N-heterocyclic carbene (NHC) derivatives of 1,3-di(benzyloxy)imidazolium salts

Article abstract of DOI:10.1515/znb-2010-0702

1-Hydroxyimidazole-3-oxide (1) was alkylated with benzyl bromide in the presence of NaHCO₃ to give the new 1,3-di(benzyloxy)imidazolium bromide 2a which was converted to the hexafluorophosphate 2b and bis(trifluoromethylsulfonyl)imide 2c. From

Full text of DOI:10.1515/znb-2010-0702

N-Heterocyclic Carbene (NHC) Derivatives of
1,3-Di(benzyloxy)imidazolium Salts
Gerhard Lausa, Klaus Wursta, Volker Kahlenbergb, Holger Kopackaa, Christoph Kreutzc,
and Herwig Schottenbergera
a Institute of General, Inorganic and Theoretical Chemistry, University of Innsbruck, 6020 Innsbruck,
Austria
b Institute of Mineralogy and Petrography, University of Innsbruck, 6020 Innsbruck, Austria
c Institute of Organic Chemistry, University of Innsbruck, 6020 Innsbruck, Austria
Reprint requests to Prof. Dr. Herwig Schottenberger. Fax: +43 512 507 2934.
E-mail: herwig.schottenberger@uibk.ac.at
Z. Naturforsch. 2010, 65b, 776 – 782; received April 6, 2010
1-Hydroxyimidazole-3-oxide (1) was alkylated with benzyl bromide in the presence of NaHCO3 to
give the new 1,3-di(benzyloxy)imidazolium bromide 2a which was converted to the hexafluorophos-
phate 2b and bis(trifluoromethylsulfonyl)imide 2c. From this cation, pyridine generated a carbene
which was trapped by sulfur or selenium to yield the respective 2-thione 3 or 2-selone 4. Bromi-
nation afforded the 2-bromo derivative 5. Reaction of the hexafluorophosphate 2b with silver oxide
gave the silver-N-heterocyclic carbene complex 6 which was transmetallated with Au(Me2S)Cl to
the gold-carbene complex 7. A rhodium-carbene complex 8 was obtained by reaction of the hexaflu-
orophosphate 2b with [Rh(cod)Cl]2 in the presence of triethylamine. Eight crystal structures were
determined by X-ray diffraction. The N-benzyloxy groups are twisted out of the plane of the imida-
zole ring in the solid state. They adopt syn conformations in the cation of the hexafluorophosphate 2b
and in the metal-carbene complexes 6 – 8, but anti conformations in the thione 3 and selone 4. Both
conformations were observed in two polymorphs of the 2-bromo compound 5.
Key words: Carbene, Gold, Imidazolium Salt, Ionic Liquid, NHC, Rhodium, Silver
Introduction
imidazolium salts and N-heterocyclic carbene (NHC)
derivatives thereof are reported.
1,3-Di(alkyloxy)imidazolium [1], 1-alkyloxy-3-al-
kylimidazolium [2], and 1-alkyl-4-(dialkylamino)tri-
azolium [3] salts are highly interesting precursors
for N-heterocyclic carbene (NHC) complexes [4] and
ionic liquids (ILs). Recently, 1,3-di(alkyloxy)imid-
azolium salts have exhibited potential as anion affin-
ity probes [5], and their suitability as precatalyst lig-
ands has been demonstrated [5]. In continuation of
our interest in these N-heterofunctionalized azolium
compounds [1 – 3], we again have entered a new ter-
ritory. In the present work, we focus on 1,3-di(benz-
yloxy)imidazolium salts. Previous attempts to synthe-
size these compounds have met with difficulties due
to unintended carbene formation under strongly ba-
sic conditions and subsequent elimination of benz-
aldehyde [6]. Therefore, only stable 2-methyl and 2-
ethyl analogs have been described [6, 7]. However, it
was found that these problems could be avoided by
a more prudent choice of the base used. Herein, the
Results and Discussion
After initial failures with several solvents and bases,
1,3-di(benzyloxy)imidazolium bromide (2a) became
readily available from 1-hydroxyimidazole-3-oxide
(1), benzyl bromide and sodium hydrogencarbonate.
