Aryl carbinols as nerve agent probes. Influence of the conjugation on the sensing properties

Article abstract of DOI:10.1039/c2nj40104e

Two new aryl carbinols (1 and 3) have been synthesised and characterised and their ability as OFF-ON probes for the chromogenic detection of the nerve agent simulant in acetonitrile has been tested. In addition compound 2 has been also studied. The carbin

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PAPER
Aryl carbinols as nerve agent probes. Influence of the conjugation on the
sensing propertiesw
Santiago Royo,ab Raul Gotor,ac Ana M. Costero,*ac Margarita Parra,ac
´
abd
abd
Salvador Gil,ac Ramon Martınez-Manez* and Felix Sancenon
´
˜
´
´
´
´
Received (in Montpellier, France) 14th February 2012, Accepted 4th April 2012
DOI: 10.1039/c2nj40104e
Two new aryl carbinols (1 and 3) have been synthesised and characterised and their ability as
OFF–ON probes for the chromogenic detection of the nerve agent simulant in acetonitrile has been
tested. In addition compound 2 has been also studied. The carbinols suffered a phosphorylation
reaction followed by an elimination process giving rise to the corresponding carbocations. This
transformation of the carbinol into the carbocation is responsible for a significant color change.
Introduction
The current increase in international concern in relation to
criminal terrorism has brought about rising interest in the
development of reliable detection techniques of chemical warfare
(CW) agents. Among them, nerve agents are especially dangerous
species and poisoning may occur through inhalation or
consumption of contaminated liquids or foods.1 Chemically,
nerve gases are highly toxic phosphoric acid esters, structurally
Scheme 1 Mechanism of the chromogenic response of carbinols 1–3
related to the larger family of organophosphate compounds.
The extreme toxicity of these compounds is due to their ability
to bind primarily and rapidly to acetylcholinesterase (AChE)
in the neuromuscular junction of the central nervous system.2
These organophosphates also have the ability to bind butyryl-
cholinesterase (BChE) in blood. The high vapour pressures of
these nerve agents and their rapid effect on the central nervous
system, combined with the low cost and unsophisticated
technology required for production, make these compounds
among the preferred choices for terrorists. For this reason,
tools which combine reliability and rapidity of response for the
detection of these lethal chemicals are strongly required. Thus, in
addition to the detection systems based on enzymatic and physical
methodologies3 the development of fluorogenic chemosensors or
reagents, as an alternative to these classical methods, has gained
interest recently.4 During the last few years we have been involved
in the presence of warfare agent simulants.
in the design and synthesis of chromo-fluorogenic chemosensors
for the detection of nerve agents.5
During this research we have recently demonstrated that
triarylcarbinols are suitable systems for detecting these reactive
compounds.6 Triarylcarbinols can be converted into their
corresponding carbocations, this reaction being directly related
to strong colour changes.7 The known reactivity of phosphate
and phosphonate with hydroxyl groups makes it possible the
sensing mechanism. The sensing protocol is shown in Scheme 1;
the hydroxyl group present in the carbinol (I) is able to
experiment phosphorylation reactions with phosphonate
substrates to form the intermediate II, which easily undergoes
an elimination reaction giving rise to the corresponding
carbocation (III). Transformation of I into III is concomitant
with the formation of a highly delocalised system that is
responsible for a significant colour modulation.
a
´
Centro de Reconocimiento Molecular y Desarrollo Tecnologico (IDM),
Unidad mixta Universidad Politecnica de Valencia – Universidad de
´
Based on this protocol, we report herein the synthesis and
characterization of three new carbinols (1–3) and their use as
probes for the detection of nerve agent simulants (see Scheme 2).
Valencia, Valencia, Spain. E-mail: Ana.Costero@uv.es
´
CIBER de Bioingenierıa, Biomateriales y Nanomedicina
(CIBER-BBN), Spain
b
c
´
´
´
Departamento de Quımica Organica, Facultad de Quımicas,
Universidad de Valencia, Doctor Moliner 50, 46100 Burjassot,
Valencia, Spain
Experimental
d
´
Departamento de Quımica, Universidad Politecnica de Valencia,
Camino de Vera s/n, 46022 Valencia, Spain
´
General procedures and materials
w Electronic supplementary information (ESI) available: Spectral
data, 1H and 13C NMR spectra. UV and changes in color of ligands
2 and 3 in the presence of DCNP. See DOI: 10.1039/c2nj40104e
All reagents commercially available were used without
purification. Silica gel 60 F254 (Merck) plates were used for TLC.
