On new N-heterocyclic carbene derived alkylidene imidazolines

Article abstract of DOI:10.1039/b921655c

A series of novel 2-alkylidene imidazolines has been synthesized from imidazolium halides and substituted alkyl halides in the presence of base. The resultant exocyclic enediamines have been characterized and their nucleophilicity has been evaluated upon NMR spectroscopic data and substitution reactions.

Full text of DOI:10.1039/b921655c

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PAPER
www.rsc.org/obc | Organic & Biomolecular Chemistry
On new N-heterocyclic carbene derived alkylidene imidazolines†‡
Christiane E. I. Knappke, Jo¨rg M. Neudo¨rfl and Axel Jacobi von Wangelin*
Received 15th October 2009, Accepted 9th January 2010
First published as an Advance Article on the web 1st February 2010
DOI: 10.1039/b921655c
A series of novel 2-alkylidene imidazolines has been synthesized from imidazolium halides and
substituted alkyl halides in the presence of base. The resultant exocyclic enediamines have been
characterized and their nucleophilicity has been evaluated upon NMR spectroscopic data and
substitution reactions.
We were particularly attracted by the property of imidazolium
Introduction
cations to act as electron-withdrawing auxiliaries. Due to their
electron-deficient aromaticity, imidazolium salts are rather stable
compounds that induce a high acidity of the proton in the 2-
position.10 The preparation of the corresponding N-heterocyclic
carbenes via deprotonation at C2 can be effected with strong
bases.11 Obviously, the electronic pull of the cationic imidazolium
moiety also enhances the acidity of 2-alkyl substituents.
Since the isolation of the first stable N-heterocyclic carbene
(NHC)1 several applications of this interesting class of heterocycles
to organic synthesis and catalysis have been reported.2,3 Over the
past decade, NHCs established themselves as ligands for transition
metal-catalyzed reactions.4 Their potential to act as nucleophilic
catalysts in umpolung reactions was initially harnessed with the
development of carbene-catalyzed benzoin and Stetter reactions.5
These biomimetic reactions are synthetic copies of acylation
reactions catalyzed by thiamine-dependent transketolases.6 The
cofactor thiamine (Vitamin B1) is embedded within the enzyme
center and bears a reactive thiazolium ion as precursor for
a nucleophilic carbene (Scheme 1, left).7 Breslow postulated a
mechanistic model that involves umpolung of an electrophilic
formyl moiety into an acyl anion equivalent.8 Various applications
of this general reactivity pattern have demonstrated the use of
carbenes for umpolung addition reactions. While a vast majority
of research effort has been devoted to carbonyl compounds for the
generation of intermediate nucleophiles via umpolung,9 alternative
reaction modes have received only little attention.
There are only a few examples of selective reactions between
alkyl halides and NHCs. Interestingly, these precedents are limited
to simple saturated alkyl halides.12,15 Intermediate alkylazolium
species are susceptible to deprotonation to afford the correspond-
ing azolylidenes12,13,14a,15 which are competent nucleophiles for fur-
ther alkylation.12,13a The underlying a1-d1 umpolung16 relates to the
formation of Breslow-type intermediates from aldehydes.8 We were
interested to expand the scope of 2-substituted imidazolylidenes
to also include unsaturated substituents that could permit further
manipulation. A vinylogous approach would result in formal a-d-
umpolung at the a- and g-carbons upon reaction of an allyl halide
with an NHC and subsequent deprotonation (Scheme 2). The
latter reaction mode relates to the umpolung of a,b-unsaturated
aldehydes and thus affords the deoxy-analogue of carbene-derived
homoenolates.9 It is important to note that the resultant exocyclic
enediamines constitute an under-utilized class of nucleophiles
with little literature precedents,12,14,17,18 but offer new avenues for
untapped umpolung reactions. This work represents the first
comprehensive study of the synthesis and characterization of a
series of novel alkylidene imidazolines demonstrating a novel entry
into NHC-mediated umpolung chemistry.
Scheme
1
Biological archetype and carbene-derived umpolung
intermediates.
Department of Chemistry, University of Cologne, Greinstr. 4, 50939, Ko¨ln,
Germany. E-mail: axel.jacobi@uni-koeln.de; Fax: +49 221 470 5057;
Tel: +49 221 6122
† This article is part of an Organic & Biomolecular Chemistry web theme
issue which highlights work by participants in the 2009 EuCheMS young
investigators workshop.
‡ Electronic supplementary information (ESI) available: NMR and
UV/Vis spectra, proposed mechanism for reaction of carbene 2 with
epibromohydrin, crystal structures. CCDC reference numbers 751976–
751978. For ESI and crystallographic data in CIF or other electronic
format see DOI: 10.1039/b921655c
Scheme 2 Umpolung of alkyl halides.
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3-trimethylsilylpropargyl bromide, cinnamyl chloride, allyl chlo-
ride, chloroacetone, bromomethyl-cyclopropane, 3,4-epoxybutyl
bromide, ethyl 2-bromomethyl-acrylate, and chloroacetonitrile,
respectively (Scheme 4). Syntheses of 7 and 8 from the organobro-
mides were less selective and led to the formation of oligomeric
by-products which could be suppressed by running the reactions
Results and discussion
NMR studies
Our initial studies were performed in sealed NMR tubes with
1,3-bis-(2,6-diisopropylphenyl)imidazolium chloride (1)19 as NHC
precursor in THF-d8 under anaerobic conditions and monitored
by 1H NMR. Reaction of 1 with equimolar KOtBu resulted
in the complete conversion to carbene 2 after 2 min at room
temperature applying ultra-sonification. Carbene 2 proved to be
stable under these conditions for > 20 days. 1H NMR monitoring
of the subsequent addition of stoichiometric amounts of p-
methylbenzyl bromide exhibited only 50% conversion to 4 but
complete disappearance of carbene 2. The identity of ene-1,1-
diamine 4 was established by NMR and ESI-MS. 4 is presumably
formed by rapid deprotonation of intermediate alkyl imidazolium
3 with carbene 2 as base.§ This observation is a consequence
of the high acidity in the a-position of the 2-alkyl substituent
(stabilization by vicinal azolium and aryl moieties). Generally, the
electron-withdrawing effect of the imidazolium moiety alone is
not sufficient to trigger deprotonation at the exocyclic a-position
by the imidazol-2-ylidene in contrast to analogous reactions
with the more basic benzimidazol-2-ylidenes and imidazolin-2-
ylidenes.12,13a,14 Upon employment of 2 equiv. of base (KOtBu),
the conversion was increased to > 80% (Scheme 3).
◦
at lower temperature (-78 C). Enediamine 9 was also obtained
from 2 and epibromohydrin under identical conditions.20
Scheme 4 Synthesis of enediamines (Ar = 2,6-diisopropylphenyl).
Scheme 3 Reaction of in situ generated carbene 2 with p-methylbenzyl
bromide (Ar = 2,6-diisopropylphenyl, R = p-tolyl).
Reaction progress and carbene consumption were best mon-
1
itored by H NMR resonance shifting of the imidazoline and
peripheral iPr moieties. Fig. 1 shows the 1H spectra of the
synthesis of benzylidene 4 from carbene 2 and p-methylbenzyl
bromide. In comparison with carbene 2, the signals of the
imidazoline ring protons (at C4 and C5) of enediamines 4–13
were shifted significantly up-field (Dd = 0.48–1.05 ppm). The iPr
septets, however, experienced a down-field shift (Dd = 0.07–0.62).
Resonances of other protons were only slightly affected by the
transformations of 2 to enediamines. Compounds 4–8 and 10–13
each showed two iPr septets. This asymmetry arises from a locked
configuration of the exocyclic double bond at room temperature
which entails the presence of a syn and an anti diisopropylphenyl
substituent (NOESY). Ketone 9 gave only one signal set for both
N-substituents, as facile keto-enol tautomerism results in rapid
interconversion of both double bond isomers on the NMR time
scale.21
We then pursued studies into the reactivity of alternative
electrophiles with imidazolium chloride 1 in the presence of two
equiv. of KOtBu in order to extend the scope of this enedi-
amine synthesis. Reaction with p-methylbenzyl chloride under
identical conditions proved much slower. A series of nine further
alkylidene imidazolines (5–13) was synthesized at room temper-
ature (with mostly 50–90% conversion) from prenyl bromide,
§ Cationic species 1 and 3 were poorly soluble in THF under the reaction
1
conditions and not detected by H NMR. 3 was independently prepared
from 4 and HI. Reaction of 1 with equimolar KOtBu and 50 mol%
of p-methylbenzylbromide cleanly gave 4 while no 1H resonances of
carbene 2 could be detected. After addition of another equiv. of base, 1H
NMR signals of carbene 2 appeared. We thus postulate the intermediacy
of imidazolium salt 3 and its deprotonation by 2 (to give 1) when
no additional base is present. UV/Vis spectroscopic monitoring of the
reaction sequence was not enlightening since p-methylbenzyl bromide,
carbene 2, and imidazolium salt 3 showed nearly the same absorption
maxima (230-238 nm), while only enediamine 4 exhibited a strong red-
shifted band at 350 nm (see also Supporting Information‡).
The electron-donating character of the five-membered 6p
electron heterocycle renders the exocyclic alkene moiety of the
synthesized enediamines strongly nucleophilic. The degree of
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Fig. 1 1H NMR monitoring of conversion from 2 to 4 (slight excess of electrophile in upper spectrum).
Table 1 13C and 1H chemical shifts of exocyclic double bonds (d in ppm)
NMR spectroscopy was also utilized to monitor chemical ex-
change phenomena in 4, 7, and 13. EXSY NMR spectra of 4 and 13
displayed a chemical exchange between the two diisopropylphenyl
groups as a result of conjugative stabilization of the zwitterionic
resonance structure and free rotation about the exocyclic double
bond. Compounds 5–12 showed no such behaviour. Synthesis of 7
from trans-cinnamyl chloride was accompanied by the formation
of minor amounts of the cis-isomer (E/Z appr. 2/1). We observed
a facile light-driven isomerization (with a maximum E/Z of 1/1
at daylight). The trans-isomer spontaneously regenerated in the
dark (E/Z 10/1 after 6 h at r.t.) or could be exclusively afforded
from a dark reaction. Electron donation from the imidazoline ring
and benzylic stabilization favour rotation about the styryl double
bond. The regioisomeric product which would arise from an SN¢
attack of the NHC onto cinnamyl chloride was not observed.
Enediamine
NCN
a-CH
a-CH
4
145.8
145.4
152.0
147.1
146.5
151.8
147.1
147.0
148.0
153.5
70.2
67.9
46.2
73.0
72.2
74.1
68.0
3.76
3.43
2.76
3.86
3.67
3.97
1.99
2.33–2.27
3.76
5
6
7
8
9
10
11
12
13
56.9 or 53.6a
66.4
33.2
2.38
a Undetermined assignment.
exocyclic double bond polarization is considered to be most
significant for the evaluation of its nucleophilic reactivity. As a first
approximation, the electronic nature of the double bond correlates
with the 13C NMR resonances. Table 1 displays the chemical
shifts of the quaternary C2 carbons (NCN) and the vicinal sp2-
hybridized a-CH moieties. Consistent with literature observations
on related compounds,12,14,17 the exocyclic double bonds are highly
polarized in 4–13. The transformations of carbene 2 (13C: d =
220.9 ppm) to alkylidene imidazolines 4–13 (13C: d = 145–152)
exert a large shielding effect onto the endocyclic NCN moieties as a
consequence of the umpolung of C2 from the nucleophilic carbene
2 to the electrophilic aminals 4–13. The strong electron-donating
property of the imidazoline ring can also be deduced from the
13C resonances of the conjugated g-position in the alkylidene
substituents vs. the organohalide. The effect is quite significant for
prenyl derivative 5 (13C(g-CH): d = 111.6 ppm (5) vs. 140.3 (prenyl
bromide)), cinnamyl derivative 7 (111.4 (7) vs. 135.0 (cinnamyl
chloride)), and allyl derivative 8 (95.1 (8) vs. 118.4 (allyl bromide))
Trapping reactions
Based upon literature precedents by Kuhn,12,17a,b,22 Bourson,23
Begtrup,13a,24 and Alt25 we set out to probe the nucleophilicity
of enediamines 4 and 5 in selected reactions with electrophiles.
