Novel optical chemosensors for anions and cations based on an amino acid core functionalised with benzimidazoles

Article abstract of DOI:10.1016/j.tet.2012.06.087

A series of novel unnatural phenylalanine derivatives bearing a benzimidazole as coordinating/reporting unit, substituted with electron donor (methoxy) or acceptor (cyano and nitro) groups were synthesised, and their evaluation as colorimetric/fluorimetric chemosensors was carried out in acetonitrile solution. They were tested for the recognition of organic and inorganic anions, (such as Br^-, F^-, CN^-, AcO^-, BzO^-, NO₃^-, ClO ₄^-, HSO₄^-, H₂PO ₄^-, and OH^-) and of alkaline, alkaline-earth, and transition metal cations, (such as Cu²⁺, Cd²⁺, Co ²⁺, Ca²⁺, Na⁺, Cr³⁺, Zn ²⁺, Hg²⁺, Fe²⁺, Fe³⁺, Ni ²⁺, and Pd²⁺). The nature of the substituent at the benzimidazole allowed the fine tuning of the sensory properties and signaling mode, as seen by spectrophotometric and spectrofluorimetric titrations, which showed that derivatives 3a-c are fluorimetric chemosensors, with 3a being selective and highly sensitive for Pd²⁺ whereas 3b was also very sensitive to this cation but not selective, while derivative 3d bearing a nitro group at the benzimidazole behaves as a colorimetric chemosensor for OH ^-.

Full text of DOI:10.1016/j.tet.2012.06.087

Tetrahedron 68 (2012) 7322e7330
Contents lists available at SciVerse ScienceDirect
Tetrahedron
journal homepage: www.elsevier.com/locate/tet
Novel optical chemosensors for anions and cations based on an amino acid core
functionalised with benzimidazoles
*
Rosa M.F. Batista, R. Cristina M. Ferreira, M. Manuela M. Raposo, Susana P.G. Costa
Centre of Chemistry, University of Minho, Campus of Gualtar, 4710-057 Braga, Portugal
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 5 May 2012
Received in revised form 14 June 2012
Accepted 21 June 2012
Available online 28 June 2012
A series of novel unnatural phenylalanine derivatives bearing a benzimidazole as coordinating/reporting
unit, substitutedwith electron donor (methoxy)oracceptor (cyano and nitro) groups were synthesised, and
their evaluation as colorimetric/fluorimetric chemosensors was carried out in acetonitrile solution. They
were tested for the recognition of organic and inorganic anions, (such as Brꢀ, Fꢀ, CNꢀ, AcOꢀ, BzOꢀ, NO3ꢀ,
ClO4ꢀ, HSO4ꢀ, H2PO4ꢀ, and OHꢀ) and of alkaline, alkaline-earth, and transition metal cations, (such as
Cu2þ, Cd2þ, Co2þ, Ca2þ, Naþ, Cr3þ, Zn2þ, Hg2þ, Fe2þ, Fe3þ, Ni2þ, and Pd2þ). The nature of the substituent at the
benzimidazole allowed the fine tuning of the sensory properties and signaling mode, as seen by
spectrophotometric and spectrofluorimetric titrations, which showed that derivatives 3aec are
fluorimetric chemosensors, with 3a being selective and highly sensitive for Pd2þ whereas 3b was also very
sensitive to this cation but not selective, while derivative 3d bearing a nitro group at the benzimidazole
behaves as a colorimetric chemosensor for OHꢀ.
Keywords:
Phenylalanine
Benzimidazole
Fluorescence
Optical chemosensors
Anion and cation sensing
Ó 2012 Elsevier Ltd. All rights reserved.
1. Introduction
the side chain of the coded amino acids, new functions, and func-
tional relationships can be generated as well as altered physico-
Molecular recognition is the basis for most biological functions,
given the high degree of control over biological systems that nature
can get.1 In recent years the design of fluorescent synthetic mole-
cules capable of recognizing and binding organic and inorganic
molecules involved in biological pathways is a current thrust.2 The
development of artificial receptors for recognizing cationic, neutral,
and anionic species is growing and has attracted much interest in
molecular recognition studies and supramolecular chemistry be-
cause they play an important role in a wide range of biological,
environmental, and chemical processes.3
In an optical sensor, the reporter is an optically active group, that
is able to change its photophysical properties (fluorescence or ab-
sorption) when selective recognition of an anionic/cationic sub-
strate occurs. Among optical sensors, the fluorescent systems offer
many advantages. Fluorescence measurements are usually very
sensitive, easy to perform, and versatile, offering subnanometer
spatial resolution with submicrometer visualization and submilli-
second temporal resolution.4
chemical properties, such as luminescence, conducting ability,
higher thermal stability, and metal ion and other analytes recog-
nition ability, among other properties.5
Benzimidazole and its derivatives have been studied in anion and
cation recognition systems that displaycolorchanges or fluorescence
quenching or enhancement upon binding.6 Anion coordination by
natural amino acids can arise from side chain OH and NH groups (in
serine, threonine, tyrosine orarginine, and tryptophan, respectively),
as well from additional binding with amide NH groups from the
backbone, which act as hydrogen-bond donors to the anion.7
Among the biologically functional anions of interest, fluoride is
of particular importance owing to its established role in dental care
and treatment of osteoporosis.8 Recently, considerable attention
has been paid to the sensing of cyanide anion for many reasons.
Cyanide is one of the most toxic anions and harmful to environment
or human health. For example, it is lethal to humans at concen-
trations of 0.5e3.5 mg per kg of body weight.9
Detecting cations is of great interest to many scientists, in-
cluding chemists, biologists, clinical biochemists, and environ-
mentalists. Sodium, potassium, magnesium, calcium are involved
in biological processes, such as transmission of nerve impulses,
muscle contraction, regulation of cell activity, etc. It is well known
that mercury, lead, and cadmium are toxic for organisms, and early
detection in the environment is desirable.10
The insertion of coded and unnatural amino acids into the
backbone of both natural and synthetic polymers is a very appealing
area of research since it can lead to the development of macro-
molecules possessing biomimetic characteristics, with unique
structural and biological properties. By synthetic manipulation at
Palladium is widely used in various materials, such as dental
crowns, catalysts, fuel cells, and jewelry. Automobile catalytic
* Corresponding author. Tel.: þ351 253 604054; fax: þ351 253 604382; e-mail
address: spc@quimica.uminho.pt (S.P.G. Costa).
