A facile synthetic route to benzimidazolium salts bearing bulky aromatic N-substituents

Article abstract of DOI:10.3762/bjoc.11.182

An atom-economic synthetic route to benzimidazolium salts is presented. The annulated polycyclic systems: 1,3-bis(2,4,6- Trimethylphenyl)-1H-benzo[d]imidazol-3-ium chloride (1-Cl), 1,3-bis(2,6-diisopropylphenyl)-1H-benzo[d]imidazol-3-ium chloride (2-Cl),

Full text of DOI:10.3762/bjoc.11.182

A facile synthetic route to benzimidazolium salts bearing
bulky aromatic N-substituents
Gabriele Grieco, Olivier Blacque and Heinz Berke*
Full Research Paper
Open Access
Address:
Beilstein J. Org. Chem. 2015, 11, 1656–1666.
Department of Chemistry, University of Zurich, Winterthurerstrasse
190, CH-8057 Zurich, Switzerland, Fax: (+41)-1-635-6802
doi:10.3762/bjoc.11.182
Received: 13 July 2015
Email:
Accepted: 01 September 2015
Published: 17 September 2015
Heinz Berke* - hberke@chem.uzh.ch
* Corresponding author
Associate Editor: T. P. Yoon
Keywords:
© 2015 Grieco et al; licensee Beilstein-Institut.
License and terms: see end of document.
benzimidazolium salts; bulky ligands; cyclization; ligand design;
N-heterocyclic carbenes (NHC); ring formation
Abstract
An atom-economic synthetic route to benzimidazolium salts is presented. The annulated polycyclic systems: 1,3-bis(2,4,6-
trimethylphenyl)-1H-benzo[d]imidazol-3-ium chloride (1-Cl), 1,3-bis(2,6-diisopropylphenyl)-1H-benzo[d]imidazol-3-ium chloride
(2-Cl), 1,3-diphenyl-1H-benzo[d]imidazol-3-ium chloride (3-Cl), and 1,3-di(pyridin-2-yl)-1H-benzo[d]imidazol-3-ium chloride
(4-Cl) were prepared in a two-step synthesis avoiding chromatographic work-up. In the key step triethyl orthoformate is reacted
with the corresponding N1,N2-diarylbenzene-1,2-diamines and then further transformed in situ, by alkoxy abstraction using
trimethylsilyl chloride (TMSCl), and concomitant imidazole ring closure.
Introduction
Imidazole-based N-heterocyclic carbenes (NHCs) are stable terial’s side NHC-type carbene–borane adducts and metal
systems serving as ancillary ligands mainly to construct complexes of them can also be used as electroluminescent ma-
organometallic complexes. These NHCs are sterically and elec- terials (OLEDs) [16-19].
tronically tunable, strongly binding ligand units in complexes in
the search for new materials [1-5] and for catalysts of homoge- After the discovery of NHCs in 1968 by Wanzlick and Öfele,
neous catalysis [6-10]. Besides that NHCs have great potential who isolated stable diamino-substituted carbenes, around
in organocatalysis [11-15], their electronic properties, σ-donor 20 years later Arduengo further stabilized these potential ligand
ability and π-back donation can be tuned and, based on these groups by embedding them into imidazole rings and increased
properties, they possess great electronic flexibility. These in addition the steric congestion [20]. It is important to recog-
ligands or catalysts reached a prominent position in the nize that synthetically the stable forms for handling of imida-
mentioned fields of chemistry. It is noteworthy that on the ma- zole carbenes are imidazolium salts. In imidazolium-based
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Beilstein J. Org. Chem. 2015, 11, 1656–1666.
NHC chemistry two synthetic aspects are important: 1) there are zole annulated systems or those with an annulated 1,3-dime-
several fundamental synthetic routes available that allowed sitylbenzene ring (1-Cl) [42,43] (Figure 1). The synthetic routes
substitution in the 4 and 5 positions of the imidazole ring; and are very sensitive to the respective substitution patterns, for
2) the routes available can be carried out as one-pot reactions instance, the procedure for 1-Cl failed to access the analogous
[21,22] or two-step preparations [23,24].
1,3-(2,6-diisopropyl)benzimidazolium salt 2-Cl.
