Stabilization of CoI by ZnII in pure acetonitrile and its reaction with aryl halides

The study of the electrochemical behavior of cobalt(II) bromide (CoBr(2)) in pure acetonitrile allowed us to demonstrate that Co(2+) is the catalyst precursor involved in the electrochemical and chemical conversions of arylhalides, ArX, to arylzinc compounds in that solvent. The reduction of Co(2+) leads to the Co(+) species, which disproportionates too rapidly to react further with aryl halides. However, the presence of zinc(II) bromide allows us to stabilize the electrogenerated cobalt(I) and to observe it on the timescale of slow cyclic voltammetry. Under such conditions, the Co(I) species has time to react with aryl halides and produce [Co(III)ArX](+) complexes that are reduced into [Co(II)ArX] by a single electron uptake at the same potential at which Co(2+) is reduced. Rate constants for the oxidative addition of ArX to Co(I) have been determined for various aryl halides and compared to the values obtained in an acetonitrile (ACN)/pyridine (9:1, v/v) mixture. It is shown that Co(I) is stabilized more by ZnBr(2) than by pyridine. A transmetallation reaction between [Co(II)ArX] and ZnBr(2) has also been observed. We finally propose a mechanism for the cobalt-catalyzed electrochemical conversion of aryl bromides into organozinc species in pure acetonitrile.     

CoI- and Co0-bipyridine complexes obtained by reduction of CoBr2bpy: electrochemical behaviour and investigation of their reactions with aromatic halides and vinylic acetates

The electrochemical behaviour of CoBr(2)bpy (bpy=2,2'-bipyridine) catalyst precursor in acetonitrile has been studied, revealing its possible reduction into the corresponding Co(I) and Co(0) complexes. These low-valent cobalt species appear to be stable on the time scale of cyclic voltammetry. In the presence of aromatic halides, both complexes undergo oxidative addition, the latter Co(0) species allowing the activation of poorly reactive aromatic chlorides. The arylcobalt(III) and arylcobalt(II) obtained are reduced at the same potential as the original Co(II) and Co(I) complexes, respectively, resulting in the observation of overall ECE mechanisms in both cases. The electrochemical study shows that vinylic acetates competitively react with electrogenerated Co(0) species, leading to a labile complex. Preparative scale electrolyses carried out from solutions containing aromatic halides (ArX), vinyl acetate (vinylOAc) and a catalytic amount of CoBr(2)bpy lead to a mixture of biaryl (Ar-Ar) and arene (ArH) as long as the potential is set on the plateau of the Co(II) right arrow over left arrow Co(I) reduction wave. The coupling product (Ar-vinyl) is formed only if the electrolysis is performed on the plateau of the Co(I)/Co(0) reduction wave. A mechanism is proposed for the overall cobalt-catalyzed coupling reaction between aromatic halides and allylic acetates.     

Cobalt-catalyzed electrochemical vinylation of aryl halides using vinylic acetates

The electroreduction of aryl halides (bromides or chlorides) allows the coupling reaction with vinylic acetates, in the presence of 2,2-bipyridine and catalytic amounts of cobalt bromide, leading to styrene derivatives in good yields.     

Electrochemical reduction of ZnBr<Subscript>2</Subscript> in the presence of organic halides

Electrochemical reduction of zinc bromide in dimethylformamide, acetonitrile, and tetrahydrofuran in the presence of organic halides (RX) results in the formation of organozinc compounds by oxidative addition of RX to electrochemically generated Zn(0). Effects of the solvent and the nature of organic halides on the electrochemical reduction of zinc (II) ions are analyzed.     