0040-4020/$ e see front matter Ó 2012 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.tet.2012.06.087

R.M.F. Batista et al. / Tetrahedron 68 (2012) 7322e7330
7323
Table 1
converters apparently release a significant amount of palladium to
the roadside environment where dust samples collected from
broad-leaved plants were found to contain palladium in about
0.3 ppm and rain may also wash Pd into local water systems. Pal-
ladium can have adverse effects on human health and the envi-
ronment, because it can bind to thiol-containing amino acids,
proteins, DNA, and other biomolecules and disturb a variety of cel-
lular processes.11 As a strong fluorescence quencher, Pd2þ could be
detected by fluorescent ligands through fluorescence quenching.12
Following our research interests that include the synthesis and
application of colorimetric and fluorimetric probes for anions and
cations based on heterocycles13 and amino acids,5,14 the synthesis
and characterization of unnatural amino acids and imidazole de-
rivatives with interesting optical properties,13e,15 we now report
innovative colorimetric/fluorimetric chemosensors for the recog-
nition of metallic cations, organic, and inorganic anions of analyt-
ical, biological, environmental, and medicinal relevance, through
the combination of a benzimidazole as coordinating/reporting unit,
substituted with electron donor or acceptor groups with an amino
acid core, namely phenylalanine.
Yields, UVevisible absorption and fluorescence data for benzimidazolyl phenylala-
nines 3aed and benzimidazoles 5b,d in acetonitrile
Cpd.
R
Yield (%)
UV/Vis
Fluorescence
lmax
log
3
lem
Stokes’
FF
shift (nm)
3a
3b
3c
3d
5b
5d
H
OMe
CN
NO2
OMe
NO2
62
56
60
50
51
60
304
319
310
335
315
320
4.43
4.41
4.46
4.22
4.38
4.13
346
381
344
371
378
369
42
62
34
36
63
49
0.66
0.80
0.58
0.001
0.76
0.001
The absorption and emission spectra of benzimidazolyl phe-
nylalanines 3aed, were measured in acetonitrile solutions
(10ꢀ6e10ꢀ5
M
) (Table 1). Compounds 3aed, differing in the sub-
stituent at position 5 of the benzimidazole, and according to the
electronic character of the substituent, displayed bathochromic
shifts, e.g., phenylalanine 3b bearing a methoxy electron donor
group displayed a bathochromic shift for the maximum wavelength
of absorption (15 nm) and emission (35 nm) when compared to
unsubstituted benzimidazolyl phenylalanine 3a (Fig. 1), while 3d
bearing a nitro electron acceptor group had a slightly larger bath-
ochromic shift for the maximum wavelength of absorption (29 nm).
The relative fluorescence quantum yields were determined by
The different substituents at the benzimidazole are intended to
improve the intramolecular electron delocalization, which will tune the
photophysical properties of new sensors and optimize of the recogni-
tion of target analytes through a greater sensitivity of fluorescence.
using a 10ꢀ6
M solution of anthracene in ethanol as standard
2. Results and discussion
(
FF¼0.27).18 For the FF determination, the fluorescence standard
was excited at the wavelengths of maximum absorption found for
each of the compounds to be tested and in all fluorimetric mea-
surements the absorbance of the solution did not exceed 0.1. Ben-
zimidazolyl phenylalanines 3aed exhibited excellent fluorescence
quantumyields in acetonitrile (FF¼0.58 and 0.80), the highest being
for 3b due to the presence of a methoxy donor group (Table 1).
Furthermore, in compound 3d using an electron acceptor as the
nitro group, known as a strong fluorescence quencher, resulted in
a strong quenching of the fluorescence, and consequently a drastic
lowering of the quantum yield of fluorescence (FF¼0.001). Stokes’
shifts directly relate to energy differences between the ground and
excited states and all compounds showed larger Stokes’ shifts (from
34 nm to 62 nm) and these values are an interesting feature for bi-
ological fluorescent probes that allows an improved separation of
the light inherent to the matrix and the light dispersed by the
sample.19 Considering the photophysical properties in acetonitrile
of phenylalanine derivatives 3aed (presented in Table 1), derivative
3b would be the more interesting candidate as chemosensor due to
the higher fluorescence quantum yield, which is important for
2.1. Synthesis
The new phenylalanines 3aed were synthesized in good yields
(50e62%) from N-t-butyloxycarbonyl-4-formyl-L-phenylalanine
methyl ester 1,16 by a one-step reaction through the Na2S2O4 re-
duction of 2-nitrobenzenamine 2a, 4-methoxy-2-nitrobenzenamine
2b, 4-amino-3-nitrobenzonitrile 2c, and 2,4-dinitrobenzenamine 2d
in ethanol at 80 ꢁC for 3 h (Scheme 1, Table 1). Precursor 1 was syn-
thesized in 85% yield by palladium-catalyzed hydroformylation of N-
t-butyloxycarbonyl-tyrosine triflate with carbon monoxide and tri-
octylsilane in the presence of Pd(OAc)2/dppp as catalyst. The in-
termediate N-t-butyloxyxarbonyl-tyrosine triflate was obtained in
excellent yield (93%) from N-t-butyloxycarbonyl-tyrosine methyl es-
ter and trifluoromethanesulfonic anhydride.17 From the results in
Table 1, itcanbeseen that theyieldfor benzimidazolyl phenylalanines
3aed was not influenced by the electronic nature of the substituent at
position 5 of the benzimidazole. All the heterocyclic phenylalanines
were fully characterised by the usual spectroscopic techniques.
R
HN
CHO
N
H2N
O2N
Na2S2O4 (3eq)
EtOH, 80ºC, 3h
a R = H
R
Boc-HN CO2Me
Boc-HN
CO2Me
b R = OMe
c R = CN
1
2
3
d R = NO2
Scheme 1. Synthesis of benzimidazolyl phenylalanine derivatives 3aed.
In order to understand better the role of the phenylalanine and
benzimidazole moieties in the sensory behavior of the amino acid
derivatives 3, arylebenzimidazoles 5b,d were also synthesized and
their sensory ability was compared with the corresponding amino
acid derivatives 3b,d. Therefore, compounds 5b,d were prepared
using the same synthetic procedure (Scheme 2) described above for
amino acids 3 using as precursors benzaldehyde 4 and ortho-
nitrobenzenamines 2b and 2d, in DMSO.
maximization of response to analyte in the analysis of very dilute
samples.
2.2. Spectrophotometric and spectrofluorimetric titrations of
3aed with metallic cations and organic/inorganic anions
The modification of phenylalanine through the introduction of
a UV-active and highly fluorescent heterocycle at its side chain was

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R.M.F. Batista et al. / Tetrahedron 68 (2012) 7322e7330
H2N
O2N
Na2S2O4 (3eq)
CHO
R
HN
DMSO, 120ºC, 4h
R
N
b R = OMe
d R = NO2
4
2
5
Scheme 2. Synthesis of 2-aryl-benzimidazoles 5b,d.
