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MS101
LIGAND DESIGN
C.Nevado
CARBENES
There are two main classes of carbene ligands Alkylidene (or Schrock carbene) ligands (A) have one or two alkyl or aryl substituents on the alpha l b
h l h
carbon atom. Fischer carbenes (B) have a heteroatom substituent on the alpha carbon atom.
MS101
LIGAND DESIGN
C.Nevado
CARBENES
Fischer carbenes are typically found on electron‐rich, low oxidation state metal complexes containing pi acceptor ligands These are electrophilic at the alpha carbon count as neutral two
containing pi‐acceptor ligands. These are electrophilic at the alpha‐carbon, count as neutral two‐
electron donor ligands, their ligation being similar to CO. We can draw a resonance structure as follows:
‐back‐donation
‐donation
MS101
LIGAND DESIGN
C.Nevado
CARBENES
Schrock carbenes are typically found on high oxidation state metal complexes. This polarizes the metal‐carbon double bond so that a partial negative charge can be assigned to the alpha carbon. Hence, Schrock alkylidenes tend to be nucleophilic at the alpha carbon. The reaction below is less likely for late‐transition metals because of the lower oxophilicity of the metal.
Schrock carbene complexes are usually electron deficient or contain strongly electron donating ligands. MS101
LIGAND DESIGN
CARBENES
In Fischer carbenes, sigma‐type electron donation of the filled methylene lone pair (singlet carbene) to an empty d‐orbital
(singlet carbene) to an empty d
orbital is is
observed alongside pi electron back donation of a filled d‐orbital to the empty p‐
orbital on carbon.
Schrock carbenes can be viewed as the coupling of a triplet state metal and triplet
triplet state metal and triplet carbene.
Both Fischer and Schrock are REACTIVE LIGANDS
Thus in terms of Ligand design for tuning metal catalyst
Th
i t
f Li d d i f t i
t l t l t
properties, not well suited = CARBENE as ANCILLARY LIGAND
C.Nevado
MS101
C.Nevado
LIGAND DESIGN
CARBENES
Thus in terms of Ligand design for tuning metal catalyst properties= CARBENE as ANCILLARY LIGAND
Persistent, Arduengo or Diamino‐Carbenes
The best‐known examples are diaminocarbenes
The
best known examples are diaminocarbenes with with
the general formula (R2N)2C:, where the 'R's are various functional groups. R2N
NR2
The groups can be bridged so that the carbon with unfilled orbitals is part of an heterocycle, such as imidazol or triazol (N‐Heterocyclic Carbenes, NHC)
NHCs are most frequently prepared via deprotonation of the corresponding
azolium salts (imidazolium, triazolium, tetrazolium, pyrazolium, benzimidazolium, oxazolium, thiazolium
hi li
salts
l ‐ pKa ‐ 21‐24)
2 2 )
MS101
LIGAND DESIGN
C.Nevado
CARBENES
Thus in terms of Ligand design for tuning metal catalyst properties= CARBENE as ANCILLARY LIGAND
Persistent, Arduengo or Diamino‐Carbenes
Imidazol‐2‐ylidenes were the first to be isolated are the most stable and Imida
ol 2 lidenes
th fi t t b i l t d
th
t t bl
d
the most well studied and understood family of persistent carbenes. Triazol‐5‐ylidenes are the “second‐best” class
MS101
LIGAND DESIGN
C.Nevado
CARBENES
Thus in terms of Ligand design for tuning metal catalyst properties= CARBENE as ANCILLARY LIGAND
Persistent, Arduengo or Diamino‐Carbenes
However there are exceptions:
However, there are exceptions:
1. No need to bridge the Nitrogen atoms
2. No need to use an aromatic precursor
3. No need to have 2 N atoms: other combinations of heteroatoms are also possible
MS101
LIGAND DESIGN
C.Nevado
CARBENES
13C NMR
C NMR
spectra are very distinctive:
spectra are very distinctive:
Fischer carbene with oxygen donors on the carbene: ‐) 290ppm to 365ppm
Fischer carbene with nitrogen donors on the carbene:
Fischer carbene
with nitrogen donors on the carbene:
‐) 185ppm to 280ppm
) 185ppm to 280ppm
N‐heterocyclic carbenes (free):
‐) 210ppm to 250ppm
S h k b
Schrock carbenes:
‐) 240ppm to 330ppm
) 240
t 330
Additionally, 1H NMR spectra can be very distinctive when hydrogen is bound to the carbene. Ch i l hif
Chemical shifts will be seen between 10ppm and 25ppm.
ill b
b
10
d 25
MS101
LIGAND DESIGN
CARBENES
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MS101
LIGAND DESIGN
NHC‐CARBENES: properties
Why are they advantageous compared to Phosphines???
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MS101
LIGAND DESIGN
CARBENES
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MS101
LIGAND DESIGN
CARBENES:
able to stabilize
high valent
Metal complexes
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MS101
C.Nevado
LIGAND DESIGN
CARBENES:
able to stabilize
high valent
Metal complexes: application in Olefin Metathesis
Grubbs:
1st generation
TOF
TON
TON
2nd generation
Chauvin
Nobel Prize in Chemistry 2005
TOF
TON
MS101
LIGAND DESIGN
C.Nevado
CARBENES:
able to participate in oxidation reactions, without being oxidized!
O
Oppenhauer‐type
h
t
alcohol oxidation
l h l id ti
Oxidative Olefin Cleavage
Y
Yamaguchi Organometallics, 2004, 1490
hi O
lli 2004 1490
Crabtree Organometallics, 2003, 1110
MS101
LIGAND DESIGN
CARBENES
CARBENES:
Key ligands for previously unprecedented/sluggish transformations
C.Nevado
MS101
LIGAND DESIGN
CARBENES
CARBENES:
In stereoselective Synthesis!
C.Nevado
MS101
LIGAND DESIGN
CARBENES:
In stereoselective Synthesis!
C.Nevado
MS101
LIGAND DESIGN
CARBENES:
In stereoselective Synthesis!
C.Nevado
MS101
LIGAND DESIGN
CARBENES:
In stereoselective Synthesis!
C.Nevado
MS101
LIGAND DESIGN
CARBENES
CARBENES:
In stereoselective Synthesis!
C.Nevado
MS101
LIGAND DESIGN
CARBENES
C.Nevado