Hydroformylation



Hydroformylation, also known as oxo synthesis, is an important industrial process for the production of aldehydes from alkenes. This chemical reaction entails the addition of a formyl group (CHO) group and a chemistry.

The process typically is accomplished by treatment of an Koch reaction.

Catalysts

The original catalyst was rhodium.[3] Subsequent research led to the development of water-soluble catalysts that facilitate the separation of the products from the catalyst.[4]

Mechanism

The overall mechanism resembles that for homogeneous hydrogenation with additional steps. The reaction begins with the generation of coordinatively unsaturated metal hydrido carbonyl complex such as HCo(CO)3 and HRh(CO)(PPh3)2. Such species bind alkenes, and the resulting complex undergoes a migatory insertion reaction to form a 16 VE alkyl complex.

Selectivity

A key consideration of hydroformylation is the "normal" vs. "iso" selectivity. For example, the hydroformylation of propene can afford two 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 was dedicated to the quest for catalyst that favored the normal isomer.

Steric effects

When the hydrogen is transferred to the carbon bearing the most hydrogen atoms (Markovnikov addition) the resulting alkyl group has a larger steric bulk close to the ligands on the cobalt. If the ligands on the cobalt are bulky (such as tributyl phosphine), then this steric effect is greater. Hence the mixed carbonyl/phosphine complexes offer a greater selectivity towards the straight chain products.

Electronic effects

In addition the more electron rich the hydride complex is, the less proton like the hydride is, thus as a result the electronic effects which favour the markovnikov addition to an alkene are less able to direct the hydride to the carbon atom bearing the most hydrogens already. Thus as a result as the metal centre becomes more electron rich the catylst becomes more selective for the straight chain compounds.

Acetyl formation

After the alkyl formation a second migatory insertion converts the alkyl into an acetyl ligand (this is when the alkyl carbon forms a bond with the carbon of a carbonyl ligand). The vacant site on the metal is filled by two hydrogens (from the oxidative insertion of a aldehyde and the complex [HCo(CO)3].

It is important to note that the rate of migatory insertion of the regioselectivity:

Asymmetric hydroformylation

Hydroformylation of internal alkenes creates new enantiomer.

Other substrates

Cobalt carbonyl and rhodium complexes catalyse the hydroformylation of pyridine). [6][7]

In the case of dicobalt octacarbonyl or Co2(CO)8 as a catalyst, 2-pentanone can arise from ethylene and CO, in the absence of hydrogen. A proposed intermediate is the ethylene-propionyl species [CH3C(O)Co(CO)3(ethylene)] which undergoes a migratory insertion to form [CH3COCH2CH2Co(CO)3]. The required hydrogen arises from the water shift reaction. For details see [8]

If the water shift reaction is not operative, the reaction affords a polymer containing alternating carbon monoxide and ethylene units. Such polymers are more conventionally prepared using palladium catalysts.[9].

References

  1. ^ Boy Cornils, Wolfgang A. Herrmann, Manfred Rasch (1994). "Otto Roelen, Pioneer in Industrial Homogeneous Catalysis". Angewandte Chemie International Edition in English 33 (21): 2144-2163. doi:10.1002/anie.199421441.
  2. ^ [1]
  3. ^ Evans D., Osborn J. A., Wilkinson G. (1968). "Hydroformylation of Alkenes by Use of Rhodium Complex Catalyst". Journal of the Chemical Society 33 (21): 3133-3142. doi:10.1039/J19680003133.
  4. ^ Cornils, B.; Herrmann, W. A. (eds.) “Aqueous-Phase Organometallic Catalysis” VCH, Weinheim: 1998
  5. ^ High-Precision Catalysts: Regioselective Hydroformylation of Internal Alkenes by Encapsulated Rhodium Complexes Kuil, M.; Soltner, T.; van Leeuwen, P. W. N. M.; Reek, J. N. H. Journal of the American Chemical Society; 2006; 128(35) pp 11344 - 11345; doi:10.1021/ja063294i
  6. ^ Chan A.S.C., Shieh H-S. (1994). "A mechanistic study of the homogeneous catalytic hydroformylation of formaldehyde: synthesis and characterization of model intermediates". Inorganica Chimica Acta 218 (1-2): 89-95. doi:10.1016/0020-1693(94)03800-7.
  7. ^ Spencer A. (1980). "Hydroformylation of formaldehyde catalysed by rhodium complexes". Journal of Organometallic Chemistry 194 (1-2): 113-123. doi:10.1016/S0022-328X(00)90343-7.
  8. ^ Murata K., Matsuda A., (1981). "Application of Homogeneous Water-Gas Shift Reaction III Further Study of the Hydrocarbonylation - A highly Selective Formation of Diethyl Keton from Ethene, CO and H2O". Bulletin of the Chemical Society Japan 54: 2089-2092.
  9. ^ J. Liu, B.T. Heaton, J.A. Iggo and R. Whyman, Angew. Chem. Int. Ed., 2004, 43, 90-94

Further reading

  • “Applied Homogeneous Catalysis with Organometallic Compounds: A Comprehensive Handbook in Two Volumes (Paperback) by Boy Cornils (Editor), W. A. Herrmann (Editor). ISBN 3-527-29594-1
  • “Rhodium Catalyzed Hydroformylation” P. W. N. M. van Leeuwen, C. Claver Eds.; Springer; (2002). ISBN 1-4020-0421-4
  • “Homogeneous Catalysis: Understanding the Art” by Piet W. N. M. van Leeuwen Springer; ISBN 2005. ISBN 1-4020-3176-9
 
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