Alkene Hydroformylation Flashcards
Why is it difficult to hydrogenation CO using a Rh catalyst
- Because CO is a very good ligand
- Therefore it is a fight between high much CO vs Hydrogen pressure is put in
What is the product of this reaction, as well as its unique properties
The product is a acid, better than HNO₃ or HCl etc
What is the alkene hydroformylation oxo reaction?
- Is a catalytic reaction in which an alkene reacts with carbon monoxide and hydrogen, in the presence of a transition metal catalyst (usually Co/Rh) to form an aldehyde
- Adds ‘H’ and ‘CHO’ units across the alkene
What is the valance electron count of Co₂(CO)₈ and why is its structure benefical for catalysis?
- 36e- = each Co has 18e-
- Forced to have a weak Co-Co single bond which is really weak
- Therefore there is energy payback to cleave the hydrogen
What is this first step of catalysis using cobalt for hydroformylation
- The product is an extremely strong acid
What is the second step of hydroformylation using a cobalt catalyst
- Need to loose a CO to generate a free coordination site
- The free coordination site is where the alkene will bind - alkene coordination
What happens in hydroformylation of an alkene using a cobalt catalyst, once the alkene has coordinated
- metal donates electron density in the π-antibonding orbital of the alkene, which breaks the pi bond
- Hydrogen forms a new bond at the now avaliable site on the alkane - cis-ligand migration/migratory insertion
What happens in the hydroformylation of an alkene using a cobalt catalyst, once migratory insertion has occured
- Migratory insertion of a CO onto the alkane
- (Reverse is suppressed by CO pressure)
What happens in the last step of hydroformylation of an alkene using a cobalt catalyst, once the second migratory insertion has occured?
- Hydrogen is added onto the carbonyl carbon, which results in an aldehyde product
- Results in the product and get the metal hydride back
What does this rate equation tell us about the following catalytic cycle?
- That the rate determining step is this one (has to involve H₂)
- AND the rate is inversely proportional to CO concentration because CO dissocation from the coordinately saturated 18e- species is required
This is the last step of alkene hydroformylation using a cobalt catalyst
Where do the hydrogens add onto the alkene
- CO is lost when H₂ is added because Co is coordinately saturated (18e-)
- H₂ is added in an oxidative addition - one adds onto alkene and one stays on the catalyst
- Alkene is formed in a reductive elimination
Why would we use a 1:1 ratio of H₂/CO to do this cycle
(rate of reaction is independent of pressure)
- HCo(CO)₄ is only stable under certain minimum CO partial pressures at a given temperature
- CO pressure ↑ = reaction rate ↓ and high ratio of linear to branched product
- CO pressure ↓ = reaction rate ↑ and branched alkyl increases (reverse β-elimination)
What are the two possible product of this alkene hydroformylation?
- The linear product is favoured due to steric reasons - reduced steric congestion the further away that R group is
- The linear one the usually the desired product too
What is the Classical Markovnikov rule?
- The ‘H’ attaches to the carbon of the double bound that is least substituted
- Driven by carbocation selectivity - not relvant to transition metal catalysis
Show the transition state for the linear product
Show the transition state for the branched product
The linear products of alkene hydroformylation are usually desired, suggest a way we can increase the proportion of them?
The addition of bulky phosphines can increase stability of catalyst, and increases proportion of linear aldehyde, e.g. PBu₃
(PBu₃ has a big enough cone angle which drives the linear product with increased steric bulk at the metal but won’t fall off)
Suggest two problematic issues that cobalt catalyst suffers from
- Suffers from poor stability
- Difficulty in separation from the product (need for a distillation column)
What are the benefits are using a Rhodium catalyst alternative for alkene hydroformylation using Cobalt
- Rhodium catalysts are much more active, allowing milder conditions (100°, up to 50 atm)
- Greater than 95% straight chain product with (RhHCO(PPh₃)₃ as catalyst
Union Carbide Hydroformylation is the commerical process as increased process efficiency offsets the increase castalyst cost (price of rhodium is higher than cobalt)
What is the first step where we activate the catalyst
- Going from 18e- to 16e-
- (note the d⁸ square planar geometry)
What happens in the Union Carbide Hydroformylation process once an active catalyst is formed
(this Rh(III) is more stable than the Co(III) equivalent, as the further down the periodic table the more stable the higher oxidation states are)
Alkene coordination
What happens in the Union Carbide Hydroformylation process once the alkene has coordinated
- Cis-ligand migration/migratory insertion
What happens in the Union Carbide Hydroformylation process once cis-ligand migration has occured
What happens in the Union Carbide Hydroformylation process once CO has bound to the metal complex
A second cis-ligand migration onto the alkyl
What happens in the Union Carbide Hydroformylation process once the second cis-ligand migration has occured?
Oxidative addition of dihydrogen
What happens in the Union Carbide Hydroformylation process once oxidative addition of dihydrogen has occured
- A third cis-ligand migration onto the alkyl, resulting in it leaving
There are times when using the Union Carbide Hydroformylation process we the branched product is desired
How is this achieved?
- Big ligands usually drive the linear forms
- To get a branched form, there is a need to balance sterics and electronics (i.e. choosing a metal which is more Lewis acidic from pi-backdonation from ligands)
E.g. Bis phosphine ligands systems developed to generate more n-selective or active catalysts
If we want to be able to determine what reaction species are present under typical operating conditions: it is difficult for heterogenous BUT possible for homogenous
What investigation methods can be used?
- IR
- UV/Vis
- NMR
- XRD
- MS
The following is a IR taken to measure alkene hydroformylation using rhodium catalyst
- The two bands of the carbonyl starting material beginning to disappear
- ‘7’ new bands appear showing the generation of new species (shown by 2 of all of them in equilibrium)
- A band for the pre-castalyst disappearing then reappearing
The following NMR is taken to measure alkene hydroformylation using rhodium catalyst
How does each spectra relate to each part of the cycle?