The use of disodium carbonate resulted in total de-
struction of the starting material. The crystalline hexa-
fluorophosphate 2b and bis(trifluoromethylsulfonyl)-
imide (‘triflimide’) 2c were obtained by ion metathesis.
The bromide and triflimide may be regarded as ionic
liquids (ILs) considering their low melting points.
The hexafluorophosphate, however, turned into the
workhorse of the present study. The corresponding
nucleophilic carbene could be generated and readily
trapped by electrophiles such as sulfur, selenium, or
bromine (Scheme 1).
Thus, 1,3-di(benzyloxy)imidazolium salts were
synthesis and crystal structures of 1,3-di(benzyloxy)- converted to 1,3-di(benzyloxy)imidazoline-2-thione
c
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G. Laus et al. · 1,3-Di(benzyloxy)imidazolium Salts
777
estingly, a higher reaction temperature did not return
good results. The same pyridine/triethylamine system
was applied for the synthesis of the 2-selone 4. We
found it crucial to employ red selenium. Black sele-
nium did not react at all. The yields of the selone were
consistently lower than those of the thione in several
experiments, possibly due to the unavoidable forma-
tion of black selenium.
Another highly interesting functionalization of the
2-position can be achieved by bromination [1] to
yield the 1,3-di(benzyloxy)-2-bromoimidazolium salt
5. This could be a very convenient intermediate for the
oxidative insertion of zero-valent metals (such as Pd or
Ni) into the C–Br bond [1].
From the hexafluorophosphate 2b, carbene-metal
complexes could be easily prepared using different
transition metal precursors and suitable bases. The re-
sulting carbenes are obviously stabilized by coordina-
tion to metal cations and do not expel benzaldehyde.
Examples include the Ag, Au and Rh complexes 6 – 8.
The silver-carbene complex 6 was obtained from 2b by
the classic silver oxide method [8 – 10] in methanol.
The NMR signal of the carbene C atom was barely
visible due to extreme line broadening. Attempts to
prepare a crystalline silver-carbene complex from the
bromide 2a were futile so far. The gold-carbene com-
plex 7 was obtained by transmetallation from the silver
complex [11]. The rhodium-carbene complex 8 was
prepared according to a general procedure as previ-
ously described [4, 12]. NMR spectroscopy revealed
that the benzylic protons in the rhodium compound
are diastereotopic, obviously due to restricted rotation
around the C–Rh bond, as noted previously [4]. The
C–Rh coupling constants were confirmed by recording
the spectra at different spectrometer frequencies.
The crystal structures of the new compounds were
determined by single crystal X-ray diffraction. Inter-
estingly, we observed two distinct conformations of the
benzyloxy groups with respect to the imidazolium ring
plane. They are twisted out of the plane in either syn
or anti conformations. The angles are defined between
the imidazole average ring and the CH2-O-N plane. For
the 1,3-di(alkyloxy)imidazolium cations, these confor-
mations have been confirmed by theoretical calcula-
Scheme 1. Reagents: a) BnBr, NaHCO3; b) ion metathesis;
c) S, pyridine, Et3N; d) Se, pyridine, Et3N; e) Br2, CH2Cl2/
H2O, NaHCO3; f) Ag2O, MeOH; g) Au(Me2S)Cl, CH2Cl2;
h) [Rh(cod)Cl]2, THF, Et3N.
(3) by stirring with sulfur in pyridine/triethylamine, tions [5].
no matter if the hexafluorophosphate or bromide was
used. This reaction was studied in more detail. It was
found that triethylamine was not necessary, but it ac-
celerated the reaction. Without it, the reaction time had
to be doubled in order to achieve the same yield. Inter-
In the cation of imidazolium salt 2b, which has
crystallographic mirror symmetry, both benzyloxy
groups are rotated syn out of the ring plane by
89.3◦ (Fig. 1). The thione 3 shows two independent
molecules (CH2O/plane anti angles of 89.7◦ and 85.5◦,
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778
G. Laus et al. · 1,3-Di(benzyloxy)imidazolium Salts
Fig. 4. Overlay of the conformational isomers of the 2-bromo
compound 5: anti (blue) and syn (red).