Cobalt-catalyzed trimethylsilylmethylmagnesium-promoted radical alkenylation of alkyl halides: a complement to the Heck reaction

A cobalt complex, [CoCl2(dpph)] (DPPH = [1,6-bis(diphenylphosphino)hexane]), catalyzes an intermolecular styrylation reaction of alkyl halides in the presence of Me3SiCH2MgCl in ether to yield beta-alkylstyrenes. A variety of alkyl halides including alkyl chlorides can participate in the styrylation. A radical mechanism is strongly suggested for the styrylation reaction. The sequential isomerization/styrylation reactions of cyclopropylmethyl bromide and 6-bromo-1-hexene provide evidence of the radical mechanism. Crystallographic and spectroscopic investigations on cobalt complexes reveal that the reaction would begin with single electron transfer from an electron-rich (diphosphine)bis(trimethylsilylmethyl)cobalt(II) complex followed by reductive elimination to yield 1,2-bis(trimethylsilyl)ethane and a (diphosphine)cobalt(I) complex. The combination of [CoCl2(dppb)] (DPPB = [1,4-bis(diphenylphosphino)butane]) catalyst and Me3SiCH2MgCl induces intramolecular Heck-type cyclization reactions of 6-halo-1-hexenes via a radical process. On the other hand, the intramolecular cyclization of the prenyl ether of 2-iodophenol would proceed in a fashion similar to the conventional palladium-catalyzed transformation. The nonradical oxidative addition of carbon(sp2)-halogen bonds to cobalt is separately verified by a cobalt-catalyzed cross-coupling reaction of alkenyl halides with Me3SiCH2MgCl with retention of configuration of the starting vinyl halides. The cobalt-catalyzed intermolecular radical styrylation reaction of alkyl halides is applied to stereoselective variants. Styrylations of 1-alkoxy-2-bromocyclopentane derivatives provide trans-1-alkoxy-2-styrylcyclopentane skeletons, one of which is optically pure.     

Electron transfer activity of a cobalt crown carbene complex

The novel cobalt(II) crown carbene complex 12(II) has been prepared and characterised by X-ray crystallography. This complex is reduced in a one-electron process to a cobalt(I) complex that acts as a powerful single electron donor, reducing aryl halides, including aryl chlorides and demonstrating the strong electron-enriching effect on cobalt of the crown carbene ligand. The metal ion is tightly held in a tetrahedral conformation by its enveloping crown ligand—this prevents what would otherwise be expected to be an easy oxidation to cobalt(III) under standard electrochemical conditions. Complex 12 is shown to be an effective catalyst in mediated electrochemical reductions of aryl iodides at room temperature and aryl bromides at 90 °C. The electrochemically produced catalyst [from 10 mol % of added Co(II) complex] also triggers reduction of aryl chlorides, although this seems at the limit of its reactivity. However, when the cobalt(II) complex is reduced by sodium amalgam, this affords stoichiometric quantities of the active cobalt reducing agent, which affords reduction of aryl iodides and bromides as above, but also reduces aryl chlorides at elevated temperatures.     

Stereochemistry of the coupling step in photo-S_(RN) 1 reaction

The photo-S_(RN) 1 reaction of (+)-camphor and aryl halides was investigated in order toestimate the stereochemistry of the coupling step of aryl radicals with a nucleophile. The ratio of endo toexo products determined by ~1H NMR and CD spectra was found to be 99:1. MNDO calculationsof the orbital parameters showed a favorable one-sided overlap of the enolate ion with the SOMO of arylradical. In addition, fragmentation of (ArX)~- was found to be related to the energy of SOMO of(ArX)~-.     

Mechanistic role of benzylic bromides in the catalytic autoxidation of methylarenes

Different pathways for benzylic bromide transformations were examined under conditions of cobalt-bromide catalysis in acetic acid. It has been shown that benzylic bromides participate in the catalytic cycle through their catalyzed and noncatalyzed oxidation, through their reaction with Co(III), and through cobalt(II)-catalyzed solvolysis. The rates of the direct reduction of Co(III) by several benzylic bromides were measured under an argon atmosphere; the reaction occurs by a mechanism involving two forms of Co(III). The same reaction under an oxygen atmosphere initiates the cobalt-bromide-catalyzed oxidation of benzyl bromide, thus leading to the regeneration of inorganic bromide and the fast reduction of Co(III). Solvolysis of benzylic bromides plays only a minor role in the regeneration of inorganic bromide in glacial acetic acid.     

