Hydroxy-1H-imidazole-3-oxides - Synthesis, kinetic acidity, and application in catalysis and supramolecular anion recognition

Article abstract of DOI:10.1515/znb-2009-0607

Using ab initio calculations (B3LYP 6-31G^*) the geometries of diethyl, dimethoxy and dimethylamino imidazolium salts were studied as representative models of imidazolium salts bearing heteroatoms directly attached to the ring nitrogen atoms of

Full text of DOI:10.1515/znb-2009-0607

Hydroxy-1H-imidazole-3-oxides – Synthesis, Kinetic Acidity, and
Application in Catalysis and Supramolecular Anion Recognition
Susan Bartza, Bettina Blumenro¨dera, Anika Kernb, Julia Fleckensteinb,
Sabine Frohnapfelb, Ju¨rgen Schatza,b, and Alexander Wagnerb
a Organic Chemistry 1, Department of Chemistry and Pharmacy, University of Erlangen,
Henkestraße 42, D-91054 Erlangen, Germany
b Department of Chemistry and Biochemistry, Ludwig-Maximilians-Universita¨t Munich,
Butenandtstraße 5 – 13 (Haus F), D-81377 Mu¨nchen-Grosshadern, Germany
Reprint requests to Prof. Dr. Ju¨rgen Schatz. Fax: +49 9131 85 24707.
E-mail: juergen.schatz@chemie.uni-erlangen.de
Z. Naturforsch. 2009, 64b, 629 – 638; received March 27, 2009
Dedicated to Professor Gerhard Maas on the occasion of his 60th birthday
Using ab initio calculations (B3LYP 6-31G*) the geometries of diethyl, dimethoxy and dimethyl-
amino imidazolium salts were studied as representative models of imidazolium salts bearing hetero-
atoms directly attached to the ring nitrogen atoms of the imidazolium core units. In all cases the syn
and anti arrangement of the substituents could be identified. In addition to the theoretical studies,
eleven dialkoxy imidazolium salts were prepared by alkylation of six 1-hydroxy-imidazole-3-oxides
using dimethyl or diethyl sulfate as strong alkylating reagents. The kinetic acidities of these com-
pounds were studied by measuring the pseudo-first order reaction rates of the H/D exchange process
of the C2-H proton of compounds 3 – 9. The observed kinetic acidities are much higher than reaction
rates observed for simple imidazolium salts; half-lifes of the H/D exchange are usually in the range
of minutes. Similar to dialkyl/aryl imidazolium salts, all prepared dialkoxy imidazolium salts could
be used as precatalysts in standard aqueous Suzuki coupling reactions. In addition, two representative
dialkoxy imidazolium salts could be used in supramolecular anion recognition, as demonstrated by
binding studies towards iodide as guest.
Key words: Imidazolium Salts, Kinetic Acidity, Catalysis, Suzuki Reaction, Anion Receptors
Introduction
In recent years, imidazolium salts of the general
structure 1 (Scheme 1) found a plethora of applications
in organic chemistry. The properties of such salts can
be easily adjusted and fine-tuned by modification of
both the substituents R1 – R5 directly attached to the
heterocyclic skeleton, and the counterion X. In cases
Scheme 1.
where non-nucleophilic, sterically demanding anions traction between the positively charged imidazolium
such as hexafluorophosphate are used, such salts can cations and the anionic guests, the directed C–H···X−
be employed as ionic liquids as alternative reaction interactions makes these salts ideal receptor moieties
media [1].
in supramolecular chemistry [3].
In addition, imidazolium salts bearing a proton at-
Owing to all these important applications, the chem-
tached at the C2 position are ideal precursors for istry of imidazolium salts is well established, and even
N-heterocyclic carbenes (NHC) which have found “abnormal” binding modes of NHCs stemming from
widespread application as “phosphine ligand-analogs” imidazolium salts are already well documented [4].
in coordination chemistry [2]. The acidic C2–H bond Surprisingly, very little is known about compounds 1
can also be exploited in imidazolium-based anion re- with heteroatoms (O, N, S) directly attached to the
ceptor molecules. Besides non-specific electrostatic at- ring nitrogen atoms, e. g. R1, R3 = OR, NHR, or
c
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630
S. Bartz et al. · Hydroxy-1H-imidazole-3-oxides
Scheme 2.
Table 1. Results of quantum chemical calculations of model
NR2. Only limited accounts on formal imidazolium
N-oxide derivatives [5], N,Nꢀ-dialkoxy imidazolium
salts [6], alkylamino [7], or N,Nꢀ-bis(alkylamino) im-
idazolium derivatives [8] are available. Recent work
of Schottenberger and colleagues attracted our atten-
tion because of our own interest in basic proper-
ties of imidazolium salts such as kinetic acidity [9]
and their use as anion receptors [10] or precatalysts
in aqueous Suzuki coupling reactions [11]. They re-
ported on the synthesis, ionic liquid behavior and use
of simple N,Nꢀ-dialkoxy imidazolium salts as NHC
precursors [12]. Inspecting the experimental NMR
data of dimethoxy and diethoxy imidazolium salts re-
vealed a difference in the C2-H proton resonance of
about 0.−1 ppm by exchanging the counterion from
e. g. PF6 to bromide [12a]. Therefore, we decided to
check whether these novel imidazolium salts are use-
ful for our application in supramolecular chemistry and
catalysis.
compounds 1 – 3 (B3LYP 6-31G∗, solvent: DMSO).
Compounds
Confor-
mation
E+ZPE
Erel
/
α
(kJ mol−1
)
kJ mol−1
(deg)a
1a (X = CH2)
2 (X = O)
anti
syn
anti
syn
anti
syn
−1007565.0
−1007565.3
−1195827.8
−1195828.1
−1091611.3
−1091614.3
0.3
= 0.0
0.3
= 0.0
3.0
70.2, 70.4
70.9, 70.9
81.2, 81.5
81.1, 82.0
61.9, 73.8
40.2, 62.4
3 (X = NH)
= 0
a
α: angle formed by the H3C–X bond against the mean plane
formed by the atoms of the imidazolium ring.
