Hydroformylation catalyzed by HCo

HCo(CO), an exceptionaUy acidic transition metal "hydride", is formed by tfie reaction of Hj and COjfCOlg. HCo(CO)g is a trigonal bipyramidal d , 18-electron complex in which the hydride occupies an apical position. High temperature is required for industrially useful rates of hydroformylation catalyzed by HCo(CO)g. Moreover, high pressure of CO is required to prevent formation of higher cobalt clusters and of metallic cobalt. The rate of hydroformylation catalyzed by cobalt carbonyl depends on [H ] and Thus, increasing the pressure of a 1 1 mixture of CO Hj has little effect on tire rate but prevents catalyst decomposition. 

The regioselectivities and rates of hydroformylation with cobalt catalysts were improved significantly by the addition of phosphines to HCo(CO) (Equation 17.6). This hydroformylation catalyzed by HCo(CO)3(PRj) complexes was developed by Slaugh and Mullineaux at... 

The basic steps of the mechanism of hydroformylation catalyzed by the combination of Rh and PPhj are similar to those of hydroformylation catalyzed by HCo(CO). However, the number of possible intermediates is much larger because many combinations of the number of phosphines and geometric orientation are possible in each intermediate complex. For example, phosphines in a five-coordinate complex can occupy the apical or equatorial positions, and these complexes are typically stereochemically nonrigid. Similarly, two phosphines in a four-coordinate intermediate can be located cis or trans to each other. A five-coordinate hydridorhodium-alkene complex containing diequatorial phosphines might be expected to convert to a four-coordinate Rh alkyl complex containing trans phosphines, but it could also lead to an alkyl complex containing cis phosphines. 

Another route to carboxylic acids from aldehyde products (once again, generally produced via hydroformylation catalysis) was discovered by Wakamatsu and coworkers. They reported the carbonylation of aldehydes and primary organic amides to produce A-acylamino acids (equation 16). The reaction is efficiently catalyzed by HCo(CO)4 at 100 °C and 140 bar of 3 2 H2/CO (hydrogen is needed to help stabilize HCo(CO)4). Yields of over 90% of the appropriate... 

Figure 7.4. Molar paraffin-to-aldehyde yield ratios in hydroformylation of n-dodecene catalyzed by HCo(CO)3Ph at 185°C as function of H2-to-CO ratio at different total pressures [11].

Example 11.1. Hydroformylation of cyclohexene with phosphine-substituted cobalt hydrocarbonyl catalyst. The most probable network of cyclohexene hydroformylation catalyzed by a phosphine-substituted cobalt hydrocarbonyl is shown on the facing page. HCo(CO)3Ph (cat) is in equilibrium with the CO-deficient HCo(CO)2Ph (cat ) and CO. For greater generality, quasi-equilibrium of these species with the 7r-complex, X, is not assumed. Actual hydroformylation olefin — aldehyde proceeds via a Heck-Breslow pathway (cycle 6.9 that includes the trihydride, X2) but without... 

After it was recognized that the hydroformylation reaction is catalyzed by a soluble species, HCo(CO)4 was proposed as the catalyst (//). Sub-... 

Saturated ketones and aldehydes have been reduced to alcohols using Co2(CO)8-phosphine (P) mixtures at high H2 pressures, probably via the coordinatively unsaturated species HCo(CO)2P (187). A reassessment of experimental data on hydrogenation of aromatics (A) catalyzed by Co2(CO)8 under hydroformylation conditions has led to the suggestion that free radicals rather than organocobalt complexes are involved (188), e.g.,... 

Hydroformylation reactions can be catalyzed by electro-generated HCo(CO)4 and HCo(C03)PBu3 species Similarly, highly active catalysts for olefin metathesis can be obtained from WClg or MoClj Aluminum is used as anode material as it forms Aid, as the required Lewis acid. 

Orchin and Roos (108) examined the isomerization of allylbenzene by HCo(CO)4 and DCo(CO)4 at ambient temperature and pressure. Both HCo(CO)4 and DCo(CO)4 catalyzed isomerization to propenylbenzene at the same rate, and when DCo(CO)4 was used as catalyst 5% of the propenylbenzene produced was found to contain a deuterium atom. Hydroformylation of propylene with residual DCo(CO)4, after an isomerization of allylbenzene, yielded RCDO with no detectable RCHO. The authors chose to reject a mechanism involving addition of D—Co to the olefinic double bond, on the grounds that the lack of an isotope effect indicated breaking of D—Co, or H—Co, was not the rate-determining step, and that only a relatively minor amount of deuterium was incorporated into the isomerized reaction product. Instead, the authors favored a mechanism expressed as... 