Fe3þ, Cr3þ, Pd2þ, and Pd2þ, the interaction of phenylalanine de-
rivatives 3aed with these cations was evaluated through UVevis
and fluorescence spectroscopy in spectrophotometric and spec-
trofluorimetric titrations in acetonitrile. The interaction with or-
ganic/inorganic anions, such as Fꢀ, Clꢀ, CNꢀ, AcOꢀ, and OHꢀ was
also evaluated. A preliminary evaluation of the chemosensing
ability was performed by addition of 100 equiv of each cation/anion
and the changes in the intensity of the UVevis absorption and
fluorescence spectra were recorded. In the UVevis absorption
spectra, no changes were seen in the bands corresponding to the
maximum wavelength of absorption of phenylalanine derivatives
3aec after addition of up to 100 equiv of each metal cation.
1
0.8
0.6
0.4
0.2
0
3a
3b
3a
3b
265
315
365
415
465
The comparative fluorimetric response of phenylalanine de-
rivatives 3aeb to several cations is summarised in Fig. 2, consid-
ering the variation of the fluorescence intensity (I/Io) upon the
addition of 100 equiv of each cation (the shorter the bar, the higher
the change in fluorescence and higher sensitivity). The effect of the
substituent at the benzimidazole is well evidenced in the sensing
studies as benzimidazole 3a is a selective fluorimetric sensor for
Pd2þ with high sensitivity, while the methoxyebenzimidazole 3b is
Wavelength (nm)
Fig. 1. Normalized UVevisible absorption (full line) and emission spectra (dotted line)
of compounds 3a and 3b in acetonitrile at T¼298
K
(3a, lexc¼304 nm; 3b,
lexc¼319 nm).
expected to provide additional binding sites for a variety of metal
ions through the heterocycle donor atoms, as well as improved
photophysical properties for the chemosensing studies. With het-
erocyclic phenylalanine derivatives 3aed it was intended to assess
the influence of the type of the substituent at the benzimidazole
system in the chemosensing ability of anions/metallic cations.
Considering the biological, environmental and analytical relevance
a fluorimetric sensor for several metal cations, such as Cu2þ, Hg2þ
Fe2þ, Fe3þ, Cr3þ, Pd2þ
,
.
Spectrofluorimetric titrations with metallic cations, such as
Hg2þ, Cr3þ, Fe2þ, Pd2þ, Fe3þ, and Cu2þ were done for phenylalanines
3a and 3b, and was possible to observe the weaker interactions
with these cations for 3b through a lower quenching of the
of transition metals, such as Cd2þ, Cu2þ, Hg2þ, Ni2þ, Zn2þ, Fe2þ
,
3a
0 eq.
3a,Cd2+, Cu2+, Hg2+
Ni2+, Zn2+, Fe2+
Fe3+, Co2+ , Cr3+
,
1
0.8
0.6
0.4
0.2
0
,
I/I0
1
0.8
0.6
0.4
0.2
0
100 eq.
3a Pd2+
365
315
415
465
Cations
Wavelength (nm)
3b
3b,Cd2+, Ni2+ Zn2+, Co2+
,
1
1
I/I0
3b Hg2+, Fe2+
Fe3+, Cr3+
,
0.8
0.8
0.6
0.4
0.2
0
0.6
0.4
0.2
0
3b Cu2+
3b Pd2+
380 430
330
480
530
Cations
Wavelength (nm)
Fig. 2. Relative fluorimetric response (I/I0) of phenylalanine derivatives 3a and 3b in the presence of 100 equiv of Cd2þ, Cu2þ, Hg2þ, Ni2þ, Zn2þ, Fe2þ, Fe3þ, Cr3þ, and Pd2þ, as
a function of metal concentration in acetonitrile.

R.M.F. Batista et al. / Tetrahedron 68 (2012) 7322e7330
7325
fluorescence intensity accompanied by a small red shift. The
quenching in the fluorescence emission was more efficient for Pd2þ
and Cu2þ, when compared to the other metals studied. The
mechanism of recognition of both cations is an inverse photoin-
duced electron transfer (PET) and occurs upon complexation with
a transition metal ion: the sensor is fluorescent in the absence of
a cation and cation binding induces an electron transfer from the
fluorophore to the cation.10b
that increases the dipole moment in the chelate ring, increasing the
positive charge at the N atom and making the cation recognition
impossible.
Spectrofluorimetric titrations for phenylalanine 3b with Cr3þ
and Fe2þ (Fig. 4) were performed and showed that for achieving
a plateau for Cr3þ almost 1.5 equiv were necessary while for Fe2þ
40 equiv were needed. In both titrations, the intensity of the
emission band was reduced (CHEQ effect), red-shifted, and fol-
lowed by fluorescence enhancement upon chelation (CHEF) effect,
with an iso-emissive point at 397 nm, that suggests the presence of
two species in the solution, the ligand and the mononuclear com-
plex. The interaction with both cations is done through photoin-
duced charge transfer process from the methoxy electron-donating
group to the imidazole group conjugated to the former, leading to
a stabilization of the excited state and therefore to a red shift of
29 nm in the emission spectra caused by the change in the dipole
moment.
Spectrofluorimetric titration of 3a and 3b with Pd2þ (Fig. 3)
showed higher selectivity among the several metallic cations
tested. In Fig. 3 is possible to see that the mechanism of Pd2þ rec-
ognition is oneoff (inverse-PET) and a strong decrease of the
fluorescence intensity (a chelation enhanced quenching, CHEQ ef-
fect) was observed for both phenylalanines 3a and 3b, with an al-
most complete fluorescence quenching, the only difference
between them being the number of metal equivalents necessary to
quench at least 90% of the initial fluorescence intensity (before
complexation) of the heterocyclic amino acid (1 equiv for 3a and
almost 2 equiv for 3b, respectively Fig. 3A and B). These results are
in agreement with the association constants of receptors 3a and 3b
for Pd2þ are 12.87ꢂ 0.021 and 13.36 ꢂ 0.028, respectively.
Studies of interaction with several anions were also carried out
by spectrophotometric and spectrofluorimetric titrations. In the
UVevis absorption spectra, no changes were seen in the bands
corresponding to the maximum wavelength of absorption of phe-
1
1
1
0.8
0.6
0.4
0.8
0.6
0.4
0.2
0
383 nm
0.8
0.6
0.4
0.2
0
0
0.4 0.8 1.2 1.6
2
[Pd(II)]/[3b]
0.2
A
B
0
315
329
379
429
479
529
365
415
465
Wavelength (nm)
Wavelength (nm)
Fig. 3. Fluorimetric titrations of phenylalanine derivatives 3a (A) and 3b (B) with Pd2þ in acetonitrile [lexc 3a¼304 nm, lexc 3b¼319 nm]. Inset: normalised emission at the
wavelength of maximum emission as a function of added metal equivalents.
