Thermochemiluminescence as a fast method to detect and characterize hydroperoxide moieties in novel 3-hydroperoxyisothiazole 1,1-dioxides

Article abstract of DOI:10.1016/j.tet.2012.05.100

Hydroperoxy compounds are important for selective oxidation of organic precursors in the chemical industry. Moreover, novel 3-hydroperoxyisothiazole 1,1-dioxides (N-phthalimidyl sultams) have been shown to be potent and specific inhibitors of acetylcholinesterase (AChE) an important enzyme in the neurotransmitter process. The inhibitory potential of these novel compounds is clearly linked to the hydroperoxide moiety. Hydroperoxides are known for their chemi- and thermochemiluminescent reactivity. In this study we tested thermochemiluminescence (TCL) as a fast method to detect and characterize the hydroperoxide moiety in structurally different synthetic organic hydroperoxides.

Full text of DOI:10.1016/j.tet.2012.05.100

Tetrahedron 68 (2012) 6765e6771
Contents lists available at SciVerse ScienceDirect
Tetrahedron
journal homepage: www.elsevier.com/locate/tet
Thermochemiluminescence as a fast method to detect and characterize
hydroperoxide moieties in novel 3-hydroperoxyisothiazole 1,1-dioxides
,
b
a
a
b
Matthias Gilbert a , Valeria Zakharova , Anna Ramenda , Christian Jebsen , Barbel Schulze ,
*
€
Christian Wilhelm a
a Institute of Biology, Department of Plant Physiology, University of Leipzig, Johannisallee 21-23, D-04103 Leipzig, Germany
b Faculty of Chemistry and Mineralogy, University of Leipzig, Johannisallee 29, D-04103 Leipzig, Germany
a r t i c l e i n f o
a b s t r a c t
Article history:
Hydroperoxy compounds are important for selective oxidation of organic precursors in the chemical
industry. Moreover, novel 3-hydroperoxyisothiazole 1,1-dioxides (N-phthalimidyl sultams) have been
shown to be potent and specific inhibitors of acetylcholinesterase (AChE) an important enzyme in the
neurotransmitter process. The inhibitory potential of these novel compounds is clearly linked to the
hydroperoxide moiety. Hydroperoxides are known for their chemi- and thermochemiluminescent re-
activity. In this study we tested thermochemiluminescence (TCL) as a fast method to detect and char-
acterize the hydroperoxide moiety in structurally different synthetic organic hydroperoxides.
Ó 2012 Elsevier Ltd. All rights reserved.
Received 8 December 2011
Received in revised form 10 May 2012
Accepted 15 May 2012
Available online 8 June 2012
Keywords:
Acetylcholinesterase inhibitor
FTIR
3-Hydroperoxyisothiazole 1,1-dioxides
N-Phthalimidyl sultams
Organic hydroperoxides
Thermochemiluminescence
1. Introduction
a-Hydroperoxy sultams, which are the product of isothiazolium
salt oxidation, have great potential as novel chemoselective elec-
trophilic oxidants.12 The selective oxidation of organic compounds
is crucial in the chemical industry for the production of necessary
oxygen containing compounds.12 Furthermore, isothiazole de-
rivatives have been described as bioactive compounds in both the
medical and agrochemical area.13e15 The novel N-phthalimidyl
sultams (1e4, Fig. 1) have been recognized as efficient inhibitors of
acetylcholinesterase (AChE).16 AChE is responsible for the hydro-
lytic destruction of acetylcholine in neurotransmission. The me-
dicinal control of this specific enzyme is important due to
‘nonclassical’ activities in disease development, e.g., Alzheimer’s
disease. The inhibitory potential of these novel compounds (1e4,
Fig. 1) is clearly associated with the hydroperoxide moiety. Sub-
stitution of the hydroperoxide group to 3-hydroxy or 3-oxo func-
tion strongly diminished inhibitory properties and AChE activity
remained >85%.16
Chemi- and thermochemiluminescent light emission (TCL) from
synthetic organic hydroperoxides is a well-known phenomenon.
Lophine (2,4,5-triphenylimidazole) was the first artificial chemi-
luminescent compound described in 1877 by Radziszewski.1
4-Hydroperoxy-2,4,5-triphenyl-4H-imidazole was determined as
the intermediate responsible for the chemiluminescence (CL) of
lophine by Sonnenberg and White,2 White and Harding3,4 and
Kimura.5 However, the luminescent reaction mechanism of lophine
has only been gradually revealed and is still under investigation.6 A
far better understanding concerning the mechanisms of chem-
iluminescence has been achieved for cyclic peroxides of the
1,2-dioxetane type.7,8 Thermal decomposition of these compounds
mainly results in the formation of triplet-excited carbonyl products
showing phosphorescence.9e11 Instead, the yield of singlet excited
carbonyl states is far lower.9e11 However, under aerated conditions
fluorescence originating from these states is responsible for the
observed direct chemiluminescence. Chemiluminescence origi-
nating from carbonyl triplet states can only be observed in the
absence of oxygen, because of triplet oxygen being a strong triplet
quencher.
In this study we tested if thermochemiluminescence (TCL) is
suitable to detect and characterize the hydroperoxide moiety in
structurally different synthetic organic hydroperoxides.
2. Results and discussion
The presence of
synthetic organic compounds (Fig. 1) was a prerequisite for
a hydroperoxide moiety in the tested
* Corresponding author. Tel.: þ49 341 9738524; fax: þ49 341 9736899; e-mail
address: mgilbert@rz.uni-leipzig.de (M. Gilbert).
