Imidazole-Based Mesoionic Carbenes
COMMUNICATION
ed with diethyl ether. Filtration and evaporation of the diethyl ether af-
forded a brown solid, which was dissolved in benzene. Then carbon mon-
oxide was bubbled through the mixture for 45 min. The solvent was
evaporated and the resulting solid was purified by column chromatogra-
phy on silica gel (eluant hexanes/ethyl acetate). Crystallization in chloro-
form/hexanes afforded the desired complexes 5b–j as yellow solids.
are comparable to those observed by varying the substituent
at the two nitrogen atoms of NHCs, as exemplified by
NHCa–c.[5g] Confirming the NMR data, the IR study shows
that the influence of the C-2 substituent is much weaker
than that of the C-4 one, which can be expected since the
latter is in direct conjugation with the carbene center. How-
ever, note that in the case of NHCs, the substituents at C-
4,5 have a significant influence on the TEP as evidenced by
comparing NHCd[5g] and NHCe.[17,18]
In summary, we have successfully improved the synthesis
of imidazole-based MICs, which allowed the placement of a
series of electro-active substituents in close proximity to the
carbene center. Electron-withdrawing groups stabilize MICs.
Electron-donating substituents do not allow the isolation of
the free carbenes, but the corresponding metal complexes
can be prepared by performing the deprotonation of imida-
zolium salts in the presence of the metal fragment. Accord-
ing to the Tolman electronic parameters, MICs are stronger
electron donors than NHCs, even when they bear electron-
withdrawing substituents. The substituent at C-2 modestly
affects the electronic properties of MICs, but the group in 4-
position does allow a significant tuning, which will be of in-
terest for the catalytic community.
Synthesis of complexes 8a,k,l: THF (3 mL) was added to a solid mixture
of imidazolium salts 3a,k,l, potassium hexamethyldisilazide (2 equiv)
and chloro-(1,5-cyclooctadiene)iridium(I) dimer (0.5 equiv) at À788C.
The mixture was stirred for 30 min at À788C, and then warmed to room
temperature and stirred overnight. Evaporation of the solvent gave a
solid residue, which was extracted with diethyl ether. Filtration and evap-
oration of the diethyl ether afforded a brown solid, which was dissolved
in benzene. The carbon monoxide was bubbled through the mixture for
45 min. The solvent was evaporated and the resulting solid was purified
by column chromatography on silica gel (eluant hexanes/ethyl acetate).
Crystallization in chloroform/hexanes afforded the desired complex as a
yellow solid.
Acknowledgements
We are grateful to the NSF (CHE-0808825) and DOE (DE-FG02–
09ER16069) for financial support of this work. Thanks are also due to B.
Donnadieu for the X-ray analysis of complex 8g.
Keywords: electronic effects · iridium · ligands · mesoionic
carbenes
Experimental Section
[1] For the first stable carbenes, see: a) A. Igau, H. Grꢁtzmacher, A.
b) A. Igau, A. Baceiredo, G. Trinquier, G. Bertrand, Angew. Chem.
[2] For reviews on different types of stable carbenes, see: a) M. Melai-
3865; f) D. Bourissou, O. Guerret, F. P. Gabbaꢃ, G. Bertrand, Chem.
[3] For reviews, see, for example: a) G. C. Vougioukalakis, R. H.
Huang, C. S. Lee, A. Bhattacharyya, W. S. Hwang, I. J. B. Lin, Chem.
Chem. Rev. 2009, 109, 3612–3676; e) M. Poyatos, J. A. Mata, E.
iek, K. Grela, Chem. Rev. 2009, 109, 3708–3742; g) W. A. L. van Ot-
[4] L. Benhamou, E. Chardon, G. Lavigne, S. Bellemin-Laponnaz, V.
[5] For tunable NHCs, see, for example: a) V. Cꢆsar, N. Lugan, G. Lav-
igne, Chem. Eur. J. 2010, 16, 11432–11442; b) U. Siemeling, C.
Farber, C. Bruhn, M. Leibold, D. Selent, W. Baumann, M. v. Hopff-
c) D. M. Khramov, E. L. Rosen, V. M. Lynch, C. W. Bielawski,
General information: All manipulations were performed under an atmos-
phere of dry argon using standard Schlenk or dry box techniques. Sol-
vents were dried by standard methods and distilled under argon.
Synthesis of imidazolium salts 3a–j: A 93:7 2-propanol/acetonitrile solu-
tion of amidine 1 or 1’, 2-bromo-1-arylethanone (1.1 equiv), and potassi-
um bicarbonate (2.4 equiv) was heated under reflux for 5 h. Hot filtra-
tion, followed by slow evaporation of the solvent under reduced pressure
induced the precipitation of products 2a–j. HCl (37% in water, 2 mL)
was added dropwise to a cold suspension (08C) of 2a–j in acetic anhy-
dride (2.2 mL) under vigorous stirring. The mixture was warmed to room
temperature and stirred overnight. Water was added until a white precip-
itate persists. The resulting suspension was extracted with dichlorome-
thane. The combined organic layer were washed with water, dried over
anhydrous magnesium sulfate, filtered, and concentrated under vacuum.
The residue was stirred with diethyl ether (15 mL) for 45 min, and then
filtered. The resulting solid was washed with diethyl ether to afford imi-
dazolium salts 3a–j.
Synthesis of imidazolium salts 3k,l: A solution of the electrophile in hex-
anes (0.5 mmol in 5 mL) was added to a solution of NHC 4 in hexanes at
À788C (0.5 mmol in 5 mL). The resulting suspension was stirred for 1 h
at room temperature. The solid was allowed to decant. Filtration, fol-
lowed by drying under vacuum afforded the desired imidazolium salts
3k,l.
Synthesis of MICs 5b–j: THF (2 mL) was added to a solid mixture of
imidazolium salts 3b–j and potassium hexamethyldisilazide (2 equiv) at
À788C. The mixture was stirred 30 min at À788C, and then warmed to
room temperature and stirred 30 min. Evaporation of the solvent gave a
solid residue, which was extracted with hexanes. Filtration and evapora-
tion of the solvent afforded the free carbene. MIC 5b,c were character-
1
ized by H and 13C spectroscopy but decomposed after a few hours in so-
lution.
Synthesis of complexes 8b–j: THF (1.5 mL) was added to a solid mixture
of MICs 5b–j and chloro-(1,5-cyclooctadiene)iridium(I) dimer (0.5 equiv)
at room temperature. The mixture was stirred overnight at room temper-
ature. Evaporation of the solvent gave a solid residue, which was extract-
Chem. Eur. J. 2011, 17, 8269 – 8272
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8271