New azaheterocyclic aromatic diphosphonates for hybrid materials for fuel cell applications

Article abstract of DOI:10.1039/c3nj00585b

New azaheterocyclic aromatic diphosphonate derivatives of benzimidazole and benzotriazole were synthesized by nickel-catalyzed Arbuzov reaction of 4,7-dibromo-2,1,3-benzothiadiazole with triethyl phosphite, followed by reductive sulfur extrusion reaction

Full text of DOI:10.1039/c3nj00585b

NJC
View Article Online
View Journal | View Issue
PAPER
New azaheterocyclic aromatic diphosphonates for
hybrid materials for fuel cell applications†
Cite this: NewJ.Chem., 2013,
37, 3084
a
a
bc
´
´
Fatima C. Teixeira,* C. M. Rangel and Antonio P. S. Teixeira
New azaheterocyclic aromatic diphosphonate derivatives of benzimidazole and benzotriazole were
synthesized by nickel-catalyzed Arbuzov reaction of 4,7-dibromo-2,1,3-benzothiadiazole with triethyl
phosphite, followed by reductive sulfur extrusion reaction and cyclization. This new strategy allowed us
to obtain these compounds with high efficiency, with the generation of these azaheterocyclic aromatic
diphosphonate derivatives in good to excellent yields, since these compounds could not be synthesized
by direct cross-coupling reactions catalyzed by palladium or nickel. All compounds were characterized
by NMR, IR spectroscopy and mass spectrometry (low and high resolution). NMR studies of compound 9
showed the presence of only one tautomeric form, on the NMR time scale, in different solvents and at
different concentrations.
Received (in Porto Alegre, Brazil)
1st June 2013,
Accepted 12th July 2013
DOI: 10.1039/c3nj00585b
www.rsc.org/njc
of new alternative membranes for the operation of fuel cells
at temperatures above 100 1C. Above this temperature, the
Introduction
The development of cleaner and sustainable sources of energy
is one of the major challenges of the 21st century. Alternative
energy systems are crucial in order to deal with the environ-
mental threat of global warming and the use of fossil energy
sources. Fuel cells are electrochemical devices that convert the
chemical energy stored directly into electrical energy. Fuel cells
can provide electrical energy with high efficiency and low environ-
mental impact. But, their performance depends crucially on the
properties of component materials.1–7
Among the various kinds of fuel cells, the proton-exchange
membrane fuel cells (PEMFCs) are considered promising power
sources, due to their high power density and high power-to-
weight ratio. A key material for the operation of PEMFCs is the
proton-exchange membrane (PEM). Usually, these membranes
are made of organic polymers containing acidic functionalities
(e.g. Nafions), but the proton transport properties of these
membranes strongly depend on their water content and, con-
sequently, limit their operation temperatures up to 90 1C. These
limitations have fostered the interest in research and development
performance of fuel cells increases, due to faster electrode
reaction without CO poisoning of the Pt electro-catalyst.1–7
New alternative membranes include polybenzimidazole (PBI)-
doped membranes, composites of Nafions and metal oxides,
sulfonated polymers based on aromatic hydrocarbons and
organosiloxane-based inorganic–organic hybrids with various
acidic species.1–7
Phosphonic acids are considered to be promising proton
carriers because of their good proton donating and accepting
properties.8–10 In addition, phosphonic acids are an alternative
to sulfonic acid groups due to their high proton conductivities,
oxidation resistance and better thermal stabilities.9–11
Arylphosphonates have numerous practical applications,
including in the design of fuel cell membranes.10,12,13 Conti-
nuous efforts have been made to construct C–P bonds directly
through metal-catalyzed C–P coupling of arylhalides to obtain
arylphosphonates.14 The Hirao reaction15 and the nickel-catalyzed
Arbuzov reaction16 are widely used methods employed for the
synthesis of arylphosphonates.
Phosphonylated azaheterocycles have gained
a lot of
interest over the years. Direct phosphonylation of aromatic
azaheterocycles increased significantly and was reviewed by
Van der Jeught and Stevens in 2009.17 Direct phosphonylation
of 6-bromobenzimidazole derivatives by Hirao reaction, using
diethyl phosphite in the presence of catalytic amounts of
tetrakis(triphenylphosphine)palladium, was reported to obtain
the (1-ethyl-2-methyl-6-benzimidazolyl)phosphonate and diethyl
(1-phenyl-2-methyl-6-benzimidazolyl)phosphonate.17 Recently
Li et al. reported the first palladium-catalyzed direct phosphonation
a
´
Laboratorio Nacional de Energia e Geologia, I.P., Estrada do Paço do Lumiar, 22,
1649-038 Lisboa, Portugal. E-mail: fatima.teixeira@lneg.pt,
carmen.rangel@lneg.pt; Fax: +351 217 162 641
b
c
´
´
´
Centro de Quımica de Evora & Departamento de Quımica,
´
ˆ
˜
Escola de Ciencias e Tecnologia, Universidade de Evora, R. Romao Ramalho,
´
59, 7000-671 Evora, Portugal
´
´
Centro de Quımica Estrutural, IST, Universidade Tecnica de Lisboa,
Av. Rovisco Pais 1, 1049-001 Lisboa, Portugal
† Electronic supplementary information (ESI) available: NMR data (1H, 13C and
31P NMR spectra) for new compounds. See DOI: 10.1039/c3nj00585b
c
3084 New J. Chem., 2013, 37, 3084--3091
This journal is The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2013

View Article Online
Paper
NJC
of azoles with dialkyl phosphite without addition of base or
1H NMR spectra of compounds 2–4 were in agreement with
acid; however N-methylbenzimidazole and N-methylindole are the literature.21,23 The NMR spectrum of 4,7-dibromobenzimid-
inactive for this direct phosphonylation.18 No synthetic routes azole 4 shows the presence of two tautomeric forms, in a fast
are known towards diphosphonylation of 4- and 7-substituted equilibrium on the NMR time scale, with equivalent proton
benzimidazoles.
atoms 5-H and 6-H, as usually described for benzimidazole.24,25
Herein, we report a novel and efficient strategy for the
The new N-1 protected 4,7-dibromobenzimidazoles 5 and 6
phosphonylation of azaheterocycles of general formula (EtO)2(O)P– were fully characterized by NMR, IR spectroscopy and mass
R–P(O)(OEt)2, where R is a benzothiadiazole, benzimidazole or spectrometry (low and high resolution). After substitution at
benzotriazole substituted at 4- and 7-positions, which could be the N-1 position by the protecting groups benzyl or THP
used as starting materials to obtain hybrid membranes for (4,7-dibromobenzimidazoles 5 and 6, respectively), NMR spectra
PEMFCs with modified properties. This strategy can also be of compounds 5 and 6 showed different shifts for all hydrogen
applied to the synthesis of other diverse phosphorylated azoles. and carbon atoms.