0040-4020/$ e see front matter Ó 2012 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.tet.2012.05.100

6766
M. Gilbert et al. / Tetrahedron 68 (2012) 6765e6771
OOH O
OOH O
OOH O
OOH O
H
H
O
H
O
H
O
Ph
Me
Me
Me
Et
N
O
N
N
O
N
N
O
N
N
O
N
S
S
S
S
Me
O
O
O
O
O
1
3
2
4
OOH
OOH
Cl
H
O
OOH
N
H
H
O
Me
Me
Me
Me
Me
COOEt
N
O
COOMe
N
S
S
S
Me
O
O
5
O
Cl
6
7
Fig. 1. Chemical structures of the studied organic hydroperoxides.
Table 1
thermochemiluminescent light emission. The isothiazolium salts
used for synthesis or structurally similar compounds carrying
a hydroxy- or keto- instead of the hydroperoxide group did not
show any thermochemiluminescent activity (data not shown).
The shape and the quantum yield of thermochemiluminescence
(TCL) from the synthetic organic hydroperoxides depended
strongly on their chemical structure, the type of metal surface and
the composition of atmosphere during the heating process (Fig. 2).
Results of TL glow curve measurements from different organic hydroperoxides
characterized by their amplitude and peak temperature Tm (meanꢂSD). The
influence of the sample holder material, copper (Cu) or gold (Au), and the atmo-
sphere (air, N2) in the sample compartment on the TL glow curve parameters is also
shown. *, **, *** indicate significant effects onTL glowcurve parameters when heating
is performed under different atmospheres on the same sample holder material. þ
,
,
þþ
þþþ
indicate significant effects on TL glow curve parameters when heating is
performed on different sample holder materials but under the same atmosphere. *,þ:
p<0.05; **, þþ: p<0.01; ***, þþþ: p<0.001; n.d.¼not detectable; n.m.¼not measured
TCL parameters of 3-hydroperoxyisothiazole 1,1-dioxides
3000
Hydroperoxide
1
Amplitude [a.u.]
Air N2
Tm [ꢀC]
Air
N2
4
1
2500
2000
1500
1000
500
Au 2583ꢂ470 4300ꢂ1212** 178.6ꢂ1.0
178.5ꢂ1.2
177.8ꢂ1.3
160.8ꢂ1.9***
n.d.
n.d.
n.d.
1
2
4
6
7
Cu 154ꢂ33þ
Au 165ꢂ24
Cu n.d.
127ꢂ16þþþ 177.0ꢂ1.1þ
2
Peak 1
Peak 2
183ꢂ32
164.7ꢂ1.9
n.d.
n.d.
Au 156ꢂ32
n.d.
n.d.
175.2ꢂ4.1
n.d.
179.5ꢂ0.4
Cu n.d.
3
4
5
6
7
Au 668ꢂ163
555ꢂ128
175.0ꢂ2.1***
Cu 129ꢂ22þþþ 118ꢂ14þþþ 145.6ꢂ3.0þþþ 140.4ꢂ1.7þþþ
**
6
Au 2243ꢂ219 2448ꢂ166
168.9ꢂ0.8
n.d.
168.4ꢂ0.6
n.d.
Cu n.d.
n.d.
Au 1947ꢂ434 2255ꢂ280
171.0ꢂ0.5
170.6ꢂ0.1
2
2
Cu n.m.
n.m.
n.m.
n.m.
Au 1060ꢂ24
1488ꢂ123** 157.5ꢂ0.5
157.6ꢂ0.7
7
Cu 341ꢂ67þþþ 384ꢂ103þþþ 153.4ꢂ0.5þþþ 152.7ꢂ0.7þþþ
*
0
Au 245ꢂ52
293ꢂ43
284ꢂ70
171.0ꢂ1.3
169.5ꢂ1.7
Cu 323ꢂ116
154.6ꢂ1.5þþþ 154.6ꢂ2.7þþþ
80
100
120
140
160
180
Temperature [°C]
thermochemiluminescence by 30e100% (Table 1). Only hydroper-
oxide 7 was unaffected by the copper surface. Hydroperoxides with
residual light emission (1, 3, 6, 7) showed a down-shift of Tm in air
by 2e34 ꢀC (Table 1). Similar to gold the Tm of some hydroperoxides
(3, 6) was down-shifted on copper by 1e5 ꢀC when N2 atmosphere
was applied instead of air (Table 1). Decomposition of hydroper-
oxides in the presence of transition metal ions or in contact with
Fig 2. TL glow curves of different micro-crystalline organic hydroperoxides in the
temperature range between 80 and 180 ꢀC (measured in air). Each hydroperoxide of
0.5
resulting in
m
mol dissolved in acetonitrile was applied to round gold plates of 0.65 cm2
a
micro-crystalline cover after evaporation of the solvent. Curve of
compound 2 was multiplied by a factor of 3 for better visibility.
In air, hydroperoxides 1, 4 and 5 showed on gold the highest
thermochemiluminescent amplitudes ranging from 2000 to 2500
units with peak temperatures Tm from 171.0 to 178.6 ꢀC (Fig. 2,
Table 1). Hydroperoxide 6 (Au, air) had an intermediate amplitude
of about 1000 units, whereas hydroperoxides 2, 3 and 7 displayed
a rather low light emission between 160 and 670 units and peak
temperatures Tm between 157.5 and 179.5 ꢀC (Table 1). Different
from the other hydroperoxides, compound 2 (Au, air) showed in-
stead of one two distinct peaks (Fig. 2, Table 1).
