine ligands proved to be crucial to promoting alkyl bromide and chloride reductive elimination. Furthermore, S_N2‐type alkyl bromide and chloride reductive elimination has also been established.     

Continuous‐Flow Synthesis of Biaryls by Negishi Cross‐Coupling of Fluoro‐ and Trifluoromethyl‐Substituted (Hetero)arenes

A continuous‐flow method for the regioselective arylation of fluoroarenes and fluoropyridines has been developed. The telescoped procedure reported here consists of a three‐step metalation, zincation, and Negishi cross‐coupling sequence, providing efficient access to a variety of functionalized 2‐fluorobiaryl products. Precise temperature control of the metalation step, made possible by continuous‐flow technology, allowed for the efficient preparation of the arylated products in high yields and short residence times. Additionally, several examples of the regioselective arylation of benzotrifluoride derivatives are also provided.     

Reductive Cross‐Coupling of Conjugated Arylalkenes and Aryl Bromides with Hydrosilanes by Cooperative Palladium/Copper Catalysis

A method for the reductive cross‐coupling of conjugated arylalkenes and aryl bromides with hydrosilanes by cooperative palladium/copper catalysis was developed, thus resulting in the highly regioselective formation of various 1,1‐diarylalkanes, including a biologically active molecule. Under the applied reaction conditions, high levels of functional‐group tolerance were observed, and the reductive cross‐coupling of internal alkynes with aryl bromides afforded trisubstituted alkenes.