Cobalt pyridine halides

Low-valent cobalt pyridine complexes, electrogenerated from CoCl2 in DMF containing pyridine and associated with a sacrificial zinc anode, are also able to activate aryl halides to form arylzinc halides.223 This electrocatalytic system has also been applied to the addition of aryl bromides containing an electron-withdrawing group onto activated alkenes224 and to the synthesis of 4-phenylquinoline derivatives from phenyl halides and 4-chloroquinoline.225 Since the use of iron as anode appeared necessary, the role of iron ions in the catalytic system remains to be elucidated. 

Synthesis of 2,6-Bis(imino)pyridine Iron(II) and Cobalt(II) Halides. 110... 

Scheme 2 Synthesis of bis(arylimino)pyridine iron(II) and cobalt(II) halides...

The capacity of bis(arylimino)pyridine iron(II) (5) and cobalt(II) halides (6) to act as precatalysts for the polymerisation and oligomerisation of ethylene was first demonstrated when toluene solutions of 5 or 6 were treated with excess MAO... 

The active species generated when bis(arylimino)pyridine iron (5) and cobalt (6) halides are activated with MAO was, by analogy with metallocene catalysts, initially considered to be a highly reactive mono-methylated cobalt(II) or iron(II) cation of the form LM-Me+ bearing a weakly coordinating counter-anion such as [X-MAO]-(X = halide, Me). To examine this theory a number of spectroscopic investigations have been directed towards identifying the active species (vide infra). 

The formation of hydrated cobalt(n) complexes of pyridine carboxylic acids and the subsequent thermal decomposition to lower hydrates has been documented.82,83 Cobalt(n) halides react with 6-methylpicolinic acid (6-mpaH), picolinic acid (paH), nicotinic acid (naH), and pyridine-2,6-dicarboxylic acid (2,6-py) to form Co(6-mpa) (6-mpaH)X (X = Cl, Br, or NCS), Co(naH)nX2 (n = 2, X = Cl, Br n = 3, X = NCS), and Co(pa)(paH)X, EtOH (X = Cl, Br, or NCS) which are all probably octahedral.83 6-Methylpicolinic acid also formed Co(6-mpaH)4X2,2HX (X = Cl or Br) which were formulated [(6-mpaH)2H]2[CoX4], since the electronic spectra show absorptions characteristic of tetrahalogenocobaltate(n) ions.83... 

Direct measurement of Co(III)/(II)(TPP) self-exchange has been possible using nmr in CDCI3 solution in the presence of pyridine. The main cobalt(III) species is the [Co(III)(TPP)(py)2, CF] ion pair and the outer-sphere self-exchange rates increase by almost three orders of magnitude on going from [Co(II)(TPP)] to [Co(II)(TPP)(py)]. The corresponding cobalt(III) halide complexes [Co(III)(TPP)X], where the halide is now metal bound/" react much more quickly by an inner-sphere pathway and show the normal order F > Br > CF for bound halide. 

The scope of this reaction is similar to that of 10-21. Though anhydrides are somewhat less reactive than acyl halides, they are often used to prepare carboxylic esters. Acids, Lewis acids, and bases are often used as catalysts—most often, pyridine. Catalysis by pyridine is of the nucleophilic type (see 10-9). 4-(A,A-Dimethylamino)pyridine is a better catalyst than pyridine and can be used in cases where pyridine fails. " Nonbasic catalysts are cobalt(II) chloride " and TaCls—Si02. " Formic anhydride is not a stable compound but esters of formic acid can be prepared by treating alcohols " or phenols " with acetic-formic anhydride. Cyclic anhydrides give monoesterified dicarboxylic acids, for example,... 

Duong and Gaudemer studied the alkylation of (presumably) [Co -(DMG)2X], where X is pyridine, aniline, or water, by the cis and trans isomers of )S-bromostyrene (PhCH=CHBr) and the methyl ester of )3-chloroacrylic acid (CHCl=CHCOOMe) in 50% aqueous methanol, and found that the configuration of the double bond remained unchanged, i.e., the halogen had simply been replaced by cobalt. They suggested that the reaction involved the addition of cobalt, followed by the elimination of the halide ion (apparently without rotation around the C—C bond), i.e.. 