Annulated polycyclic NHCs can, however, not always be In this article we describe a two-step synthesis for the sterically
prepared in straightforward ways, especially when the com- hindered benzimidazolium salts: 1,3-bis(2,4,6-trimethylphenyl)-
pounds are designed to bear bulky aromatic moieties as N1,N2- 1H-benzo[d]imidazol-3-ium chloride (1-Cl) [42,43], 1,3-
substituents. As mentioned before the synthetic pre-stages of bis(2,6-diisopropylphenyl)-1H-benzo[d]imidazol-3-ium
NHCs are always imidazolium salts, but benzimidazolium salts, chloride (2-Cl), 1,3-diphenyl-1H-benzo[d]imidazol-3-ium
which possess two aromatic N1,N2-substituents are rare. More chloride (3-Cl) [44,45] and 1,3-di(pyridin-2-yl)-1H-
common are benzimidazolium salts having different N1,N2- benzo[d]imidazol-3-ium chloride (4-Cl) [45] (Figure 2). The
substituents, where one of the N-substituent is an aromatic or a synthesis starts from suitable aryl diamines applying the combi-
benzylic group and the other substituent an alkyl [25-30] or nation of the reagents trimethylsilyl chloride and triethyl ortho-
benzyl [31,32] group. A synthetic strategy for the preparation of formate as C1 component to accomplish concomitantly cycliza-
sterically demanding monoaryl benzimidazolium salts starts tion to the imidazole rings. We expected high yields and
from the corresponding benzimidazoles possessing a bulky aryl reduced overall reaction times when compared with the previ-
group introducing the other bulky N-substituent by means of an ously reported synthetic pathways.
N-alkylation [33-37]. Indeed benzimidazolium salts that bear
N-alkyl or N-alkenyl substituents can be accessed synthetically Excluding the strongly sterically encumbered benzimidazolium
by simple routes in comparison with those possessing two salt 2-Cl, the benzannulated NHCs 1-Cl, 3-Cl and 4-Cl were
N-aromatic substituents.
prepared earlier, however, 1-Cl was prepared by a relatively
complex reaction scheme. Moreover, the benzimidazolium salts
We therefore sought the preparation of sterically demanding 1-Cl to 4-Cl possessing a variety of N1,N2-substituents were
N1,N2-benzimidazolium systems as, for instance, those accessed designed to eventually allow facile release of the benzoimida-
by Chianese and co-workers [38]. But for the access to other zole carbenes acting eventually as ligands in complexes by
aryl-substituted benzimidazolium species we had to face com- deprotonation avoiding additional complications that could arise
plex synthetic pathways, for which we intended to simplify from the anions of the benzimidazolium salts.
these as much as possible.
Results and Discussion
In addition to the 1,3-benzoimidazolium salts prepared by The starting point for the syntheses of the benzannulated NHCs
Chianese and co-workers, polycyclic diaminocarbenes were 1-Cl, 2-Cl and 3-Cl was the development of a straightforward
reported possessing mesityl substituents in the N1,N2-positions palladium-catalyzed preparation of the known N1,N2-diphenyl-
and in addition benzoquinone [39,40] or quinone [41] imida- benzene-1,2-diamine compounds 5, 6 and 7 in dependence on
Figure 1: Sterically demanding benzannulated NHCs bearing mesityl rings. From the left side:, 4,9-dihydro-4,9-dioxo-1,3-bis(2,4,6-trimethylphenyl)-
1H-naphth[2,3-d]imidazolium chloride [39], 3,4,5,8-tetrahydro-4,8-dioxo-1,3,5,7-tetrakis(2,4,6-trimethylphenyl)-benzo[1,2-d:4,5-d']diimidazolium bro-
mide [41], 1,3-bis(2,4,6-trimethylphenyl)-1H-benzo[d]imidazol-3-ium chloride (1-Cl) [42,43] and 1,3-bis(2,6-diisopropylphenyl)-1H-benzo[d]imidazol-3-
ium chloride (2-Cl).
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Beilstein J. Org. Chem. 2015, 11, 1656–1666.
Figure 2: Benzannulated NHCs of this work. Benzannulated imidazolium chloride salts 1-Cl, 2-Cl, 3-Cl and 4-Cl of various steric demands and
ligating properties in complexes.
an earlier synthetic route [46] (Scheme 1). In the case of com- zolium salts by ring closure. 3-Cl and 4-Cl could principally be
pound 5 a better yield (89%) could be accomplished when the obtained from the corresponding N1,N2-diarylbenzene-1,2-di-
reaction was carried out at 92 °C instead of 120 °C as reported amines via two different cyclization pathways leading to the
[47].
benzimidazolium salts 3-Cl [44] and 4-BF4 [48] (Scheme 2).