Only in hydroperoxides 1 and 6 the amplitude was significantly
affected by the composition of atmosphere. When air (Au) was
replaced by N2 the amplitudes increased by 70 and 40%, re-
spectively (Table 1). In hydroperoxides 2 (peak 1) and 3, N2 atmo-
sphere (Au) down-shifted Tm by 4e5 ꢀC.
metal surfaces of the respective elements (Co2þ, Cu2þ, Fe2þ/Fe3þ
)
has already been described in other research studies.17 Further-
more, the copper-induced down-shift of Tm (Table 1) indicates to
a decrease in activation energy and a catalytic mechanism by
transition metals in the decomposition process.18,19
IR spectra (Fig. 3) clearly showed that the hydroperoxide group
is almost completely lost after heating as can be seen in the
quenching of the rather broad band at 3270 cmꢁ1. This band is
typical for a hydrogen bonded OeH stretching mode of the hy-
droperoxide group.20,21 Since no eOeH groups and also no eNeH
groups are present in the chemical structures of the studied
hydroperoxides a contribution of OeH or NeH stretching modes
can be excluded in this frequency band.22 There is also an almost
complete loss of the 871 cmꢁ1 vibration, which is ascribed to one of
the OeO stretching modes of hydroperoxides.21
When samples were prepared on copper instead of gold most
hydroperoxides showed a strong or even complete reduction in

M. Gilbert et al. / Tetrahedron 68 (2012) 6765e6771
6767
0.06
0.05
0.04
0.03
0.02
0.01
0.00
fully resolved at the long wavelengths part compared to that in
Fig. 4A. TCL emission scans from 300 to 450 nm and 600 to 750 nm
were also performed yielding only very noisy light emission well
below 1000 cps of uncertain origin (Supplementary data; Fig. S-1).
At least, from these experiments it can be deduced that the main
light emission of TCL takes place in the range from 460 to 600 nm.
Beside the measurements of TCL emission spectra we also car-
ried out steady state fluorescence emission and excitation scans of
the reaction products of compound 4 by re-dissolving them in
acetonitrile. Reaction products formed after heating were a yel-
lowish amorphous solid on the gold plates and a white fume de-
posit on the protective glass filter (Supplementary data; Fig. S-2)
shielding the photomultipliers in the TL apparatus. The latter
deposit clearly indicated that due to vaporization/sublimation and/
or reactions in the gas phase volatile molecular species were pro-
duced, which at least partially deposited at the cold glass filter
surface above the sample holder. When the white fume deposit was
re-dissolved in acetonitrile a clearly structured double band
between 500 and 600 nm in the fluorescence emission scan could
be resolved showing a peak at 525 nm and a shoulder about
555 nm (Fig. 4C). Excitation spectra between 220 and 450 nm at
Em¼525 and 555 nm showed rather similar shapes and intensities
with highest excitation between 300 and 350 nm (Fig. 4D). For the
thermolytic yellowish decomposition product from the gold plate
we only found a rather unstructured large fluorescence emission
band peaking about 400 nm including a broad shoulder between
480 and 600 nm (Ex¼300 nm, Supplementary data; Fig. S-3). When
diluting the latter deposit down to the linear range of fluorescence
intensity the broad emission shoulder between 480 and 600 nm
could not be any longer resolved. In the non-heated control sample
the broad emission shoulder between 480 and 600 nm was miss-
ing. Here the broad emission band peaking about 400 nm, common
for all samples, declined monotonously with a long tail to higher
wavelengths (Supplementary data; Fig. S-4). In the excitation
control
after heating to 180°C
4000
3500
1500
1000
Wavenumber (cm-1)
Fig. 3. FTIR spectra of micro-crystalline hydroperoxide 5 before and after heating to
180 ꢀC.
The thermochemiluminescent light emission in air induced by
the heat-induced decomposition of hydroperoxides is dominated
by blue-green and green light (Fig. 4A, B) in the spectral range
between 460 and 600 nm (4e6). However, due to scanning of the
emission wavelength in parallel to the heating gradient, the spectra
represent
a convolution of both time-dependent parameters
(c.f. Section 4.3.). Hence the peaks displayed in the spectra do not
exactly represent true emission peak(s). This can be observed when
the spectrum is scanned once from 450 to 600 nm (Fig. 4A) and the
other time from 370 to 520 nm (Fig. 4B), yielding emission peaks at
550 and 485 nm, respectively. However, the intensity of the TCL
emission in Fig. 4B is significantly lower as well as the band is not
4000
3500
3000
2500
2000
1500
1000
500
4000
A
B
3500
3000
2500
2000
1500
1000
500
0
0
460
480
500
520
540
560
580
600
380
400
420
440
460
480
500
520
25000
20000
15000
10000
5000
0
500000
400000
300000
200000
100000
0
D
C
Em = 555 nm
Ex = 340 nm
400
450
500
550
600
650
250
300
350
400
450
Wavelength [nm]
Wavelength [nm]
Fig. 4. A, B: TCL emission spectra of hydroperoxide 4 in air atmosphere. The displayed wavelength range was scanned in parallel with the heating range from 160 to 175 ꢀC. C, D:
fluorescence emission (C) and excitation (D) spectra of the fume deposit from 4 in acetonitrile after heating, excited at 300 nm (C) and detected at 555 nm (D), respectively. For
details, see Section 4.3.

6768
M. Gilbert et al. / Tetrahedron 68 (2012) 6765e6771
spectra of control samples no significant excitation with
300e350 nm was observed at Em¼525 or 555 nm. This clearly in-
dicates that the fluorescence emission spectrum resolved between
500 and 600 nm in the re-dissolved fume deposit represents re-
action products not present in the untreated hydroperoxide 4. Since
the broad shoulder in the emission spectrum of the reaction
products formed on the gold plate covers the same wavelength
range it might be possible that also in this non-volatile deposit
products with the same or similar molecular structure occur as in
the fume deposit, which seem in the latter more selectively con-
centrated. The most striking result is that the TCL emission occurs
in the same spectral range as the new steady state fluorescence
emission of the products in the fume deposit. Hence, the light
emitting molecular species in the TCL might be identical with the
species emitting fluorescence between 500 and 600 nm when ex-
cited in the UV.