c
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to give a yellow oil. The oil was stirred with hexane to afford 1
as a green solid (220 mg, 48%). M. p. 133 1C. 1H NMR
(300 MHz, CDCl3) d 7.91 (d, J = 1.7 Hz, 1H), 7.77–7.67
(m, 4H), 7.42 (dd, J = 8.6, 1.8 Hz, 1H), 7.40–7.35 (m, 2H),
7.23 (dd, J = 8.9, 3.6 Hz, 4H), 6.62–6.57 (m, 5H), 6.46
(d, J = 15.9 Hz, 1H), 2.87 (s, 6H), 2.86 (s, 6H). 13C NMR
(75 MHz, CDCl3) d 150.6, 150.2, 144.6, 134.7, 133.5, 132.8,
131.8, 129.6, 128.7, 128.6, 128.1, 128.0, 127.9, 126.4, 126.3,
126.3, 126.2, 125.7, 125.3, 112.8, 112.5, 79.9, 41.0. HRMS (EI):
m/z calc. for C29H29N2 [M–OH]+: 405.2331, [M–OH]+ found:
405.2334.
Synthesis of 1,1-bis(4-(dimethylamino)phenyl)-3-phenylprop-2-
in-1-ol (2)8
Phenylacetylene (200 mL, 1.82 mmol) was dissolved in freshly
distilled THF (15 mL) and the solution was kept under an inert
atmosphere. Then, BuLi (1.32 M in hexane, 1.52 mL, 1.1 equiv.)
was added to the solution and the mixture was stirred for
25 minutes. After this, 4,40-bis(dimethylamino)benzophenone
(488 mg, 1.82 mmol) dissolved in dry THF (10 mL) was added
at 0 1C. The reaction was kept at room temperature overnight
and then water (50 mL) was added to obtain a white-yellow
solid. The product was purified by column chromatography
on silica gel (hexane : ethyl acetate : Et3N 8 : 3 : 1) to yield 2
(296 mg, 44%). 1H NMR (300 MHz, acetone) d 7.39–7.30
(m, 6H), 7.27–7.21 (m, 3H), 6.56 (d, J = 9.0 Hz, 4H), 2.77
(s, 12H). 13C NMR (75 MHz, acetone) d 150.3, 135.2, 132.0,
131.7, 128.9, 128.6, 127.3, 112.2, 94.7, 85.2, 73.7, 40.2. HRMS
(EI): m/z calc. for C25H27N2O [M + H]+: 371.2123, [M + H]+
found: 371.2119.
Scheme 2 Chemical structures of carbinols 1–3 and their corres-
ponding carbocations 1a–3a.
1H and 13C NMR spectra were recorded with the deuterated
solvent as the lock and the residual solvent as the internal
reference. High-resolution mass spectra were recorded in the
positive ion mode on a VG-AutoSpec. UV-vis spectra were
recorded using a 1 cm path length quartz cuvette.
Synthesis of p-dimethylaminonaphthylideneketone (4)
4-Dimethylaminobenzaldehyde (97%, 1.044 g, 6.79 mmol)
and 2-acetylnaphthalene (1.160 g, 6.79 mmol) were dissolved
in EtOH (10 mL). Then NaOH (2 mL, 40% in water–EtOH
(4 : 1)) was added to the mixture and then it was stirred
overnight at room temperature. The reaction mixture was
poured over water (100 mL) to get an orange solid. The solid
was dried in the oven at 60 1C and recrystallized from MeOH
to obtain 4 (1.2 g, 59%) as an orange solid. 1H NMR (300 MHz,
d6-acetone) d 8.66 (d, J = 0.7 Hz, 1H), 8.02 (dd, J = 8.6, 1.8 Hz,
1H), 7.99 (dd, J = 7.0, 1.4 Hz, 1H), 7.89 (d, J = 8.2 Hz, 1H),
7.86 (d, J = 7.9 Hz, 1H), 7.67 (s, 2H), 7.58 (d, J = 9.0 Hz, 2H),
7.50 (dqd, J = 8.5, 6.9, 1.6 Hz, 2H), 6.67 (d, J = 9.0 Hz, 2H),
2.93 (s, 6H). 13C NMR (75 MHz, d6-acetone) d 188.9, 152.8,
145.4, 136.9, 135.8, 133.3, 130.9, 129.9, 129.8, 128.6, 128.5, 128.1,
127.0, 124.9, 123.1, 116.7, 112.2, 39.7.
