Reduction Of Alkyl Halides Research Articles

Anti‐Selective Carbosilylation: Nickel‐Catalyzed Multicomponent Reaction of Solid Me3SiZnI

AbstractThe stereodefined and highly substituted vinylsilanes are essential building blocks for constructing complex organic molecules. Transition metal‐mediated silylmetalation of alkynes was developed to overcome the limitations of conventional hydrosilylations; however, a very limited study was carried out to utilize transient vinylmetal species in cross‐coupling reactions. Moreover, they produce syn‐adduct, and the anti‐selective cross‐coupling is still unknown and highly desired. Silylzinc reagents are highly functional group tolerant, however, their synthesis from pyrophoric silyllithium and dissolved lithium salts hampers cross‐coupling reactions. Our novel solid silylzinc reagents circumvent these constraints are employed in the anti‐selective synthesis of vinylsilanes via a multi‐component reaction involving Me3SiZnI, terminal alkynes, and activated alkyl halides. An intensive computational and experimental investigation of the mechanism reveals an equilibrium between the intermediate syn‐ and anti‐adducts; the greater barrier at the single electron reduction of alkyl halides and the thermodynamic stability of the Ni(III) adduct determine the anti‐selectivity.

Mesoionic Carbene‐Catalyzed Formyl Alkylation of Aldehydes

AbstractKetones are among the most useful functional groups in organic synthesis, and they are commonly encountered in a broad range of compounds with various applications. Herein, we describe the mesoionic carbene‐catalyzed coupling reaction of aldehydes with non‐activated secondary and even primary alkyl halides. This metal‐free method utilizes deprotonated Breslow intermediates derived from mesoionic carbenes (MICs), which act as super electron donors and induce the single‐electron reduction of alkyl halides. This mild coupling reaction has a broad substrate scope and tolerates many functional groups, which allows to prepare a diversity of simple ketones as well as bio‐active molecules by late‐stage functionalization.

Mesoionic Carbene-Catalyzed Formyl Alkylation of Aldehydes.

Ketones are among the most useful functional groups in organic synthesis, and they are commonly encountered in a broad range of compounds with various applications. Herein, we describe the mesoionic carbene-catalyzed coupling reaction of aldehydes with non-activated secondary and even primary alkyl halides. This metal-free method utilizes deprotonated Breslow intermediates derived from mesoionic carbenes (MICs), which act as super electron donors and induce the single-electron reduction of alkyl halides. This mild coupling reaction has a broad substrate scope and tolerates many functional groups, which allows to prepare a diversity of simple ketones as well as bio-active molecules by late-stage functionalization.

In Memoriam of Jean‐Michel Savéant (1933–2020)

In Memoriam of Jean‐Michel Savéant (1933–2020)

Controlled Single-Electron Transfer via Metal-Ligand Cooperativity Drives Divergent Nickel-Electrocatalyzed Radical Pathways.

Electrocatalysis enables the construction of C-C bonds under mild conditions via controlled formation of carbon-centered radicals. For sequences initiated by alkyl halide reduction, coordinatively unsaturated Ni complexes commonly serve as single-electron transfer agents, giving rise to the foundational question of whether outer- or inner-sphere electron transfer oxidative addition prevails in redox mediation. Indeed, rational design of electrochemical processes requires the discrimination of these two electron transfer pathways, as they can have outsized effects on the rate of substrate bond activation and thus impact radical generation rates and downstream product selectivities. We present results from combined synthetic, electroanalytical, and computational studies that examine the mechanistic differences of single electron transfer to alkyl halides imparted by Ni metal-ligand cooperativity. Electrogenerated reduced Ni species, stabilized by delocalized spin density onto a redox-active tpyPY2Me polypyridyl ligand, activates alkyl iodides via outer-sphere electron transfer, allowing for the selective activation of alkyl iodide substrates over halogen atom donors and the controlled generation and sequestration of electrogenerated radicals. In contrast, the Ni complex possessing a redox-innocent pentapyridine congener activates the substrates in an inner-sphere fashion owning to a purely metal-localized spin, thereby activating both substrates and halogen atom donors in an indiscriminate fashion, generating a high concentration of radicals and leading to unproductive dimerization. Our data establish that controlled electron transfer via Ni-ligand cooperativity can be used to limit undesired radical recombination products and promote selective radical processes in electrochemical environments, providing a generalizable framework for designing redox mediators with distinct rate and potential requirements.

