Angela Casini Flashcards
Bullet point the influence of substituents on the ground state multiplicity of NHCs
- Electronic (Inductive and mesomeric)
- Steric effects
Inductive effect
- Can be understood in terms of electronegativity of the atom bonded to the carbene atom.
- The more electronegative the atom, the lower the sigma orbital (i.e increase in singlet/triplet gap).
Mesomeric effect
- Can be understood in terms of Pi-acidity/basicity of the substituent adjacent to the carbene
- Pi electrons from the substituent to the empty pi orbital on the carbene increase the energy of the Ppi orbital.
- As no effect on the sigma, the singlet/triplet gap increase
Steric effects
- Bulky substituents favour the triplet state by opening the X-C-X angle. The closer to 180degrees the more degenerate the two empty orbitals.
- However in NHCs the bond angle is constrained to give a bent singlet
Describe the nature of the following carbenes:
N-C-N
P-C-Si
B-C-B
Push-Push/Singlet/bent
Push-Pull/Singlet/linear
Pull-Pull/Singlet/linear
Four factors that contribute to the stability of NHCs
Nitrogen heteroatoms: -Sigma electron withdrawing -Pi electron donating -inductive and mesomeric stabilisation Backbone: -Electronic stablisation from backbone -Substituents effect carbene electronics N-Substituents -Kinetic stabilisation from steric bulk -Electronic influence Ring size: -cyclic structure favours bent singlet ground state
Synthetic methods of NHCs
-Deprotonation of the azolium salt
Firstly imidazolium salt can be built up and then deprotonation of the carbene precursor salt
-Dechalcogenation
Reductive desulfurisation of thioureas with molten potassium in boiling THF
-Pyrolysis
Removal of neutral volatile by-products by vacuum pyrolysis
Routes to NHC metal complexes
- Insertion of a metal into the C=C bond of NHC dimer
- Use of protected forms of NHCs
- Preformed, isolated free carbene
- In situ deprotonation of azolium salt with base
- Deprotonation with metal complex containing basic ligand
- Transmetallation from a silver-NHC complex
- Oxidative activation of the C2-X
Cyclometallation
-Cyclometallation refers to the transition metal-mediated
activation of a C-R bond to form a metallacycle comprising a new metal-carbon σ bond
-reaction consists of two consecutive steps: initial coordination of the metal centre via a donor group and subsequent intramolecular activation of the C-R bond, which closes the metallacycle.
List three factors for successful cyclometallation
- Metal Precursor
- Donor site to E
- Type of C-R bond to be activated
Metal precursor requirements for transmetallation
Must provide a coordination site for the heteroatom
- Dimeric and polymeric complexes (readily cleaved into monomeric complexes in presence of donor
- Precursors possessing lossely bound ligands (H2)
- Contains anionic ligands that are easily replace by neutral ligands
- Contains ligands to assist the C-R activation (e.g. acetate ligand coordinates to Hydrogen, weakening the C-H bond)
Influence of the donor group in transmetallation
- Hard soft acid base principle (draw scheme)
- Often another donor group may substitute weakly bound ligands
- Too strong and too stable bonding stops ligand dissociation from complex, while too weak shifts equilibrium back to the starting materials
- For amines, generally, latge N-substituents weaken metal coordination substantially, and primary amines bind to strong
- For softer donors (phosphorus and sulfur), metal coordination is well known for bulky substituents. steric effects ensure M-P bond does not get to strong
Influence of the C-R bond activation
- 5 membered metacycles are preferred
- C-SiMe3 is a directing group and activation is favoured over C-H
- Sp2 bond activation favoured over Sp3
Role of NHCs is cross coupling reaction
- Electron donating ability of the NHC aids oxidative addition
- Bulky NHC aid reductive elimination
What orientation do gold(III) complexes prefer
square planor
