NHCs

Question 1

Oxidation state of carbon

Answer 1

+2

Question 2

Ground state of NHCs

Answer 2

Singlet
N = EDG so raises the energy of the LUMO, which increases the stability of the singlet state

Question 3

Donor/acceptor properties of NHCs

Answer 3

Strong sigma-donors (stronger than phosphines)
Weak pi-acceptors (weaker than alkylidenes)

Carbene diagram

Question 4

Function of NHCs

Answer 4

Used as spectator ligands to tweak the steric and electronic properties of a complex

Question 5

NHC backbone

Answer 5

Provides electronic stabilisation from aromaticity

Can be substituted with EDG/EWG to affect carbene electronics

Question 6

N-substituents in NHCs

Answer 6

Modify the electronics
Provide steric bulk to influence kinetic stability
Can be substituted with chiral functionalities to make NHC asymmetric

Question 7

NHC ring size

Answer 7

The cyclic structure favours the bent singlet ground state

Increasing the ring size and (therefore increasing the N-C-N angle) would start to favour the triplet state

Question 8

Synthesis of NHCs

Answer 8

Draw

Question 9

Why are free NHCs reactive?

Answer 9

Due to the naked carbene carbon

Stabilised through ligation to a metal centre

Question 10

NHCs in reactions

Answer 10

Can be used to stabilise high oxidation state and high valent metal centres
Draw

Question 11

Reactivity of NHC complexes

Answer 11

Draw

Highly active in cross-coupling, metathesis, oxidation and dehalogenation reactions
The free carbenes (i.e. when not ligated to a TM) are excellent organocatalysts