Synthesis of (E)-3-(4-(dimethylamino)phenyl)-1-(naphthalene-2-
yl)prop-2-en-1-ol (3)
p-Dimethylaminonaphthylideneketone (4) (50 mg, 0.166 mmol)
was dissolved in MeOH (10 mL). Then, NaBH4 (63 mg,
1.66 mmol) was added in small portions to this solution and
the mixture was stirred at room temperature overnight. Finally,
water (20 mL) was added to the solution and it was extracted with
dichloromethane (3 ꢀ 20 mL). The organic phase was washed
with brine (20 mL), dried with anhydrous MgSO4 and evaporated
to give 3 as a dark oil (47 mg, 93%). 1H NMR (300 MHz,
acetone) d 8.02 (d, J = 0.7 Hz, 1H), 7.93–7.86 (m, 3H), 7.66
(dd, J = 8.6, 1.7 Hz, 1H), 7.49 (pd, J = 6.8, 1.8 Hz, 2H), 7.32
(d, J = 8.8 Hz, 2H), 6.74–6.65 (m, 3H), 6.33 (dd, J = 15.8,
6.8 Hz, 1H), 5.57 (d, J = 6.8 Hz, 1H), 4.76 (s, 1H), 2.89 (s, 6H).
13C NMR (75 MHz, acetone) d 150.7, 142.9, 134.0, 133.3, 130.4,
128.9, 128.4, 128.3, 128.1, 127.9, 126.4, 126.0, 125.7, 125.6, 124.9,
112.8, 75.3, 40.1. HRMS (EI): m/z calc. for C21H20N [M–OH]+:
286.1590, [M–OH]+ found: 286.1596.
Synthesis of (E)-1,3-bis(4-(dimethylamino)phenyl)-1-
(naphthalene-2-yl)prop-2-en-1-ol (1)
N,N,N0,N0-Tetramethylethylenediamine (332 mL, 2.18 mmol)
was dissolved in ethyl ether (20 mL) under an inert atmosphere.
Then, n-butyllithium (1.3 M in hexane, 1.68 mL, 2.18 mmol) was
added to the previously prepared solution and the mixture was
stirred for 45 min to form the BuLi–TMEDA complex. Then
4-bromo-N,N-dimethylaminoaniline (451 mg, 2.18 mmol) in
diethyl ether (10 mL) was added to the BuLi–TMDA complex.
After 45 min at room temperature, p-dimethylaminenaphthylide-
neketone (4) (327 mg, 1.09 mmol) dissolved in THF (30 mL) was
added and the mixture was stirred for 5 hours at room
temperature. Then, the reaction mixture was poured in water
(100 mL) and the aqueous phase was extracted with dichloro-
methane (3 ꢀ 20 mL). The organic phase was washed with
brine (20 mL), dried with anhydrous MgSO4 and evaporated
General procedure for detection-limit determinations
To the corresponding carbinol dissolved in acetonitrile,
increasing amounts of the corresponding simulant solution
(DFP and DCNP) were added. UV-visible spectra were
recorded in 1 cm path length cells at 20 1C (thermostated).
Representation of absorbance at the appropriate wavelength
versus concentration of the simulant allows the calculation of
the detection limits.
c
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Regeneration experiments
10 mL of DCNP were added to 10 mL of a 9.0 ꢀ 10ꢁ4 mol dmꢁ3
solution of carbinol 3 in H2O/MeCN (99/1) and the solution
was kept stirring at room temperature for 2 minutes. After this
time the UV spectrum of the solution was registered. Then 1 M
TBAOH in MeOH was added until the solution fades. The
solution was kept stirring for 30 seconds and then the process
was started again.
Results and discussion
Compounds 1–3 were prepared following the pathways
described in Scheme 3. For the preparation of 1 and 3, first
4-(N,N-dimethylamino)benzaldehyde was condensed with
2-acetylnaphthalene in order to obtain the intermediate ketone 4.9
The reaction of this ketone with 4-(N,N,-dimethylamino)-
phenyllithium yielded compound 1 (48%),10 whereas reduction of
the same ketone allowed us to obtain compound 3 with 93% yield.
