Cobalt complexes aryl halides

Perfluoroalkyl or -aryl halides undergo oxidative addition with metal vapors to form nonsolvated fluonnated organometallic halides and this topic has been die subject of a review [289] Pentafluorophenyl halides react with Rieke nickel, cobalt, and iron to give bispentafluorophenylmetal compounds, which can be isolated in good yields as liquid complexes [290] Rieke nickel can also be used to promote the reaction of pentafluorophenyl halides with acid halides [297] (equation 193)... 

Palladium complexes also catalyze the carbonylation of halides. Aryl (see 13-13), vinylic, benzylic, and allylic halides (especially iodides) can be converted to carboxylic esters with CO, an alcohol or alkoxide, and a palladium complex. Similar reactivity was reported with vinyl triflates. Use of an amine instead of the alcohol or alkoxide leads to an amide. Reaction with an amine, AJBN, CO, and a tetraalkyltin catalyst also leads to an amide. Similar reaction with an alcohol, under Xe irradiation, leads to the ester. Benzylic and allylic halides were converted to carboxylic acids electrocatalytically, with CO and a cobalt imine complex. Vinylic halides were similarly converted with CO and nickel cyanide, under phase-transfer conditions. ... 

Structural types for organometallic rhodium and iridium porphyrins mostly comprise five- or six-coordinate complexes (Por)M(R) or (Por)M(R)(L), where R is a (T-bonded alkyl, aryl, or other organic fragment, and Lisa neutral donor. Most examples contain rhodium, and the chemistry of the corresponding iridium porphyrins is much more scarce. The classical methods of preparation of these complexes involves either reaction of Rh(III) halides Rh(Por)X with organolithium or Grignard reagents, or reaction of Rh(I) anions [Rh(Por)] with alkyl or aryl halides. In this sense the chemistry parallels that of iron and cobalt porphyrins. 

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. 

The synthesis of metalloporphyrins which contain a metal-carbon a-bond can be accomplished by a number of different methods(l,2). One common synthetic method involves reaction of a Grignardreagent or alkyl(aryl) lithium with (P)MX or (PMX)2 where P is the dianion of a porphyrin macrocycle and X is a halide or pseudohalide. Another common synthetic technique involves reaction of a chemically or electrochemically generated low valent metalloporphyrin with an alkyl or aryl halide. This latter technique is similar to methods described in this paper for electrosynthesis of cobalt and rhodium a-bonded complexes. However, the prevailing mechanisms and the chemical reactions... 

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... 

Since Wakamatsu serendipitously discovered amidocarbonylation while performing the cobalt-catalyzed hydroformyla-tion of olefins in 1971, this unique carbonylation reaction, affording a-amino acids directly from aldehydes, has been extensively studied.More recently, palladium-catalyzed processes have been developed to expand the scope of this reaction.The Pd-catalyzed amidocarbonylation has been applied to aldehydes,aryl halides, and imines. As a related reaction, lactamization " of aryl halides catalyzed by a rhodium complex has also been developed. 

The amount of cobalt complex in this step influences the reaction rate, but not the yields. Indeed, with only 0.3 equivalent of cobalt catalyst, the arylzinc compound is consumed after 24 h instead of 10 h when 1 equivalent was used. An excess of the activated olefin is required to optimize the yield of the conjugate addition. Under these conditions, this process has been studied with various aryl halides (X = Br, Cl) and activated olefins. Yields range from 40 to 80%. 

An asymmetric preparation of alkenylcyclopropanes has also been realized by the use of palladium(O) complexes carrying chiral ferrocenylphosphine ligands (equation 22)38. The requisite rt-allyl palladium intermediates can also be generated from allene and meth-ylenecyclopropane derivatives, 1839 and 1940, in the presence of palladium(O) complex and alkenyl or aryl halide (equations 23 and 24). The cobalt complexes, 20, similarly afford the corresponding alkenylcyclopropanes upon exposure to LDA (equation 25)41. 

