Hydroformylation reactions stoichiometric

In studies of the isomerization of olefins by HCo(CO)4, it must be borne in mind that the catalyst HCo(CO)4 is consumed stoichiometrically via the hydroformylation reaction with the formation of aldehydes and dicobalt octacarbonyl, as shown by Kirch and Orchin (16) ... 

The hydroformylation reaction of vinyl aromatics (Table 4)60 lends itself to the synthesis of a number of 2-arylpropionic acids in high enantiomeric excess that are nonsteroidal antiinflammatory agents.61 Previous asymmetric syntheses of these acids required the use of stoichiometric amounts of chiral auxiliaries, which in most cases are not easily recovered. The branched aldehyde was oxidized to (S)-(+)-na-proxen,62 in 84% yield. 

Along with studies of the catalyst solution and stoichiometric reaction mixtures, the hydroformylation reaction was studied online under typical reaction conditions by connecting a pressurized autoclave (20 bar) directly to the mass spectrometer via a splitter. While this allowed them to identify new reaction intermediates they did not extract any kinetic data from the observed intermediates over time. Nevertheless, a new hydroformylation reaction mechanism for self-assembling ligands (in which the ligands play an active role in H2 activation) was considered based on... 

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... 

HCo(CO)4 in Hydroformylation Reactions. As previously mentioned, HCo(CO)4 is a key intermediate in the homogeneous hydroformylation of alkenes with CO/H2 to aldehydes. Using HCo(CO)4 as the CO component, hydroformylation can be made stoichiometric when run under N2 as an inert atmosphere. 

The prediction that tetracarbonylcobalt hydride would act as a catalyst in hydroformylation reactions 133, 224) has been amply verified for example, the stoichiometric hydroformylation, using HCo(CO)4, of 1-pen-tene at room temperature affords a variety of isomeric aldehydes 187). Also, HCo(CO)4 is formed under the high pressure (100 atm. 1 1 H2 CO) and temperature (100°-300°C) conditions of the hydroformylation reaction 183, 226). 

Cobalt, nickel, iron, ruthenium, and rhodium carbonyls as well as palladium complexes are catalysts for hydrocarboxylation reactions and therefore reactions of olefins and acetylenes with CO and water, and also other carbonylation reactions. Analogously to hydroformylation reactions, better catalytic properties are shown by metal hydrido carbonyls having strong acidic properties. As in hydroformylation reactions, phosphine-carbonyl complexes of these metals are particularly active. Solvents for such reactions are alcohols, ketones, esters, pyridine, and acidic aqueous solutions. Stoichiometric carbonylation reaction by means of [Ni(CO)4] proceeds at atmospheric pressure at 308-353 K. In the presence of catalytic amounts of nickel carbonyl, this reaction is carried out at 390-490 K and 3 MPa. In the case of carbonylation which utilizes catalytic amounts of cobalt carbonyl, higher temperatures (up to 530 K) and higher pressures (3-90 MPa) are applied. Alkoxylcarbonylation reactions generally proceed under more drastic conditions than corresponding hydrocarboxylation reactions. 

The negative reaction order in ligand L has implications for truly complex mass-transfer effects. In particular, lower mass-transfer rates can actually increase the overall observed reaction rate. This issue of negative reaction orders is seldom observed or mentioned in the context of stoichiometric and heterogeneous catalytic reactions. But it is frequently noted in homogeneous catalytic studies. Chaudhari has discussed this problem in detail for the gas-liquid mass-transfer control of CO in the hydroformylation reaction (21). 

Aldehydes are stoichiometrically decarbonylated by reaction with (XL) under mild conditions (77, 98,110,113). Aromatic aldehydes yield aromatic hydrocarbons whereas aliphatic aldehydes form saturated hydrocarbons and olefins. The latter minor products can be considered to arise from a reverse hydroformylation reaction. The initial step of this reaction is probably the oxidative addition of an aldehyde C—H bond to the rhodium(I) complex. However, a stable adduct of this type has not yet been reported. The driving force in these reactions is derived from the stability of the carbonyl (LXIX). 