Radical Reactions in Cavitands Unveil the Effects of Affinity on Dynamic Supramolecular Systems.

Radical reduction of alkyl halides and aerobic oxidation of alkyl aromatics are reported using water-soluble container compounds (1 and 2). The reductions involve α,ω-dihalides (4-8 and 10) with radical initiators in cavitand hosts with varied binding affinities. Product distributions lead to general guidelines for the use of dynamic supramolecular systems with fast reactions. The binding of guest substrates in the hosts must show high affinities (Ka > 103 M-1) to ensure that the reactions take place under confinement in the containers.

A Free‐Radical Reduction and Cyclization of Alkyl Halides Mediated by FeCl2

AbstractIron mediated catalytic reactions are of great interest in the field of organic synthesis because they are economic and naturally abundant. However, the use of iron catalyst in the field of free radical cyclization or reduction of alkyl halides remains limited. Here we describe the use of an unprecedented combination of iron and zinc in the reduction and 5‐exo‐trig radical cyclization of alkyl halides under mild condition in the absence of added ligands or additives. The method is distinguished by its wide scope, functional group tolerance and the use of 1,4‐cyclohexadiene as the source of hydrogen, which aids easy purification.

Kinetic solvent effects in the reduction of alkyl halides by {Sm[N(SiMe3)2]2(THF)2}

The rate of reduction of two representative alkyl halides, 1-bromododecane and 1-chlorododecane by {Sm[N(SiMe3)2]2(THF)2} was examined in five different solvents. Reductions were several orders of magnitude faster in nonpolar, non-coordinating solvents than in polar coordinating solvents. Good correlations between the rate of reduction and coordination index of each solvent was obtained whereas excellent correlations were obtained between rates and dielectric (as a measure of solvent polarity). The basis of this effect is proposed to be a consequence of solvent coordination to the inner sphere of Sm(II) leading to a deceleration of ET in coordinating solvents and possible transition state stabilization by nonpolar solvents.

Photoinduced Charge-Transfer State of 4-Carbazolyl-3-(trifluoromethyl)benzoic Acid: Photophysical Property and Application to Reduction of Carbon-Halogen Bonds as a Sensitizer.

The photoinduced persistent intramolecular charge-transfer state of 4-carbazolyl-3-(trifluoromethyl)benzoic acid was confirmed. It showed a higher catalytic activity in terms of yield and selectivity in the photochemical reduction of alkyl halides compared to the parent carbazole. Even unactivated primary alkyl bromides could be reduced by this photocatalyst. The high catalytic activity is rationalized by considering the slower backward single-electron transfer owing to the spatial separation of the donor and acceptor subunits.

Copper-catalyzed alkyl-alkyl cross-coupling reactions using hydrocarbon additives: efficiency of catalyst and roles of additives.

Cross-coupling of alkyl halides with alkyl Grignard reagents proceeds with extremely high TONs of up to 1230000 using a Cu/unsaturated hydrocarbon catalytic system. Alkyl fluorides, chlorides, bromides, and tosylates are all suitable electrophiles, and a TOF as high as 31200 h(-1) was attained using an alkyl iodide. Side reactions of this catalytic system, i.e., reduction, dehydrohalogenation (elimination), and the homocoupling of alkyl halides, occur in the absence of additives. It appears that the reaction involves the β-hydrogen elimination of alkylcopper intermediates, giving rise to olefins and Cu-H species, and that this process triggers both side reactions and the degradation of the Cu catalyst. The formed Cu-H promotes the reduction of alkyl halides to give alkanes and Cu-X or the generation of Cu(0), probably by disproportionation, which can oxidatively add to alkyl halides to yield olefins and, in some cases, homocoupling products. Unsaturated hydrocarbon additives such as 1,3-butadiene and phenylpropyne play important roles in achieving highly efficient cross-coupling by suppressing β-hydrogen elimination, which inhibits both the degradation of the Cu catalyst and undesirable side reactions.