Fig. 1. Molecular structure of 2b. For clarity, H atoms have
been omitted.
Fig. 2. Molecular structure of the thione 3. For clarity, H
atoms have been omitted. Only one of the two independent
molecules is shown.
Fig. 5. Molecular structure of the silver-carbene complex 6.
For clarity, H atoms have been omitted.
Fig. 3. Molecular structure of the selone 4. For clarity, H
atoms have been omitted.
88.1◦ and 87.7◦, respectively) in the asymmetric unit
(Fig. 2). In crystals of the selone 4 (Fig. 3), the sub-
stituents are also anti oriented (CH2O/plane angles of
89.5◦ and 85.9◦). We were fortunate to obtain single
crystal data of two polymorphs of 1,3-di(benzyloxy)-2-
bromoimidazolium hexafluorophosphate 5, one adopt-
ing the anti conformation with CH2O/plane angles of
81.3◦ and 82.6◦ and the other syn with respective an-
gles of 85.3◦ and 71.5◦ (Fig. 4). The bulk material con-
sisted of a mixture of the two conformers according
to powder X-ray diffraction. The silver complex 6 ex-
hibits a linear arrangement of the two carbene ligands
and the silver atom at a center of inversion (Fig. 5) with
CH2O/plane syn angles of 83.4◦ and 88.2◦. The iso-
morphic gold complex 7 has syn angles of 88.5◦ and
85.6◦. Two independent molecules were also observed
in crystals of the rhodium complex 8 with the pertinent
syn angles 82.9 and 83.0◦, 86.0 and 88.7◦, respectively.
The rhodium atom shows the expected square-planar
coordination geometry (Fig. 6).
Fig. 6. Molecular structure of the rhodium-carbene complex
8. For clarity, H atoms have been omitted. Only one of the
two independent molecules is shown.
than in the cations, as discussed earlier [13, 14]. The
pertinent N–C–N angles are as follows: imidazolium
salt 2b, 105.5◦; thione 3, 101.2◦ and 101.4◦; selone
4, 101.7◦; bromo compound 5 (anti), 106.5◦; bromo
compound 5 (syn), 105.3◦; silver complex 6, 100.9◦;
gold complex 7, 101.5◦; rhodium complex 8, 100.2◦
and 100.9◦.
Conclusion
Another interesting structural feature is the N–C–
An appropriate choice of the conditions facilitated
N angle which is significantly smaller in the carbenes the synthesis of the elusive 1,3-di(benzyloxy)imidazol-
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G. Laus et al. · 1,3-Di(benzyloxy)imidazolium Salts
779
ium salts. The hexafluorophosphate was converted into bath for 1 h. The organic phase was dried over Na2SO4 and
the solvent evaporated. The residue solidified upon standing.
Yield: 0.42 g (64 %). M. p. 50 – 51 ◦C. – 1H NMR (300 MHz,
[D6]DMSO): δ = 5.43 (s, 4H), 7.45 (m, 10H), 8.13 (d,
J = 2.0 Hz, 2H), 10.12 (t, J = 2.0 Hz, 1H). – 13C NMR
(75 MHz, [D6]DMSO): δ = 83.5 (2C), 118.0 (2C), 119.5 (q,
J = 322 Hz, 2C), 128.9 (4C), 130.1 (6C), 130.4, 131.9 (2C). –
IR (neat): ν = 3134, 1551, 1457, 1347, 1327, 1178, 1132,
1052, 1011, 943, 905, 844, 789, 753, 740, 698, 653, 612,
598, 569, 510 cm−1. – C19H17F6N3O6S2 (561.48): calcd.
C 40.64, H 3.05, N 7.48; found C 40.72, h 2.90, N 7.42.
NHC derivatives. Applications of carbene complexes
are envisaged in the fields of catalysis [15, 16] and
medicine [17]. Further work on this promising ligand
is in progress.
Experimental Section
1-Hydroxyimidazole-3-oxide (1) was prepared accord-
ing to references [18, 19]. NMR spectra were recorded with
Bruker AC 300 and Varian Unity 500 spectrometers. IR spec-
tra were obtained with a Nicolet 5700 FT instrument. Ele-
mental analyses were conducted at the University of Vienna.
