User:Benjah-bmm27/degree/3/PGP

Organometallic Chemistry and Homogeneous Catalysis 2, PGP

Feedstocks

• Synthesis gas

→ aldehydes from alkenes, by Rh catalysis (hydroformylation)

→ methanol by Cu catalysis

→ alkanes and alkenes by Fe catalysis (Fischer-Tropsch)

• • Methanol

→ alkanes, alkenes, aromatics by zeolite catalysis

→ formaldehyde and ethers, e.g. MTBE

→ acetic acid by Ir catalysis (Cativa process)

Hydroformylation

Hydroformylation is formally the addition of a molecule of formaldehyde across the C=C double bond of an alkene.

R₂C=CR₂ + H–CHO → R₂CH–CR₂CHO

Formaldehyde isn't actually used. Synthesis gas (a mixture of hydrogen and carbon monoxide, H₂ + CO) is used instead.

Regioselectivity

Hydroformylation of monosubstituted alkenes (vinyl alkanes, if you like: RHC=CH₂) can give two possible regioisomers: linear (n) and branched (iso)

• • The regioselectivity of a particular hydroformylation reaction is expressed as the ratio of linear to branched, i.e. n : iso
  • Example: hydroformylation of propene can yield both butyraldehyde (linear) and isobutyraldehyde (branched)
  • Butyraldehyde is a precursor for two very important molecules
    • hydrogenation of butyraldehyde gives n-butanol, an important industrial solvent (4 Mt/year, industry prefers it to EtOH, which is more of a fire risk and is heavily taxed)
    • aldol reaction followed by hydrogenation gives 2-ethylhexanol (2Mt/year, used to make bis(2-ethylhexyl) phthalate, the most important phthalate plasticizer for polymers)

Cobalt carbonyl catalysis of hydroformylation

• Cobalt added as either dicobalt octacarbonyl, Co₂(CO)₈, or as a cobalt(II) salt
• Synthesis gas is a good reducing agent, and rapidly reduces Co(II) to Co₂(CO)₈
• Co₂(CO)₈ + H₂ → 2HCo(CO)₄
• HCo(CO)₄ is cobalt tetracarbonyl hydride
  • Co(I), d⁸, 18 e, trigonal bipyramidal molecular geometry
  • volatile
  • unstable above 20 °C unless an atmosphere of CO is present
• Co catalysis requires high pressures of CO
• Mechanism begins with the precatalyst HCo(CO)₄

1. Dissociation of a CO ligand generates HCo(CO)₃: Co(I), d⁸, 16 e, vacant coordination site
2. Coordination of alkene generates HCo(CO)₃(alkene): Co(I), d⁸, 18 e, coordinatively saturated
3. Migration of hydride from Co to alkene C generates Co(CO)₃(alkyl): Co(I), d⁸, 16 e, coordinatively unsaturated
4. Coordination of a new CO ligand to Co gives Co(CO)₄(alkyl): Co(I), d⁸, 18 e, coordinatively saturated
5. Migratory insertion of alkyl (formally R⁻) to one of the CO ligands gives Co(CO)₃(acyl): Co(I), d⁸, 16 e, coordinatively unsaturated
6. Oxidative addition of H₂ generates H₂Co(CO)₃(acyl): Co(III), d⁶, 18 e, coordinatively saturated, octahedral molecular geometry
7. Reductive elimination of acyl hydride (aldehyde!) regenerates HCo(CO)₃

Acetic acid synthesis by methanol carbonylation

• MeOH + CO → MeCO₂H
• Catalysed by {Co, Rh, Ir}-iodine species
  • BASF, 1960: Co-I
  • Monsanto process, 1970: Rh-I
  • Cativa process, 2000: Ir-Ru-I

MMA synthesis

• Methyl methacrylate for poly(methyl methacrylate) production