For both the imidazolium salts 1a and 2 the energy
differences between the syn and anti arrangement are
small (0.3 kJ mol−1). This is in good agreement with
the experimental observation of both isomers for the
−
dimethoxy imidazolium hexafluorophosphate 2-PF6
in the solid state – with a dominance of the syn con-
former in the bulk material [12]. The geometries of
the syn and anti isomers of 1a and 2 are compara-
ble: The angles formed between the Me–CH2 bond
and the mean plane formed by the atoms of the im-
idazolium ring of the diethyl derivate 1a are about 70◦,
in the anti arrangement only one ethyl substituent is
flipped around to the other side of the imidazolium
ring used as a reference plane. For the dimethoxy im-
idazolium salt 2 the MeO-imidazolium ring angles are
somewhat larger (81◦ – 82◦). Interestingly, the syn con-
former identified by the DFT optimization is very sim-
ilar to the corresponding isomer found in the solid
state [12]. Here, the angles between the Me–O bond
and the imidazolium ring are 79.9◦ and 82.6◦ versus
81.1◦ and 82.0◦ calculated. A distinctive difference
could be found for anti-2. In the experimental geom-
etry, the angles of the substituents against the hetero-
cyclic plane are quite imbalanced. One methoxy group
is nearly perpendicular (88.8◦) whereas the second one
is at an angle of 63.2◦. In the calculated structure such
a mismatch could not be observed, both angles are
about 81◦.
Results and Discussion
Quantum chemical calculations
To get a first insight into the influence of N,N het-
erofunctionalization on the electronic properties of im-
idazolium salts, model quantum chemical calculations
were performed on the three model compounds 1a – 3
(Scheme 2).
Here, the conformational space of all three cations
– neglecting any counterion – was explored at a pm3
semi-empirical level using the “conformer distribu-
tion” methodology as implemented in the program
package SPARTAN’08 [13]. In every case two major
conformers could be identified with the methyl group
of the X-CH3 substituents pointing towards the same
side (syn) or different sides (anti) of the imidazolium
core unit used as a reference plane. Based on these two
major isomers further optimizations assuming DMSO
as solvent were performed using density functional
methods (DFT, B3LYP 6-31G* basis set); both the
syn and the anti arrangements were characterized as
For the bis(methylamino) derivate 3 the energy dif-
minima structures using frequency calculations on the ference between the syn and anti isomer is about
same level of theory. The results of this approach are 10 times bigger than in 1a and 2. This is also re-
summarized in Table 1.
flected in the geometries: in the syn isomer the MeN-
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S. Bartz et al. · Hydroxy-1H-imidazole-3-oxides
631
Table 2. Yields of 1-hydroxy-
imidazole-3-oxides 3 and alkyla-
tion products 4 – 9 thereof.
Compound
R1
R2
R3
Yield (%)
Compound
R1
R2
R3
Yield (%)
3a
3c
3e
4a
5a
6a
7a
8a
9
H
Et
Me
H
Me
Et
Me
Me
Ph
H
Et
n-Pr
H
Me
Et
Et
–
–
–
Me
Me
Me
Me
Me
Me
41
16
25
31
31
31
83
26
50
3b
3d
3f
4b
5b
6b
7b
8b
Me
Me
Ph
H
Me
Et
Me
Et
Ph
H
Me
Et
Et
–
–
–
Et
Et
Et
Et
Et
26
24
20
10
90
68
38
8
Me
Me
n-Pr
Ph
n-Pr
imidazolium ring plane angles are 62.4◦ and 40.2◦, in
In case 4-nitro benzyl chloride or benzyl chloride
the anti isomer 73.8◦ and 61.9◦, respectively. Inspect- was used, no alkylation could be observed with any of
ing the syn conformer it became apparent that the lone the 1-hydroxy-imidazole-3-oxides 3 – 7; only starting
pair of electrons located at the methylamino group is material could be recovered.
nearly perfectly parallel to the C2–H bond probably
balancing the positive partial charge of that part of the
molecule. One can assume that this small favorable
interaction gives the syn-3 isomer an extra stabiliza-
tion increasing the energy difference between syn and
anti, and thus makes the bis(methylamino) derivate 3
an interesting candidate for further applications known
from simple alkyl imidazolium salts.
Kinetic acidity of oxy-imidazolium salts
A key reaction of imidazolium salts is often the de-
protonation of C2-H to give N-heterocyclic carbenes
(NHC) used as ligands in metal-organic chemistry or
catalysis. Despite these manifold applications consid-
erably less information is available about fundamen-
tal acid-base properties of such salts. Both experi-
mental and theoretical investigations have character-
ized nucleophilic carbenes as strong bases (pKa ∼
Synthesis
For the syntheses of various alkoxy imidazolium 20 – 30) [14] with a high proton affinity [14d]. How-
salts of general structure 2 the established protocol ever, for the application of azolium salts as precur-
[12] could be adapted. Starting from symmetrically sor of nucleophilic carbenes not only the thermody-
or unsymmetrically substituted 1,2-dicarbonyl com- namics of the deprotonation are important but also
pounds, condensation with hydroxylamine hydrochlo- its kinetics. Because protonation of the NHC can be
ride and formaldehyde under acidic conditions gave assumed to be fast compared to the formation of
rise to six 1-hydroxyimidazole-3-oxides 3a – f in mod- the NHC [15b], the H/D exchange rates of the C2-
est yields (Table 2). However, owing to the simplic- H group gives an indirect estimate for the reaction
ity of the synthetic approach reasonable quantities of rate of its formation. Here, only very limited data are
the desired compounds could easily be prepared. Alky- available [9, 15]. Therefore, we measured the over-
lation was possible using methyl or ethyl sulfate and all H/D exchange rates kobs by monitoring the de-
yielded the dialkoxy derivatives4 – 9. By the alkylation crease of the integral of the C2-H proton resonance.
process, the C2-H proton resonance of the 1-hydroxy- As solvent pure deuterated methanol was used, and
imidazolium-3-oxides3, which usually appears at 8.2– the concentrations of compounds 1, 3 – 9 were fixed
8.4 ppm, is shifted by roughly 2 ppm towards 10.2 – at 10 millimolar to facilitate the comparison of the
10.7 ppm for the alkylated products 4 – 9. In our hands, kinetic data. In all cases, a clear pseudo-first order
this alkylation step is somewhat limited in scope: only decrease of the C2 proton resonance could be ob-
strong alkylating agents could be used successfully.