We observed protonation of benzylidene 4 at the exocyclic a-
carbon. Treatment of 4 with non-aqueous hydrogen iodide at
room temperature gave imidazolium iodide 3. (HI was generated
stoichiometrically from reaction of TMSI with the t-butanol
containing reaction mixture. Protonation of 4 with gaseous HCl
=
also afforded compound 3, with X Cl.) The crystal structure of
the resultant square, slightly yellow prisms is depicted in Fig. 2. It
shows the expected planarity of the five-membered heterocycle; the
bond lengths confirm its aromatic delocalization. All three carbon-
based substituents are slightly tilted out of the heterocycle plane.
The aryl substituents are perpendicular to the imidazolium ring,
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Fig. 2 Crystal structure of 3 (iodide is omitted for clarity). Selected
◦
˚
bond lengths [A] and angles [ ]:C2-N1 1.349(2), C2-N3 1.346(2), N1-C5
1.391(3), N3-C4 1.390(2), C4-C5 1.335(3), N1-C6 1.452(2), C2-C18
1.493(3), C18-C19 1.537(3), C2-C18-C19 117.3(2), N1-C2-C18 124.2(2),
N3-C2-C18 128.4(2), C2-N1-C6 126.6(2), C2-N1-C5 109.1(2), C2-N3-C4
109.1(2), N1-C5-C4 107.2(2), N3-C4-C5 107.6(2), N1-C2-N3 107.0(2),
N1-C2-C18-C19 130.3(2), C5-N1-C2-C18 172.4(2), C4-N3-C2-C18 -172.0
(2), C2-N1-C6-C7 -87.8(2), N1-C5-C4-N3 -0.1(2), C5-N1-C2-N3 1.1(2),
N3-C2-N1-C6 -178.6(2).
Scheme 5 Secondary substitution products (Ar = 2,6-diisopropylphenyl).
while the tolyl moiety spans an angle of 74◦ with the heterocycle.
The unit cell contains two molecules with an inversion centre.
2-(p-Methylbenzyl)imidazolium iodide 3 crystallized in the
¯
same space group (P1) as the structurally related 2-iodomethyl-
1,3,4,5-tetramethylimidazolium triiodide.17b
Upon employment of carbon-based electrophiles, new im-
idazolium derivatives with exocyclic a,a-di-substitution were
prepared. Reactions of 2 with methyl iodide, prenyl bromide,
p-methylbenzyl bromide, and p-nitrobenzyl bromide at room
temperature afforded the desired 2-alkyl imidazolium halides 14–
17, respectively (Scheme 5). Prenylidene 5 was methylated in the
a-position by reaction with methyl iodide to afford 18. When
NaH (instead of KOtBu) was employed for the formation of 5,
further methylation in g-position was observed as side reaction
via deprotonation of 18 by excess NaH and subsequent second
methylation to give 2-alkenylimidazolium 19. In the presence of
excess KOtBu, citenzyl imidazolium 16 was converted into its
conjugated base 20. NMR spectra indicated the presence of a
polarized tetra-substituted double bond with a 13C NMR reso-
nance of the a-carbon at d = 80.1 ppm. Analogous deprotonation
of mixed prenyl-benzyl derivative 15 gave enediamine 21 which
showed a similar 13C NMR resonance at d = 79.8 ppm.
Scheme 6 Synthesis and benzylation of N,N-dialkyl enediamine 24.
deprotonation to the corresponding base 24 effected with KOtBu
24 was subjected to further alkylation with methyl iodide and p-
methylbenzyl bromide to give imidazolium 25 (quant.) and 26,
respectively, with quaternary exocyclic a-carbons. Treatment of
25 with base at elevated temperature resulted in the formation
of stilbene 27 with 80% yield by formal elimination of 1-ethyl-3-
methylimidazolium iodide (Scheme 7).
Variation of N-substituents
In order to investigate the influence of the N-substituents of the
NHC on the reactivity, analogous substitution reactions were per-
formed with 1-ethyl-3-methylimidazolium bromide 22 (Scheme 6).
Benzylation of 22 proceeded under identical conditions as shown
for 1. Bibenzyl derivative 23 was synthesized in 55% yield and its
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to carbene via peak integration against TMS. 99.998% argon
from Linde was used for manipulations under inert atmosphere.
IR-ATR spectroscopy was performed on a Perkin-Elmer 100
Paragon FT-IR. ESI-MS were measured with a Finnigan MAT
900S and an Agilent LC/MSD VL G1956A, respectively. Exact
masses (HRMS) were determined by peak matching method. GC-
MS were run on an Agilent 6890 N Network GC system with
Agilent 7683B series injector and Agilent 5975 inert mass-selective
detector; stationary phase HP-5MS (L = 30 m, Ø = 0.25 mm)
from Macherey-Nagel, mobile phase H2, temperature program:
Scheme 7 Stilbene synthesis by base-mediated NHC elimination.
◦
50 ◦C (2 min), 25 C min-1 (10 min), final temperature 300 ◦C
Conclusions
(5 min). UV/Vis absorptions were recorded on an OceanOptics
USB2000 Fiber Optic. Crystal structure data were collected on
a Nonius KappaCCD diffractometer using monochromated Mo-
Ka radiation, and the structures refined by shelxs97 and shelxl97.27
We have developed an operationally simple protocol for the
synthesis of conjugated 2-alkylidene imidazolines from imida-
zolium halides and unsaturated alkyl halides in the presence of
2 equivalents of base. This work constitutes the first systematic
study of the synthesis and stereo-electronic properties of such
enediamines that contain exocyclic alkene, diene, enyne, and
styryl moieties and further unsaturation in the side chain (keto,
cyclopropyl, epoxy, ester, nitrile functions). The general structural
motif bears a close conceptual relationship to the Breslow-type
homoenolates and can be viewed as their deoxy-counterpart.
Detailed NMR experiments document the presence of polar-
ized exocyclic double bonds that render the a- and g-positions
highly nucleophilic as a consequence of an overall base-mediated
umpolung process. Initial model studies proved their ability to
engage in facile substitution reactions with electrophiles (H+,
methyl, prenyl, benzyl) to give access to new imidazolium cations
with branched side chains. Imidazolium salts with quaternary
substitution of the exocyclic a-carbon are subject to base-mediated
carbene elimination to give tri-substituted olefins. Further inves-
tigations into the reactivity of alkylidene imidazolines toward
other electrophiles and the development of umpolung processes
for carbene-mediated carbon-carbon bond-forming reactions are
currently being undertaken.
In situ NMR studies of enediamines
General procedure. Under anaerobic conditions, an NMR
tube was charged with 1,3-bis-(2,6-diisopropyl-phenyl)-
imidazolium chloride (1) (recrystallized from CHCl3 and
azeotropized with CH2Cl2), sealed with a rubber septum, and
transferred to a glove box where KOtBu was added. Alternatively,
KOtBu was dissolved in THF-d8 under anaerobic conditions
and the solution added to 1. Addition of THF-d8 and ultra-
sonification gave a colourless solution after 5 min to which the
electrophile was added.
1,3-Bis-(2,6-diisopropylphenyl)-2-(4-methylbenzylidene)-2,3-
dihydro-1H-imidazole (4). 1 (1.5 mg, 0.0059 mmol) and KOtBu
(0.012 mmol) were dissolved in THF-d8 (0.6 mL with TMS). A
solution of p-methylbenzyl bromide (0.0059 mmol) in THF-d8
was added. The colour of the solution turned yellow and a yellow
precipitate formed. Conversion: 80%. 1H-NMR (600 MHz, THF-
3
3
d8): d = 7.38 (t; 1H, JHH = 7.8 Hz, H-9¢), 7.30 (d; 2H, JHH
=
3
7.8 Hz, H-8¢, H-10¢), 7.25 (t; 1H, JHH = 7.8 Hz, H-9), 7.08 (d;
2H, 3JHH = 7.8 Hz, H-8, H-10), 6.38 (d; 1H, 3JHH = 2.4 Hz, H-4),
3
3
6.31 (d; 1H, JHH = 2.4 Hz, H-5), 6.28 (d; 2H, JHH = 8.2 Hz,
Experimental section
3
H-21, H-23), 5.97 (d; 2H, JHH = 8.2 Hz, H-20, H-24), 3.76 (s;
General
1H, H-18), 3.35 (sep; 2H, 3JHH = 6.7 Hz, H-12, H-15), 3.17 (sep;
2H, 3JHH = 6.7 Hz, H-12¢, H-15¢), 1.95 (s; 3H, H-25), 1.28 (d; 6H,
3JHH = 6.8 Hz, H-13¢, H-16¢), 1.23 (d; 6H, 3JHH = 6.8 Hz, H-14¢,
All chemicals were purchased from commercial suppliers and
used without further purification unless otherwise noted. THF
3
H-17¢), 1.19 (d; 6H, JHH = 6.8 Hz, H-13, H-16), 1.08 (d; 6H,
was distilled over sodium and benzophenone under an argon
3JHH = 6.8 Hz, H-14, H-17); 13C-APT-NMR (151 MHz, THF-d8):
d = 149.5 (d; 2C, C7¢, C11¢), 147.8 (d; 2C, C7, C11), 145.8 (d; 1C,
C2), 137.7 (d; 1C, C6), 136.3 (d; 1C, C19), 135.3 (d; 1C, C6¢), 130.0
(u; 1C, C9¢), 129.4 (u; 1C, C9), 128.4 (d; 1C, C22), 127.8 (u; 2C,
C21, C23), 126.2 (u; 2C, C20, C24), 125.2 (u; 2C, C8¢, C10¢), 124.3
(u; 2C, C8, C10), 118.0 (u; 1C, C5), 116.2 (u; 1C, C4), 70.2 (u; 1C,
C18), 29.4 (u; 2C, C12¢, C15¢), 29.2 (u; 2C, C12, C15), 25.0 (u; 2C,
C13, C16), 24.4 (u; 2C, C13¢, C16¢), 23.9 (u; 2C, C14¢, C17¢), 22.7
(u; 2C, C14, C17), 20.8 (u; 1C, C25); ESI-MS: m/z 493 [M+H+];
UV/Vis lmax(THF)/nm = 350.
1
˚
atmosphere. THF-d8 was dried over molecular sieve (4 A). H and
13C NMR spectra were recorded on a Bruker Avance II 600 (600.20
and 150.94 MHz), a Bruker DRX 500 (500.13 and 125.77 MHz)
and a Bruker Avance 300 (300.13 and 75.48 MHz) at 298 K unless
otherwise stated. TopSpin (Version 1.3, 2.0 and 2.1) was used.
Chemical shifts (d in ppm) are referenced to tetramethylsilane
(TMS) or solvent residual peaks (THF-d8 at dH 3.58, dC 67.4;
CD3OD at dH 3.35, dC 49.3; CDCl3 at dH 7.24, dC 77.0; DMSO-
1
d6 at dH 2.49, dC 39.7). Abbreviations for H-NMR data: s =
singlet, d = doublet, t = triplet, sep = septet, m = multiplet,
“t” = pseudo triplet. Abbreviations for 13C-APT/DEPTQ-NMR26
data: u = up (CH3 or CH), d = down (CH2 or Cq). Peaks
were assigned based on H,H-COSY, H,C-HMQC, H,C-HMBC,
and H,H-NOESY/EXSY (mixing time d8 = 500 ms) correlation
spectra. For atom numbering, please see Schemes 4–6 and Fig. 1.