Electron Impact Ionization (EI) Mass Spectrometry spectra of
compounds 5 and 6 show the molecular ion of both compounds,
which confirmed the proposed molecular formulae.
Results and discussion
The Hirao reaction and the nickel-catalyzed Arbuzov reac-
Initial studies were carried out in order to obtain new diphos-
tion are widely used methods employed for the synthesis of
phonylated 4,7-benzimidazoles. The formation of a carbon–
arylphosphonates.15,16 Our first studies to achieve the construction
phosphorus bond in these compounds was performed starting
of C(sp2)–P bonds were performed by the Hirao cross-coupling
from the 4,7-dibromobenzimidazole 4, which was protected,
previous to the phosphonylation reactions, at the N-1 position
with a benzyl or a tetrahydropyranyl (THP) group. Our strategy
for the synthesis of the protected 4,7-dibromobenzimidazoles 5
reaction of diethyl phosphite with 4,7-dibromobenzimidazole 5
or 6, using Pd(PPh3)4 as a catalyst and triethylamine as base
(Scheme 2). However these reactions failed under the mentioned
conditions and the cross-coupling reaction did not occur.
Tolmachev used this procedure for the synthesis of diethyl
(1-ethyl-2-methyl-6-benzimidazolyl)phosphonate and diethyl
(1-phenyl-2-methyl-6-benzimidazolyl)phosphonate, starting from
and 6 is summarized in Scheme 1.
Compound 2 was prepared in high yield by bromination of
commercially available 2,1,3-benzothiadiazole 1.19 Reduction of 2
with NaBH4 gave 1,2-diamine 3 in moderate yield.20–22 Cyclization
a 6-bromobenzimidazole derivative, a meta-substituted benz-
of 1,2-diamine 3 to 4,7-dibromobenzimidazole 4 was performed in
excellent yield by reaction with trimethyl orthoformate under
imidazole, with no steric effect caused by ortho-groups.17
A second attempt was made using the Arbuzov nickel-
catalyzed cross-coupling reaction of 4,7-dibromobenzimidazole
catalytic acid conditions, using (Æ)-camphorsulfonic acid (CSA).
Treatment of 4,7-dibromobenzimidazole 4 with benzylbromide
5 or 6 with triethyl phosphite and NiBr2, but that also failed
and K2CO3 in DMF gave compound 5 in 89% yield and treatment
(Scheme 2). In the case of reaction of 4,7-dibromobenzimid-
of 4,7-dibromobenzimidazole 4 with dihydropyran in CH2Cl2 in
the presence of a catalytic amount of p-TsOH gave compound 6 in
91% yield (Scheme 1).
azole 6, a deprotection occurs and compound 4 was isolated
instead.
The failure of both reactions probably occurred due to the
steric hindrance of ortho groups as previously reported for aryl
substrates.16,26–28 It has been observed for Arbuzov reaction
that a b branch in the alkyl halide retards the reaction.27
In view of these results, an alternative strategy was devised
by us. This strategy involved C–P cross coupling bond forma-
tion, followed by reductive sulfur extrusion and cyclization,
starting from 4,7-dibromo-2,1,3-benzothiadiazole 2 (Scheme 3 –
strategy 2). Compound 2 is a very common intermediate in the
C–C and C–N cross-coupling reactions, but no C–P cross-
coupling reaction was yet described.29
Under Hirao’s conditions, no product was observed in the reac-
tion of 2,4-dibromo-2,1,3-benzothiadiazole 2 and diethyl phosphite,
Scheme 2 Attempted synthesis of benzimidazole diphosphonates 7 and 8 by
Scheme 1 Synthesis of new N-1 protected 4,7-dibromobenzimidazoles 5 and 6.
Hirao reaction and Arbuzov nickel-catalyzed reaction.
c
This journal is The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2013
New J. Chem., 2013, 37, 3084--3091 3085

View Article Online
NJC
Paper
Scheme 5 Synthesis of benzimidazole diphosphonate 9.
Reduction of azadiphosphonate 10 with NaBH4 gave
1,2-diamine 11 in moderate yield (62%) (Scheme 5). Cyclization
reaction of diamine diphosphonate 11 to the desired 4,7-benz-
imidazole diphosphonate 9 was performed quantitatively by
reaction with trimethyl orthoformate in the presence of a
catalytic amount of CSA (Scheme 5).
Scheme 3 Two strategies to obtain 4,7-benzimidazole diphosphonate 9 used in
this work.
The 1H NMR spectrum of diamine diphosphonate 11 shows
a triplet ( J = 7.1 Hz) and a multiplet centered at 1.32 and
4.04–4.17 ppm, characteristic of the four methyl groups and
eight methylene protons of the phosphonate groups, respec-
tively. The two aromatic protons were observed at 6.94 ppm as a
two proton double doublet. In the 31P NMR spectrum a singlet
was noted at 20.3 ppm assigned to the phosphorus atom in the
aromatic phosphonate group.
Scheme 4 Synthesis of 2,1,3-benzothiadiazole diphosphonate 10.
The prototropic tautomerism of benzimidazole is well
using Pd(PPh3)4 as the catalyst and triethylamine as base. known and seems to be taking place fast enough on the NMR
However, using the Arbuzov reaction conditions in the pre- time scale, but the NMR spectra usually presents broad signals
sence of triethyl phosphite and a catalytic amount of NiBr2, because of this tautomeric equilibrium.24,25,30
2,1,3-benzothiadiazole 2 was readily phosphorylated to give
the desired 4,7-diphosphonate compound 10 in 86% yield an axis of symmetry with the corresponding proton, carbon and
(Scheme 4). phosphorus atoms appearing as magnetically equivalent.
