1. Stability and Reactions of N-heterocyclic Carbenes
Özlem Urcan,
F. Richard Kurth,
Jonas Renner,
University of Utah

2. Introduction and History
Carbenes are two-coordinate carbon compounds with a sextet of electrons on the carbon. Most of them the
indicate instability and high reactivity. Stable carbenes lead to new possibilities and fields.
Imidazol-carbene synthesized by Wanzlick
Could only be isolated as a dimer
Reversible dimerization of carbene
(Wanzlick equilibrium)
Equilibrium could not be confirmed by cross studies with two different aromatic substituents
1,3-di-1-adamantylimidazolium chloride deprotonated with sodium hydride
Synthesis of first stable carbene
(steric effects hinder dimerization)
[1] Wanzlick, H.-W.; Schikora, E. Angew. Chem. Int. Ed. 1960, 72 (14), 494. [2] Lemal, D. M.; Lovald, R. A.; Kawano, K. I. J. Am. Chem. Soc. 1964, 86 (12), 2518–2519. [3] Arduengo, A. J.; Harlow, R. L.; Kline, M. J. Am. Chem. Soc. 1991, 113 (1), 361–363.

3. Stability
High stability is based on the mesomeric and inductive effect of nitrogen.
strong p-donor and weak s-acceptor
LUMO
Backbone DB expands the p-system
electronic effect on p-orbital on the carbon increases
HOMO
p-electron-donation
s-electron-withdrawing
The empty p-orbital is stabilized due to π-donation from the nitrogen (The LUMO-energy increases)
Electronic NHC-Carbenes favor singlet state (lower energy compared to triplet state)
[4] Herrmann,W. A.; Köcher, C. Angew. Chem. 1997, [5] Herrmann, W. A.;Elison, M.; Fischer, J.; Köcher,C.; Artus; Georg, R. J. Chem. Eur. J. 1996, 2, [6] Amyes, T. L.; Diver, S. T.; Richard, J. P.; Rivas, F. M.; Toth, K. J. Am. Chem. Soc. 2004, 126 (13), 4366–4374.

4. Properties and Thermodynamics
Acidity:
Basicicity of NHC’s, ranking the pKa’s of the conjugate acids
Thermodynamics of dissociation:
Bis-(benzimidazol-2-ylidene):
ΔH°=13.7 ± 0.6 𝑘𝑐𝑎𝑙 𝑚𝑜𝑙
ΔS°=30.4 ± 1.7 𝑐𝑎𝑙 𝑚𝑜𝑙∗𝐾
carbene observed in situ by NMR
[7] Massey, R. S.; Collett, C. J.; Lindsay, A. G.; Smith, A. D.; O'Donoghue, A. C. J. Am. Chem. Soc. 2012, 134 (50), 20421– [8] Liu, Y.; Lindner, P. E.; Lemal, D. M. J. Am. Chem. Soc , 121 (45), 10626– [9] Böhm, V. P. W.; Herrmann, W. A. Angew. Chem. Int. Ed. 2000, 39 (22), 4036–4038.

5. Transition Metal Complexes
[10] New J Chem. 1998, 77; Chem. Ber. 199, 130, [11] Angew. Chem. 1995, 107, 2602; J. Organomet. Chem. 1998, 557, 93; J. Organomet. Chem. 1997, 532, 261; Chem. Ber. 1972, 105, 529; J. Organomet. Chem. 1968, 12. [12] Organometallics. 1999, 18, 3228; J. Organomet. Chem. 1988, 358, 185. [13] J. Organomet. Chem. 1998, 358, 185. [14] J. Organomet. Chem. 1996, 15, [15] J. Organomet. Chem. 1999, 18, 2145; Organometallics. 1999, 17, 972. [16] Organometallics. 2003, 22, 907.

6. Sample applications Hydrosylation of Ketones Hydrogenation of Olefines
Heck Olefination
Furan Synthesis
Olefin Metathesis
[17] J. Organomet. Chem. 1977, 137, 293; Transition Metal Chemistry. Verlag Chemie, Weinheim, [18] Angew. Chem. 1995, 107, [19] Angew. Chem. Int. Ed. Eng. 1995, 34, [20] J. Mol. Catal. A. 1997, 118, L1. [21] J. Organomet. Chem. 1998, 358, 185. [22] Angew. Chem. Int. Ed. Eng. 1997, 36, 2162.

7. Questions
What is the Gibbs free energy difference for the Wanzlick equilibrium for the Bis-(benzimidazol-2-ylidene) below at room temperature (25 °C)?
What is Keq?
Rank the compounds for their acidity (low to high pKa):
Why are NHC-ligands useful in organometallic C-C coupling reactions? (Stille, Suzuki, Kumada, Heck, Sonogashira)? Which step in the catalytic cycle is supported by NHC-ligands and why?
Rank the compounds for their stability (strongest to weakest). Explain briefly.
[23] Massey, R. S.; Collett, C. J.; Lindsay, A. G.; Smith, A. D.; O'Donoghue, A. C. J. Am. Chem. Soc. 2012, 134 (50), 20421– [24] Higgins, E. M.; Sherwood, J. A.; Lindsay, A. G.; Armstrong, J.; Massey, R. S.; Alder, R. W.; O'Donoghue, A. C. Chemical communications (Cambridge, England). 2011, 47 (5), 1559–1561.

8. Contributed by:
Özlem Urcan, F. Richard Kurth, Jonas Renner (Undergraduates)
University of Utah, 2016