Metal-NHC Complexes
N H N [HCr(CO)5] - H2 N N Cr(CO)5
Ph N 2 H N Ph ClO4 Hg(OAc)2 - 2 AcOH
Ph N Hg N Ph
Ph N 2 ClO4N Ph
Currently used as ligands for transition metal catalysts
– Pd catalyzed reactions – Metathesis – Hydrosilylation
10
Herrmann, W. Angew. Chem. Int. Ed. 2002, 41, 1290
Isolation of a Stable Carbene
H
N H + NaH N+ Cl-
HCl
Ph
N Cl H
N
Ph
1st N-heterocyclic carbene (NHC) synthesized by extrusion of chloroform from the imidazolidene adduct
9
Wanzlick, H. W. Angew. Chem. Int. Ed. 1962, 1, 75.
8
Wanzlick, H. W.; Schonher, H. J. Angew. Chem. Int. Ed., 1968, 7, 141.
The First N-Heterocyclic Carbene
Ph N H N CCl3 Ph - CHCl3 N N N Ph N Ph N Ph Ph N Ph Ph
H THF cat. DMSO
N + H2 (g) + NaCl (s) N
H
H
1991 – Arduengo isolates first stable carbene Stable in absence of O2 and H2O Does not decompose upon melting (240°C)
Arduengo, A. J.; Rasika Dias, H. V.; Dizon, D. A.; Harlow, R. L.; Klooster, W. T.; Koetzle, T. F. J. Am. Chem. Soc. 1994, 116, 6812.
14
Evidence for Carbenes
11
Arduengo, A. J.; Harlow, R. L.; Kline, M. J. Am. Chem. Soc., 1991, 113, 361
Stable Carbenes
N Cl m.p. = 153 - 155 °C θN-C2-N = 101.7° N N Cl m.p. = 109 - 110 °C θN-C2-N = 101.5° N
Arduengo, A. J.; Goerlich, J. R.; Marshall, W. J.; J. Am. Chem. Soc. 1995, 117, 11207. Enders, D.; Breuer, K.; Raabe, G.; Runsink, J.; Teles, J. H.; melder, J. P.; Ebel, K.; Brode, S. Angew. Chem. Int. Ed. 1995, 34, 1021. Arduengo, A. J.; Goerlich, J. R.; Marshall, W. J.; Leibigs Ann. 1997, 365.
CH
Friedman, L.; Shechter, H. J. Am. Chem. Soc., 1960, 82, 1002
5
Singlet vs. Triplet Reactivity
Triplet carbenes show radical reactivity
– Triplet ground state favored when substituents σ electron donors
15
Evidence for Ylides
CH2
20 kcal/mol (σ- stabilization)
Heterocyclic Carbenes
– Discovery – Structure/Stability – Reactivity
Applications of Stabilized Carbenes in Organocatalysis
– Reactions involving acyl anions – Acylation catalysis
Heterocyclic Carbenes in Biological Systems
– Thiamine catalyzed reactions
7
A Quest for Stable Carbenes
R C R R R R N C N R R N N R
C C R R
C N R or
C O
1960’s – Wanzlick attempts isolation of a stable carbene – Electrophilic nature of carbenes will be decreased if substituents are strong π-donors – Generate nucleophilic, singlet carbenes
[H+]
S
N S
13
Carbenes or Ylides?
R
N
+
N
R
R
N
N
R
R
N
N
+
R
Carbene center stabilized by a “push-pull” effect
– Electronegative nitrogens “pull” electron density away from the carbene center – σ-stabilization – Nitrogen lone pairs can “push” electron density into the empty p-orbital – π-stabilization
Stable Carbenes
First commercially available carbene Ph N N Ph N N N Ph
Decomposes at 150 °C m.p. = 107 - 109 °C θN-C2-N = 104.7 ° θN-C2-N = 100 °
Dipp S N Dipp N Dipp m.p. = 111 - 114 °C θN-C2-N = 104.2°
Heterocyclic Carbenes in Biological Systems
– Thiamine catalyzed reactions
2
Carbenes – A Definition
Two-coordinate carbon compounds that have two nonbonding electrons and no formal charge on carbon
4
Bauschlicher, C. W.; Schaefer, H.F. III; Bagus, P.S.; J. Am. Chem. Soc., 1977, 7106.
Carbenes Insert into Bonds
C=C
Cl C Cl + Cl
Doering, W. E.; Hoffmann, A.K.; J. Am. Chem. Soc. 1954, 76, 6162.
R1 C R1 C H H H H R2 R2
Me
Me
Me
Me
6
Skell, S.; Woodworth, R. C. J. Am. Chem. Soc., 1956, 78, 4496.
Outline
Introduction to Carbenes
– Carbene Structure – Carbene Reactivity
R1 C R1 H H H R2 C H H R1 C H R2 R1 C R2 Me R2
+
H
Me
Me
Me
Me
Me
Me
Me
H
Singlet carbenes show electrophilic/nucleophilic reactivity
– Singlet ground state favored when substituents are π electron donors
Chemistry of Heterocyclic Carbenes
Eric C. Hansen
University of Wisconsin - Madison
Outline
Introduction to Carbenes
– Carbene Structure – Carbene Reactivity
N N
Neutron and X-ray diffraction data suggest low electron density for carbene p-orbital Stability of carbenes is kinetic
Arduengo, A. J.; Rasika Dias, H. V.; Dixon, D. A.; Harlow, R. L.; Klooster, W. T.; Koetzle, T. F. J. Am. Chem. Soc., 1994, 116, 6812