Thioureas as reporting elements for metal-responsive fluorescent chemosensors

Article abstract of DOI:10.1021/jo4003129

Proof that sulfur is a viable reporting element for the development of fluorescent chemosensors for metal ions is presented. To date, the majority of metal-responsive fluorescent chemosensors have relied on metal-nitrogen coordination to provide a fluores

Full text of DOI:10.1021/jo4003129

Article
pubs.acs.org/joc
Thioureas as Reporting Elements for Metal-Responsive Fluorescent
Chemosensors
Mireille Vonlanthen and Nathaniel S. Finney*,†
Institute of Organic Chemistry, University of Zurich, Winterthurerstrasse 190, 8057 Zurich, Switzerland
S
* Supporting Information
ABSTRACT: Proof that sulfur is a viable reporting element for the development of
fluorescent chemosensors for metal ions is presented. To date, the majority of metal-
responsive fluorescent chemosensors have relied on metal−nitrogen coordination to
provide a fluorescence response, most commonly by suppressing photoinduced
electron transfer (PET) quenching. While chemosensors with direct application to
biology, medicine, and analytical chemistry have been so developed, reliance on the
coordination chemistry of nitrogen remains a practical and conceptual limitation. Building on the fact that thioureas can quench
fluorescence emission by PET, it is shown that the quenched emission of thiourea-appended naphthalimides can be restored by
metal binding and that metal affinity and selectivity can be controlled through structural modification of the thiourea substituents.
Further, such chemosensors can function in aqueous media and, unlike nitrogen-based chemosensors, are unresponsive to
increases in [H+]. Given that the coordination properties of sulfur are distinct from those of nitrogen, this work lays the
foundation for the development of a new class of interesting and useful metal-responsive fluorescent probes.
that they focus on anion- rather than cation-induced changes in
emission, do not involve direct coordination of the thiourea
sulfur atom, generally rely on fluorescence quenching rather
than fluorescence enhancement for signaling, and have not
been shown to function in hydroxylic/aqueous solvents.
INTRODUCTION
■
Chemosensors that provide a fluorescent response to reversible
metal ion binding have broad biological and environmental
application.1,2 One of the most general approaches to
developing such fluorescent chemosensors relies on photo-
induced electron transfer (PET) from an amine appended to a
reporting fluorophore.3,4 In the absence of metal ion, PET from
the amine lone pair quenches fluorescence, and emission is
restored upon metal coordination. Beyond the generality of the
signaling mechanism, the appeal of the aminofluorophore motif
stems from the fact that metal ion affinity and selectivity can be
controlled by variation of the nitrogen subsitutents. However,
reliance on nitrogen and its coordination properties for
signaling is necessarily a limitation of this approach, and the
validation of alternative reporting elements is clearly desirable.
Of the neighboring heteroatoms that might be considered as
PET quenchers, oxygen is an insufficient donor and
phosphorus undergoes ready oxidation.5 While sulfur is a
viable candidate, there are no well-defined examples of metal-
responsive fluorescent chemosensors that rely directly on sulfur
as a reporting element.6−9 We describe here a well-defined
thiourea-based system which functions by suppression of PET,
in protic media and show that metal binding selectivity and
affinity can be controlled by structural variation. We anticipate
that this will facilitate the design and development of a range
new fluorescent chemosensors.
RESULTS AND DISCUSSION
■
Thiourea/Fluorophore Conjugates Studied. A series of
thioureas based on a naphthalimide fluorophore (1−8, Figure
1)10 were prepared, the synthesis of 6 being representative
Figure 1. Thiourea-based chemosensors and control compounds (Py
= 2-pyridyl).
(Scheme 1). Phenylthioureas 1−3 vary in alkyl spacer length; 4
serves as an N-alkyl fluorophore control; and 5−8 bear side
chains with additional metal-coordinating functionality.11
Optical Properties and Quenching Mechanism. Other
than variation in quantum yield, the optical properties of 1−8
are essentially the same as those of the naphthalimide:
substituent effects are minimal, and all compounds have
Known Thiourea-Based Fluorescent Chemosensors.
Thioureas have been used in the development of anion-
responsive fluorescent chemosensors.6 In such systems, hydro-
gen bonding of anions by thiourea N−H groups increases the
electron density of the thiourea and enhances PET quenching
of a proximate fluorophore, leading to reduction in fluorescence
emission. These efforts are distinct from the present work in
Received: February 14, 2013
© XXXX American Chemical Society
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a
Scheme 1. Synthesis of 6, a Representative Thiourea (Py = 2-Pyridyl, Im = Imidazolyl)
a
See the Experimental Section for full synthetic details.
identical absorption and emission maxima (Table 1). Thioureas
1−3 all show diminished fluorescence relative to control 4, and
a
Table 1. Relevant Optical Properties of 1−8
b
b
ε/103 M−1 cm−1
ϕ
ε/103 M−1cm−1
ϕ
1
2
3
4
11.1
12.8
12.8
13.3
0.02
0.08
0.15
0.92
5
6
7
8
13.5
13.3
12.6
13.0
0.03
0.05
0.05
0.03
a
All spectra measured in CH3OH. Emission spectra acquired at 3.3
μM. Longest λ absorption/excitation maxima 367 nm; emission
b
maxima 446 nm. Quantum yields relative to anthracene (ϕ = 0.30).
emission intensity increases as a function of increased
fluorophore/thiourea separation, as expected for PET quench-
ing.3 This distance dependence, and electrochemical measure-
ments on 1 and 4,12 strongly support PET quenching of
fluorescence emission in these thioureas.
Figure 2. Maximum metal-induced ratiometric fluorescence enhance-
ment of 1 and 5−8 (3.3 μM in CH3OH; λem = 446 nm). Ratiometric
enhancement in the presence of 4 equiv of metal ion shown by cross-
hatching to emphasize Hg2+ selectivity. See Tables 2 and 3.
Metal Ion Response in Methanol. Methanolic solutions
of thioureas 1 and 5−8 were titrated with ZnCl2, HgCl2, CdCl2,
AgClO4, and Pb(NO3)2 (see the Experimental Section). The
intrinsic binding preferences of the thiourea fragment are
illustrated by 1, which shows a strong, low-affinity response to
Zn2+, a weak response to Cd2+, and no significant response to
the other ions (Figure 2, Tables 2 and 3).13 Titrations with
alkaline and alkaline earth metal salts lead to no change in
fluorescence emission.
The affinity and selectivity of the metal response can be
dramatically altered by extension with known metal coordinat-
ing groups, in the present case alkylpyridines or a thiacrown
ether (Figure 2, Tables 2 and 3). The titration of 5 in CH3OH
with ZnCl2 is representative in showing a continuous increase
in fluorescence emission up to a maximum I/I0 (Figure 3),14
after which emission remains constant. Plots of I/I0 vs
log[metal ion] are consistent with simple saturation binding,
and nonlinear least-squares fitting allows ready extraction of
apparent first-order Kd values (Table 2);15,16 titrations of 1 and
5−7 with ZnCl2 are representative (Figure 4). Metal binding is
reversible,17 and importantly, fluorescence responses are
insensitive to the addition of excess acid (TFA).
Table 2. Apparent log(Kd) (M) for Titrations of 1 and 5−
a,
b
8
Zn2+
Cd2+
Hg2+
Ag+
Pb2+
1
5
6
7
8
−1.3
−3.2
−2.1
−2.9
−1.2
−1.6
−3.2
−2.3
−3.1
-
-
-
-
-
−5.8
−5.3
−6.1
−5.2
−2.5
−2.3
-
-
-
-
−4.4
a
b
Titrations in CH3OH at 3.3 μM chemosensor. Entries marked “-”
indicate binding too weak to allow Kd determination.
∼20-fold represent a significant dynamic range; the associated
maximum effective quantum yields (ϕmax = ϕinitial × (I/I0)max
of up to 0.6−0.7 indicate that substantial recovery of quenched
emission is possible (Figure 2, Table 3; compare to 4, ϕ =
0.92).
Second, the identity of the additional metal-coordinating
group(s) determines the metal-binding affinity and selectivity.
The highest affinities observed in 5−8 are for Hg2+, and 5−8
are all highly Hg2+-selective chemosensors (Figure 1, Table 2).