served, and from these data pseudo-first order rate
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632
S. Bartz et al. · Hydroxy-1H-imidazole-3-oxides
Table 3. Observed, pseudo-first order rate constants of the
In case of the dialkoxy salts 4 – 9 the additional
cationic charge in the heterocycle usually leads to an
acceleration of the H/D exchange process by a factor
of > 50. In most cases the NMR method used was
too slow to get reliable reaction rates. Here, only the
residual proton content could be detected after a few
minutes necessary to obtain the first NMR spectra, in-
dicating an estimate for kobs > 800 d−1. However, this
trend is not clear-cut. Salts 6b, 7b, and 8a exhibit re-
action rates in the same order of magnitude as the
corresponding basic skeletons 3; for the bis(phenyl)-
substituted derivative 9 only half of the rate compared
to 3f was found. Currently, there is no easy explanation
for this observation, but the previous study with simple
imidazolium salts already proved that subtle structural
changes can have significant influence on these acid-
base properties [9]. However, it can be assumed that the
comparatively slow exchange of salts 6b, 7b, 8a, and 9
could be based on sterical reasons. Substituents R4 and
R5 in the “back” of the heterocyclic ring could force
the methoxy or ethoxy groups to the front. This kind
of buttressing effect would lead to a sterical shielding
of the C2-H position hampering the attack of deuterium
cations.
H/D exchange in [D4]MeOD at r. t. (csalt = 10.0 mmol L−1)a.
No.
X
R1
R3
R4
R5
kobs (d−1
)
1b
1c
3a
3b
3c
3d
3e
3f
Cl
Cl
–
–
–
–
–
–
PF6
PF6
PF6
PF6
PF6
PF6
PF6
PF6
PF6
PF6
PF6
Mes
Dip
OH
OH
OH
OH
OH
OH
OMe
OEt
OMe
OEt
OMe
OEt
OMe
OEt
OMe
OEt
OMe
Mes
Dip
H
H
H
Me
Et
Et
Pr
Ph
H
H
H
H
H
Me
Et
Me
Me
Ph
H
too fast
0.0580
46.3
14.3
15.5
14.2
11.6
8.37
too fast
too fast
too fast
too fast
too fast
11.4
too fast
14.7
13.9
O−
O−
O−
O−
O−
O−
4a
4b
5a
5b
6a
6b
7a
7b
8a
8b
OMe
OEt
OMe
OEt
OMe
OEt
OMe
OEt
OMe
OEt
OMe
H
Me
Me
Et
Me
Me
Et
Et
Et
Et
Pr
Et
Me
Me
Me
Me
Ph
Pr
Ph
too fast
3.05
9
a
Mes = mesityl, Dip = 2,6-diisopropylphenyl.
Oxyimidazolium salts in Suzuki cross coupling reac-
tions
constants kobs were obtained by non-linear curve fits
(Table 3) [9].
For the synthetic organic chemist the formation
The imidazolium salts 1b and 1c were included of C–C bonds is presumably the most important
as representatives for well established NHC precur- and challenging task. Many transition metal-catalyzed
sors used in metal-organic chemistry. Here – as al- coupling protocols have been introduced during the
ready reported before [9] – it is obvious that small last decades. Amongst them, the palladium-catalyzed
changes in the salt structure have a significant in- Suzuki-Miyaura reaction [16] has attracted much at-
fluence on the H/D exchange rates. In case of the tention because this reaction allows aryl-aryl couplings
mesityl (mes)-substituted imidazolium salt 1b the ex- using basically non-toxic staring materials, mild con-
change is too fast to be measured reliably using the ditions and tolerates many functional groups. In recent
aforementioned NMR technique. In contrast, the 2,6- years, much effort has been made to investigate NHCs
diisopropylphenyl (dip) substituents in 1c seem to be as ligands for palladium-catalyzed cross-coupling re-
large enough to shield the C2-H group very effectively; actions of aryl halides, and here especially aryl chlo-
the exchange is highly retarded, and the half-life is in- rides [17].
creased from minutes to about 12 d switching from 1b
to 1c.
Owing to our ongoing interest in organic transfor-
mations in aqueous solution we recently reported that
Simple imidazolium salts usually exhibit slow to water-soluble calixarenes bearing imidazolium moi-
mediocre H/D exchange rates [9] with pseudo-first eties are beneficial for the Suzuki cross-coupling in
order rate constants ranging from 0 – 12 d−1 corre- aqueous media [11]. Therefore, it was obvious to
sponding to half-lifes in the range of several hours or test the novel dialkoxy imidazolium salts in standard
days. Introducing two oxygen atoms at the heterocyclic Suzuki coupling reactions. This idea was supported
skeleton speeds up the exchange process, typical half- by a very recent finding that salt 5a can enhance the
lifes values for compounds 3 are in the range of 30 – ruthenium-catalyzed metatheses in pure water using
60 min.
the Grubbs-II catalyst [18].
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S. Bartz et al. · Hydroxy-1H-imidazole-3-oxides
633
Table 4. Suzuki coupling reaction
of 4-chloro-toluene with benzene
boronic acid catalyzed by an
in situ catalytic system (Mes =
mesityl).
Salt
X
R1a
R3a
R4
R5
Rel. yields
Salt
X
R1a
R3a
R4
R5
Rel. yields
1b
3a
3c
3e
4a
5a
6a
7a
8a
9
Cl
Mes
OH
OH
Mes
O−
H
H
Et
Pr
H
Me
Et
Me
Me
Ph
H
H
Et
Me
H
Me
Et
Et
Pr
Ph
=1
0.35
0.10
0.10
0.15
0.15
0.30
0.20
0.25
0.10
3b
3d
3f
4b
5b
6b
7b
8b
–
OH
OH
OH
OEt
OEt
OEt
OEt
OEt
O−
O−
Me
Et
Ph
H
Me
Et
Me
Me
Me
Me
Ph
H
Me
Et
0.10
0.10
0.20
0.25
0.30
0.10
0.10
0.15
0
O−
OH
O−
O−
PF6
PF6
PF6
PF6
PF6
PF6
OMe
OMe
OMe
OMe
OMe
OMe
OMe
OMe
OMe
OMe
OMe
OMe
PF6
PF6
PF6
PF6
PF6
OEt
OEt
OEt
OEt
OEt
Et
Pr
without ligand precursor
To test the performance of simple dialkoxy imi- in the Suzuki coupling reaction with mainly the same
dazolium salts 4 – 9 and the parent heterocyclic sys- activity as compared to cationic species 4 – 9.