Due to this prototropic tautomerism, compound 9 presents
The structure of diphosphonate 10 was confirmed by 31P, 1H The 1H NMR spectrum in CDCl3 presents a broad singlet at
and 13C NMR spectroscopy. The 1H NMR spectrum of com- 7.76 ppm, attributed to 5-H and 6-H protons and a singlet at
pound 10 reveals that two ethyl ester groups are chemically 8.26 ppm attributed to the 2-H proton. The 13C NMR spectrum is
equivalent, as a triplet ( J = 7.1 Hz) and a multiplet centered at in agreement with the 1H NMR spectrum as it presents the
1.38 and 4.24–4.49 ppm, respectively, characteristic of the four carbon atoms C4 and C7, C5 and C6 and C3a and C7a equiva-
methyl groups and eight methylene protons of the two phos- lents. The 31P NMR spectrum presents a singlet attributed to the
phonate groups. The two proton double doublets at 8.31 ppm two magnetically equivalent phosphorus atoms at 15.6 ppm.
( J = 13.5 and 5.9 Hz) are attributed to the aromatic ring system
However, under certain conditions, the NMR spectra of
(5-H and 6-H), due to the coupling of the 5-H (6-H) proton with compound 9 also allow the identification of one single tautomer
one phosphorus atom of the phosphonate groups at ortho and as if the tautomeric equilibrium occurs at slower speed than the
meta positions.
NMR time scale. The 1H NMR spectra of compound 9 (with only
The 13C NMR spectrum of compound 10 shows a double one tautomeric form, on the NMR time scale) generally present
doublet at 126.8 ppm ( JCP = 188.2 and 3.5 Hz), consistent with broad signals or multiplets for aromatic protons, due to the
the signal of a quaternary carbon bearing a phosphonate group, asymmetric structure caused by the ‘‘fixed’’ position of the
coupling with other phosphorus atoms at para positions. Also, N–H proton, as is frequently observed for other benzimidazole
the observed carbon triplets signals at 16.4 ppm ( JCP = 3.1 Hz) derivatives.31–36 The 31P NMR spectra (Fig. 1 and 2) show two
and 63.3 ppm ( JCP = 2.7 Hz) are characteristic of the methyl singlets assigned to the two unequivalent phosphorus atoms,
and methylene carbon atoms, respectively, of the ethyl ester which is in agreement with the observed 1H NMR spectra,
phosphonate group.
attributed to an asymmetric molecular structure of compound 9.
The 31P NMR spectrum presents a singlet at 11.6 ppm, The 13C NMR spectrum of compound 9 in acetone-d6 presents
assigned to the phosphorus atom in the aromatic phosphonate broad signals due to the slow proton exchange between N1
group.
and N3, which allows the identification of carbon atoms as
c
3086 New J. Chem., 2013, 37, 3084--3091
This journal is The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2013

View Article Online
Paper
NJC
Scheme 6 Synthesis of benzotriazole diphosphonate 12.
Fig. 1 The 31P NMR spectra of compound 9 in different solvents in 0.01 M
solutions: (a) CD2Cl2, (b) CDCl3, (c) acetone-d6, (d) MeOD, (e) DMSO-d6.
tautomeric equilibrium probably due to the establishment of a
hydrogen bond between the N–H proton and the oxygen atoms,
as observed by the other authors for benzimidazole.33,34,41
The same strategy regarding the synthesis was used in the
preparation of benzotriazole derivatives. Cyclization of the diamine
diphosphonate 11 in the presence of sodium nitrite in acetic
acid gave the 4,7-benzotriazole diphosphonate 12 in good yield
(Scheme 6).
1
The H NMR spectrum of compound 12 in CDCl3 shows a
double doublet at 7.95 ppm attributed to the two aromatic
protons. In the 31P NMR spectrum a singlet was observed at
12.8 ppm, assigned to the two equivalent phosphorus atoms in
the aromatic phosphonate groups. This is in agreement with
1
the H NMR spectrum, which shows the two aromatic protons
to be magnetically equivalent. The 13C NMR spectrum presents
broad signals due to the slow proton exchange between N atoms;
C4 and C-7, C5 and C6, and C3a and C7a are magnetically
equivalent. The 1H, 13C and 31P NMR spectra recorded in
acetone-d6 present similar signals to those recorded in CDCl3.
It is known that the prototropic tautomerism of the amine
proton of benzotriazole, which moves between the N-1 and N-2,
and N-3 positions, causes the benzotriazole to exist in three
tautomeric forms, the asymmetric N(1)–H and N(3)–H and the
symmetric N(2)–H forms.42–44 Unlike some 31P and 1H NMR
spectra of benzimidazole 9, in this case, all NMR spectra are in
agreement with the three tautomeric forms of benzotriazole
equilibrating rapidly on the NMR time scale.
The new diphosphonates 9–12 were also characterized by IR
spectroscopy and mass spectrometry (low and high resolution).
Electrospray Ionization (ESI) methods were used to show the
molecular ion of all diphosphonates 9–12, which confirmed the
proposed molecular formula of these new compounds.
The IR spectra of compounds 9–12 display intensive bands
characteristic of phosphonate groups, including the PQO stretching
band at 1227–1250 cmÀ1, and in the region 1021–1050 cmÀ1
corresponding to stretching bands of the P–O–C group.
Fig. 2 The 31P NMR spectra of compound 9 at different concentrations: (1) CDCl3:
(a) 0.1 M (after redissolution of the sample), (b) 0.1 M, (c) 0.05 M, (d) 0.01 M,
(2) acetone-d6: (e) 0.01 M, (f) 0.1 M and (3) MeOD: (g) 0.01 M, (h) 0.1 M.
magnetically unequivalent corresponding to the presence of
only one tautomer on the NMR time scale.
The presence of fast or slow tautomeric equilibria was
observed in different solvents (Fig. 1), in the same solvent but
at different concentrations (Fig. 2) and even for NMR samples
prepared with the same solvents and at the same concentration
but at different times (Fig. 2); no relation was observed between
temperature and the speed of the tautomeric equilibrium,
as observed by other authors for similar compounds.37,38 The
presence of one tautomer was observed in spectra in solvent
acetone-d6 and low concentration MeOD and CDCl3 samples
(Fig. 2). NMR spectra of samples with higher concentration
in CDCl3 and MeOD show both tautomers as well as the low
concentration CD2Cl2 and DMSO-d6 spectra (Fig. 1 and 2).