Direct vs Indirect Grafting of Alkyl and Aryl Halides

The direct and indirect electrochemical grafting of alkyl and aryl halides (RX, ArX) on carbon, metal and polymer surfaces is examined. Their electrochemical reduction occurs at highly negative potential in organic solvents and very often produces carbanions because the reduction potentials of RX and ArX are more negative than those of their corresponding radicals. Therefore, direct electrografting of alkyl and aryl radicals generated from RX and ArX is not easy to perform. This obstacle is overcome using aryl radicals derived from the 2,6-dimethylbenzenediazonium salt (2,6-DMBD), which do not react on the electrode surface due to their steric hindrance but react in solution by abstracting an iodine or bromine atom from RX (X=I, Br) or ArI to give alkyl or aryl radicals. As a consequence, alkyl and aryl radicals are generated at very low driving force by diverting the reactivity of aryl radicals derived from an aryl diazonium salt; they attack the electrode surface and form strongly attached organic layers. This strategy applies to the chemical modification of polymers (polyethylene, polymethylmethacrylate) by alkyl halides under heating.     

Homocoupling of aryl halides promoted by an NiCl2/bpy/Mg system in DMF

Homocoupling of aryl halides (2 ArX → Ar―Ar) promoted by NiCl₂/2,2′‐bipyridine (bpy)/Mg mixtures in DMF has been studied. Mixtures of NiCl₂, bpy and Mg in DMF promoted homocoupling of aryl halides such as phenyl bromide and p‐tolyl bromide to give the coupling products in good (e.g. approximately 60–75%) yields, and the homocoupling products were easily isolated from the reaction mixtures. Application of this homocoupling to dibromo‐aromatic compounds (Br–arylene–Br: 2,5‐dibromopyridine, 2,7‐dibromo‐9.9‐dioctylfluorene and 2,7‐dibromo‐9,10‐dioctyl‐9,10‐dihydrophenanthrene) gave the corresponding π‐conjugated polymers, –(arylene)_n–, in good yields. Organometallic processes for the homocoupling are discussed. Copyright © 2014 John Wiley & Sons, Ltd.     

Nickel‐Catalyzed Cross‐Coupling of Alkyl Zinc Halides for the Formation of C(sp2)?C(sp3) Bonds: Scope and Mechanism

Optimal conditions for a general Ni‐catalysed Negishi cross‐coupling of alkyl zinc halides with aryl, heteroaryl and alkenyl halides have been determined. These conditions allow the reaction to take place smoothly, with low catalyst loading, and in the presence of a wide variety of functional groups to afford products in good yields at room temperature. DFT studies on the mechanism support the occurrence of a catalytic cycle involving transmetalation of the alkyl zinc halide to Ni^I followed by oxidative addition of the haloarene and C? C reductive elimination.     

Well-Defined Ni(0) and Ni(II) Complexes of Bicyclic (Alkyl)(Amino)Carbene (MeBICAAC): Catalytic Activity and Mechanistic Insights in Negishi Cross-Coupling Reaction

Negishi cross-coupling reaction of organozinc compounds as nucleophiles with aryl halides has drawn immense focus for C−C bond formation reactions. In comparison to the well-established library of Pd complexes, the C−C cross-coupling of this particular approach is largely primitive with nickel-complexes. Herein, we describe the syntheses of Ni(II) complexes, [(^MeBICAAC)₂NiX₂] (X=Cl ( 1 ), Br ( 2 ), and I ( 3 )) by employing the bicyclic (alkyl)(amino)carbene (^MeBICAAC) ligand. The reduction of complexes 1 – 3 using KC₈ afforded the two coordinate low valent, Ni(0) complex, [(^MeBICAAC)₂Ni(0)] ( 4 ). Complexes 1 – 4 have been characterized by spectroscopic techniques and their solid-state structures were also confirmed by X-ray crystallography. Furthermore, complexes 1 – 4 have been applied in a direct and convenient method to catalyze the Negishi cross-coupling reaction of various aryl halides with 2,6-difluorophenylzinc bromide or phenylzinc bromide as the coupling partner in the presence of 3 mol % catalyst. Comparatively, among all-pristine complexes, 1 exhibit high catalytic potential to afford value-added C−C coupled products without the use of any additive. The UV-vis studies and HRMS measurements of controlled stochiometric reactions vindicate the involvement of Ni(I)−NI(III) cycle featured with a penta-coordinated Ni(III)-aryl species as the key intermediate for 1 whereas Ni(0)/Ni(II) species are potentially involved in the catalytic cycle of 4 .     

