Cobalt alkyl halides

Polymers containing 90-98% of a c 5-1,4-structure can be produced using Ziegler-Natta catalyst systems based on titanium, cobalt or nickel compounds in conjuction with reducing agents such as aluminium alkyls or alkyl halides. Useful rubbers may also be obtained by using lithium alkyl catalysts but in which the cis content is as low as 44%. 

The corresponding reactions of transient Co(OEP)H with alkyl halides and epoxides in DMF has been proposed to proceed by an ionic rather than a radical mechanism, with loss of from Co(OEP)H to give [Co(TAP), and products arising from nucleophilic attack on the substrates. " " Overall, a general kinetic model for the reaction of cobalt porphyrins with alkenes under free radical conditions has been developed." Cobalt porphyrin hydride complexes are also important as intermediates in the cobalt porphyrin-catalyzed chain transfer polymerization of alkenes (see below). 

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. 

Cobaloxime(I) generated by the electrochemical reductions of cobaloxime(III), the most simple model of vitamin Bi2, has been shown to catalyze radical cyclization of bromoacetals.307 Cobalt(I) species electrogenerated from [ConTPP] also catalyze the reductive cleavage of alkyl halides. This catalyst is much less stable than vitamin Bi2 derivatives.296 It has, however, been applied in the carboxylation of benzyl chloride and butyl halides with C02.308 Heterogeneous catalysis of organohalides reduction has also been studied at cobalt porphyrin-film modified electrodes,275,3 9-311 which have potential application in the electrochemical sensing of pollutants. 

When an alkyl halide is treated with phenylmagnesium bromide in the presence of cobaltous chloride, the products obtained are partly those to be expected from unrearranged alkyl radicals and partly from rearranged ones.80... 

A second interfacial exchange reaction of the o-acylcobalt complex with hydroxide ion leads to the production of the alkanecarboxylate anion, which migrates into the aqueous phase, leaving the cobalt tetracarbonyl anion in the organic phase for subsequent reaction (Scheme 8.2). Optimum yields of the carboxylic acids are obtained with ca. 40 1 ratio of the alkyl halide to dicobalt octacarbonyl. Co(Ph,P)2Cl2 can also be used and has the advantage that the cobalt can be recycled easily [5]. 

Selected examples of the conversion of alkyl halides into ethyl esters using cobalt carbonyl complexes... 

Vitamin B12 reacts with alkyl halides to form a cobalt (III) alkyl intermediate. Irradiation with visible light leads to the expulsion of a carbon-centered radical and a cobalt (II) species. The latter is easily reduced at —0.8 V to reconvert it to a cobalt (I) intermediate that reenters the catalytic cycle by reacting with a second molecule of the halide. The radical is capable of undergoing a number of interesting transformations, including conjugate addition to a Michael acceptor. The example illustrated in Scheme 9 provided a straightforward route to ester... 

Typically, the reactions are carried out using a controlled potential, with the potential set to a value that allows the selective reduction of Co(III) in the presence of the alkyl halide and Michael acceptor [26]. Only 1-20 mol % of the cobalt mediator is needed. The reactions are conducted in a medium in which lithium perchlorate... 

Scheme 11 Carbanion formation from alkyl halides and cobalt (III) species.

Vitamin B12 derivatives and their model compounds have recently been used as recyclable electrocatalysts for the reduction of alkyl halides since low-valent Co species are good nucleophiles toward organic substrates [367-369]. Examples of such elec-trocatalysts are the vitamin B12 derivatives aquocobalamin (230), dibromo[l-hydr-oxy-2,2,3,3,7,7,8,8,12,12,13,13,17,17,18,18-hexadecamethyl-10,20-diazaoctahydropor-phinato]cobalt(III) (231), and cobaloxim (232). The above Co(I) complexes can be... 

A novel Michael-type addition of alkyl halides to a,/3-enones (239) has been realized using either aquocobalamin (232) or dibromo[l-hydroxy-8H-HDP]cobalt(111)... 

Carbonvlation of Benzyl Halides. Several organometallic reactions involving anionic species in an aqueous-organic two-phase reaction system have been effectively promoted by phase transfer catalysts(34). These include reactions of cobalt and iron complexes. A favorite model reaction is the carbonylation of benzyl halides using the cobalt tetracarbonyl anion catalyst. Numerous examples have appeared in the literature(35) on the preparation of phenylacetic acid using aqueous sodium hydroxide as the base and trialkylammonium salts (Equation 1). These reactions occur at low pressures of carbon monoxide and mild reaction temperatures. Early work on the carbonylation of alkyl halides required the use of sodium amalgam to generate the cobalt tetracarbonyl anion from the cobalt dimer(36). 