tems 3a – f in the Suzuki coupling a catalytic sys-
Currently, we are exploring new synthetic pathways
tem was used, in which the catalytic species was pre- to dialkoxy imidazolium salts with considerably higher
pared, as reported before [11], in situ from ligand pre- steric bulk at C1 and C3 because simple alkylation
cursors, palladium acetate as a source of palladium methods proved not useful for that purpose in our
and cesium carbonate as a standard base. As reaction hands. Here, recent, preliminary findings showed that
medium a mixture of dioxane/water approximately dialkoxy imidazolium salts bearing such sterically de-
5 : 1 (cf. Experimental Section) was used. Under these manding groups can compete with or even excel the
non-optimized standard conditions all ligand precur- IMes system in Suzuki reactions.
sors were tested in the Suzuki coupling of 4-chloro-
toluene with benzene boronic acid yielding 4-methyl-
biphenyl. As by-products 4,4-dimethyl-biphenyl and
toluene stemming from homo-coupling and dehalo-
Oxy imidazolium salts as novel anion recognition ele-
ments in supramolecular chemistry
genation could be expected and were observed in traces
in all reaction runs.
We were also interested in the basic supramolecular
properties of the synthesized imidazolium derivatives.
The observed high kinetic acidity of the imidazolium
derivatives of interest is, however, a major drawback
for the application in anion recognizing moieties; by
this exchange process the testing probe for the inter-
action with anions is lost in protic solvents. Neverthe-
less, the exchange process is slow in solvents such as
DMSO or acetonitrile often used to study anion bind-
ing processes. In these media standard NMR titration
The relative yields of coupled product are summa-
rized in Table 4; the yield of IMes 1b as ligand precur-
sor was used as a “benchmark”, and all yields obtained
for salts 3 – 9 were related to this yield. Here, we were
not interested in optimizing the catalytic system or get
high conversions. Main target of this study was to get
a quick qualitative insight into the catalytic abilities of
the novel oxy imidazolium salts 3 – 9.
As expected, the catalytic abilities of all ligand pre- experiments were possible yielding association con-
cursors were inferior to the standard and often used stants Kass (L mol−1) and complexation-induced chem-
IMes system 1b. This is mainly due to the lack of ical shifts ∆δ (ppm) by non-linear curve fits of the ex-
steric bulk near the carbenoid center which is neces- perimental data as reported before [10a]. The obtained
sary to speed up the catalytic cycle. However, even data are summarized in Table 5.
with these simple systems about 10 – 30 % of the per-
In acetonitrile as solvent, a high response as ex-
formance of IMes could be observed; even the neutral pressed in the complexation-induced shift (CIS) ∆δ
1-hydroxy-imidazolium-3-oxides 3a – f could be used (i. e. the difference of the C2-proton resonance of the
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634
S. Bartz et al. · Hydroxy-1H-imidazole-3-oxides
Experimental Section
Table 5. Association constants of imidazolium salts with io-
dide (NaI) determined by standard NMR titration experi-
ments (errors ∼ 10 %).
Melting points were determined on an Electrothermal
IA9100 apparatus and are uncorrected. Infrared (IR) spec-
tra were obtained on an ASI React IR-1000 instrument
using ATR. Absorption maxima are given in wave num-
bers (cm−1). NMR spectra were recorded on Varian 200
(200.10 MHz for 1H and 50.32 MHz for 13C), Bruker
Entry
Salt
Solvent
Kass (M−1
)
∆δ (ppm)
1
2
3
4
5
6
7
8
9
4a
4a
4a
8a
8a
4a
4b
5a
5b
CD3CN
32
n. c.a
−
−0.69
−0.02
–
−0.85
−0.08
−0.04
−0.03
−0.06
−0.08
CD3CN / D2O 80 : 20
CD3CN / D2O 50 : 50
CD3CN
CD3CN / D2O 80 : 20
DMSO
DMSO
DMSO
29
20
10
10
5
1
Avance 300 (300.13 MHz for H and 75.47 MHz for 13C),
Bruker Avance 400 (400.13 MHz for 1H and 100.62 MHz
for 13C), Joel JNM EX400 (400.05 MHz for 1H and
100.50 MHz for 13C), or Joel JNM EX400 (400.13 MHz
for 1H and 100.62 MHz for 13C) instruments. Tetra-
methylsilane was used as internal standard (δ = 0.00 ppm)
for the 1H NMR spectra and the solvent signals for the
13C NMR spectra [δC(CDCl3) = 77.0, δC([D6]DMSO) =
39.5, δC([D4]methanol) = 49.3 ppm]. For measurements in
D2O, the solvent signal was used as reference (δH(D2O) =
4.80 ppm) and a trace amount of [D4]methanol for 13C spec-
tra. Chemical shifts (δ) are given in ppm and coupling con-
stants (J) in Hz. Mass spectra were obtained with spectrom-
eters Finnigan MAT 95 XP (EI), Micromass Zabspec (FAB),
Shimadzu Biotech AXIMA Confidence (MALDI-TOF spec-
tra), or Thermo Finnigan LTQ FT (ESI). Matrices for FAB
and MALDI ionization are given individually. Microanaly-
DMSO
5
a Not calculated owing to the small change in chemical shift leading
to non-reliable values.
pure host with the chemical shift extrapolated towards
quantitative complex formation) of the salts 4a and
8a using iodide as a guest molecule could be ob-
served (Table 5, entries 1 and 4). By the complex-
ation process, the C2-H resonance is shifted high-
field by 0.7 – 0.8 ppm. No other protons in the imi-
dazolium salts are affected indicating that the bind-
ing process takes place directly at this C–H acidic
bond. The observed association constants are low
compared to designed imidazolium-based anion re- ses were performed on a Ce Instruments EA 1119 CHNS.
ceptors. However, taking into account that these data
were obtained with very simple salts without any ra-
tional design and optimization towards binding, the
anion receptor properties can be regarded as very
promising.
