Heck-Breslow mechanism

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). 

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. 

Synthesis of intermediates. An excellent technique for confirming or refuting a postulated pathway is to synthesize intermediates and use them as starting materials. Often, a key intermediate that is reactive enough to remain at trace level under reaction conditions is stable at very low temperatures (e.g., that of liquid nitrogen) and can be synthesized. If the reaction starting with the postulated intermediate yields the same products in the same ratios, this can be taken as evidence in favor of the presumed pathway. For example, the essential features of the Heck-Breslow mechanism of hydroformylation (see Example 6.2 in Section 6.3) with cobalt hydrocarbonyl catalysts have been verified in this way by synthesis and use of the alkyl-and acyl-cobalt species [42]. 

The first calculation of the complete hydroformylation cycle with Rh-phosphine catalysts (substrate = ethylene, model ligand = PH3) was published in 1997 [3]. The QM methods used are HF and MP2, respectively (cf. Section 3.1.2.1). Hybrid DFT methods such as B3LYP [4], however, are more appropriate in terms of both accuracy and efficiency [5, 6] (cf. Section 3.1.2.1). Therefore, the same model system was recalculated [7] on the level B3LYP functional/DZVP basis set [8]/quasi-relativistic pseudopotentials on rhodium [9]. Since homologous Ir catalysts are interesting alternatives from an economic point of view [10], calculations with the central metal Ir were also made. This comparative treatment is supported by the experimental assumption of a common mechanism [11], which equals the Heck-Breslow mechanism of the cobalt-catalyzed reaction [12],... 

A key intermediate in the Heck-Breslow mechanism for hydroformylation is proposed to be HCo(CO)3. This 16 e coordinatively unsaturated intermediate would be produced by dissociation of CO as shown in Equation 10.15 and rapidly bind olefins as shown in Equation 10.16. 

In spite of the prominence of 16/18 e complexes in the Heck-Breslow mechanism, a number of observations suggested a possible role, under some conditions of temperature and pressure, for the Co(CO)4 radical in the hydroformylation process. Photolysis studies in frozen gas matrices gave evidence31 for the infrared spectrum of the proposed radical species at low temperatures ... 

A series of carefully designed model reactions, simulations, analogies with stoichiometric reactions, kinetic and IR spectroscopic studies at the same temperature and pressure as those of the industrial Oxo Process confirmed the validity of the Heck-Breslow mechanism with some modifications. For instance, IR spectroscopic studies under industrial Oxo Process conditions have revealed the virtually complete conversion of Co2(CO)g (1) to HCo(CO)4 (2). Although the formation of alkyl- and acyl-cobalt carbonyl complexes can be observed in model reactions, no alkyl-cobalt complexes have been detected under the conditions of the industrial process, i.e., only acyl-Co(CO)4 8 is observed. ... 

In the Heck-Breslow mechanism, formal reductive cleavage of the acyl-Co complex 8 with molecular hydrogen or HCo(CO)4 (2) is proposed. However, it is more than likely that the actual acyl-Co complex that reacts with molecular hydrogen is the coordinatively unsaturated (16-electron) acyl-Co(CO)3 7, and the oxidative adduct 10 is formed from 7, which then reductively eliminates to give aldehyde and HCo(CO)3 (3) (Scheme... 

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). 

The hydroformylation mechanism for phosphine-modified rhodium catalysts follows with minor modifications the Heck-Breslow cycle. HRh(CO)(TPP)3 [11] is believed to be the precursor of the active hydroformylation species. First synthesized by Vaska in 1963 [98] and structurally characterized in the same year [99], Wilkinson introduced this phosphine-stabilized rhodium catalyst to hydroformylation five years later [100]. As one of life s ironies, Vaska even compared HRh(CO)(TPP)3 in detail with HCo(CO)4 as an example of structurally related hy-drido complexes [98]. Unfortunately he did not draw the conclusion that the rhodium complex should be used in the oxo reaction. According to Wilkinson, two possible pathways are imaginable the associative and the dissociative mechanisms. Preceding the catalytic cycle are several equilibria which generate the key intermediate HRh(CO)2(TPP)2 (Scheme 4 L = ligand). 

The extent to which hydrogenolysis takes place by reaction with HCo(CO)4 rather than with H2, as in the conventional Heck and Breslow mechan-ism " and under what conditions, and whether it plays a role at all in rhodium hydroformylations, remains to be established by kinetic and spectroscopic experiments. The situation with cobalt is also complicated by the occurrence under some conditions of odd-electron pathways. Ungvary and co-workers have shown that reactions of HCo(CO)4 with olefins can be catalyzed by Co2(CO)g, implying a significant role for Co(CO)4. 

The Heck and Breslow Mechanism of the Cobalt-Catalyzed Olefin Hydroformylation... 

By analogy with the Heck and Breslow mechanism of the cobalt-catalyzed olefin hydroformylation, similar mechanisms involving the hydrido-, alkyl-, and acyl-complex sequence have been proposed for the other catalysts as well. 

Unmodified Rhodium Catalyst. The strong similarities of the results of the kinetics (91,92,106,107) and spectroscopic observations (106-109) with the unmodified cobalt case suggest that the Heck and Breslow mechanism can be applied according to Scheme 4 for the unmodified rhodium case as well with the exception that a binuclear elimination as a product-forming step can be excluded (106). The up to four orders of magnitude higher activity of the rhodium catalyst has been explained by the much greater tendency of rhodium to form the key 16-electron complexes. 

Recent results of model reactions with isolated hydrido-, alkyl-, and acylcobalt carbonyls, and their kinetic and spectroscopic investigations have provided a more detailed picture of the classic Heck and Breslow mechanism. This may serve toward a better understanding of the hydroformylation mechanism with other metal catalysts. 

Modeling the Formate Ester Formation. Formate esters up to 4% of the total products are side products mainly in the unmodified cobalt-catalyzed alkene hydroformylation. Their formation has been explained in analogy to the Heck and Breslow mechanism of olefin hydroformylation (203). By the addition of HCo(CO)4 to the aldehyde carbonyl group, either an a-hydroxyaUgrl- or an alkoxy-cobalt carbonyl is formed. The latter complex converts with carbon monoxide into an (alkoxycarbonyl)cobalt carbonyl. Reduction of this complex gives the formate product (Scheme 15). 

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 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 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... 

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

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). 

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. 

Under pressure of CO and H2, the cobalt catalyst precursor is transformed into cobalt carbonyl hydride, HCo(CO)4. The main steps of the reaction mechanism, first elucidated by D. S. Breslow and R. F. Heck, involve (a) /3-hydrogen transfer to the coordinated olefin, (b) the insertion of CO to form an acyl intermediate, and (c) the hydrogenolysis of the acyl, with formation of the aldehyde product ... 

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... 

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