Cobalt, alkylcobalt complexes

More recently Schrauzer, Weber, and Beckham (159) showed the existence of equilibria involving the loss of a proton from the (r-alkylcobalt(III) complex to give a Tr-olefin-cobalt(I) complex, i.e.. 

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. 

The n complex 1 has not been isolated or observed directly, but its involvement is strongly supported by indirect evidence. In the second step the alkene inserts into the cobalt-hydrogen bond to yield an alkylcobalt complex (2), which is transformed via the migratory insertion of CO into a coordinatively unsaturated acylcobalt complex (3). 

When simple terminal alkenes are used as acceptors, the cyclic primary alkyl cobalt species are stable, and can often be isolated and purified by standard techniques.145 Scheme 32 shows some of the transformations that Pattenden has accomplished with the cyclic alkylcobalt complex (55).146 In addition to standard elimination to an alkene, the complexes can be converted to alcohols, halides, oximes, and phe-... 

Alkylcobalt(III) complexes can also be synthesized in aqueous solution. Two of the best-known systems are methylcobalamin and a group of related cobaloximes, and alkylcobalt(III) complexes having ancillary cyanide ligands. As with the chromium(III) system, alkyl cobalt(III) complexes having dimethylglyoxime (DMG) or cyanide ligands can be synthesized by reaction of the cobalt(II) precursor with alkyl halides (Scheme... 

Reduction of Co2(CO)g with sodium-amalgam in THF solution gives Na[Co(CO)4]. Na[Co(CO)4] has high nucleophilicity, and the reaction with alkyl halides gives alkyl-cobalt complexes. It is generally difficult to isolate the alkylcobalt complexes, but in the presence of alkenes and alkynes, acylated compounds are obtained in good yields (eq (10)) [13]. 

Whereas homolytic ligand dissociation is not commonly observed for inorganic complexes, it has been identified as an important process in organometallic chemistry where it is favored by the characteristic weakness of transition metal-alkyl tr-bonds. Recent determinations yield metal alkyl band dissociation energies for CH3— Mn(CO)s (ca. 120 kJ/mol) and for several alkylcobalt complexes (ca. 80-100 kJ/mol) . Homolytic dissociation of such complexes results in the formation of free radicals and in the opening up of free radical catalytic pathways, e.g., for hydrogenation". Important biochemical examples of free radical catalytic mechanisms, initiated by the homolytic dissociation of a transition metal-carbon bond (i.e., the 5 -deoxyadenosy 1-cobalt bond of coenzyme 8,2) are provided by the coenzyme B,2-promoted rearrangements (see Section... 

Simple alkylcobalt complexes (i.e. those without heteroatomic substituents on the alkyl ligand) are also known to undergo eliminations of cobalt(I) species. Thus Grate and Schrauzer [42] have studied the decomposition of unstable, sterically strained secondary alkyl- and cycloalkyl-cobalamins to form olefins and hydridocobalamin in neutral and acidic solution. In this case the formation of hydridocobalamin was inferred from the observation of monodeuteriohydrogen gas when undeuterated alkylcobalamins were decomposed in DCl/DjO, presumably via Eqns. 42 and 43. 

Finally it should be pointed out that a number of studies have been published purporting to show the nucleophilic displacement of cobalt(I) chelates from alkylcobalt complexes by thiolate anions in base (Eqn. 44) [77,78]. 

Acetylene and monosubstituted acetylenes appear to give some of the 7T-(penteno-4-lactonyl)cobalt tricarbonyl complexes on reaction with alkylcobalt or acylcobalt tetracarbonyls also but other products are formed too. These other products have not been characterized but are thought to be linear, low molecular weight polymers of the acetylene or of the acetylene and carbon monoxide with an acyl group at one end of the polymer chain and a cobalt carbonyl group at the other. The formation of cyclic products from the more-substituted compounds and cyclic and linear ones from the less-substituted compounds is explainable because substitution is known to improve many cyclization reactions. 

Alkylcobalt(III) compounds have been reduced to alkanes and cobalt(II) complexes by NaBH4, by catalytic hydrogenation, and by electrolysis. A polarographic wave at — 1.7 V (vs. Ag/AgN03 in CH3CN) was assigned to C—Co bond cleavage 23). 

The trapping of these cations with carbon nucleophiles has made them useful for synthesis. The carbon nucleophiles must be stable to the mildly acidic conditions used to generate the cations. Examples are trisubstituted alkenes, in an intramolecular fashion, allyl silanes (Scheme 6.130) and pyrrole. The ti -alkylcobalt complex 6.354 produced may be converted to an alcohol 6.355 by free radical methods as the carbon-cobalt bond undergoes photochemical homolysis. 