As expected, adding small amounts of D2O (Table 5,
entries 2, 3, and 5) basically interrupts the binding
process, and only a small response could be observed
towards the added anion. The association constants
dropped owing to competition of water in the solva-
tion process of the anion. Furthermore, the fast H/D
exchange process in the protic environment strongly
interfered with the measurements. In DMSO as an al-
Solvent mixtures used for chromatography or recrystal-
lization are volume/volume (v/v) mixtures. Solvents were
dried by standard procedures. All reaction mixtures were
stirred magnetically, unless otherwise noted.
General procedure for the preparation of 1-hydroxy-1H-
imidazole-3-oxide derivatives
In methanol, the appropriate diketone (1 equivalent) and
paraformaldehyde (1.2 equivalent) were dissolved. After
cooling, the reaction mixture was stirred for 20 min, and
then an aqueous solution of hydroxylamine hydrochloride
and 10 N HCl was added and the mixture stirred for further
20 h at r. t. Subsequently the pH value of the solution was ad-
ternative medium the salts 4 and 5 exhibited only slight justed to 5 by using 12 N NaOH. The formed precipitate was
filtered, washed with cold water, cold MeOH and cold Et2O,
and finally dried in vacuo.
affinity towards iodide as an anion (Table 5, entries
6 – 9).
In summary, dialkoxy imidazolium salts proved to
be a valuable extension of the well known and estab-
lished family of imidazolium salts. The heterofunc-
tionalization on the core imidazolium skeleton modi-
fies the electronic properties, and the kinetic acidity of
the C2-proton is highly increased. This feature could be
exploited in first Suzuki coupling reactions using these
salts as ligand precursors. Here, further optimization of
the catalytic performance of ligand precursors by the
introduction of bulky substituents can be envisaged.
1-Hydroxy-4,5-dimethyl-1H-imidazole-3-oxide (3b)
A mixture of diacetyl (8.70 mL, 100 mmol), paraform-
aldehyde (3.60 g, 120 mmol), hydroxylamine hydrochloride
(14.0 g, 200 mmol) and 10 N HCl (2 mL) was stirred for 20 h
at r. t. A colorless product was obtained (3.37 g, 26.0 mmol,
26 %). M. p. 204 – 205 ◦C. – IR: ν = 3100, 2360, 2340,
1440, 1391, 1233, 1191, 993, 854, 765, 726, 635 cm−1. –
1H NMR (200 MHz, [D6]DMSO): δ = 2.12 (s, 6 H), 8.22
(s, 1 H). – 13C NMR (100 MHz, [D6]DMSO): δ = 6.72,
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S. Bartz et al. · Hydroxy-1H-imidazole-3-oxides
635
120.32, 121.34. – MS ((+)-ESI): m/z = 129 (calcd. 129.1 for m, ArH), 8.42 (1 H, s, NCHN). – MS ((+)-ESI): m/z = 253
C5H9N2O2, [M + H]+).
(calcd. 252.3 for C15H12N2O2, [M + H]+).
1-Hydroxy-4,5-diethyl-1H-imidazole-3-oxide (3c)
General procedure for the alkylation of 1-hydroxy-1H-
imidazole-3-oxide derivatives
3,4-Hexadion (5.32 mL, 43.8 mmol), paraformaldehyde
(1.58 g, 52.6 mmol), hydroxylamine hydrochloride (6.09 g,
The corresponding 1-hydroxy-1H-imidazole-3-oxide de-
87.6 mmol) and 10 N HCl (1 mL) were stirred together rivative 3 (1 equivalent) was added to dimethylsulfate or di-
for 20 h at r. t. A colorless product was obtained (1.07 g, ethylsulfate (2 equivalents) and the mixture stirred for 2 h.
7.00 mmol, 16 %). M. p. 132 – 133 ◦C. – IR: ν = 3113, 2976, After addition of NaHCO3 (1 equivalent) stirring was con-
2939, 2877, 1457, 1328, 1228, 1116, 982 cm−1. – 1H NMR tinued for further 20 h. First H2Odest, then NH4PF6 (1 equiv-
(200 MHz, D2O): δ = 1.13 (t, J = 7.5 Hz, 6H), 2.59 (q, J = alent) were added, and the reaction mixture was placed in an
7.5 Hz, 4H), 8.18 (s, 1 H). – 13C NMR (100 MHz, D2O): ultrasonic bath. The white precipitate was filtered, washed
δ = 12.60, 14.73, 121.70, 126.80. – MS ((+)-ESI): m/z = 157 with H2Odest, recrystallized several times from methanol,
(calcd. 157.1 for C7H13N2O2, [M + H]+).
and finally dried in vacuo.
5-Ethyl-1-hydroxy-4-methyl-1H-imidazole-3-oxide (3d)
1,3-Dimethoxy-1H-imidazol-3-ium hexafluorophosphate
(4a)
2,3-Pentadion (5.22 mL, 50.0 mmol), paraformaldehyde
(1.80 g, 60.0 mmol), hydroxylamine hydrochloride (6.94 g,
1-Hydroxy-1H-imidazole-3-oxide (3a, 8.01 g, 80.0
100 mmol) and 10 N HCl (1 mL) were stirred together mmol), dimethylsulfate (15.2 mL, 160 mmol) and NaHCO3
for 20 h at r. t. A colorless product was obtained (1.72 g, (6.71 g, 80.0 mmol) were stirred together for 20 h, and
12.0 mmol, 24 %). M. p. 154 – 155 ◦C. – IR: ν = 3106, 2980, H2Odest (20 mL) and NH4PF6 (13.1 g, 80.0 mmol) were
2940, 2890, 1461, 1339, 1226, 1106, 997, 772, 612 cm−1. – added. A colorless product was obtained (6.86 g, 25.0 mmol,
1H NMR (200 MHz, D2O): δ = 1.14 (t, J = 7.6 Hz, 3 H), 31 %). M. p. 84 – 85 ◦C (lit. [12] 83 – 84 ◦C). – IR: ν =
2.15 (s, 3 H), 2.61 (q, J = 7.6 Hz, 2 H), 8.23 (s, 1 H). – 3163, 2958, 1556, 1456, 1445, 1376, 1015, 944, 760, 719,
13C NMR (100 MHz, D2O): δ = 5.98, 12.10, 14.75, 121.51, 707 cm−1. – 1H NMR (200 MHz, [D6]DMSO): δ = 4.25
121.93, 126.99. – MS ((+)-ESI): m/z = 143 (calcd. 143.1 for (d, J = 0.7 Hz, 6H), 8.29 (dd, J = 2.0 Hz, 0.7 Hz, 2 H),
C6H11N2O2, [M + H]+).
