c.nevado ms101 ligand design - uzhffffffff-9e0a-90d9-ffff-ffffa...alkylidene(or schrock carbene)...
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LIGAND DESIGN
CARBENES
There are two main classes of carbene ligands
Fischer carbenes (B) have a heteroatom substituent on the alpha carbon atom.
Alkylidene (or Schrock carbene) ligands (A) have one or two alkyl or
l b h l haryl substituents on the alpha carbon atom.
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LIGAND DESIGN
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 twocontaining 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
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LIGAND DESIGN
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.
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LIGAND DESIGN
CARBENES
In Fischer carbenes, sigma‐type electron donation of the filled methylene lone pair (singlet carbene) to an empty d‐orbital is
Schrock carbenes can be viewed as the coupling of a triplet state metal and triplet(singlet carbene) to an empty d orbital is
observed alongside pi electron back donation of a filled d‐orbital to the empty p‐orbital on carbon.
triplet state metal and triplet carbene.
Both Fischer and Schrock are REACTIVE LIGANDS
Th i t f Li d d i f t i t l t l tThus in terms of Ligand design for tuning metal catalystproperties, not well suited = CARBENE as ANCILLARY LIGAND
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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 with R2N NR2The best‐known examples are diaminocarbenes 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 correspondingazolium salts (imidazolium, triazolium, tetrazolium, pyrazolium, benzimidazolium, oxazolium, hi li l 2 2 )thiazolium salts ‐ pKa ‐ 21‐24)
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LIGAND DESIGN
CARBENES
Thus in terms of Ligand design for tuning metal catalyst properties= CARBENE as ANCILLARY LIGAND
Imida ol 2 lidenes th fi t t b i l t d th t t bl d
Persistent, Arduengo or Diamino‐Carbenes
Imidazol‐2‐ylidenes were the first to be isolated are the most stable and the most well studied and understood family of persistent carbenes.
Triazol‐5‐ylidenes are the “second‐best” class
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LIGAND DESIGN
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
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LIGAND DESIGN
CARBENES
13C NMR spectra are very distinctive:13C NMR spectra are very distinctive:
Fischer carbene with oxygen donors on the carbene: ‐) 290ppm to 365ppm
Fischer carbene with nitrogen donors on the carbene: ) 185ppm to 280ppmFischer carbene with nitrogen donors on the carbene: ‐) 185ppm to 280ppm
N‐heterocyclic carbenes (free): ‐) 210ppm to 250ppm
S h k b ) 240 t 330Schrock carbenes: ‐) 240ppm to 330ppm
Additionally, 1H NMR spectra can be very distinctive when hydrogen is bound to the carbene. Ch i l hif ill b b 10 d 25Chemical shifts will be seen between 10ppm and 25ppm.
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LIGAND DESIGNCARBENES
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LIGAND DESIGN
NHC‐CARBENES: properties
Why are they advantageous compared to Phosphines???
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LIGAND DESIGN
CARBENES
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LIGAND DESIGN
CARBENES:able to stabilizehigh valentMetal complexes
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LIGAND DESIGN
CARBENES:able to stabilizehigh valentMetal complexes: application in Olefin Metathesis
Grubbs:1st generation
TON
2nd generationChauvin
TOF TON
Nobel Prize in Chemistry 2005 TOF TON
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LIGAND DESIGN
CARBENES:able to participate in oxidation reactions, without being oxidized!
O h t l h l id tiOppenhauer‐type alcohol oxidation
Y hi O lli 2004 1490Yamaguchi Organometallics, 2004, 1490Oxidative Olefin Cleavage
Crabtree Organometallics, 2003, 1110
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LIGAND DESIGN
CARBENESCARBENES:Key ligands for previously unprecedented/sluggish transformations
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LIGAND DESIGN
CARBENESCARBENES:In stereoselective Synthesis!
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LIGAND DESIGN
CARBENES:In stereoselective Synthesis!
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LIGAND DESIGN
CARBENES:In stereoselective Synthesis!
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LIGAND DESIGN
CARBENES:In stereoselective Synthesis!
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LIGAND DESIGN
CARBENESCARBENES:In stereoselective Synthesis!
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LIGAND DESIGN
CARBENES