The spectral analysis of TCL and UV excited fluorescence emis-
sion indicate to the formation of excited carbonyl or keto singlet
and triplet states representing this type of molecular species.
Compared to our TCL emission spectra, rather similar chem-
iluminescence emission spectra were reported for lophine hydro-
peroxide and its derivates17 having rather broad emission bands
with peaks between 490 and 580 nm.3,4,17 Light emission in the
spectral domain between 450 and 550 nm has been attributed to
excited carbonyl triplet and singlet states formed in the
decomposition process of peroxidic compounds.5,6,23 From syn-
thesis of the respective 3-hydroperoxides studied in the present
paper it is known that they react with DMSO forming the
3-hydroxy compounds and thermolysis in ethanol results in oxi-
dation to the corresponding 3-ones.16 Hence, a thermolytic mech-
anism leading to an excited triplet or singlet keto group in position-
3 in the isothiazole heterocycle is possible.
Beside the formation of excited carbonyl and keto singlet and
triplet states also the formation of excited singlet oxygen can take
place,24 showing three distinct dimol emission bands at 634, 703
and 785 nm.25 Due to emission intensities well below 1000 cps in
that spectral range of our experiments (Supplementary data;
Fig. S-1) the signal is of uncertain origin and cannot be attributed
to the formation of considerable amounts of singlet oxygen. How-
ever, triplet oxygen is known especially as a strong quencher of
excited carbonyl and keto triplet states. But since only two of the
seven hydroperoxides showed a significant increase in TCL in-
tensity in oxygen free atmosphere this principal type of quenching
mechanism seems to be only of secondary significance in our ex-
periments (Table 1). Furthermore, this observation indicates that
the mechanism of thermolytic breakdown obviously yield signifi-
cant amounts of excited singlet states. This in turn agrees with
findings for cyclic ketones in the gas phase, which showed only
has a melting point significantly above the maximal heating
temperature.
The TCL curves (Fig. 2, Table 1) are characterized by two im-
portant parameters, their peak temperature and their intensity
(amplitude, integral). Several factors will influence these parame-
ters shifting the peak to lower or higher temperature and de-
creasing or increasing the TCL intensity. One major factor is the
melting point associated with the molecular weight (MW) of the
respective compound but also with the level of symmetry and
intra- and intermolecular attractive forces within and between the
molecules. The latter often override the primary dependency of the
melting point on the MW. Another major factor is the activation
energy for the thermolytic decomposition of the hydroperoxy
group, which also depends on the above-described molecular
qualities governing the electronic configuration of the molecule. A
higher activation energy will upshift the TCL peak and a lower one
will result in a down-shift. The molecular structure will also de-
termine the mechanism of the chemiluminescent reaction and the
lifetimes of excited product states. Different mechanisms for the
chemiluminescent decomposition of hydroperoxides have been
described in literature, e.g., selfreaction of peroxylradicals (‘Russel
mechanism’), formation of dioextanes as intermediates and de-
hydration of allylic hydroperoxides (for review c.f. Cilento and
Adam).24 Competition between radiative and radiationless de-
activation pathways of excited states in turn influence the intensity
of TCL. However, also the character of the environment can in-
fluence the quantum yield of chemiluminescent reactions. Life-
times of excited states strongly depend on the collisional rate with
neighbouring molecules, which strongly differs in the solid, liquid
and the gas phases. As already described other molecule species
operating as quenchers and present in the reaction environment,
e.g., triplet oxygen, can lower the intensity of TCL. This short
overview shows that the parameters of TCL curves can be
determined in a rather complex way.
However, when viewing the structures of the studied hydro-
peroxides (Fig. 1) they are displaying two homogeneous structural
groups (1e4 and 5e7) with differences in only a few residues.
Hence, a good chance is given that differences in the TCL parame-
ters can be explained on the basis of a few predominant factors.
When looking at the melting points of the hydroperoxides 5e7
these do not follow the order of MWs. Hydroperoxide 7 with an
intermediate MW of 324.18 g molꢁ1 has the highest melting point
of all. Its structure is the most symmetric of the three compounds
favouring a more highly ordered crystal structure. However, the
main reason for the rather high mp of compound 7 is the two heavy
chlorine atoms resulting in strong dipolaric intermolecular forces.
The peak temperature of TCL for this hydroperoxide is well below
the mp indicating that the activation energy of thermolytic de-
composition of the hydroperoxide moiety is reached much earlier.