Finally, derivative 2 was prepared from 4,40-bis(dimethylamino)-
benzophenone by reaction with lithium phenylacetylide (44%).11
The prepared compounds 1–3 can be converted into their
corresponding cations 1a–3a (see Scheme 2) by reaction with
perchloric acid. Table 1 shows the absorption bands observed
for these compounds in acetonitrile. As can be seen the
transformation of the carbinols into the corresponding carbo-
cations results in a significant bathochromic shift. Thus,
whereas the carbinols show absorption maxima in the UV
zone (i.e. lmax = 274, 266, 284 nm for 1, 2 and 3, respectively),
the corresponding carbocations display absorption bands in
the visible range (i.e. 715, 689 and 510 nm for 1a, 2a and 3a,
respectively). Moreover, the trisubstituted cations 1a and 2a
show absorption bands at longer wavelengths than cation 3a,
which only contains two aromatic substituents.
Scheme 4 Chemical structures of nerve agents (sarin, soman and
tabun), their simulants (DFP and DCNP) and the organophosphonates
used for the interference assay.
used as models for the design of indicators and sensing systems, as
they have similar reactivity but lack their acute toxicity. Initially
studies were carried out with compound 1 in acetonitrile (1.0 ꢀ
10ꢁ5 mol dmꢁ3) which displays an absorption band at 274 nm.
The addition of DFP or DCNP to acetonitrile solutions of 1
resulted in the appearance of a new absorption band at 715 nm
(Fig. 1) and colour modulation from colourless to blue. The
observed results are fully consistent with the phosphorylation and
subsequent elimination shown in Scheme 1.
Nearly the same spectroscopic behaviour was observed
upon addition of DFP and DCNP to acetonitrile solutions
of carbinols 2 and 3, namely the apparition of a red shifted
absorption band (689 and 510 nm for 2 and 3, respectively)
due to the formation of the corresponding carbocation (data
not shown). These red shifted absorptions were responsible for
the colour changes observed with probe 2 (colourless solution
of this carbinol turned green-blue upon addition of DFP
and DCNP, see Fig. 2), and with carbinol 3 (the colourless
initial acetonitrile solution turned pink upon addition of both
simulants, data not shown).
Signalling studies were carried out with diethylcyanophos-
phonate (DCNP) and diisopropylfluorophosphate (DFP) in
acetonitrile (see Scheme 4 for their structures). Arising from
the high toxicity of the nerve agents, sarin, soman and tabun,12
the related compounds DFP and DCNP have been typically
In a second step, and in order to evaluate the applicability of
these carbinols for the detection of nerve agent simulants, the
limits of detection (LOD) for the three carbinols were determined.
Fig. 1 UV-visible spectra of carbinol 1 alone and in the presence of
100 equiv. of DCNP in acetonitrile (1.0 ꢀ 10ꢁ5 mol dmꢁ3).
Scheme 3 Synthetic procedure for the preparation of carbinols 1–3.
Table 1 Maxima of the UV-visible spectra of carbinols 1–3 and their
corresponding carbocations 1a–3a in acetonitrile (1.0 ꢀ 10ꢁ5 mol dmꢁ3
)
Fig. 2 Colour changes observed for probe 2 (1.0 ꢀ 10ꢁ5 mol dmꢁ3) in
acetonitrile upon addition of 100 equivalents of DCNP (middle) and
100 equivalents of DFP (right).
Probe
1
2
3
1a
2a
3a
510
l/nm
274
266
284
715
689
c
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Table 2 Limit of detection, LOD (ppm), and kinetic data for probes
1–3 with DCNP and DFP (1.0 ꢀ 10ꢁ5 mol dmꢁ3 in acetonitrile)
Probe
Simulant
Detection limit/ppm
k/sꢁ1
t1/2/s
1
DFP
DCNP
DFP
DCNP
DFP
DCNP
35
85
264
165
1700
268
0.0230
0.0144
0.0025
0.0026
0.0013
0.0010
30.2
48.2
277.3
266.6
533.2
693.2
2
3
In this respect, LOD for probes 1, 2 and 3 in the presence of
DCNP and DFP are summarized in Table 2. Compounds
2 and 3 showed lower LOD values for DCNP than for DFP in
agreement with the trend observed using other chromoreagents.4c,5a
However, this tendency was the opposed for compound 1.
Moreover, it is also evident from Table 2 that trisubstituted
carbinols (i.e. 1 and 2) gave lower detection limits than
the corresponding disubstituted carbinol 3. Finally, it is
also apparent from Table 2 that lower detection limits were
observed for probe 1 when compared with 2 and 3.
Fig. 3 Changes in the absorbance band (visible zone) vs. time for
carbinols 1–3, in acetonitrile (1.0 ꢀ 10ꢁ5 mol dmꢁ3), upon addition of
100 equiv. of DCNP.