Conversions of in situ NMR experiments were determined relative
1,3-Bis-(2,6-diisopropylphenyl)-2-(3-methylbut-2-enylid-ene)-2,
3-dihydro-1H-imidazole (5). 1 (12.3 mg, 0.029 mmol) and KOtBu
(6 mg, 0.05 mmol) were dissolved in THF-d8 (0.6 mL). Distilled
1-bromo-3-methylbut-2-ene (3.3 mL, 0.029 mmol) in THF-d8 was
added. The colour of the solution turned yellow and cloudy. 1H-
NMR (600 MHz, THF-d8): d = 7.36–7.33 (m; 2H, H-9, H-9¢),
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7.26 (d; 2H, 3JHH = 7.8 Hz, H-8¢, H-10¢), 7.22 (d; 3JHH = 7.7 Hz,
C26), 122.1 (u; 1C, C24), 118.1 (u; 1C, C5), 116.9 (u; 1C, C4), 111.4
(u; 1C, C20), 73.0 (u; 1C, C18), 29.6 (u; 2C, C12, C15), 29.4 (u;
2C, C12¢, C15¢), 24.3 (u; 4C, CH(CH3)2), 23.9 (u; 2C, CH(CH3)2),
23.5 (u; 2C, CH(CH3)2); ESI-MS: m/z 505 [M+H+].
3
2H, H-8, H-10), 6.30 (d; 1H, JHH = 2.4 Hz, H-4), 6.24 (d, 1H,
3JHH = 2.4, H-5), 4.77 (d; 1H, 3JHH = 12.0 Hz, H-19), 3.43 (d; 1H,
3JHH = 12.0 Hz, H-18), 3.25 (sep; 2H, 3JHH = 6.9 Hz, H-12, H-15),
3
3.15 (sep; 2H, JHH = 6.9 Hz, H-12¢, H-15¢), 1.24–1.16 (m; 30H,
1,3-Bis-(2,6-diisopropylphenyl)-2-(prop-2-enylidene)-2,3-dihydro-
1H-imidazole (8). 1 (10.4 mg, 0.024 mmol) and KOtBu (6 mg,
0.05 mmol) were dissolved in THF-d8 (0.6 mL). Distilled allyl
chloride (6.0 mL, 0.072 mmol) was added whereupon the solution
turned slightly green. 1H-NMR (600 MHz, THF-d8): d =
7.39–7.35 (m; 2H, H-9, H-9¢), 7.27–7.24 (m; 4H, H-8, H-10, H-8¢,
H-10¢), 6.35 (s; 1H, H-4), 6.30 (s; 1H, H-5), 5.29 (m; 1H, H-19),
CH3); 13C-APT-NMR (151 MHz, THF-d8): d = 149.4 (d; 2C, C7¢,
C11¢), 148.8 (d; 2C, C7, C11), 145.4 (d; 1C, C2), 137.3 (d; 1C, C6),
134.9 (d; 1C, C6¢), 129.8 (u; 1C, C9), 129.7 (u; 1C, C9¢), 125.0 (u;
2C, C8 and C10 or C8¢ and C10¢), 124.5 (u; 2C, C8 and C10 or C8¢
and C10¢), 120.9 (u; 1C, C19), 117.3 (u; 1C, C5), 116.0 (u; 1C, C4),
111.6 (d; 1C, C20), 67.9 (u; 1C, C18), 29.4 (u; 2C, C12, C15), 29.3
(u; 2C, C12¢, C15¢), 26.1 (u; 1C, C21), 24.3 (u; 2C, C13 and C16
or C13¢ and C16¢), 24.3 (u; 2C C13 and C16 or C13¢and C16¢),
24.1 (u; 2C, C14¢, C17¢), 23.3 (u; 2C, C14, C17), 17.1 (u; 1C, C22);
ESI-MS: m/z 457 [M+H+].
3
3
3.78 (d; 1H, JHH = 16.0 Hz, cis-H-20), 3.67 (d; 1H, JHH
=
11.8 Hz, H-18), 3.38 (d; 1H, 3JHH = 9.9 Hz, trans-H-20), 3.21 (sep;
3
3
2H, JHH = 6.9 Hz, H-12, H-15), 3.09 (sep; 2H, JHH = 6.9 Hz,
H-12¢, H-15¢), 1.29–1.16 (m; 24H, CH(CH3)2); 13C-APT-NMR
(151 MHz, THF-d8): d = 149.2 (d; 2C, C7¢, C11¢), 148.7 (d; 2C,
C7, C11), 146.5 (d; 1C, C2), 136.9 (d; 1C, C6), 134.7 (d; 1C, C6¢),
132.7 (u; 1C, C19), 130.0 (u; 2C, C9, C9¢), 125.0 (u; 2C, C8 and
C10 or C8¢ and C10¢), 124.8 (u; 2C, C8 and C10 or C8¢ and C10¢),
117.6 (u; 1C, C5), 116.2 (u; 1C, C4), 95.1 (d; 1C, C20), 72.2 (u;
1C, C18), 29.5 (u; 2C, C12, C15), 29.4 (u; 2C, C12¢, C15¢), 24.3 (u;
2C, CH(CH3)2), 23.9 (u; 2C, CH(CH3)2), 23.7 (u; 2C, CH(CH3)2),
23.4 (u; 2C, CH(CH3)2); ESI-MS: m/z 429 [M+H+].
1,3-Bis-(2,6-diisopropylphenyl)-2-(3-trimethylsilylprop-2-ynyli-
dene)-2,3-dihydro-1H-imidazole (6). 1 (10.9 mg, 0.026 mmol) and
KOtBu (6 mg, 0.05 mmol) were dissolved in THF-d8 (0.6 mL).
1-Bromo-3-(trimethylsilyl)-prop-2-yne (3.7 mL, 0.026 mmol) was
added whereupon the solution turned dark purple and cloudy.
3
1H-NMR (600 MHz, THF-d8): d = 7.38 (t; 1H, JHH = 7.9 Hz,
H-9¢), 7.35 (t; 1H, 3JHH = 7.7 Hz, H-9), 7.28 (d; 2H, 3JHH = 7.9 Hz,
H-8¢, H-10¢), 7.16 (d; 2H, 3JHH = 7.7 Hz, H-8, H-10), 6.49 (d; 1H,
3JHH = 2.4 Hz, H-4), 6.45 (d; 1H, 3JHH = 2.4 Hz, H-5), 3.13 (sep;
3
3
1-(1,3-Bis-(2,6-diisopropylphenyl)imidazole-2-ylidene)-propa-
none (9). 1 (11.9 mg, 0.028 mmol) and KOtBu (7 mg, 0.06 mmol)
were dissolved in THF-d8 (0.6 mL with TMS). Epibromohydrin
(2.4 mL, 0.028 mmol) was added whereupon the solution turned
pink then green. A precipitate formed. Conversion (after 4 h): 75%.
1H-NMR (600 MHz, THF-d8): d = 7.33 (t; 2H, 3JHH = 7.8 Hz, H-
9, H-9¢), 7.21 (d; 4H, 3JHH = 7.8 Hz, H-8, H-10, H-8¢, H-10¢), 6.71
(s; 2H, H-4,H-5), 3.97 (s; 1H, H-18), 2.89 (sep; 4H, 3JHH = 6.9 Hz,
2H, JHH = 6.9 Hz, H-12, H-15), 2.99 (sep; 2H, JHH = 7.0 Hz,
H-12¢, H-15¢), 2.76 (s; 1H, H-18), 1.35 (d; 6H, 3JHH = 6.9 Hz, H-
13, H-16), 1.27 (d; 6H, 3JHH = 7.0 Hz, H-13¢, H-16¢), 1.21 (d; 6H,
3
3JHH = 6.9 Hz, H-14, H-17), 1.19 (d; 6H, JHH = 7.0 Hz, H-14¢,
H-17¢), -0.26 (s; 9H, H-21); 13C-APT-NMR (151 MHz, THF-d8):
d = 152.0 (d; 1C, C2), 148.8 (d; 4C, C7, C11, C7¢, C11¢), 135.2
(d; 1C, C6), 133.9 (d; 1C, C6¢), 130.4 (u; 1C, C9 or C9¢), 130.1 (u;
1C, C9 or C9¢), 125.1 (u; 2C, C8¢, C10¢), 123.9 (u; 2C, C8, C10),
118.1 (u; 1C, C5), 116.7 (u; 1C, C4), 108.0 (d; 1C, C19), 90.9 (d;
1C, C20), 46.2 (u; 1C, C18), 29.6 (u; 2C, C12, C15), 29.4 (u; 2C,
C12¢, C15¢), 24.4 (u; 2C, CH(CH3)2), 24.3 (u; 2C, CH(CH3)2), 23.9
(u; 2C, CH(CH3)2), 23.8 (u; 2C, CH(CH3)2), 1.01 (u; 3C, C21);
ESI-MS: m/z 499 [M+H+], 427 [M+H+-TMS].
H-12, H-15, H-12¢, H-15¢), 1.39 (s; 3H, H-20), 1.23 (d; 12H, 3JHH
=
3
6.9 Hz, H-13, H-16, H-13¢, H-16¢), 1.19 (d; 12H, JHH = 6.9 Hz,
H-14, H-17, H-14¢, H-17¢); 13C-APT-NMR (151 MHz, THF-d8):
d = 182.9 (d; 1C, C19), 151.8 (d; 1C, C2), 147.0 (d; 4C, C7, C11,
C7¢, C11¢), 136.0 (d; 2C, C6, C6¢), 129.8 (u; 2C, C9, C9¢), 124.3
(u; 4C, C8, C10, C8¢, C10¢), 119.0 (u; 2C, C4, C5), 74.1 (u; 1C,
C18), 29.6 (u; 4C, C12, C15, C12¢, C15¢), 29.3 (u; 1C, C20), 24.4
(u; 4C, C14, C17, C14¢, C17¢), 23.6 (u; 4C, C13, C16, C13¢, C16¢);
HRMS(ESI): calcd. for C30H41N2O: 445.3219 ([M+H]+); found:
445.322 ( 0.0015).
(E)-1,3-Bis-(2,6-diisopropylphenyl)-2-(3-phenylprop-2-enyli-
dene)-2,3-dihydro-1H-imidazole (7). 1 (11.2 mg, 0.026 mmol)
and KOtBu (0.054 mmol in THF-d8) were mixed in THF-d8
(0.5 mL). Distilled (trans)-cinnamyl chloride (3.6 mL, 0.026 mmol)
1
was added. The colour of the solution turned yellow. H-NMR
3
(600 MHz, THF-d8): d = 7.53 (t; 1H, JHH = 7.8 Hz, H-9), 7.41
2-Cyclopropylmethylidene-1,3-bis-(2,6-diisopropylphenyl)-2,3-
dihydro-1H-imidazole (10). 1 (10.9 mg, 0.026 mmol) and KOtBu
(0.055 mmol) were dissolved in THF-d8 (0.6 mL with TMS).