1,3-Di(benzyloxy)imidazoline-2-thione (3)
A solution of 2b (5.0 g, 12 mmol), sulfur (0.38 g, 1
equiv.) and Et3N (1.7 mL, 12 mmol) in pyridine (100 mL)
was stirred for 24 h at r. t. The mixture was poured into
H2O (500 mL) and stirred for 10 min. The colorless prod-
uct was filtered, washed with H2O (20 mL) and dried (3.1 g,
85 %). Single crystals were obtained from EtOH. A lower
yield (65 %) was obtained when the bromide 2a was used in-
stead of 2b. M. p. 92 ◦C. – 1H NMR (300 MHz, [D6]DMSO):
δ = 5.30 (s, 4H), 7.13 (s, 2H), 7.4 – 7.5 (m, 10H). – 13C
NMR (75 MHz, [D6]DMSO): δ = 78.3 (2C), 112.8 (2C),
128.5 (4C), 129.3 (2C), 129.9 (4C), 133.5 (2C), 153.5. – IR
(neat): ν = 3153, 3094, 3061, 2956, 2924, 2875, 1556, 1497,
1454, 1404, 1371, 1212, 1131, 1048, 1009, 959, 917, 849,
1,3-Di(benzyloxy)imidazolium bromide (2a)
A
mixture of 1-hydroxyimidazole-3-oxide (5.0 g,
0.05 mol) and ◦benzyl bromide (17.0 g, 0.10 mol) was stirred
for 2 h at 60 C and for 3 h at r. t. Then NaHCO3 (4.2 g,
0.05 mol) and H2O (20 mL) was slowly added, and stir-
ring was continued for 17 h at r. t. The phases were sepa-
rated, and the dense liquid was washed with H2O (10 mL)
and Et2O (10 mL), dissolved in CH2Cl2 (50 mL), and dried
over MgSO4. Volatiles were removed in vacuo to yield crude
2a as a clear syrup (15.2 g, 85 %) which crystallized af-
ter 3 months. M. p. 60 – 70 ◦C. – 1H NMR (300 MHz,
[D6]DMSO): δ = 5.52 (s, 4H), 7.46 (m, 10H), 8.25 (d, J =
2.0 Hz, 2H), 10.38 (t, J = 2.0 Hz, 1H). – 13C NMR (75 MHz,
[D6]DMSO): δ = 83.3 (2C), 117.9 (2C), 128.8 (4C), 130.0
(2C), 130.1 (4C), 130.4, 131.8 (2C). – IR (neat): ν = 3031,
1547, 1496, 1455, 1367, 1213, 1014, 944, 906, 843, 752, 696,
756, 734, 686, 640, 590, 523, 500, 473 cm−1
.
1,3-Di(benzyloxy)imidazoline-2-selone (4)
594 cm−1
.
A solution of 2b (1.5 g, 3.5 mmol), red selenium (0.28 g,
1 equiv.) and Et3N (0.50 mL, 3.5 mmol) in pyridine (45 mL)
was stirred for 24 h at r. t. The mixture was poured into H2O
(150 mL) and stirred for 10 min. The grey product was fil-
tered, washed with H2O (10 mL) and dried. The crude prod-
uct was dissolved in acetone, the solution was filtered to re-
move traces of black selenium, and the solvent was evap-
orated to give an off-white powder (0.66 g, 52 %). Single
crystals were grown from heptane. M. p. 104 ◦C. – 1H NMR
(300 MHz, [D6]DMSO): δ = 5.35 (s, 4H), 7.37 (s, 2H), 7.4 –
7.6 (m, 10H). – 13C NMR (75 MHz, [D6]DMSO): δ = 78.8
(2C), 115.2 (2C), 128.6 (4C), 129.4 (2C), 130.1 (4C), 133.3
(2C), 145.7. – IR (neat): ν = 3149, 3087, 3048, 2935, 1545,
1534, 1496, 1455, 1386, 1369, 1212, 1102, 1033, 1010,
956, 939, 906, 845, 738, 693, 634, 585, 493, 447 cm−1. –
C17H16N2O2Se (359.28): calcd. C 56.83, h 4.49, N 7.80;
found C 56.52, H 4.34, N 7.78.