Finally, the reactivity of carbinols 1–3 with other organo-
phosphorous compounds (OP1–OP4) (C = 250 equiv.) shown
in Scheme 4 was studied in acetonitrile. In all cases, the
solutions of carbinols 1–3 remained colorless, which indicated
that no reaction occurred between them and these phosphate
derivatives. The lack of response of carbinols 1–3 in the
presence of OP1–OP4 is clearly related with the absence
of a good leaving group in the structure of the latter.
The impossibility of phosphorylation reaction precludes the
formation of the highly coloured carbocations 1a–3a.
To achieve a better understanding of the reaction, studies on
the reactivity of compounds 1–3 in the presence of the mimics
DCNP and DFP in acetonitrile solutions, by using an excess of
the corresponding simulant to ensure to be under pseudofirst
order conditions, were carried out. Monitorization of the
changes in the absorbance intensity of the carbocation band
and the plotting of ln[(A0 ꢁ A)/A0] versus time (A0 being the
final absorbance, and A the absorbance at a given time)
allowed the determination of the rate constant (k) and the
half time (t1/2 = ln 2/k) for the corresponding reaction of the
nerve-gas simulants with the probes. For example, the absorbance
changes at the UV/Vis band maximum for acetonitrile solutions
of probes 1, 2 and 3 in the presence of DCNP are shown in Fig. 3.
The kinetic data are summarized in Table 2. The obtained data
showed that the reaction of the simulants DCNP and DCP with
probe 1 is quicker than with 2 and 3.
Fig. 4 Absorbance of the 510 nm band upon consecutive exposure of
probe 3 to DCNP in acetonitrile. After each reaction probe 3 is
retrieved by treatment of the corresponding carbocation 3a with
TBAOH.
reversed and it has been reported that in the presence of
hydroxide the arylcarbinol derivatives could be retrieved.13
Therefore the attractive possibility of attaining re-usable
colorimetric probes was tested via recuperation of the sensing
arylcarbinol by reaction of the arylcation derivative with basic
solutions. Thus, after having accomplished the reaction
between the probes 1–3 and the simulants, the carbinols
obtained (1a–3a) were treated with TBAOH. Upon this treatment,
the probes 1–3 were fully regenerated and could be used for
another measurement. Taking this reaction into account the
possible use of the prepared chemodosimeter in subsequent
sensing cycles has been explored. Fig. 4 shows the absorbance
at 510 nm for probe 3 in the presence of DCNP after successive
regeneration cycles. After 10 cycles probe 3 is still able to
display colour modulations in the presence of nerve agent
simulants. The same regeneration cycles were applied to
carbinols 1 and 2 and are also able to display colour modifications
upon addition of DCNP (data not shown).
The detection of DCNP or DFP using compounds 1, 2 and 3
follows a chemodosimeter approach in which the presence of
the analyte is signalled through a specific chemical reaction
between the dosimeter molecule and the target species, leading
to the formation of a fluorescent or coloured product. An
advantage of this approach is the remarkable selectivity
usually achieved, however a disadvantage of dosimeters is that
in most cases the reactions are irreversible and thus provide
only single-use assays. However, in certain circumstances, it is
possible to take advantage of the favourable features of
chemodosimeters and also reuse them.
Conclusions
One particular feature of the formation of arylcations from
the corresponding arylcarbinols is that the process can be
In summary, a family of new reagents for the chromogenic
detection of nerve agent simulants has been prepared. The sensing
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protocol relies on the reactivity of the nerve agent simulants
with arylcarbinols to produce, upon phosphorylation and
elimination, the corresponding highly coloured arylcations.
Additionally the synthesis of the arylcarbinols is easy and the
approach is highly modular bearing in mind that a number of
chemical modifications can be carried out on the probes that
might result in a modulation of their reactivity with the nerve
agent simulants and also a modulation in the chromogenic
response. In all cases the reaction of the probes with the nerve
agent mimics results in remarkable OFF–ON chromogenic
behaviours. It is also noteworthy that these reagents react only
with DFP and DCNP that show close chemical structures to
sarin, soman and tabun, whereas they do not react with other
organophosphorous derivatives such as OP1–OP4. Moreover
the fact that the probes retain their signaling ability upon
regeneration with TBAOH is an additional issue of interest.
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Acknowledgements
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Financial support from the Spanish Government (project
MAT2009-14564-C04-01) and the Generalitat Valencia (project
PROMETEO/2009/016) is gratefully acknowledged. R.G. is
grateful to the Spanish Ministry of Education and S.R. to
Generalitat Valencia for their grants.
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