An unexpected synthesis of dihydrophenazines en route to benzimidazolium salts

Article abstract of DOI:10.1002/adsc.201100890

The oxidation of various N,N'-diarylbenzene-1,2-diamines bearing bulky aromatic substituents with sodium periodate on wet silica gel afforded a series of five new dihydrophenazines instead of the expected cyclohexadiene-1,2- diimines. The reaction most likely proceeds via a 1,6-electrocyclic path and provides a convenient access to an important class of nitrogen heterocycles. Subsequent treatment of the mesityl derivative with chloromethyl pivalate and silver triflate led to the corresponding benzimidazolium salt. Copyright

Full text of DOI:10.1002/adsc.201100890

FULL PAPERS
DOI: 10.1002/adsc.201100890
An Unexpected Synthesis of Dihydrophenazines en Route to
Benzimidazolium Salts
Yannick Borguet,a Guillermo Zaragoza,b Albert Demonceau,a
and Lionel Delaudea,*
a
Laboratory of Macromolecular Chemistry and Organic Catalysis, Universitꢀ de Liꢁge, Institut de Chimie (B6a),
Sart-Tilman par 4000 Liꢁge, Belgium
Fax : (+32)-4-366-3497; e-mail: l.delaude@ulg.ac.be
Unidade de Difracciꢂn de Raios X, Universidade de Santiago de Compostela, Edificio CACTUS, Campus Vida, 15782
b
Santiago de Compostela, Spain
Received: November 16, 2011; Revised: January 19, 2012; Published online: April 26, 2012
Dedicated to Dr. Christian Bruneau on the occasion of his 60th birthday.
Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/adsc.201000890.
Abstract: The oxidation of various N,N’-diarylben- mesityl derivative with chloromethyl pivalate and
zene-1,2-diamines bearing bulky aromatic substitu- silver triflate led to the corresponding benzimidazoli-
ents with sodium periodate on wet silica gel afforded um salt.
a series of five new dihydrophenazines instead of the
expected cyclohexadiene-1,2-diimines. The reaction
most likely proceeds via a 1,6-electrocyclic path and Keywords: electrocyclic reactions; ligand design; ni-
provides a convenient access to an important class of trogen heterocycles; oxidation; sigmatropic rear-
nitrogen heterocycles. Subsequent treatment of the rangement
Introduction
late benzimidazolium salts bearing bulky aromatic
substituents on their nitrogen atoms.[9] This is most
Stable N-heterocyclic carbenes (NHCs) have become unfortunate because large aryl groups, such as the me-
ubiquitous ligands in organometallic chemistry and in sityl (2,4,6-trimethylphenyl) or 2,6-diisopropylphenyl
homogeneous catalysis.[1] They have already afforded groups, usually provide the right balance of steric pro-
an impressive range of transition metal complexes tection and electronic donation to many NHC-based
that have found countless applications in organic syn- catalytic systems, while avoiding unwanted ortho-met-
thesis and in polymer chemistry.[2] They have also ac- alation side-reactions.[10]
quired a place on their own as reagents and organoca-
To the best of our knowledge, the only efficient
talysts, since they behave as powerful nucleophiles.[3] strategy developed so far for installing mesityl sub-
Currently, the NHCs most commonly encountered are stituents onto polyannulated NHCs involved the con-
based on the imidazole ring system.[4] They are usual- densation of N,N’-dimesitylformamidine with halo-
ly obtained by deprotonation of either imidazolium genated quinones to afford quinoimidazolium salts.[11]
(A) or imidazolinium salts (C) with a strong base
(Figure 1).[5] Various experimental and theoretical
studies have shown that subtle differences between
“unsaturated” and “saturated” NHCs could signifi-
cantly affect their catalytic properties.[6]
Fused polycyclic NHCs, including benzimidazolyli-
dene species derived from “aromatic” benzimidazoli-
um salts (B), offer further options to fine-tune the
steric and electronic properties of carbene ligands.[8]
However, their synthesis is often challenging. Indeed,
Figure 1. Structure of various azolium salts used as NHC
several research groups have reported failures to iso- precursors.[7]
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An Unexpected Synthesis of Dihydrophenazines en Route to Benzimidazolium Salts
In this article, we disclose our endeavors to isolate
the up-to-now elusive 1,3-dimesitylbenzimidazolium
tetrafluoroborate (2a) and we show that its synthesis
proceeds via the unexpected formation of dihydro-
phenazines.
