Alkene Hydrogenation Reactions Flashcards
How strong is a H-H bond, as seen in H₂?
- Around 100kcal mol⁻¹
- Same as an unactivated C-H bond (pretty strong)
There are two types of mechanism that hydrogenation of an alkene can go by
What are they….
- Dihydride
- Monohydride
What are the key fundamental steps of alkene hydrogenation by monohydrides
1) substrate coordination
2) cis ligand migration
3) σ-bond metathesis
The catalyst shown for hydrogenation of an alkene is called Wilkinson’s catalyst
Why is it quite slow?
- The reaction is inhibited by added PPh₃ and other π-donor ligands
- Two mechanisms operate, with the slower one altering the order of addition of the H₂ and alkene (olefin mechanism)
- The rate is very slow with sterically hindered alkenes
What happens if we dissolve Wilkionson’s catalyst in benezen/ethanol solvent (S)
- Dissociation of PPh₃ (too big) gives a low equilirbium concentration of a three-coordinate intermediate which is stabilised by solvent coordination
- (the S stands for a free space take up by the solvent)
What happens once we dissolve Wilkionson’s castalyst in solvent during industrial hydrogenation?
- The intermediate undergoes oxidative addition of H₂ >10⁴ times faster than RhCl(PPh₃)₃ itself
- This reaction industrially is undertaken at pressure (high conc H₂) which drives this oxidative addition
- (note the PPh₃ in equatorial region)
What is the trans effect and how does it relate to this complex?
- The trans effect is a concept in coordination chemistry that refers to the influence to the influence that a ligand in a square planar/octehedral complex has on the ligand positions trans (opposite) it
- Hydride is a ligand which desire a lot of electron density from the metal, which weakens the solvent bond trans to it
Once we have formed this complex during industrial hydrogenation of an alkene, what happens next?
- Alkene coordination
- Note the trans relationship now beween hydride and alkene - both bonds are going to be relatively weak
Once we have formed this complex during industrial hydrogenation of an alkene, what happens next?
- Migratory insertion (cis-ligand migration)
- Rate determining step
Once we have formed this complex during industrial hydrogenation of an alkene, what happens next?
- Reductive elimination of the alkene and regeneration of the catalyst
Within this cycle of industrial hydrogenation of an alkene, why does the rate of reaction decrease with excess PPh₃?
Supports initial dissociation of PPh₃
Within this cycle of industrial hydrogenation of an alkene, why do strong π acids (acceptor) poison the catalyst?
- Strongly binds to the Rh(I) centre)
- (e.g. CO, CN)
What does this tell us about where the hydrogens come from in alkene hydrogenation?
- Minimal scrambling of H/D products using H₂/D₂ mixtures
- Formation of a dihydrido species that transfers both H⁻ ligands to the alkene and that the final reductive elimination is fast
The mechansim of alkene hydrogenation predicts syn addition of the hydrogen to the double bond. This can be confirmed by deuterium labelling studies
What is the benefit of this
- Some hydrogenation catalyst have strong directing effects which makes them useful in organic synthesis
- For example, H₂ only adds to one side on the alkene when there is a coordinating group present on that face (e.g. OH, COMe, OMe)
- Results in a single steroisomers
RhCl(PPh₃)₃ has some really large ligands which generate steric clashes when we try to coordinate to the alkene
What does this mean in terms of hydrogenation of different alkenes?
You can generally hydrogenate mono- and disubstitued alkenes
But likely not tri- or tetra-substituted alkene
(could use heterogenous catalysis of Pd on carbon support to reduce all C=C bonds - brings a degree of selectivity in reduction)
- C=O and C=N are only reduced by certain homogenous catalysts. CN, NO₂, Ph and CO₂Me are not usually reduced by homogenous catalysts (heterogenous catalysis can)
- Instead the following substrate-controlled hydrogenation can be used
- What are the two things which are special about this catalyst?
- The availability of a second coordination site on the catalust allows the coordination of a donor in addition to the alkene
- “Directing group” binding is important for product selevtivity
- (Very similar to Wilkinson catalyst)
What is the structure of the intermediate formed during this selective alkene hydrogenation with Ir catalyst
- The metal complex can do this because of its Lewis acidicity (due to the overall positive charge)
- Using OH as a directing group (temporary ligand to bind the alkene to the metal)
Draw the intermediates for the following reaction (general)
Substitued alkenes are prochiral, why?
The π-faces which can be approached from opposite sides and delivery of H₂ to one side or the other results in enantiomeric products
What type of catalyst is used for asymmetric hydrogenation?
- Catalyst types for asymmetric hydrogenation with the general form: [RHL₂S₂]⁺
- where s = polar solvent molecule (e.g. THF, MeCN
- L₂ = chiral diphosphine (pure) which generates stereochemical control
- Generally catalyst is generated in situ from (COD)RhL₂⁺
What are the intermediates for asymmetric hydrogenation
- Note the alkene this time binds first, this is due to the control of pressure and sticky R groups on the alkene which is better for making chiral drugs
- 1 - Formation of active species, 2 - Binding of alkene, 3 - Oxidative addition of H₂, 4 - Migratory insertion, 5 - Reductive elimination - release of product
How can diasteroisomeric complexes occur from a complex with a chiral phosphine combined with an incoming prochiral alkene
- the prochiral alkene can bind in one of two ways (as show)
- If you add some chirality into the phosphines, it will result in them being diasteroisomeric complexes
Why does the minor diastereoisomer in solution lead to the major product in this asymmetric synthesis
- Ketone is a poor donor so will easily dissociate, therefore can flip easily between the two steroisomers
- The minor diasteroisomer is 580-fold more reactive towards H₂ oxidative addition, which offset the lower conc in solution. Resulting in a 60:1 product ratio for R-enantiomer
The rarest type, e.g. Co(CN)₅³⁻, d⁷, 17e-
Co(II) is a metal centred radical and will only reduce activated alkenes
How does it work?
- Aqueous/aqueous alcoholic solution of cobalt (II) cyanide rapidly absorbs hydrogen to give a species capable of catalysing the hydrogenation of an alkene
- Works to replace the Rh in catalyst (not much Rh left)
For alkene hydrogenation to work, Rhodium need to have…
- Need to be a cation (e.g. Rh(I) )
- (Or need to have an anionic ligand)
- It need two free coordination site