10.31 (td, J = 2.0 Hz, 0.7 Hz, 1 H). – 13C NMR (100 MHz,
[D6]DMSO): δ = 69.38, 116.90, 124.61. – MS ((+)-ESI):
m/z = 129 (calcd. 129.07 for C5H9N2O2, [M]+). – MS
((–)-ESI): m/z = 145 (calcd. 144.9 for PF6, [PF6]−).
1-Hydroxy-4-methyl-5-propyl-1H-imidazole-3-oxide (3e)
2,3-Hexadion (3.21 mL, 26.3 mmol), paraformaldehyde
(950 mg, 31.5 mmol), hydroxylamine hydrochloride (3.65 g,
52.5 mmol) and 10 N HCl (1 mL) were stirred together for
20 h at r. t. A beige pr◦oduct was obtained (1.04 g, 6.70 mmol,
1,3-Diethoxy-1H-imidazol-3-ium hexafluorophosphate (4b)
1-Hydroxy-1H-imidazole-3-oxide (3a, 1.01 g, 10.0
25 %). M. p. 65 – 66 C. – IR: ν = 3112, 3021, 2956, 2918, mmol), diethylsulfate (2.60 mL, 20.0 mmol) and NaHCO3
2868, 1615, 1462, 1228, 1112, 899, 772, 684 cm−1. – (0.840 g, 10.0 mmol) were stirred together for 20 h,
1H NMR (200 MHz, D2O): δ = 0.77 (t, J = 7.4 Hz, 3 H), and H2Odest (2.5 mL) and NH4PF6 (1.63 g, 10.0 mmol)
1.48 (m, 2 H), 2.07 (s, 3 H), 2.49 (t, J = 7.3 Hz, 2 H), 8.40 (s, were added. A colorless product was obtained (400 mg,
1 H). – 13C NMR (100 MHz, D2O): δ = 6.18, 12.46, 20.97, 1.00 ◦ mmol, 10 %). M. p. 101 – 102 ◦C (lit. [12] 99 –
22.89, 122.23, 122.28, 125.60. – MS ((+)-ESI): m/z = 157.1 102 C). – IR: ν = 3157, 3002, 1559, 1479, 1446, 1395,
(calcd. 157.1 for C7H12N2O2, [M + H]+).
1120, 1006, 802, 726, 599 cm−1. – 1H NMR (200 MHz,
[D6]DMSO): δ = 1.33 (t, J = 7.0 Hz, 6 H), 4.50 (q, J =
7.0 Hz, 4H), 8.27 (d, J = 2.0 Hz, 2 H), 10.27 (t, J = 2.0 Hz,
1 H). – 13C NMR (100 MHz, [D6]DMSO): δ = 12.73, 78.14,
117.68, 130.15. – MS ((+)-ESI): m/z = 157 (calcd. 157.1 for
C7H13N2O2, [M]+). – MS ((–)-ESI): m/z = 145 (calcd. 144.9
for PF6, [PF6]−).
1-Hydroxy-4,5-diphenyl-1H-imidazole-3-oxide (3f)
To a solution of benzil (42.1 g, 200 mmol) in glacial
acetic acid (160 mL), paraformaldehyde (9.00 g, 300 mmol)
and hydroxylamine hydrochloride (27.8 g, 400 mmol) were
◦
added. After stirring for 8 h at 50 C a colorless precipitate
was formed, isolated by filtration, washed with water, and
recrystallized from chloroform yielding the colorless prod-
uct (9.10 g, 40.0 mmol, 20 %). M. p. 227 – 229 ◦C. – IR: ν =
3391, 3151, 2983, 1443, 1212, 1074, 1028, 914, 839, 749,
1,3-Dimethoxy-4,5-dimethyl-1H-imidazol-3-ium hexafluoro-
phosphate (5a)
Compound 3b (1.71 g, 13.0 mmol), dimethylsulfate
686 cm−1. – 1H NMR (200 MHz, CDCl3): δ = 7.34 (10 H, (2.53 mL, 27.0 mmol) and NaHCO3 (1.09 g, 13.0 mmol)
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636
S. Bartz et al. · Hydroxy-1H-imidazole-3-oxides
were stirred together for 20 h, and H2Odest (3.4 mL) and 1206, 1109, 1006, 825, 740 cm−1. – 1H NMR (200 MHz,
NH4PF6 (2.12 g, 13.0 mmol) were added. A colorless prod- [D6]DMSO): δ = 1.18 (t, J = 7.5 Hz, 6 H), 1.37 (t, J = 7.0 Hz,
uct was obtained (1.23 g, 4.00 mmol, 31 %). M. p. 130 – 6 H), 2.73 (q, J = 7.5 Hz, 4 H), 4.48 (q, J = 7.0 Hz, 4 H),
131 ◦C. – IR: ν = 3161, 3011, 2960, 1640, 1554, 1461, 1450, 10.20 (s, 1 H). – 13C NMR (100 MHz, [D6]DMSO): δ =
1394, 1119, 1084, 948, 812, 779, 572 cm−1. – 1H NMR 12.91, 12.94, 14.67, 78.57, 126.81, 128.03. – MS ((+)-ESI):
(200 MHz, [D6]DMSO): δ = 2.29 (s, 6H), 4.21 (s, 6 H), m/z = 213 (calcd. 213.2 for C11H21N2O2, [M]+). – MS
10.21 (s, 1 H). – 13C NMR (100 MHz, [D6]DMSO): δ = ((–)-ESI): m/z = 145 (calcd. 144.9 for PF6, [PF6]−).
6.13, 69.41, 121.98, 127.26. – MS ((+)-ESI): m/z = 157
(calcd. 157.1 for C7H13N2O2, [M]+). – MS ((–)-ESI): m/z =
1,3-Dimethoxy-5-ethyl-4-methyl-1H-imidazol-3-ium hexa-
fluorophosphate (7a)
145 (calcd. 144.9 for PF6, [PF6]−).