Results and Discussion
To begin with, we decided to reinvestigate the classi-
cal amination/cyclization path most commonly adopt-
ed to prepare 1,3-diarylbenzimidazolium salts.[12]
Thus, a range of aniline derivatives substituted in
ortho and para positions were coupled with 1,2-dibro-
mobenzene in a palladium-catalyzed Buchwald–Hart-
wig amination reaction (Scheme 1).[13] We then tried
to condense N,N’-bismesityl-1,2-benzenediamine (1a)
into benzimidazolium salt 2a using triethyl orthofor-
mate and tetrafluoroboric acid in a microwave reactor
(Scheme 2, path I). Previous work from our laborato-
ry had shown that recourse to microwave irradiation
usually provides a fast and efficient access to cyclic
amidinium salts.[14] In this particular case, however,
model substrate 1a was recovered unchanged even
Scheme 2. Failed attempts to cyclize diamine 1a into NHC
precursors 2a–4a. Reaction conditions: HC(OEt)3, HBF4,
2008C (mW), 4 h (path I); S=CCl2, THF, room temperature,
4 h (path II); O=C(OCCl3)2, Na2CO3, THF, 708C, 24 h
(path III).
AHCTUNGTRENNUNG
AHCTUNGTRENNUNG
AHCTUNGTRENNUNG
At this stage, we turned our attention to the cycli-
when rather drastic conditions were applied (4 h at zation of diimine precursors into imidazolium salts
2008C). using a combination of chloromethyl pivalate and
Recourse to stronger electrophilic reagents was silver triflate. Since it was first reported by Glorius
also explored to promote the cyclization of starting et al. in 2002,[17] this method has been widely used for
material 1a into suitable NHC precursors. A first set the synthesis of strained and sterically encumbered
of experiments inspired by related work on N,N’-di- imidazolium salts.[18] Because its application to the
ACHTUNGTRENNUNGalkylbenzimidazol-2-ylidenes was conducted with thi- synthesis of 2a would require the preliminary conver-
ophosgene (Scheme 2, path II).[15] Despite several at- sion of diamine 1a into the corresponding 3,5-cyclo-
tempts to tinker with the reaction conditions by hexadiene-1,2-diimine, we searched for a suitable oxi-
changing the nature of the base (Et3N or Na2CO3) dizing agent to perform this additional reaction. Re-
and the temperature (room temperature or 708C), the ports from the literature dealing with the oxidation of
expected thione 3a could not be isolated. Alternative- ortho-phenylenediamines are scarce,[19] but we felt en-
ly, we also tried to perform a cyclization of substrate couraged by related work from the groups of
1a with triphosgene in the presence of Na2CO3 in Harlan[20] and Bielawski[21] who showed that 1,2,4,5-
THF at 708C (Scheme 2, path III).[16] Once again, the tetraaminobenzene derivatives 5 had a high propensi-
desired product 4a was not obtained.
ty to oxidize into the corresponding 2,5-diamino-1,4-
benzoquinonediimines 6 under aerobic conditions
(Scheme 3). An equilibrium between these azophena-
zines and their ortho-quinoid tautomers 7 was then
postulated to expose the vicinal diamino and diimino
fragments that ultimately led to the benzobis(imida-
zolium) salts 8 in a one-pot cyclization/oxidation reac-
tion sequence.[21]
Attempts to oxidize diamine 1a under aerobic con-
ditions showed that the reaction was very sluggish,
most likely because this substrate is not as electron-
rich as the analogous tetraamine derivative 5. Thus,
we decided to use a stronger oxidizing agent. Sodium
periodate on wet silica gel was chosen, because it is
an efficient, easy to handle reagent for the oxidation
of hydroquinones into quinones.[22] Indeed, a complete
conversion of substrate 1a occurred within 24 h using
a heterogeneous system based on NaIO4/SiO2 and
Scheme 1. Synthesis of N,N’-diaryl-1,2-benzenediamines 1a–f
via Buchwald–Hartwig amination. Reaction conditions:
Pd
G
PACTHNGUETRNU(NG t-Bu)3 (12 mol%), NaO-t-Bu,
PhCH3, 1108C, overnight.
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To probe the generality of this new oxidative cycli-
zation path, we carried out additional reactions start-
ing from diamines 1b–f instead of the mesityl deriva-
tive 1a. These experiments showed that the presence
of methyl groups on both ortho positions of the
remote aryl substituents was mandatory to achieve
the clean formation of a single product. Indeed, dihy-
drophenazines 9b and 9c were isolated in high yields
when substrates 1b or 1c were treated with the per-
AHCTUNGTRENNUNG
iodate reagent (Scheme 4 and Figure 2).[23] Structural
variations on the para-position of the 2,6-dimethyl-
phenyl ring (1a: Me, 1b: H, 1c: Cl) did not seem to
have any significant influence on the outcome of the
reaction. Besides, the spectral properties and molecu-
lar structures of compounds 9a–c were very similar
(see the Supporting Information for details). Con-
versely, substrates 1d and 1e that were obtained from
2-chloro-4,6-dimethylaniline and 2-tert-butylaniline,
respectively, afforded only complex mixtures of un-
identified products.