Their apparent affinities lie at the limit of determination in the
present system.18 (As our routine measurements must be made
at micromolar chemosensor concentrations, this represents the
lower limit of the Kd values we can determine; brighter probes
suitable for use at lower concentration will be required to
determine actual Kd values fluorimetrically.) The variations in
Zn2+ affinities of 5−8 are instructive. They indicate that only
)
Methanolic Metal Ion Titrations. Several aspects of these
data provide insight for the further development and use of
thioureas in fluorescent chemosensors. First, as titrations of 1
indicate, the thiourea itself is not a strongly coordinating ligand,
even for thiophilic metals. However, it is clearly a viable
reporting element for metal detection: the maximum
ratiometric fluorescence enhancements, (I/I0)max, of up to
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a,b,c
Table 3. Values of (I/I0)max and (ϕmax) for Titrations of 1 and 5−8
Zn2+
Cd2+
Hg2+
-
Ag+
Pb2+
1
5
6
7
8
20.5 (0.41)
20.5 (0.62)
13 (0.65)
13 (0.65)
7 (0.21)
5.5 (0.11)
21.7 (0.65)
5.4 (0.27)
5 (0.25)
-
-
-
19 (0.57)
15.6 (0.78)
7.4 (0.37)
20.8 (0.62)
-
6.3 (0.19)
7 (0.35)
-
-
-
-
11.2 (0.33)
a
b
c
Maximum observed I/I0 as shown in Figure 2. ϕmax = ϕinitial × (I/I0)max. ϕmax in parentheses. Entries marked “-” indicate that saturated binding,
and thus I/I0(max), were not reached.
Finally, it is instructive to consider the differences in behavior
between 5 and 7. These ligands exhibit nearly identical Kd
values for a given metal (Zn, Cd, or Hg) and presumably
provide nearly identical coordination environments. However,
(I/I0)max is consistently lower for complexes of 7 than 5,
indicating the influence of additional factors. We propose that
one of these factors is the position of the s-cis/s-trans
conformational equilibrium of nonemissive metal-complexed
species in which thiourea is not ligated (Figure 5). The
Figure 3. Titration of 5 (3.3 μM in CH3OH) with ZnCl2.
Figure 5. Minimal form of the s-cis/s-trans equilibria in metal
complexes of 5 and 7 (Fl = naphthalimide fragment; M = metal ion; R
= CH3 (5), CH2Py (7)).
presence of such species is consistent with the low intrinsic
affinity of the thiourea fragment for metal ions. In 5·M (Figure
5, R = CH3), the s-cis isomer should be favored over the s-trans
isomer for steric reasons,21 while in 7·M the isomer ratio should
be closer to unity (Figure 5, R = CH2Py).22 An increase in the
relative population of the s-cis conformation should be
accompanied by an increase in the population of the emissive
thiourea-coordinated state, increasing the observed (I/I0)max
.
This would thus account for the fluorescence enhancements of
5 being larger than those of 7.
If correct, this provides an important guideline for the
development of future thiourea chemosensors: monosubsti-
tuted or conformationally constrained analogues should exhibit
the largest increases in fluorescence upon metal binding by
favoring the s-cis conformation.
Metal Ion Response in Aqueous Media. The solubility
of 5−8 in H2O is not sufficient to prepare the requisite
micromolar solutions by serial dilution in pure water. However,
dilution of methanolic stock solutions allows preparation of
suitable aqueous samples. Titrations of 5−7 carried out in 9:1
H2O/CH3OH reveal promising and useful information (Table
4).
Although the dipicolylamine moiety is known to have high
affinity for aqueous Zn2+ and Cd2+, as noted above, the
conjugation of the amine nitrogen with the thiocarbonyl
precludes simultaneous engagement of both pyridine fragments
and coordination of the thiourea. The diminished aqueous
affinity of 5−7 for these metals is thus not surprising. The
strong response of 5 to Zn2+ ((I/I0)max = 9) indicates that
thiourea-based signaling functions effectively in aqueous
Figure 4. Binding curves from titration of 1 and 5−7 with ZnCl2.
one of the N-substituents on the thiourea contributes to metal
binding (5 vs 7), reflecting the planarity of the thiourea N
resulting from conjugation to the thiocarbonyl,19 and that even
small changes in the relative orientation of the coordinating
heteroatom lone pairs have a significant impact on metal affinity
(5 vs 6).20
Third, while there is significant variation in (I/I0)max, there is
no clear correlation with metal ion identity. The difference in
(I/I0)max for 1·Zn2+ and 1·Cd2+ indicates that coordinated
metals do not have an equal intrinsic impact on PET
quenching. That the presence of an additional ligand can
increase (I/I0)max (1·Cd2+ vs 5·Cd2+) indicates that details of
the metal coordination environment beyond thiourea binding
must also play a role in the fluorescence response. This
sensitivity to structural variation holds promise for fine control
of fluorescence response in future chemosensors.
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responsive fluorescent chemosensors in protic media. This is
significant in that most known metal-responsive chemosensors
rely on binding to a nitrogen atom, and the coordination
chemistry of nitrogen has dominated fluorescent chemosensor
development to date. Given its distinct coordination prefer-
ences, the addition of sulfur to the group of viable reporting
heteroatoms is significant expansion. In addition, unlike amine-
based fluorescent probes, the thioureas reported here do not to
respond to the addition of acid, indicating that they will be less
susceptible to pH-dependent variation in emission. We expect
these findings will have an impact beyond the present work.
It is anticipated that further variation of binding domain and
fluorophore will lead to the development of practical probes for
detection and visualization of aqueous Zn2+ and Hg2+. To this
end, we are currently preparing water-soluble congeners of 5−8
that differ in the orientation and spacing of the chelating groups
relative to the thiourea reporter as well as exploring probes
based on other recognition elements and evaluating more
strongly absorptive fluorophores.
Table 4. Apparent log(Kd) (M) and ((I/I0)max) for Aqueous
Titrations of 5−7
a,b
Zn2+
Cd2+
Hg2+
Ag+
Pb2+
5
6
7
7
−1.1 (9.3)
−0.4 (5.0)
−0.9 (5.7)
−2.1 (4.6)
−1.7 (3.0)
−2.5 (4.5)
−5.9 (4.4)
−5.8 (6.8)
−6.0 (8.6)
−7.8 (8.3)
-
-
-
-
-
-
c
d
a
b
Titrations in 9:1 H2O/CH3OH at 3.3 μM chemosensor. Entries
marked “-” indicate binding too weak to allow Kd determination.
c
log(Kd) extrapolated; maximum response could not be reached.
Measured at 33 nM chemosensor. See text and ref 23.
d
solution and that alternate chemosensor architectures should
permit the development of higher affinity Zn2+ and Cd2+
probes. It is noteworthy that, as in pure CH3OH, the addition
of excess acid (TFA) does not alter fluorescence emission,
underscoring this advantage of thiourea-based fluorescent
probes relative to the majority of known nitrogen-based
systems.
The high affinity of 5−7 for Hg2+ is retained in aqueous
media, with log(Kd) values at or near the limits of
determination. The maximum observed fluorescent enhance-
ments of 5 and 6 are diminished relative to titrations in
EXPERIMENTAL SECTION
■
General Methods. Synthetic procedures were carried out under an
inert atmosphere, in dry solvent, using standard Schlenk techniques,
unless otherwise noted. All reagents and solvents were reagent grade
and were used without further purification unless otherwise specified.
Flash chromatographic purification was performed using silica gel
Merck 60 (particle size 0.040−0.063 mm), deactivated (20%
triethylamine in hexane) silica gel Merck 60 (particle size 0.040−
0.063 mm), or deactivated (5% water by weight) neutral aluminum
oxide Sigma-Aldrich, Brockmann I, packed in glass columns; eluting
solvent for each purification was determined by thin layer
chromatography (TLC). Analytical thin-layer chromatography was
performed using Merck TLC silica gel 60 F254 or Macherey-Nagel
POLYGRAM ALOX N/UV254.
CH3OH, but the maximum enhancement for 7 ((I/I0)max
=
8.5) actually increases slightly. The naphthalimide chromo-
phore is not sufficiently absorptive to allow routine titration at
probe concentrations much below the micromolar level,
although results with 100-fold more dilute solutions ([7] =
33 nM) indicate that log(Kd) for 7·Hg2+ is at least −7.8.23
These data suggest that variation of the reporting fluorophore,
without further modification of the metal-binding domain could
readily lead to selective fluorescence detection of aqueous Hg2+
at analytically useful concentrations,18 especially as there is no
response to the potential competing ions Ag+ and Pb2+.
1H NMR chemical shifts are reported in parts per million (ppm)
relative to the solvent residual peak (CDCl3, 7.26 ppm). Multiplicities
are given as: s (singlet), d (doublet), t (triplet), q (quartet), dd
(doublet of doublets), m (multiplet), and the coupling constants, J, are
given in Hz. 13C NMR chemical shifts are reported relative to the
solvent residual peak (CDCl3, 77.0 ppm).
CONCLUSIONS
This work demonstrates that modulation of PET quenching by
thioureas provides a basis for the development of metal-
■
Scheme 2. Synthesis of Phenylthioureas 1−3
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IR frequencies are given in cm−1. HRMS data were acquired on
Bruker maXis UHPLC-HR-MS QTOF instrument with an ESI source.
All solid synthetic products were noncrystalline (oils or sticky solids),
precluding melting point determination.
Cyclic voltammograms conditions: 1 mM compound, 0.1 M
Bu4NClO4 as supporting electrolyte in CH3CN, scan rate 100 mV
s−1, glassy carbon working electrode (Ø = 0.3 cm), Pt wire counter
electrode, Ag/AgCl reference electrode, added ferrocene (Fc) as
internal reference.
