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 carbon atom. Fischer carbenes (B) have a heteroatom substituent on the alpha carbon atom.
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 twoelectron donor ligands, their ligation being similar to CO. We can draw a resonance structure as follows: back donation donation
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.
In Fischer carbenes, sigma type electron donation of the filled methylene lone pair (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. Schrock carbenes can be viewed as the coupling of a triplet state metal and triplet carbene. Both Fischer and Schrock are REACTIVE LIGANDS Th i t f Li d d i f t i tl tl t Thus in terms of Ligand design for tuning metal catalyst properties, not well suited = CARBENE as ANCILLARY LIGAND
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 R 2 N NR 2 the general formula (R 2 N) 2 C:, where the 'R's are various functional groups. 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 salts pka 21 24) 2
Thus in terms of Ligand design for tuning metal catalyst properties= CARBENE as ANCILLARY LIGAND Persistent, Arduengo or Diamino Carbenes Imidazol 2 ylidenes lidenes were the first to be isolated are the most stable tbl and the most well studied and understood family of persistent carbenes. Triazol 5 ylidenes are the second best class
Thus in terms of Ligand design for tuning metal catalyst properties= CARBENE as ANCILLARY LIGAND Persistent, Arduengo or Diamino Carbenes However, there areexceptions: 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
13 C NMR spectra are very distinctive: Fischer carbene with oxygen donors on the carbene: Fischer carbene with nitrogen donors on the carbene: N heterocyclic carbenes (free): ) 290ppm to 365ppm )) 185ppm to 280ppm ) 210ppm to 250ppm Sh Schrock carbenes: )) 240ppm to 330ppm Additionally, 1 H NMR spectra can be very distinctive when hydrogen is bound to the carbene. Chemical shifts will be seen between 10ppm and 25ppm.
NHC : properties Why are they advantageous compared to Phosphines???
: able to stabilize high valent Metal complexes
: able to stabilize high valent Metal complexes: application in Olefin Metathesis Grubbs: 1 st generation TON Chauvin TOF TON 2 nd generation Nobel Prize in Chemistry 2005 TOF TON
: able to participate in oxidation reactions, without being oxidized! Oppenhauer type alcohol loxidation Oxidative Olefin Cleavage Yamaguchi hiorganometallics, 2004, 1490 Crabtree Organometallics, 2003, 1110
: Key ligands for previously unprecedented/sluggish transformations
: In stereoselective Synthesis!
: In stereoselective Synthesis!
: In stereoselective Synthesis!
: In stereoselective Synthesis!
: In stereoselective Synthesis!