CO2 as Both a Selective Agent and Reaction Media in Palla-dium-Catalyzed Reductive Ullmann-Type Coupling Reaction

Carbon dioxide as both a selective agent and reaction media in the palladium-catalyzed Ullmann-type coupling has been described. The results showed that aryl chlorides could be easily activated in the presence of carbon dioxide and the chemoselectivity shifted toward the palladium-catalyzed Ullmann-type coupling reaction. In liquid carbon dioxide, homocoupling reactions of aryl halides, including less reactive aryl chlorides, were carried out smoothly in moderate to good yields using Pd/C, zinc, and H2O as the catalytic system at room temperature.     

Reduction of aryl halides at transition metal cathodes. Conditions for aryl–aryl bond formation: The Ullmann's reaction revisited

The cathodic reduction of some aryl halides ArX (1-naphthyl halides NpI and NpBr, iodo benzene and bromobenzene PhI and PhBr taken as model substrates) was achieved essentially in propylene carbonate (PC) considered for its high dielectric permittivity. Different electrode materials such as copper, silver, palladium, silver palladium alloy and nickel were used. Such conditions permit the activation of the C–X bond by metal (the step featuring similarity with Ullmann's reaction). Electron transfer to organometallic intermediates generated at the metal interface activates the formation of Ar–Ar linkages often in good yields, especially in the case of aryl iodides.     

Palladium-catalyzed cross-coupling of pyrrole anions with aryl chlorides, bromides, and iodides

[reaction: see text] A general method for the conversion of pyrrole anions to 2-arylpyrroles has been developed. Using a palladium precatalyst and sterically demanding 2-(dialkylphosphino)biphenyl ligands, (pyrrolyl)zinc chloride may be cross-coupled with a wide range of aryl halides, including aryl chlorides and aryl bromides, at low catalyst loadings and under mild conditions. A high degree of steric hindrance is tolerated. Certain ring-substituted pyrrole anions have also been arylated with aryl bromide substrates.     

One pot direct synthesis of β-ketoesters via carbonylation of aryl halides using cobalt carbonyl

A direct method for the synthesis of β-ketoesters from aryl halides (iodide, bromide) has been described by using cobalt carbonyl as carbon monoxide source in microwave irradiation. Using this protocol, a wide variety of substituted aryl halides has been successfully converted to corresponding β-ketoesters.     

Cobalt co-catalysis for cross-electrophile coupling: diarylmethanes from benzyl mesylates and aryl halides

The nickel-catalyzed cross-coupling of aryl halides with alkyl radicals derived from alkyl halides has recently been extended to couplings with carbon radicals generated by a co-catalyst. In this study, a new co-catalyst, cobalt phthalocyanine (Co(Pc)), is introduced and demonstrated to be effective for coupling substrates not prone to homolysis. This is because Co(Pc) reacts with electrophiles by an S_N2 mechanism instead of by the electron-transfer or halogen abstraction mechanisms previously explored. Studies demonstrating the orthogonal reactivity of (bpy)Ni and Co(Pc), applying this selectivity to the coupling of benzyl mesylates with aryl halides, and the adaptation of these conditions to the less reactive benzyl phosphate ester and an enantioconvergent reaction are presented.     