Stolzenberg and coworkers have used electrogenerated nickel(I) tetrapyrrole complexes for the catalytic reduction of dichloromethane and methyl iodide [364], alkyl halides [365-367], and aryl halides [367], and Lexa and coworkers [368] have discussed the catalytic reduction of frm75 -l,2-dibromocyclohexane to cyclohexene by electrogenerated nickel(I), cobalt(I), and iron(I) porphyrin complexes. 

In contrast, nucleophilic attack at an sp -carbon center with anionic 18-electron species like [Co(CO)4] and [Fe(CO)4] usually fails to activate aromatic or vinylic halides. Thus, catalytic carbonylations of aryl halides are initiated by oxidative addition of a C-X bond to an electronically unsaturated metal complex, normally a palladium [7], cobalt [8] or nickel complex [9]. The rate of this oxidative addition decreases along the sequence... 

An alternative route used in organometallic chemistry is the reaction of low valent organometallic derivatives with alkyl (aryl) halides. The two electron oxidative addition of alkyl (aryl) halides or cyclopropane derivatives to metalloporphyrins such as [M (Por)] leads to metal alkyl (aryl) o-bonded porphyrins of cobalt " rhodium and iridium ° (Scheme 2). Substitution of aryl and vinyl halides by electrochemically generated iron(I) porphyrins also leads to o-bonded Fe complexes ... 

Since the researcher normally looks to the chemistry of soluble complexes in designing polymer-bound catalysts, it is notable that some areas that have proven fruitful in homogeneous catalysis have been omitted from investigations using polymer-bound catalysts. One of these areas concerns the reactions of arenes. Benzene, for example, may be hydrogenated with homogeneous cobalt phosphite and ruthenium phosphine complexes, but the corresponding supported versions are not reported. Aryl halides may be carboxylated in the presence of a soluble palladium catalyst ... 

In methanol, Cl and 7tH+ ions exhibit a retarding effect. The stabilities of nickel(o)-phosphine complexes have been assessed these seem to depend more on the size of the phosphine and electronic effects on bond strengths are of secondary importance. In the oxidative addition of aryl halides to nickel(o)-phosphine complexes the reaction appears to proceed via an initial slow dissociation step to give Ni(PR3)2 which then attacks the organic species. The mechanism of oxidative elimination of these nickel species thus contrasts with that for the platinum(o)-phosphines where the dissociation of the ligand is rapid and the rate-determining step is that involving the redox interaction. The oxidation of tetrahedral cobalt(i) complexes with carbon tetrachloride has been described ... 

The catalytic system proved not only applicable to alkyl hahdes, but also allowed for the intramolecular conversion of aryl halides. Interestingly, the corresponding Mizoroki-Heck-type cyclization products were formed selectively, without traces of reduced side-products (Scheme 10.27) [55]. Therefore, a radical reaction via a single electron-transfer process was generally disregarded for cobalt-catalysed Mizoroki-Heck-type reactions of aromatic hahdes. Instead, a mechanism based on oxidative addition to yield an aryl-cobalt complex was suggested [51]. 

A series of papers have also reported the coupling of alkyl and aryl electrophiles with aryl Grignard reagents catalyzed by iron (Equation 19.15) and cobalt complexes. These reactions build upon Kochi s and Molander s early results on coupling reactions catalyzed by complexes of these metals. The recent reactions have been conducted with simple metal salts in many cases and with discrete metal complexes as catalyst precursors in others. Although little recent mechanistic data is available on these reactions, they have earlier been shown to involve radical intermediates. Kochi concluded that the catalytic process occurs by an Fe(I)-Fe(III) couple reactions of optically active alkyl halides generate racemic coupled products and reactions of diastereomerically pure aUcyl halides generate equal ratios of diastereomeric products, as depicted in Equations 19.16 and 19.17. ... 

The use of cobalt species to catalyze cross-coupling reactions of activated or unactivated alkyl halides with alkynyl Grignard reagents in order to form C(sp)-G(sp ) bonds is also of interest. Cobalt compleres promote the crossbenzyl chlorides and bromides react with 2-trimethylsilylethynyl- and 1-octyn-l-ylmagnesium halides to provide l-aryl-2-alkynes in the presence of a catalytic amount of a cobalt complex [100,101]. 

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