Hydroformylation is the transformation of an alkene to an aldehyde via the addition of both H₂ and CO. The aldehyde has one more carbon atom than the precursor alkene.C. Elschenbroich, A. Salzer ”Organometallics : A Concise Introduction” (2nd Ed) (1992) from Wiley-VCH: Weinheim. ISBN 3527281657 Hydroformylation is an important industrial process and is also useful for organic synthesis. The development of hydroformylation is considered one of the premier achievements of 20th century industrial chemistry. The developments are considered to originate within German coal-based industry of the 1930's.

The process typically involves high pressure (between 10 to 100 atmospheres) and temperature between 40 and 300°C. The process requires transition metal catalysts, that typically contain cobalt or rhodium.

Utility of aldehydes

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Hydroformylation is important industrially because the product aldehyde, is easily converted into many important secondary products. The increasing interest in this process has led to continuous growth in production. The production capacity was 5.2×10⁶ tons per year in 1980 and reached 6.6×10⁶ tons per year in 1995. In the SHOP process, aldehydes are hydrogenated to alcohols which are converted to detergents. At the other end of the scale, some specialized fragrances are prepared by hydroformylation.

Normal vs. iso selectivity

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A key consideration of hydroformylation is the "normal" vs. "iso" selectivity. The issue is illustrated in the hydroformylation of propene. Two isomeric products can result - butyraldehyde]] or isobutyraldehyde:

H₂ + CO + CH₃CH=CH₂ → CH₃CH₂CH₂CHO ("normal")

vs.

H₂ + CO + CH₃CH=CH₂ → (CH₃)₂CHCHO ("iso")

These isomers result from the differing ways of inserting the alkene into the M-H bond. Of course, both products are not equally desirable. Much research has been dedicated to the quest for catalyst that favored the normal isomer.

Catalysts

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The original catalyst was HCo(CO)₄, discovered by Roelen. Subsequent work demonstrated that the ligand PPh₃ improved the normal/iso selectivity, in part by rendering the metal bulkier and thus disfavoring the formation of Co-C bonds where the latter is secondary. The active species were HCo(CO)_4-x(PPh₃)_x. In the 1960's, a new generation of catalysts was generated based on HRh(CO)₂(PPh₃)₃.D. Evans, J. A. Osborn, G. Wilkinson “Hydroformylation of Alkenes by Use of Rhodium Complex Catalyst” Journal of the Chemical Society (A) (1968) pages 3133-3142. Subsequent research led to the development of water-soluble catalysts that streamlined the separation of the products.B. Cornils, W. A. Herrman (eds.) “Aqueous-Phase Organometallic Catalysis” VCH, Weinheim 1998

Asymmetric hydroformylation

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Hydroformylation of internal alkenes creates new stereocenters. Using chiral phosphine ligands, the hydroformylation can be tailored to favor one enantiomer.

References

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Further reading

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• “Applied Homogeneous Catalysis with Organometallic Compounds: A Comprehensive Handbook in Two Volumes (Paperback) by Boy Cornils (Editor), W. A. Herrmann (Editor). ISBN 3527295941
• “Rhodium Catalyzed Hydroformylation” P. W. van Leeuwen, C. Claver Eds.; Springer; (2002). ISBN 1402004214
• “Homogeneous Catalysis: Understanding the Art” by P. W. van Leeuwen Springer; ISBN 2005. ISBN 1402031769

Organic reactions | Organometallic chemistry | Homogeneous catalysis