Alkylcobalt

Conjugated dienes can be acylated by treatment with acyl- or alkylcobalt tetracarbonyls, followed by base-catalyzed cleavage of the resulting jt-allyl carbonyl derivatives. The reaction is very general. With unsymmetrical dienes, the acyl... 

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.. 

There are many biomimetic model Co complexes of the cobalamins.1149 The primary criterion for an effective B12 model has been that the complex may be reduced to the monovalent state and undergo facile oxidative addition to generate a stable alkylcobalt(III) complex. The two main classes of B12 model complexes that have been investigated are Co oximes and Schiff base complexes. The former class shares the planar CoN4 array of their biological analogs whereas the majority of effective Schiff base Bi2 model complexes comprise equatorial czj-N202 donor sets. 

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. 

Interception of the reaction sequence at the alkylcobalt carbonyl stage before carbonyl insertion, and hydrogenation of this intermediate, produces an alkane. This undesired side reaction is only minor (1-3%) in cobalt-catalyzed hydroformylation of a nonfunctional olefin, but may become predominant with phenyl- or acyl-substituted olefins. Ethylbenzene has been obtained in >50% yield from styrene (37), and even more alkane was obtained from a-methylstyrene (35). 

Stoichiometric, nucleophilic addition of alkylcobalt compounds to carbonyl compounds were reviewed in 1996.374 This chapter focuses on recent progress in the reactions of cobalt enolates with carbonyls and cobalt-catalyzed coupling reactions. 

The pH value in a solid tumor is typically 0.2-0.5 units lower than in normal tissues. Therefore it may be possible to design pH-depen-dent cell-selective antitumor agents. Some free radicals are known to damage biological targets and especially to cleave nucleic acids (233). Alkylcobalt(III) complexes such as 53 generate alkyl radicals via ho-... 

Rearrangement of Alkylcobalt Carbonyls Takegami et al. [25) reported that when ethyl ot-bromopropionate is treated with KCo(CO)4 in toluene at 0° in the presence of 1 atm of CO and the product cleaved with iodine, both the expected ester, ethyl methyl-malonate, and the rearranged ester, diethyl succinate, are formed, the latter in smaller quantity. At 25°, however, the succinate predominates. 

This scheme is particularly attractive because Heck and Breslow (22a) reacted methyl acrylate with HCo(CO)4 at 0° in pentane in 1 atm of CO and obtained both products, the malonate in 25% yield and the succinate in 6% yield. In view of the coincidence of yield and of distribution of products, one must consider the possibility that a dehydrohalogenation to acrylate occurred prior to the formation of alkylcobalt carbonyls ... 

The alkylcobalt tetracarbonyls react completely analogously with carbon monoxide, forming acyl cobalt tetracarbonyls (43). 

Similarly, alkylcobalt tetracarbonyls react with triphenylphosphine (44, 45) or with phosphites (36) to give high yields of acylcobalt tricarbonyl triphenylphos-phines or phosphites. 

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). 

The reduction potential for the reductive elimination can be lowered to —0.9 V (vy. SCE) if the alkylcobalt(III) complex is photochemically activated by visible light 225-227,228) (pp( photo-Elcctro-Catalysis see Scheme 4). 

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). 

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. 

Considering the results obtained in the reaction of orthoformic esters with CO and H2 (13) and the postulated instability of secondary and tertiary alkylcobalt carbonyls, the suggested role of alkylcobalt carbonyls in the course of this reaction seems questionable. Although a small amount of olefin is detected among the reaction products of sec-butyl-orthoformate with CO and H2, probably because of the low stability of secondary alkylcobalt carbonyls, and an 80 20 ratio is found between 2-methylbutanal and n-pentanal formed (13), it is still to be explained why the hydroformylation of 2-butene under the same conditions involving the same alkylcobalt intermediates gives a 29 71 ratio (4) of the same aldehydes. 

The main reaction product, 2-phenylbutane, shows no detectable optical activity between 590 and 350 m. The 2-phenylbutane does not arise from the alkylcobalt carbonyls as reported (10) since these give rise to the optically active 3-phenylpentanal or 4-phenylpentanal. Taking into account the peculiar behavior under oxo conditions of double bonds conjugated with phenyl rings (14), it might be assumed that electronic effects produce predominately the disubstituted benzylcobalt complex CH3... 

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 ... 

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