But we also planned to access the chloride compounds 1-Cl,
3-Cl and 4-Cl reported previously as tetrafluoroborate [48] and
chloride [45] salts for two reasons: we wished to find a faster
way to access them and it seemed advantageous to aim prefer-
entially at the synthesis of benzimidazolium chlorides than at
[BF4]− salts, since in case the carbene will be generated in “in
situ” reactions in the presence of metal complexes, the lower
stability of the [BF4]− anion could lead to complications gener-
ating side-products [50-52]. The preparations of both 3-Cl and
4-Cl started along the given lines with the syntheses of the
respective aryl-substituted diamines, which had to undergo ring
closure in the presence of a C1 component. For 3-Cl and 4-Cl
the synthetic procedures proceed without noticeable problems,
while complications were faced in the synthesis for 1-Cl
and 2-Cl, particularly during the benzimidazole ring
closures. Excluding the procedure of Borguet [42,43] all the
known procedures to accomplish imidazole ring formation
Scheme 1: Synthesis of the N1,N2-diaryl-1,2-benzenediamines 5, 6, 7
and 8. i) Pd(dba)2, P(t-Bu)3, t-BuONa, toluene, 92 °C. ii) Pd(dba)2,
P(t-Bu)3, t-BuONa, toluene, 115 °C. iii) SNAr reaction: 2-chloropyridine,
neat, 185 °C, microwave.
Somewhat modified approaches were used for the syntheses of [44,45,48,53,54] of either 6 and 7 failed.
the N1,N2-diarylbenzene-1,2-diamines 6 and 7 compared with
the preparation of the N1,N2-di(pyridine-2-yl)benzen-1,2-di-
amine (8). The Buchwald–Hartwig amination was applied in the
syntheses of 5 and 6 where 1,2-dichlorobenzene was coupled
with aniline and 2,4,6-trimethylaniline, respectively
(Scheme 1). Attempting the synthesis of the N1,N2-bis(2,6-
diisopropylphenyl)benzene-1,2-diamine (7) and using 1,2-
dichlorobenzene as the starting compound, the mono-substi-
tuted product was obtained. To avoid this complication 1,2-
dibromobenzene had to be applied to eventually approach the
preparation of 7. For 8 a reported synthetic procedure was used
[48,49] consisting of nucleophilic aromatic substitutions (SNAr)
of benzene-1,2-diamine at 2-chloropyridine (Scheme 1). Once
Scheme 2: Previous synthesis of the benzannulated NHCs 3-Cl and
4-BF4. Ring closure. i) (EtO)3CH, HCl (conc.), HCOOH, 80 °C [44].
ii) Microwave assisted synthesis: NH4BF4, (EtO)3CH, 160 °C [47].
all the different N1,N2-diarylbenzene-1,2-diamines were For instance, the method of Hintermann [23] that uses the
prepared a method had to be developed to build up the imida- combination of paraformaldehyde and trimethylsilyl chloride
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Beilstein J. Org. Chem. 2015, 11, 1656–1666.
(TMSCl) gave satisfying results in the preparation of the known assumed to abstract an alkoxide group and to deliver at the
IMes and IPr derivatives, but failed for the synthesis of both same time the chloride as the preferred counterion for the
1-Cl and 2-Cl. In fact there are examples of N1,N2- imidazolium salts.
bisarylethandiamines, which could be cyclisized to benzimida-
zolium salts in the presence of air, paraformaldehyde and As said, temperature plays a decisive role in the formation of
hydrochloric acid [55], and the success of this synthetic ap- stage d, requiring the N,N’-diarylbenzene-1,2-diamines to be
proach was presumably crucially depending on the mode of heated in triethyl orthoformate at almost reflux temperature
action of O2 oxidizing the aminal intermediate. The failures to (145 °C) for reaction times between 10 and 20 min. Then
access 1-Cl and 2-Cl may originate from the high steric TMSCl was added in large excess all at once leading to precipi-
hindrance of the 2,6-substituents of the N1,N2-diarylbenzene- tation of greyish products, which indicated the formation of the
1,2-diamines preventing initial aminal formation. Involving benzimidazolium chlorides. In this way not only the known
instead triethyl orthoformate has the advantage that this C1 compound 1-Cl could be accessed in a short total reaction
building block provides the right oxidation state for the cycliza- sequence (2 steps, one isolated intermediate product) instead of
tion process making the involvement of an oxidizing agent the 4 steps of the synthetic route reported earlier [42,43]
unnecessary [23] and, moreover, it possesses high elec- (Table 1). The application of new reagents avoided at the same
trophilicity required for this reaction. The method of Chianese time tedious column chromatography. Even the more sterically
et al. [38] demonstrated that the cyclization of N1,N2-diarylben- encumbered and elusive compound 2-Cl could obtained in this
zene-1,2-diamine can be achieved with bulky substituents in the way (X-ray diffraction structure displayed in Figure 3). The
3-, 4- and 5-positions of the N1,N2-phenyl rings, but this study yields of 1-Cl and 2-Cl were 68% and 28%, respectively.
clearly showed also that cyclization of the diarylbenzene di-
amines get difficult when bulkier groups are in 2- and 6-pos- The previous synthesis of 1-Cl by Borguet and co-workers [42]
ition of the N1,N2-substituents.