Synthesis, Reactivity, and Catalytic Application of a Nickel Pincer Hydride Complex

The nickel(II) hydride complex [(MeN2N)Ni-H] (2) was synthesized by the reaction of [(MeN2N)Ni-OMe] (6) with Ph2SiH2 and was characterized by NMR and IR spectroscopy as well as X-ray crystallography. 2 was unstable in solution, and it decomposed via two reaction pathways. The first pathway was intramolecular N–H reductive elimination to give MeN2NH and nickel particles. The second pathway was intermolecular, with H2, nickel particles, and a five-coordinate Ni(II) complex [(MeN2N)2Ni] (8) as the products. 2 reacted with acetone and ethylene, forming [(MeN2N)Ni-OiPr] (9) and [(MeN2N)Ni-Et] (10), respectively. 2 also reacted with alkyl halides, yielding nickel halide complexes and alkanes. The reduction of alkyl halides was rendered catalytically, using [(MeN2N)Ni-Cl] (1) as catalyst, NaOiPr or NaOMe as base, and Ph2SiH2 or Me(EtO)2SiH as the hydride source. The catalysis appears to operate via a radical mechanism.

Uncovering the Mechanistic Role of HMPA in the Samarium Barbier Reaction

The presence of HMPA is critical for the selective coupling of alkyl halides and ketones by SmI2. Although previous rate studies have shown that HMPA dramatically accelerates the reduction of alkyl halides over ketones, the basis of this rate acceleration is unknown. In this communication, we report experimental and computational evidence that demonstrate that the selectivity observed in the samarium Barbier reaction is in part a result of activation of the alkyl halide bond by HMPA.

ChemInform Abstract: Radical Chain Reduction of Alkyl Halides, Dialkyl Sulfides and O-Alkyl S-Methyl Dithiocarbonates to Alkanes by Trialkylsilanes.

ChemInform Abstract: Radical Chain Reduction of Alkyl Halides, Dialkyl Sulfides and O-Alkyl S-Methyl Dithiocarbonates to Alkanes by Trialkylsilanes.

Electrochemistry of substituted salen complexes of nickel(II): Nickel(I)-catalyzed reduction of alkyl and acetylenic halides

Dimethyl-, diethyl-, and diphenyl-substituted analogues of nickel(II) salen have been synthesized, and the cyclic voltammetric behavior of each compound at a glassy carbon electrode in dimethylformamide (DMF) containing tetra- n-butylammonium tetrafluoroborate (TBABF 4) has been compared with that of nickel(II) salen itself. Differences in the cathodic peak potentials for these species have been rationalized with the aid of theoretical computations based on density functional theory. Cyclic voltammograms for reduction of dimethylated nickel(II) salen reveal that placing a methyl group on the carbon atom of each imino (C N) bond of the ligand improves the performance of electrogenerated dimethylated nickel(I) salen as a catalyst for reduction of 1-iodobutane. This conclusion is supported by experiments that combine controlled-potential electrolysis with high-performance liquid chromatography–electrospray ionization–mass spectrometry (HPLC–ESI–MS) to detect and analyze the post-electrolysis nickel-containing species arising from the bulk catalytic reduction of 1-iodooctane. Additional studies have been made of the use of electrogenerated dimethylated nickel(I) salen for the catalytic reduction of alkyl halides and for the reductive intramolecular cyclization of acetylenic halides.

ChemInform Abstract: Organotin Reagents Supported on Ionic Liquid: Highly Efficient Catalytic Free Radical Reduction of Alkyl Halides.

AbstractChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 200 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.

ChemInform Abstract: Reduction of Alkyl Halides by Triethylsilane Based on a Cationic Iridium Bis(phosphinite) Pincer Catalyst: Scope, Selectivity and Mechanism.

AbstractChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 200 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.

