Autoxidation of acridanyl and benzhydryl carbanions adjacent to sulfur groups leading to C-S bond cleavage with weak chemiluminescence

Article abstract of DOI:10.1002/hc.10025

Autoxidation of acridanyl and benzhydryl carbanions stabilized by sulfur groups gave the corresponding ketones via radical intermediates, being accompanied by a weak light emission. While oxidation of 9-sulfenylacridanes (1a and 1b) in the presence of tert-BuOK showed direct chemiluminescence (CL) due to excited 10-methylacridone, that of benzhydryl phenyl sulfide (2) and benzhydryldimethyl-sulfomium bromide (3) displayed CL in the presence of a fluorescer, 9, 10-dibromoanthracene (DBA), due to excited benzophenone. The yields of excited species in the present oxidation are low but comparable to acyclic peroxide systems. The bimolecular energy transfer from triplet benzophenone to DBA was established in CL of the sulfonium ylide from 3.

Full text of DOI:10.1002/hc.10025

Heteroatom Chemistry
Volume 13, Number 3, 2002
A
utoxidation of Acridanyl and Benzhydryl
Carbanions Adjacent to Sulfur Groups
Leading to C---S Bond Cleavage with
Weak Chemiluminescence
Jiro Motoyoshiya, Hisashi Inoue, Yutaka Takaguchi,
and Hiromu Aoyama
Department of Chemistry, Faculty of Textile Science and Technology, Shinshu University,
Ueda 386-8567, Japan
Received 31 August 2000; revised 27 October 2000
of carbanions. The common species formed initially
ABSTRACT: Autoxidation of acridanyl and benz-
are the peroxy radicals and anions, which are
hydryl carbanions stabilized by sulfur groups gave
transformed into the corresponding hydroperoxides,
the corresponding ketones via radical intermediates,
carbinols, or carbonyl compounds, according to the
being accompanied by a weak light emission. While
neighboring groups. For instance, while the reaction
oxidation of 9-sulfenylacridanes (1a and 1b) in the
of molecular oxygen with triarylmethanides gave the
presence of tert-BuOK showed direct chemilumines-
hydroperoxides and/or carbinol [1], that of diphenyl-
cence (CL) due to excited 10-methylacridone, that of
methanide produced benzophenone and benzyl al-
benzhydryl phenyl sulfide (2) and benzhydryldimethyl-
cohol [2,3] probably via the Russell mechanism in-
sulfomium bromide (3) displayed CL in the pres-
volving tetraoxides formed by combination of peroxy
ence of a fluorescer, 9,10-dibromoanthracene (DBA),
radicals [4,5]. Autoxidation of some stabilized car-
due to excited benzophenone. The yields of excited
banions is accompanied by a peculiar phenomenon,
species in the present oxidation are low but com-
chemiluminescence (CL), which is ascribed to the
parable to acyclic peroxide systems. The bimolec-
excited carbonyl compounds formed from decompo-
ular energy transfer from triplet benzophenone to
sition of peroxides [6]. Namely, CL in the above men-
DBA was established in CL of the sulfonium ylide
tioned autoxidation of diphenylmethane in the pres-
ence of strong base has been observed [2], in which
from 3. �C 2002 Wiley Periodicals, Inc. Heteroatom Chem
1
3:252–257, 2002; Published online in Wiley Interscience
excited benzophenone was produced by peroxy rad-
ical disproportion. It has been sometimes suggested
that tetraoxides could be the precursors for emitting
species [2,7].
On the other hand, little attention has been paid
to autoxidation of the sulfur-stabilized carbanions,
in spite of some detailed reports on oxidation of sul-
fides from the mechanistic and biochemical view-
points [8,9]. In contrast, autoxidation of the car-
banions stabilized by phosphorus groups has been
relatively well studied [10–12] because of its syn-
thetic value. Previously, we found that autoxidation
of some phosphonate carbanions showed weak CL
(
www.interscience.wiley.com). DOI 10.1002/hc.10025
INTRODUCTION
Autoxidation of the stabilized carbanions gives var-
ious oxidation products, depending on the source
Correspondence to: Jiro Motoyoshiya; e-mail: jmotoyo@giptc.
shinshu-u.ac.jp.
Contract grant sponsor: Ministry of Education, Science, Sports,
and Culture of Japan.
Contract grant number: 10CE2003.
�
c 2002 Wiley Periodicals, Inc.
252