These could be explained by an intermolecular migration of
the hydrogen atom in the prototropic tautomerism, which is
facilitated in more concentrated solutions or with the assis-
Conclusions
tance of their own solvent, like protic solvents, or the presence New azaheterocyclic aromatic diphosphonate derivatives were
of impurities, such as water.31–33,39,40 The presence of a phos- synthesized to become precursors for novel membrane materials
phonate group at the ortho position could also slow down the for PEMFCs with modified properties.
c
This journal is The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2013
New J. Chem., 2013, 37, 3084--3091 3087

View Article Online
NJC
Paper
The direct phosphonylation reaction of benzimidazole cata- the mixture was stirred at room temperature for 3 h. The
lyzed by transition-metals (Pd and Ni) was not successful reaction mixture was cooled to 0 1C and H2O (100 mL) was
presumably due to the ortho-steric effects of amine groups. A added slowly. After removing the solvent under reduced pres-
new strategy was idealized with the direct phosphonylation of sure, the residue was diluted with Et2O (150 mL), washed with
4,7-dibromo-2,1,3-benzothiadiazole by nickel Arbuzov reaction, brine and dried over MgSO4. The solvent was removed under
followed by reduction sulfur extrusion and cyclization to afford reduced pressure, resulting in the very air-unstable diamine
the new azaheterocyclic diphosphonates derived from benz- compound 3 (2.78 g, 88%) as a white solid. Diamine 3 was used
imidazole and benzotriazole, in good yields. This strategy can be immediately in the next step to avoid fast decomposition.
applied to the synthesis of other diverse phosphorylated azoles. mp 94–95 1C (lit.21 94–95 1C). 1H NMR (300 MHz, CDCl3):
New compounds were fully characterized by NMR, IR spectro- d (ppm) = 3.90 (br s, 4H, NH2), 6.84 (s, 2H, ArH, 5-H and 6-H).
scopy and mass spectrometry (low and high resolution).
Synthesis of 4,7-dibromo-1H-benzimidazole 4. To a solution
Compound 9 shows, on the NMR time scale, the presence of of diamine 3 (2.16, 8.11 mmol) in dichloromethane (70 mL) was
one or both tautomeric forms according to the solvent and the added HC(OEt)3 (1.06 mL, 9.73 mmol) and the solution was
concentration of the solution.
stirred for 10 min. A catalytic quantity of (+)-camphorsulfonic
These azoles will be incorporated into proton conductive acid (CSA) (20 mg) was added and the solution stirred for 1 h.
inorganic–organic hybrid membranes of mesoporous silica to A white powder precipitate was formed. The precipitate was
produce novel membrane materials with potentially high proton filtered and washed with Et2O to afford compound 4 (2.73 g,
conductivity for intermediate temperature PEMFCs.
98%) as a white solid. mp 264–266 1C (lit.23 266–268 1C (decomp.)).
1H NMR (300 MHz, DMSO-d6): d (ppm) = 7.37 (s, 2H, ArH, 5-H and
6-H), 8.39 (s, 1H, ArH, 2-H).
Synthesis of 1-benzyl-4,7-dibromo-1H-benzimidazole 5. A solution
of 4,7-dibromo-1H-benzimidazole 4 (200 mg, 0.724 mmol) and
Experimental section
General remarks
NMR spectra were recorded on Bruker Avance II 300 (1H 300 MHz, K2CO3 (300 mg, 2.171 mmol) in DMF (3 mL) was stirred at
13C 75 MHz, 31P 121 MHz), on Bruker Avance II 400 and Bruker 80 1C. After 30 min, BrCH2Ph (0.17 mL, 145 mmol) was added
Avance 400 MHz Ultra-shield (1H 400 MHz, 13C 100 MHz, and the reaction mixture was refluxed for 2.5 h. Upon cooling,
31P 162 MHz) spectrometers. Chemical shifts (d) are reported the mixture was acidified with 10% aqueous HCl solution and
in ppm and coupling constants ( J) in Hz.
the aqueous layer was extracted with EtOAc. The combined
Infrared spectra were recorded on a Perkin Elmer FT-IR organic extracts were washed with brine and dried over anhydrous
system spectrum BX Fourier Transform spectrometer, using MgSO4. After filtration, the solvent was removed under reduced
KBr discs or film.
pressure. The resulting oil was purified by column chromato-
Low resolution and high resolution (HRMS) mass spectra graphy (1 : 1 EtOAc:petroleum ether) to give compound 5
analyses were performed at the ‘C.A.C.T.I. – Unidad de Espectro- (237 mg, 89%) as a white solid. mp 155 1C. nmax (KBr) (cmÀ1):
metria de Masas’ at the University of Vigo, Spain, on a VG 3069, 3029, 2969, 2923, 1600, 1495, 1476, 1448, 1391, 1369,
AutoSpect M, MicroTOF (Bruker Daltonics) or APEX-Q (Bruker 1354, 1334, 1310, 1274, 1227, 1209, 1162, 1104, 1073, 1029, 969,
Daltonics) instrument.
Melting points were determined on a Reichert Thermovar 1H NMR (400 MHz, CDCl3): d (ppm) = 5.77 (s, 2H, NCH2),
melting point apparatus and are not corrected. 7.07 (d, J = 6.8, 2H, ArH, 5-H and 6-H), 7.27–7.36 (m, 5H, PhH),
2,1,3-Benzothiadiazole 1 is commercially available (Aldrich) 7.97 (s, 1H, ArH, 2-H).13C NMR (100 MHz, CDCl3): d (ppm) =
919, 899, 880, 799, 769, 754, 729, 690, 656, 634, 534, 454.
and was used without further purification.
49.9 (NCH2), 102.5, 113.6 (Ar:C4,C7), 126.5, 129.1 (Ph:Cortho
,
Synthesis of 4,7-dibromobenzothiadiazole 219. A mixture of
C
meta), 126.6, 128.3, 128.8 (Ph:Cpara, Ar:C5,C6), 131.7 (Ar:C7a),
1,2,5-benzothiadiazole 1 (5.00 g, 36.72 mmol) in aq. HBr (48%, 136.5 (Ph:Cipso), 144.2 (Ar:C3a), 145.0 (Ar:C2). MS (EI): m/z = 368
+
15 mL) was heated to reflux with stirring while Br2 (5.7 mL, (M+ + 4, 13%), 366 (M+ + 2, 29%), 364 (M+, 14%), 91 (PhCH2 ,
111.25 mmol) was added dropwise for 30 min. The mixture was 100%). HRMS (EI) m/z calcd for C14H10Br2N2 363.9211 (Br-79
refluxed for 3 h. The mixture was allowed to cool to room isotope) [M]+, found 363.9205.