In the Shell process (SHOP) phosphine-modified cobalt-catalyzed hydrofor-mylation is one of the steps in the synthesis of linear alcohols with 12-15 carbon atoms (see Section 7.4.1). Two important characteristics of this reaction should be noted. First, the phosphine-modified precatalyst HCo(CO)3(PBu3) is less active for hydroformylation than HCo(CO)4 but more active for the subsequent hydrogenation of the aldehyde. In this catalytic system both hydroformylation and hydrogenation of the aldehyde are catalyzed by the same catalytic species. Second, the phosphorus ligand-substituted derivatives are more stable than their carbonyl analogues at higher temperatures and lower pressures (see Table 5.1). 

The hydroformylation of alkenes was accidentally discovered by Roelen [4] while he was studying the Fischer-Tropsch reaction (conversion of syn-gas to liquid fuels with a heterogeneous cobalt catalyst) in the late thirties. In what w s probably designed as a mechanistic experiment Roelen examined whether alkenes were intermediates in the Aufbau process for converting syn-gas (from coal, Germany 1938) to fuel. It took more than a decade before the reaction was taken further, but now it was the conversion of petrochemical hydrocarbons into oxygenates that was the driving force. It was discovered that the reaction was not catalyzed by the supported cobalt, but in fact by HCo(CO)4 which was formed in the liquid state. 

Example 7.5. Olefin hydrvformylation with paraffin by-product formation [7,9]. Hydroformylation of olefins to aldehydes, catalyzed by a phosphine-substituted cobalt hydrocarbonyl, HCo(CO)3Ph (Ph = tertiary organic phosphine), has been used for illustration in examples 5.2 and 5.3 in Sections 5.2 and 5.3. The catalyst also promotes hydrogenation, so aldehyde produced from olefin is converted to alcohol, and paraffin is formed from olefin as by-product ... 

An example is the hydroformylation reaction of cyclohexene catalyzed by the unsaturated compound HCo(CO)3 which is formed under reaction conditions from the precursor HCo(CO)4. Following the usual mechanism (see, e. g., [18]), the catalytic cycle is depicted in Scheme 1. Since the oxidative addition of H2 to the acylcobalt complex is the rate-determining step in this case the rate equation follows eq. (2) (cf. Section 2.1.1) ... 

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. 

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. 

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). 

Hydroformylation involves the addition of both H2 and CO to a terminal alkene to create an aldehyde. It can be catalyzed by many transition metal complexes, but a classic example is the use of HCo(CO)4 (Eq. 12.73). 

It may be assumed that dialdehyde formation can be achieved because the isomerisation of the intermediate nonconjugated unsaturated aldehyde to the conjugated aldehyde (which would only give monoaldehyde by hydrogenation) is not catalyzed by the modified rhodium catalyst (see also chapter on reaction mechanism of the hydroformylation reaction, p. 4), whereas HCo(CO)4 catalyzes the isomerisation. 

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. 

Figure 22.13 shows the scheme used to describe the hydroformylation process. The active catalyst is HCo(CO)3, which is a 16-electron species that is coordinatively unsaturated. After that species is generated, the first step of the catalyzed process involves the addition of the alkene to the catalyst. In the next step, an insertion reaction occurs in which the alkene is inserted in the Co-H bond (nucleophilic attack by H on the alkene would accomplish the same result as described... 

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... 

The HCo(CO)4-catalyzed hydrocarboxylation of alkenes has also been known for a long time. The mechanism is analogous to that presented for hydroformyla-tion (Scheme 1), except that H2O is used instead of H2. Hydrocarboxylation is generally slower than hydro-formylation, and it is believed that the concentrations of the intermediate species are quite low relative to those seen for hydroformylation. Pyridine has a rateenhancing effect that is believed to be due to the facile cleavage of the (acyl)Co(CO)4 intermediate. This reaction forms [pyridine-acyl] + [Co(CO)4] , which is more rapidly hydrolyzed by water to form the product carboxylic acid and HCo(CO)4. 

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