The decomposition takes either place in the solid or, if sublimation
will occur, in the gas phase. When transfer to the gas phase is
prerequisite to the decomposition process the low intensity of TCL
in compound 7 might be due to the very high mp not allowing
significant sublimation. However, another factor might more likely
explain the rather low TCL yield of 7, the so-called ‘heavy atom
effect’. This effect induced typically in halogenated organic fluo-
rophores lead to a strong quenching of singlet excited states,27
which might be also responsible for the low TCL intensity in
compound 7. Hydroperoxides 5 and 6 are even more similar to
each other differing only in the p-CO2Me and p-CO2Et residues,
respectively. Also for these two structures the melting points
do not follow the MWs of the two compounds. Compound 6
having a MW of 327.36 g molꢁ1 melts significantly at lower tem-
UV-induced fluorescence on UV-irradiation and
a negligible
influence of oxygen on the quantum yield.26 From the results of the
latter study26 also the conclusion might be drawn that at least
a considerable part of the TCL in our experiments has its origin in
the hot gas phase above the solid phase of the decomposing hy-
droperoxide on the gold plate, either due to the liberation (vapor-
ization/sublimation) of the excited state species into the gas phase
or due to the thermolytic reaction taking place in the gas phase
itself. However, since several hydroperoxides have melting points
well above the maximum temperature of heating (180 ꢀC, c.f.) and
sublimation will contribute far less to such a molecular transfer
than vaporization probably all phases in the thermolytic de-
composition process might contribute to the TCL signal. In case of
hydroperoxides 3, 5 and 6 the liquid phase is reached before TCL
reaches its peak temperature (Table 1). Here, vaporization will
probably transfer more material into the gas phase. However, the
white fume deposit on the protective filter glass of hydroperoxide 4
obviously shows that sublimation takes place, since this compound
perature (153e155 ꢀC) than
5
with MW¼313.33 g molꢁ1 and
mp¼166e168 ꢀC. The more symmetric compound 5 seems to form
a more stable crystal structure. This is supported by X-ray structure

M. Gilbert et al. / Tetrahedron 68 (2012) 6765e6771
6769
analysis. Compound 5 forms highly compact centrosymmetric
head-to-tail dimers held together by strong C]O/HeOO hydro-
gen bonds further stabilized by interactions of the phenyl
4. Experimental section
p
/p
4.1. Synthesis and characterization of organic hydroperoxides
rings.28 The significantly lower mp of 6 indicates, when dimers are
formed at all they must be far less stable. The reason for this be-
haviour can be seen in the far less symmetric ethoxyester group
compared to the methoxyester group in 5. Unfortunately, no X-ray
structures of 6 are available. The highly stable dimers of 5 might
also be responsible for the significantly higher peak temperature of
TCL. Due to the strong hydrogen bonds and other intermolecular
forces the hydroperoxy group is probably better protected from
decomposition than in its monomeric form. Also after melting, the
dimeric forces will reduce the vaporization of compound 5 to the
gas phase. However, according to their rather similar monomeric
structures activation energies for the hydroperoxy-decomposition
should be very similar. In this example the peak temperatures of
the TCL seem to be strongly governed by the differences in in-
termolecular forces.
Compounds 1e7 (Fig. 1) were synthesized by the cyclo-
condensation of b-thiocyanatovinyl aldehydes with corresponding
amine derivative followed by the oxidation of the isothiazolium
perchlorate by hydrogen peroxide. 3-Hydroperoxyisothiazole-1,1-
dioxides 1, 4e7 and their detailed synthesis were described else-
where.29,16 Synthetic procedure and characteristics for new com-
pounds 2, 3 are presented below.
Melting points were determined on Boetius micro-melting-
point apparatus. 1H and 13C NMR spectra were recorded at 300 or
400 MHz (1H) and 75 or 100 MHz (13C) using Varian Mercury Plus
300 or 400 NMR spectrometers in DMSO-d6 or acetone-d6 solution
with TMS as internal standard; J in hertz. IR spectra were recorded
on a spectrophotometer Genesis FTIR Unicam Analytical System
(ATI Mattson) with KBr pellets; values in cmꢁ1. UV/vis spectra were
Similar considerations can be drawn for the second homoge-
neous group of hydroperoxides (1e4). Also in this group the
melting points do not follow the order of MWs and the symmetry
factor obviously overrides the dependency on MW (c.f. Section 4.1.3
and Fig. 1). The mps follow the order 1>2>4>3, which also
resembles the level of symmetry with 1 being the most symmetric
and 3 with its outwardly directed large phenyl group the least
symmetric one. However, the differences in peak temperatures and
intensities of TCL seem to be determined in this homogeneous
group of hydroperoxides in a more complex manner. Especially the
extreme differences between 1 and 2 characterized only by
the exchange of a methyl- by an ethyl-residue in position-5 of the
heterocycle cannot be explained easily. Additionally, due to the lack
of X-ray crystal data a discussion of this group is difficult.
measured on Beckmann DU650, lmax in nanometre (log ). Ele-
3
mental analysis was performed on a Heraeus CHNO Rapid Analyser.
Mass spectra were determined on Quadrupol-MS VG 12-250;
70 eV.
4.1.1. Preparation of isothiazoliumperchlorates; general procedure. To
a magnetically stirred solution of corresponding b-thiocyanatovinyl
aldehyde (1 mmol) in glacial acetic acid, (2 ml) N-amino-
phthalimide (1 mmol, 0.162 g) was added under argon atmosphere.
The reaction mixture was stirred for 15 min and perchloric acid
(0.4 ml) was added. After stirring for 50 min, the reaction mixture
was diluted with 20 ml of diethyl ether. The precipitate of iso-
thiazolium perchlorate was filtered off, washed several times with
diethyl ether and dried on air.
4.1.1.1. 5-Ethyl-4-methyl-2-(phthalimid-1-yl)isothiazolium per-
chlorate. Yield: 90%; white solid; mp 207e211 ꢀC; IR (KBr):
n
¼1756
)¼221 nm (4.46),
282 nm (3.98); 1H NMR (300 MHz, DMSO-d6):
¼1.32 (t, 3H,
(CO), 1089 (ClO4) cmꢁ1; UV (CH3CN): lmax (log
3
3. Conclusions
d
J¼7.5 Hz, CH3), 2.39 (s, 3H, CH3), 3.26 (q, 2H, J¼7.5 Hz, CH2), 8.04 (d,
2H, J¼7.8 Hz, 2 arom. CH), 8.14 (d, 2H, J¼7.8 Hz, 2 arom. CH), 9.34
Most of the synthetic organic hydroperoxides show rather ho-
mogeneously shaped TCL curves peaking in the temperature range
between 140 and 180 ꢀC. (Fig. 2, Table 1). From the presented re-
sults no general reaction mechanisms can be deduced. However,
the data clearly show that the hydroperoxide moiety is a pre-
requisite for thermochemiluminescent light emission. The quan-
tum yield and the peak temperature of this TCL strongly depend on
the chemical structure, the type of metal surface and the atmo-
spheric environment in which the reaction takes place. Also the
intermolecular forces play a major role for differences in crystal
structure and/or the dimerization probability in the liquid phase
governing in turn the TCL curve parameters via the character of
phase transition. The TCL quantum yield for most hydroperoxides is
not influenced by the presence or absence of oxygen and the TCL
emission spectrum is dominated by blue-green and green light.