Fig. 4. Fluorimetric titrations of phenylalanine derivative 3b with Cr3þ (A) and Fe2þ (B) in acetonitrile [lexc 3b¼319 nm]. Inset: normalised emission at 384 nm and 413 nm as
a function of added metal equivalents.
Benzimidazolyl phenylalanines 3c and 3d were not affected by
interaction with Pd2þ and this fact could be explained probably by
the electron-acceptor nature of the cyano and nitro groups at the
benzimidazole moiety, which could change the coordination ca-
pacity of the chelate ring involving the nitrogen atom of the im-
idazole, probably due to the ICT to benzimidazole by the former,
nylalanine derivatives 3aec after addition of up to 100 equiv of each
organic/inorganic anion.
In the spectrofluorimetrictitrations in acetonitrile, a fluorescence
quenching was seen especially for the fluoride and hydroxide ions.
Additionally, in order to evaluate the sensing ability of these com-
pounds in acetonitrile/water mixtures, a study was performed as an

7326
R.M.F. Batista et al. / Tetrahedron 68 (2012) 7322e7330
example for phenylalanine 3b with Fꢀ, in a 9:1 and a 1:9 proportion
(Fig. 5). It was found that in the presence of water, there was a neg-
ligible difference of the fluorescence intensity upon interaction with
the anion, when compared to the titration in acetonitrile.
ascribed to electronic excitation through charge transfer from the
donor to the acceptor substituents (-NO2) of chromophore, i.e, by
an ICT. The excited state would be more stabilized by anion binding,
resulting in a bathochromic shift in the absorption maxima by
a new gradually emerging CT band at 414 nm, resulting in a change
from colorless to yellow, which was clearly visible to the naked eye
after addition of several anions, such as Fꢀ, AcOꢀ, CNꢀ, and OHꢀ to
I/a.u.
a 100 m
M solution of compound 3d (see Fig. 6B for OHꢀ).
1.60E+06
3b (ACN)
In Fig. 7, the fluorescence titrations of 3aec with the addition of
OHꢀ can be seen, and the observed can be attributed to the
deprotonation of the imidazole nitrogen, inducing a small red shift.
The interaction constants of ligands 3aec with OHꢀ were calcu-
lated and are summarized in Table 2. In these spectrofluorimetric
titrations with OHꢀ, a less pronounced CHEQ effect of the fluores-
cence intensity was observed for all the phenylalanine derivatives.
In Fig. 7, the effect of ion complexation is evident in the band
centered at the wavelength of maximum emission at 346 nm,
380 nm, and 343 nm, for 3a, 3b, and 3c, respectively. The only
difference between them is being the number of anion equivalents
necessary to quench at least 50% of the initial fluorescence intensity
(before complexation) of the benzimidazolyl phenylalanines
(70 equiv for 3a, 200 equiv for 3b, and 4 equiv for 3c). These results
suggest that the greater the electron acceptor effect from the sub-
stituent group in the benzimidazole moiety, the lower the number
of equivalents required for the interaction, and the introduction of
the electron donor group in the compound 3b can enhance the
electron density of the imidazole ring and consequently increase
the basicity of the hydrogen-bond donors, so deprotonation be-
comes more difficult. Similar results were obtained for phenylala-
nines 3aec in the spectrofluorimetric titrations with Fꢀ. Derivatives
3b and 3c also demonstrated sensitivity for other anions, such as 3b
for CNꢀ or 3c for Clꢀ and AcOꢀ and the results are shown in Table 2.
In order to compare the influence of the size of the anions, ti-
trations with some halides was done, and it was found that the
fluoride ion bound strongly with phenylalanines 3aed because of
its highest electronegativity and smaller size. Spectrofluorimetric
titration of compound 3c with the halides suggested good accor-
dance with the ordering of diameter size of these halides as a higher
selectivity for Fꢀ than for Clꢀ was found as shown by Kass (Table 2).
3b + F- (ACN)
3b (9:1 – ACN:H2O)
1.20E+06
3b + F- (9:1 – ACN:H2O)
3b (1:9 – ACN:H2O)
3b + F- (1:9 – ACN:H2O)
8.00E+05
4.00E+05
0.00E+00
320
370
420
470
520
Wavelength (nm)
Fig. 5. Fluorimetric titrations of phenylalanine derivative 3b with Fꢀ (addition of
100 equiv) in acetonitrile and in mixtures of acetonitrile/water in a 9:1 and a 1:9
proportion [lexc 3b¼319 nm].
It was anticipated that the nitro functionality present at the
benzimidazole in compound 3d would be responsible for color
changes and for an increase in the hydrogen bond donor tendency,
but also for a strong fluorescence quenching, resulting in a non-
fluorescent compound and thus possibly yielding a colorimetric
sensor. In fact, selectivity and sensitivity for anions was seen, as
shown in the spectrophotometric titration with OHꢀ (Fig. 6B). In
the absence of anions, the spectrum of receptor 3d showed two
bands at lmax¼273 and 337 nm. Upon addition of increasing
amounts of Fꢀ, CNꢀ, AcOꢀ or OHꢀ, the intensities of the bands at
273 nm and 337 nm decreased while new two bands appeared at
299 and 414 nm, with three perfect isosbestic points at 282 nm,
315 nm and 364 nm. The band at the 337 nm, the pep* transition of
2.3. 1H NMR titrations
the chromophore, disappeared gradually, and a new band formed
at 414 nm, which is the charge transfer (CT) band. The pale yellow
color of the sensor solution turned yellow at the same time. Both
isosbestic points and Hyperquad Software suggest the coexistence
of two species in equilibrium (stoichiometry 1:2), the free phe-
nylalanine and the dinuclear complex. The optical response may be
1H NMR was used to investigate the nature of anion coordination,
and experiments were carried out in DMSO-d6 and CDCl3. It has been
known that fluoride and hydroxide ions induce deprotonation in
many receptors and hence they are responsible for color changes.20 In
3d
AcO- CN-
F-
OH-
0.6
0.4
0.2
1
0.8
0.6
0.4
0.2
0
1
337 nm
414 nm
0
0.8
0.6
0.4
0.2
0
3d F-, OH-, CN-
3dAcO-
0
0.5
1
1.5
2
[OH-]/[3d]
3d
A
B
250
300
350
400
450
500
250
300
350
400
450
Wavelength (nm)
Wavelength (nm)
Fig. 6. (A): Absorption spectra of phenylalanine 3d (100
([3d]¼10 M, T¼298 K, [TBAOH]¼0e2.5 equiv). Inset: normalised absorption at 337 nm and 414 nm as a function of added anion equivalents.