Bromomethylcyclopropane (2.8 mL, 0.029 mmol) was added. After
3 d, the solution was bright yellow and a white precipitate had
formed. Conversion: >90%. 1H-NMR (600 MHz, THF-d8): d =
7.29–7.26 (m; 2H, H-9, H-9¢), 7.19 (d; 2H, 3JHH = 7.7 Hz, H-8¢, H-
(t; 1H, 3JHH = 7.8 Hz, H-9¢), 7.38 (d; 2H, 3JHH = 7.8 Hz, H-8, H-
10), 7.31 (d; 2H, 3JHH = 7.8 Hz, H-8¢, H-10¢), 6.79 (t; 2H, 3JHH
=
3
7.8 Hz, H-23, H-25), 6.57 (t; 1H, JHH = 7.8 Hz, H-24), 6.52 (d;
1H, 3JHH = 2.4 Hz, H-4), 6.51 (d; 1H, 3JHH = 2.4 Hz, H-5), 6.41 (d;
2H, 3JHH = 7.8 Hz, H-22, H-26), 5.95 (dd; 1H, 3JHH = 14.4 Hz and
12.6 Hz, H-19), 5.16 (d; 1H, 3JHH = 14.4 Hz, H-20), 3.86 (d; 1H,
3JHH = 12.6 Hz, H-18), 3.19 (sep; 2H, 3JHH = 6.6 Hz, H-12, H-15),
10¢), 7.17 (d; 2H, 3JHH = 7.7 Hz, H-8, H-10), 6.13 (d; 1H, 3JHH
=
3.07 (sep; 2H, 3JHH = 6.6 Hz, H-12¢, H-15¢), 1.26 (d; 6H, 3JHH
=
2.4 Hz, H-4), 6.12 (d; 1H, JHH = 2.4 Hz, H-5), 3.44 (sep; 2H,
3JHH = 6.9 Hz, H-12, H-15), 3.20 (sep; 2H, 3JHH = 6.9 Hz, H-12¢,
3
6.6 Hz, CH(CH3)2), 1.24 (d; 6H, 3JHH = 6.6 Hz, CH(CH3)2), 1.22
3
3
3
3
(d; 6H, JHH = 6.6 Hz, CH(CH3)2), 1.21 (d; 6H, JHH = 6.6 Hz,
CH(CH3)2); 13C-DEPTQ-NMR (151 MHz, THF-d8): 149.1 (d;
2C, C7¢, C11¢), 148.3 (d; 2C, C7, C11), 147.1 (d; 1C, C2), 142.7 (d;
1C, C21), 136.6 (d; 1C, C6), 134.2 (d; 1C, C6¢), 130.4 (u; 1C, C9),
130.3 (u; 1C, C9¢), 128.4 (u; 2C, C23, C25), 126.5 (u; 1C, C19),
125.3 (u; 2C, C8, C10), 125.2 (u; 2C, C8¢, C10¢), 123.7 (u; 2C, C22,
H-15¢), 1.99 (d; 1H, JHH = 8.2 Hz, H-18), 1.33 (d; 6H, JHH
=
3
6.9 Hz, H-13, H-16), 1.24 (d; 6H, JHH = 6.9 Hz, H-14, H-17),
3
3
1.23 (d; 6H, JHH = 6.9 Hz, H-13¢, H-16¢), 1.19 (d; 6H, JHH
=
6.9 Hz, H-14¢, H-17¢), 0.32 (m; 1H, H-19), 0.06 (m; 2H, H-20a,
H-21a), -0.44 (m; 2H, H-20b, H-21b); 13C-APT-NMR (151 MHz,
THF-d8): d = 149.7 (d; 2C, C7¢, C11¢), 149.2 (d; 2C, C7, C11),
1700 | Org. Biomol. Chem., 2010, 8, 1695–1705
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3
147.1 (d; 1C, C2), 137.7 (d; 1C, C6), 135.5 (d; 1C, C6¢), 129.4 (u;
1C, C9 or C9¢), 129.2 (u; 1C, C9 or C9¢), 124.9 (u; 2C, C8¢, C10¢),
124.1 (u; 2C, C8, C10), 116.6 (u; 1C, C5), 115.3 (u; 1C, C4), 68.0
(u; 1C, C18), 29.3 (u; 2C, C12, C15), 29.1 (u; 2C, C12¢, C15¢),
24.7 (u; 2C, C13, C16), 24.4 (u; 2C, C13¢, C16¢), 24.0 (u; 2C, C14¢,
C17¢), 23.3 (u; 2C, C14, C17), 9.1 (d; 2C, C20, C21), 7.4 (u; 1C,
C19); ESI-MS: m/z: 443.2 [M+H+].
H-10¢), 7.21 (d; 2H, JHH = 7.8 Hz, H-8, H-10), 6.70 (d; 1H,
3JHH = 2.2 Hz, H-4), 6.68 (d; 1H, 3JHH = 2.2 Hz, H-5), 2.98 (sep;
3
3
2H, JHH = 6.8 Hz, H-12, H-15), 2.88 (sep; 2H, JHH = 6.9 Hz,
3
H-12¢, H-15¢), 2.38 (s; 1H, H-18), 1.34 (d; 6H, JHH = 6.8 Hz,
H-13, H-16), 1.27–1.20 (m; 18H, H13¢, H-16¢, H-14, H-16, H-14¢,
H-16¢); 13C-APT-NMR (151 MHz, THF-d8): d = 153.5 (d; 1C,
C2), 148.4 (d; 2C, C7, C11), 148.4 (d; 2C, C7¢, C11¢), 133.4 (d; 1C,
C6), 133.0 (d; 1C, C6¢), 130.9 (u; 1C, C9 or C9¢), 130.0 (u; 1C, C9
or C9¢), 125.4 (u; 2C, C8¢, C10¢), 124.4 (u; 2C, C8, C10), 120.0 (d;
1C, C19), 118.7 (u; 1C, C5), 117.5 (u; 1C, C4), 33.2 (u; 1C, C18),
29.7 (u; 2C, C12, C15), 29.5 (u; 2C, C12¢, C15¢), 24.5 (u; 2C, C13,
C16), 24.3 (u; 2C, C13¢, C16¢), 23.8 (u; 2C, C14¢, C17¢), 23.5 (u; 2C,
C14, C17); HRMS(ESI): calcd. for C29H38N3: 428.3066 ([M+H]+);
found: 428.306 ( 5 ppm).
1,3-Bis-(2,6-diisopropylphenyl)-2-(2-oxiranylethylidene)-2,3-di-
hydro-1H-imidazole (11). 1 (13.8 mg, 0.032 mmol) and KOtBu
(7.6 mg, 0.068 mmol) were dissolved in THF-d8 (0.6 mL with
TMS). 1-Bromo-3,4-epoxybutane28 (5.6 mL, 0.059 mmol) was
added whereupon a white precipitate formed. After 2 d, the
solution turned dark green and enediamine 11 started to slowly
decompose. Observed conversion to 11: 25% (instability of 11!).
1H-NMR (600 MHz, THF-d8): d = 7.34–7.30 (m; 2H, H-9, H-9¢),
7.21 (d; 4H, H-8, H-10, H-8¢, H-10¢), 6.17 (d; 1H, 3JHH = 2.4 Hz,
Secondary substitution products
3
3
1,3-Bis-(2,6-diisopropylphenyl)-2-(4-methylbenzyl)-imidazolium
iodide (3). 1 (88.2 mg, 0.208 mmol) was stirred for 5 min with
KOtBu (57 mg, 0.51 mmol in 2.7 mL THF) at room temperature.
p-Methylbenzyl bromide (53 mg, 0.29 mmol in 2.1 mL THF) was
added. The resulting yellow solution was stirred for 1 h and kept
over night without stirring. The resulting white precipitate was
filtered off under inert conditions. Trimethylsilyl iodide (71 mL,
0.50 mmol) was added whereupon the solution turned dark. After
standing at rt overnight, square yellow crystals of imidazolium
iodide 3 (9 mg, 7%) had formed. 1H-NMR (600 MHz, CD3OD):
d = 8.32 (s; 2H, H-4, H-5), 7.76 (t; 2H, 3JHH = 7.8 Hz, H-9, H-9¢),
7.57 (d; 4H, 3JHH = 7.8 Hz, H-8, H-10, H-8¢, H-10¢), 6.97 (d; 2H,
H-4), 6.12 (d; 1H, JHH = 2.4 Hz, H-5), 3.35 (sep; 2H, JHH
=
6.9 Hz, H-12, H-15), 3.22–3.16 (m; 2H, H-12¢, H-15¢), 2.33-2.27
(m; 2H, H-18, H-20), 2.23 (“t”; JHH = 4.4 Hz, 1H, H-21a), 1.91 (dd;
1H, 3JHH = 5.6 Hz, 2JHH = 2.5 Hz, H-21b), 1.68–1.60 (m; 2H, H-
19a), 1.38–1.32 (m; 1H, H-19b), 1.34–1.05 (m; 24H, CH(CH3)2);
13C-APT-NMR (151 MHz, THF-d8): d = 149.7 (d; 2C, C7¢, C11¢),
149.1 (d; 2C, C7, C11), 147.0 (d; 1C, C2), 137.5 (d; 1C, C6), 135.4
(d; 1C, C6¢), 129.6 (u; 2C, C9, C9¢), 125.0 (u; 2C, C8¢, C10¢), 124.5
(u; 2C, C8, C10), 116.7 (u; 1C, C4), 115.7 (u; 1C, C5), 56.9 (u; 1C,
C18 or C20), 53.6 (u; 1C, C18 or C20), 46.2 (d; 1C, C21), 29.3 (u;
2C, C12 and C15 or C12¢ and C15¢), 29.2 (u; 2C, C12 and C15
or C12¢ and C15¢), 28.6 (d; 1C, C19) 24.8–23.0 (multiple u; 8C,
CH(CH3)2); ESI-MS: m/z 459 [M+H+].
3JHH = 7.9 Hz, H-21, H-23), 6.39 (d; 2H, JHH = 7.9 Hz, H-20,
3
H-24), 3.95 (s; 2H, H-18), 2.35 (sep; 4H, 3JHH = 6.8 Hz, H-12, H-
15, H-12¢, H-15¢), 2.25 (s; 3H, H-25), 1.28 (d; 12H, 3JHH = 6.8 Hz,
CH(CH3)2), 1.28 (d; 12H, 3JHH = 6.8 Hz, CH(CH3)2); 13C-DEPTQ
(151 MHz, CD3OD): d = 149.0 (d; 1C, C2), 146.9 (d; 4C, C7, C7¢),
140.0 (d; 1C, C22), 134.1 (u; 2C, C9, C9¢), 131.2 (u; 2C, C21, C23),
131.1¶ (d; 2C, C6, C6¢), 130.5 (u; 2C, C20, C24), 128.9 (d; 1C,
C19), 127.4 (u; 2C, C4, C5), 126.8 (u; 4C, C8, C8¢, C10, C10¢),
31.7 (d; 1C, C18), 31.1 (u; 4C, C12, C15, C12¢, C15¢), 26.3 (u; 4C,
C13, C16, C13¢, C16¢), 22.7 (u; 4C, C14, C17, C14¢, C17¢), 21.2 (u;
1C, C25), HRMS: calcd. for C35H45N2 (M–I-): 493.3582; found:
493.358; ATR-IR: 2963, 2921, 2866, 1734, 1599, 1551, 1493, 1457,
1364, 1238, 1102, 1059, 933, 806, 752; UV/Vis lmax(THF)/nm =
234; for crystal structure data see Table 2.
Ethyl 2-[1,3-Bis-(2,6-diisopropylphenyl)-1,3-dihydro-imidazol-2-
ylidenemethyl]acrylate (12). 1 (12.5 mg, 0.029 mmol) and KOtBu
(6.8 mg, 0.061 mmol) were dissolved in THF-d8 (0.6 mL with
TMS). a-Bromomethyl-ethylacrylate (4.0 mL, 0.029 mmol) was
added whereupon the solution turned brown. Conversion: 56%.
3
1H-NMR (600 MHz, THF-d8): d = 7.36 (t; 1H, JHH = 7.7 Hz,
H-9¢), 7.30 (t; 1H, 3JHH = 7.8 Hz, H-9), 7.27 (d; 2H, 3JHH = 7.7 Hz,
H-8¢, H-10¢), 7.17 (d; 2H, 3JHH = 7.8 Hz, H-8, H-10), 6.46 (d; 1H,
3
3JHH = 1.9 Hz, H-5), 6.38 (d; 1H, JHH = 1.9 Hz, H-4), 4.69 (s;
1H, H-21a), 3.83 (q; 2H, 3JHH = 7.1 Hz, H-22), 3.76 (s; 1H, H-18),
3
3.37 (s; 1H, H-21b), 3.28 (sep; 2H, JHH = 7.1 Hz, H-12, H-15),
3
3.06 (sep; 2H, JHH = 6.9 Hz, H-12¢, H-15¢), 1.27–1.19 (m; 24H,
3
CH(CH3)2), 1.01 (t; 3H, JHH = 7.1 Hz, H-23); 13C-APT-NMR
1,3-Bis-(2,6-diisopropylphenyl)-2-(1-p-tolylethyl)-imidazolium
iodide (14). 1 (113.8 mg, 0.268 mmol) was reacted with a
solution of KOtBu (75 mg, 0.67 mmol) in THF (3.5 mL) and
p-methylbenzyl bromide (70 mg, 0.38 mmol in 1.7 mL THF).