Hence, the thermal decomposition most likely results in a singlet
excited carbonyl state representing an intermediate or product of
reaction.
(s, 1H, CH); 13C NMR (75 MHz, DMSO-d6):
(2CO), 160.6 (C-3), 136.1 (2 arom. CH), 130.8 (C-4), 129.3 (2 arom.
C), 124.8 (2 arom. CH), 21.6 (CH2), 14.4 (CH3), 10.6 (CH3).
d
¼179.3 (C-5), 162.1
4.1.1.2. 5-Methyl-4-phenyl-2-(phthalimid-1-yl)isothiazolium per-
chlorate. Yield: 81%; white solid; mp 225e226 ꢀC; IR (KBr):
n
¼1757
)¼220 nm (4.67),
¼2.93 (s, 3H, CH3),
(CO), 1090 (ClO4) cmꢁ1; UV (EtOH): lmax (log
299 nm (3.88); 1H NMR (400 MHz, DMSO-d6):
3
d
7.31e7.88 (m, 5H, 5 arom. CH), 8.06 (d, 2H, J¼7.8 Hz, 2 arom. CH),
8.19 (d, 2H, J¼7.8 Hz, 2 arom. CH), 9.72 (s, 1H, CH); 13C NMR
(100 MHz, DMSO-d6):
d
¼173.4 (C-5), 162.0 (2CO), 159.5 (C-3), 136.1
(2 arom. CH), 134.8e128.3 (5 arom. CH, 3 arom. C, C-4), 124.9
(2 arom. CH), 15.4 (CH3).
4.1.2. Synthesis of 2,3-dihydro-3-hydroperoxy-2-(phthalimid-1-yl)
isothiazole 1,1-dioxides 2,3; general procedure. H2O2 (2.8 ml, 30%) was
added at room temperature to a stirred suspension of 5-ethyl-4-
methyl-2-(phthalimid-1-yl)isothiazolium perchlorate or 5-methyl-
4-phenyl-2-(phthalimid-1-yl)isothiazolium perchlorate (0.7 mmol)
in AcOH (4.2 ml). After 24e48 h the solution was diluted with cold
water, colourless crystals of compounds 2,3 were filtered off and
recrystallized from ethanol/water.
Concerning the applied area of organic chemistry TCL might be
supplementary as a simple and fast method to detect and charac-
terize novel hydroperoxy compounds. For example, the shelf life-
time of novel hydroperoxides could be tested in future. Moreover,
TCL has the advantage compared to the popular characterization by
FTIR that it can detect the presence of hydroperoxide groups spe-
cifically due to their TCL. However, detection by FTIR will be re-
stricted if eOH or eNH groups occur beside the hydroperoxide
moiety contributing to the broad wavenumber range between
4.1.2.1. 5-Ethyl-2,3-dihydro-3-hydroperoxy-4-methyl-2-(phthali-
mid-1-yl)isothiazole 1,1-dioxide (2). Yield: 59%; white solid; mp
194e197 ꢀC; IR (KBr):
UV (CH3CN): lmax (log
n
¼1731 (CO), 1314 (SO2), 1173 (SO2) cmꢁ1
;
3000 and 4000 cmꢁ1
.
3
)¼216 nm (4.47), 220 nm (4.46), 295 nm

6770
M. Gilbert et al. / Tetrahedron 68 (2012) 6765e6771
(3.37); 1H NMR (400 MHz, acetone-d6):
d
¼1.26 (t, 3H, J¼7.5 Hz,
evaporation of the solvent. The hydroperoxide covering plates
were transferred to the sample holder and incubated at 20 ꢀC for
5 min in air or N2 atmosphere and then heated in the respective
atmosphere from 20 to 180 ꢀC at a rate of 20 ꢀC minꢁ1. Light
emission was detected by means of channel photomultipliers
(CPM). The different TCL curves were characterized by their
peak temperatures and maximal amplitudes (relative quantum
yield).
CH3), 2.10 (s, 3H, CH3), 2.62 (q, 2H, J¼7.5 Hz, CH2), 5.98 (d, 1H,
3-H), 7.98 (m, 4H, 4 arom. H), 11.67 (s, 1H, OOH); 13C NMR
(100 MHz, acetone-d6):
d
¼165.9 (CO), 165.4 (CO), 139.4 (C-5),
138.4 (C-4), 136.3 (2 arom. CH), 130.6 (arom. C), 130.5 (arom. C),
124.8 (arom. CH), 124.6 (arom. CH), 94.5 (C-3), 18.0, 12.9, 12.4;
EIMS: m/z¼320 ([MꢁH2O]þꢃ); C14H14N2O6S (338.34); calcd (%): C
49.70, H 4.17, N 8.28, S 9.48; found (%): C 50.05, H 4.09, N 8.52, S
9.71.
