History
The process was discovered by the German chemist Otto Roelen in 1938 in the course of investigations of the Fischer–Tropsch process. Aldehydes and diethylketone were obtained when ethylene was added to an F-T reactor. Through these studies, Roelen discovered the utility of cobalt catalysts. HCo(CO)4, which had been isolated only a few years prior to Roelen's work, was shown to be an excellent catalyst. The term oxo synthesis was coined by the Ruhrchemie patent department, who expected the process to be applicable to the preparation of both aldehydes and ketones. Subsequent work demonstrated that the ligand tributylphosphine (PBu3) improved the selectivity of the cobalt-catalysed process. The mechanism of Co-catalyzed hydroformylation was elucidated by Richard F. Heck and David Breslow in the 1960s.. In 1968, highly active rhodium-based catalysts were reported. Since the 1970s, most hydroformylation relies on catalysts based on rhodium.J. F. Hartwig; Organotransition metal chemistry – from bonding to catalysis. University Science Books. 2009. 753, 757–578. . Water-soluble catalysts have been developed. They facilitate the separation of the products from the catalyst.Mechanism
Selectivity
A key consideration of hydroformylation is the "normal" vs. "iso" selectivity. For example, the hydroformylation of propylene can afford twoSteric effects
Markovnikov's rule addition of the cobalt hydride to primary alkenes is disfavored by steric hindrance between the cobalt centre and the secondary alkyl ligand. Bulky ligands exacerbate this steric hindrance. Hence, the mixed carbonyl/phosphine complexes offer a greater selectivity for anti-Markovnikov addition, thus favoring straight chain products (''n''-) aldehydes. Modern catalysts rely increasingly on chelating ligands, especially diphosphites.Electronic effects
Additionally, electron-rich the hydride complex are less proton-like. Thus, as a result, the electronic effects that normally favour the Markovnikov addition to an alkene are less applicable. Thus, electron-rich hydrides are more selective.Acyl formation
To suppress competing isomerization of the alkene, the rate of migratory insertion of the carbonyl into theAsymmetric hydroformylation
Hydroformylation of prochiral alkenes creates newProcesses
The industrial processes vary depending on the chain length of the olefin to be hydroformylated, the catalyst metal and ligands, and the recovery of the catalyst. The original Ruhrchemie process produced propanal from ethylene and syngas using cobalt tetracarbonyl hydride as the catalyst. Today, industrial processes based on cobalt catalysts are mainly used for the production of medium- to long-chain olefins, whereas the rhodium-based catalysts are usually used for the hydroformylation of propene. The rhodium catalysts are significantly more expensive than cobalt catalysts. In the hydroformylation of higher molecular weight olefins the separation of the catalyst from the produced aldehydes is difficult.BASF-oxo process
The BASF-oxo process starts mostly with higher olefins and relies on cobalt carbonyl-based catalyst. By conducting the reaction at low temperatures, one observes increased selectivity favoring the linear product. The process is carried out at a pressure of about 30 MPa and in a temperature range of 150 to 170 °C. The cobalt is recovered from the liquid product by oxidation to water-soluble Co2 +, followed by the addition of aqueous formic orExxon process
The Exxon process, also Kuhlmann- or PCUK – oxo process, is used for the hydroformylation of C6–C12 olefins. The process relies on cobalt catalysts. In order to recover the catalyst, an aqueous sodium hydroxide solution or sodium carbonate is added to the organic phase. By extraction with olefin and neutralization by addition ofShell process
The Shell process uses cobalt complexes modified with phosphine ligands for the hydroformylation of C7–C14 olefins. The resulting aldehydes are directly hydrogenated to the fatty alcohols, which are separated by distillation, which allows the catalyst to be recycled. The process has good selectivity to linear products, which find use as feedstock for detergents. The process is carried out at a pressure of about 4 to 8 MPa and at a temperature range of about 150–190 °C.Union Carbide process
The Union Carbide (UCC) process, also known as low-pressure oxo process (LPO), relies on a rhodium catalyst dissolved in high-boiling thick oil, a higher molecular weight condensation product of the primary aldehydes, for the hydroformylation of propene. The reaction mixture is separated in a falling film evaporator from volatile components. The liquid phase is distilled and butyraldehyde is removed as head product while the catalyst containing bottom product is recycled to the process. The process is carried out at about 1.8 MPa and 95–100 °C. 244px, is representative diphosphite ligand popularized by workers at Union Carbide.">BiPhePhos is representative diphosphite ligand popularized by workers at Union Carbide.Ruhrchemie/Rhone–Poulenc process