required 4 steps. All the other earlier preparations and the
preparation of this paper shown in Table 1 were accomplished
A reaction course is proposed for the ring closure of 5, 6, 7 and in two steps. The overall reaction time required for the earlier
8 forming the benzimidazolium salts 1-Cl, 2-Cl, 3-Cl and 4-Cl, synthetic access of 1-Cl was quite high (112 h), as well, in com-
which is suggested to pass through stages a and b with alcohol parison with the use of the TMSCl–triethyl orthoformate
elimination (Scheme 3) and eventually then cyclization is initi- reagent presented in this work. By this the total reaction time
ated via the 2-ethoxy-1,3-diaryldihydrobenzimidazole species c could be shortened to 17 h and the overall yields raised from
and d enforced by the applied higher temperatures. TMSCl is 23% to 55%. Despite the fact that 2-step syntheses are
Scheme 3: Proposed ring-closure mechanism for 1-Cl, 2-Cl, 3-Cl and 4-Cl.
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Table 1: Comparison of various preparations of the benzannulated NHCs: 1-Cl, 2-Cl, 3-Cl and 4-Cl, and of 1-BF4, 3-BF4 and 4-BF4.
Starting
materials
Reaction
conditions
Intermediate product
Reaction
conditions
Final
product
(B)
Time Stepsb Overall Ref.
(A)
(h)a
yield
(%)
115 °C, 15 h,
Pd(dba)2,
P(t-Bu)3,
t-BuONa,
toluene
145 °C,
1,25 h,
HC(OEt)3,
neat,
This
work
16.25
2
55
TMSCl
yield 82%
yield 67%
6
1-Cl
115 °C, 15 h,
Pd(dba)2,
P(t-Bu)3,
t-BuONa,
toluene
145 °C,
3.3 h,
HC(OEt)3,
neat,
This
work
18.3
2
16
TMSCl
yield 58%
yield 28%
7
2-Cl
92 °C, 4 h,
Pd(dba)2,
P(t-Bu)3,
toluene
145 °C, 1 h,
HC(OEt)3,
neat,
This
work
5
2
74
TMSCl
yield 89%
Yield 83%
5
3-Cl
145 °C, 1 h,
HC(OEt)3,
neat,
TMSCl
yield 98%
Microwave,
185 °C,
0.58 h, neat,
yield 91%
This
workc
1.75
2
89
8
4-Cl
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Table 1: Comparison of various preparations of the benzannulated NHCs: 1-Cl, 2-Cl, 3-Cl and 4-Cl, and of 1-BF4, 3-BF4 and 4-BF4. (continued)
80 °C, 2 h,
80 °C, 18 h,
HC(OEt)3,
DMSO, CuI,
HCl conc.,
L-Pro,
K2CO3
20
2
27
[44]
HCOOH
cat.
yield 30%
yield 91%
5
3-Cl
3-Cl
4-Cl
110 °C, 15 h,
Pd2(dba)3,
BINAP,
NaOt-Bu,
toluene
80 °C, 2 h,
HC(OEt)3,
HCl conc.,
HCOOH
cat.
17
2
77
[45]
yield 86%
yield 90%
5
110 °C, 15 h,
Pd2(dba)3,
BINAP,
NaOt-Bu,
toluene
80 °C, 2 h,
HC(OEt)3,
HCl conc.,
HCOOH
cat.
17
2
85
[45]
yield 91%
yield 94%
8
110 °C, 14 h,
Pd(OAc)2,
BINAP,
NaOt-Bu,
toluene
three
stepsd
total time
97 h
overall
yield 29%
111
4
25
[42,43]
yield 86%
6
1-BF4
−78 °C,
THF,
12-crown-4,
n-BuLi,
TMSCl,
Cr(CO)6
yield 13%
60 °C, 15 h,
Pd(OAc)2,
BINAP,
NaOt-Bu,
toluene
17
2
11
[53]e
yield 86%
6
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Table 1: Comparison of various preparations of the benzannulated NHCs: 1-Cl, 2-Cl, 3-Cl and 4-Cl, and of 1-BF4, 3-BF4 and 4-BF4. (continued)
110 °C, 5 h,
145 °C,
Pd2(dba)3,
18 h, neat,
Xantphos,
NaOt-Bu,
toluene
NH4BF4,
HC(OEt)3
yield 91%
23
2
82
[54]
yield 91%
5
3-BF4
microwave,
160 °C,
0.75 h,
microwave,
185 °C,
0.58 h, neat,
yield 91%
neat,
2
2
90
[48]
NH4BF4,
HC(OEt)3
yield 99%
8
4-BF4
aTotal reaction time: The overall reaction times refer to reaction times and do not take into account the work-up times needed in order to obtain either
the intermediate and final products. bTotal number of steps: Steps with isolated intermediate or final products. cCompound 8, precursor to 4-Cl or
4-BF4, was prepared according to Ref. [47]. d1st step: NaIO4, SiO2, CH2Cl2/H2O, 24 h, room temperature, Yield 90%; 2nd step: a) PivOCH2Cl, AgOTf,
KOAc, CH2Cl2, 48 h, 50 °C. b) PivOCH2Cl, AgOTf, 24 h, 50 °C (further 0.5 equiv of the pivalate/silver salt solution); 3rd step: HBF4, H2O, 1 h, room
temperature, yield 32% (over two steps). eThe intermediate 6 reacts with a carbamoyl chromate intermediate to give rise directly to the organometallic
complex (1,3-bis(2’,4’,6’-trimethylphenyl)benzimidazol-2-ylidene)pentacarbonyl chromium(0).