Compound 3d (1.60 g, 11.3 mmol), dimethylsulfate
(2.14 mL, 22.6 mmol) and NaHCO3 (0.950 g, 11.3 mmol)
1,3-Diethoxy-4,5-dimethyl-1H-imidazol-3-ium hexafluoro-
phosphate (5b)
were stirred together for 20 h, and H2Odest (3 mL) and
Compound 3b (1.28 g, 10.0 mmol), diethylsulfate NH4PF6 (1.84 g, 11.3 mmol) were added. A colorless prod-
(2.60 mL, 20.0 mmol) and NaHCO3 (0.840 g, 10.0 mmol) uct was obtained (2.96 g, 9.40 mmol, 83 %). M. p. 116 –
were stirred together for 20 h, and H2Odest (5 mL) and 118 ◦C. – IR: ν = 3155, 2992, 2964, 1628, 1552, 1456, 1444,
NH4PF6 (1.63 g, 10.0 mmol) were added. A colorless prod- 1213, 1101, 1008, 998, 948, 877, 810, 740, 576, 555 cm−1. –
uct was obtained (3.11 g, 9.00 mmol, 90 %). M. p. 124 – 1H NMR (200 MHz, [D6]DMSO): δ = 1.17 (t, J = 7.5 Hz,
125 ◦C. – IR: ν = 3159, 2991, 2945, 1632, 1540, 1479, 3 H), 2.31 (s, 3 H), 2.73 (q, J = 7.5 Hz, 2 H), 4.23 (d,
1447, 1396, 1120, 1086, 1008, 880, 813, 740, 576 cm−1. – J = 4.2 Hz, 6 H), 10.23 (s, 1 H). – 13C NMR (100 MHz,
1H NMR (200 MHz, [D6]DMSO): δ = 1.35 (t, J = 7.0 Hz, [D6]DMSO): δ = 6.21, 12.51, 14.58, 69.40, 69.76, 121.73,
6H), 2.28 (s, 6H), 4.44 (q, J = 7.0 Hz, 4H), 10.17 (s, 1 H). – 126.58, 127.27. – MS ((+)-ESI): m/z = 171 (calcd. 171.1 for
13C NMR (100 MHz, [D6]DMSO): δ = 6.51, 12.84, 78.12, C8H15N2O2, [M]+). – MS ((–)-ESI): m/z = 145 (calcd. 144.9
122.42, 128.03. – MS ((+)-ESI): m/z =185 (calcd. 185.1 for for PF6, [PF6]−).
C9H17N2O2, [M]+). – MS ((–)-ESI): m/z = 145 (calcd. 144.9
for PF6, [PF6]−).
1,3-Diethoxy-5-ethyl-4-methyl-1H-imidazol-3-ium hexa-
fluorophosphate (7b)
4,5-Diethyl-1,3-dimethoxy-1H-imidazol-3-ium hexafluoro-
Compound 3d (2.00 g, 14.1 mmol), diethylsulfate
phosphate (6a)
(3.69 mL, 28.1 mmol) and NaHCO3 (1.18 g, 14.1 mmol)
Compound 3c (0.920 g, 5.92 mmol), dimethylsulfate were stirred together for 20 h, and H2Odest (4 mL) and
(1.12 mL, 11.8 mmol) and NaHCO3 (0.500 g, 11.8 mmol) NH4PF6 (2.29 g, 14.1 mmol) were added. A colorless prod-
were stirred together for 20 h, and H2Odest (2 mL) and uct was obtained (1.81 g, 5.26 mmol, 38 %). M. p. 74 –
NH4PF6 (0.970 g, 5.92 mmol) were added. A colorless prod- 75 ◦C. – IR: ν = 3159, 2986, 2942, 1480, 1445, 1396, 1215,
uct was obtained (0.600 g, 1.81 mmol, 31 %). M. p. 120 – 1117, 1009, 885, 818, 741 cm−1. – 1H NMR (200 MHz,
122 ◦C. – IR: ν = 3152, 2998, 2953, 2888, 1452, 1327, [D6]DMSO): δ = 1.17 (t, J = 7.5 Hz, 3 H), 1.36 (td, J =
1233, 1209, 1108, 950, 877, 820, 741, 555 cm−1. – 1H NMR 7.0 Hz, 1.6 Hz, 6H), 2.30 (s, 3 H), 2.72 (q, J = 7.5 Hz, 2 H),
(200 MHz, [D6]DMSO): δ = 1.18 (t, J = 7.5 Hz, 6H), 2.74 4.46 (qd, J = 7.0 Hz, 3.0 Hz, 4H), 10.18 (s, 1 H). – 13C NMR
(q, J = 7.5 Hz, 4H), 4.25 (s, 6H), 10.27 (s, 1 H). – 13C NMR (100 MHz, [D6]DMSO): δ = 6.41, 12.41, 12.86, 12.91,
(100 MHz, [D6]DMSO): δ = 13.54, 15.08, 70.26, 78.15, 14.70, 78.15, 78.55, 122.18, 126.97, 127.98. – MS ((+)-ESI):
126.92, 127.84. – MS ((+)-ESI): m/z = 185 (calcd. 185.1 for m/z = 199 (calcd. 199.1 for C10H19N2O2, [M]+). – MS
C9H17N2O2, [M]+). – MS ((–)-ESI): m/z = 145 (calcd. 144.9 ((–)-ESI): m/z = 145 (calcd. 144.9 for PF6, [PF6]−).
for PF6, [PF6]−).
