Heck reaction cobalt

Fig. 54 Cobalt-catalyzed Heck reactions of alkyl halides and styrenes...

Some of the most exciting reactions in organic chemistry are based on transition metals. How about these two for example The first is the Heck reaction, which allows nucleophilic addition to an unactivated alkene. Catalytic palladium (Pd) is needed to make the reaction go. The second, the Pauson-Khand reaction, is a special method of making five-membered rings from three components an alkene, an alkyne, and carbon monoxide (CO). It requires cobalt (Co). Neither of these reactions is possible without the metal. 

An organometallic reaction which shows a close similarity to the Mizoroki-Heck reaction is the cobalt-catalysed [46] reaction between alkenes and organic halides. The use of [CoCKPPhsls] (59) as catalyst for intermolecular arylations of methyl acrylate (1) and styrene (2) was reported by Iyer [47] (Scheme 10.20). The para-substituted aryl iodides could be employed with this homogeneous catalyst, but the more sterically hindered ortho-substituted iodoarenes failed to undergo the desired substitution reaction. Aryl bromides and chlorides, as well as alkyl-substituted halides, proved unreactive under these reaction conditions. 

Affo, W., Ohmiya, H., Fujioka, T. et al. (2006) Cobalt-catalyzed trimethylsilylmethylmagnesium-promoted radical alkenylation of alkyl halides a complement to the Heck reaction. J. Am. Chem. Soc., 128, 8068-77. 

As a Generator of Alkyl Radicals from Alkyl Halides in the Presence of Cobalt Catalysts. A conceptually novel use of Me3SiCH2MgCl recently emerged in cobalt-catalyzed Heck-type transformations. The palladium-catalyzed Heck reaction always etT5)loys aryl- or alkenyl halides. In eontrast, alkyl halides are not available for use as starting material, due to predominant 0-hydride elimination from the corresponding alkylpalladium... 

Other metals can catalyze Heck-type reactions, although none thus far match the versatility of palladium. Copper salts have been shown to mediate the arylation of olefins, however this reaction most probably differs from the Heck mechanistically. Likewise, complexes of platinum(II), cobalt(I), rhodium(I) and iridium(I) have all been employed in analogous arylation chemistry, although often with disappointing results. Perhaps the most useful alternative is the application of nickel catalysis. Unfortunately, due to the persistence of the nickel(II) hydride complex in the catalytic cycle, the employment of a stoichiometric reductant, such as zinc dust is necessary, however the nickel-catalyzed Heck reaction does offer one distinct advantage. Unlike its palladium counterpart, it is possible to use aliphatic halides. For example, cyclohexyl bromide (108) was coupled to styrene to yield product 110. 

Mechanism ofLP Oxo Rea.ction. The LP Oxo reaction proceeds through a number of rhodium complex equilibria analogous to those ia the Heck-Breslow mechanism described for the ligand-free cobalt process (see Fig. 1). 

Heck has studied the reaction of triphenylphospbine22-24 and trimethylol-propane phosphite25 with the substituted cobalt carbonyls listed in Tables 1-4. The general mechanism for the reaction of the acyl cobalt carbonyls shown in Table 1 in the presence of triphenylphosphine is... 

Figure 2 shows the generally accepted dissociative mechanism for rhodium hydroformylation as proposed by Wilkinson [2], a modification of Heck and Breslow s reaction mechanism for the cobalt-catalyzed reaction [3]. With this mechanism, the selectivity for the linear or branched product is determined in the alkene-insertion step, provided that this is irreversible. Therefore, the alkene complex can lead either to linear or to branched Rh-alkyl complexes, which, in the subsequent catalytic steps, generate linear and branched aldehydes, respectively. 

The first catalyst used in hydroformylation was cobalt. Under hydroformylation conditions at high pressure of carbon monoxide and hydrogen, a hydrido-cobalt-tetracarbonyl complex (HCo(CO)4) is formed from precursors like cobalt acetate (Fig. 4). This complex is commonly accepted as the catalytic active species in the cobalt-catalyzed hydroformylation entering the reaction cycle according to Heck and Breslow (1960) (Fig. 5) [20-23]. 

One final interesting isomerization achieved in the cobalt carbonyl system should be mentioned. Heck and Breslow (22b) found that acylcobalt tetracarbonyl compounds undergo alcoholysis with the formation of HCo(CO)4. With methanol, the reaction proceeds at 50° ... 

In 1961 Heck and Breslow presented a multistep reaction pathway to interpret basic observations in the cobalt-catalyzed hydroformylation.28 Later modifications and refinements aimed at including alternative routes and interpreting side reactions.6 Although not all the fine details of hydroformylation are equally well understood, the Heck-Breslow mechanism is still the generally accepted basic mechanism of hydroformylation.6,17,19,29 Whereas differences in mechanisms using different metal catalysts do exist,30 all basic steps are essentially the same in the phosphine-modified cobalt- and rhodium-catalyzed transformations as well. 

The present discussion is limited to the rhodium catalyzed hydroformylation. In the widely accepted mechanism proposed by Wilkinson et ah (36), on the basis of suggestions by Breslow and Heck (37) for the cobalt catalyzed hydroformylation, the reaction steps, with the exception of the hydrogenolysis of the acyl-rhodium complexes, correspond to equilibria (Scheme 10). 

For 1-pentene, Karapinka and Orchin (73) found that Eq. (2) was strongly inhibited by carbon monoxide at 0° C. This was confirmed by Heck and Breslow (62) who noted that the inhibition also retarded the formation of alkyl- and acylcobalt carbonyls as well as aldehydes. Thus, about 30% less alkyl- and acylcobalt carbonyls were formed in 15 minutes under 1 atm of carbon monoxide than under 1 atm of nitrogen. These results should not, of course, be taken as implying that nitrogen promotes the reaction. Takegami et al. (147) have noted that under nitrogen a side reaction consumes cobalt... 

Some light has been thrown on this unusual reaction by a study of the reaction of cobalt hydrocarbonyl with olefins under nitrogen (14). It has also be discussed recently by Heck (59). 

An unusual synthesis of acyldienes from conjugated dienes, carbon monoxide, and alkyl or acyl halides using cobalt carbonylate anion as a catalyst should be mentioned here (57). The reaction apparently involves the addition of an acylcobalt carbonyl to a conjugated diene to produce a l-acylmethyl-7r-allylcobalt tricarbonyl, followed by elimination of cobalt hydrocarbonyl in the presence of base. The reaction can thus be made catalytic. Since the reaction was discussed in detail in the recent review by Heck (59), it will not be pursued further here. 

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