Fluorescence measurements were carried out in spectroscopic grade
CH3OH using a 450 W xenon lamp excitation with 1 nm excitation
and 1 nm emission slit widths. Emission spectra were obtained by
exciting at the longest-wavelength absorption maxima. Quantum yields
were determined by standard methods24 using anthracene (ϕ = 0.30)
in CH3OH.25 The samples were diluted to optical transparency (A ≤
0.05), and the integrated emission intensity was compared to an iso-
absorptive solution of the standards in degassed solvent.
For extinction coefficient determination, four independent solutions
of different concentration were prepared, with absorption between
0.04 and 0.10 AU. The value of ε was calculated by linear least-squares
fitting of plots of A vs concentration. All fits gave R2 values of ≥0.98.
General Protocol for Metal Ion Titrations. Chemosensor
solutions (3.00 mL, ca. 3.3 μM in CH3OH, prepared by serial
dilution) were placed in a quartz cuvette, and the initial fluorescence
emission spectra were recorded. Aliquots of metal salt (3 × 5 μL, 20
μM−2 M in 10-fold increments, in CH3OH) were then added
sequentially (i.e., 3 × 5 μL × 20 μM, then 3 × 5 μL × 20 μM, etc.)
until maximum fluorescence increase or a total volume of 4 mL was
reached. After each addition of the metal ion solutions, the
fluorescence emission spectra were recorded.
(A) N-Boc-protected amine 9 (1.0 equiv) was dissolved in DMF
and triethylamine (1.2 equiv) was added. The reaction mixture was
stirred at room temperature for 30 min. 4-Bromo-1,8-naphthalic
anhydride (1.0 equiv) was added as a solid and the reaction mixture
was heated to 100 °C for 48 h. The solvent was removed and the
crude product was purified by column chromatography to give 10.
(B) A solution of 10 (1 equiv) and potassium carbonate (10 equiv)
in methoxyethanol was heated to 100 °C for 2 h. After the solution
was cooled to room temperature, water was added and the product
was extracted with CH2Cl2. The organic phase was washed with water,
dried over MgSO4, and concentrated under vacuum. The crude
product was purified by column chromatography to give 11.
(C) A solution of 11 (1 equiv) in CH2Cl2 was cooled to 0 °C, and
trifluoroacetic acid (25 equiv) was added. The reaction mixture was
stirred for 2 h at room temperature. The reaction was quenched by the
adding of a solution of NaOH (2 N), and the product was extracted
with CH2Cl2. The organic phase was dried over MgSO4 and
concentrated under vacuum. The product, 12, did not require further
purification.
(D) Phenyl isothiocyanate (1.0 equiv) was added to a suspension of
12 (1 equiv) in acetonitrile. The reaction mixture was stirred at room
temperature for 30 min. The resulting solid was isolated by filteration
and purified by recrystallization from 2-propanol to give 1.
(E) 1,1-Thiocarbonyldiimidazole and 12 were mixed in acetonitrile,
and the reaction mixture was stirred for 1 h at room temperature. The
solid product 13 was obtained by filtration and did not require
purification.
(F) To a mixture of 13 (1 equiv) and acetonitrile was added a
solution of the appropriate amine (1.0 equiv) in acetonitrile. The
reaction mixture was refluxed until clarified. After cooling to room
temperature, the reaction mixture was concentrated under vacuum.
The residue was solved in CH2Cl2 (20 mL), washed with water, dried
over MgSO4, and concentrated under vacuum. The crude was purified
to give 5−8 as yellow solids.
Synthetic Schemes. Scheme 2−4 show the synthetic schemes for
each synthesis.
Scheme 3. Synthesis of Reference Compound 4
Synthetic Details and Tabulated Spectroscopic Data. N-Boc-
diamine 9. Prepared according to the literature.26 The 1H NMR data
1
are consistent with those previously reported. H NMR (δ ppm, 400
MHz, CDCl3): 4.92 (s, br, 1H), 3.15 (q, 2H, J = 5.8), 2.78 (t, 2H, J =
5.8), 1.43 (s, 9H), 1.19 (s, br, 2H).
N-Boc-bromonaphthalimide Amine 10. Prepared according to
general procedure A: N-Boc-1,2-diaminoethane 9 (400 mg, 2.71
mmol) in DMF (20 mL); triethylamine (328 mg, 3.25 mmol); 4-
bromo-1,8-naphthalic anhydride (750 mg, 2.71 mmol). The crude
product was purified by column chromatography (CH2Cl2/MeOH
1
99.5/0.5) to give 10 as a yellow solid (782 mg, 69%). The H NMR
data are consistent with those previously reported.27 1H NMR (δ ppm,
400 MHz, CDCl3): 8.66 (dd, 1H, J = 7.3, J = 0.9), 8.57 (dd, 1H, J =
8.4, J = 0.8), 8.42 (d, 1H, J = 7.8), 8.04 (d, 1H, J = 7.8), 7.86 (dd, 1H, J
= 8.4, J = 7.3), 4.93 (s, br, 1H), 4.35 (t, 2H, J = 5.7), 3.55−3.52 (m,
2H), 1.28 (s, 9H).
General Procedures. The following are general protocols,
illustrated with the transformation of 9→13. Complete experimental
procedures follow.
N-Boc-methoxyethoxynaphthalimide Amine 11. Prepared accord-
ing to general procedure B: 10 (800 mg, 1.90 mmol); potassium
Scheme 4. Synthesis of Chemosensors 5−8 (Py = 2-Pyridyl)
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carbonate (2.63 g, 19.00 mmol); methoxyethanol (50 mL). The crude
product was purified by column chromatography (CH2Cl2/EtOAc 9/
1) to give 11 as a light yellow solid (665 mg, 84%). 1H NMR (δ ppm,
400 MHz, CDCl3): 8.61 (dd, 1H, J = 1.2, J = 8.3), 8.60 (dd, 1H, J =
1.2, J = 7.4), 8.54 (d, 1H, J = 8.3), 7.70 (dd, 1H, J = 7.4, J = 8.3), 7.04
(d, 1H, J = 8.3), 5.02 (s, 1H, br), 4.44−4.42 (m, 2H), 4.35 (t, 2H, J =
5.7), 3.95−3.93 (m, 2H), 3.54−3.49 (m, 5H), 1.31 (s, 9H). 13C NMR
(δ ppm, 100 MHz, CDCl3): 165.0, 164.4, 160.3, 156.1, 133.7, 132.0,
129.7, 129.1, 126.1, 123.7, 122.3, 115.2, 106.1, 79.2, 70.8, 68.6, 59.5,
40.1, 39.8, 28.4 (3). IR (KBr) cm−1: 3346m, 2979m, 2932m, 2889m,
1698s, 1655s, 1622m, 1595s, 1582s, 1535s, 1453m, 1428m, 1390s,
1362s, 1270s, 1237s, 1180s, 1129m, 1087m, 1054m. HRMS-ESI: calcd
for C22H26N2NaO6 [M + Na]+ 437.16831, found 437.16787. Rf
(CH2Cl2/AcOEt 9/1): 0.14.
product was purified by column chromatography (CH2Cl2/MeOH
99/1) to give 11a as a light yellow solid (720 mg, 73%). H NMR (δ
1
ppm, 500 MHz, CDCl3): 8.63 (dd, 1H, J = 1.2, J = 8.3), 8.61 (dd, 1H,
J = 1.2, J = 7.4), 8.55 (d, 1H, J = 8.3), 7.72 (dd, 1H, J = 7.4, J = 8.3),
7.06 (d, 1H, J = 8.3), 5.31 (s, 1H, br), 4.45−4.43 (m, 2H), 4.26 (t, 2H,
J = 6.5), 3.95−3.93 (m, 2H), 3.52 (s, 3H), 3.18−3.14 (m, 2H), 1.93
(quint, 2H, J = 6.4), 1.45 (s, 9H). 13C NMR (δ ppm, 100 MHz,
CDCl3): 165.0, 164.4, 160.3, 156.2, 133.7, 131.9, 129.6, 129.2, 126.1,
123.7, 122.3, 115.2, 106.2, 79.1, 70.8, 68.6, 59.5, 37.6 (3), 28.6 (3). IR
(KBr) cm−1: 3309m, 2930m, 2895w, 2819w, 1688s, 1658s, 1623w,
1598m, 1538m, 1442m, 1398m, 1363s, 1324w, 1290m, 1267m, 1244s,
1180m, 1165m, 1127s, 1097m, 1084m, 1060w, 1036w. HRMS-ESI:
calcd for C23H28N2NaO6 [M + Na]+ 451.18369, found 451.18363. Rf
(CH2Cl2/EtOAc 9/1): 0.16.