Organotin reagents supported on ionic liquid: highly efficient catalytic free radical reduction of alkyl halides

Ionic liquid-supported organotin reagents were prepared in good yield and demonstrated a convenient catalytic free radical reduction of alkyl halides. A variety of alkyl and aryl halides could be reduced in high to excellent yields and selectively to afford the desired products with simple work-up procedure.

Reduction of Alkyl Halides by Triethylsilane Based on a Cationic Iridium Bis(phosphinite) Pincer Catalyst: Scope, Selectivity and Mechanism

AbstractA highly efficient procedure for the reduction of a broad range of alkyl halides by triethylsilane based on a cationic iridium bis(phosphinite) pincer catalyst has been discovered and developed. This reduction chemistry is chemoselective and has unique selectivities compared with conventional radical‐based processes and the aluminum trichloride/triethylsilane (AlCl3/Et3SiH) and triphenylmethyl tetrakis[pentafluorophenyl]borate/triethylsilane {[Ph3C] [B(C6F5)4]/Et3SiH} systems. Reductions use three equivalents of triethylsilane relative to the halide and can be carried out with very low catalyst loadings and in a solvent‐free manner, which may provide an environmentally attractive and safe alternative to many currently practiced methods for reduction of alkyl halides. Mechanistic studies reveal a unique catalytic cycle. The cationic iridium hydride 2,6‐bis[di‐(tert‐butyl)phosphinyloxy)phenyl(hydrido)iridium, (POCOP)IrH+ {POCOP= 2,6‐[OP(t‐Bu)2]2C6H3} binds and activates the silane. This complex serves as a potent silylating reagent to generate silyl halonium ions, Et3SiXR+, which are reduced by the neutral iridium dihydride to yield alkane product and regenerate the cationic (POCOP)IrH+, thus closing the catalytic cycle. All key intermediates have been identified by in situ NMR monitoring and kinetic studies have been completed. An application of this reduction system to the catalytic hydrodehalogenation of a metal chloride complex is also described.

Iridium-Catalyzed Reduction of Alkyl Halides by Triethylsilane

A highly active cationic Ir(III) hydride complex catalyzes the reduction of primary, secondary, and tertiary chlorides, bromides, and iodides as well as certain fluorides by Et3SiH. The catalytic cycle appears to operate by a unique process in which an electrophilic iridium silane complex acts as a silylating reagent to produce a silyl-substituted halonium ion which is then readily reduced by the nucleophilic dihydride formed following silyl transfer.

Catalytic reduction of 1-iodooctane by nickel(I) salen electrogenerated at carbon cathodes in dimethylformamide: Effects of added proton donors and a mechanism involving both metal- and ligand-centered one-electron reduction of nickel(II) salen

In dimethylformamide containing tetramethylammonium tetrafluoroborate, 1-iodooctane is reduced catalytically by nickel(I) salen electrogenerated at a glassy carbon cathode. Cyclic voltammograms for the nickel(II) salen–1-iodooctane system recorded in the absence as well as in the presence of a proton or deuteron donor (1,1,1,3,3,3-hexafluoro-2-propanol (HFIP), phenol, or D2O) exhibit enhanced cathodic peaks and diminished anodic peaks for the nickel(II) salen–nickel(I) salen couple, along with a new cathodic peak attributed to reduction of a nickel(II) salen species for which an imino bond of the ligand is octylated. Without a proton donor, controlled-potential catalytic reduction of 1-iodooctane by nickel(I) salen affords hexadecane, octane, and 1-octene. For electrolyses performed in the presence of either HFIP or phenol, the yields of hexadecane and octane are decreased and increased, respectively, whereas that of 1-octene remains unchanged. Bulk electrolyses done in the presence of D2O give a product distribution similar to that obtained when no proton donor is added; none of the octane is deuterated, indicating that octyl radicals (not octyl carbanions) are precursors for the formation of octane. Theoretical calculations involving density functional theory have been employed to establish that nickel(II) salen can undergo either a metal- or ligand-centered one-electron reduction. A mechanistic scheme is proposed that invokes both metal- and ligand-centered reduction of nickel(II) salen to explain the effects of proton donors on the catalytic reduction of alkyl halides as well as the pathway for alkylation of the imino bonds of the ligand.