m
M) upon addition of 10 equiv of TBA salt of various anions. (B): Spectrophotometric titration of 3d with OHꢀ in acetonitrile
m

R.M.F. Batista et al. / Tetrahedron 68 (2012) 7322e7330
7327
1
0.8
0.6
0.4
0.2
0
1
0.8
0.6
0.4
0.2
0
1
0.8
0.6
0.4
0.2
0
1
346 nm
380 nm
0.8
0
40 80 120 160 200
0
20
40
60
80
[OH-]/[3b]
[OH-]/[3a]
0.6
0.4
0.2
0
A
B
314
364
414
464
314
364
414
464
514
Wavelength (nm)
Wavelength (nm)
1.0
0.8
0.6
0.4
0.2
0.0
1
0.8
0.6
0.4
0.2
343 nm
0
1
2
3
4
[OH-]/[3c]
C
0
320
370
420
470
Wavelength (nm)
Fig. 7. Fluorimetric titrations of phenylalanines 3a (A), 3b (B) and 3c (C) with OHꢀ in acetonitrile [lexc 3a¼304 nm, lexc 3b¼319 nm, lexc 3c¼310 nm]. Inset: normalised emission at
the wavelength of maximum emission as a function of added anion equivalents.
(NeH)þ/(OH)ꢀ hydrogen bonds. The cyano and nitro groups in de-
rivatives 3c and 3dpresumably greatly strengthen this hydrogen bond
because they both enhance the positive charge at the benzimidazole
NeH and the dipole moment along the NeH bond. Two effects are
Table 2
Association constants (Kass) for the interaction of phenylalanines
3aed with several anions/cations in acetonitrile (L:M or L:A
stoichiometry suggested from Hyperquad is 1:2)
1
responsible for the H NMR changes upon NHe and CeH hydrogen
Ligand
Interaction
Log Kass
bond formation to the hydroxide: through-bond effects, which in-
crease the electron density of the phenyl ring and promote upfield
shifts, and by through-space effects, which polarize the CeH bond in
proximity to the hydrogen bond, creating a partial positive charge at
the proton, and causing a downfield shift. As shown in the partial 1H
NMR hydroxide titration spectra of 3c in CDCl3 (Fig. 8) upon addition
of 4 equiv of the anion, a downfieldshiftofNHeBocisobserved,which
3a
Fꢀ
7.8670ꢂ1.77ꢃ10ꢀ02
7.4538ꢂ8.30ꢃ10ꢀ03
12.8664ꢂ2.14ꢃ10ꢀ02
8.0522ꢂ2.34ꢃ10ꢀ02
8.4435ꢂ1.51ꢃ10ꢀ02
7.0305ꢂ6.00ꢃ10ꢀ03
10.5191ꢂ2.48ꢃ10ꢀ02
7.5784ꢂ3.81ꢃ10ꢀ02
10.0554ꢂ2.93ꢃ10ꢀ02
13.6920ꢂ5.30ꢃ10ꢀ02
13.4672ꢂ5.08ꢃ10ꢀ02
13.3572ꢂ2.82ꢃ10ꢀ02
8.1078ꢂ2.12ꢃ10ꢀ02
4.1604ꢂ3.59ꢃ10ꢀ02
9.2931ꢂ1.25ꢃ10ꢀ02
9.2576ꢂ2.62ꢃ10ꢀ02
10.0684ꢂ5.97ꢃ10ꢀ02
9.2796ꢂ6.60ꢃ10ꢀ03
8.1674ꢂ1.19ꢃ10ꢀ02
10.3865ꢂ1.84ꢃ10ꢀ02
8.9998ꢂ6.14ꢃ10ꢀ02
OHꢀ
Pd2þ
CNꢀ
Fꢀ
3b
OHꢀ
Cu2þ
Cr3þ
Fe2þ
Fe3þ
Hg2þ
Pd2þ
AcOꢀ
Clꢀ
is ascribed to the through-space effects. On the other hand,
shifted upfield, due to the proximity to the hydrogen bond between
NHeBoc and anion, which polarize the CeH bond. Similarly, all the
a-H is
a
aromatic protons are shifted upfield, which indicates the increase of
the electron density at the phenyl ring owing to the through-bond
effects. At the end of the titration, the complete disappearance of
the signals for the NH protons was also observed, confirming depro-
tonation of the NHeBoc and NH of the imidazole ring.
In the case of 1H NMR hydroxide titration spectra of 3c in
DMSO-d6 (Fig. 9) is possible to see that after adding 4 equiv of
hydroxide the gradual formation of the complex occurred and
consequently the disappearance of the free ligand. It has been
reported that DMSO interacts with one set of urea-based anion
binding site and leaves other similar sites free.20b The NH-Boc
signal is overlapping with the signal corresponding to H3 and H5,
and both signals were shifted to a much lesser extent when com-
pared to the other signals. This small shift can be attributed to the
3c
CNꢀ
Fꢀ
OHꢀ
AcOꢀ
CNꢀ
Fꢀ
3d
OHꢀ
our present work we report a type of 1H NMR spectral changes upon
addition of OHꢀ to a solution of phenylalanine 3c in DMSO-d6 or CDCl3
(Figs. 8 and 9, respectively). The results suggest that the interaction
between phenylalanine 3c and OHꢀ is formed by (CeH)þ and

7328
R.M.F. Batista et al. / Tetrahedron 68 (2012) 7322e7330
CN
CN
4´
4´
6´
N
N
6´
7´
5
6
OH-
6
7´
5
N
N
H
CDCl3
3
2
2
3
H
Boc
MeO2C
NBoc
MeO2C
N
0 equiv
OH-
(a)
(b)
(c)
(d)
(e)
4 equiv
OH-
l
Fig. 8. Partial H NMR (300 MHz) spectra of 3c (100 m .