Methoxyethoxynaphthalimide Amine 12a. Prepared according to
general procedure C: 11a (350 mg, 0.82 mmol) in CH2Cl2 (8 mL);
trifluoroacetic acid (2.33 g, 38.6 mmol). Compound 12a was obtained
without as a yellow solid without need for further purification (220
mg, 81%). 1H NMR (δ ppm, 500 MHz, CDCl3): 8.63 (dd, 1H, J = 1.2,
J = 8.3), 8.61 (dd, 1H, J = 1.2, J = 7.4), 8.55 (d, 1H, J = 8.3), 7.72 (dd,
1H, J = 7.4, J = 8.3), 7.06 (d, 1H, J = 8.3), 4.45−4.42 (m, 2H), 4.28 (t,
2H, J = 6.8), 3.95−3.93 (m, 2H), 3.53 (s, 3H), 2.76 (t, 2H, J = 6.7).
13C NMR (δ ppm, 125 MHz, CDCl3): 164.9, 164.3, 160.2, 133.6,
131.9, 129.6, 129.1, 126.1, 123.7, 122.4, 115.3, 106.1, 70.8, 68.5, 59.6,
39.5, 37.7, 32.3. IR (KBr) cm−1: 3372w, 3308w, 2955w, 2934w,
2896w, 2866,w 2822w, 1695s, 1658s, 1621m, 1594s, 1579m, 1515w,
1472w, 1454w, 1431m, 1389s, 1354s, 1271s, 1240m, 1199m, 1129m,
1094m, 1044m, 1034m. HRMS-ESI: calcd for C18H20N2O4 [M + H]+
329.14958, found 329.14967.
Methoxyethoxynaphthalimide Amine 12. Prepared according to
general procedure C: 11 (640 mg, 1.54 mmol) in CH2Cl2 (5 mL);
trifluoroacetic acid (4.40 g, 38.60 mmol). Compound 12 was obtained
1
without purification as a yellow solid (393 mg, 81%). H NMR (δ
ppm, 400 MHz, CDCl3): 8.62 (dd, 1H, J = 1.2, J = 8.3), 8.60 (dd, 1H,
J = 1.2, J = 7.4), 8.54 (d, 1H, J = 8.3), 7.70 (dd, 1H, J = 7.4, J = 8.3),
7.04 (d, 1H, J = 8.3), 4.44−4.42 (m, 2H), 4.28 (t, 2H, J = 6.5), 3.95−
3.93 (m, 2H), 3.52 (s, 3H), 3.08 (t, 2H, J = 6.5). 13C NMR (δ ppm,
100 MHz, CDCl3): 165.0, 164.4, 160.3, 133.7, 131.9, 129.6, 129.1,
126.1, 123.7, 122.4, 115.3, 106.1, 70.8, 68.6, 59.5, 43.1, 40.8. IR (KBr)
cm−1: 3366m, 3299w, 2953w, 2878w, 2821w, 1696s, 1656s, 1622m,
1514s, 1473m, 1457m, 1427m, 1386s, 1356s, 1310m, 1263s, 1233s,
1200m, 1172m, 1124s, 1106s, 1093s, 1075s, 1033s. HRMS-ESI: calcd
for C17H19N2O4 [M + H]+ 315.13393, found 315.13342.
Phenylthiourea Naphthalimide 1. Prepared according to general
procedure D: Phenyl isothiocyanate (86 mg, 0.64 mmol); 12 (200 mg,
0.64 mmol); acetonitrile (8 mL). The reaction mixture was stirred at
room temperature for 30 min. The solvent was removed, and the
crude material was purified by recrystallization from 2-propanol to give
1 as a cream white solid (185 mg, 65%). 1H NMR (δ ppm, 400 MHz,
CDCl3): 8.65 (dd, 1H, J = 1.2, J = 8.3), 8.54 (dd, 1H, J = 1.2, J = 7.4),
8.48 (d, 1H, J = 8.3), 7.72 (dd, 1H, J = 7.4, J = 8.3), 7.43−7.34 (m,
3H), 7,20 (s, br), 7.07 (d, 1H, J = 8.3), 6.85 (s, 1H, br), 4.46−4.41 (m,
4H), 4.01−3.94 (m, 4H), 3.53 (s, 3H). 13C NMR (δ ppm, 125 MHz,
CDCl3): 181.4, 165.1, 164.5, 160.6, 136.1, 134.0, 132.1, 130.1 (2),
129.6, 129.5, 127.5, 126.1, 126.0 (2), 123.7, 122.0, 114.8, 106.2, 70.8,
68.6, 59.6, 46.0, 38.7. IR (KBr) cm−1: 3361m, 3193m, 3003w, 2935w,
2885w, 1690s, 1651s, 1620m, 1595s, 1549s, 1515s, 1471m, 1453m,
1427m, 1398m, 1385s, 1362s, 1345s, 1321s, 1298m, 1271s, 1238s,
1202m, 1174m, 1124s, 1081s, 1033m. HRMS-ESI: calcd for
C24H23N3NaO4S [M + Na]+ 472.13015, found 437.12983. Rf
(deactivated silica gel, CH2Cl2/hexane/Et3N 3.5/1.5/0.1): 0.42.
Naphthalimide Phenylthiourea 2. Prepared according to general
procedure D: Phenyl isothiocyanate (37 mg, 0.27 mmol); 12a (90 mg,
0.27 mmol); acetonitrile (6 mL). The reaction mixture was stirred at
room temperature for 30 min, the volatiles were removed, and the
crude product was purified by recrystallization from 2-propanol to give
2 as a yellow solid (75 mg, 59%). 1H NMR (δ ppm, 400 MHz,
CDCl3): 8.63 (d, 1H, J = 8.3), 8.52 (d, 1H, J = 7.4), 8.46 (d, 1H, J =
8.3), 7.70 (dd, 1H, J = 7.4, J = 8.3), 7.56 (s, 1H, br), 7.52−7.48 (m,
2H), 7.35−7.30 (m, 3H), 7.05 (d, 1H, J = 8.3), 4.44−4.42 (m, 2H),
4.16 (t, 2H, J = 6.1), 3.95−3.93 (m, 2H), 3.69−3.67 (m, 2H), 3.52 (s,
3H), 2.09−2.06 (m, 2H). 13C NMR (δ ppm, 125 MHz, CDCl3):
180.64, 165.05, 164.41, 160.42, 136.29, 133.87, 132.03, 130.21 (2),
129.52, 129.45, 127.19, 126.10, 125.32 (2), 123.64, 122.08, 114.88,
106.14, 70.75, 68.57, 59.54, 42.30, 37.36, 27.45. IR (KBr) cm−1:
3308m, 3198m, 3104w, 3025w, 2932w, 1689s, 1653s, 1591s, 1548s,
1532s, 1513s, 1457m, 1395s, 1376m, 1357s, 1343s, 1327s, 1315s,
1266s, 1237s, 1171s, 1123m, 1081s, 1051m. HRMS-ESI: calcd for
C25H25N3NaO4S [M + Na]+ 486.1458, found 486.14522. Rf
(deactivated silica gel, CH2Cl2/hexane/Et3N 3.5/1.5/0.1): 0.44.
N-Boc-diamine 9a. Prepared according to the literature.28 The H
1
1
NMR data are consistent with those previously reported. H NMR (δ
ppm, 400 MHz, CDCl3): 4.91 (s, br, 1H), 3.23−3.15 (m, 2H), 2.75 (t,
2H, J = 6.6), 1.60 (quint, 2H, J = 6.6), 1.43 (s, 9H), 1.29 (s, br, 2H).
N-Boc-bromonaphthalimide Amine 10a. Prepared according to
general procedure A: N-Boc-1,3-diaminopropane 9a (1.00 g, 5.74
mmol) was dissolved in DMF (20 mL); triethylamine (0.58 g, 5.74
mmol); 4-bromo-1,4-naphthalic anhydride (1.59 g, 5.74 mmol) The
crude product was purified by column chromatography (CH2Cl2/
MeOH 99.5/0.5) to give 10a as a yellow solid (2.38 g, 94%). 1H NMR
(δ ppm, 400 MHz, CDCl3): 8.66 (dd, 1H, J = 7.3, J = 1.2), 8.58 (dd,
1H, J = 8.5, J = 1.2), 8.41 (d, 1H, J = 7.8), 8.05 (d, 1H, J = 7.8), 7.85
(dd, 1H, J = 8.5, J = 7.3), 5.20 (s, br, 1H), 4.26 (t, 2H, J = 6.6), 3.19−
3.14 (m, 2H), 1.93 (quint, 2H, J = 6.5), 1.45 (s, 9H). 13C NMR (δ
ppm, 125 MHz, CDCl3): 164.0 (2), 156.0, 133.5, 132.3, 131.4, 131.2,
130.7, 130.5, 129.0, 128.1, 122.9, 122.0, 79.1, 37.8 (2), 37.5, 28.4 (3).