4.1.2.2. 2,3-Dihydro-3-hydroperoxy-5-methyl-4-phenyl-2-
(phthalimid-1-yl)isothiazole 1,1-dioxide (3). Yield: 58%; white
4.3. Emission spectra of thermochemiluminescence and
fluorescence emission and excitation spectra
solid; mp 170e172 ꢀC; IR (KBr):
n
¼1739 (CO), 1329 (SO2) 1190
(SO2) cmꢁ1; UV (CH3CN): lmax (log
)¼218 nm (4.64), 221 nm
(4.63), 292 nm (3.55); 1H NMR (400 MHz, acetone-d6):
3
Emission spectra were recorded with a Spectrofluorometer
Fluoromax-4 (Horiba Jobin Yvon, Edison, NJ, U.S.A). Sample heat-
ing was performed with a watercooled Peltier unit detached from
a TL measuring device described by Ducruet.31 This unit was
mounted on the original solid sample holder of the spectrofluo-
rometer positioned with the sample plate centrally at the focus
position of the fluorometer’s excitation/emission beam. The
sample plate was in vertical position and its surface plane facing
at an angle of 90ꢀ to the emission beam and the entrance gate of
the detector unit. The sample plates were kept in place on the
Peltier element by means of a Teflon diaphragm. The wavelength
range waꢁs1 scanned in parallel to the heating gradient
(20 ꢀC min ), the latter covering the range from 160 to 175 ꢀC and
therefore also the peak of the TCL. The wavelength range covered
150 nm for different spectral areas, e.g., 370e520 nm or
450e600 nm. The instrument settings were: slit 20 nm, increment
2 nm and integration time 0.6 s. It has to be emphasized that this
procedure is a compromise due to the lack of enough material,
which would have allowed otherwise to perform several mea-
surements at constant different emission wavelengths. The ap-
d
¼1.27
(s, 3H, CH3), 6.50 (s, 1H, 3-H), 7.53e7.63 (m, 5H, 5 arom. H), 7.98
(m, 4H, 4 arom. H), 11.70 (s, 1H, OOH); 13C NMR (100 MHz, ac-
etone-d6):
d
¼165.7 (CO), 165.6 (CO), 139.2 (C-5), 136.4 (2 arom.
CH), 135.9 (C-4), 130.9 (arom. C), 130.8 (2 arom. CH), 130.6
(arom. C), 130.5 (arom. C), 129.7 (arom. CH), 129.5 (2 arom. CH),
124.9 (arom. CH), 124.9 (arom. CH), 94.8 (C-3), 9.32 (CH3); EIMS:
m/z¼368 ([MꢁH2O]þꢃ). C18H14N2O6S (386.39); calcd (%):
C
55.95, H 3.65, N 7.25, S 8.30; found (%): C 56.38, H 3.71, N 7.42, S
8.04.
4.1.3. Melting points of hydroperoxides and sample prepara-
tion. Since several hydroperoxides have melting points lower than
the peak temperature of their respective TCL glow curve the mps of
all hydroperoxides are listed below together with their molecular
weights (MW) in overview:
(1) mp¼215e217 ꢀC (MW 324.32);
(2) mp¼194e197 ꢀC (MW 338.34);
(3) mp¼170e172 ꢀC (MW 386.38);
(4) mp¼184e187 ꢀC (MW 350.35);
(5) mp¼166e168 ꢀC (MW 313.33);
(6) mp¼153e155 ꢀC (MW 327.36);
(7) mp¼207e209 ꢀC (MW 324.18).
plied procedure results in
a convolution of the TCL, being
a function of temperature/time, and the emission scan being
a function of time. An exact determination of the emission spec-
trum localizing true peaks is therefore not possible. The applied
procedure can only deliver an estimate of the spectral range
where the main light emission occurs (see also Results and
discussion).
Due to the large distance between the detector and the sample
plate as well as to the omnidirectional chemiluminescent light
emission the absolute amount of hydroperoxide had to be in-
Whereas hydroperoxides (1), (2), (4) and (7) will stay in the solid
state, hydroperoxides (3), (5) and (6) will undergo a solid/liquid
phase transition during the heating process to 180 ꢀC.
Sample preparation:
Fig. 1, Table 1: 0.5 mmol of each hydroperoxide dissolved in
creased to 6
tion 4.1.3.).
mmol to achieve a good signal-to-noise ratio (c.f. Sec-
acetonitrile (Prolabo HPLC grade VWR 20060.320) was applied to
round gold or copper plates of 0.65 cm2 resulting in a micro-
crystalline cover after evaporation of the solvent.
Steady state fluorescence emission and excitation spectra were
also performed with the Spectrofluorometer Fluoromax-4 (Horiba
Jobin Yvon, Edison, NJ, U.S.A). To avoid Raman peaks of the solvent
acetonitrile, excitation was restricted to the range of 220e340 nm
and emission scanned from 390 to 750 nm. To avoid distortions by
higher order light a cut-on order filter opening at 350 nm having
a constant transmission of 91% from 390 to 750 nm was placed in
front of the emission site. Settings were: slit width 5 nm, increment
1 nm, integration time 0.1 s.
Fig. 4A, B: for recording of TCL emission spectra amounts of
mol were applied (c.f. Section 4.3.).
Fig. 4C, D: for measurements of fluorescence emission and
6
m
excitation spectra four heated gold plates each covering 0.5 mmol
were rinsed with acetonitrile (Prolabo, VWR 20060.320, HPLC
grade) and diluted to a final volume of 2.6 ml. For controls
non-heated gold plates were rinsed and diluted in the same man-
ner. For analysis of the fume deposit covering the protective glass
filter, seven gold plates, each covering 0.5 mmol were heated to
180 ꢀC and then rinsed from the glass filter with acetonitrile to
a final volume of 2,6 ml.