After filtration, the solvent was removed under reduced pressure,
the residue dissolved in THF (6 mL) and methyl iodide (15 mL,
0.24 mmol) added. The resultant precipitate was collected and
dissolved in MeOH. After removal of the solvent, a white solid
(23.7 mg, 14%) was obtained. 1H-NMR (600 MHz, CD3OD): d =
8.19 (s; 2H, H-4, H-5), 7.74 (“t”; 2H, 3JHH = 7.9 Hz, H-9, H-9¢),
7.60 (dd; 2H, 3JHH = 7.9 Hz, 4JHH = 1.1 Hz, H-10, H-10¢), 7.45 (dd;
(151 MHz, THF-d8): d = 169.0 (d; 1C, C20), 149.2 (d; 2C, C7¢,
C11¢), 148.2 (d; 2C, C7, C11), 148.0 (d; 1C, C2), 137.2 (d; 1C, C6),
134.8 (d; 1C, C6¢), 134.2 (d; 1C, C19), 130.1 (u; 1C, C9¢), 129.7 (u;
1C, C9), 125.1 (u; 2C, C8¢, C10¢), 124.0 (u; 2C, C8, C10), 118.2 (u;
1C, C4), 116.9 (u; 1C, C5), 107.5 (d; 1C, C21), 66.4 (u; 1C, C18),
60.0 (d; 1C, C22), 29.4 (u; 2C, C12¢, C15¢), 29.2 (u; 2C, C12, C15),
24.7 (u; 2C, C13, C16), 24.4 (u; 2C, C13¢, C16¢), 23.9 (u; 2C, C14¢,
C17¢), 22.9 (u; 2C, C14, C17), 14.5 (u; 1C, C23).
[1,3-Bis-(2,6-diisopropylphenyl)-1,3-dihydroimidazol-2-ylidene]-
acetonitrile (13). 1 (9.4 mg, 0.022 mmol) and KOtBu (6 mg,
0.05 mmol) were dissolved in THF-d8 (0.6 mL with TMS).
Chloroacetonitrile (2.8 mL, 0.044 mmol) was added whereupon
the solution turned dark brown. Conversion: 50%. 1H-NMR
(600 MHz, THF-d8): d = 7.43 (t; 1H, 3JHH = 7.8 Hz, H-9¢), 7.37
2H, 3JHH = 7.9 Hz, 4JHH = 1.1 Hz, H-8, H-8¢), 6.95 (d; 2H, 3JHH
=
3
8.0 Hz, H-21, H-23), 6.43 (d; 2H, JHH = 8.0 Hz, H-20, H-24),
4.30 (q; 1H, 3JHH = 7.6 Hz, H-18), 2.50 (sep; 2H, 3JHH = 6.8 Hz,
3
3
(t; 1H, JHH = 7.8 Hz, H-9), 7.32 (d; 2H, JHH = 7.8 Hz, H-8¢,
¶ determined from H,C-HMBC
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Table 2 Crystal structure data for compounds 3, 16, and 19
Compound reference
3
16
19
Chemical formula
Formula Mass
Crystal system
C35H45N2·I
620.63
Triclinic
C43H53N2·Br
677.78
C34H49N2·I
612.65
Monoclinic
14.5466(18)
16.0087(16)
20.286(3)
90.00
128.613(8)
90.00
3691.3(8)
100(2)
P21/c
4
9022
6004
Orthorhombic
17.0042(8)
18.6505(13)
20.0765(14)
90.00
90.00
90.00
6367.0(7)
100(2)
Pbca
8
29869
6958
0.1138
0.0799
0.2172
0.2076
˚
a/A
9.9198(3)
10.5205(4)
16.6770(6)
83.068(2)
85.616(2)
69.015(2)
1612.08(10)
90(2)
˚
b/A
˚
c/A
a (◦)
b (◦)
g (◦)
3
˚
Unit cell volume/A
T/K
Space group
¯
P1
No. of formula units per unit cell, Z
No. of reflections measured
No. of independent reflections
Rint
Final R1 values (I > 2s(I))
Final wR(F2) values (I > 2s(I))
Final R1 values (all data)
Final wR(F2) values (all data)
2
11853
7035
0.0262
0.0294
0.0629
0.0399
0.0655
0.0501
0.0581
0.1186
0.1496
0.1393
0.2583
3
CH(CH3)2), 2.23 (s; 3H, H-25), 2.17 (sep; 2H, JHH = 6.8 Hz,
120.1 (u; 1C, C27), 43.5 (u; 1C, C18), 31.6 (d; 1C, C26), 30.6 (u;
2C, C12, C12¢), 30.4 (u; 2C, C15, C15¢), 27.2 (u; 2C, C13, C13¢),
26.7 (u; 2C, C16, C16¢), 25.4 (u; 1C, C29), 22.2 (u; 2C, C14, C14¢),
21.9 (u; 2C, C17, C17¢), 20.9 (u; 1C, C25), 18.7 (u; 1C, C30);
HRMS(ESI): m/z: calcd. for C40H53N2 (M–Br-): 561.4209; found:
561.420 ( 0.0015).
CH(CH3)2), 1.46 (d; 3H, 3JHH = 7.6 Hz, H-26), 1.45 (d; 6H, 3JHH
=
6.8 Hz, CH(CH3)2), 1.25 (d; 6H, 3JHH = 6.8 Hz, CH(CH3)2), 1.17
3
3
(d; 6H, JHH = 6.8 Hz, CH(CH3)2), 1.05 (d; 6H, JHH = 6.8 Hz,
CH(CH3)2); 13C-APT-NMR (151 MHz, CD3OD): d = 151.0 (d;
1C, C2), 147.3 (d; 2C, C7, C7¢), 146.9 (d; 2C, C11, C11¢), 140.3 (d;
1C, C22), 134.2 (u; 2C, C9, C9¢), 133.9 (d; 1C, C19), 131.8 (d; 2C,
C6, C6¢), 131.0 (u; 2C, C21, C23), 129.6 (u; 2C, C20, C24), 127.8
(u; 2C, C4, C5), 126.7 (u; 4C, C8, C8¢, C10, C10¢), 38.2 (u; 1C,
C18), 31.2 (u; 2C, CH(CH3)2), 31.0 (u; 2C, CH(CH3)2), 27.0 (u;
2C, CH(CH3)2), 26.6 (u; 2C, CH(CH3)2), 22.4 (u; 2C, CH(CH3)2),
22.2 (u; 2C, CH(CH3)2), 21.2 (u; 1C, C25), 19.4 (u; 1C, C26);
HRMS(ESI): m/z calcd. for C36H47N2 (M–I-): 507.3739; found:
507.374 ( 0.0015).
Corresponding
base:
1,3-Bis(2,6-diisopropylphenyl)-2-(4-
methyl-1-p-tolylpent-3-enyliden)-2,3-dihydro-1H-imidazole (21).
According to the general procedure for NMR studies, 1 (9.8 mg,
0.023 mmol) was reacted with KOtBu (7.7 g, 0.069 mmol in
0.6 mL THF-d8 (with TMS). p-Methylbenzyl bromide (6.8 mg,
0.037 mmol in THF-d8) was added. After reaction control by
NMR, distilled prenyl bromide (4.8 mg, 0.032 mmol) was added.
1
The yellow solution exhibited 53% conversion to 21. H-NMR
1,3-Bis-(2,6-diisopropylphenyl)-2-(4-methyl-1-p-tolylpent-3-enyl)-
imidazolium bromide (15). 1 (61.2 mg, 0.144 mmol) was
reacted with KOtBu (40 mg, 0.36 mmol in 1.7 mL THF) and
p-methylbenzyl bromide (37 mg, 0.20 mmol in 1.7 mL THF).
After filtration, distilled prenyl bromide (16 mL, 0.14 mmol) was
added. After 4 d, the solvent was removed to give an oily yellow
(600 MHz, THF-d8): d = 7.30 (t; 1H, 3JHH = 7.6 Hz, H-9¢), 7.23
3
3
(d; 2H, JHH = 7.6 Hz, H-8¢, H-10¢), 6.93 (t; 1H, JHH = 7.6 Hz,
H-9), 6.82 (d; 2H, 3JHH = 7.6 Hz, H-8, H-10), 6.55 (d; 2H, 3JHH
=
3
8.0 Hz, H-20, H-24), 6.44 (d; 2H, JHH = 8.0 Hz, H-21, H-23),
3
3
6.13 (d; 1H, JHH = 2.4 Hz, H-4), 6.12 (d; 1H, JHH = 2.4 Hz,
3
3
H-5), 4.49 (t; 1H, JHH = 6.1 Hz, H-27), 3.53 (sep; 2H, JHH
=
1
residue (yield n. d.). H-NMR (600 MHz, THF-d8): d = 9.24 (s;
7.0 Hz, H-12¢, H-15¢), 3.30 (sep; 2H, 3JHH = 6.8 Hz, H-12, H-15),
2H, H-4, H-5), 7.65 (“t”; 2H, 3JHH = 7.8 Hz, H-9, H-9¢), 7.54 (d;
2.59 (d; 2H, JHH = 6.1 Hz, H-26), 1.98 (s; 3H, H-25), 1.35 (d;
3
3
3
3
2H, JHH = 7.8 Hz, H-10, H-10¢), 7.37 (d; 2H, JHH = 7.8 Hz,
6H, JHH = 7.0 Hz, H-14¢, H-17¢), 1.32 (s; 3H, H-29), 1.25 (d;
H-8, H-8¢), 6.86 (d; 2H, 3JHH = 8.1 Hz, H-21), 6.24 (d; 2H, 3JHH
=
6H, JHH = 6.8 Hz, H-13, H-16), 1.23 (d; 6H, JHH = 7.0 Hz,
3
3
3
3
8.1 Hz, H-20), 4.34 (t; 1H, JHH = 5.4 Hz, H-27), 3.98 (dd; 1H,
3JHH = 13.2 Hz and 4.2 Hz, H-18), 2.69 (m; 2H, H-26), 2.64 (sep;
H-13¢, H-16¢), 1.10 (d; 6H, JHH = 6.8 Hz, H-14, H-17), 0.94 (s;
3H, H-30); 13C-APT-NMR (151 MHz, THF-d8): d = 148.4 (d;
2C, C7¢, C11¢), 147.2 (d; 2C, C7, C11), 145.1 (d; 1C, C2), 140.4
(d; 1C, C19), 139.0 (d; 1C, C6¢), 138.5 (d; 1C, C6), 131.7 (d; 1C,
C22), 130.4 (u; 2C, C20, C24), 129.1 (u, 1C, C9¢), 128.4 (d; 1C,
C28), 128.1 (u; 1C, C9), 127.9 (u; 2C, C21, C23), 127.0 (u; 1C,
C20), 125.0 (u; 2C, C8¢, C10¢), 124.1 (u; 2C, C8, C10), 119.3 (u;
1C, C5), 118.2 (u; 1C, C4), 79.8 (d; 1C, C18), 32.1 (d; 1C, C26),
29.3 (u; 2C, C12¢, C15¢), 28.9 (u; 2C, C12, C15), 26.0 (u; 2C, C14,
C17), 25.7 (u; 1C, C29), 25.5 (u; 2C, C13¢, C16¢), 23.1 (u; 2C,
C14¢, C17¢), 22.5 (u; 2C, C13, C16), 21.0 (u; 1C, C25), 17.6 (u;
1C, C30); ESI-MS m/z 561 [M+H]+.