Autoxidation of Acridanyl and Benzhydryl Carbanions Adjacent to Sulfur Groups 253
[13,14] with elimination of phosphorus groups. Our
continuous interest in autoxidation of the carban-
ions stabilized by heteroatom groups prompted us
to investigate oxidation of the sulfur-stabilized car-
banions and their CL. We report here chemilumi-
nescent autoxidation of the carbanions adjacent to
sulfur groups that generates ketones via C S bond
cleavage.
RESULTS AND DISCUSSION
Autoxidation
When 9-phenylthio-10-methylacridane (1a) was re-
acted with atmospheric oxygen in the presence of
tert-BuOK in tetrahydrofuran (THF), N-methylacri-
done (NMA) and diphenyl disulfide (PhSSPh) were
obtained in 49 and 44% yield, respectively. Interest-
ingly, this reaction at very high concentration was
accompanied by a weak light emission observable
by naked eyes in the dark. Although the detection
of emitting light by a fluorescence spectrometer is
unsuccessful because of the difficulty in measuring
at such high concentration, this CL seems to be as-
cribed to the fluorescence of NMA, since the absorp-
tion in the UV–vis spectrum of the reaction mixture
agreed with that of authentic NMA and no other ab-
sorption in the visible region was observed. As has
been established in autoxidation of some stabilized
carbanions [1–3], this reaction is also expected to
involve a radical intermediate. Additionally, an adja-
cent sulfenyl group can stabilize a radical as a donor
FIGURE 1 ESR spectrum (a) observed in DMSO and (b)
computer-simulated spectrum.
ple spectrum with ten-lines led us to guess equiv-
alence of some hyper fines [16]. Because of well
agreement with a computer simulation, the observed
spectrum was identified to that of 9-phenylsulfenyl-
acridanyl ketyl radical.
The previously documented mechanisms con-
cerning autoxidation help us to consider the paths
of the present reaction (Scheme 1). If the Russell
mechanism for self-reaction of peroxy radicals [5,17]
is applied to the present reaction, the formation of
[15]. In fact, the colored reaction mixture of 1a and
tert-BuOK in dimethyl sulfoxide (DMSO) showed an
ESR spectrum as shown in Fig. 1. This relatively sim-
SCHEME 1