IR (KBr) cm−1: 3359m, 2968m, 2926w, 1704s, 1684s, 1652s, 1618m,
1592m, 1571m, 1526s, 1460m, 1447m, 1436m, 1361s, 1349s, 1283s,
1269s, 1251m, 1230s, 1171s, 1101m, 1080m, 1065m, 1044m, 1001w.
HRMS-ESI: calcd for C20H21BrN2NaO4 [M + Na]+ 455.05769, found
455.05746. Rf (CH2Cl2/MeOH 99.5/0.5): 0.23.
N-Boc-diamine 9b. Prepared according to the literature.29 The H
1
1
NMR data are consistent with those previously reported. H NMR (δ
ppm, 400 MHz, CDCl3): 4.61 (s, br, 1H), 3.15−3.11 (m, 2H), 2.71 (t,
2H, J = 6.6), 1.50−1.46 (m, 4H), 1.44 (s, 9H), 1.31 (s, br, 2H).
N-Boc-bromonaphthalimide Amine 10b. Prepared according to
general procedure A: N-Boc-1,4-diaminobutane 9b (1.30 g, 6.91
mmol) was dissolved in DMF (20 mL); triethylamine (0.70 g, 6.91
mmol); 4-bromo-1,4-naphthalic anhydride (1.91 g, 6.90 mmol). The
crude product was purified by column chromatography (CH2Cl2/
MeOH 99.5/0.5) to give 10b as a light yellow solid (2.10 g, 68%). 1H
NMR (δ ppm, 400 MHz, CDCl3): 8.65 (dd, 1H, J = 7.3, J = 1.1), 8.57
(dd, 1H, J = 8.5, J = 1.1), 8.41 (d, 1H, J = 7.8), 8.04 (d, 1H, J = 7.8),
7.85 (dd, 1H, J = 8.5, J = 7.3), 4.61 (s, br, 1H), 4.19 (t, 2H, J = 7.4),
3.20 (q, 2H, J = 6.5), 1.80−1.73 (m, 2H), 1.65−1.57 (m, 2H), 1.43 (s,
9H). 13C NMR (δ ppm, 125 MHz, CDCl3): 163.8 (2), 156.1, 133.5,
132.2, 131.4, 131.3, 130.8, 130.5, 129.2, 128.2, 123.2, 122.4, 79.2, 40.4,
40.2, 28.6 (3), 27.7, 25.55. IR (KBr) cm−1: 3358m, 2971w, 2934w,
1703s, 1682s, 1650s, 1591m, 1570m, 1532m, 1460w, 1435w, 1389m,
1364m, 1263m, 1233m, 1177m, 1104w, 1066m. HRMS-ESI: calcd for
C21H23BrN2NaO4 [M + Na]+ 469.07334, found 469.07344. Rf
(CH2Cl2/MeOH 99.5/0.5): 0.23.
N-Boc-methoxyethoxynaphthalimide Amine 11a. Prepared ac-
cording to general procedure B: 10a (1.00 g, 2.30 mmol); potassium
carbonate (3.18 g, 23.00 mmol); methoxyethanol (60 mL). The crude
F
dx.doi.org/10.1021/jo4003129 | J. Org. Chem. XXXX, XXX, XXX−XXX

The Journal of Organic Chemistry
Article
N-Boc-methoxyethoxynaphthalimide Amine 11b. Prepared ac-
cording to general procedure B: 10b (1.75 g, 3.91 mmol); potassium
carbonate (5.4 g, 39.1 mmol); methoxyethanol (60 mL). The crude
product was purified by column chromatography (CH2Cl2/AcOEt 9/
1) to give 11b as a light yellow solid (1.37 g, 79%). 1H NMR (δ ppm,
500 MHz, CDCl3): 8.62 (dd, 1H, J = 1.2, J = 8.3), 8.61 (dd, 1H, J =
1.2, J = 7.4), 8.54 (d, 1H, J = 8.3), 7.71 (dd, 1H, J = 7.4, J = 8.3), 7.06
(d, 1H, J = 8.3), 4.63 (s, 1H, br), 4.45−4.43 (m, 2H), 4.18 (t, 2H, J =
7.4), 3.95−3.93 (m, 2H), 3.52 (s, 3H), 3.20−3.19 (m, 2H), 1.80−1.74
(m, 2H), 1.64−1.58 (m, 2H), 1.43 (s, 9H). 13C NMR (δ ppm, 100
MHz, CDCl3): 164.7, 164.1, 160.2, 156.1, 133.5, 131.8, 129.6, 129.0,
126.1, 123.7, 122.5, 115.4, 106.1, 79.1, 70.8, 68.5, 59.5, 40.4, 39.9, 28.6
(3), 27.7, 25.6. IR (KBr) cm−1: 3347m, 2980m, 2936m, 2893w, 1683s,
1652s, 1623m, 1596m, 1541s, 1453m, 1393s, 1358s, 1292m, 1274s,
1248s, 1175s, 1125m, 1097m, 1085m, 1052m, 1032m. HRMS-ESI:
calcd for C24H30N2NaO6 [M + Na]+ 465.19961, found 465.19935. Rf
(CH2Cl2/AcOEt 9/1): 0.16.
was purified by column chromatography (CH2Cl2 to CH2Cl2/MeOH
99/1) to give 4 as a light yellow solid (71 mg, 72%). 1H NMR (δ ppm,
400 MHz, CDCl3): 8.67−8.59 (m, 2H), 8.55 (d, 1H, J = 8.2), 7.71 (t,
1H, J = 7.8), 7.05 (d, 1H, J = 8.3), 4.51−4.36 (m, 2H), 4.17−4.18 (m,
2H), 4.04−3.87 (m, 2H), 3.52 (s, 3H), 1.88−1.66 (m, 2H), 1.01 (t,
3H, J = 7.4). 13C NMR (δ ppm, 125 MHz, CDCl3): 164.7, 164.2,
160.1, 133.5, 131.8, 129.6, 128.9, 126.1, 123.7, 122.7, 115.5, 106.1,
70.8, 68.5, 59.6, 42.0, 21.6, 11.7. IR (KBr) cm−1: 2960w, 2931w,
2900w, 2875w, 1699s, 1660s, 1595s, 1580m, 1514w, 1455m, 1432m,
1387s, 1354s, 1287s, 1236s, 1200m, 1130m, 1091s, 1045m, 1037m.
HRMS-ESI: calcd for C18H19NNaO4 [M + Na]+ 336.12063, found
336.12065. Rf (CH2Cl2/MeOH 99/1): 0.28.
Naphthalimide Imidazolylthiourea 13. Prepared according to
general procedure E: thiocarbonyldiimidazole (187 mg, 1.05 mmol),
12 (330 mg, 1.05 mmol), acetonitrile (5 mL). 13 was obtained as a
1
yellow solid without need for purification (365 g, 82%). H NMR (δ
ppm, 400 MHz, CDCl3): 9.13 (s, 1H), 8.62 (dd, 1H, J = 1.2, J = 8.3),
8.60 (dd, 1H, J = 1.2, J = 7.4), 8.54 (d, 1H, J = 8.3), 8.42 (s, 1H), 7.70
(dd, 1H, J = 7.4, J = 8.3), 7.64 (s, 1H), 7.04 (d, 1H, J = 8.3), 4.63−4.60
(m, 2H), 4.47−4.44 (m, 2H), 4.05−4.04 (m, 2H), 3.96−3.93 (m, 2H),
3.52 (s, 3H). 13C NMR (δ ppm, 100 MHz, CDCl3): 166.0, 161.1,
136.7, 134.6, 132.6, 130.1, 129.7, 126.4, 123.7, 121.7, 117.0, 114.4,
106.5, 70.7, 68.8, 59.6, 47.8, 38.8. HRMS-ESI: calcd for C21H20N4O4S
[M + H]+ 425.12780, found 425.12724.
N-Methyl-2-picolylthiourea Naphthalimide 5. Prepared according
to general procedure F: 13 (150 mg, 0.35 mmol); acetonitrile (10
mL); N-methyl-2-picoline (43 mg, 0.35 mmol); acetonitrile (3 mL).
The reaction mixture was stirred for 1 h at reflux, the volatiles were
removed, and the crude product was purified by column
chromatography (silica gel deactivated with 20% Et3N/hexanes,
CH2Cl2/hexane/Et3N 3.5/1.5/0.1) to give 5 as a yellow solid (138
mg, 82%). 1H NMR (δ ppm, 400 MHz, CDCl3): 8.65 (dd, 1H, J = 1.2,
J = 8.4), 8.58 (dd, 1H, J = 1.2, J = 7.3), 8.53 (d, 1H, J = 8.3), 8.44−8.43
(m, 1H), 7.72 (dd, 1H, J = 7.3, J = 8.4), 7.58 (td, J = 1.6, J = 7.6, 1H),
7.37−7.32 (m, 2H), 7.14−7.11 (m, 1H), 7.06 (d, 1H, J = 8.4), 5.13 (s,
2H), 4.55−4.52 (m, 2H), 4.46−4.43 (m, 2H), 4.04−4.01 (m, 2H),
3.96−3.94 (m, 2H), 3.53 (s, 3H), 3.21 (s, 3H). 13C NMR (δ ppm, 100
MHz, CDCl3): 182.9, 165.6, 165.0, 160.6 (2), 149.3, 137.0, 134.0,
132.2, 129.7, 129.5, 126.2, 123.7, 122.5, 122.3, 122.1, 114.9, 106.3,
70.8, 68.7, 59.6, 58.6, 47.4 (2), 39.4. IR (KBr) cm−1: 3343w, 2925w,
1693s, 1652s, 1621m, 1593s, 1534s, 1472m, 1436m, 1386s, 1354s,
1269s, 1236s, 1179m, 1125m, 1081s, 1031m. HRMS-ESI: calcd for
C25H26N4O4S [M + H]+ 479.17475, found 479.17452. Rf (deactivated
silica gel, CH2Cl2/hexane/Et3N 3.5/1.5/0.1): 0.26.