Acrylonitrile, polymerization, 120 Activity of phase-transfer catalysts Sjj2 reactions, 170-175 weak-nucleophile Sj.Ar reactions, 175-182 Acyltetracarbonyl cobalt compound, cleavage in the carboxyalkylation of alkyl halides, 150 Addition reactions, Michael, catalytic asymmetric, 69,70f... 

Quite similar results have been found recently in the reaction of the cobalt(i) form of vitamin B,2 (Bus) with alkyl halides with n-butyl iodide, bromide and chloride, ethyl bromide and benzyl chloride the representative data point of vitamin B s falls several orders of magnitude above the outer sphere dissociative electron-transfer line (Walder, 1989). 

The chemistry takes place via an initial reduction of vitamin B12 or a similar cobalt (III) species 275, in a process that sees the conversion of cobalt from the +3 to the -1-1 oxidation state, and the opening of two sites of unsaturation, to afford 276 [74], This very reactive, highly nucleophilic intermediate reacts rapidly with the alkyl halide to form the octahedral complex 277, and reestablish... 

Control experiments establish that the initial process converting the dibromide 275 to 276 takes place in two steps with E1/2 for extrusion of the first and second axial ligands occurring at —0.36 and —1.08 V, respectively [75]. After reaction with the alkyl halide, the resulting octahedral complex 277 is further reduced in the range of -1.4 to -1.7 V to form a cobalt (II) complex which decays via the addition of an additional electron, cleavage of the C-Co and Co-Y bonds, and reaction with the Michael acceptor. 

As a last point, it should be mentioned that cobalt(II) salen (26) and cobalt(II) salophen (27) are known to interact with molecular oxygen in solution. Dioxygen forms a peroxo bridge between two cobalt centers, resulting in an often-undesired dicobalt(III) species that can attenuate the efficiency of electrogenerated cobalt(I) salen or cobalt(I) salophen for other reactions of interest, for example, the catalytic reduction of alkyl halides discussed in the following text. 

Other pubKcations dealing with the catalytic ability of electrogenerated cobalt(I) species have appeared. Cob(I)alamin reacts with 1,4-dibromobutane to yield a tet-ramethylene-1,4-di-Co -cobalamin complex [138]. Alkyl radicals (which arise from the oxidative addition of cobalt(I) tetraphenyl porphyrin to an alkyl halide) have been found to migrate from the cobalt center to a nitrogen of a pyrrole ring [139]. 

Incidentally, I should say that the thermodynamic data on cobalt(III) indicates that it is a hard acid, but just barely so, and one might say borderline. It is a little harder than tetrahedral carbon and alkyl halides. So hydroxide ion in that sense would be expected perhaps to be a good nucleophilic reagent, as hard acids would like hydroxide ion. 

The cocatalytic activity of alkyl halides in the cationic polymerization of styrene in the presence of stannic chloride (17), in the polymerization of butadiene with Et2AlCl-cobalt compound (10) and R3Al-cobalt compound (23) catalyst systems and in the cationic polymerization of isobutylene (12) and styrene (13) in the presence of Et2AlCl is well documented. It is reasonable to propose that a reaction between Et2AlCl and a labile chlorine atom on PVC results in the generation of a carbonium ion on the polymer backbone. 

Transition metal-catalyzed cross-coupling reactions between vinyl organometallic compounds and unactivated alkyl halides that can be usually performed with palladium, nickel and cobalt are of particular synthetic interest [37-39]. Recently, the groups of Cahiez [48] and Cossy [49] concurrently reported the first iron-catalyzed reaction of alkenyl Grignard compounds with primary and secondary alkyl halides (X=Br, I) (Scheme 5.15). The two protocols basically differ in the iron source... 

Heck demonstrated Eq. (89) for a number of acylcobalt carbonyls, preparing them from the corresponding alkyl halide and sodium cobalt carbonylate. In the presence of bases, cobalt hydrocarbonyl regenerated cobalt carbonylate ion and a catalytic reaction resulted at atmospheric pressure and at temperatures from 0° to 100° C. Thus the following reaction was reported in 56% yield at 50° C ... 

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