Figure 3: Molecular structure of 2-Cl. The solvate molecule, the counterion and the hydrogen atoms are omitted for clarity. The displacement ellip-
soids are drawn at the 30% probability. Selected bond lengths (Å) and angles (°): N2–C1–N1 109.3(2), N2–C1–H1 126.1(17), N1–C1–H1 124.6(17),
N1–C1 1.334(3), N2–C1 1.330(3), N1–C2 1.392(3), N2–C7 1.397(3), N1–C8 1.448(3), N2–C20 1.447(3), C1–H1 0.97(3).
available for 3-Cl and 4-Cl, Table 1 suggests that the introduc- overall reaction time (19 hours) and the low number of steps (2)
tion of the TMSCl–triethyl orthoformate reagent has advan- together with the fact that this is the only way to access this
tages in terms of overall yields and reaction times. The overall compound, makes the given route an acceptable synthetic
achieved yield of 16% for 2-Cl is indeed low, but the short pathway.
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Conclusion
or Bruker DRX 500 spectrometer using 5 mm diameter NMR
We presented a new synthetic route for the synthesis of benz- tubes. Chemical shifts are expressed in parts per million (ppm).
annulated imidazolium salts bearing bulky aromatic 1H and 13C{1H} NMR spectra were referenced to the residual
N-substituents. The synthetic approach applied allowed to proton or 13C resonances of the deuterated solvent. Signal
prepare the as yet inaccessible 1,3-diisopropylphenylbenzimida- patterns are reported as follows: s, singlet; d, doublet; dd,
zolium chloride 2-Cl in two steps starting from 1,2-halo-substi- doublet of doublets; t, triplet; m, multiplet, q, quartet. ESIMS
tuted benzenes avoiding purification by column chromatog- spectrometric data were obtained from an HCT Esquire Bruker
raphy. In addition the known 1,3-dimesitylbenzimidazolium Daltonics instrument, while the EIMS spectrometric data were
chloride 1-Cl could be synthesized in better yields and much obtained from a Varian 450GG-Saturn 2000 GC/MS/MS instru-
shorter reaction times when compared with the previously ment using a Brechbühler column (30 m, ZB-5ms). The reac-
reported method. The known benzannulated NHCs 1,3- tions that required the use of a microwave reactor were carried
diphenylbenzimidazolium chloride 3-Cl and the 1,3-di(pyridin- out using an Anton Paar Monowave 300. The high resolution
2-yl)benzimidazolium chloride 4-Cl could be prepared also in ESI were performed by the Mass Service of the University of
excellent overall yields and reduced reaction times, if compared Zurich using a Bruker maXis apparatus. The compounds 5 [44],
with the synthetic access by previous methods. The synthetic 6 [52], 7 [46], 8 [47,48] were prepared according to literature
access presented in this paper possesses various advantages with small modifications.
over the conventional methodologies for the synthesis of
benzimidazolium salts with bulky N-substituents. The Synthesis of 1,3-bis(2,4,6-trimethylphenyl)-1H-
developed synthetic route may show greater generality and benzo[d]imidazol-3-ium chloride (1-Cl). In a 125 mL two
therefore constitutes a valid alternative to the earlier synthetic neck round-bottomed flask was weighed N1,N2-bis(2,4,6-
accesses.
trimethylphenyl)-1,2-benzenediamine (6, 600 mg, 1.74 mmol)
and triethyl orthoformate (60 mL) was added and a fractional
distillation apparatus equipped with a Vigreux column and a
thermometer on the head of the latter was connected with the
Experimental
X-ray diffraction study
Crystallographic data for the structure of 2-Cl has been central neck of the flask. The green solution was stirred at
deposited with the Cambridge Crystallographic Data Centre as 145 °C until the color switched to orange (15 min) and then a
supplementary publication number CCDC 1018301. Copies of small flux of nitrogen was passed through the solution until
the data can be obtained free of charge, on application to 30 mL of a mixture of EtOH and HC(OEt)3 distilled out
CCDC, 12 Union Road, Cambridge CB2 1EZ, UK (e-mail: (1 hour). Then the temperature was lowered to 90 °C and
deposit@ccdc.cam.ac.uk).
trimethylsilyl chloride (10 mL, 76.43 mmol) was added at once.