Laboratory process
Recipes have been developed for the hydroformylation on a laboratory scale, e.g. ofSubstrates other than alkenes
Cobalt carbonyl and rhodium complexes catalyse the hydroformylation of formaldehyde and ethylene oxide to give hydroxyacetaldehyde and 3-hydroxypropanal, which can then be hydrogenated to ethylene glycol and propane-1,3-diol, respectively. The reactions work best when the solvent is basic (such as pyridine). In the case of dicobalt octacarbonyl or Co2(CO)8 as a catalyst, pentan-3-one can arise from ethylene and CO, in the absence of hydrogen. A proposed intermediate is the ethylene-propionyl species 3C(O)Co(CO)3(ethylene)">H3C(O)Co(CO)3(ethylene)which undergoes a migratory insertion to form 3COCH2CH2Co(CO)3">H3COCH2CH2Co(CO)3 The required hydrogen arises from the water shift reaction. For details, see If the water shift reaction is not operative, the reaction affords a polymer containing alternating carbon monoxide and ethylene units. Such aliphatic polyketones are more conventionally prepared using palladium catalysts. Functionalized olefins such as allyl alcohol can be hydroformylated. The target product 1,4-butanediol and its isomer is obtained with isomerization free catalysts such as rhodium-triphenylphosphine complexes. The use of the cobalt complex leads by isomerization of the double bond to n- propanal. The hydroformylation of alkenyl ethers and alkenyl esters occurs usually in the α-position to the ether or ester function. The hydroformylation of acrylic acid and methacrylic acid in the rhodium-catalyzed process leads to the Markovnikov product in the first step.Jürgen Falbe, Ch. R. Adams: ''Carbon Monoxide in Organic Synthesis'', Springer Verlag, 1970, By variation of the reaction conditions the reaction can be directed to different products. A high reaction temperature and low carbon monoxide pressure favors the isomerization of the Markovnikov product to the thermodynamically more stable β-isomer, which leads to the n-aldehyde. Low temperatures and high carbon monoxide pressure and an excess of phosphine, which blocks free coordination sites, can lead to faster hydroformylation in the α-position to the ester group and suppress the isomerization.Side- and consecutive reactions
Tandem carbonylation-water gas shift reactions
Side reactions of the alkenes are the isomerization and hydrogenation of the double bond. While the alkanes resulting from hydrogenation of the double bond do not participate further in the reaction, the isomerization of the double bond with subsequent formation of the n-alkyl complexes is a desired reaction. The hydrogenation is usually of minor importance; However, cobalt-phosphine-modified catalysts can have an increased hydrogenation activity, where up to 15% of the alkene is hydrogenated.Tandem hydroformylation-hydrogenation
Using tandem catalysis, systems have been developed for the one-pot conversion of akenes to alcohols. The first step is hydroformylation.Ligand degradation
Conditions for hydroformylation catalysis can induce degradation of supporting organophosphorus ligands. Triphenylphosphine is subject to hydrogenolysis, releasing benzene and diphenylphosphine. The insertion of carbon monoxide in an intermediate metal-phenyl bond can lead to the formation of benzaldehyde or by subsequent hydrogenation to benzyl alcohol. Arno Behr: ''Angewandte homogene Katalyse'', Wiley-VCH. Weinheim, One of the ligands phenyl-groups can be replaced by propene, and the resulting diphenylpropylphosphine ligand can inhibit the hydroformylation reaction due to its increased basicity.Metals
Although the original hydroformylation catalysts were based on cobalt, most modern processes rely on rhodium, which is expensive. There has therefore been interest in finding alternative metal catalysts. Examples of alternative metals include iron and ruthenium.See also
* Koch reaction - related reaction of alkenes and CO to form carboxylic acidsReferences
Further reading
*"Applied Homogeneous Catalysis with Organometallic Compounds: A Comprehensive Handbook in Two Volumes (Paperback) by Boy Cornils (Editor), W. A. Herrmann (Editor). *"Rhodium Catalyzed Hydroformylation" P. W. N. M. van Leeuwen, C. Claver Eds.; Springer; (2002). *"Homogeneous Catalysis: Understanding the Art" by Piet W. N. M. van Leeuwen Springer; 2005. * Imyanitov N.S./ Hydroformylation of Olefins with Rhodium Complexes // Rhodium Express. 1995. No 10–11 (May). pp. 3–62 (Eng) {{Authority control Organometallic chemistry Homogeneous catalysis Formylation reactions Carbon monoxide