M) in CDCl3 in (a) the absence and in the presence of (b) 1.0, (c) 2.0, (d) 3.0, and (e) 4.0 equiv of OHꢀ
H
CN
6´ H
7´ H
CN
4´
H
N
N
5
OH-
H
N
H
N
H
6
3H
O
DMSO
2
H
H
MeO2C
N
MeO2C
H
N
S
Boc
Boc
H2, H6
7´H
H3, H5
6´H
0 equiv
OH-
H4´
(a)
(b)
(c)
LIGAND
(d)
H2, H6
H3, H5
7´H
H4´
6´H
4 equiv
OH-
(e)
COMPLEX
l
Fig. 9. Partial H NMR (300 MHz) spectra of 3c (100 m
M) in DMSO-d6 in (a) the absence and in the presence of (b) 1.0, (c) 2.0, (d) 3.0, and (e) 4.0 equiv of OHꢀ salt.
protons that are already in an hydrogen-bonded environment
established with DMSO-d6. The signa1 corresponding to H7’ was
shifted by Dd¼0.39 ppm by through-bond effects. Similarly the
signal corresponding to H2 and H6’ wasshifted by Dd¼0.38 ppm,
which is ascribed to through-bond effects, due to the high density
in the phenyl ring promoting upfield shifts.
The binding stoichiometries of benzimidazolylephenylalanines
3aed were determined from Job’s plots and the binding affinity was
calculated with Hyperquad software (see Supplementary data, Fig. S1).
In order to understand better the contribution of the benz-
imidazole and the phenylalanine moieties in the recognition
event, spectrofluorimetric and spectrophotometric titrations of
arylebenzimidazoles 5b,d were performed in acetonitrile solutions
with Fꢀ and Cu2þ, as representative examples (see Supplementary
data, Figs. S2eS4). As can be seen by comparison between the
benzimidazole derivatives 5b,d, with the corresponding amino
acids 3b,d, there was a similar sensory behavior that can be related
to the major role of the benzimidazole moiety compared to the
phenylalanine residue as the receptor.
3. Conclusions
A family of novel optical chemosensors for anions and cations
based on a phenylalanine functionalised with benzimidazole were

R.M.F. Batista et al. / Tetrahedron 68 (2012) 7322e7330
7329
synthesised in good yields (50e62%). The sensory ability was
evaluated for several cations by spectrofluorimetric titrations in
acetonitrile, and it was found that benzimidazolylephenylalanines
3aeb were suitable chemosensors for Pd2þ, showing higher sen-
4.2.1. N-t-Butyloxycarbonyl-4-(1H-benzo[d]imidazol-2-yl)phenylala-
nine methyl ester (3a). Starting from 1 (0.100 g, 0.32ꢃ10ꢀ3 mol) and
2a (0.044 g, 0.32ꢃ10ꢀ3 mol), compound 3a was obtained as a white
solid (0.078 g, 62%); mp¼211.0e213.0 ꢁC; 1H NMR (400 MHz,
sitivity for this cation when compared to Cd2þ, Cu2þ, Hg2þ, Ni2þ
,
CDCl3):
d¼1.41 (s, 9H, C(CH3)3), 3.00e3.15 (m, 2H,
b
-CH2), 3.67 (s,
Zn2þ, Fe2þ, Fe3þ, and Cr3þ, as an almost complete fluorescence
quenching was achieved with the addition of two metal equiva-
lents. The results indicated that if the substituent at position 5 of
the benzimidazole moiety was a cyano or a nitro acceptor group,
respectively 3c and 3d, no interaction was observed with metallic
cations. However, these later benzimidazolylephenylalanines had
strong sensitivity for anions, such as Fꢀ, Clꢀ, CNꢀ, AcOꢀ, and OHꢀ. It
should also be noted that 3c behaves as a fluorimetric sensor while
3d is a colorimetric sensor for anions, as seen from spectro-
fluorimetric and spectrophotometric titrations, respectively. The 1H
NMR titration study corroborated that the mechanism of anion
recognition of phenylalanines 3c in aprotic solvent (CDCl3 and
DMSO-d6) was deprotonation of both NH, namely the NHeBoc and
benzimidazole NH. Comparison of the sensory ability of 2-
arylbenzimidazoles 5b,d with the corresponding amino acid de-
rivatives 3b,d revealed that the benzimidazole moiety is the main
responsible for the recognition event.
3H, OCH3), 4.55e4.58 (m, 1H,
a-H), 5.12 (d, J 8.0 Hz, 1H, NH Boc),
7.16 (d, J 8.0 Hz, 2H, H2 and H6), 7.23e7.27 (m, 2H, H50and H60),
7.60e7.62 (m, 2H, H40and H70), 7.98 (d, J 8.0 Hz, 2H, H3 and H5); 13C
NMR (100.6 MHz, CDCl3):
(OCH3), 54.4 (
d
¼28.3 (C(CH3)3), 38.2 (
b-CH2), 52.3
a
-C), 80.2 (C(CH3)3),115.2 (C40 and C70), 123.0 (C50 and
C600), 126.8 (C3 and C5),128.4 (C4),129.9 (C2 and C6),138.5 (C1, C30a,
C7 a), 151.5 (C20), 155.2 (C]O Boc), 172.1 (C]O ester); IR (Nujol,
cmꢀ1):
n
¼3199, 1751, 1681, 1552, 1293, 1280, 1260, 1166, 1057, 829,
741; UV/Vis (acetonitrile, nm): lmax (log
3 )¼304 (4.43); MS: m/z
(ESI) 396 ([MþH]þ, 100); HMRS: m/z (ESI) calcd for C22H26N3O4
396.19178, found 396.19,174.