A white-grey solid started to form after 10 minutes. Then the
2-Cl: Crystal data for C32H41N2Cl3 (M = 560.02 g/mol): mono- solution was cooled down to room temperature and the precipi-
clinic, space group P21/n (no. 14), a = 12.6905(3) Å, b = tate was filtered off and washed with Et2O (3 × 15 mL) then
18.5911(3) Å, c = 14.3067(4) Å, β = 113.687(3)°, V = dried to afford in 455 mg of 1-Cl as an off-white powder
3091.02(15) Å3, Z = 4, T = 183(2) K, μ(Cu Kα) = 2.843 mm−1, (1.16 mmol, 390.95 g/mol). Yield 67%. 1H NMR (CD3OD,
Dcalc = 1.203 g/cm3, 40306 reflections measured (9.5° ≤ 2Θ ≤ 300 MHz) δ 10.09 (s, 1H, N-CH1-N), 7.84 (dd, J = 3.2 Hz, J =
178.9°), 7060 unique (Rint = 0.0323, Rsigma = 0.0199) which 6.3 Hz, 2H, H-4, H-5), 7.56 (dd, J = 3.2 Hz, J = 6.3 Hz, 2H,
were used in all calculations. The final R1 was 0.0777 (I > H-3, H-6), 7.30 (s, 4H, H-10, H-12, H-16, H-18), 2.45 (s, 6H,
2σ(I)) and wR2 was 0.2142 (all data).
p-CH3, mesityl), 2.11 (s, 12H, o-CH3, mesityl);
13C{1H}(CD3OD, 75 MHz) δ 143.55 (N-C1-N), 136.54
(N-C2=C7-N), 132.94 (C4=C5), 131.27 (N-C8, C11), 130.24
General procedures
All manipulations were carried out under an atmosphere of dry (C3, C6), 129.34 (C10, C12), 114.93 (C9, C13), 21.34 (p-CH3),
nitrogen using standard Schlenk techniques or in a glove box 17.45 (o-CH3); HRMS–ESI (MeOH): calcd for C25H27N2+,
(M. Braun 150B-G-II) filled with dry nitrogen. Solvents were 355.21688; found, 355.21644.
freshly distilled under N2 by employing standard procedures
and were degassed by freeze–thaw cycles prior to use. All Synthesis of 1,3-bis[2,6-bis(1-methylethyl)phenyl]-1H-
chemicals used were purchased from Sigma-Aldrich and used benzo[d]imidazol-3-ium chloride (2-Cl). In a 50 mL two neck
as received. The deuterated solvents were dried with sodium/ round-bottomed flask was weighed N1,N2-bis[2,6-bis(1-
benzophenone and vacuum transferred for storage in Schlenk methylethyl)phenyl]-1,2-benzenediamine (7, 100 mg,
flasks fitted with Teflon valves. 1H NMR, 13C{1H} NMR data 0.23 mmol) and triethyl orthoformate (20 mL) was added and a
were recorded on a Varian Gemini-300, a Varian Mercury 200 fractional distillation apparatus equipped with a Vigreux
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Beilstein J. Org. Chem. 2015, 11, 1656–1666.