As mentioned above, hydroformylation reactions occur under atmospheric pressure at normal temperature with stoichiometric amounts of cobalt carbonyls. However, with catalytic amounts of cobalt catalysts a minimum CO partial pressure is necessary for reformation and stability of Co2(CO)8, or HCo(CO)4, as the case may be (see page 15). A small increase of the CO partial pressure above this value first results in an increase of the reaction velocity until a maximum is reached depending on temperature and olefin structure. However, further increase of the CO-partial pressure causes a decrease in the reaction velocity [38, 40, 120], (see also section on reaction mechanism). 

More difficult hydroformylation processes such as the hydroformylation of acetylene (to form propenal) have been studied theoretically. It has been demonstrated that the hydrogenation of acetylene to yield ethane is not a concurrent reaction to hydroformylation under stoichiometric conditions. The propenal hydrogenation under HCo(CO)3-catalyzed hydroformylation conditions was also studied.Phenylacetylene led to 2-phenylpropanal under mild carbonylation conditions (hexane or benzene as solvent, 1 atm of CO, RT). This reaction needed an excess of HCo(CO)4 to be realized. The authors found that a radical mechanism was involved." ... 

Modeling of the hydroformylation kinetics was started by constracting a stoichiometric scheme for the process. An detailed reaction pattern is shown in Scheme 29.1. 

Hydroformylation of 1-butene in the presence of the Rh catalyst gave pentanal (P) and 2-methyl bntanal as the main products. Just trace amounts of c/5-and trans-1-butene were detected as by-prodncts. No butane was detected in experiments, where a stoichiometric ratio of CO and H2 were used. Based on preliminary considerations of prodnct distribntions, a kinetic model was developed. The kinetic parameters obtained from the model were well identified and physically reasonable. The prodnct concentrations are predicted very well by the kinetic model. The kinetic model can be further refined by considering detailed reaction mechanisms and extending it to the domain of lower partial pressures of CO and H2. 

Stoichiometric model reactions in alkene hydroformylation by platinum-tin systems have been studied for the independent steps involved in the hydroformylation process, insertion of the alkene, insertion of CO, and hydrogenolysis, with use of Pt-Sn catalysts and 1-pentene as alkene at low pressure and temperature.92... 

Today, iridium compounds find so many varied applications in contemporary homogeneous catalysis it is difficult to recall that, until the late 1970s, rhodium was one of only two metals considered likely to serve as useful catalysts, at that time typically for hydrogenation or hydroformylation. Indeed, catalyst/solvent combinations such as [IrCl(PPh3)3]/MeOH, which were modeled directly on what was previously successful for rhodium, failed for iridium. Although iridium was still considered potentially to be useful, this was only for the demonstration of stoichiometric reactions related to proposed catalytic cycles. Iridium tends to form stronger metal-ligand bonds (e.g., Cp(CO)Rh-CO, 46 kcal mol-1 Cp(CO)Ir-CO, 57 kcal mol ), and consequently compounds which act as reactive intermediates for rhodium can sometimes be isolated in the case of iridium. 

Although Eq. (3) indicates that CO absorption is required for aldehyde formation, it has been shown by Karapinka and Orchin 18) that at 25° and with a moderate excess of olefin the rate of reaction and the yield of aldehyde are similar when either 1 atm of CO or 1 atm of Nj is present. Obviously CO is not essential for the reaction and a CO-deficient intermediate, probably an acylcobalt tricarbonyl, can be formed under these conditions. The relative rates of HCo(CO)4 cleavage of tricarbonyl and tetracarbonyl are not known, and thus the stage at which CO is absorbed in the stoichiometric hydroformylation of olefins under CO is not known with certainty. Heck (19) has shown conclusively that acylcobalt tetracarbonyls are in equilibrium with the acylcobalt tricarbonyl ... 