3
3
2H, JHH = 6.8 Hz, H-12, H-12¢), 2.26 (sep; 2H, JHH = 6.8 Hz,
H-15, H-15¢), 2.19 (s; 3H, H-25), 1.43 (br; 3H, H-30), 1.42 (d; 6H,
3JHH = 6.8 Hz, H-14, H-14¢), 1.36 (br; 3H, H-29), 1.30 (d; 6H,
3JHH = 6.8 Hz, H-13, H-13¢), 1.21 (d; 6H, JHH = 6.8 Hz, H-16,
3
H-16¢), 0.93 (d; 6H, 3JHH = 6.8 Hz, H-17, H-17¢); 13C-APT-NMR
(151 MHz, THF-d8): d = 148.1 (d; 1C, C2), 147.3 (d; 2C, C11,
C11¢), 146.8 (d; 2C, C7, C7¢), 139.1 (d; 1C, C22), 135.9 (d; 1C,
C28), 132.9 (u; 2C, C9, C9¢), 131.7 (d; 2C, C6, C6¢), 131.0 (d; 1C,
C19), 130.3 (u; 2C, C20, C24), 130.1 (u; 2C, C21, C23), 129.3 (u;
2C, C4, C5), 125.7 (u; 2C, C8, C8¢), 125.6 (u; 2C, C10, C10¢),
1702 | Org. Biomol. Chem., 2010, 8, 1695–1705
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1,3-Bis-(2,6-diisopropylphenyl)-2-(1,2-di-p-tolylethyl)-imidazo-
lium bromide (16). Under anaerobic conditions, KOtBu (0.48 g,
4.3 mmol in 20 mL THF) was added to 1 (0.97 g, 2.0 mmol),
followed by the addition of p-methylbenzyl bromide (0.43 g,
2.4 mmol in 10 mL THF). The reaction mixture was kept overnight
C6¢), 134.3 (d; 1C, C22), 132.0 (d; 1C, C30), 130.6 (u; 2C, C28,
C32), 129.4 (u; 1C, C9¢) 129.2 (u; 2C, C20, C24), 129.1 (u; 2C,
C8¢, C10¢) 128.6 (u; 2C, C21, C23), 128.3 (u; 1C, C9), 127.7 (u;
2C, C29, C31), 124.0 (u; 2C, C8, C10), 119.8 (u; 1C, C4), 117.8 (u;
1C, C5), 80.1 (d; 1C, C26), 39.1 (d; 1C, C18), 29.3 (u; 2C, C12¢,
C15¢), 29.0 (u; 2C, C12, C15), 25.9 (u; 2C, CH(CH3)2), 25.8 (u;
2C, CH(CH3)2), 23.2 (u; 2C, CH(CH3)2), 22.4 (u; 2C, CH(CH3)2),
21.0 (u; 1C, C25), 20.9 (u; 1C, C33); ESI-MS: m/z 597 [M+H+].
◦
at 2 C. The yellow solution was then separated from the white
precipitate. p-Methylbenzyl bromide (0.31 g, 1.7 mmol in 10 mL
THF) was added. After 3 d at rt, the solvent was removed under
reduced pressure to afford a slightly orange solid (1.06 g, 80%).
1H-NMR (600 MHz, THF-d8): d = 9.22 (s; 2H, H-4, H-5), 7.69
(“t”; 2H, 3JHH = 7.9 Hz, H-9, H-9¢), 7.57 (dd; 2H, 3JHH = 7.8 Hz,
1,3-Bis-(2,6-diisopropylphenyl)-2-[2-(4-nitrophenyl)-1-p-tolylethyl]-
imidazolium bromide (17). Under anaerobic conditions,
1
(578.2 mg, 1.315 mmol) was stirred with KOtBu (310 mg,
2.76 mmol) in 14.5 mL THF. p-Methylbenzyl bromide (259 mg,
2.76 mmol in 8.8 mL THF) was added whereupon the solution
turned yellow. After 2.5 h at rt, p-nitrobenzyl bromide (0.287 g,
1.33 mmol in 6 mL THF) was added. The resulting purple
solution was stirred for 1.25 h. The reaction mixture was then
filtered and the solvent removed under reduced pressure to give
3
4
4JHH = 1.0 Hz, H-8, H-8¢), 7.40 (dd; 2H, JHH = 7.8 Hz, JHH
=
1.0 Hz, H-10, H-10¢), 6.76 (d; 2H, 3JHH = 8.0 Hz, H-21, H-23), 6.72
(d; 2H, 3JHH = 8.0 Hz, H-29, H-31), 6.38 (d; 2H, 3JHH = 8.0 Hz,
3
H-28, H-32), 6.26 (d; 2H, JHH = 8.0 Hz, H-20, H-24), 4.30 (dd;
1H, 3JHH = 13.4 Hz, 3JHH = 2.4 Hz, H-18), 3.21 (dd; 1H, 2JHH
=
3
13.4 Hz, JHH = 2.4 Hz, H-26a), 3.07 (“t”; 1H, JHH = 13.4 Hz,
H-26b), 2.66 (sep; 2H, 3JHH = 6.7 Hz, H-15, H-15¢), 2.35 (sep; 2H,
3JHH = 6.7 Hz, H-12, H-12¢), 2.12 (s; 3H, H-25), 2.06 (s; 3H, H-
1
a dark blue solid (0.992 mg, > 99% yield). H-NMR (600 MHz,
THF-d8): d = 8.86 (s; 2H, H-4, H-5), 7.79 (d; 2H, 3JHH = 8.2 Hz,
H-29, H-31), 7.67 (t; 2H, 3JHH = 7.7 Hz, H-9, H-9¢), 7.55 (d; 2H,
33), 1.42 (d; 6H, 3JHH = 6.7 Hz, H-17, H-17¢), 1.30 (d; 6H, 3JHH
=
3
6.7 Hz, H-16, H-16¢), 1.24 (d; 6H, JHH = 6.7 Hz, H-13, H-13¢),
3
3JHH = 7.7 Hz, H-8, H-8¢), 7.41 (d; 2H, JHH = 7.7 Hz, H-10,
3
1
0.99 (d; 6H, JHH = 6.7 Hz, H-14, H-14¢); H-NMR (300 MHz,
CD3OD): d = 8.38 (s; 2H, H-4, H-5), 7.83 (“t”; 2H, 3JHH = 7.8 Hz,
H-9, H-9¢), 7.71 (dd; 2H, 3JHH = 7.8 Hz, 4JHH = 1.0 Hz, H-8, H-8¢),
3
H-10¢), 7.02 (d; 2H, JHH = 8.2 Hz, H-28, H-32), 6.81 (d; 2H,
3
3JHH = 7.7 Hz, H-21, H-23), 6.26 (d; 2H, JHH = 7.7 Hz, H-20,
3
3
H-24), 4.35 (d; 1H, JHH = 13.9 Hz, H-18), 4.03 (d; 1H, JHH
=
3
4
7.53 (dd; 2H, JHH = 7.8 Hz, JHH = 1.0 Hz, H-10, H-10¢), 6.81
(“t”; 4H, JHH = 7.9 Hz, H-21, H-23, H-29, H-31), 6.33 (d; 2H,
13.9 Hz, H-26), 3.28 (t; 1H, 3JHH = 13.9 Hz, H-26), 2.88 (sep; 2H,
3JHH = 6.6 Hz, H-15, H-15¢), 2.59 (sep; 2H, 3JHH = 6.7 Hz, H-12,
3JHH = 7.9 Hz, H-28, H-32), 6.32 (d; 2H, JHH = 7.9 Hz, H-20,
3
3
H-12¢), 2.14 (s; 3H, H-25), 1.38 (d; 6H, JHH = 6.6 Hz, H-16,
3
3
H-24), 4.37 (dd; 1H, JHH = 9.0 Hz, JHH = 6.8 Hz, H-18), 3.12
3
H-16¢), 1.27 (d; 6H, JHH = 6.6 Hz, H-17, H-17¢), 1.21 (d; 6H,
3
(m; 2H, H-26), 2.64 (sep; 2H, JHH = 6.8 Hz, H-15, H-15¢), 2.28
3JHH = 6.7 Hz, H-13, H-13¢), 1.00 (d; 6H, JHH = 6.7 Hz, H-14,
3
(sep; 2H, 3JHH = 6.8 Hz, H-12, H-12¢), 2.18 (s; 3H, H-25), 2.13 (s;
3H, H-33), 1.55 (d; 6H, 3JHH = 6.8 Hz, H-17, H-17¢), 1.31 (d; 6H,
H-14¢); 13C-APT-NMR (151 MHz, THF-d8): d = 148.1 (d; 1C,
C2), 147.7 (d, 1C, C30), 147.5 (d; 2C, C11, C11¢), 147.2 (d; 2C,
C7, C7¢), 145.7 (d; 1C, C27), 139.5 (d; 1C, C22), 133.1 (u; 2C, C9,
C9¢), 131.9 (d; 2C, C6, C6¢), 130.7 (u; 2C, C28, C32), 130.5 (u;
2C, C20, C24), 130.3 (u; 2C, C21, C23), 130.2 (d; 1C, C19), 128.4
(u; 2C, C4, C5), 126.1 (u; 2C, C8, C8¢), 125.9 (u; 2C, C10, C10¢),
123.8 (u; 2C, C29, C31), 44.9 (u; 1C, C18), 37.9 (d; 1C, C26), 30.5
(u; 4C, C12, C12¢, C15, C15¢), 27.0 (u; 2C, C17, C17¢), 26.9 (u;
2C, C13, C13¢), 22.4 (u; 2C, C16, C16¢), 22.2 (u; 2C, C14, C14¢),
20.9 (u; 1C, C25); ESI-MS: m/z 628 (M–Br-).
3JHH = 6.8 Hz, H-16, H-16¢), 1.25 (d; 6H, JHH = 6.8 Hz, H-13,
3
H-13¢), 1.07 (d; 6H, JHH = 6.8 Hz, H-14, H-14¢); 13C-DEPTQ
3
(151 MHz, THF-d8): d = 148.0 (d; 1C, C2), 147.0 (d; 2C, C11,
C11¢), 146.5 (d; 2C, C7, C7¢), 139.0 (d; 1C, C22), 136.9 (d; 1C,
C30), 134.1 (d; 1C, C27), 133.1 (u; 2C, C9, C9¢), 131.7 (d; 2C, C6,
C6¢), 130.4 (u; 2C, C20, C24), 130.2 (d; 1C, C19), 130.0 (u; 2C,
C21, C23), 129.6 (u; 2C, C29, C31), 129.2 (u; 2C, C4, C5), 129.0
(u; 2C, C28, C32), 125.9 (u; 2C, C10, C10¢), 125.8 (u; 2C, C8, C8¢),
45.2 (u; 1C, C18), 38.1 (d; 1C, C26), 30.6 (u; 2C, C15, C15¢), 30.5
(u; 2C, C12, 12¢), 27.0 (u; 2C, C16, C16¢), 26.9 (u; 2C, C13, C13¢),
22.2 (u: 2C, C17, C17¢), 22.0 (u; 2C, C14, C14¢), 20.8 (u; 2C, C25,
C33); HRMS(ESI): calcd for C43H53N2 (M–Br-): 597.4209; found:
597.420 ( 0.0015); for crystal structure data see Table 2.
1,3-Bis-(2,6-diisopropylphenyl)-2-(1,3-dimethylbut-2-enyl)imida-
zolium iodide (18). Under anaerobic conditions, 1 (350 mg,
0.823 mmol) and KOtBu (196 mg, 1.75 mmol) were stirred at rt
in THF (11 mL) for 2 h. Prenyl bromide (105 mL, 0.905 mmol)
was added to the cloudy solution whereupon it turned yellow.