column and a thermometer on the head of the latter was Synthesis of 1,3-di(pyridin-2-yl)-1H-benzo[d]imidazol-3-ium
connected with the central neck of the flask. The colourless chloride (4-Cl). In a 125 mL two neck round-bottomed flask
suspension was stirred at 145 °C then after 20 minutes the was weighed N1,N2-di(pyridin-2-yl)benzene-1,2-diamine (8,
colour switched to green. A small flux of nitrogen was passed 300 mg, 1.14 mmol) and triethyl orthoformate (30 mL) was
through the solution until 15 mL of a mixture of EtOH and added and a fractional distillation apparatus equipped with a
HC(OEt)3 distilled out (40 min). Then fresh triethyl orthofor- Vigreux column and a thermometer on the head of the latter
mate (3 mL) was added to the solution, followed by the was connected with the central neck of the flask. The pink
trimethylsilyl chloride (4 mL, 31.51 mmol) added at once. The suspension was stirred at 145 °C until the suspension became a
solution, that switched from dark green to dark red, was stirred red solution (10 min), then a small flux of nitrogen was passed
at 50 °C for 3 hours, then the solvent was removed and the red through the solution until 20 mL of a mixture of EtOH and
solid was triturated with Et2O (20 mL) and the precipitate was HC(OEt)3 distilled out (50 min). Then trimethylsilyl
filtered off, washed with further Et2O (3 × 10 mL), then tritu- chloride (6 mL, 47.27 mmol) was added at once and the solu-
rated with acetone (3 mL) and finally dried to afford 31 mg tion switched from deep red to blue and a blue precipitate
of 2-Cl as a grey powder (0.065 mmol, 475.11 g/mol). formed immediately. The solution was cooled down to room
Yield 28%. 1H NMR (CD2Cl2, 300 MHz) δ 12.95 (s, 1H, temperature and the precipitate was filtered off, washed with
N-CH1-N), 7.74 (m, 6H, H-10, H-11, H12, H14, H-15, H-16), Et2O (3 × 10 mL) and then dried to afford 344 mg of 4-Cl as a
7.52 (d, J = 7.8 Hz, 2H, H-3, H-6), 7.39 (dd, J = 3.2 Hz, J = 6.3 blue powder (1.12 mmol, 308.76 g/mol). Yield 98%. 1H NMR
Hz, 2H, H-4, H-5), 2.29 (sept., J = 9Hz, 4H, CH3-CH-CH3 iso- (CD3CN, 300 MHz) δ 10.07 (s, N-C1H1-N, 1H), 8.84 (dd, J =
propyl), 1.33 (d, J = 9 Hz, 12H, CH3-isopropyl), 1.17 (d, J = 9 1.8Hz, J = 4.8Hz, 2H, H-12, H-16), 8.48 (dd, J = 3.2 Hz, J =
Hz, 12H, CH3-isopropyl); 13C NMR (CDCl3, 75 MHz) δ 6.4 Hz, 2H, H-4, H-5), 8.28 (td, J = 1.8 Hz, J = 8.1 Hz, 2H,
146.80 (N-C1-N), 146.02 (N-C8), 132.21 (N-C2=C7-N), H-10, H-14), 8.03 (dd, J = 1.8 Hz, J = 8.1Hz, 2H, H-9, H-13),
128.5 (C9, C13, C15, C19), 127.45 (C10, C12, C16, C18), 7.89 (dd, J = 3.2 Hz, J = 6.4 Hz, 2H, H-3, H-6), 7.77 (td, J =
124.97 (C11, C17), 113.42 (C4=C5), 29.42 (CH3-CH-CH3, iso- 3.9 Hz, J = 8.1 Hz, 2H, H-11, H-15); 13C{1H}(CDCl3,
propyl), 24.73 (CH3, isopropyl), 23.10 (CH3, isopropyl); 75 MHz) δ 150.0 (s, N-C1-N), 147.52 (C8, C13), 140.73 (C10,
HRMS–ESI (MeOH): calcd for C31H39N2+, 439.31078; found, C12), 130.96 (N-C2=C7-N), 128.52 (C4=C5), 125.89 (C3, C6),
439.31042.
117.81 (C11, C16), 116.10 (C9, C14); HRMS–ESI
(MeOH/H2O 1:1): calcd for C17H13N4, 273.11347; found,
Synthesis of 1,3-diphenylbenzimidazolium chloride (3-Cl). 273.11308.
In a 50 mL two neck round-bottomed flask was weighed N,N’-
diphenylbenzene-1,2-diamine (5, 50 mg, 0.19 mmol) and Synthesis of N1,N2-diphenylbenzene-1,2-diamine (5). In a
triethyl orthoformate (15 mL) was added and a fractional distil- glove-box a 250 mL Schlenk flask was charged with Pd(dba)2
lation apparatus equipped with a Vigreux column and a ther- (298 mg, 0.52 mmol) and P(t-Bu)3 (100 mg, 1.04 mmol).