Ethylpyridylthiourea Naphthalimide 6. Prepared according to
general procedure F: 13 (150 mg, 0.35 mmol); acetonitrile (10 mL);
2-(aminoethyl)pyridine (43 mg, 0.35 mmol); acetonitrile (3 mL). The
reaction mixture was stirred for 1 h at reflux, the solvent was removed
under vacuum, and the solid crude product was washed with
acetonitrile and purified by recrystallization from 2-propanol to give
6 as a yellow solid (96 mg, 57%). 1H NMR (δ ppm, 500 MHz,
CDCl3): 8.64 (dd, 1H, J = 1.2, J = 8.4), 8.60 (dd, 1H, J = 1.2, J = 7.3),
8.55 (d, 1H, J = 8.3), 8.54−8.52 (m, 1H), 7.72 (dd, 1H, J = 7.3, J =
8.4), 7.63 (td, J = 1.6, J = 7.6, 1H), 7.24−7.22 (m, 2H), 7.16−7.13 (m,
1H), 7.06 (d, 1H, J = 8.4), 5.13, 4.46−4.42 (m, 4H), 3.96−3.93 (m,
4H), 3.80−3.70 (m, 2H), 3.53 (s, 3H), 3.15−3.12 (m, 2H). 13C NMR
(δ ppm, 100 MHz, CDCl3): 181.7, 165.3, 164.7, 160.4 (2), 149.3,
136.8, 134.1, 132.2, 129.6, 126.2, 123.7, 123.6, 122.0, 121.8, 114.7,
106.3, 70.8, 68.6, 59.6, 53.6, 38.8, 36.7, 25.5. IR (KBr) cm−1: 3350
(m), 3210w, 2928w, 1692s, 1657s, 1593s, 1581s, 1518s, 1474m,
1425m, 1386s, 1346s, 1272s, 1293s, 1196m, 1124m, 1083s, 1054m.
HRMS-ESI: calcd for C25H26N4O4S [M + H]+ 479.17475, found
479.17497. Rf (deactivated silica gel, CH2Cl2/hexane/Et3N 3.5/1.5/
0.1): 0.09.
Methoxyethoxynaphthalimide Amine 12b. Prepared according to
general procedure C: 11b (560 mg, 1.26 mmol) in CH2Cl2 (10 mL);
trifluoroacetic acid (3.6 g, 31.6 mmol). Compound 12b was obtained
1
without purification as a yellow solid (560 mg, 84%). H NMR (δ
ppm, 500 MHz, CDCl3): 8.57 (dd, 1H, J = 1.2, J = 8.3), 8.55 (dd, 1H,
J = 1.2, J = 7.4), 8.49 (d, 1H, J = 8.3), 7.67 (dd, 1H, J = 7.4, J = 8.3),
7.02 (d, 1H, J = 8.3), 4.42−4.40 (m, 2H), 4.17 (t, 2H, J = 7.3), 3.94−
3.92 (m, 2H), 3.52 (s, 3H), 2.91 (t, 2H, J = 7.0), 1.85−1.79 (m, 2H),
1.72−1.66 (m, 2H). 13C NMR (δ ppm, 100 MHz, CDCl3): 164.7,
164.1, 160.2, 133.5, 131.8, 129.6, 129.0, 126.1, 123.7, 122.5, 115.4,
106.1, 70.8, 68.5, 59.5, 41.9, 40.0, 31.1, 25.5. IR (KBr) cm−1: 3340w,
2928w, 1694s, 1657s, 1622m, 1594s, 1514m, 1454m, 1429m, 1389s,
1354s, 1271s, 1240m, 1200m, 1181m, 1129m, 1094m, 1080m, 1032m,
1053m. HRMS-ESI: calcd for C19H22N2O4 [M + H]+ 343.16523,
found 343.6529.
Naphthalimide Phenylthiourea 3. Prepared according to general
procedure D: Phenyl isothiocyanate (100 mg, 0.73 mmol); 12b (252
mg, 0.73 mmol); acetonitrile (10 mL). The reaction mixture was
stirred at room temperature for 30 min, the solvent evaporated and the
crude product was purified by recrystallization from 2-propanol to give
3 as a cream white solid (175 mg, 50%). 1H NMR (δ ppm, 400 MHz,
CDCl3): 8.63 (dd, 1H, J = 1.2, J = 8.3), 8.56 (dd, 1H, J = 1.2, J = 7.4),
8.49 (d, 1H, J = 8.3), 7.71 (dd, 1H, J = 7.4, J = 8.3), 7.56 (s, 1H, br),
7.44−7.40 (m, 2H), 7.31−7.22 (m, 3H), 7.05 (d, 1H, J = 8.3), 6.26 (s,
1H, br), 4.45−4.43 (m, 2H), 4.17 (t, 2H, J = 7.0), 3.95−3.93 (m, 2H),
3.78−3.73 (m, 2H), 3.52 (s, 3H), 1.81−1.68 (m, 4H). 13C NMR (δ
ppm, 125 MHz, CDCl3): 180.9, 164.7, 164.1, 160.3, 136.3, 133.7,
131.9, 130.3 (2), 129.6, 129.2, 127.4, 126.1, 125.6 (2), 123.7, 122.4,
115.2, 106.1, 70.8, 68.6, 59.6, 45.1, 39.5, 26.4, 25.5. IR (KBr) cm−1:
3379m, 3159m, 2933m, 1692s, 1650s, 1595s, 1578s, 1541s, 1513s,
1450m, 1397s, 1386s, 1358s, 1320s, 1266s, 1235s, 1189m, 1126s,
1087s, 1060m, 1029m. HRMS-ESI: calcd for C26H27N3NaO4S [M +
Na]+ 500.16145, found 500.16158. Rf (deactivated silica gel, CH2Cl2/
hexane/Et3N 3.5/1.5/0.1): 0.36.
N-Propylbromonaphthalimide 9c. 1-Aminopropane (43 mg, 0.72
mmol) was added to 4-bromo-1,8-naphthalic anhydride (200 mg, 0.72
mmol) in ethanol (15 mL), resulting in an orange solution which was
refluxed overnight. After the mixture was cooled to room temperature
a solid precipitated and was isolated by filtration. Purification by
column chromatography (CH2Cl2/hexane 1/1) to give 9c as a white
solid (200 mg, 88%). 1H NMR (δ ppm, 400 MHz, CDCl3): 8.67 (dd,
1H, J = 1.1, J = 7.3), 8.58 (dd, 1H, J = 1.1, J = 8.5), 8.42 (d, 1H, J =
7.9), 8.05 (d, 1H, J = 7.9), 7.85 (dd, 1H, J = 7.3, J = 8.5), 4.17−4.13
(m, 2H), 1.82−1.73 (m, 2H), 1.02 (t, 3H, J = 7.4). 13C NMR (δ ppm,
125 MHz, CDCl3): 163.7 (2), 133.3, 132.1, 131.3, 131.2, 130.8, 130.3,
129.2, 128.2, 123.3, 122.4, 42.2, 21.5, 11.6. IR (KBr) cm−1: 3054w,
2950m, 2869w, 1699s, 1659s, 1586s, 1568s, 1503m, 1460m, 1437m,
1397m, 1358s, 1287m, 1240s, 1165m, 1152w, 1101w, 1072s, 1044m,
1015w. HRMS-ESI: calcd for C15H12BrNNaO2 [M + Na]+ 339.99436,
found 339.99412. Rf (CH2Cl2/hexane 1/1): 0.42.
