Cobalt reductive coupling

The reductive coupling of allyl halides to 1,5-hexadiene at glassy C electrodes was catalyzed by tris(2, 2,-bipyridyl)cobalt(II) and tris(4,4 -dimethyl-2, 2/-bipyridyl)cobalt(II) in aqueous solutions of 0.1 M sodium dodecylsulfate (SDS) or 0.1 M cetyltrimethylammonium bromide (CTAB).48 An organocobalt(I) intermediate was observed by its separate voltammetric reduction peak in each system studied. This intermediate undergoes an internal redox reaction to form 1,5-hexadiene... 

Rhodium and rhodium-cobalt based catalysts using silanes as the stoichiometric reductant were initially reported in 1992 to reductively couple enyne substrate (Eq. 30) [90,91]. Further investigation showed this reaction to be an effective method for the cyclization of enyne substrates 129 to... 

The highly oxophilic nature of the cobalt powder was readily demonstrated by its reaction with nitrobenzene at room temperature. Reductive coupling was quickly effected by 2 to give azo- and azoxy derivatives. Nitrobenzene reacted with 2 to give azobenzene in yields up to 37%. In some cases small amounts of azoxybenzene were also formed. With 1,4-diiodonitrobenzene, 2 reacted to give low yields of 4,4-diiodoazoxy-benzene and 4,4-diiodoazobenzene. 

The electrochemistry of cobalt-salen complexes in the presence of alkyl halides has been studied thoroughly.252,263-266 The reaction mechanism is similar to that for the nickel complexes, with the intermediate formation of an alkylcobalt(III) complex. Co -salen reacts with 1,8-diiodo-octane to afford an alkyl-bridged bis[Co" (salen)] complex.267 Electrosynthetic applications of the cobalt-salen catalyst are homo- and heterocoupling reactions with mixtures of alkylchlorides and bromides,268 conversion of benzal chloride to stilbene with the intermediate formation of l,2-dichloro-l,2-diphenylethane,269 reductive coupling of bromoalkanes with an activated alkenes,270 or carboxylation of benzylic and allylic chlorides by C02.271,272 Efficient electroreduc-tive dimerization of benzyl bromide to bibenzyl is catalyzed by the dicobalt complex (15).273 The proposed mechanism involves an intermediate bis[alkylcobalt(III)] complex. 

Though several intermolecular catalytic reductive aldol additions are reported, corresponding reductive cyclizations have received less attention. The first reported reductive aldol cyclization involves use of a (diketonato)cobalt(ll) precatalyst in conjunction with PhSiHj as terminal reductant.48,486 The reductive cyclization is applicable to aromatic and heteroaromatic enone partners to form five- and six-membered rings. As demonstrated by the reductive cyclization of mono-enone mono-aldehyde 65a to afford aldol 65b, exceptionally high levels of ty -diastereoselectivity are observed. Interestingly, exposure of the substrate 65a to low-valent nickel in the presence of excess Et2Zn provides the isomeric homoaldol cyclization product 65c via reductive coupling to the enone /3-position (Scheme 43).47a... 

In the past, there has been controversy [1] regarding both the true identity of the cobalt(III) species present in an aqueous medium and the standard potential for its one-electron reduction to cobalt(II). However, according to Wam-qvist [7], as well as Davies and Wam-qvist [8], there is good reason to conclude that Co(H20)6 is the predominant species in a 3 M perchloric acid solution and that the cobalt( 111) -cobalt( 11) couple in this medium can be represented as follows ... 

Several investigations have been made of the reduction of cobalt(II) to cobalt(O) in molten salt media. Eor a eutectic melt of LiCl-KCl at 450°C[10], a 1 1 NaCl-KCl melt at 450°C[11], and a MgCh-NaCl-KCl (50 30 20 mol%) mixture at 475 °C [12], the apparent standard potentials for the cobalt(II)-cobalt(0) couple have been deduced to be —1.207 V, — 1.277 V, and—1.046 V, respectively, each with respect to a chlorine-chloride ion reference electrode. 

Reduction potentials for /z-superoxo//z-peroxo-cobalt(III) couples have recently been obtained by cyclic voltammetry.739,740 Decomposition or dissociation of one or the other of the components of the couple, which frustrated measurements by other techniques, may be overcome by the use of fast scan rates. Protonation of the /r-peroxo group stabilizes the complex and EB values are pH dependent below pH 3. A similar stabilization on protonation of the peroxo bridge has been noted in the Ct2+-, V2+- and Eu2+-promoted reduction705 of the [(NHj)5Co(02)Co(NH3)5]4+ ion. The protonated species has Kh 10 dm3 mol-1 at 25 °C.705... 

Reductive coupling of allylic halides. This cobalt complex (1 equiv.) effects reductive coupling of allylic halides to form 1,5-dienes with preservation of the geometry of the double bonds/ The major product from coupling of terpenoid allylic halides is that formed by head-to-head coupling. The triphenylphosphine liberated during the reaction is removed as methyltriphenylphosphonium iodide, obtained by reaction with methyl... 

The mononuclear cobalt complexes are stable and are able to be isolated in both 2+ and 3+ oxidation states. Cyclic voltammetric studies reveal reversible waves for both Co " 2+ and Co + i reduction couples. These redox couples are shifted anodically as the ligand substituents are changed from methyl to phenyl. Electrolytic and cyclic voltammetric studies before and after electrolysis support the idea that the integrity of the complexes is maintained during electrolytic cycles of the 2+/3+ oxidation states. The IpJIpa values of the Co + 2+ couple for the binuclear cobalt complexes are identical to those observed for the oxidation of the analogous iron complex. Attempts to produce the binuclear cobalt(III) species by exhaustive electrolysis have been limited by adsorption of the cobalt(III) complexes on the electrode surface [186, 187],... 

Dialkyl ketones/ The reductive coupling of alkenes and carbon monoxide at room temperature is promoted by CoCl2-Ph(Et)2N BH3 (7 examples, 50-70%). The generation of ClBHj as hydroborating agent for the alkenes is implicated, and the subsequent carbonylation is assisted by cobalt carbonyl species formed in situ. 

Electrocatalytic reductive coupling of aryl chlorides to afford biphenyls can be accomplished with dichloro(l,2-bis(di-propylphosphino)benzene)nickel(II) in yields as high as 96% with 2 mol % of the catalyst in polar, coordinating solvents (55). Similar couplings can also be achieved with nickel-2,2 -bipyridyl and Pd(PPh3)2Cl2 as catalysts (56, 57). Indirect electrochemical reduction of vicinal dibromides to alkenes occurs efficiently with iron and cobalt porphyrins as mediators (58). Vitamin B12 is a mediator for the indirect electrochemical reduction of a-halo acids (59). 

Bromocyclizadon led to the formatitHi of tetracyclic 680 as a single stereoisomer. Kinetic deprotonatitm followed by treatment with methyl iodide furnished derivative 681. Compound 682 was obtained by exchanging the silyl ether with acetate. Again, cobalt(l)-mediated reductive coupling afforded the dimeric diketopiperazine 683. 

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