C-C activation

Question 1

Why is C-C activation so difficult?

Answer 1

Because C-C bonds are so strong

Question 2

Accounting for the reactivity of C-C activation in cyclopropane

Answer 2

Draw MOs

Although the M-C bonds formed are weak (endothermic), the reaction is thermodynamically feasible due to relief of steric strain

Question 3

Example of non-strained, unactivated C-C bonds undergoing C-C activation

Answer 3

Draw

Question 4

Carbene complexes

Answer 4

= “metal alkylidenes”

Involve the formal bonding of carbenes to the metal centre to form a M=C double bond

Question 5

Two types of carbene complexes

Answer 5

1. Metal in low oxidation state - bonding is reminiscent to CO (Fischer-type carbenes)
2. Metal in high oxidation state (Schrock-type carbenes, = alkylidene)

Question 6

Fischer carbenes

Answer 6

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

Question 7

Schrock carbenes

Answer 7

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

Question 8

13C NMR characterisation of carbenes

Answer 8

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)

Question 9

What type of metals tend to form M-C multiple bonds (i.e. carbene complexes)?

Answer 9

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

Question 10

Synthesis of Fischer carbenes

Answer 10

Generally start with carbonyl complex and nucleophilic alkyl reagent (e.g. organolithium, Grignard), which will attack carbonyl carbon
Draw

Question 11

Synthesis of Schrock carbenes

Answer 11

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
Draw

Question 12

Reactivity of Fischer carbenes

Answer 12

Fischer carbenes react with nucleophiles
Insertions and cyclisation are also possible
Draw examples

Question 13

Reactivity of Schrock carbenes

Answer 13

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)

Question 14

Carbene complexes in catalysis

Answer 14

Used in metathesis as pre-catalysts

e.g. Schrock, Grubbs I and Grubbs II

Question 15

What is meant by metathesis

Answer 15

Swapping the groups around on alkenes

Question 16

How is Grubbs II an improvement on Grubbs I

Answer 16

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)

Question 17

Why are mid/late TMs the most successful metals for synthetic organic transformations?

Answer 17

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

Question 18

Different types of metathesis

Answer 18

Simple metathesis
Cross metathesis
Ring-closing/ring-opening metathesis
Acyclic diene metathesis (ADMET)
Ring opening metathesis polymerisation (ROMP)

See flashcards

Question 19

Simple metathesis

Answer 19

RHC=CH2 –[cat]–> RHC=CHR + H2C=CH2

i.e. unsymmetrical alkene reacts with itself to give 2 homocoupled products

Question 20

Cross metathesis

Answer 20

Generally unsymmetrical alkene + symmetric alkene

Question 21

Ring-closing/ring-opening metathesis

Answer 21

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)

Question 22

Acyclic diene metathesis (ADMET)

Answer 22

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

Question 23

Ring opening metathesis polymerisation (ROMP)

Answer 23

Ring opening driven by ring strain (ring wants to spring open)
Metathesis catalyst catalyses both the ring opening and the polymerisation

Question 24

Proposed mechanisms of metathesis

Answer 24

1. Pairwise mechanism
2. Non-pairwise (Chauvin)

Draw

Question 25

How can the mechanism of metathesis be proven?

Answer 25

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

Question 26

New double cross-over experiment

Answer 26

Draw
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