Stirring was continued for 18 h, then methyl iodide (155 mL,
2.47 mmol) was added and the reaction stirred for 3 h. All
volatiles were removed under reduced pressure, the residue
triturated with toluene producing a white solid (366 mg, 56%).ꢀ
1H-NMR (600 MHz, CD3OD): d = 8.25 (s; 2H, H-4, H-5), 7.78
Corresponding base 1,3-Bis-(2,6-diisopropylphenyl)-2-(1,2-
diphenylethylidene)-2,3-dihydro-1H-imidazole (20). 1H-NMR
(600 MHz, THF-d8): d = 7.26 (t; 1H, 3JHH = 7.8 Hz, H-9¢), 7.19
3
3
(d; 2H, JHH = 7.8 Hz, H-8¢, H-10¢), 6.94 (t; 1H, JHH = 7.7 Hz,
H-9), 6.81 (d; 2H, 3JHH = 7.7 Hz, H-8, H-10), 6.71 (d; 2H, 3JHH
=
3
3
3
(t; 2H, JHH = 7.8 Hz, H-9, H-9¢), 7.63 (d; 2H, JHH = 7.8 Hz,
7.8 Hz, H-21, H-23), 6.53 (d; 2H, JHH = 7.8 Hz, H-20, H-24),
3
3
3
H-8, H-8¢), 7.60 (d; 2H, JHH = 7.8 Hz, H-10, H-10¢), 4.87 (d;
6.44 (d; 2H, JHH = 8.0 Hz, H-28, H-32), 6.33 (d; 2H, JHH
=
3
3
1H, JHH = 9.1 Hz, H-19), 3.94 (m; 1H, H-18), 2.49 (sep; 2H,
8.0 Hz, H-29, H-31), 6.17 (d; 1H, JHH = 2.5 Hz, H-4), 6.15 (d;
3JHH = 6.9 Hz, H-15, H-15¢), 2.42 (sep; 2H, 3JHH = 6.9 Hz, H-12,
3
3
1H, JHH = 2.5 Hz, H-5), 3.56 (sep; 2H, JHH = 6.9 Hz, H-12¢,
3
3
H-12¢), 1.61 (s; 3H, H-21), 1.48 (d; 6H, JHH = 6.9 Hz, H-16,
H-15¢), 3.36 (sep; 2H, JHH = 6.8 Hz, H-12, H-15), 3.21 (s; 2H,
3
H-16¢), 1.37 (d; 6H, JHH = 6.9 Hz, H-13, H-13¢), 1.31 (d; 6H,
H-26), 2.12 (s; 3H, H-25), 1.94 (s; 3H, H-33), 1.28–1.13 (m; 24H,
CH(CH3)2); 13C-APT-NMR (151 MHz, THF-d8): d = 148.4 (d;
2C, C7, C11), 147.1 (d; 2C, C7¢, C11¢), 145.6 (d; 1C, C2), 141.1
(d; 1C, C19), 140.0 (d; 1C, C27), 139.3 (d; 1C, C6), 138.7 (d; 1C,
3JHH = 6.9 Hz, H-14, H-14¢), 1.29 (d; 6H, JHH = 6.9 Hz, H-17,
3
ꢀ Product contains 1 equiv. KCl and 1 equiv. KBr.
Org. Biomol. Chem., 2010, 8, 1695–1705 | 1703
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C7); HRMS(ESI): calcd. for C22H27N2 (M–Br-): 319.2174, found:
319.218 ( 0.0015); ATR-IR nmax/cm-1 3044, 3023, 2976, 2914,
1609, 1578, 1515, 1446, 1381, 1232, 1194, 1109, 1034, 1017, 810,
783, 749.
3
H-17¢), 1.07 (s; 3H, H-22), 1.06 (d; 3H, JHH = 7.3 Hz, H-23);
13C-DEPTQ-NMR (151 MHz, CD3OD): d = 151.3 (d; 1C, C2),
147.2 (d; 2C, C11, C11¢), 146.9 (d; 2C, C7, C7¢), 139.2 (d; 1C,
C20), 134.2 (u; 2C, C9, C9¢), 131.7 (d; 2C, C6, C6¢), 127.5 (u; 2C,
C4, C5), 126.8 (u; 2C, C10, C10¢), 126.7 (u; 2C, C8, C8¢), 120.7 (u;
1C, C19), 33.3 (u; 1C, C18), 31.2 (u; 2C, C12, C12¢), 31.1 (u; 2C,
C15, C15¢), 26.5 (u; 2C, C14, C14¢), 26.3 (u; 2C, C17, C17¢), 26.2
(u; 1C, C21), 22.8 (u; 2C, C16, C16¢), 22.5 (u; 2C, C13, C13¢), 19.3
(u; 1C, C23), 17.5 (u; 1C, C22); HRMS(ESI): calcd. for C33H47N2
(M–I-): 471.3739; found: 471.374 ( 3 ppm).
Corresponding base 2-(1,2-Di-p-tolylethylidene)-1-ethyl-3-
methyl-2,3-dihydro-1H-imidazole (24). Under in situ NMR
conditions, 23 (9.6 mg, 0.024 mmol) and KOtBu (2.7 mg,
0.024 mmol) were reacted in THF-d8 (0.6 mL). 1H-NMR
(500 MHz, THF-d8): d = 7.13 (d; 2H, 3JHH = 7.7 Hz, H-19, H-23),
3
3
6.92 (d; 2H, JHH = 7.7 Hz, H-20, H-22), 6.76 (d; 2H, JHH
=
3
1,3-Bis-(2,6-diisopropylphenyl)-2-(1,3,3-trimethylbut-1-enyl)-
imidazolium iodide (19). Under anaerobic conditions, 1 (82.4 mg,
0.194 mmol) and NaH (10.5 mg, 0.438 mmol) were stirred in
THF (5.0 mL) for 1.5 h. Freshly distilled prenyl bromide (24.6 mL,
0.213 mmol) was added to the cloudy reaction mixture whereupon
it turned yellow. After stirring overnight at rt, methyl iodide (36 mL,
0.58 mmol) was added and stirring was continued for 4 h which
resulted in partial decolorization. All volatiles were removed under
reduced pressure, the residue dissolved in benzene, and the yellow
solution filtered through a pad of silica. Crystals were collected
from the filtrate (yield n. d.). 1H-NMR (600 MHz, CD3OD): d =
8.26 (s; 2H, H-4, H-5), 7.73 (t; 2H, 3JHH = 7.8 Hz, H-9, H-9¢), 7.57
(d; 4H, 3JHH = 7.8 Hz, H-8, H-8¢, H-10, H-10¢), 5.64 (s; 1H, H-19),
2.47 (sep; 4H, 3JHH = 6.9 Hz, H-12, H-12¢, H-15, H-15¢), 1.80 (s;
3H, H-24), 1.40 (d; 12H, 3JHH = 6.9 Hz, H-14, H-14¢, H-17, H-17¢),
8.7 Hz, H-12, H-14), 6.74 (d; 2H, JHH = 8.7 Hz, H-11, H-15),
3
3
6.30 (d; 1H, JHH = 2.3 Hz, H-4), 6.28 (d; 1H, JHH = 2.3 Hz,
3
H-5), 3.89 (s; 2H, H-17), 3.35 (q; 2H, JHH = 7.2 Hz, H-6), 2.93
(s; 3H, H-8), 2.21 (s; 3H, H-24), 2.13 (s; 3H, H-16), 1.05 (t; 3H,
3JHH = 7.2 Hz, H-7); 13C-APT (126 MHz, THF-d8): d = 152.0
(d; 1C, C2), 143.0 (d; 1C, C10), 142.1 (d; 1C, C18), 134.7 (d; 1C,
C21), 129.2 (u; 2C, C20, C22), 129.1 (u; 2C, C12, C14), 128.9 (u;
2C, C19, C23), 127.4 (d; 1C, C13), 124.3 (u; 2C, C11, C15), 119.3
(u; 1C, C4), 116.3 (u; 1C, C5), 70.6 (d; 1C, C9), 44.4 (d; 1C, C6),
37.9 (d; 1C, C17), 37.7 (u; 1C, C8), 21.0 (u; 2C; C16, C24), 14.4
(u; 1C, C7).
1-Ethyl-3-methyl-2-(1-methyl-1,2-di-p-tolylethyl)-imidazolium
bromide (25). Under anaerobic conditions, 23 (386 mg,
0.967 mmol) was reacted with KOtBu (130 mg, 1.16 mmol) in
11 mL THF. After stirring for 1 h at ambient temperature, methyl
iodide (180 mL, 2.90 mmol) was added. The reaction mixture was
stirred for 42 h, then all volatiles were removed under reduced
pressure. The white residue (0.61 g, quant.)** contained the desired
product in a mixture with 1 equiv. of KBr. 1H-NMR (600 MHz,
3
1.32 (d; 12H, JHH = 6.9 Hz, H-13, H-16, H-13¢, H-16¢), 0.96 (s;
9H, H-21, H-22, H-23); 13C-APT-NMR (151 MHz, CD3OD): d =
154.5 (u; 1C, C19), 150.4 (d; 1C, C2), 146.6 (d; 4C, C7, C11, C7¢,
C11¢), 133.7 (u; 2C, C9, C9¢), 132.1 (d; 2C, C6, C6¢), 126.9 (u; 2C,
C4, C5), 126.8 (u; 4C, C8, C10, C8¢, C10¢), 118.3 (d; 1C, C18), 72.4
(d; 1C, C20), 31.0 (u; 4C, C12, C15, C12¢, C15¢), 30.0 (u; 3C, C21,
C22, C23), 26.1 (u; 4C, C13, C16, C13¢, C16¢), 23.0 (u; 4C, C14,
C17, C14¢, C17¢), 15.7 (u; 1C, C24); for crystal structure data, see
Table 2.
CD3OD): d = 7.66 (d; 1H, 3JHH = 2.1 Hz, H-5), 7.51 (d; 1H, 3JHH
=
3
2.1 Hz, H-4), 7.34 (d; 2H, JHH = 8.1 Hz, H-12, H-14), 7.30 (d;
2H, 3JHH = 8.1 Hz, H-11, H-15), 7.14 (d; 2H, 3JHH = 7.8 Hz, H-20,
3
H-22), 6.74 (d; 2H, JHH = 7.8 Hz, H-19, H-23), 4.03–3.96 (m;
2
1H, H-6), 3.86–3.79 (m; 1H, H-6), 3.75 (d; 1H, JHH = 13.1 Hz,
2
2-(1,2-Di-p-tolylethyl)-1-ethyl-3-methylimidazolium
bromide
H-17a), 3.56 (d; 1H, JHH = 13.1 Hz, H-17b), 3.40 (s; 3H, H-8),
(23). Under anaerobic conditions, 22 (1.10 g, 5.76 mmol) and
KOtBu (1.36 g, 12.1 mmol) were stirred in THF (36 mL) for
1.5 h at rt. Upon addition of p-methylbenzyl bromide (1.16 g,
6.29 mmol in 20 mL THF), the reaction turned from cloudy
white to yellow. Stirring was continued for 23 h. The solution
was separated from the precipitate and p-methylbenzyl bromide
(1.05 g, 5.67 mmol in 15 mL THF) added. After stirring for 1.5 h,
a yellow oil precipitated. The resultant oil was separated, dried in
vacuo to afford 23 (1.26 g, 55%). 1H-NMR: (600 MHz, CD3OD):
d = 7.58 (d; 1H, 3JHH = 2.0 Hz, H-5), 7.49 (d; 1H, 3JHH = 2.0 Hz,
2.41 (s; 3H, H-16), 2.35 (s; 3H, H-24), 1.94 (s; 3H, H-25), 1.03
(t; 3H, 3JHH = 7.3 Hz, H-7); 13C-APT-NMR (151 MHz, CD3OD):
d = 150.3 (d; 1C, C2), 144.2 (d; 1C, C10), 139.3 (d; 1C, C13), 139.0
(d; 1C, C21), 134.1 (d; 1C, C18), 131.6 (u; 2C, C12, C14), 131.4
(u; 2C, C19, C23), 130.7 (u; 2C, C20, C22), 127.2 (u; 2C, C11,
C15), 126.8 (u; 1C, C4), 123.4 (u; 1C, C5), 50.0 (d; 1C, C9), 47.1
(d; 1C, C6), 44.7 (d; 1C, C17), 39.9 (u; 1C, C8), 30.3 (u; 1C, C25),
21.4 (u; 1C, C16 or C24), 21.3 (u; 1C, C16 or C24), 15.1 (u; 1C,
C7); HRMS(ESI): calcd. for C23H29N2: 333.2331 (M–I-), found:
333.233 ( 0.001).
