L10: Catalysis of alkene hydroformylation

Question 1

Oxo process

Answer 1

• Formation of aldehydes by addition of CO and H₂ to an alkene
• Thermodynamically favourable
• Huge scale use in industry (biggest use of homogenous catalysis)
• Selectivity to linear vs branched products is key

Question 2

Uses of hydroformylation process (Oxo)

Answer 2

• Key product is n-butanal, iso-butanal also produced but less desirable
• Surfactants
• Biodegradable detergents (lin alkyl w/ SO₃- grp)
• Plasticisers (di-pthalate esters)

Question 3

Hydroformylation: which step controls n:iso selectivity? How could this be improved?

Answer 3

• Largely the alky migration step
• Favours migration of linear alkyl over branched on steric grounds
• TS for migration of a linear alkyl group is less sterically crowded
• The selectivity could be improved by making the metal centre more crowded; replace CO with a bulky phosphine ligand as in other hydroformylation processes

Question 4

Phosphine modified catalyst: give the catalyst precursor, consequences for conditions and selectivity

Answer 4

• HCo(CO)₃ PR₃
• Stronger Co-CO bonds, lower CO/H₂ pressures (30-50 atm) needed to prevent decomposition of Co metal and higher temperatures can be used
• Selectivity of ~8:1 (increased steric crowding in alkyl migration step)

Question 5

Differences for phosphine-modified catalyst hydroformylation (besides selectivity)

Answer 5

• Modified catalyst is 5-10x less active..
  CO dissociation and initial alkene binding is less facile
  alkyl migration step (alkyl to acyl) is slower and becomes the new rds
• Oxidative addition of H₂ is faster with HCo(CO)₃PR₃ as it is a more active hydrogenation catalyst
• As a result, alkene hydrogenation becomes a significant sie reaction (10-15%)
• primary alcohols not n-aldehydes are the major final product

Question 6

HRh(CO)₄ as a hydroformylation catalyst

Answer 6

• around 10x more active than Cobalt based analogue
• Selectivity only 1:1 so commercially useless

Question 7

HRh(CO)(PPh₃)₃ w/ xs PPh₃ as a hydroformylation catalyst

Answer 7

• > 20 mol equiv of PPh₃ required
• ~15:1 selectivity
• > 100 times more active than Co catalysts
• Lower temperatures and lower CO/H₂ pressures needed
• Negligible production of alkane or alcohol by-products
• Long lived and stable catalyst
• Steps of mechanism same as Cobalt analogue
• Expensive

Question 8

Why is an excess of PPh₃ required in Rhodium-based hydroformylation?

Answer 8

• Ensures n:iso selectivity remains high
• Without it, there would be further loss of phosphine from active 16e- catalyst
• Makes reaction more active but less selective
• It is essential to have 2 phosphine ligands coordinated to the active Rh centre as their steric bulk ensures the high n:iso selectivity

Question 9

Use of chelating phosphine ligands in Rh-catalysed hydroformylation

Answer 9

• chelating bis(phopshine) ligand: typically results in catalysts with much lower rates and selectivity
• BISBI: gives an active and extremely selective catalyst, >30:1 with double the activity of a normal rhodium based system. Forms a -membered chelate ring with Rh centre, 120 degree bond angle may be important for taking up equatorial sites

Question 10

Aqueous-phase Rh-catalysed hydroformylation

Answer 10

• Water soluble, TPPTS
• Used commercially for hydroformylation of propene in a liquid/liquid biphasic system
• Catalyst is easily separated from organic due to aq phase
• Rate slower due to low alkene conc. in aq phase; only short chains sufficiently water soluble for viability