Heck and Breslow

A typical example of this is the dicobalt octacarbonyl catalyzed hydroformylation of olefins to yield aldehydes. According to the classical mechanism proposed by Heck and Breslow /29/ (Equations 28-31), the cobalt carbonyl reacts with hydrogen to form hydrido cobalt tetracarbonyl, which is in equilibrium with the coordinatively unsaturated HCo(C0)2. The tricarbonyl coordinates the olefin, and rearranges to form the alkyl cobalt carbonyl. 

The Heck and Breslow mechanism was widely accepted until evidence was found that Co(CO), radicals may be involved in the hydrofor-mylation /30, 31/ (Equations 32-34). 

Another important line of investigation concerned the carbonyl insertion reaction, which was best defined in manganese chemistry (75, 16) and extended to acylcobalt tetracarbonyls by Heck and Breslow. The insertion may be through three-membered ring formation or by nucleophilic attack of an alkyl group on a coordinated CO group. 

The mechanism offered by Heck and Breslow (17, 18) has been the one most accepted as representing the probable reaction course. This is outlined in Eqs. (7)—(11) ... 

For the phosphine-substituted cobalt carbonyl hydroformylations, it is probable that the mechanism follows the pathway of Heck and Breslow (77, 18), although the possibility of an associative mechanism has been raised (7). The increased stability of the HCo(CO)3PR3 complexes toward loss of CO was cited as being suggestive of a nondissociative pathway. 

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]. 

The stoichiometry of Eq. (2) requires the absorption of 1 mole of CO per 2 moles of HCo(CO)4. However, Heck and Breslow (17) showed that, when olefin is used as the solvent, the absorption of CO approaches 1 mole per mole of HCo(CO)4, and they further showed that the 1 1 1 HCo(CO)4 CO olefin complex suggested as a possible intermediate by Kirch and Orchin (16) was in fact an isolable intermediate, namely, an acylcobalt tetracarbonyl, RCOCo(CO)4. Accordingly, the formation of aldehyde and dicobalt octacarbonyl proceeds ... 

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 ... 

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° ... 

The essential features of the alkene hydroformylation mechanism proposed by Heck and Breslow [61] remain intact, after many investigations using a variety of techniques. The cycle shown in Scheme 3.3 is that for the unmodified, cobalt... 

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 most widely accepted mechanism for the catulytic cycle is the following one proposed by Heck and Breslow 7 ... 

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... 

Heck and Breslow preferred to postulate this occurring in two stages involving an initial SN1 reaction... 

Heck and Breslow (62) have postulated equilibria between acylcobalt carbonyls and olefin-hydride complexes. 

Heck and Breslow (60) demonstrated the feasibility of Eq. (22) by the successful reduction of CH3COCo(CO)4 with hydrogen at 3000 psi and... 

In support of the existence of an acylcobalt tricarbonyl, Heck and Breslow cited the appearance of an infrared band at 5.8 p, similar to that occurring in acylcobalt tetracarbonyls when alkylcobalt tetracarbonyls are examined in solution. They postulated the equilibrium, Eq. (20). There is now some doubt of the value of this evidence since the 5.8 /x band is due in part at least to the acylcobalt tetracarbonyl formed by some kind of disproportionation, or decomposition during the preparation (53). However, evidence for Eq. (23) has since been found in a study of the reaction of acylcobalt tetracarbonyls with triphenylphosphine, where a first-order dissociation was indicated (52). 

Piacenti et al. suggested that the different results at low and high carbon monoxide pressure were due to different catalytic intermediates (A and B) under the two sets of conditions. Thus at low pressures A caused a rapid olefin isomerization and the formation of similar product distributions of aldehydes from 1- and 2-pentene. At high pressures little olefin isomerization occurred and 1-olefin yielded significantly more straight-chain aldehyde than 2-olefin. This would seem consistent with Heck and Breslow s mechanism (62) if A were an acylcobalt tricarbonyl in equilibrium with isomeric olefin-cobalt hydrocarbonyl complexes and B were an acylcobalt tetracarbonyl. 