4.2.2. N-t-Butyloxycarbonyl-4-(50-methoxy-1H-benzo[d]imidazol-2-
yl)phenylalanine methyl ester (3b). Starting from
1 (0.100 g,
0.32ꢃ10ꢀ3 mol) and 2b (0.054 g, 0.32ꢃ10ꢀ3 mol), compound 3b was
obtained as a white solid (0.078 g, 56%); mp¼203.0e205.0 ꢁC; 1H
NMR (400 MHz, CDCl3):
d
¼1.40 (s, 9H, C(CH3)3), 2.98e3.13 (m, 2H,
b-
CH2), 3.65 (s, 3H, OCH3 ester), 3.77 (s, 3H, OCH3), 4.54e4.56 (m, 1H,
4. Experimental
a
-H), 5.17 (d, J 8.4 Hz,1H, NH Boc), 6.86 (dd, J 8.8 and 2.4 Hz,1H, H60),
7.03 (s,1H, H40), 7.12 (d, J 8.0 Hz, 2H, H2 and H6), 7.46 (d, J 8.8 Hz, 2H,
4.1. Synthesis general
H70), 7.96 (d, J 8.0 Hz, 2H, H3 and H5); 13C NMR (100.6 MHz, CDCl3):
d
¼28.2 (C(CH3)3), 38.1 (
b-CH2), 52.3 (OCH3 ester), 54.4 (a-C), 55.7
All melting points were measured on a Stuart SMP3 melting
point apparatus. TLC analyses were carried out on 0.25 mm thick
precoated silica plates (Merck Fertigplatten Kieselgel 60F254) and
spots were visualised under UV light. Chromatography on silica gel
was carried out on Merck Kieselgel (230e240 mesh). IR spectra
were determined on a BOMEM MB 104 spectrophotometer. NMR
spectra were obtained on a Bruker Avance III 400 at an operating
frequency of 400 MHz for 1H and 100.6 MHz for 13C using the sol-
vent peak as internal reference at 25 ꢁC. All chemical shifts are given
in parts per million using dH Me4Si¼0 ppm as reference and J values
are given in Hertz. Assignments were made by chemical shifts, peak
multiplicities and J values and were supported by spin decoupling-
double resonance and bidimensional heteronuclear correlation
techniques. Low and high resolution mass spectrometry analyses
were performed at the ‘C.A.C.T.I.dUnidad de Espectrometria de
Masas’, at University of Vigo, Spain. Fluorescence spectra were
collected using a FluoroMax-4 spectrofluorometer. UVevisible ab-
sorption spectra (200e700 nm) were obtained using a Shimadzu
UV/2501PC spectrophotometer. Luminescence quantum yields
were measured using an ethanol solution of anthracene as standard
(OCH3), 80.2 (C(CH3)3), 97.4 (C40), 112.4 (C60), 116.2 (C70), 126.6 (C3
and C5), 128.7 (C4), 129.8 (C2 and C6), 138.1 (C1, C30a, C70a), 151.3
(C20), 155.2 (C]O Boc), 156.6 (C50), 172.1 (C]O ester); IR (Nujol,
cmꢀ1):
n
¼3206, 1749, 1680, 1632, 1549, 1227, 1163, 1118, 1059, 1029,
997, 960, 834, 758, 688; UV/Vis (acetonitrile, nm): lmax (log
3 )¼319
(4.41); MS: m/z (ESI) 426 ([MþH]þ, 100), 245 (20); HMRS: m/z (ESI)
calcd for C23H28N3O5 426.20235, found 426.20,276.
4.2.3. N-t-Butyloxycarbonyl-4-(50-cyano-1H-benzo[d]imidazol-2-yl)
phenylalanine methyl ester (3c). Starting from
1 (0.100 g,
0.32ꢃ10ꢀ3 mol) and 2c (0.052 g, 0.32ꢃ10ꢀ3 mol), compound 3c was
obtained as a white solid (0.085 g, 60%); mp¼190.0e192.0 ꢁC; 1H
NMR (400 MHz, CDCl3):
d
¼1.42 (s, 9H, C(CH3)3), 3.01e3.19 (m, 2H,
b-CH2), 3.70 (s, 3H, OCH3), 4.56e4.58 (m, 1H, a-H), 5.20 (d, J 8.4 Hz,
1H, NH Boc), 7.22 (d, J 7.8 Hz, 2H, H2 and H6), 7.50 (dd, J 8.3 and
1.6 Hz, 1H, H60), 7.66 (d, J 8.3 Hz, 1H, H70), 7.91 (s, 1H, H40), 8.00 (d, J
7.8 Hz, 2H, H3 and H5); 13C NMR (100.6 MHz, CDCl3):
d¼28.3
(C(CH3)3), 38.3 (b-CH2), 52.5 (OCH3), 54.49 (a-C), 80.5 (C(CH3)3),
105.5 (C50), 116.1 (C70), 120.0 (C40), 126.3 (C60), 127.2 (C3 and C5),
127.5 (C4),130.0 (C2 and C6),139.6 (C1, C30a, C70a),154.4 (C20),155.4
(
FF¼0.27).18 All commercially available reagents were used as re-
(C]O Boc), 172.0 (C]O ester); IR (Nujol, cmꢀ1):
1679, 1625, 1542, 1312, 1222, 1160, 1061, 1029, 988, 959, 832, 811,
n
¼3197, 2227, 1748,
ceived. Compound 1 was synthesised as reported elsewhere.16
701; UV/Vis (acetonitrile, nm): lmax (log
3
)¼310 (4.46); MS: m/z
4.2. General procedure for the synthesis of benzimidazolyl
phenylalanines 3aed
(ESI) 421 ([MþH]þ, 100); HMRS: m/z (ESI) calcd for C23H25N4O4
421.18703, found 421.18,748.
A solution of N-t-butyloxycarbonyl-4-formyl-
L
-phenylalanine
4.2.4. N-t-Butyloxycarbonyl (4-(5-nitro-1H-benzo[d]imidazol-2-yl)
phenyl) phenylalanine methyl ester (3d). Starting from 1 (0.100 g,
0.32ꢃ10ꢀ3 mol) and 2d (0.059 g, 0.32ꢃ10ꢀ3 mol), compound 3d
was obtained as a brown solid (0.070 g, 50%); mp¼183.0e185.0 ꢁC;
methyl ester 1 (1 equiv) and 2-nitrobenzenamine 2a, 4-methoxy-
2-nitrobenzenamine 2b, 4-amino-3-nitrobenzonitrile 2c and 2,4-
dinitrobenzenamine 2d (1 equiv) in absolute ethanol (3 mL) was
treated with Na2S2O4 (3 equiv), dissolved in a small volume of
water, and heated at 80 ꢁC with stirring for 3 h. The mixture was
poured into water (20 mL) and extracted with ethyl acetate
(3ꢃ50 mL). The organic layer was dried with magnesium sulfate
and evaporated under reduced pressure to give the crude
benzimidazolyl phenylalanines 3aed, which were submitted to
silica gel column chromatography using mixtures of ethyl acetate
and light petroleum of increasing polarity. The fractions containing
the purified product were collected and evaporated under vacuum.