The stability constants for the coordination of pyridine or substituted pyridines to various alkylcobalt porphyrin systems have been reported. This study has been done in order to observe the m-influence of hydroporphyrin macrocycles on axial position and alkyl exchange reactions. The alkyl exchange reactions of organocobalt(lll) porphyrins with a cobalt(ll) complex of a distinguishable porphyrin or tetrapyrrolc have been studied. The equilibrium constants for the alkyl transfer have been reported. The exchange of the axial ligand is reversible and it follows a bimolecular mechanism. Thermodynamic and activation parameters for homolytic Co-C bond dissociation have been obtained on the (TAP)CoC(CH3)2CN complex (TAP = tetraanisylporphirinato). ... 

The electroreduction of organocobalt complexes generally leads to cleavage of the cobalt-carbon bond - and this also seems to be the case for alkylcobalt complexes of (TPP)Co(R) i. In contrast, the cobalt-carbon bond cleavage for (TPP)Co(R) species with a-bonded aryl groups is slower and reversible half-wave potentials have been reported. ... 

Cobalt hydrocarbonyl reacts rapidly with conjugated dienes, initially forming 2-butenylcobalt tetracarbonyl derivatives. These compounds lose carbon monoxide at 0°C. or above, forming derivatives of the relatively stable l-methyl-ir-allyl-cobalt tricarbonyl. As with normal alkylcobalt tetracarbonyls, the 2-butenyl derivatives will absorb carbon monoxide, forming the acyl compounds but these acyl compounds also slowly lose carbon monoxide at 0°C. or above, forming 7r-allyl complexes. The acyl compounds can be isolated as the monotriphenylphosphine derivatives (47). 

Alkylcobalt carbonyl isomerization via the formation of olefincobalt-carbonyl hydride complexes with retention of the olefin attached to the cobalt atom has been suggested as the mechanism of formation, of the necessary precursors of the products with high stereospecificity. 

The cyclization reactions of organocobalt complexes are very useful, and they offer an excellent alternative to the tin hydride method when reduced products are not desired. Most cobalt cyclizations have been conducted with nucleophilic radicals. Precursors are prepared by alkylation of cobalt(I) anions, and are usually (but not always) isolated. One suspects that alkylcobalt precursors should be useful for slow cyclizations because there are no rapid competing reactions that would consume the initial radical (coupling of the initial radical with cobalt(II) regenerates the starting complex). 

The conditions under which cobalt hydrocarbonyl was reacted with olefin were also found to affect the distribution of products and the extent of isomerization of excess olefin (62, 73, 147). At low temperatures (0° C) under carbon monoxide (1 atm) very little isomerization of excess 1-pentene occurred and the main product was the terminal aldehyde. Under nitrogen or under carbon monoxide at 25° C, extensive olefin isomerization occurred and the branched aldehyde was mainly produced. The olefin isomerization is most satisfactorily accounted for by an equilibrium between alkylcobalt and olefin-hydride cobalt complexes [Eqs. (9) and (10)]. The carbon monoxide inhibition is most easily explained if the isomerization proceeds via the tricarbonyls rather than tetracarbonyls. This also explains why ethylcobalt tetracarbonyl is not in equilibrium with hydrocarbonyl and ethylene under conditions where the isomerization is rapid (62, 73). 

It was suggested that such derivatives would have unusual stability like the corresponding 7r-allyl complexes and would be further reduced by hydrocarbonyl rather than undergoing carbonylation. Heck (63) has, however, attempted the preparation of similar derivatives from chloroacetone and phenacyl bromide with sodium cobalt tetracarbonylate, Instead of finding unusually stable complexes he reported an unusual instability. It seems likely that in fact normal alkylcobalt carbonyls are formed, e.g.,... 

Only two reports deal with the reactions of cobalt complexes with HFA. Insertion into the cobalt-hydrogen bond of a hydride complex affords a cobalt hexafluoroisopropylate 136a). An oxolene(2) is formed from an alkylcobalt compound and HFA 66). For mechanistic reasons the authors favor the depicted structure 157 over the isomeric oxolene (3) ring reported for the analogous iron complex 149 174). 

Their view that cobalt hydrotricarbonyl, instead of die hydrotetra-carbonyl, is the reactive species is based on evidence that the formation of alkylcobalt tetracarbonyl is inhibited by carbon monoxide more fundamentally, initial complexing with olefin would presumably require the participation of a coordinately unsaturated carbonyl. 

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