Dipicolylthiourea Naphthalimide 7. Prepared according to general
procedure F: 13 (150 mg, 0.35 mmol); acetonitrile (10 mL); di(2-
picolyl)amine (70 mg, 0.35 mmol); acetonitrile (3 mL). The reaction
mixture was stirred for 1 h at reflux, the solvent was removed, and the
crude product was purified by column chromatography (silica gel
deactivated with 20% Et3N/hexanes, CH2Cl2/hexane/Et3N 3.5/1.5/
N-Propylmethoxyethoxynaphthalimide 4. Prepared according to
general procedure B: 9c (100 mg, 0.32 mmol); potassium carbonate
(435 mg, 19.00 mmol); methoxyethanol (10 mL). The crude product
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0.1) to give 7 as a yellow solid (180 mg, 82%). 1H NMR (δ ppm, 500
MHz, CDCl3): 8.77 (t, 1H, J = 4.8), 8.63 (dd, 1H, J = 1.2, J = 8.4),
8.52 (dd, 1H, J = 1.2, J = 7.3), 8.48 (d, 1H, J = 8.3), 8.32−8.28 (m,
2H), 7.68 (dd, 1H, J = 7.3, J = 8.4), 7.48 (td, J = 1.8, J = 7.6, 2H),
7.26−7.23 (m, 2H), 7.06−7.02 (m, 3H), 4.97 (s, 4H), 4.55−4.53 (m,
2H), 4.45−4.42 (m, 2H), 4.16−4.12 (m, 2H), 3.96−3.93 (m, 2H),
3.53 (s, 3H). 13C NMR (δ ppm, 100 MHz, CDCl3): 184.9, 165.1,
164.4, 160.3 (3), 149.0 (2), 136.9 (2), 133.7, 131.9, 129.7, 129.1,
126.1, 123.7, 122.9 (2), 122.5 (2), 122.4, 115.3, 106.2, 70.8, 68.6, 59.5,
45.9, 45.6 (2), 39.6. IR (KBr) cm−1: 3374w, 2924m, 1695s, 1656s,
1592s, 1516m, 1473m, 1440m, 1384s, 1355s, 1267s, 1325s, 1201m,
1179m, 1125m, 1081s, 1031m. HRMS-ESI: calcd for C30H29N5O4S
[M + H]+ 556.2013, found 556.20181. Rf (deactivated silica gel,
CH2Cl2/Hexane/Et3N 3.5/1.5/0.1): 0.26.
589−598. (f) Rurack, K.; Resch-Genger, U. Chem. Soc. Rev. 2002, 31,
116−127. (g) Bren, V. A. Russ. Chem. Rev. 2001, 70, 1017−1036.
(h) de Silva, A. P.; Gunarathe, H. Q. N.; Gunnlaugsson, T.; Huxley, A.
J. M; McCoy, C. P.; Rademacher, J. T.; Rice, T. E. Chem. Rev. 1997,
97, 515.
(2) We differentiate between chemosensors, which provide a signal in
response to a reversible binding event, and chemodosimeters, which
provide a signal in response to a transformation that is irreversible. For
an early articulation of this important distinction, see: Fluorescent
Chemosensors for Ion and Molecule Recognition; Czarnik, A. W., Ed.;
American Chemical Society: Washington, DC, 1993.
(3) For systematic evaluations of intramolecular PET in aminoalkyl
fluorophores, including variation of quenching efficiency with chain
length, see: (a) Shizuka, H.; Nakamura, M.; Morita, T. J. Phys. Chem.
1979, 83, 2019−2024. (b) Vanderawera, P.; DeSchryver, F. C.; Weller,
A.; Winnik, M. A.; Zachariasse, K. A. J. Phys. Chem. 1984, 88, 2964−
2970. (c) Beeson, J. C.; Huston, M. E.; Pollard, D. A.; Venkatachalam,
T. K.; Czarnik, A. W. J. Fluoresc. 1993, 3, 65−67. For overviews of
PET-based fluorescent chemosensors, see: (d) Bissell, R. A.; de Silva,
A. P.; Gunaratne, H. Q. N.; Lynch, P. L. M.; Maguire, G. E. M.;
McCoy, C. P.; Sandanayake, K. R. A. S. Top. Curr. Chem. 1993, 168,
223−264. (e) da Silva, A. P.; Uchiyama, S. Top. Curr. Chem. 2011, 300,
1−28.
Thiacrown Amine 8a. Prepared according to the literature.30 The
1H NMR data are consistent with those previously reported.31 1H
NMR (δ ppm, 400 MHz, CDCl3): 3.02−2.99 (m, 8H), 2.82−2.79 (m,
4H).
Thiacrown Thiourea Naphthalimide 8. Prepared according to
general procedure F: 13 (50 mg, 0.12 mmol); acetonitrile (3 mL);
thiacrown amine 8a (19 mg, 0.12 mmol); acetonitrile (3 mL). The
reaction mixture was stirred for 30 min at reflux. The solvent was
removed under vacuum and the solid crude product was washed with
acetonitrile and purified by recrystallization from 2-propanol to give 8
as a yellow solid (42 mg, 69%). 1H NMR (δ ppm, 400 MHz, CDCl3):
8.66 (dd, 1H, J = 1.2, J = 8.4), 8.62 (dd, 1H, J = 1.2, J = 7.4), 8.56 (d,
1H, J = 8.3), 7.73 (dd, 1H, J = 7.4, J = 8.4), 7.33 (s, 1H), 7.07 (d, 1H, J
= 8.4), 4.55−4.53 (m, 2H), 4.45−4.43 (m, 2H), 4.02−3.94 (m, 8H),
3.53 (s, 3H), 3.31−3.30 (m, 2H), 2.80 (s, 4H). 13C NMR (δ ppm, 100
MHz, CDCl3): 184.0, 165.4, 164.8, 160.5, 133.9, 132.0, 129.5, 129.4,
126.1, 123.6, 122.0, 114.8, 106.1, 70.6, 68.5, 59.4, 56.5 (2), 46.8, 39.2,
34.0, 30.7 (2), 25.4. IR (KBr) cm−1: 3399m, 2921w, 1691s, 1647s,
1619m, 1594s, 1580s, 1525s, 1470m, 1441m, 1421m, 1384s, 1345s,
1330s, 1265s, 1238s, 1198m, 1166m, 1134s, 1082s, 1054m, 1031m.
HRMS-ESI: calcd for C24H29N3NaO4S3 [M + Na]+ 542.12124, found
542.12083. Rf (deactivated silica gel, CH2Cl2/hexane/Et3N 3.5/1.5/
0.1): 0.64.
(4) Other signaling mechanisms, such as modulation of intra-
molecular charge transfer (ICT), are also important. For an overview,
see: Demchenko, A. P. Introduction to Fluorescence Sensing; Springer
Science: London, 2009; Chapter 6. See also ref 1.
(5) PET-based chemodosimeters based on phosphorus have been
reported in which phosphine oxidation to the phosphine oxide leads to
significant fluorescence enhancement. For examples, see: (a) Akasaka,
K.; Suzuki, T.; Ohrui, H.; Meguro, H. Anal. Lett. 1987, 20, 731−745.
(b) Onoda, M.; Uchiyama, S.; Endo, A.; Tokuyama, H.; Santa, T.;
Imai, K. Org. Lett. 2003, 5, 1459−1461. (c) Lemieux, G. A.; de
Graffenried, C. L.; Bertozzi, C. R. J. Am. Chem. Soc. 2003, 125, 4708−
4709. (d) Fang, A. G. Development of novel fluorescent chemo-
sensors. Ph.D. Thesis, University of California, San Diego, La Jolla,
CA, 2003. We have described the use of sulfoxide fluorophores as
chemodosimeters for peroxide detection and noted that they also
respond to added metal ion: (e) Malashikhin, S. A.; Finney, N. S. J.
Am. Chem. Soc. 2008, 130, 12846−12847.
(6) For reviews of anion coordination by ureas/thioureas, see:
(a) Gunnlaugsson, T.; Ali, H. D. P.; Glynn, M.; Kruger, P. E.; Hussey,
G. M.; Pfeffer, F. M.; Santos, C. M. G.; Tierney, J. J. Fluoresc. 2005, 15,
287−299. (b) Amendola, V.; Bonizzoni, M.; Esteban-Gomez, D.;
Fabbrizzi, L.; Licchelli, M.; Sancenon, F.; Taglietti, A. Coord. Chem.
Rev. 2006, 250, 1451−1470. (c) Gunnlaugson, T.; Glynn, M.; Tocci,
G. M.; Kruger, P. E.; Pfeffer, F. M. Coord. Chem. Rev. 2006, 250,
3094−3117. (d) Li, A.-F.; Wang, J.-H.; Wang, F.; Jiang, Y.-B. Chem.
Soc. Rev. 2010, 39, 3729−3745.
(7) Fluorescent chemodosimeters based on thioureas have been
reported. These most frequently involve desulfurization by Hg2+; for
representative examples, see especially ref 1a,b. There are two reports
of Hg2+-responsive fluorescent chemosensors based on amino-
methylanthracene thioureas in which PET has been stated as the
operant signaling mechanism: (a) Profatilova, I. A.; Bumber, A. A.;
Tolpygin, I. E.; Rybalkin, V. P.; Gribanova, T. N.; Mikhailov, I. E.;
Bren, V. A. Russ. J. Gen. Chem. 2005, 75, 1774−1781. (b) Tolpygin, I.
