Everything about Hydroformylation totally explained
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
hydrogen atom to a carbon-carbon double bond. This process has witnessed continuous growth since its invention in the 1930’s: production capacity reached 6.6×10
6 tons in 1995. It is important because the resulting aldehydes are easily converted into many secondary products. For example, the resulting aldehydes are hydrogenated to
alcohols which are converted to
detergents. Hydroformylation is also used in specialty chemicals, relevant to the
organic synthesis of
fragrances and natural products. The development of hydroformylation, which originated within the German coal-based industry, is considered one of the premier achievements of 20th century industrial
chemistry.
The process typically is accomplished by treatment of an
alkene with high pressures (between 10 to 100
atmospheres) of
carbon monoxide and hydrogen at temperatures between 40 and 200 °C. Transition metal
catalysts are required.
Catalysts
The original catalyst was
HCo(CO)4, discovered by
Otto Roelen. Subsequent work demonstrated that the ligand
tributylphosphine (PBu
3) improved the selectivity of the cobalt-catalysed process. Since the 1970’s, most hydroformylation relies on catalysts based on
rhodium.
Subsequent research led to the development of water-soluble catalysts that facilitate the separation of the products from the catalyst.
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)(PPh
3)
2. Such species bind alkenes, and the resulting complex undergoes a
migatory insertion reaction to form a alkyl complex.
Selectivity
A key consideration of hydroformylation is the "normal" vs. "iso" selectivity. For example, the hydroformylation of
propylene can afford two
isomeric products,
butyraldehyde or
isobutyraldehyde:
» H
2 + CO + CH
3CH=CH
2 → CH
3CH
2CH
2CHO ("normal")
vs.
» H
2 + CO + CH
3CH=CH
2 → (CH
3)
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
hydrogen molecule. One of these
hydrides then takes part in a
reductive elimination to form the molecule of the
aldehyde and the complex [HCo(CO)
3].
It is important to note that the rate of migatory insertion of the
carbonyl into the
carbon-
metal bond of the alkyl is fast; in systems where the migatory insertion doesn't occur (such as nickel hydride tris
triphenyl phosphite) the reaction of the hydride with the alkene is reversible. This results in the isomerisation of the alkene, in this way oct-2-ene could be converted into a mixture of both oct-1-ene and oct-2-ene by a
beta hydride elimination from the alkyl. In the system below the rate of insertion of the carbonyl carbon into the C-M bond is likely to be greater than the rate of beta-hydride elimination. If the converse was true then some n-C
8H
17CHO would have been formed. Hydroformylation of 2-octene: the
rhodium catalyst is coordinated to
acac and
carbon monoxide and encapsulated in a
molecular self-assembly process by
zinc tetraphenylporphyrin or Zn-tpp and the
pyridine analogue of
triphenylphosphine. In this process very much like the way
enzymes work encapsulation of the catalytic site explains the observed
regioselectivity:
Asymmetric hydroformylation
Hydroformylation of internal alkenes creates new
stereocenters. Using
chiral phosphine
ligands, the hydroformylation can be tailored to favor one
enantiomer.
Other substrates
Cobalt carbonyl and rhodium complexes catalyse the hydroformylation of
formaldehyde and
ethylene oxide to give 2-hydroxy
acetaldehyde and 3-hydroxy
propanaldehyde, which can then be hydrogenated to
ethylene glycol and
1,3-propanediol, 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, 2-pentanone can arise from ethylene and CO, in the absence of hydrogen. A proposed intermediate is the ethylene-propionyl species [CH
3C(O)Co(CO)
3(ethylene)] which undergoes a
migratory insertion to form [CH
3COCH
2CH
2Co(CO)
3]. The required hydrogen arises from the
water shift reaction. For details see
If the water shift reaction isn't operative, the reaction affords a polymer containing alternating carbon monoxide and ethylene units. Such polymers are more conventionally prepared using
palladium catalysts..
Further Information
Get more info on 'Hydroformylation'.
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