mometer on the head of it, was connected with the central neck Subsequently was added toluene (10 mL) and the solution was
of the flask. The light blue suspension was stirred at 145 °C stirred for 10 min at room temperature. Then 1,2-dichloroben-
until the suspension became a light green solution (20 min), zene (0.65 mL, 6.67 mmol) was added followed by aniline addi-
then a small flux of nitrogen was passed through the solution tion (1.61 mL, 17.32 mmol) and the t-BuONa reagent
until 10 mL of a mixture of EtOH and HC(OEt)3 distilled out (1664 mg, 17.32 mmol). An additional amount of toluene
(ca. 45 min). Then the trimethylsilyl chloride (3.0 mL, (40 mL) was used to rinse the walls of the Schlenk vessel and
23.64 mmol) was added at once and a light blue precipitate the solution was then heated at 92 °C. The reaction was moni-
formed immediately. The solution was cooled down to room tored by GC–MS and after completion (4 h) to the solution were
temperature and the precipitate was filtered off, washed with added AcOEt (100 mL) and water (100 mL). The organic layer
Et2O (3 × 10 mL) and then dried to afford 48 mg of 3-Cl as a was washed with brine and afterwards dried over MgSO4 and
slightly green powder (0.157 mmol, 306.79 g/mol). Yield 83%. filtered. After the evaporation of the solvent under reduced
1H NMR (CDCl3, 500 MHz) δ 11.51 (s, 1H, N-C1H1-N,), 8.19 pressure we obtained 1547 mg of a deep blue solid (5.94 mmol,
(d, J = 8 Hz, 4H, H9, H13, H15, H19), 7.79 (dd, J = 3.5 Hz, J Mw = 260,33). The product has a sky-blue color when in solu-
= 6.3 Hz, 2H, H4, H5), 7.68 (m, 6H, H10, H11, H12, H16, tion, which is due to a small amount of impurities. The
H17, H18), 7.61 (t, J = 7.4 Hz, 2H, H3, H6); 13C{1H}(CDCl3, 1H NMR analysis confirmed that 5 was pure enough to be used
125 MHz) δ 139.90 (N-C1-N), 130.68 (N-C2=C7-N), 129.39 for the next synthetic step. Yield 89.1%; GC–MS (EI+): 11.074
(C8), 128.80 (C4=C5), 128.53 (C3), 126.08 (C10), 123.32 min [260.3–261.2]; (98%). 1H NMR (CDCl3, 300 MHz) δ 7.28
(C11), 111.96 (C9); HRMS–ESI (MeOH/H2O 1:1): calcd for (m, 6H), 6.97 (m, 8H), 5.65 (s, 2H, NH); 13C{1H} NMR
C19H15N2, 271.12297; found, 271.12281.
(CDCl3, 75 MHz) δ 144.8 (s, HN-C1=C6-NH), 135.0 (s, C7),
1664

Beilstein J. Org. Chem. 2015, 11, 1656–1666.
129.3 (s, C3=C4), 122.35 (s, C2), 120.3 (s, C9), 119.4 (s, C10), (CD2Cl2, 75 MHz) δ 145.77 (HN-C1=C2-NH), 137.25 (NH-
116.56 (C8).
C7), 137.12 (C8, C12), 126.49 (C10), 124.14 (C9, C11), 120.20
(C4,C5), 114.69 (C6), 28.61 (CH3-CH-CH3, isopropyl), 24.848
Synthesis of N1,N2-bis(2,4,6-trimethylphenyl)benzene-1,2- (CH3, isopropyl), 23.29 (CH3, isopropyl).
diamine (6). Inside a glove box a 250 mL Schlenk flask was
charged with Pd(dba)2 (23 mg, 0.04 mmol), the P(t-Bu)3
Supporting Information
(9.0 mg, 0.093 mmol) and subsequently toluene (10 mL). The
solution was stirred for 10 minutes at room temperature. Then
Supporting Information File 1
1,2-dichlorobenzene (0.322 mL, 3.33 mmol) was added fol-
lowed by 2,4,6-trimethylaniline (1.146 mL, 7.99 mmol) and
t-BuONa (768 mg, 7.99 mmol) and an additional amount of
toluene (40 mL) was used to rinse the walls of the Schlenk tube.
The solution was then heated at 115 °C for 15 hours and the
solid material that precipitated out of the solution was isolated
by suction filtration. The so obtained cake was washed with
water and ether and then dried to afford 941 mg of 6 as a
grayish powder (2.73 mmol, 344.49 g/mmol). Yield 82%. The
1H NMR analysis confirmed that 6 was pure enough to be used
CIF file of compound 5.
[http://www.beilstein-journals.org/bjoc/content/
supplementary/1860-5397-11-182-S1.cif]
Supporting Information File 2
NMR and HRMS–ESI analyses.
[http://www.beilstein-journals.org/bjoc/content/
supplementary/1860-5397-11-182-S2.pdf]
for the next step without further purification. MS–EI+: 12.485 Acknowledgements
min [344.5–345.3]; 1H NMR (CDCl3, 300 MHz) δ 7.025 (s, 4H, Funding from the Swiss National Science Foundation (SNSF)
H-9, H-11, H-15, H-17), 6.72 (dt, J = 3.5 Hz, J = 5.7 Hz, 2H, and the University of Zurich (UZH) is gratefully acknowledged.
H-4, H-5), 6.37 (dt, J = 3.5 Hz, J = 5.7 Hz, 2H, H-3, H-6),
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