Scheme 3. Synthesis of benzobis(imidazolium) salts via an
oxidation/cyclization route.
We also examined the reactivity of diamine 1f bear-
ing 2,6-diisopropylphenyl substituents on its nitrogen
atoms. In this case, the reaction with NaIO4/SiO2 took
Scheme 4. Synthesis of dihydrophenazines 9a–c.
a single product was isolated in 90% yield after chro-
matographic purification (Scheme 4). NMR analysis
of this compound revealed, however, that it possessed
a much more unsymmetrical structure than expected.
Indeed, there were no less than 24 different carbon
atoms and 14 peaks for hydrogen atoms visible on 13C
and 1H NMR spectra, respectively. Recourse to
DEPT, HMBC, and HSQC sequences helped us
assign these various signals to racemic dihydrophen-
ACHTUNGTRENNUNG
Scheme 5. Synthesis of dihydrophenazines 10 and 11.
Figure 2. Molecular structures of dihydrophenazines 9a–c in the solid state (hydrogen atoms were omitted for clarity).
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An Unexpected Synthesis of Dihydrophenazines en Route to Benzimidazolium Salts
Scheme 7. Synthesis of benzimidazolium salt 2a.
also Table S2 in the Supporting Information). This
stereochemistry is in good agreement with the disro-
tatory cyclization path expected for a thermal p6s
electrocyclic process.[25] When the diamine 1f served
as starting material, an additional sigmatropic rear-
rangement may be invoked to explain the migration
of the angular isopropyl group onto its final position
Figure 3. Molecular structures of dihydrophenazines 10 and
11 in the solid state (hydrogen atoms were omitted for clari-
ty).[24]
a different course and afforded a mixture of 1,5-dihy- in products 10 and 11. This would release the steric
drophenazine 10 and its 5,10-isomer 11 in a 3:1 molar pressure exerted by the bulky 2,6-diisopropylphenyl
ratio (Scheme 5). The structures of these two products substituent located on the proximal 5-N heterocyclic
1
were established by H and 13C NMR spectroscopy position. The formation of the major 1,5-
and further confirmed by X-ray diffraction analysis dihydrophen
ACHTUNGTRNEaNUNG zine 10 may be explained by a [1,5]-
(Figure 3).[23,24]
shift, whereas the minor 5,10-isomer 11 would result
The formation of 4a,5-dihydrophenazines 9a–c prob- from a thermally less favored [1,3]-migration of the
ably begins with the oxidation of the N,N’-diarylben- alkyl substituent.
zene-1,2-diamines 1a–c into the corresponding di-
Last but not least, we were very pleased to find
ACHTUNGTRENNUNG
tected in the reaction media. We tentatively propose pivalate and silver triflate as initially planned to
that they further react via a 6 eÀ electrocyclic process afford the desired 1,3-dimesitylbenzimidazolium salt
involving the diimine moiety and a neighboring aryl 2a in 32% isolated yield (Scheme 7). This result is in
substituent to afford ring closure (Scheme 6). Al- accordance with the reversible nature of an electrocy-
though the nitrogen atom bearing the remaining aryl clic process and is reminiscent of the equilibrium
group featured an almost planar geometry indicative postulated by Bielawski between structures 6 and 7,
of a substantial sp2 character, an anticlinal conforma- although the latter one was never observed.[21] It
tion between the 5-aryl group and the angular 4a- should be pointed out that the modest yield obtained
methyl substituent was clearly visible in the X-ray for product 2a is due in part to its tedious purification
crystal structures of compounds 9a–c (cf. Figure 2, see from tarry ionomer by-products.[10c,26]
Conclusion and Perspectives
Altogether, the synthetic path that we have devised
paves the way to benzimidazol-2-ylidene species bear-
ing bulky aromatic substituents on their nitrogen
atoms, and we are currently investigating the ligand
properties of these new NHCs. This study also opens
new vistas in the chemistry of dihydrophenazines. The
recognition that pericyclic reactions occur readily on
these planar, highly conjugated nitrogen heterocycles
could ease the access to this important family of bio-
logically active compounds.[27] Furthermore, the pres-
ence of a quaternary chiral center in products 9a–c is
an attractive feature for future developments in asym-
metric organic synthesis.
Scheme 6. Possible mechanism for the formation of dihydro-
phenazines 9a–c, 10 and 11.
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Yannick Borguet et al.