temperature and a saturated solution of NaHSO3 (aq.) was
Synthesis of 4,7-dibromo-1-(tetrahydro-2H-pyran-2-yl)-1H-
added to consume completely any excess of Br2. The solid benzimidazole 6. To a solution of 4,7-dibromo-1H-benzimid-
was filtered and washed with water. The solid was then washed azole 4 (400 mg, 1.450 mmol) in dichloromethane (15 mL) were
once with cold Et2O and dried under reduced pressure to afford added 3,4-dihydro-2H-pyran (0.4 mL, 4.384 mmol) and p-TsOH
compound 2 (10.11 g, 94%) as a white solid. mp 188–189 1C (28 mg, 0.145 mmol). The solution was stirred for 3 h at room
(lit.19 188–189 1C). 1H NMR (400 MHz, DMSO-d6): d (ppm) = 7.94 temperature. The solution was further diluted with dichloro-
(s, 2H, ArH, 5-H and 6-H). 1H NMR (300 MHz, CDCl3): d (ppm) = methane, washed with saturated Na2HCO3 and dried over
7.73 (s, 2H, ArH, 5-H and 6-H).
anhydrous MgSO4. After filtration, the solvent was removed
Synthesis of 3,6-dibromobenzene-1,2-diamine 320. To a sus- under reduced pressure. The resulting oil was purified by
pension of compound 2 (3.50 g, 11.91 mmol) in EtOH (400 mL) column chromatography (1 : 1 EtOAc : petroleum ether) to give
and THF (40 mL) at 0 1C, under N2, was added portion-wise compound 6 (477 mg, 91%) as a white solid. mp 119–121 1C.
NaBH4 (8.11 g, 211.31 mmol). After stirring at 0 1C for 10 min, nmax (KBr) (cmÀ1): 3119, 3084, 3018, 2965, 2939, 2870, 2849,
c
3088 New J. Chem., 2013, 37, 3084--3091
This journal is The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2013

View Article Online
Paper
NJC
1600, 1490, 1475, 1454, 1440, 1388, 1342, 1319, 1280, 1250, 120.9 (Ar:C5,C6), 141.1 (dd, JCP = 16.3 and 9.3, Ar:C2,C3).
1218, 1185, 1168, 1125, 1105, 1080, 1054, 1044, 995, 943, 922, 31P NMR (162 MHz, H3PO4/CDCl3): d (ppm) = 20.3. MS (ESI):
898, 871, 849, 824, 786, 759, 722, 693, 654, 634, 620, 559, 484, m/z = 381 (MH+, 100%). HRMS (ESI) m/z calcd for C14H27N2O6P2
1
454. H NMR (400 MHz, CDCl3): d (ppm) = 1.64–1.80 (m, 3H, 381.13389 [MH]+, found 381.13474.
CH), 1.94–2.10 (m, 2H, CH), 2.26 (d, J = 12.1, 1H, CH), 3.81
Synthesis of tetraethyl 1H-benzo[d]imidazole-4,7-diylbis-
(t, J = 11.2, 1H, CH), 4.16 (d, J = 11.3, 1H, CH), 6.24 (d, J = 10.5, (phosphonate) 9. To a solution of diamine 11 (150 mg,
1H, CH), 7.32 (m, 2H, ArH, 5-H and 6-H), 8.26 (s, 1H, ArH, 2-H). 0.394 mmol) in dichloromethane (10 mL) was added HC(OEt)3
13C NMR (100 MHz, CDCl3): d (ppm) = 23.2, 24.9, 32.2, 68.5 (0.05 mL, 0.473 mmol) and the solution was stirred for 10 min.
(CH2), 83.2 (CH), 102.4, 113.6 (Ar:C4,C7), 126.5, 129.1 (Ar:C5,C6), A catalytic quantity of (+)-camphorsulfonic acid (10 mg) was
131.0 (Ar:C7a), 141.9 (Ar:C2), 144.3 (Ar:C3a). MS (EI): m/z = added and the solution stirred for 24 h at room temperature.
362 (M+ + 4, 1.5%), 360 (M+ + 2, 3%), 358 (M+, 1.5%), 278 The solvent was removed and the resulting oil was purified by
(362-C5H9O + H+, 68%), 276 (360-C5H9O + H+, 100%), 274 column chromatography (1 : 1 EtOAc : acetone) to give compound
(358-C5H9O + H+, 77%). HRMS (EI) m/z calcd for C12H12Br2N2O 9 (154 mg, 100%) as a pale white solid. mp 158–159 1C.
357.9316 (Br-79 isotope) [M]+, found 357.9322.
nmax (KBr) (cmÀ1): 3147, 3095, 2985, 2936, 2905, 1598, 1478,
Synthesis of tetraethyl benzo[c][1,2,5]thiadiazole-4,7-diylbis- 1460, 1443, 1393, 1370, 1334, 1294, 1250, 1240, 1201, 1165, 1124,
(phosphonate) 10. A mixture of 4,7-dibromobenzothiadiazole 2 1100, 1049, 967, 920, 805, 760, 685, 645, 604, 579, 530, 477, 446.
(1.00 g, 3.345 mmol) and nickel(II) bromide (73 mg, 0.334 mmol) MS (ESI): m/z = 391 (MH+, 100%). HRMS (ESI) m/z calcd for
was heated at 135 1C under N2. Triethyl phosphite (3 mL,
17.495 mmol) was added dropwise. The mixture was heated
C
15H25N2O6P2 391.11824 [MH]+, found 391.11826.
1st attempt: H NMR (400 MHz, CDCl3) (0.1 M): d (ppm) =
1
at the same temperature and stirred under N2 for 24 h. After 1.32 (m, 12H, 4Â OCH2CH3), 4.06–4.29 (m, 8H, 4Â OCH2CH3),
cooling to room temperature, the mixture was filtered through 7.58 (m, 1H, ArH, 5-H or 6-H), 7.92 (m, 1H, ArH, 5-H or 6-H),
Celite. The solvent was removed under reduced pressure. 8.25 (s, 1H, ArH, 2-H), 11.30 (br s, 1H, NH). 31P NMR (162 MHz,
The resulting oil was purified by column chromatography H3PO4/CDCl3): d (ppm) = 15.4, 15.7.
