L9: Catalysis of alkene hydrogenation

Question 1

Why does alkene hydrogenation require catalysis?

Answer 1

• Whilst it is thermodynamically favourable reaction, it doesn’t occur at a finite rate unless catalysed

Question 2

Metal surfaces as alkene hydrogenation catalysts; give examples, why aren’t they used instead of Wilkinson’s catalyst?

Answer 2

• Finely divided metal surfaces: Raney Ni, Pd Black, Pt, Cr etc)
• They still have industrial uses but suffer from relatively poor selectivity (other unsaturated groups in the molecule may be unavoidably hydrogenated, too)
• These groups include C=O, -NO₂ etc.
• Conversely, these functional groups are tolerated by WC

Question 3

Wilkinson’s catalyst: formula, key features, solvation

Answer 3

• (Ph₃P)₃RhCl
• Used to achieve the first catalysis of alkene hydrogenation ar room temperature and pressure
• Rh(I) is square planar, d8 and 16e-, sterically congested
• Typically solvated using toluene containing some acetone/EtOH
• This is since the addition of a weakly coordinating solvent doubles rate

Question 4

Common reactivity themes for WC

Answer 4

• Oxidative addition to 18e- complexes of Rh(III)
• Loss of a phosphine and its replacement by a less bulky ligand
• Both may be reversible, depends which ligand is added; O₂ is an example of irreversible, means that it must be excluded when using WC for alkene hydrogenation

Question 5

Examples of potential ligands for promoting loss of phosphine from WC

Answer 5

• C₂H₄ (k~0.5)
• 2-Butyne
• CO
  *w/ CO, Cl(CO)(PPh₃)₂Rh is formed which, under xs triphenylphosphine forms a lewis acid catalyst for alkene hydroformylation

Question 6

Key steps for mechanism of alkene hydrogenation via WC

Answer 6

1. Oxidative addition of H₂
2. Alkene coordination
3. Hydride migration (slow - RDS)
4. Reductive elimination of alkane from cis-alkyl hydride

Question 7

Sterics: effect on WC selectivity and rate of hydrogenation

Answer 7

• Alkene coordination site is cis to 2 large triphenylphosphine ligands; catalysts show selectivity for coordination of less sterically hindered alkenes (alkynes >cis alkenes> trans alkenes)
• Rates of hydrogenation are under steric control

Question 8

Conjugation: effect on rate

Answer 8

• Isolated double bonds are rapidly hydrogenated over conjugated dienes

Question 9

Alkene hydrogenation: hydride migration step, changing rate

Answer 9

• RDS
• modifications to catalyst that increase delta- charcater of H should increase rate
• Aiming to make halide less EQ and use more ED-phosphines..
  1. Rate increases as halide becomes less e- withdrawing (Cl<Br<I)
  2. Rate increases if phosphine made slightly more electron donating (e.g. F-phenyl grp instead of phenyls is less electron donating to Rh so slower than normal WC)
  3. If the phosphine ligand is too electron donating, all catalytic activity is lost i.e. product upon cis addition of H is stable, has no phosphine dissociation, no vacant site for alkene binding

Question 10

Alkene hydrogenation via WC: rate of reductive elimination

Answer 10

• The reductive elimination step is intramolecular
• Fast
• Irreversible

Question 11

Pro-chiral alkenes

Answer 11

• If an alkene has 2 different substituents on one end it is pro-chiral
• The addition of H₂ to just one face generates an optically active product
• Requires a chiral catalyst. Optical activity is expected in the product