Bis(imino)pyridine Iron Halide/MAO and Cobalt Halide/MAO Catalysts. 120... 

Studies conducted to examine the mode of activation of MAO with bis(imino) pyridine cobalt halide systems have shown some intriguing findings. With regard to 6a/MAO, initial reduction of the cobalt(II) precatalyst to cobalt halide followed by conversion to a cobalt methyl and ultimately to a cobalt cationic species has been demonstrated (see Sect. 2.6) [108, 109], Addition of ethylene affords an eth-... 

Heteroatom transfer in metallacyclopentadienes was first developed in the context of cobalt chemistry in the mid-1970s [27]. Cobaltacyclopentadienes were converted into various five-membered heterocyclic compounds such as pyrrole and thiophene, and into six-mem-bered heterocyclic compounds such as pyridine and pyridone derivatives. In the case of zirconacydopentadienes, the heteroatom compound must bear at least two halide substituents, since the Cp2Zr moiety is re-converted to the stable Cp2ZrX2. Indeed, this is the driving force behind the heteroatom transfer of zirconacydopentadienes. 

Numerous d cobalt(III) complexes are known and have been studied extensively. Most of these complexes are octahedral in shape. Tetrahedral, planar and square antiprismatic complexes of cobalt(lII) are also known, but there are very few. The most common ligands are ammonia, ethylenediamine and water. Halide ions, nitro (NO2) groups, hydroxide (OH ), cyanide (CN ), and isothiocyanate (NCS ) ions also form Co(lII) complexes readily. Numerous complexes have been synthesized with several other ions and neutral molecular hgands, including carbonate, oxalate, trifluoroacetate and neutral ligands, such as pyridine, acetylacetone, ethylenediaminetetraacetic acid (EDTA), dimethylformamide, tetrahydrofuran, and trialkyl or arylphosphines. Also, several polynuclear bridging complexes of amido (NHO, imido (NH ), hydroxo (OH ), and peroxo (02 ) functional groups are known. Some typical Co(lll) complexes are tabulated below ... 

Electroreduction of the cobalt(II) salt in a mixture of either dimethylform-amide-pyridine or acetonitrile-pyridine as solvent, often in the presence of bipyridine, produces a catalytically active cobalt(I) complex which is believed to be cobalt(I) bromide with attached bipyridine ligands (or pyridine moieties in the absence of bipyridine). As quickly as it is electrogenerated, the active catalyst reduces an aryl halide, after which the resulting aryl radical can undergo coupling with an acrylate ester [141], a different aryl halide (to form a biaryl compound) [142], an activated olefin [143], an allylic carbonate [144], an allylic acetate [144, 145], or a... 

The formation of arylzinc reagents can also be accomplished by using electrochemical methods. With a sacrificial zinc anode and in the presence of nickel 2,2-bipyridyl, polyfunctional zinc reagents of type 36 can be prepared in excellent yields (Scheme 14) . An electrochemical conversion of aryl halides to arylzinc compounds can also be achieved by a cobalt catalysis in DMF/pyridine mixture . The mechanism of this reaction has been carefully studied . This method can also be applied to heterocyclic compounds such as 2- or 3-chloropyridine and 2- or 3-bromothiophenes . Zinc can also be elec-trochemically activated and a mixture of zinc metal and small amounts of zinc formed by electroreduction of zinc halides are very reactive toward a-bromoesters and allylic or benzylic bromides . ... 

Et2AlCl could be replaced by the sesquichloride or by a mixture of a trialkylaluminum and a reactive halide such as benzyl chloride or tert-butyl chloride. The effective cobalt compounds were those which are known to yield cis-1,4-polybutadiene—e.g. cobalt stearate, cobalt acetyl-acetonate, cobalt bis(salicylaldehyde imine), cobalt chloride-pyridine, etc. Et2AlCl concentration could be varied within the range 0.3-5% by weight based on PVC, and the cobalt compound concentration was 0.002-0.01 mole per mole of Et2AlCl. 

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