1
(1 : 1 EtOAc : acetone) to give compound 10 (1.18 g, 86%) as a
2nd attempt: H NMR (400 MHz, CDCl3) (0.1 M): d (ppm) =
white solid. mp 74–75 1C. nmax (KBr) (cmÀ1): 3132, 3084, 3054, 1.32 (t, J = 7.1, 12H, 4Â OCH2CH3), 4.11–4.26 (m, 8H, 4Â
2984, 2938, 2905, 2868, 1655, 1560, 1535, 1476, 1444, 1391, OCH2CH3), 7.75 (dd, J = 11.6 and 5.9, 2H, ArH, 5-H and 6-H),
1366, 1295, 1246, 1209, 1165, 1099, 1047, 1023, 973, 941, 887, 8.32 (s, 1H, ArH, 2-H). 13C NMR (100 MHz, CDCl3): d (ppm) =
854, 794, 754, 731, 664, 622, 570, 550, 528, 514, 491, 455, 422. 16.4 (t, JCP = 3.1, CH3), 62.9 (t, JCP = 2.4, OCH2), 119.4 (d, JCP
=
1H NMR (400 MHz, CDCl3): d (ppm) = 1.38 (t, J = 7.1, 12H, 184, Ar:C4,C7), 126.5 (m, Ar:C5,C6), 139.5 (Ar:C3a,C7a), 143.4
4Â OCH2CH3), 4.24–4.49 (m, 8H, 4Â OCH2CH3), 8.31 (dd, J = (Ar:C2). 31P NMR (162 MHz, H3PO4/CDCl3): d (ppm) = 15.4.
13.5 and 5.9, 2H, ArH, 5-H and 6-H). 13C NMR (100 MHz,
1H NMR (400 MHz, CDCl3) (0.05 M): d (ppm) = 1.33 (t, J = 7.1,
CDCl3): d (ppm) = 16.4 (t, JCP = 3.1, CH3), 63.3 (t, JCP = 2.7, 12H, 4Â OCH2CH3), 4.14–4.27 (m, 8H, 4Â OCH2CH3), 7.76
OCH2), 126.8 (dd, JCP = 188.2 and 3.5, Ar:C4,C7), 135.3 (br s, 1H, ArH, 5-H and 6-H), 8.26 (s, 1H, ArH, 2-H). 31P NMR
(m, Ar:C5,C6), 153.5 (t, JCP = 7.8, Ar:C3a,C7a). 31P NMR (162 MHz, (162 MHz, H3PO4/CDCl3): d (ppm) = 15.6 (br s).
H3PO4/CDCl3): d (ppm) = 11.6. MS (ESI): m/z = 431 (MNa+, 100%),
1H NMR (400 MHz, CDCl3) (0.01 M): d (ppm) = 1.34 (t, J = 7.1,
409 (MH+, 44%). HRMS (ESI) m/z calcd for C14H23N2O6P2S 12H, 4Â OCH2CH3), 4.15–4.23 (m, 8H, 4Â OCH2CH3), 7.65 (br s,
409.07466 [MH]+, found 409.07470.
1H, ArH, 5-H or 6-H), 7.88 (br s, 1H, ArH, 5-H or 6-H), 8.25
Synthesis of tetraethyl (2,3-diamino-1,4-phenylene)bis- (s, 1H, ArH, 2-H), 10.9 (br s, 1H, NH). 31P NMR (162 MHz,
(phosphonate) 11. To a solution of compound 10 (200 mg, H3PO4/CDCl3): d (ppm) = 15.7 (br s).
0.490 mmol) in EtOH (10 mL) at 0 1C, under N2, was added
1H NMR (400 MHz, DMSO-d6) (0.01 M): d (ppm) = 1.24
portion-wise NaBH4 (334 mg, 8.829 mmol). After stirring at 0 1C (t, J = 7.0, 12H, 4Â OCH2CH3), 4.03–4.12 (br m, 8H, 4Â
during 10 min, the mixture was stirred at room temperature for OCH2CH3), 7.69 (br s, 2H, ArH, 5-H and 6-H), 8.37 (s, 1H,
2 h. The reaction mixture was cooled to 0 1C and H2O (5 mL) ArH, 2-H), 12.59 (br s, 1H, NH). 31P NMR (162 MHz, H3PO4/
was added slowly. After removing the solvent under reduced DMSO-d6): d (ppm) = 14.5.
pressure, the residue was diluted with Et2O, washed with brine
1H NMR (400 MHz, MeOD) (0.01 M): d (ppm) = 1.33 (t, J = 7.0,
and dried over MgSO4 and the solvent removed under reduced 12H, 4Â OCH2CH3), 4.18–4.26 (m, 8H, 4Â OCH2CH3), 7.79–7.87
pressure. The resulting oil was purified by column chromato- (br m, 2H, ArH, 5-H and 6-H), 8.37 (s, 1H, ArH, 2-H). 31P NMR
graphy (1 : 1 EtOAc : acetone) to give compound 11 (116 mg, (162 MHz, H3PO4/MeOD): d (ppm) = 15.0, 15.6.
62%) as a pale yellow oil. nmax (film) (cmÀ1): 3421, 3310, 3244,
1H NMR (400 MHz, MeOD) (0.1 M): d (ppm) = 1.33 (t, J = 7.1,
3073, 2984, 2945, 2914, 1678, 1652, 1623, 1533, 1454, 1448, 12H, 4Â OCH2CH3), 4.16–4.23 (m, 8H, 4Â OCH2CH3), 7.85 (dd,
1393, 1371, 1301, 1233, 1164, 1151, 1099, 1049, 1021, 966, 789, J = 11.7 and 6.1, 2H, ArH, 5-H and 6-H), 8.49 (s, 1H, ArH, 2-H).
769, 756, 632. 1H NMR (400 MHz, CDCl3): d (ppm) = 1.32 (t, J = 31P NMR (162 MHz, H3PO4/MeOD): d (ppm) = 15.2.
7.1, 12H, 4Â OCH2CH3), 4.02–4.17 (m, 8H, 4Â OCH2CH3), 5.06
1H NMR (400 MHz, acetone-d6) (0.01 M): d (ppm) = 1.29 (dt,
(br s, 4H, NH), 6.94 (dd, J = 10.8 and 6.6, 2H, ArH, 5-H and 6-H). J = 7.0 and 3.0, 12H, 4Â OCH2CH3), 4.07–4.29 (m, 8H, 4Â OCH2CH3),
13C NMR (100 MHz, CDCl3): d (ppm) = 16.4 (t, JCP = 3.4, CH3), 7.67 (ddd, J = 13.8, 7.5 and 3.1, 1H, ArH, 5-H or 6-H), 7.85 (ddd,
62.4 (t, JCP = 2.7, OCH2), 112.4 (dd, JCP = 179.7 and 2.6, Ar:C1,C4), J = 14.2, 7.5 and 3.5, 1H, ArH, 5-H or 6-H), 8.37 (s, 1H, ArH, 2-H).
c
This journal is The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2013
New J. Chem., 2013, 37, 3084--3091 3089

View Article Online
NJC
Paper
31P NMR (162 MHz, H3PO4/acetone-d6): d (ppm) = 14.0 (d, J = 4.9),
14.8 (d, J = 4.9).