H-4), 7.33 (d; 2H, 3JHH = 8.1 Hz, H-12, H-14), 7.29 (d; 2H, 3JHH
=
1-Ethyl-3-methyl-2-[1-(4-methylbenzyl)-1,2-di-p-tolyl-ethyl]-
imidazolium bromide (26). Under anaerobic conditions, 23
(49.6mg, 0.146mmol)was reactedwithKOtBu(20mg, 0.18mmol)
in 2 mL THF. After stirring for 1.5 h at ambient temperature, p-
methylbenzyl bromide (44.1 mg, 0.238 mmol in 2.5 mL THF)
was added, the mixture stirred for 6 h, and filtered. The solvent
was removed under reduced pressure to afford a pasty, slightly
3
8.1 Hz, H-11, H-15), 7.14 (d; 2H, JHH = 7.9 Hz, H-20, H-22),
3
3
7.03 (d; 2H, JHH = 7.9 Hz, H-19, H-23), 5.24 (dd; 1H, JHH
=
11.9 Hz, 3JHH = 5.1 Hz, H-9), 4.08–4.01 (m; 1H, H-6), 3.93–3.86
2
3
(m; 1H, H-6), 3.80 (dd; 1H, JHH = 13.3 Hz, JHH = 5.1 Hz,
H-17a), 3.53 (“t”; 1H, JHH = 12.3 Hz, H-17b), 3.51 (s; 3H, H-8),
2.41 (s; 3H, H-16), 2.33 (s; 3H, H-24), 1.06 (t; 3H, 3JHH = 7.2 Hz,
H-7); 13C-DEPTQ-NMR (151 MHz, CD3OD): d = 148.3 (d; 1C,
C2), 139.8 (d; 1C, C13), 138.8 (d; 1C, C21), 135.8 (d; 1C, C18),
134.7 (d; 1C, C10), 131.4 (u; 2C, C12, C14), 131.0 (u; 2C, C20,
C22), 130.1 (u; 2C, C19, C23), 128.6 (u; 2C, C11, C15), 125.3 (u;
1C, C4), 122.6 (u; 1C, C5), 45.1 (d; 1C, C6), 43.5 (u; 1C, C9), 36.9
(d; 1C, C17), 36.6 (u; 1C, C8), 21.4 (u; 2C, C16, C24), 15.2 (u; 1C,
1
yellow residue (yield n. d.). H-NMR (600 MHz, CD3OD): d =
3
3
7.20 (d; 2H, JHH = 7.5 Hz, H-12, H-14), 7.08 (d; 4H, JHH
=
7.8 Hz, H-20, H-22, H-28, H-30), 6.97 (d; 2H, 3JHH = 7.5 Hz, H-11,
** Under consideration of KBr contamination.
1704 | Org. Biomol. Chem., 2010, 8, 1695–1705
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H-15), 6.84 (d; 4H, 3JHH = 7.8 Hz, H-19, H-23, H-27, H-31), 3.72
(d; 2H, 2JHH = 13.0 Hz, H-17a, H-25a), 3.61 (d; 2JHH = 13.0 Hz,
H-17b, H-25b), 2.39 (s; 3H, H-16), 2.32 (s; 6H, H-24, H-32)††;
4 (a) W. A. Herrmann, Angew. Chem., 2002, 114, 1342–1363, (Angew.
Chem., Int. Ed., 2002, 41, 1290); (b) N-Heterocyclic Carbenes (NHC)
in Transition Metal Catalysis, ed. F. Glorius, Top. in Organomet. Chem.,
Vol. 28, Springer-Verlag, Berlin, 2006.
5 (a) D. Enders and T. Balensiefer, Acc. Chem. Res., 2004, 37, 534–
541; (b) Recent developments in NHC-catalyzed benzoin and Stetter
reactions:D. Enders and T. Balensiefer, Angew. Chem., 2006, 118, 1491–
1495, (Angew. Chem., Int. Ed., 2006, 45, 1327); (c) J. R. de Alaniz, M. S.
Kerr, J. L. Moore and T. Rovis, J. Org. Chem., 2008, 73, 2033–2040;
(d) Q. Liu, S. Perreault and T. Rovis, J. Am. Chem. Soc., 2008, 130,
14066–14067.
1H-NMR (600 MHz, DMSO-d6, 348 K): d = 7.71 (d; 1H, 3JHH
=
3
3
1.7 Hz, H5), 7.61 (d; 1H, JHH = 1.7 Hz), 7.15 (d; 2H, JHH
=
3
8.0 Hz, H-12, H-14), 7.03 (d; 4H, JHH = 7.9 Hz, H-20, H-22,
H-28, H-30), 6.99 (d; 2H, 3JHH = 8.0 Hz, H-11, H-15), 6.74 (d; 4H,
3JHH = 7.9 Hz, H-19, H-23, H-27, H-31), 3.76 (br; 2H, H-6), 3.63
(d; 2H, 2JHH = 13.1 Hz, H-17a, H-25a), 3.53 (d; 2JHH = 13.1 Hz,
H-17b, H-25b), 3.44 (br; 3H, H-8), 2.32 (s; 3H, H-16), 2.26 (s; 6H,
6 (a) F. Jordan, Nat. Prod. Rep., 2003, 20, 184–201; (b) D. Enders and
A. A. Narine, J. Org. Chem., 2008, 73, 7857–7870.
7 U. Nilsson, L. Meshalkina, Y. Lindqvist and G. Schneider, J. Biol.
Chem., 1997, 272, 18350–1869.
H-24, H-32), 0.97 (t; 3H, JHH = 7.0 Hz, H-7); 13C-APT-NMR
3
(151 MHz, CD3OD): d = 151.3 (d; 1C, C2), 139.6 (d; 1C, C13),
138.7 (d; 2C, C21, C29), 133.5 (d; 2C, C18, C26), 132.0 (u; 4C,
C19, C23, C27, C31), 131.1 (u; 2C, C12, C14), 130.4 (u; 4C, C20,
C22, C28, C30), 128.9 (u; 2C; C11, C15), 41.6 (d; 2C, C17, C25),
21.3 (u; 3C, C16, C24, C32)**; HRMS(ESI): calcd. for C30H35N2:
423.2800 (M–Br-), found: 423.281 ( <4 ppm)
8 R. Breslow, J. Am. Chem. Soc., 1958, 80, 3719–3726.
9 (a) For reviews, see:N. Marion, S. D´ıez-Gonza´lez and S. P. Nolan,
Angew. Chem., 2007, 119, 3046–3058, (Angew. Chem., Int. Ed., 2007,
46, 2988); (b) D. Enders, O. Niemeier and A. Henseler, Chem. Rev.,
2007, 107, 5606–5655; (c) selected examples:C. Burstein and F. Glorius,
Angew. Chem., 2004, 116, 6331–6334, (Angew. Chem., Int. Ed., 2004,
43, 6205); (d) A. Chan and K. A. Scheidt, Org. Lett., 2005, 7, 905–
908; (e) M. He, J. R. Struble and J. W. Bode, J. Am. Chem. Soc.,
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1473.
10 Average pKa values range from 19.7 (1-methyl-3-phenyl-imidazolium)
to 23.4 (1,3,4,5-tetramethylimidazolium):Y. Chu, H. Deng and
J.-P. Cheng, J. Org. Chem., 2007, 72, 7790–7793, and references cited
therein.
11 Bases such as KOtBu, NaH, K2CO3:(a) A. J. Arduengo, III, Acc.
Chem. Res., 1999, 32, 913–921; (b) A. J. Arduengo III, H. V. R. Dias,
R. L. Harlow and M. Kline, J. Am. Chem. Soc., 1992, 114, 5530–
5534.
(E)-a,4,4¢-Trimethylstilbene (27). Under anaerobic condi-
tions, 25 (55 mg, 0.096 mmol) and KOtBu (16 mg, 0.14 mmol)
were suspended in 2 mL toluene and heated to 100 ◦C for
19 h. Then, the solvent was removed under reduced pressure and
the residue purified by silica gel flash column chromatography
1
(cyclohexane/ethylacetate 20/1) to afford 27 in 80% yield. H-
3
NMR (300 MHz, CDCl3): d = 7.40 (d; 2H, JHH = 8.1 Hz, Ar),
3
3
7.25 (d; 2H, JHH = 7.8 Hz, Ar), 7.16 (d; 4H, JHH = 8.1 Hz,
4
Ar), 6.77 (s; 1H, CH), 2.36 (s; 6H, p-CH3), 2.25 (d; 3H, JHH
=
1.1 Hz, CH3); 13C-APT-NMR (75 MHz, CDCl3): d = 141.2 (d;
1C, CHCCH3), 136.8 (d; 1C, Cq-arom.), 136.5 (d; 1C, Cq-arom.),
136.0 (d; 1C, Cq-arom.), 135.6 (d; 1C, Cq-arom.), 129.0 (u; 1C,
CH-arom.), 129.0 (u; 1C, CH-arom.), 128.8 (u; 1C, CH-arom.),
126.9 (u; 1C, CHCCH3), 125.8 (u; 1C, CH-arom.), 21.2 (u; 1C,
p-CH3), 21.2 (u; 1C, p-CH3), 17.4 (u; 1C, CHCCH3); GC-MS:
9.38 min; m/z: 223, 222[M+·], 208, 207, 206, 193, 192, 191, 189,
165, 129, 119, 117, 115, 91.
12 N. Kuhn, M. Go¨hner and M. Steimann, Z. Naturforsch., 2002, 57b,
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20 See Supporting Information for a mechanistic proposal.
21 A. Fu¨rstner, M. Alcarazo, R. Goddard and C. W. Lehmann, Angew.
Chem., 2008, 120, 3254–3258, (Angew. Chem., Int. Ed., 2008, 47, 3210).
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23 (a) J. Bourson, Bull. Soc. Chim. Fr., 1971, 152–159; (b) J. Bourson, Bull.
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24 M. Begtrup, J. Chem. Soc., Chem. Commun., 1975, 334–335.
25 G. H. Alt, J. Org. Chem., 1968, 33, 2858–2861.
26 For pulse sequence, see:P. Bigler, R. Ku¨mmerle and W. Bermel, Magn.
Reson. Chem., 2007, 45, 469–472.
Acknowledgements
This work was supported by the Emmy-Noether program of the
Deutsche Forschungsgemeinschaft (DFG). We thank the Fonds
der Chemischen Industrie (FCI) for a scholarship (to C.E.I.K.),
and Dr N. Schlo¨rer and Prof. H.-G. Schmalz (both University of
Cologne) for excellent technical support.
Notes and references
1 (a) A. J. Arduengo III, R. L. Harlow and M. Kline, J. Am. Chem. Soc.,
1991, 113, 361–363; (b) A. J. Arduengo III and R. Krafczyk, Chem.
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and C. Ko¨cher, Angew. Chem., 1997, 109, 2256–2282, (Angew. Chem.,
Int. Ed. Engl., 1997, 36, 2162); (d) F. E. Hahn and M. C. Jahnke, Angew.
Chem., 2008, 120, 3166–3216, (Angew. Chem., Int. Ed., 2008, 47, 3122).
3 N-Heterocyclic Carbenes in Synthesis, ed. S. P. Nolan, Wiley-VCH,
Weinheim, 2006.
27 G. M. Sheldrick, Acta Crystallogr., Sect. A: Found. Crystallogr., 2008,
64, 112–122.
28 This compound was synthesized according to:P. Thanei-Wyss and P. G.
Waser, Helv. Chim. Acta, 1983, 66, 2198–2205.
†† The NMR-signals of the imidazolium moiety are not detectable at 298 K
because they are close to coalescence at this temperature. Measurement at
348 K reveals the missing 1H signals.
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The Royal Society of Chemistry 2010
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