COMMUNICATION
Table 1. 4,5-Dimethylimidazolium salts (1) obtained according to
Scheme 1.
ably affect the donor capacity of the ligands. Moreover,
steric interactions of the substituents with the tertiary
groups (buttressing effect) would determine spatial arrange-
ment in the vicinity of the C2 nucleophilic centre.[15]
A new, general and modular approach for the efficient
synthesis of hitherto unknown imidazolium cations 1 with
tertiary groups at the nitrogen atoms and substituents at po-
sitions 4 and 5 is reported herein. Preparation, unprecedent-
ed properties and reactivity of the archetype of the new,
highly sterically encumbered, yet electron-donating imida-
Product tR1
Methylating
agent
tR2
Yield[a]
[%]
[b]
1a·BF4
CMe3
CMe3
CMe2CH2CMe3 MeOTf
1d·OTf 1-Ad[c]
MeOTf
N
CMe3
42 (80)
[b]
1b·BF4
CMe2CH2CMe3 34 (66)
CMe2CH2CMe3 44 (75)
1c·OTf
1-Ad[c]
48 (69)
[a] Overall yields and yields starting from the corresponding formamides
ꢀ
are given in brackets. [b] Obtained by an exchange of MeOSO3 with aq
NaBF4. [c] 1-Ad=1-Adamantyl.
zol-2-ylidenes,
1,3-di-tert-butyl-4,5-dimethylimidazol-2-yli-
dene (2a), are discussed.
The requisite imidazolium cations are routinely produced
by condensation of formaldehyde or its equivalents with 1,2-
clean and the progress can be conveniently monitored by
GC or H NMR spectroscopy.
The substitution of the OMe group (operation 4) is ac-
companied by fast 5-exo-dig cyclisation, whereas the inter-
mediate 4-methylene imidazolium cations (Scheme 1) isom-
1
1
[2b]
ꢀ
diimines [RN=C(R ) ]2 under acidic conditions. However,
an attempted TiCl4-induced condensation of butan-2,3-dione
with tert-butylamine did not produce the desired 1,2-diimine
building block.[16] Moreover, no imidazolium precursors for
the compounds of type II (see Figure 1, R’¼H) have been
reported by recently developed general and efficient meth-
ods.[17] Our own initial attempts to extend the earlier general
method[18] to the preparation of 1,3-di(adamantan-1-yl)-4,5-
dimethylimidazole-2-thione, another plausible precursor of
type II imidazol-2-ylidenes, proved futile.
AHCTUNGTREGeNNNU rise to the final products 1 only slowly, despite the gain in
aromatic stabilisation.[20]
The method could easily be extended to imidazolium salts
with different substituents at positions 4 and 5 by a modular
assembly of the propargylamine moiety. This procedure
would start from the easily available N-tert-alkyl aldoni-
trones,[21] as exemplified in Scheme 2.
Clearly, steric crowding owing to two tertiary nitrogen
atoms poses a significant challenge for assembly of the 4,5-
disubstituted imidazole core. A stepwise assembly of steri-
cally unhindered building blocks is thus likely to be advanta-
geous.
Gratifyingly, this new approach culminated in the devel-
opment of the general and efficient synthesis of the unpre-
cedented subfamily of 1,3-di-tert-alkyl-4,5-dimethylimidazo-
lium salts 1a–d starting from rac-but-3-yn-2-yl methanesul-
fonate[19] (Scheme 1 and Table 1).
Scheme 2. Synthesis of dissimilar substituted imidazolium salts 1e,f.
[22]
ꢁ
a) PhC CLi, THF/hexane, ꢀ788C to RT; b) CS2, RT; for 2–4 see the
legend of Scheme 1; 5) MeSO2OH added.
The solid-state structures of the salts 1a·OTf (Figure 2)
and 1b·I[23] were characterised by single-crystal X-ray analy-
sis.[24]
The most notable feature of the highly symmetrical X-ray
structure of cation 1a is the almost equidistant arrangement
ꢀ
of the apical methyl substituents of the N CMe3 groups and
the methyl substituents in the imidazolium ring, evidently
due to the steric interactions known as the allylic 1,3-
strain.[25] This buttressing effect forces the equatorial Me
Scheme 1. A new stepwise method for synthesis of the imidazolium salts
1. 1) tR1NH2 in MeCN or DMF; 2) AcOCH=O in THF, RT; 3) MeOTf in
ClCH2CH2Cl, ꢀ208C to RT or (MeO)2SO2 in MeCN, 558C; 4) tR2NH2,
RT; 5) heating at 508C.
ꢀ
substituents of the N CMe3 groups to remain almost per-
fectly in the plane of the imidazolium ring thus efficiently
ꢀ
shielding the C2 H bond.
Chemical shifts of the C2 H signal in the H NMR spec-
1
ꢀ
This new three-step method (operations 3–5 can be per-
formed in one pot) features excellent reproducibility and
modularity. It can easily be scaled up as no chromatographic
purification is required in any step. All the acyclic inter-
mediates, including the methoxy-substituted immonium
salts, are stable, storable compounds. The reactions are
tra of 1a·Br and 1a·Cl (d=8.75 and 8.98 ppm, respectively,
in CDCl3) were found significantly further upfield in com-
parison to published values of similar N,N’-dialkylimidazoli-
um salts.[23,26] This is probably indicative of very attenuated
interactions of the acidic hydrogen of the imidazolium salt
with anions and solvents;[27] these interactions are strongly
Chem. Eur. J. 2011, 17, 96 – 100
ꢂ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
97