Cobalt complexes, double carbonylation

Not unexpectedly, alkylation of the double carbonylated complex proceeds via a base-catalysed interfacial enolization step, but it is significant that the initial double carbonylation step also involves an interfacial reaction, as it has been shown that no pyruvic acid derivatives are obtained at low stirring rates. Further evidence comes from observations of the cobalt-catalysed carbonylation of secondary benzyl halides [8], where the overall reaction is more complex than that indicated by Scheme 8.3. In addition to the expected formation of the phenylacetic and phenylpyruvic acids, the reaction with 1-bromo-l-phenylethane also produces 3-phenylpropionic acid, 2,3-diphenylbutane, ethylbenzene and styrene (Scheme 8.4). The absence of secondary carbonylation of the phenylpropionylcobalt tetracarbonyl complex is consistent with the less favourable enolization of the phenylpropionyl group, compared with the phenylacetyl group. 

With cobalt complex catalysts, in polar, aprotic solvents like DME it is often possible to get a-keto acids by controlled double carbonylation874-877. Alternatively, a-hydroxy acids are formed when benzyl halides are carbonylated in the presence of calcium hydroxide, in aqueous media878. Presumably the initially formed a-keto acid is reduced in the Meerwein-Ponndorf fashion to give the a-hydroxy group878. 

The Pauson-Khand reaction starts with the replacement of two CO molecules, one from each Co atom, with the alkyne to form a double a complex with two C-Co a bonds, again one to each Co atom. One CO molecule is then replaced by the alkene and this n complex in its turn gives a a complex with one C-Co a bond and one new C-C a bond, and a C-Co bond is sacrificed in a ligand coupling reaction. Then a carbonyl insertion follows and reductive elimination gives the product, initially as a cobalt complex. 

Alkylcobalts form in stoichiometric reactions of pentacyanocobalt(III) hydride and alkenes. This reaction occurs both for halogenated alkenes such as tetrafluoroethylene and for alkenes that contain other electron-withdrawing groups such as carbonyls, nitriles and arenes as substituents (see Table 6) . The addition is regiospedfic, forming the more substituted alkylcobalt. Prior coordination of alkene to cobalt to form an alkene(hydrido)cobalt complex, an intermediate in hydrometalation reactions, is not important. This reaction is a radical process however, by NMR, additions of [HCo(CN)5 ] " to diastereomeric alkenes such as fumaric and maleic add salts lead to a cr-alkylcobalt by stereospecific cis addition of Co and H to the double bond . The overall reduction is not stereospecific. (r-Alkylcobalt bond formation proceeds by either a concerted addition or a rapid collapse of a radical cage. 

The proposed mechanism, which is based on the double carbonylation of styrene oxide, is shown in Scheme 6.2. The generation of an acylcobalt carbonyl complex from the reaction of cobalt tetracarbonyl anion with an alkyl halide is followed by reaction with a thiirane. This species can undergo carbonylation, the thioester function can undergo hydrolysis to reveal a sulfido nucleophile, and intramolecular cydization then produces thietan-2-one. The thietan-2-one can undergo ring cleavage and the mercapto acid results by protonahon. 

As we mentioned in Sect. A, the first catalytic double carbonylation reaction was found to take place by using cobalt carbonyl complex as catalyst. In fact, there are a few other transition metals that can promote the double carbonylation process in addition to palladium complexes. The following is a brief description of the double carbonylation process promoted by transition metals other than palladium. 

H.i. Double Carbonylation Reactions Catalyzed by Cobalt Complexes... 

The double carbonylation process was first found in the conversion of substituted benzyl halides to corresponding arylpyruvic acids catalyzed by cobalt carbonyl complexes in a phase transfer system (Eq. 27). ... 

In their report of crotylation reactions with cobalt carbonyl complexed aldehyde not only the enantioselectivity is improved, but it is also reversed. As can be seen in Scheme 3.47 the reaction of aldehyde 223 with the crotylation reagent 224 results in moderate enantioselectivities. If in contrast, cobalt complex 227 of the parent aldehyde 223 was used the opposite enantiomer was obtained, and in the latter case an improved selectivity was achieved. The double bond introduced via this crotylation is required for the intramolecular Pauson-Khand reaction that Roush and coworkers carried out [75]. 

C-19 dicarboxyhc acid can be made from oleic acid or derivatives and carbon monoxide by hydroformylation, hydrocarboxylation, or carbonylation. In hydroformylation, ie, the Oxo reaction or Roelen reaction, the catalyst is usually cobalt carbonyl or a rhodium complex (see Oxo process). When using a cobalt catalyst a mixture of isomeric C-19 compounds results due to isomerization of the double bond prior to carbon monoxide addition (80). 

The carbon dioxide anion-radical was used for one-electron reductions of nitrobenzene diazo-nium cations, nitrobenzene itself, quinones, aliphatic nitro compounds, acetaldehyde, acetone and other carbonyl compounds, maleimide, riboflavin, and certain dyes (Morkovnik and Okhlobystin 1979). The double bonds in maleate and fumarate are reduced by CO2. The reduced products, on being protonated, give rise to succinate (Schutz and Meyerstein 2006). The carbon dioxide anion-radical reduces organic complexes of Co and Ru into appropriate complexes of the metals(II) (Morkovnik and Okhlobystin 1979). In particular, after the electron transfer from this anion radical to the pentammino-p-nitrobenzoato-cobalt(III) complex, the Co(III) complex with thep-nitrophenyl anion-radical fragment is initially formed. The intermediate complex transforms into the final Co(II) complex with the p-nitrobenzoate ligand. 

Even though the X-ray data suggest the presence of two a bonds and one 7T bond between metal and olefin as was postulated for the butadiene-iron carbonyls, the UV spectra of the triphenyltropone complexes closely resemble that of the free olefin (80) suggesting that the bonding of the complex may indeed be intermediate between that of structure (96) and simple rr bonding to two olefinic double bonds (80). This concept is more fully discussed in the section on cobalt (Section VII, A). 

Acrylonitrile and related compounds displace all the carbonyl groups from nickel carbonyl to form [(RCH CHCN)2Ni], in which the nitrile bonds through the olefinic double bond 222, 418). The bis(acrylonitrile) complex catalyzes many reactions, including the conversion of acrylonitrile and acetylene to heptatrienenitrile and the polymerization of acetylene to cyclooctatetraene 418). Cobalt carbonyl gave a brown-red amorphous material with acrylonitrile, which had i cn absorptions typical of uncoordinated nitrile groups, but interestingly, the presence of C=N groups was also indicated 419). In acidic methanol, cobalt carbonyl converts a,j8-unsaturated nitriles to saturated aldehydes 459). 

The third compound was made by a Pauson-Khand reaction using the same starting material = the first. The only difference between these two target molecules is the position of the double bor. In the Nazarov reaction, it goes into the thermodynamically most favourable position but in 1-Pauson-Khand reaction it goes where the alkyne was. So we simply react the cyclic ether wci acetylene cobalt carbonyl complex. The cis stereochemistry is inevitable. 

The resulting heterocycles in the complex may be further reduced or desilylated (either in the complex or after demetallation). Further synthetic potential exists in the use of the primary products, obtained by cobalt-mediated cycloadditions, as synthons in organic chemistry. For example, indole derivatives have been co-cyclized at the j/ -Cp-cobalt catalyst to give 4a,9a-dihydro-9//-carbazoles or, after oxidation, precursors for strychnine [50]. Remarkably, the cycloaddition of acrolein in the presence of a small amount of methyl acetate occurs at the carbonyl, rather than at the C=C double bond, to give vinylpyran selectively (eq. (19)) [48]. 

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