2 V. Di Noto, T. A. Zawodzinski, A. M. Herring, G. A. Giffin,
E. Negro and S. Lavina, Int. J. Hydrogen Energy, 2012, 37,
6120–6131.
3 L. Zhang, S.-R. Chae, Z. Hendren, J.-S. Park and M. R.
Wiesner, Chem. Eng. J., 2012, 204–206, 87–97.
4 H. Zhang and P. K. Shen, Chem. Soc. Rev., 2012, 41, 2382–2394;
H. Zhang and P. K. Shen, Chem. Rev., 2012, 112, 2780–2832.
5 C. Laberty-Robert, K. Valle, F. Pereira and C. Sanchez,
Chem. Soc. Rev., 2011, 40, 961–1005.
6 Y. Wanga, K. S. Chen, J. Mishler, S. C. Cho and X. C.
Adroher, Appl. Energy, 2011, 88, 981–1007.
1H NMR (400 MHz, acetone-d6) (0.1 M): d (ppm) = 1.29 (t, J =
7.0, 12H, 4Â OCH2CH3), 4.22 (br m, 8H, 4Â OCH2CH3), 7.70
(br s, 1H, ArH, 5-H or 6-H), 7.85 (br s, 1H, ArH, 5-H or 6-H), 8.43
(s, 1H, ArH, 2-H), 12.00 (br s, 1H, NH). 13C NMR (100 MHz,
acetone-d6): d (ppm) = 16.6 (CH3), 63.1 (d, JCP = 24.5, OCH2),
116.4 (d, JCP = 186, Ar:C4 or C7), 125.4 (d, JCP = 183, Ar:C4 or C7),
126.3, 127.2 (Ar:C5,C6), 135.8 (Ar:C3a or C7a), 145.0 (Ar:C2).
31P NMR (162 MHz, H3PO4/acetone-d6): d (ppm) = 14.2, 14.8.
1H NMR (400 MHz, CD2Cl2) (0.01 M): d (ppm) = d 1.31 (t, J =
7.1, 12H, 4Â OCH2CH3), 4.12–4.21 (m, 8H, 4Â OCH2CH3), 7.72
(br s, 2H, ArH, 5-H and 6-H), 8.22 (s, 1H, ArH, 2-H), 11.33 (br s,
1H, NH). 31P NMR (162 MHz, H3PO4/CD2Cl2): d (ppm) = 14.9 (br s).
Synthesis of tetraethyl 1H-benzo[d][1,2,3]triazole-4,7-diylbis-
(phosphonate) 12. To a solution of diamine 11 (116 mg,
0.304 mmol) in AcOH (1.2 mL) was added a solution of NaNO2
(23 mg, 0.335 mmol) in H2O (0.6 mL). After 30 min stirring at
room temperature, the solution was diluted with H2O and the
aqueous layer was extracted with dichloromethane. The com-
bined organic extracts were washed with brine, dried over
anhydrous MgSO4 and the solvent removed under reduce
pressure. The resulting oil was purified by column chromato-
graphy (1 : 1 EtOAc : acetone) to give compound 12 (118 mg,
99%) as a white solid. mp 172–174 1C; nmax (KBr) (cmÀ1): 3042,
2985, 2912, 2828, 1599, 1560, 1479, 1460, 1444, 1394, 1370,
1354, 1270, 1254, 1227, 1165, 1099, 1050, 1025, 974, 941, 874,
7 S. J. Peighambardoust, S. Rowshanzamir and M. Amjadi,
Int. J. Hydrogen Energy, 2010, 35, 9349–9384.
8 H. Steininger, M. Schuster, K. D. Kreuer, A. Kaltbeitzel,
¨
B. Bingol, W. H. Meyer, S. Schauff, G. Brunklaus, J. Maier
and H. W. Spiess, Phys. Chem. Chem. Phys., 2007, 9,
1764–1773.
9 M. Schuster, T. Rager, A. Noda, K. D. Kreuer and J. Maier,
Fuel Cells, 2005, 5, 355–365.
10 B. Lafitte and P. Jannasch, in Advances in Fuel Cells, ed.
T. S. Zhao, K.-D. Kreuer and T. V. Nguyen, Elsevier, Oxford,
2007, ch. 3, vol. 1, pp. 119–185.
11 S. J. Paddison, K. D. Kreuer and J. Maier, Phys. Chem. Chem.
Phys., 2006, 8, 4530–4542.
¨
12 T. Bock, H. Mohwald and R. Mu¨lhaupt, Macromol. Chem.
Phys., 2007, 208, 1324–1340.
13 E. Abouzari-Lotf, H. Ghassemi, A. Shockravi, T. Zawodzinski
and D. Schiraldi, Polymer, 2011, 52, 4709–4717.
14 C. S. Demmer, N. Krogsgaard-Larsen and L. Bunch, Chem.
Rev., 2011, 111, 7981–8006.
15 For example: Y. Belabassi, S. Alzghari and J.-L. Montchamp,
J. Organomet. Chem., 2008, 693, 3171–3178.
1
798, 756, 684, 587, 530, 487, 444. H NMR (300 MHz, CDCl3):
d (ppm) = 1.34 (t, 12H, 4Â OCH2CH3), 4.18–4.40 (m, 8H, 4Â
OCH2CH3), 7.95 (dd, J = 12.0 and 5.6, 2H, ArH, 5-H and 6-H).
13C NMR (75 MHz, CDCl3): d (ppm) = 16.4 (t, JCP = 3.1, CH3),
63.5 (t, JCP = 2.7, OCH2), 120.4 (br d, JCP = 180, Ar:C4,C7),
130.0 (t, JCP = 10.6, Ar:C5,C6), 139.1 (br s, Ar:C3a,C7a). 31P NMR
16 For example: G. Yang, C. Shen, L. Zhang and W. Zhang,
Tetrahedron Lett., 2011, 52, 5032–5035.
1
(162 MHz, H3PO4/CDCl3): d (ppm) = 12.8. H NMR (400 MHz,
17 S. Van der Jeught and C. V. Stevens, Chem. Rev., 2009, 109,
2672–2702.