Heck and Breslow (63) obtained evidence relating to Eq. (52) for the case of cobalt. They reacted a number of alkyl and acyl halides carrying functional groups with sodium cobalt tetracarbonylate under carbon monoxide. Normal acylcobalt carbonyls were readily formed except for the case of chloroacetonitrile, where a cyanomethylcobalt carbonyl was isolated. Apparently an a-nitrile group is sufficiently electronegative to stabilize the alkylcobalt carbonyl against carbonylation. 

The reaction sequence as proposed by Heck and Breslow [19] is shown in Scheme 6.1. 

An alternate bimetallic pathway was also suggested, but not favored, by Heck and Breslow (also shown in Scheme 1). The acyl intermediate could react with HCo(CO)4 to undergo intermolecular hydride transfer, followed by reductive elimination of aldehyde to produce the Co-Co bonded dimer Co2(CO)s. A common starting material for HCo(CO)4-catalyzed hydroformylation, Co2(CO)g is well-known to react with H2 under catalysis reaction conditions to form two equivalents of HCo(CO)4. The bimetallic hydride transfer mechanism is operational for stoichiometric hydroformylation with HCo(CO)4 and has been proposed to be a possibility for slower catalytic hydroformylation reactions with internal alkenes.The monometallic pathway involving reaction of the acyl intermediate with H2, however, has been... 

Heck and Breslow considered that this first stage involves at least three distinct steps, as follows ... 

When 5,6-anhydro-l,2-0-isopropylidene-a-I>-glucofuianose (67) in ether was allowed to react with carbon monoxide (12 atmospheres, at room temperature) in the presence of sodium cobalt tetracarbonyl for 3 days, the stoichiometric amount of carbon monoxide was absorbed. The mixture was cooled to —5 , and subsequent treatment with methanol and iodine by the procedure of Heck and Breslow resulted in the formation (in 80 % yield) of the methyl uronate (70) and, in a yield of about 10%, the 6-deoxy-hexos-5-ulose (69). Reduction of the methyl uronate (70) and of the dialdose derivative (68) with lithium aluminum hydride yielded identical sugars. 

In the early 1960s Heck and Breslow formulated the generally accepted hydroformylation cycle depicted in Scheme 3 [89]. Originally formulated for cobalt catalysts, the mechanism is valid for unmodified rhodium complexes as well. The elemental steps (Scheme 3) are ... 

Heck and Breslow found that alkyl halides, sulfates, and sulfonates undergo carboalkoxylation in the presence of carbon monoxide, an alcohol, a base, and a catalytic amount of sodium cobalt carbonylate, as illustrated for the reaction of 1-iodooctane in methanol with the strongly basic hindered amine dicyclohexylethyl-amine as base. Use of an unhindered amine leads to formation of the amide. 

Two pathways have been proposed for step e, which may be the rate-limiting stage in the cycle (this notion is supported by the observed overall rate law for hydroformylation, which is typically first-order in H2 concentration). Equation 9.10 illustrates an oxidative addition followed by reductive elimination sequence, as originally proposed. Since Heck and Breslow s work, a bimolecular process described by equation 9.11 has been suggested.23... 

The generally accepted mechanism for the hydroformylation of olefins catalyzed by Co2(CO)g was first proposed by Heck and Breslow and is depicted in Scheme 1 for the formation of a linear aldehyde. The proposed mechanism includes the generation of HCo(CO)4 from Co2(CO)g and hydrogen as the first step, followed by the three crucial unit processes mentioned above. Instead of hydrogenolysis of the acyl-cobalt species, RCH2CH2CO-Co(CO)4, reductive cleavage of the acyl-cobalt species with HCo(CO)4 is also possible to regenerate Co2(CO)g. 

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