1,3-Di(benzyloxy)imidazolium hexafluorophosphate (2b)
To a well stirred mixture of the bromide 2a (5.0 g,
14 mmol) and H2O (30 mL) was slowly added a solution
of NH4PF6 (2.5 g, 15 mmol) in H2O (10 mL). The result-
ing slurry was stirred for 30 min at r. t., filtered, washed
with H2O (20 mL), and dried to give the colorless prod-
uct (5.1 g, 86 %). Single crystals were obtained by slow
evaporation of a solution in MeOH. M. p. 137 ◦C. – 1H
NMR (300 MHz, [D6]DMSO): δ = 5.44 (s, 4H), 7.45 (m,
10H), 8.13 (s, 2H), 10.13 (s, 1H). – 13C NMR (75 MHz,
[D6]DMSO): δ = 83.5 (2C), 118.0 (2C), 128.9 (4C), 130.1
(6C), 130.4, 131.9 (2C). – IR (neat): ν = 3178, 3149, 1552,
1463, 1457, 1388, 1371, 1216, 1011, 940, 911, 831, 809, 750,
701, 607, 555 cm−1. – C17H17F6N2O2P (426.29): calcd.
C 47.90, H 4.02, N 6.57; found C 47.98, H 3.82, N 6.54.
1,3-Di(benzyloxy)-2-bromoimidazolium hexafluorophos-
phate (5)
1,3-Di(benzyloxy)imidazolium triflimide (2c)
A mixture of the hexafluorophosphate 2b (0.50 g,
1.2 mmol) and LiNTf2 (0.37 g, 1.3 mmol) in CH2Cl2
To a solution of hexafluorophosphate 2b (5.0 g, 12 mmol)
(10 mL) and H2O (10 mL) was agitated in an ultrasonic in CH2Cl2 (500 mL) and H2O (50 mL) were added bromine
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G. Laus et al. · 1,3-Di(benzyloxy)imidazolium Salts
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G. Laus et al. · 1,3-Di(benzyloxy)imidazolium Salts
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(0.65 mL, 12.5 mmol) and NaHCO3 (3.0 g, 36 mmol) during (s, 4H), 7.35 (m, 10 H), 7.85 (s, 2H). – 13C NMR (75 MHz,
2 h. The mixture was stirred for additional 2 h at r. t.. The [D6]DMSO): δ = 82.3 (2C), 117.5 (2C), 128.7 (4C), 129.6
CH2Cl2 was evaporated, Et2O (100 mL) was added, and the (2C), 130.1 (4C), 132.6 (2C), 170.4. – IR (neat): ν = 3178,
crystalline product was collected by filtration, washed with 3159, 3032, 2951, 2934, 2891, 1455, 1367, 1220, 1026, 980,
Et2O and dried to yield 5.1 g (85 %) of 5. Single crystals were 959, 910, 818, 745, 711, 693, 648, 555 cm−1
obtained from acetone (anti conformer) and CH2Cl2/Et2O
.
(syn conformer). M. p. 121 – 122 ◦C. – 1H NMR (300 MHz,
[D6]DMSO): δ = 5.45 (s, 4H), 7.47 (m, 10H), 8.43 (s, 2H). –
1H NMR (300 MHz, [D6]acetone): δ = 5.61 (s, 4H), 7.52 (m,
10H), 8.28 (s, 2H). – 13C NMR (75 MHz, [D6]acetone): δ =
85.7, 119.2, 121.2, 130.6, 132.1, 132.2, 133.2. – IR (neat):
Chloro(η4-1,5-cyclooctadiene)[1,3-di(benzyloxy)imid-
azolin-2-ylidene]rhodium(I) (8)
To a solution of 1,3-di(benzyloxy)imidazolium hexaflu-
orophosphate 2b (100 mg, 0.23 mmol) and [RhCl(cod)]2
(58 mg, 0.12 mmol) in dry THF (2 mL) was added Et3N
(33 µL, 0.23 mmol), and the mixture was stirred under Ar
for 24 h at r. t. The solvent was removed and the residue
treated with iso-pentane (2 × 1 mL). Then the residue was
extracted with Et2O (3 × 2 mL) and the solvent evaporated
ν = 3152, 1045, 837, 734, 696, 646, 557 cm−1
.
