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.     

Chemoselective vinylation of aminophenols with acetylene catalyzed by sodium aminophenolates in aqueous DMSO

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Alkenylation of C(sp3)−H Bonds by Zincation/Copper‐Catalyzed Cross‐Coupling with Iodonium Salts

α‐Vinylation of phosphonates, phosphine oxides, sulfones, sulfonamides, and sulfoxides has been achieved by selective C?H zincation and copper‐catalyzed C(sp³)?C(sp²) cross‐coupling reaction using vinylphenyliodonium salts. The vinylation transformation proceeds in high efficiency and stereospecificity under mild reaction conditions. This zincative cross‐coupling reaction represents a general alkenylation strategy, which is also applicable for α‐alkenylation of esters, amides, and nitriles in the synthesis of β,γ‐unsaturated carbonyl compounds.     

Palladium-catalyzed vinylation of cyclohexenes using a directing-group strategy

Palladium-promoted vinylation of cyclohexenes via employment of a directing-group strategy to yield the coupled vinyl cyclohexenes with excellent regio- and stereoselectivity was studied. Typically, reaction of 2-(cyclohex-2-en-1-yl)-N-tosylacetamide ( 1a ) with (Z)-styryl bromides ( 2 ) gave cis-2-[(E)-styryl]cyclopent-3-en-1-yl-N-tosylacetamides in good to excellent yields. It is noticed that (Z)-styryl moiety was inverted into (E)-form in products. Unfortunately, (E)-styryl bromide substrates were not suitable for this reaction under the condition investigated. Further studies on norbornene system, we found that palladium-catalyzed reaction of endo-N-tosylbicyclo[2.2.1]hept-5-ene-2-carboxamide ( 6 ) with styryl bromides gave the Aza-Heck type products.     

Catalytic Carbonyl Addition through Transfer Hydrogenation: A Departure from Preformed Organometallic Reagents

Classical protocols for carbonyl allylation, propargylation and vinylation typically rely upon the use of preformed allyl metal, allenyl metal and vinyl metal reagents, respectively, mandating stoichiometric generation of metallic byproducts. Through transfer hydrogenative C C coupling, however, carbonyl addition may be achieved from the aldehyde or alcohol oxidation level in the absence of stoichiometric organometallic reagents or metallic reductants. Here, we review transfer hydrogenative methods for carbonyl addition, which encompass the first catalytic protocols enabling direct C H functionalization of alcohols.     

Superbase-promoted multi-molecular acetylene/arylamine self-organization to 1-arylpyrroles

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Enantioselective Formal C(sp2)−H Vinylation

An enantioselective formal C(sp²)?H vinylation of prochiral 2,2‐disubstituted cyclopentene‐1,3‐dione is presented. This vinylative desymmetrization is realized by using a two‐step procedure that consists of a catalytic enantioselective vinylogous Michael addition of deconjugated butenolides to cyclopentene‐1,3‐dione and a base‐mediated decarboxylation. The overall process utilizes deconjugated butenolides as the source of the highly substituted vinyl unit. Five‐membered carbocycles containing a remote all‐carbon quaternary stereogenic center are obtained in good yields and with good to high enantioselectivities.     

Vinylierung und 91Zr-NMR-Spektren von Substituierten Zirconocendichloriden

Vinylation and ⁹¹Zr N.M.R. Spectra of substituted Zirconocene Dichlorides Substituted zirconocene dichlorides react with vinyl lithium with formation of zirconacyclopent-2-enes, Cp₂ZrCH = CHCH₂CH₂, or zirconocene butadiene complexes, Cp₂Zr(C₄H₆). The compounds obtained were characterized by their ¹H and ¹³C n.m.r. spectra. The ⁹¹Zr n.m.r. chemical shifts of substituted zirconocene dichlorides correlate with the bond angles Cp′? Zr? Cp′ and Cl? Zr? Cl respectively. They can be used to estimate the reaction behaviour of zirconocene dichlorides.