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Stability and Reactions of N-heterocyclic Carbenes

Özlem Urcan,F. Richard Kurth,

Jonas Renner,University of Utah

Introduction and History

[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.

N

NPh

Ph

HCCl3

2

N

NPh

Ph

-CHCl32

N

NPh

PhN

NPh

PhWanzlick-equlibrium

N

N

N

NAr

Ar

Ar

ArN

N

N

NAr'

Ar'

Ar'

Ar'N

N

N

NAr

Ar

Ar'

Ar'

N

NH

N

N

No dimerisation

2 2N

N

N

NNaH

Cl-H2-NaCl

1,3-di-1-adamantylimidazolium chloride deprotonated with sodium hydrideSynthesis of first stable carbene (steric effects hinder dimerization)

Equilibrium could not be confirmed by cross studies with two different aromatic substituents

Carbenes are two-coordinate carbon compounds with a sextet of electrons on the carbon. Most of them theindicate instability and high reactivity. Stable carbenes lead to new possibilities and fields.

Imidazol-carbene synthesized by WanzlickCould only be isolated as a dimerReversible dimerization of carbene (Wanzlick equilibrium)

Stability

[4] Herrmann,W. A.; Köcher, C. Angew. Chem. 1997, 2256-2282. [5] Herrmann, W. A.;Elison, M.; Fischer, J.; Köcher,C.; Artus; Georg, R. J. Chem. Eur. J. 1996, 2, 772-780. [6] Amyes, T. L.; Diver, S. T.; Richard, J. P.; Rivas, F. M.; Toth, K. J. Am. Chem. Soc. 2004, 126 (13), 4366–4374.

p-electron-donation s-electron-withdrawing

N

NR

RN

NR

R

High stability is based on the mesomeric and inductive effect of nitrogen.strong p-donor and weak s-acceptor

HOMO

LUMO

Backbone DB expands the p-system electronic effect on p-orbital on the carbon increases

Electronic NHC-Carbenes favor singlet state (lower energy compared to triplet state)

The empty p-orbital is stabilized due to -donation from the nitrogen (The LUMO-energy increases)

Properties and Thermodynamics

[7] Massey, R. S.; Collett, C. J.; Lindsay, A. G.; Smith, A. D.; O'Donoghue, A. C. J. Am. Chem. Soc. 2012, 134 (50), 20421–20432. [8] Liu, Y.; Lindner, P. E.; Lemal, D. M. J. Am. Chem. Soc.. 1999, 121 (45), 10626–10627. [9] Böhm, V. P. W.; Herrmann, W. A. Angew. Chem. Int. Ed. 2000, 39 (22), 4036–4038.

Thermodynamics of dissociation: Bis-(benzimidazol-2-ylidene):

ΔH°=13.7 ± 0.6 ΔS°=30.4 ± 1.7

carbene observed in situ by NMR

N

N

N

NEt

Et

Et

Et

2N

NEt

Et

Acidity:Basicicity of NHC’s, ranking the pKa’s of the conjugate acids

Transition Metal Complexes

[10] New J Chem. 1998, 77; Chem. Ber. 199, 130, 1253. [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, 1055. [15] J. Organomet. Chem. 1999, 18, 2145; Organometallics. 1999, 17, 972. [16] Organometallics. 2003, 22, 907.

Sample applications

[17] J. Organomet. Chem. 1977, 137, 293; Transition Metal Chemistry. Verlag Chemie, Weinheim, 1981. [18] Angew. Chem. 1995, 107, 2602. [19] Angew. Chem. Int. Ed. Eng. 1995, 34, 2371. [20] J. Mol. Catal. A. 1997, 118, L1. [21] J. Organomet. Chem. 1998, 358, 185. [22] Angew. Chem. Int. Ed. Eng. 1997, 36, 2162.

Hydrosylation of Ketones

Heck Olefination

Furan Synthesis

Olefin Metathesis

Hydrogenation of Olefines

Questions

[23] Massey, R. S.; Collett, C. J.; Lindsay, A. G.; Smith, A. D.; O'Donoghue, A. C. J. Am. Chem. Soc. 2012, 134 (50), 20421–20432. [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.

• 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.

N

N

N

NEt

Et

Et

Et

NNN

Ph

H

NN

HMeO OMeNN

N

Ph

H

Ph

PhNN

HN N

H

N N RR N N RRN N RR

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Contributed by: Özlem Urcan, F. Richard Kurth, Jonas Renner (Undergraduates)

University of Utah, 2016