1H NMR (400 MHz, CDCl3):
d
¼1.46 (s, 9H, C(CH3)3), 3.03e3.22 (m,
2H, b-CH2), 3.72 (s, 3H, OCH3), 4.59e4.61 (m, 1H, a-H), 5.15 (d, J
8.0 Hz, 1H, NH Boc), 7.25 (d, J 8.4 Hz, 2H, H2 and H6), 7.66 (d, J
8.8 Hz, 1H, H70), 8.00 (d, J 8.4 Hz, 2H, H3 and H5), 8.20 (dd, J 8.8 and
2.4 Hz, 1H, H60), 8.53 (s, 1H, H40); 13C NMR (100.6 MHz, CDCl3):
d
¼28.3 (C(CH3)3), 38.4 (
b-CH2), 52.5 (OCH3), 54.5 (a-C), 80.4
(C(CH3)3), 112.4 (C40), 118.8 (C60), 120.0 (C70), 127.1 (C3 and C5),
127.4 (C4), 130.1 (C2 and C6), 139.8 (C1, C30a, C70a), 143.8 (C50), 155.3
(C20), 155.5 (C]O Boc), 172.0 (C]O ester); IR (Nujol, cmꢀ1):

7330
R.M.F. Batista et al. / Tetrahedron 68 (2012) 7322e7330
n
¼3349, 1714, 1676, 1624, 1516, 1337, 1310, 1250, 1160, 1054, 1013,
Supplementary data
971, 951, 880, 816, 739; UV/Vis (acetonitrile, nm): lmax (log
3 )¼335
(4.22); MS: m/z (ESI) 441 ([MþH]þ, 100), 201 (31); HMRS: m/z (ESI)
Supplementary data associated with this article can be found, in
the online version, at http://dx.doi.org/10.1016/j.tet.2012.06.087.
calcd for C22H25N4O6 441.17686, found 441.17,626.
4.3. General procedure for the synthesis of 2-
arylbenzimidazoles 5b,d
References and notes
A solution of benzaldehyde 4 (1 equiv) and 4-methoxy-2-
nitrobenzenamine 2b and 2,4-dinitrobenzenamine 2d (1 equiv) in
DMSO (3 mL) was treated with Na2S2O4 (3 equiv), dissolved in
a small volume of water, and heated at 120 ꢁC with stirring for 4 h.
The mixture was poured into water (20 mL) and a precipitate was
formed and collected by filtration, washed with water, dried, and
recrystallized from ethanol.
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3.84 (s, 3H, OCH3), 7.01 (dd, J 8.7 and 2.4 Hz, 1H, H60), 7.17 (d, J
2.1 Hz, 1H, H40), 7.60e7.63 (m, 4H, H70, H3, H4 and H5), 8.12e8.15
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148.0e150.1 ꢁC (lit.22 147.1e149.0 ꢁC). 1H NMR (400 MHz, DMSO-
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d
7.58e7.61 (m, 3H, H3, H4 and H5), 7.77 (d, J 8.8 Hz, 1H, H70),
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4.4. Spectrophotometric/spectrofluorimetric titrations and
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Costa, S. P. G.; Lodeiro, C.; Raposo, M. M. M. Org. Lett. 2007, 9, 3201e3204; (e)
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14. (a) Costa, S. P. G.; Batista, R. M. F.; Raposo, M. M. M. Tetrahedron 2008, 64,
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M.; Raposo, M. M. M. Dyes Pigm. 2009, 80, 329e336; (c) Pina, J.; de Melo, J. S. S.;
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M. M. M. Phys. Chem. Chem. Phys. 2010, 12, 9719e9725; (e) Pina, J.; de Melo, S.
S.; Burrows, H. D.; Batista, R. M. F.; Costa, S. P. G.; Raposo, M. M. M. J. Phys. Chem.
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Solutions of phenylalanine derivatives 3aed and benzimid-
azoles 5b,d (ca. 1.0ꢃ10ꢀ5 to 1.0ꢃ10ꢀ6 M) and of the anions/cations
under study (ca. 1.0ꢃ10ꢀ1 to 1.0ꢃ10ꢀ3 M) were prepared in UV-
grade acetonitrile or in mixtures of acetonitrile with water in the
form of hexahydrated tetrafluorborate salts for Cu2þ, Co2þ, Ni2þ
,
,
and Pd2þ, perchlorate salt for Cd2þ, Ca2þ, Naþ, Cr3þ, Zn2þ, Hg2þ
Fe2þ, and Fe3þ, and tetrabutylamonium salts for Fꢀ, Clꢀ, CNꢀ, AcOꢀ,
and OHꢀ. Titration of the compounds with the several metallic
cations was performed by the sequential addition of equivalents of
cation/anion to the amino acid derivative solution, in a 10 mm path
length quartz cuvette and emission spectra were measured by ex-
citation at the wavelength of maximum absorption for each com-
pound, indicated in Table 1. The binding stoichiometry of the
phenylalanine derivatives with the anions/metal cations was de-
termined by using Job’s plots, by varying the molar fraction of the
anion/cation while maintaining constant the total phenylalanine
derivative and anion/metal cation concentration. The association
constants were obtained from Hyperquad Software.
16. Morera, E.; Ortar, G.; Varani, A. A. Synth. Commun. 1998, 28, 4279e4285.
17. Shieh, W.-C.; Carlson, J. A. J. Org. Chem. 1992, 57, 319e381.
18. (a) Borlman, I. B. Handbook of Fluorescence Spectra of Aromatic Molecules, 2nd
ed.; Academic: New York, 1971; (b) Montalti, L.; Credi, A.; Prodi, T.; Gandolfi, M.
ꢀ
T. Handbook of Photochemistry, 3rd ed.; Taylor and Francis: Boca Raton, 2006.
19. Holler, M. G.; Campo, L. F.; Brandelli, A.; Stefani, V. J. Photochem. Photobiol. A.
Chem. 2002, 149, 217e225.
20. (a) Amendola, V.; Esteban-Gomez, D.; Fabbrizzi, L.; Licchelli, M. Acc. Chem. Res.
Acknowledgements
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2006, 39, 343e353; (b) Burns, D. H.; Calderon-Kawasaki, K.; Kularatne, S. J. Org.
Chem. 2005, 70, 2803e2807; (c) Peng, X.; Wu, Y.; Fan, J.; Tian, M.; Han, K. J. Org.
Chem. 2005, 70, 10524e10531.
Thanks are due to the Foundation for the Science and Technol-
ogy (FCT, Portugal) for financial support to the NMR Portuguese
network (PTNMR, Bruker Avance III 400-Univ. Minho), FCT, and
FEDER (European Fund for Regional Development)-COMPETE-
QREN-EU for financial support to the Research Centre, CQ/UM
[PEst-C/QUI/UI0686/2011 (FCOMP-01-0124-FEDER-022716)] and
a post-doctoral grant to R.M.F. Batista (SFRH/BPD/79333/2011).
21. Yamamoto, Y.; Mizuno, H.; Tsuritani, T.; Mase, T. J. Org. Chem. 2008, 74,
1394e1396.
ꢀ
22. Navarrete-Vazquez, G.; Hidalgo-Figueroa, S.; Torres-Piedra, M.; Vergara-Galicia,
ꢀ
J.; Rivera-Leyva, J. C.; Estrada-Soto, S.; Leon-Rivera, I.; Aguilar-Guardarrama, B.;
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Rios-Gomez, Y.; Villalobos-Molina, R.; Ibarra-Barajas, M. Bioorg. Med. Chem.
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