4.4. Fourier transform infrared (FTIR) spectroscopy
4.2. Thermochemiluminescence measurements
Samples of 1 mL from re-dissolved hydroperoxide (acetonitrile)
prepared before and after heating (solid state, gold plates) were
placed on a microtiter plate and dried at room temperature for
10 min. Infrared (IR) spectra (Vector 22, Bruker Optics, Karlsruhe,
Germany) were recorded in the range of 4000e700 cmꢁ1 in
transmission mode with 32 scans oversampling to enhance the
signal-to-noise ratio (Vector 22 laser unit, HTS-XT microtiter
Thermochemiluminescence (TCL) was measured in a set-up
already described according to Gilbert et al.30 The artificial hy-
droperoxides were dissolved in acetonitrile (spectroscopic
grade); 30
m
L of the solution was applied to gold or copper
micro-crystalline layer of 0.5 mol after
plates forming
a
m

M. Gilbert et al. / Tetrahedron 68 (2012) 6765e6771
6771
11. Wilson, T.; Golan, D. E.; Harris, M. S.; Baumstark, A. L. J. Am. Chem. Soc. 1976, 98,
1086e1091.
12. Makota, O.; Wolf, J.; Trach, Y.; Schulze, B. Appl. Catal., A: Gen. 2007, 323,
174e180.
13. Davis, M. Organic Compounds of Sulfur, Selenium and Tellurium; Chemical Soci-
ety: London, 1984; p 345.
module, OPUS and OPUSLab v5.0 software, Bruker Optics, Karls-
ruhe, Germany).
Acknowledgements
14. Pain, D. L.; Peart, B. J.; Wooldridge, K. R. H. In Comprehensive Heterocyclic
Chemistry; Katritzky, A. R., Rees, C. W., Eds.; Pergamon: Oxford, 1984; 6, p 131.
15. Asish, D. In Progress in Medicinal Chemistry; Ellis, G. P., West, G. B., Eds.; Elsevier:
Amsterdam, 1981; 18, pp 117e133.
16. Zakharova, V.; Gonzalez Tanarro, C.; Gutschow, M.; Hennig, L.; Sieler, J.;
Schulze, B. Synthesis 2008, 2008, 1133e1141.
The authors thank Claudia Matzke for technical assistance in the
laboratory.
Supplementary data
ꢀ
€
17. Kimura, M.; Tsunenaga, M.; Takami, S.; Ohbayashi, Y. Bull. Chem. Soc. Jpn. 2005,
78, 929e931.
18. Weiss, J. Trans. Faraday Soc. 1935, 31, 1547e1557.
Supplementary data associated with this article can be found in
the online version, at http://dx.doi.org/10.1016/j.tet.2012.05.100.
19. O’Brien, P. J. Biochem. Cell Biol. 1969, 47, 485e492.
20. Walling, C.; Heaton, L. J. Am. Chem. Soc. 1965, 87, 48e51.
21. Shurvell, H. F.; Southby, M. C. Vib. Spectrosc. 1997, 15, 137e146.
22. Smith, B. C. Infrared spectral Interpretation: A Systematic Approach; CRC: Boca
Raton, 1999.
23. Miyazawa, T.; Usuki, R.; Kaneda, T. Agric. Biol. Chem. 1982, 46, 1671e1672.
24. Cilento, G.; Adam, W. Free Radical Biol. Med. 1995, 19, 103e114.
25. Krasnovsky, A. A., Jr. J. Photochem. Photobiol., A: Chem. 2008, 196, 210e218.
26. Berces, T. In Comprehensive Chemical Kinetics; Bamford, C. H., Tipper, C. F. H.,
Eds.; Elsevier: Amsterdam, 1972; 5, pp 234e380.
References and notes
1. Radziszewski, B. R. Ber. Dtsch. Chem. Ges. 1877, 10, 70e75.
2. Sonnenberg, J.; White, D. M. J. Am. Chem. Soc. 1964, 86, 5685e5686.
3. White, E. H.; Harding, M. J. C. J. Am. Chem. Soc. 1964, 86, 5686e5687.
4. White, E. H.; Harding, M. J. C. Photochem. Photobiol. 1965, 4, 1129e1155.
5. Kimura, M.; Nishikawa, H.; Kura, H.; Lim, H.; White, E. H. Chem. Lett. 1993, 22,
505e508.
6. Kimura, M.; Lu, G.; Iga, H.; Tsunenaga, M.; Zhang, Z.; Hu, Z. Tetrahedron Lett.
2007, 48, 3109e3113.
7. Baader, W. J.; Stevani, C. V.; Bastos, E. L. PATAI’S Chemistry of Functional Groups;
John Wiley & Sons: New York, 2008.
27. Solovyov, K. N.; Borisevich, E. A. Phys. Usp. 2005, 48, 231e253.
28. Schulze, B.; Taubert, K.; Gelalcha, F. G.; Hartung, C.; Sieler, J. Z. Naturforsch.
2002, 57b, 383e392.
29. Taubert, K.; Sieler, J.; Hennig, L.; Findeisen, M.; Schulze, B. Helv. Chim. Acta 2002,
8. Kopecky, K. R.; Lockwood, P. A.; Gomez, R. R.; Ding, J.-Y. Can. J. Chem. 1981, 59,
851e858.
9. Abdel-Shafi, A. A.; Worrall, D. R.; Wilkinson, F. J. Photochem. Photobiol., A: Chem.
85, 183e195.
30. Gilbert, M.; Wagner, H.; Weingart, I.; Skotnica, J.; Nieber, K.; Tauer, G.; Bergmann,
F.; Fischer, H.; Wilhelm, C. J. Plant Physiol. 2004, 161, 641e651.
31. Ducruet, J. M. J. Exp. Bot. 2003, 54, 2419e2430.
2001, 142, 133e143.
10. McCapra, F. Methods in Enzymology; Academic: San Diego, 2000; 305, pp 3e47.