Angela Casini Flashcards

1
Q

Bullet point the influence of substituents on the ground state multiplicity of NHCs

A
  • Electronic (Inductive and mesomeric)

- Steric effects

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2
Q

Inductive effect

A
  • 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).
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3
Q

Mesomeric effect

A
  • 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
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4
Q

Steric effects

A
  • 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
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5
Q

Describe the nature of the following carbenes:
N-C-N
P-C-Si
B-C-B

A

Push-Push/Singlet/bent
Push-Pull/Singlet/linear
Pull-Pull/Singlet/linear

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6
Q

Four factors that contribute to the stability of NHCs

A
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
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7
Q

Synthetic methods of NHCs

A

-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

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8
Q

Routes to NHC metal complexes

A
  • 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
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9
Q

Cyclometallation

A

-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.

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10
Q

List three factors for successful cyclometallation

A
  • Metal Precursor
  • Donor site to E
  • Type of C-R bond to be activated
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11
Q

Metal precursor requirements for transmetallation

A

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)
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12
Q

Influence of the donor group in transmetallation

A
  • 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
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13
Q

Influence of the C-R bond activation

A
  • 5 membered metacycles are preferred
  • C-SiMe3 is a directing group and activation is favoured over C-H
  • Sp2 bond activation favoured over Sp3
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14
Q

Role of NHCs is cross coupling reaction

A
  • Electron donating ability of the NHC aids oxidative addition
  • Bulky NHC aid reductive elimination
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15
Q

What orientation do gold(III) complexes prefer

A

square planor

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16
Q

Three types of pinsar ligand for gold(III)

A

C^N^C
N^C^N
N^N^C

17
Q

What type of C-H activation do Gold prefer

A

Nitrogen assisted C(aryl)-H using HAuCl4

18
Q

3 methods for cylcoauration

A

Direct cycloauration:

  • N coordination to HAuCl4
  • Heat in a polar solvent

Silver assisted cycloauration:

  • Abraction of chloride forms reactive coordination site
  • Formation of AgCl (precipitate) thermodynamic driving force

Transmetallation:

  • Add Hg(OAc)2 and LiCl
  • Transfer of organic group from mercury to Gold
  • HgCl2 released
19
Q

Why is acetate a good ligand favouring cyclometallation reaction?

A
  • Acts as a base to deprotonate the NHC

- facillitates the approach of the second NHC by forming hydrogen bonds and acting as a template