254 Motoyoshiya et al.
the products, NMA and PhSSPh, can be explained as
follows: The initially formed carbanion is oxidized to
the ketyl radical A by electron transfer to molecular
oxygen, which reacts with oxygen to form a peroxy
radical. Dimerization to the tetraoxide B followed by
decomposition would give an excited state NMA and
PhSSPh with release of oxygen. On the other hand,
Ando [18,19] and Clennan [9] reported recently that
oxidation of some sulfides with singlet oxygen gave
the corresponding carbonyl compounds along with
disulfides and others. Therefore, decomposition of
the intermediary ␣-perhydroxy sulfide C into NMA
and the thiol may also be involved in the present
reaction. This mechanism seems not to be chemi-
luminescent but can be involved in the present sys-
tem, because the thiolate anion could be trapped by
methyl iodide to give thioanisole in 22% yield in the
reaction of 1a. We believe at present that both reac-
tion pathways, the former being luminescent and the
latter not, are competing in this oxidation, but the
nonluminescent path seems to be preferable because
of very low quantum efficiency of light emission, as
described later.
tert-butylnitrone, giving a typical feature of signals
scattering three doublets. To our knowledge, there
has been no document for the nature of the radi-
cal adjacent to a sulfonium group so far. It should
be noted that the sulfur ylide derived from 3 is
known to undergo the Stevens rearrangement to
give (2-methylthiomethylphenyl)phenylmethane in
the absence of oxygen [20,21]. However, since the
rearranged product could not be detected, the in-
teraction of oxygen with the carbanion formed by
the abstraction of a proton by the base seemed to be
faster than the rearrangement. Such a high reactiv-
ity of the carbanion toward oxygen was reported by
Russell in the reaction of triphenyl methanide [1]. In
these reactions weak CL was observed in the pres-
ence of 9,10-dibromoanthracene (DBA). As shown
in Scheme 2, the observed CL is expected to occur
from the dioxirane formed from the peroxy radical
[22–24]. However, all attempts to detect the products
expected from dioxirane were unsuccessful.
Chemiluminescence
The similar reaction of 9-methylthio-10-methyl-
acridane (1b) also gave NMA and showed weak
CL. Oxidation of 10-methylacridanylsulfonium ylide
is of interest also because of the higher ability of
the sulfonium cation as the leaving group. How-
ever, the preparation of the sulfonium salt from
In autoxidation of the carbanions of 1a and 1b, ex-
cited NMA that emits blue light was directly formed
(direct CL). The singlet excited yield is defined as
�
�
8
8
CL = 8
r
8
S
F
�
�
where 8CL , 8
r
, 8
S
, and 8
F
are the total CL yield,
1
b and methyl iodide was unsuccessful because
chemical yield of NMA, the singlet excited yield, and
the fluorescence efficiency of NMA, respectively [25].
The values of 8CL were estimated by the photo-
of the lability of the salt that readily afforded 10-
methylacridinium iodide by elimination of dimethyl
sulfide.
�
counting method using luminol CL as a standard
Next, we examined autoxidation of benzhydryl
phenyl sulfide (2) and benzhydryldimethylsulfonium
bromide (3) in the presence of tert-BuOK. Oxidation
of both 2 and 3 gave benzophenone as a major prod-
uct in 87 and 46% yield, respectively (Scheme 2).
While the detection of the ketyl radical derived
from 2 by ESR measurement was unsuccessful, the
radical from 3 could be detectable on the nitrox-
ide radical, formed by the trapping with phenyl-N-
[26], and 8
r
was determined by the UV spectrum
�
of the reaction mixtures. The total CL yields (8CL
)
�
8
� 8
were 8.8 � 10 einstein/mol for 1a and 2.9 � 10
einstein/mol for 1b, respectively. According to the
�
above equation, each 8
S
is calculated to be 1.8 �
�
7
� 8
10 einstein/mol for 1a and 4.9 � 10 einstein/mol
for 1b, respectively, using 8
for 1b, and 8
r
= 0.49 for 1a, 8
r
= 0.6
�
F
= 0.98. The values for 8
S
are much
less than those for 1,2-dioxetane decomposition
SCHEME 2

Autoxidation of Acridanyl and Benzhydryl Carbanions Adjacent to Sulfur Groups 255
of the acridane systems 4 [27] but almost equal
to the reported values for acridanylperoxyester 5
formed from the oxidation of 9-arylmethylene-10-
methylacridane with m-chloroperbenzoic acid [28].
On the other hand, CL from the reaction of 2
or 3 is the result of energy transfer from excited
benzophenone to DBA. Generally, the CL quantum
FIGURE 2 Reciprocal plot of CL quantum yield (8CL) against
concentration of DBA for 3.
Flu.
yield (8CL ) in the presence of the fluorescers is es-
timated from a linear relationship between a dou-
ble reciprocal plot of emission intensity versus fluo-
rescer concentration [25]. Such a linear relationship
means that light emission is ascribed to a bimolec-
ular interaction between the excited species and the
fluorescer. Therefore, emission intensity in the oxida-
tion of 2 and 3 was measured in the presence of DBA
as a triplet energy acceptor [25]. While a linear rela-
tionship for the CL of 3 and DBA could be obtained,
plotting for 2 displayed a random fashion in our sev-
eral attempts. This would be due to the interaction
between the peroxide anion and DBA [29] as well as
the unsettled concentration of the semistabilized car-
banion of 2, depending on the circumstances. Never-
theless, DBA showed higher emission intensity than
DPA (a singlet energy acceptor) in both CL of 2 and
� 5
1
0
einstein/mol. This value is very small compared
to CL from thermal dioxetane decomposition, but
comparable to CL from the similar base promoted
autoxidation of diphenylmethane generating triplet
benzophenone [3].
In conclusion, we have investigated autoxidation
of the sulfur-stabilized carbanions, which resulted
in carbon–sulfur bond cleavage to give the corre-
sponding ketones accompanied by weak chemilumi-
nescence. The reactions involved the radical inter-
mediates formed by the electron transfer from the
carbanions to oxygen. The resulting radical reacted
with oxygen to form the peroxy radical, which fur-
ther underwent elimination of the sulfur group to
afford the excited ketone, though the yields of which
were low.
3
at each concentration of the fluorescers, in agree-
ment with the favored formation of benzophenone
in a triplet excited state.
EXPERIMENTAL
The linear relationship in the reaction of 3 in the
Preparation of 9-Phenylthio-10-methylacridane
presence of DBA is shown in Fig. 2, which permits
(
1a)
Flu.(∞)
us to estimate 8CL
as well as the yield of triplet
DBA(∞)
benzophenone in this reaction. The value of 8CL
To a solution of sodium ethoxide prepared from
sodium (0.7 g, 30.4 mmol) in ethanol (100 ml) was
added thiophenol (0.32 ml, 3.09 mmol). After 10 min,
10-methylacridinium iodide (1.0 g, 3.11 mmol) was
added and a red color of the acridinium salt dis-
appeared immediately. After the mixture had been
stirred for 1 h and water added, the product was ex-
tracted with chloroform and the extract was dried
at the infinite concentration of DBA could be calcu-
�
7
lated from the intercept as 8.5 � 10 einstein/mol.
The total CL quantum yield is represented as
DBA(∞)
�
r T TS F
8 8 8
DBA
8
CL
= 8
�
DBA
where 8
r
,8
T
,8TS, and 8
F
are the chemical yields
of formed benzophenone, the yield of triplet ben-
zophenone, the limiting triplet–singlet energy trans-
fer efficiency, and the fluorescence quantum yield of
DBA (0.1), respectively [25]. On the assumption that
over Na
2
SO . After evaporation of the solvent, the
4
residue was chromatographed using silica gel and
chloroform as an eluant to give 1a, which was recrys-
talized from hexane–chloroform to give white nee-
�
1
8
TS is relatively insensitive to triplet ketone struc-
ture and generally constant (ca. 0.3) [30] and use
of 8 * as 6.2 �
= 0.46 allowed us to calculate 8
dles (0.29 g, 31%). mp 139–141 C. H NMR (60 MHz,
CDCl
3
) � 3.10 (s, 3H, SMe), 5.45 (s, 1H, 9-H), 6.70–
+
7.28 (m, 8H, aromatic H), m/z 303 (M ). Anal. Calcd.
r
T

