C-C activation Flashcards
Why is C-C activation so difficult?
Because C-C bonds are so strong
Accounting for the reactivity of C-C activation in cyclopropane
Draw MOs
Although the M-C bonds formed are weak (endothermic), the reaction is thermodynamically feasible due to relief of steric strain
Example of non-strained, unactivated C-C bonds undergoing C-C activation
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Carbene complexes
= “metal alkylidenes”
Involve the formal bonding of carbenes to the metal centre to form a M=C double bond
Two types of carbene complexes
- Metal in low oxidation state - bonding is reminiscent to CO (Fischer-type carbenes)
- Metal in high oxidation state (Schrock-type carbenes, = alkylidene)
Fischer carbenes
Low oxidation state metal - generally mid-late TMs (group 8+), electron-rich, 10-18 VE
Ligands on metal are generally pi-acceptors e.g. CO
Heteroatom (or aryl) alpha to carbene carbon to act as electron donor
Carbene carbon electrophilic, prone to nucleophilic attack
Draw orbitals
Schrock carbenes
High oxidation state metal e.g. Zr4+, Ta5+ - generally early TMs, electron-poor, 18 VE
Ligands on metal are generally good sigma/pi donors e.g. Cp, Cl - this allows for good metal to carbene back-donation
No heteroatom alpha to carbene carbon
Carbene carbon generally nucleophilic in nature
13C NMR characterisation of carbenes
Carbene carbon in Fischer carbenes has a more downfield shift because it is more electrophilic (deshielded by heteroatom)
Carbene carbon in Schrock carbenes are more upfield because they are more shielded (nucleophilic character)
What type of metals tend to form M-C multiple bonds (i.e. carbene complexes)?
Those that also form M-M multiple bonds (i.e. middle, heavy d-block)
e.g. Ta, Cr, Mo, W, Re
Less common for Fe, Co, Ni
Synthesis of Fischer carbenes
Generally start with carbonyl complex and nucleophilic alkyl reagent (e.g. organolithium, Grignard), which will attack carbonyl carbon
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Synthesis of Schrock carbenes
Start with pentahalide compound and form pentaalkyl compound via salt metathesis
But… if a bulky alkyl group is used and steric bulk is increased, alpha-H abstraction can occur
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Reactivity of Fischer carbenes
Fischer carbenes react with nucleophiles
Insertions and cyclisation are also possible
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Reactivity of Schrock carbenes
Schrock carbenes react with electrophiles
LnM=CR2 react similarly to phosphorus ylides - react with carbonyl compounds to release an alkene (useful)
Tebbe’s reagent = protected form of Cp2Ti=CH2 (reactive Schrock carbene for installing CH2 groups)
Carbene complexes in catalysis
Used in metathesis as pre-catalysts
e.g. Schrock, Grubbs I and Grubbs II
What is meant by metathesis
Swapping the groups around on alkenes
How is Grubbs II an improvement on Grubbs I
One of the phosphine ligands was replaced with an N-heterocyclic carbene, which stabilises the system through pi-back-donation
Also leads to improved reactivity because the NHC is a strong sigma-donor in a trans position so promotes PCy3 dissociation which is essential for reactivity (need a vacant site on metal for catalysis)
Why are mid/late TMs the most successful metals for synthetic organic transformations?
Because they are tolerant of a wide range of FGs
Early TMs react preferentially with heteroatoms (esp. O e.g. Schrock’s catalyst) RCOOH > ROH > R2CO > RCO2R > C=C
W and Mo are intermediate in reactivity
Rh/Ir are too late and the metathesis is not clean - get ‘overreaction’ of the alkenes
Ru = best, reacts preferentially with C=C bonds over heteroatoms
C=C > RCOOH > ROH > R2CO > RCO2R
Different types of metathesis
Simple metathesis Cross metathesis Ring-closing/ring-opening metathesis Acyclic diene metathesis (ADMET) Ring opening metathesis polymerisation (ROMP)
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Simple metathesis
RHC=CH2 –[cat]–> RHC=CHR + H2C=CH2
i.e. unsymmetrical alkene reacts with itself to give 2 homocoupled products
Cross metathesis
Generally unsymmetrical alkene + symmetric alkene
Ring-closing/ring-opening metathesis
Ring-closing metathesis favoured by dilute solutions (conc. solutions would favour polymerisation), also favoured for specific ring sizes e.g. 5/6 membered
Ring-opening metathesis = need ethene in excess (positive pressure)
Acyclic diene metathesis (ADMET)
Basically polymerisation of a diene, so favoured by conc. solutions (too dilute = ring closing)
Can also avoid RCM by using unattractive ring sizes e.g. 8/9
Ring opening metathesis polymerisation (ROMP)
Ring opening driven by ring strain (ring wants to spring open)
Metathesis catalyst catalyses both the ring opening and the polymerisation
Proposed mechanisms of metathesis
- Pairwise mechanism
- Non-pairwise (Chauvin)
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How can the mechanism of metathesis be proven?
By a double cross-over experiment
If the reaction occurs via a pairwise mechanism, there will be 2 alkenes in the product mixture
If the reaction occurs via a non-pairwise mechanism, there will be 3 alkenes in the the product mixture
However, this is not completely conclusive
If some of the metal catalyst holds onto some alkene, it could allow a 3rd product to form from the pairwise mechanism
Furthermore, one of the alkene products from the pairwise mechanism could react again to give the 3rd product
New double cross-over experiment
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Need to make a product that cannot metathesise e.g. an aromatic product - aromaticity cannot be easily broken
Also need an ethylene product that is inactive with the chosen Mo catalyst
