Acylcobalt bond

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 Co reagent 192, prepared by the reaction of Co2(CO)8 with sodium, is reactive, and the acylcobalt complex 193 is formed by the reaction of acyl halides. Insertion of butadiene at the Co-acyl bond generates the 7r-allylcobalt complex 194, from which the acylbutadiene 195 is formed by deprotonation with a base [82]. Based on this reaction, various acyldienes are prepared by Co2(CO)8-catalysed reaction of active alkyl halides, conjugated dienes and CO. The Co-catalysed reaction can be carried out smoothly under phase-transfer conditions. For example, 6-phenyl-3,5-hexadien-2-one (197) was prepared in 86% yield by the reaction of Mel, 1-phenylbutadiene (196) and CO in the presence of cetyltrimethylammonium bromide [83]. 

In addition to alkyl, aryl and vinylcobalt/radical species, acid chlorides may be converted into acylcobalt species by reaction with the cobalt(I) salens. These generate acyl radicals under photolysis, which participate in similar reactions to the alkyl radicals in most cases (equations 184 and 185). Acylcobalt(III) species bearing an a-aryl or vinyl substituent, on the other hand, undergo concomitant decarbonylation to afford a benzylic or allyl radical, which then may undergo a number of bond-forming processes, including homocoupling361. 

When the acylcobalt species is derived from a compound containing halogen on an activated carbon (e.g. an a-halo ester or nitrile) an elimination may occur to introduce an exocyclic double bond in the final product. This sequence, leading to lactones of pentadienoic acids, is general for both terminal and internal alkynes in the presence of amine bases (equation 17). ... 

The mechanism of this reaction was investigated to find out if carbon monoxide dissociation is the rate-determining step (16). The rate of the reaction of acetylcobalt tetracarbonyl with iodine is too fast to measure under conditions which allow dissociation rate to be measured easily. Thus, dissociation is not rate-determining, and the acylcobalt tetracarbonyl and the iodine must be reacting directly. Further studies with the less reactive acyl(triphenylphosphine)cobalt tricarbonyls showed that the first step in the reaction with iodine is a rapid cleavage of the cobalt-carbon bond to form acyl iodide and iodo(triphenylphosphine)cobalt tricarbonyl. 

The addition of acylcobalt carbonyls to conjugated dienes is a very general reaction (14, 15). Some information on the reactivity of differently substituted double bonds in the diene system can be obtained from the structures of the l-acylmethyl-7r-allylcobalt tricarbonyls formed from them. The acyl group is reasonably assumed to add to the least-hindered and most reactive double bond. Isoprene reacts with methylcobalt tetracarbonyl to form l-acetylmethyl-2-methyl-7r-allylcobalt tricarbonyl. 

Insertion reactions of alkylcobalt or acylcobalt tetracarbonyls with saturated aldehydes or ketones have not been observed. Carbonyl insertions do occur in some unsaturated carbonyl systems, however. The cyclization of the intermediate acylacrylylcobalt tricarbonyls, formed from acetylenes and alkylcobalt or acylcobalt tetracarbonyls, to butenolactone derivatives, as described above, is one example of the reaction. Another example is the addition of alkylcobalt or acylcobalt tetracarbonyls to a, -unsaturated aldehydes or ketones. In this reaction an acyl group from the cobalt compound is added to a carbonyl oxygen and the cobalt carbonyl group forms a iT-allyl system with the carbonyl carbon and the double bond 19). 

Water and alcohols cleave some acyl-metal bonds to give carboxylic acid derivatives and a metal hydride, another example of an inverse cleavage. This reaction has been studied with acylcobalt carbonyl derivatives such as... 

The insertion of an olefin into a C—M bond is a critical step in some olefin dimerization and polymerization reactions (Section IV,D), but studies of this reaction have not been very fruitful. The insertion of isobutylene into the C—Ti bond of CH3TiCl3 to give a neopentyl derivative is one of the few straightforward examples of this reaction (133). Olefin, diene, and acetylene insertion in acylcobalt compounds have been reported by Heck in an elegant series of papers sununarized in his review of this area (3). These insertions are often quite complex, as illustrated by the butadiene insertion. 

Besides nickel and cobalt, almost all of the catalysts discussed in the last chapter which were suited for the formation of free acids can be applied, e. g. rhodium, palladium and, with certain restrictions, iron. Cobalt hydrocarbonyl catalyzes the stoichiometric ester synthesis at mild reaction conditions [35, 121]. The initially formed acylcobalt carbonyls react rapidly with alcohols even at 50 °C and, in the presence of Na-alcoholate, even at 0 °C to give esters [121]. Dienes with isolated double bonds react with carbon monoxide and alcohols at mild reaction conditions in the presence of Pd/HCl to give unsaturated monocarboxylic acid esters and at more severe conditions to give saturated dicarboxylic acid esters [508]. 

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