Cobalt-catalyzed cross-coupling reactions of alkyl halides with aryl Grignard reagents and their application to sequential radical cyclization/cross-coupling reactions

Reactions of alkyl halides with arylmagnesium bromides in the presence of cobalt(II)(diphosphine) complexes are discussed. Treatment of 1-bromooctane with phenylmagnesium bromide with the aid of a catalytic amount of CoCl₂(dppp) [DPPP=1,3-bis(diphenylphosphino)propane] yielded octylbenzene in good yield. The reaction mechanism would include single electron transfer from an electron-rich cobalt complex to alkyl halide to generate the corresponding alkyl radical. The mechanism was justified by CoCl₂(dppe)-catalyzed [DPPE=1,2-bis(diphenylphosphino)ethane] sequential radical cyclization/cross-coupling reactions of 6-halo-1-hexene derivatives that yielded benzyl-substituted cyclopentane skeletons.     

Dechlorination of beta-methylallyl chloride by electrogenerated [Co(I)(bipyridine)3]+: an electrochemical study in the presence of cationic surfactants

We have investigated the electrocatalytic dehalogenation of beta-methylallyl chloride (beta-mAC), widely used in the polymer industry, using [Co(I)(bpy)3]+ (where bpy=2,2'-bipyridine) electrochemically generated in situ from [Co(II)(bpy)3]2+ at a glassy carbon electrode in the presence of three different cationic surfactants in aqueous solution. Cetyltrimethylammonium bromide (CTAB), tetradecyltrimethylammonium bromide (TDTAB), and cetylbenzyldimethylammonium chloride (CBDAC) were employed in the present investigation. The [Co(II)(bpy)3]2+-cationic surfactant systems show excellent electrocatalytic activity toward dehalogenation of beta-mAC. The dependence of the catalytic current, the corresponding potential, and the current function on the potential scan rate has been analyzed to assess the nature of the catalytic reaction. The second-order rate constant, kchem, for the reaction between the beta-mAC substrate and the electrogenerated-micelle stabilized-Co(I) complex has been calculated by a cyclic voltammetry technique. The reduction products after 3 h of bulk electrolysis have been identified by GC/MS to be one nonchloro compound (2-methyl-1,5-hexadiene (IV)) and two chloro compounds (1-chloro-2,5-dimethyl-2,5-hexadiene (V) and spiro[1.2]cylopropyl-6-chloro-5-methyl-hex-4-ene (VI)). Based on the electrochemical results and the mass spectral data, a reaction scheme involving all the reduction products has been proposed. Finally, a good correlation between the catalytic efficiency and the structural features of the surfactant molecules is demonstrated. The present study emphasizes the need for further optimization work to achieve maximum yield of nonchloro compound (IV) to employ the present [Co(II)(bpy)3]2+-cationic surfactant systems with a high catalytic efficiency as promising for possible applications.     

Aryl transfer between Pd(II) centers or Pd(IV) intermediates in Pd-catalyzed domino reactions

A computational study has been performed to determine the mechanism of the key steps of Pd-catalyzed domino reactions in which C(sp2)-C(sp2) are formed from aryl and alkenyl halides. DFT calculations were done on model complexes of the proposed intermediates, with PH3 and H2O as ancillary ligands, to explore two possible mechanisms: the oxidative addition of aryl or alkenyl halides to palladacycles to give Pd(IV) intermediates, and the transmetalation-type reaction of aryl or alkenyl ligands between two Pd(II) centers, a palladacycle, and a Pd(II) complex formed by oxidative addition of aryl or alkenyl halides to Pd0. We have shown that oxidative addition of iodoethylene to Pd0 precursors is more favorable than oxidative addition to Pd(II) palladacycles, whereas transmetalation-type reactions between Pd(II) complexes are facile. Similar results were obtained with iodobenzene instead of iodoethylene and formamide as the ancillary ligand. These results suggest that Pd(IV) intermediates are not involved in these reactions.