Bis[1,3-di(benzyloxy)imidazolin-2-ylidene]silver(I) hexa-
fluorophosphate (6)
A suspension of 2b (1.0 g, 2.3 mmol) and Ag2O (0.33 g, to leave 90 mg (73 %) of 8 as a yellow powder. Single crys-
1.4 mmol) in MeOH (25 mL) was stirred at r. t. for 24 h. The tals were obtained by slow evaporation of a solution in Et2O.
◦
1
precipitate was collected by filtration and recrystallized from M. p. 130 C. – H NMR (300 MHz, CDCl3): δ = 1.96 (m,
hot MeOH and cooling of the solution to −20 ◦C to give the 4H), 2.43 (m, 4H), 3.80 (m, 2H), 5.16 (m, 2H), 5.74 and
product 6 (0.76 g, 80 %) as colorless needles. M. p. 164 ◦C. – 5.84 (AB, J = 10.2 Hz, 4H), 6.37 (s, 2H), 7.40 (m, 6H), 7.67
1H NMR (300 MHz, [D6]DMSO): δ = 5.28 (s, 4H), 7.34 (m, (m, 4H). – 13C NMR (75 and 125 MHz, CDCl3): δ = 29.2
10H), 7.77 (s, 2H). – 13C NMR (75 MHz, [D6]DMSO): δ = (2C), 32.9 (2C), 69.9 (d, JC-Rh = 13.9 Hz, 2C), 81.8 (2C),
81.8 (2C), 117.2 (2C), 128.6 (4C), 129.4 (2C), 129.9 (4C), 97.5 (d, JC-Rh = 7.7 Hz, 2C), 115.8 (2C), 128.5 (4C), 129.2
132.9 (2C), 167 (br). – IR (neat): ν = 3176, 3157, 3031, (2C), 130.4 (4C), 134.2 (2C), 172.6 (d, JC-Rh = 49.7 Hz). –
2952, 2934, 2892, 2877, 1498, 1455, 1367, 1220, 1183, IR (neat): ν = 3171, 3149, 2939, 2931, 2915, 2878, 2829,
1159, 1097, 1022, 981, 964, 911, 851, 831, 818, 745, 710, 1491, 1463, 1452, 1431, 1384, 1367, 1330, 1216, 1175,
693, 555 cm−1
.
1152, 1076, 1012, 950, 908, 848, 754, 708, 682, 649, 575,
503 cm−1
.
Bis[1,3-di(benzyloxy)imidazolin-2-ylidene]gold(I) hexa-
fluorophosphate (7)
Crystal structure determination
The crystal structures were determined using Nonius Kap-
paCCD, Oxford Diffraction Gemini-R Ultra, and Stoe IPDS
2 diffractometers with graphite-monochromatized MoKα ra-
diation (λ = 0.71073 A). The experimental conditions and
crystallographic data are listed in Table 1.
CCDC 771352–771359 contain the supplementary crys-
tallographic data for this paper. These data can be obtained
free of charge from The Cambridge Crystallographic Data
Centre via www.ccdc.cam.ac.uk/data request/cif.
A mixture of silver-carbene complex 6 (100 mg,
0.12 mmol) and Au(Me2S)Cl (40 mg, 0.14 mmol) in CH2Cl2
(3 mL) was stirred at r. t. for 2.5 h. The precipitate was re-
moved by filtration, and the solution was concentrated. The
product was crystallized by addition of Et2O (2 mL). Sin-
gle crystals were grown by diffusion of Et2O into a solution
of 7 in CH2Cl2. Yield: 100 mg (90 %). The reaction gave a
comparable yield when carried out with MeOH as solvent.
M. p. 175 ◦C. – 1H NMR (300 MHz, [D6]DMSO): δ = 5.35
˚
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