FULL PAPERS
508C under an inert atmosphere in the dark. After cooling
to room temperature, the reaction mixture was quenched
with methanol and the solvents were evaporated under
vacuum. The resulting black oil was purified by preparative
TLC on alumina using H2O/MeOH (6/4 v/v) as eluents. The
product (Rf =0.4) was desorbed with methanol to afford
a light-yellow oil, which was taken up with water (10 mL)
and diethyl ether (2 mL). 48% Aqueous HBF4 (0.5 mL) was
added and the biphasic mixture was stirred for 1 h at room
temperature. Upon evaporation of diethyl ether, a light-
yellow precipitate was obtained. It was filtered and water
was removed by azeotropic distillation with benzene. The
precipitate was further dried for one night under high
vacuum to afford the title compound as a light-yellow solid;
Experimental Section
General Information
Toluene was distilled from sodium under N2. Aniline deriva-
tives were stirred over KOH and distilled under vacuum
prior to use. N,N’-Diaryl-1,2-benzenediamines 1a–f were
synthesized according to literature procedures.[20] Palladi-
ACHTUNGTRENNUNGum(II) acetate, tri-tert-butylphosphine (1M solution in tolu-
ene), 1,2-dibromobenzene, sodium tert-butoxide, sodium
periodate, silver(I) triflate, chloromethyl pivalate, potassium
acetate and tetrafluoroboric acid (48% solution in water)
were purchased from Aldrich and used as received. Petrole-
um ether refers to the hydrocarbon fraction of bp 40–608C
and was purchased from Labotec. Column chromatography
was performed on silica gel 60 (60 ꢄ nominal pore diameter,
0.063–0.200 mm particle size) supplied by Biosolve. Prepara-
tive TLC was performed on PLC aluminum oxide 60 F254
glass plates (20ꢅ20 cm, 1.5 mm thickness) supplied by
˜
yield: 205 mg (32%); mp 2318C (dec.). IR (KBr): n=3116
(m), 3053 (m), 2912 (m), 1811 (w), 1613 (m), 1542 (m), 1484
(m), 1467 (m), 1384 (w), 1319 (w), 1288 (w), 1225 (m), 1060
(s), 909 (w), 861 (m), 758 cmÀ1 (m); 1H NMR (400 MHz,
CDCl3): d=9.59 (s, 1H, Im C2), 7.68 (dd, J=6.3 and 3.1 Hz,
2H), 7.33 (dd, J=6.3 and 3.1 Hz, 2H), 7.06 (s, 4H, Mes m-
CH), 2.33 (s, 6H, Mes p-CH3), 1.98 (s, 12H, Mes o-CH3);
13C NMR (101 MHz, CDCl3): d=142.8, 141.6, 134.8, 131.4,
130.0, 128.8, 127.6, 113.4, 21.0, 17.0; HR-MS (ESI): m/z=
355.21653, calcd. for C25H27N2: 355.21688 [(MÀBF4)+].
1
Merck. H and 13C NMR spectra were recorded on a Bruker
DRX 400 or a Bruker Avance 250 spectrometer at 298 K
unless otherwise specified. Chemical shifts are listed in parts
per million downfield from TMS and are referenced from
the solvent peaks or TMS. IR spectra were recorded with
a Perkin–Elmer Spectrum One FT-IR spectrometer. Mass
spectral analyses were performed on a Bruker Daltonics mi-
crOTOF-Q spectrometer at the Plateforme d’Analyses Chi-
miques et de Synthꢁse Molꢀculaire of the University of
Bourgogne.
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Typical Procedure for the Oxidation of Benzene-1,2-
diamines into Dihydrophenazines
A 0.65M aqueous solution of NaIO4 (8.70 mL, 5.65 mmol)
was added dropwise to a vigorously stirred suspension of
chromatographic grade silica gel (8.70 g) in CH2Cl2 (70 mL),
whereupon a flaky suspension was obtained. A solution of
1a (1.50 g, 4.35 mmol) in CH2Cl2 (15 mL) was then added
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imidazolium Tetrafluoroborate (2a)
Chloromethyl pivalate (0.29 mL, 1.35 equiv.) was added to
a suspension of silver triflate (506.4 mg, 1.35 equiv.) in
CH2Cl2 (10 mL) under argon. The resulting suspension was
stirred for 45 min at room temperature. The supernatant so-
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of dihydrophenazine 9a (500 mg, 1 equiv.) and potassium
acetate (157.6 mg, 1.1 equiv.) in CH2Cl2 (10 mL). The result-
ing suspension was stirred for 48 h at 508C under an inert
atmosphere in the dark. Next, a further 0.5 equiv. of the piv-
alate/silver salt solution were added at room temperature
and the resulting suspension was further stirred for 24 h at
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Adv. Synth. Catal. 2012, 354, 1356 – 1362
ꢃ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
asc.wiley-vch.de
1361

Yannick Borguet et al.
FULL PAPERS
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