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Hydroformylation
The reaction of an alkene with synthesis gas (CO + H2) to give an aldehyde is called hydroformylation. It is also known as the "OXO reaction".
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Compounds of several transition metals catalyse hydroformylation to some extent, but the main interest lies in catalysis by cobalt or rhodium compunds. Industrially, the aldehyde products are usually hydrogenated to alcohols either directly or after aldol condensation.
The reaction conditions required for hydroformylation with cobalt carbonyl as catalyst are quite severe, typically 200-300 atm at 130-170oC, which leads to a high plant cost for the industrial process.
The following cycle is animated each step lasts ~15 seconds |
One difficulty with the hydroformylation reaction is that the product is usually a mixture of isomers because the insertion of the alkene into the Co-H bond can occur in two ways..
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The two possibilities are usually described as Markownikoff and anti-Markownikoff additions. The direction of addition is important in many commercial hydroformylation reactions, and for most applications the anti-Markownikoff orientation leading to the straight chain aldehyde is preferred. A measure of the desirability of a catalyst is the normal/iso ratio of the product (typically 3:1). If a tertiary phosphine is added to the cobalt catalyst, a higher normal/iso ratio can be achieved. This is the basis of the Shell process for hydroformylation and is usually carried out at a lower pressure, but slighty higher temperature than unmodified process (50-100 atm at 175oC). A disadvantage of the Shell process is that some of the alkene is hydrogenerated to alkene (up to 15%).
A more marked improvement in the normal/iso ratio can be achieved under yet milder conditions using a rhodium phosphine complex such as HRh(CO)(PPh3)3 as catalyst. A normal /iso ratio of 15 is obtained at 12.5 atm at 125oC when triphenylphosphine is used as ligand and solvent. Th rhodium-catalysed reaction has been used widely in synthesis. A mechanism for rhodium-catalysed hydroformylation is given in the next catalytic cycle.
The following cycle is animated each step lasts ~15 seconds |
It is interesting to compare this with the cobalt-catalysed proces in the previous catalytic cycle. Both meganisms involve two insertion steps, but the rhodium case the metal undergoes a series of oxidative additions and reductive eliminations, while in the cobalt case the oxidation state is +1 throughout the cycle. Not all aspects of the mechanism of hydroformylation are understood. For example, it is not certain that two phosphines ligands remain coordinated to rhodium throughout the cycle. Finally, it should be noted that a study of the comparable iridium (iridium, rhodium and cobalt are in the same group in the periodic table) system in which the individual steps occur more slowly has provided general support for the mechanisme given above.