Essentially the same sequence of reactions was proposed (22a) to explain the isomerization of olefins which accompanies the stoichiometric hydroformylation of olefins. In particular, it has been suggested that the active catalyst is cobalt hydrotricarbonyl, which first adds by Markownikoff addition and is then eliminated in the opposite direction ... 

Studies of stoichiometric hydroformylation, spectroscopic identification, isolation, and transformation of intermediates provided valuable information of the understanding of the catalytic reaction. Despite the complexity of the process, important conclusions were also drawn from kinetic studies. 

C and a total pressure of 1-300 atm (the pressure of the hydro-formylation reaction), depending on the reactivity of the olefin. After the cobalt carbonyl hydride has passed from the aqueous phase into the organic phase (Reaction 3), stoichiometric hydroformylation (6, 7, 8) takes place (Reactions 4-8). 

The treatment of a cobalt(II) salt with synthesis gas generates sequentially Co2(CO)8 then HCo(CO>4. This catalyst is generated only at 120-140 C for the carbonylation to proceed smoothly 200-300 bar is required to stabilize the catalyst. If the hydridocobalt catalyst is prepared separately and then introduced into the reaction, temperatures as low as 90 C can be used for the hydrocarbonylation. An important consideration in industrial reactions is the normal to branched nib ratio to give the desired straight chain aldehyde, the hydridocobalt catalyst providing an nib ratio of -4 in the hydroformylation of propene under the lower temperature conditions. This catalyst will stoichiometrically hydroformylate 1-alkenes under ambient conditions. 

A useful study has just been completed by Roos and Orchin (125), who have examined the effect of ligands such as benzonitrile on the stoichiometric hydroformylation of olefins. A variety of such reagents (acetonitrile, anisole) were found to act in a similar manner to carbon monoxide by suppressing the formation of branched products and the isomerization of excess olefin. The yield of aldehyde was also increased by increasing ligand concentration up to 2 moles per mole of cobalt hydrocarbonyl. Benzonitrile was not found to affect the rate of the reaction of cobalt hydrocarbonyl with acylcobalt tetracarbonyl, so the ligand must have affected an earlier step in the reaction sequence. It seems most likely that cobalt hydrocarbonyl reacts with olefin in the presence of benzonitrile to form an acylcobalt tricarbonyl-benzonitrile complex which is reduced more rapidly than the acylcobalt tetracarbonyl. 

The preceding stoichiometric reactions are clearly closely related to the catalytic hydroformylation of epoxides. Somewhat less clear is their relation... 

Using hydroformylation and other catalytic or stoichiometric reactions, how could the following transformations be achieved in one or more steps (a) Ethylene to 2-methylpentanol (b) butadiene to 1,6-hexanediol (c) allyl alcohol to butane 1,4-dicarboxylic acid (d) allyl alcohol to 4-carboxylic butanal. 

The study of stoichiometric CO insertions into transition metal complexes is of great importance because this reaction is the first step m the catalytic conversion of carbon dioxide. Hence, these investigations can lead to the possibility of introducing carbon dioxide into transition metal-catalyzed synthetic processes. Analogies with carbon monoxide chemistry may be drawn, for instance. from the CO insertion into metal alkyl bonds leading to such important industrial processes as hydroformylation and carbonylalion. 

Monsanto acetic acid synthesis 4), and the hydroformylation or 0X0 reaction (5). A key mechanistic step in catalytic carbonylation reactions is the migration of an alkyl group onto an adjacent carbonyl ligand. This reaction involves the formation of a new carbon-carbon bond and has been termed a carbonyl insertion reaction since a CO ligand has been formally inserted into the transition metal-carbon (r-bond. Because of the industrial and commercial importance of these catalytic reactions, the search for stoichiometric systems in which this step can be observed directly has been, and still is, one of great endeavor. 

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