18 C. Hou, Y. Ren, R. Lang, X. Hu, C. Xia and F. Li, Chem.
Commun, 2012, 48, 5181–5183.
acetone-d6): d (ppm) = 1.31 (t, J = 7.1, 12H, 4Â OCH2CH3),
4.21–4.30 (m, 8H, 4Â OCH2CH3), 8.03 (dd, J = 11.9 and 5.6, 2H,
ArH, 5-H and 6-H). 13C NMR (100 MHz, acetone-d6): d (ppm) =
16.6 (t, JCP = 3.1, CH3), 63.6 (t, JCP = 2.7, OCH2), 121.8 (br d, JCP
=
19 K. Pilgram, M. Zupan and R. Skiles, J. Heterocycl. Chem.,
1970, 7, 629–633.
20 Y. Tsubata, T. Suzuki, T. Myashi and Y. Yamashita, J. Org.
Chem., 1992, 57, 6749–6765.
21 M. J. Edelmann, J.-M. Raimundo, N. F. Utesch, F. Diederich,
C. Boudon, J.-P. Gisselbrecht and M. Gross, Helv. Chim. Acta,
2002, 85, 2195–2213.
22 B. A. D. Neto, A. S. Lopes, M. Wu¨st, V. E. U. Costa, G. Ebeling
and J. Dupont, Tetrahedron Lett., 2005, 46, 6843–6846.
23 T. Yamamoto, K. Sugiyama, T. Kanbara, H. Hayashi and
H. Etori, Macromol. Chem. Phys., 1998, 199, 1807–1813.
24 V. I. Minkin, A. D. Garnosvskii, J. Elguero, A. R. Katritzky
and O. V. Denisko, in Advances in Heterocyclic Chemistry, ed.
A. R. Katritzky, Academic Press, New York, 2000, vol. 76,
pp. 157–323.
200, Ar:C4,C7), 130.8 (t, JCP = 9.7, Ar:C5,C6), 139.7 (br s,
Ar:C3a,C7a). 31P NMR (162 MHz, H3PO4 acetone-d6): d (ppm) =
12.2. MS (ESI): m/z = 414 (MNa+, 28%), 392 (MH+, 100%). HRMS
(ESI) m/z calcd for C14H24N3O6P2 392.11348 [MH]+, found
392.11420.
Acknowledgements
The authors thank FCT and FEDER (QREN-COMPETE) for provi-
sion of funding (PTDC/CTM-CER/109843/2009) and Portuguese
NMR Network (IST-UTL Center) for providing access to the
NMR facilities.
References
1 A. Chandan, M. Hattenberger, A. El-Kharouf, S. Du, A. Dhir, 25 R. M. Claramunt, C. Lopez, M. D. Santa Maria, D. Sanz and
V. Self, B. G. Pollet, A. Ingram and W. Bujalski, J. Power
Sources, 2013, 231, 264–278.
J. Elguero, Prog. Nucl. Magn. Reson. Spectrosc., 2006, 49,
169–206.
c
3090 New J. Chem., 2013, 37, 3084--3091
This journal is The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2013

View Article Online
Paper
NJC
´
´
26 C. Shen, G. Yang and W. Zhang, Org. Biomol. Chem., 2012, 35 E. Alcalde, M. Gisbert and L. Perez-Garcıa, Tetrahedron,
10, 3500–3505. 1995, 51, 13365–13378.
27 Q. Yao and S. Levchik, Tetrahedron Lett., 2005, 47, 36 E. Alcalde, M. Gisbert, J. M. Pons and L. Perez-Garcıa,
277–281. Tetrahedron, 1996, 52, 15197–15208.
28 Y. L. Zhao, G. J. Wu, Y. Li, L. X. Gao and F. S. Han, 37 E. Dall’Oglio, M. B. Caro, J. C. Gesser, C. Zucco and
Chem.–Eur. J., 2012, 18, 9622–9627. M. C. Rezende, J. Braz. Chem. Soc., 2002, 13, 251–259.
29 B. A. D. Neto, A. A. M. Lapis, E. N. da Silva Ju´nior and 38 A. E. Ceniceros-Gomez, A. Ramos-Organillo, J. Hernandez-
´
´
´
´
´
´
J. Dupont, Eur. J. Org. Chem., 2013, 228–255.
30 R. Faure, E.-J. Vincent and J. Elguero, Heterocycles, 1983, 20,
1713–1716.
Dıaz, J. Nieto-Martınez, R. Contreras and S. E. Castillo-
Blum, Heteroat. Chem., 2000, 11, 392–398.
39 L. A. Fedorov, P. Savarino, G. Viskardi, A. I. Rebrov and
´
31 V. A. Mamedov, N. A. Zhukova, V. V. Syakaev, A. T.
E. Barni, Russ. Chem. Bull., 1992, 41, 223–230.
´
Gubaidullin, T. N. Beschastnova, D. I. Adgamova, A. I. 40 M. Boiani, H. Cerecetto, M. Gonzalez, O. E. Piro and
Samigullina and S. K. Latypov, Tetrahedron, 2013, 69,
1403–1416.
32 I.-S. H. Lee, E. H. Jeoung and C. K. Lee, J. Heterocycl. Chem.,
1996, 33, 1711–1716.
E. E. Castellano, J. Phys. Chem. A, 2004, 108, 11241–11248.
41 C. D. Edlin, D. Parker, J. J. B. Perry, C. Chartroux and
K. Gloe, New J. Chem., 1999, 23, 819–826.
42 N. Jagerovic, M. L. Jimeno, I. Alkorta, J. Elguero and
R. M. Claramunt, Tetrahedron, 2002, 58, 9089–9094.
33 V. Sridharan, S. Saravanan, S. Muthusubramanian and
´
S. Sivasubramanian, Magn. Reson. Chem., 2005, 43, 551–556. 43 J. Poznanski, A. Najda, M. Bretner and D. Shugar, J. Phys.
´˜
´
34 A. de la Hoz, I. Almena, C. Foces-Foces, M. Yanez, O. Mo,
Chem. A, 2007, 111, 6501–6509.
´
´
´
M. Alcamı, N. Jagerovic and J. Elguero, J. Phys. Chem. B, 44 R. W˛asik, P. Winska, J. Poznanski and D. Shugar, J. Phys.
2001, 105, 12759–12770.
Chem. B, 2012, 116, 7259–7268.
c
This journal is The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2013
New J. Chem., 2013, 37, 3084--3091 3091