1,3-Dimethoxy-4-methyl-5-propyl-1H-imidazol-3-ium hexa-
1,3-Diethoxy-4,5-diethyl-1H-imidazol-3-ium hexafluoro-
phosphate (6b)
fluorophosphate (8a)
Compound 3e (8.00 g, 51.2 mmol), dimethylsulfate
Compound 3c (2.00 g, 12.8 mmol), diethylsulfate (9.71 mL, 102 mmol) and NaHCO3 (4.30 g, 51.2 mmol) were
(3.35 mL, 25.6 mmol) and NaHCO3 (1.08 g, 12.8 mmol) stirred together for 20 h, and H2Odest (10 mL) and NH4PF6
were stirred together for 20 h, and H2Odest (4 mL) and (8.35 g, 51.2 mmol) were added. A beige product was ob-
NH4PF6 (2.09 g, 12.8 mmol) were added. A colorless prod- tained (4.29 g, 13.0 mmol, 26 %). M. p. 40 – 42 ◦C. – IR: ν =
uct was obtained (3.12 g, 8.72 mmol, 68 %). M. p. 86 – 3486, 3148, 2972, 2874, 1631, 1443, 1213, 1106, 1004, 948,
87 ◦C. – IR: ν = 3149, 2989, 2944, 1482, 1455, 1333, 845, 820, 733 cm−1. – 1H NMR (200 MHz, [D6]DMSO):
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S. Bartz et al. · Hydroxy-1H-imidazole-3-oxides
637
δ = 0.91 (t, J = 7.4 Hz, 3 H), 1.58 (m, 2 H), 2.30 (s, 3 H), 281.1 for C15H17N2O2, [M]+). – MS ((–)-ESI): m/z = 145
2.67 (t, J = 7.4 Hz, 2 H), 4.23 (d, J = 1.8 Hz, 6H), 10.25 (calcd. 144.9 for PF6, [PF6]−).
(s, 1 H). – 13C NMR (100 MHz, [D6]DMSO): δ = 6.38,
13.07, 21.04, 22.62, 69.38, 69.72, 122.23, 125.17, 127.36. –
MS ((+)-ESI): m/z = 185 (calcd. 185.1 for C9H17N2O2,
[M]+). – MS ((–)-ESI): m/z = 145 (calcd. 144.9 for PF6,
[PF6]−).
General procedure for the Suzuki coupling reactions
In a small vial Cs2CO3 (652 mg, 2.00 mmol) was added to
a solution of the ligand precursor (0.03 mmol) in 0.400 mL of
water and heated to 80 ◦C. After that, a solution of Pd(OAc)2
in dioxane [0.600 mL, concentration of the stock solution
c = 0.05 mol L−1, 0.03 mmol Pd(OAc)2] was added. The
1,3-Diethoxy-4-methyl-5-propyl-1H-imidazol-3-ium hexa-
fluorophosphate (8b)
◦
resulting mixture was stirred at 80 C for 30 min. Then, 4-
chloro-toluene (118 µL, 1.00 mmol) and benzene boronic
acid dissolved in dioxane (1.5 mL, concentration of the stock
solution c = 1.5 mol L−1) were◦added, and the resulting
mixture was stirred for 2 h (80 C). After cooling to r. t.,
the aqueous phase was extracted with chloroform, the or-
Compound 3e (8.00 g, 51.2 mmol), diethylsulfate
(13.4 mL, 102 mmol) and NaHCO3 (4.30 g, 51.2 mmol) were
stirred together for 20 h, and H2Odest (10 mL) and NH4PF6
(8.35 g, 51.2 mmol) were added. A colorless product was
obtained (1.55 g, 4.53 mmol, 8 %). M. p. 70 – 71 ◦C. –
IR: ν = 3153, 2968, 2939, 2874, 1632, 1452, 1397, 1214,
1104, 1009, 948, 822, 740 cm−1. – 1H NMR (200 MHz,
[D6]DMSO): δ = 0.91 (t, J = 7.4 Hz, 3 H), 1.36 (td, J =
7.0 Hz, 0.7 Hz, 6 H), 1.58 (m, 2 H), 2.29 (s, 3 H), 2.70 (t,
J = 7.4 Hz, 2 H), 4.46 (q, J = 7.0 Hz, 4H), 10.25 (s, 1 H). –
13C NMR (100 MHz, [D6]DMSO): δ = 6.57, 12.87, 12.92,
13.08, 20.94, 22.75, 78.13, 78.50, 122.65, 125.53, 128.17. –
MS ((+)-ESI): m/z = 213 (calcd. 213.2 for C11H21N2O2,
[M]+). – MS ((–)-ESI): m/z = 145 (calcd. 144.9 for PF6,
[PF6]−).
R
ꢀ
ganic phases filtered over Celite and dried using molec-
˚
ular sieves (3 A). A 0.3 mL aliquot of the chloroform extract
was diluted with CDCl3 (0.3 mL), and the relative content
of 4-chloro-toluene, 4-methyl-biphenyl as well as the (po-
tential) by-products 4,4ꢀ-dimethyl-biphenyl and toluene was
determined using 1H NMR spectroscopy. Using IMes 1b as
ligand precursor in this experimental setup, after 2 h reaction
time 30 % conversion was observed. By blank experiments
using premixed samples of starting material and all potential
products it could be proven that the extraction process did
not affect the composition of the samples.
1,3-Dimethoxyy-4,5-diphenyl-1H-imidazol-3-ium hexa-
fluorophosphate (9)
General procedure for NMR titration experiments
Compound 3f (5.00 g, 20.0 mmol), dimethylsulfate
All 1H NMR titration experiments were performed in the
(9.48 mL, 100 mmol) and NaHCO3 (1.68 g, 20.0 mmol) deuterated solvent as follows: The host concentration was
were stirred together for 20 h, and H2Odest (5.5 mL) and kept constant at 2.50 × 10−3 mol L−1. The guest solutions
NH4PF6 (3.26 g, 20.0 mmol) were added. A colorless prod- were added from stock solutions (0.15 mol L−1) in the ap-
uct was obtained (3.80 g, 10.0 mmol, 50 %). M. p. 178 – propriate solvent; the guest concentration was varied with the
◦
180 C. – IR: ν = 3145, 2772, 1448, 1199, 1050, 954, 824, ratio host/guest ranging from about 1 to 10. Each experiment
756, 692 cm−1. – 1H NMR (200 MHz, [D6]DMSO) δ = consisted of 10 – 15 points. In all cases, the C2-H signal was
4.09 (s, 6 H), 7.49 (m, 10 H), 10.67 (s, 1 H). – 13C NMR followed, and association constants (Kass) were calculated
(100 MHz, [D6]DMSO): δ = 69.56, 122.56, 125.42, 128.82, using a non-linear curve fit of the observed chemical shifts
129.10, 130.00, 130.60. – MS ((+)-ESI): m/z = 281 (calcd. using the program CHEMEQUI [19].
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