E.; Shepelenko, E. N.; Revinskii, Y. V.; Tsukanov, A. V.; Dubonosov,
A. D.; Bren, V. A.; Minkin, V. I. Russ. J. Gen. Chem. 2010, 80, 756−770.
Reversibility is not demonstrated, and the possibility of desulfurization
is not addressed in these reports. Further, recent transient absorption
measurements reveal no evidence of a PET process in the
aminomethylanthracene phenylthiourea: (c) Del Giacco, T.; Carlotti,
B.; De Solis, S.; Barbafina, A.; Elisei, F. Phys. Chem. Chem. Phys. 2010,
12, 8062−8070.
ASSOCIATED CONTENT
■
S
* Supporting Information
Brief note on the instability of des-methyl 5; copies of 1H NMR
spectra for all new compounds. This material is available free of
charge via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
*E-mail: finney@oci.uzh.ch.
■
Present Address
†Department of Chemistry, North Carolina State University,
Raleigh, NC. E-mail: nsfinney@ncsu.edu.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We acknowledge financial support from the Institute of Organic
Chemistry (University of Zurich) and the Swiss National
Science Foundation.
REFERENCES
■
(1) For representative reviews of fluorescent chemosensors and
probes, see: (a) Formica, M.; Fusi, V.; Giorgi, L.; Micheloni, M. Coord.
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Bren, V. A.; Minkin, V. I. Chem. Heterocycl. Compd. 2008, 44, 899−
923. (d) Callan, J. F.; de Silva, A. P.; Magri, D. C. Tetrahedron 2005,
61, 8551−8588. (e) Bell, T. W.; Hext, N. M. Chem. Soc. Rev. 2004, 33,
(8) Metal coordination by thioureas with an N-acyl group as an
additional chelating element has been studied in depth. For a review,
see: (a) Koch, K. R. Coord. Chem. Rev. 2001, 216−217, 473−488. The
use of an N-naphthyl ferrocenyl thiosemicarbazone has been reported
H
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as metal-responsive electrochemical and fluorescent chemosensor.
Reversibility is not demonstrated, and the direct conjugation of the
thiourea and the naphthyl fragment precludes involvement of PET:
(b) Liu, W.; Li, X.; Li, Z.; Zhang, M.; Song, M. Inorg. Chem. Commun.
2007, 10, 1485−1488. During the course of this work, an
aminomethylpyrene-derived N-acylthiourea was reported as a
fluorescent chemosensor for Hg2+ in 1:1 H2O/CH3CN: (c) Li, X.
L.; He, Y. W.; Yang, S. I. Bull. Korean Chem. Soc. 2011, 32, 338−340
While reversibility of the Hg2+ response is demonstrated, in light of ref
7c it is unlikely that this chemosensor relies on PET quenching..
(9) By “well-defined”, we mean examples in which metal binding has
been shown to be reversible (a requirement for a molecule being
denoted as a chemosensor) and in which the mechanism of
fluorescence enhancement has been evaluated rather than merely
asserted.
(10) Naphthalimides are versatile and hydrolytically stable
fluorophores. For an overview of their properties and applications,
see: Duke, R. M.; Veale, E. B.; Pfeffer, F. M.; Kruger, P. E.;
Gunnlaugson, T. Chem. Soc. Rev. 2010, 39, 3936−3953 and references
therein.
(11) For a review of bis(2-pyridylmethyl) amines in molecular
recognition, see: (a) Kruppa, M.; Koenig, B. Chem. Rev. 2006, 106,
3520−3560. For metal ion coordination by 7-aza-1,4-dithiacyclono-
nane, see: (b) McAuley, A.; Subramanian, S. Inorg. Chem. 1990, 29,
2830−2837. (c) Blake, A. J.; Danks, J. P.; Fallis, I. A.; Harrison, A.; Li,
W.-S.; Parsons, S.; Ross, S. A.; Whittaker, G.; Schroeder, M. J. Chem.
Soc., Dalton Trans. 1998, 3969−3976. (d) van de Water, L. G. A.; ten
Hoonte, F.; Driessen, W. L.; Reedijk, J.; Sherrington, D. C. Inorg.
Chim. Acta 2000, 303, 77−85.
of fluorescence detection of Zn2+, see: (b) Jiang, P.; Guo, Z. Coord.
Chem. Rev. 2004, 248, 205−229.
(19) This was not a foregone conclusion, given that the barriers to
thiourea C−N bond rotation in 5−8 are likely only ∼11−13 kcal/mol.
See: (a) Sullivan, R. H.; Price, E. Org. Magn. Reson. 1975, 7, 143−150.
(b) Haushalter, K. A.; Lau, J.; Roberts, J. D. J. Am. Chem. Soc. 1996,
118, 8891−8896.
(20) The observed binding profiles for Ag+ and Pb2+ are intriguing,
but their origin is presently unclear. These responses dissipate
completely in aqueous media.
(21) In N-methylthiourea, the s-cis isomer is favored over the s-trans
isomer by a factor of 2.1−2.3 in D2O, and this ratio is reported as
being “higher” in methanol. This is similar to the 1.4- to 4-fold
variance of (I/I0)max seen between 5 and 7. See: Tompa, A. S.;
Barefoot, R. D.; Price, E. J. Phys. Chem. 1969, 73, 435−438.
(22) A further test of this proposal would be comparison of the (I/
I0)max values for metal titrations of 5 and its des-methyl analogue.
Unfortunately, des-methyl 5 is unstable due to elimination/
carbodiimide formation. See the Supporting Information.
(23) At [7]0 = 33 nM (999:1 H2O/CH3OH) summation of five
emission scans at each Hg2+ concentration is required for reliable
measurement. Half-maximum response still occurs at [Hg2+]total = [7]0.
Using Kd(apparent) = [Hg2+]·[7]/[7·Hg2+], at half-maximum [7] =
[7·Hg2+] and Kd(apparent) = [Hg2+] = [Hg2+]total − [7·Hg2+] =
[Hg2+]total − 0.5 × [7]0 = 0.5 × [7]0 = 17 nM. Thus, log Kd is at least
−7.8.
(24) Lakowicz, J. R. Principles of Fluorescence Spectroscopy; Springer:
New York, 2006.
(25) Berlman, B. I. Handbook of Fluorescent Spectra; Academic Press:
New York, 1965.
(12) The driving force for PET, ΔGET, is estimated as follows. The
reduction potential of 4 is taken as the absolute LUMO energy of the
naphthalimide (−1.80 V vs Fc/Fc+). The energy corresponding to the
(0,0) band of the naphthalimide (396 nm; E00 = 3.13 eV) is used to
estimate the absolute energy of the naphthalimide HOMO (+1.33 eV).
The E0 value for oxidation of 1 is taken as the absolute HOMO energy
of the thiourea fragment (+0.90 V). Assuming that the energy levels of
the singly occupied orbitals in the naphthalimide excited state are the
same as the ground-state HOMO and LUMO energies, the difference
between the HOMO values of the thiourea and the naphthalimide
represents the driving force for electron transfer; in the present case,
ΔGET ≈ −0.43 eV = 9.91 kcal/mol. While this is an approximation, it
is well established; see: Handbook of Photochemistry; Montalti, M.,
Credo, A., Prodi, L., Gandolfi, M. T., Eds.; CRC Press: Boca Raton,
FL, 2006.
(26) Eisenfuhr, A.; Arora, P. S.; Sengle, G.; Takaoka, L. R.; Nowick, J.
̈
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(13) We observe increased emission from 1 in CH2Cl2 in response to
added HgCl2, indicating that the thiourea fragment is (as expected)
capable of binding to Hg2+ in nonpolar solvent. However, this intrinsic
affinity is apparently insufficient to compete with solvation by
CH3OH.
(14) The recurring deflection at ∼450 nm is a Wood’s anomaly
characteristic of our fluorimeter.
(15) Binding is illustrated with ZnCl2 to allow inclusion of 1, which
does not bind HgCl2 in CH3OH.13
(16) Binding constants were determined by nonlinear least-squares
fitting of plots of emission intensity versus log[M] using the program
Prism3 (Graphpad, Inc., San Diego, CA). Fitting was consistent with
formation of 1:1 M·L complexes and has been confirmed for titrations
of 5 with ZnCl2 and HgCl2, and we have obtained the X-ray structure
of a 1:1 complex between ZnCl2 and a ligand related to 7. While other
binding events have been characterized in less detail, variation in the
stoichiometry does not affect any conclusions reached here.
(17) We have found that 1 and 7 can be recovered untransformed
following exposure to Zn2+ and Hg2+ for several hours in CH3OH.
This excludes the possibility that fluorescence enhancements are the
result of desulfurization of the thiourea. The addition of chelating
agents (EDTA or dipicolyl amine) reverses the observed fluorescence
enhancements.
(18) For an overview of optical Hg2+ detections, see: (a) Nolan, E.
M.; Lippard, S. J. Chem. Rev. 2008, 108, 3443−3480. For an overview
I
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