Alkylcobalt complexes

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). 

The carboalkoxylation of saturated aliphatic halides may give mixtures of isomeric products if carried out above about 75°, at least with tetra-carbonylcobalt anion as catalyst. Isomerization occurs because the intermediate alkylcobalt complex isomerizes competitively with the carbonylation at the higher temperatures. The isomerization probably involves stepwise loss of carbon monoxides to the tricarbonylalkylcobalt(I) stage. This complex then may reversibly rearrange by a hydride elimination to a hydride-olefin-71 complex. The hydride may also add back in the reverse direction and produce an isomeric alkyl. Subsequent readdition of carbon monoxides and alcoholysis would produce isomerized ester ... 

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 complexes provide an easy bridge between two-electron ionic chemistry and one-electron radical chemistry. This has made them popular and useful radical precursors ionic reactions provide alkylcobalt complexes which then provide alkyl radicals via C-Co bond homolysis. In the mid 1970s and early 1980s, radical chain Sh2 reactions of allylcobaloximes were studied (eq 1). These reactions were not applied to specific synthetic problems. ... 

The reaction of alkylcobalt complexes (RCoSalphen.H20, R = Me, Et) with C3F7I gives the complex [158, R = H], parameters for which... 

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... 

Products of the photolysis of alkylcobalt complexes depend on the organic radical and whether anaerobic or aerobic conditions are used. The Bjj coenzyme forms the organic radical 1 which, in the absence of Oj, yields cyclic 2 and in the presence of O2 oxidizes to 3a and 3b (Scheme I) ... 

This side reaction, during the preparation of alkylcobalt complexes from Co(I) species, lowers the yield. rr-Complexes are formed between electron deficient olefins and Co(I) species under strongly basic conditions . 

Perhaps the best studied mode I cleavages are the base-catalyzed /8-elimination reactions of 2-substituted alkylcobalt complexes with electron-withdrawing substituents (Eqn. 34). 

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]. 

Figure 9 An alkylcobalt complex 15 that undergoes rapid degradation at the pH characteristic of many cancer cells producing alkyl radicals that cause cell death.

An alternative to this interpretation is based on the formation of a hydridocobalt(III) species, which adds across the double bond, and the alkylcobalt complex formed reacts with 0 to afford a peroxocobalt... 

Giese B, Hartung J, He Jianing H, Hiiter O, Koch A (1989) On the formatitni of free radicals from alkylcobalt complexes. Angew Chem Int Ed Engl 28 325-327... 

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 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. ... 

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