Alkene Metathesis Flashcards
Where does the reactivity of carbenes come from?
The Pz orbital
(see the similarity to a ketone - similar to a attack on the antibonding orbital)
What impacts the strength of the M-C bond in a carbene?
- If it is a late transition metal: Rh, Cu, it has a relatively week interaction between M-C - very reactive
- If group 6-8 it is a stronger bond more representing M=C bond - higher stability
- A heteroatom which has LPs available will stabilise the carbene
What is metathesis?
Metal-catalysed redistribution of alkylidene groups (=CR₂) in alkenes
What is the basic product of cross-metathesis?
What is the basic product of this ring-closing metathesis
- Opposite reaction is the ring-opening metathesis
What is the basic product of this ring-opening metathesis polymerisation (ROMP)
Alkene metathesis are (almost) thermoneutral equilibria, shifting to desired direction achieved by removal of volatile components
What does this mean for designing these reactions
This requires modification of the reagents to drive the reaction to completion
i.e. desiging a product which is a gas, which therefore will be removed from the system, shifting equilibrium
This cycle is the mechanism of cross metathesis of an internal alkene with ethene
Fill in the intermediates
1) Internal alkene coordinates to alkylidene
2) [2+2] cycloaddition to form metallacyclobutane
3) Cycloreversion gives a alkylidene and a new 1-alkene
4) The 1-alkene is displaced by ethene
5) [2+2] cycloaddition gives new four-membered ring from the alkylidene and ethene
6) Release of the second 1-alkene and formation of the starting complex
This was the first Schrock catalyst made in 1990
Indentify the core properties of it?
- Highly reactive, hence poor tolerance of functional groupd in the substrate
- Catalytic activity increases with increasing electron-withdrawing nature of R
- Metathesis of tri- and tetra-substituted olefins is possible
This was Grubbs 1st Generation catalyst (1995)
Identify the core properties of it?
(note this is the most general/easy accessible routine catalyst)
- Stable towards air + moisture (due to metal) with an increased temperature stability
- Selectivity towards unhindered olefins + strained olefins
- Tri and tetra-substituted olefins not attacked
- (it is massively stabilised by the carbene)
How would you hypothetically form a carbene from the following species
What allows this carbene to be so stable?
What is the most common deactivation pathway for this Grubbs 1st gen precatalyst
- (note high reactive/unstable, meaning if it doesn’t capture an alkene substrate it decomposes)
- but it can be stabilised under ethylene atmopshere + fluorinated solvents
This cycle is for ring closing metathesis (RCM)
1) The α,ω-diene coordinates to the alkylidene
2) [2+2] cycloaddition forms metallacyclobutane
3) Cycloreversion forming ethene and a new alkylidene
4) Ethene dissociates from complex giving vacant coordination site for coordination of the second double bond of the diene - driving force
5) Intramolecular [2+2] cycloaddition forming a new, four-membered ring
6) The cycloalkene is released, reforming the starting complex
Small and medium rings: the Z-isomer will be perferred (due to ring strain energy differences for cis- and trans-isomers in these rings)
Despite the large entropic barrier to forming large rings, how can we select for it?
- By using directing groups to help ring closure
- For example than ether or carbonyl
- In macrocycles, the E-isomer is the thermodynamic product as E-isomers are more stable compared to the Z-isomer
The thermodynamic product (E or Z) will depend on…
… the structure
Fill in the blanks for this alkene metathesis cycle for a smaller ring
Why is metathesis scientifically important?
Metathesis can considerable improve selectivity to a desired biologically active steroisomer or enantiomer in total synthesis
It allows for the required steroselectivity
In ring opening metathesis polymerisation (ROMP) the new alkene that is generated remains attached to the catalyst as part of a growing polymer chain
What is the driving force of this reaction?
- Driving force is the relief of ring strain
- Cyclic alkenes must have sufficient ring strain (>5 kcal/mol) to make ROMP possible
- (Alkenes such as cyclohexene or benzene have little or no ring strain and cannot be polymerised as it is around 0 kcal mol⁻¹)
Describe the initation of the ROMP reaction
Note: we want the precursor to be an 8-12C ring
- Coordination of a transition-metal alkylidene to a cyclic olefin
- [2+2]-cycloaddition forms a four-membered metallacyclobutane intermediate which is the beginning of a growing polymer chain
- This intermediate undergoes a cycloreversion reaction to afford a new metal alkylidene
Describe the Propagation of the ROMP reaction?
- Although the resulting complex has increased in size, its reactivity towards cyclic olefins is similar to the initiator
- Analogous steps are repeated during the propagation stage until polymerisation stops
Describe the termination of a ROMP reaction?
- Living ROM reactions are commonly terminated deliberately through the addition of a specialised reagent
- The function of this reagent is too:
- Selectively remove and deactivate the transition metal from the end of the growing polymer chain, and
- Install a known functional group in place of the metal
How can we direct ring closing metathesis?
- By using a ligand to steer in the substrate (i.e. directing group)
- Usually a weak ligand as it will never be remove from Ru/Rh
- Usually NR₃