256 Motoyoshiya et al.
for C20
H
17NS: C, 79.17; H, 5.65; N, 4.62%. Found:
DPA and DBA, the emission efficiency was calibrated
based on the characteristic curve of the photomulti-
plier. All measurements were done under the same
conditions.
7
9.30; H, 5.43; N, 4.56%.
Preparation of 9-Methylthio-10-methylacridane
1b)
A solution (1 ml) of tert-BuOK (2.0 � 10� mol/l
2
(
in DMSO) was added to each solution (2 ml) of
�
3
To a solution of 10-methylacridinium iodide (5 g,
5.6 mmol) in DMSO (60 ml) was added an aque-
acridanes 1a and 1b (1.00 � 10 mol/l in DMSO) in a
quartz cell, and photons produced during 204 s were
counted, which led to estimation of the collected
photons by comparison with the above described lu-
minol luminescence. The resulting solution was di-
luted at once with a mixed solution of DMSO (18 ml)
and t-BuOH (9 ml) for the UV measurement. The
yield of NMA was calculated by comparison of the
absorption at 401 nm with the standard solution of
1
ous solution of sodium methylthiolate (5%, 50 ml),
and the reaction mixture was stirred for 1 h. The
product was extracted with chloroform and the ex-
tract was dried over Na
for purification as described above gave pure 1b as
2
SO . The similar procedure
4
�
pale yellow needles (1.96 g, 49%). mp 114–115 C.
1
H NMR (60 MHz, CDCl ) � 1.87 (s, 3H, SMe), 3.35 (s,
3
�
4
3
H, NMe), 5.03 (s, 1H, 9-H), 6.68–7.33 (m, 8H, aro-
NMA (1 � 10 mol/l) in the same solvent. The CL
quantum yield was calculated as a quotient of pho-
tons per produced NMA. The quantum yields of CL
in the presence of DBA were measured for the so-
+
matic H), m/z 241 (M ). Anal. Calcd. for C15
C, 74.65; H, 6.26; N, 5.80%. Found: C, 74.64; H, 6.21;
N, 5.77%.
Benzhydryl phenyl sulfide (2) [31] and benzhy-
dryldimethylsulfomium bromide (3) [19] are known
compounds.
H15NS:
�
4
lutions (2 ml) of 2 and 3 (1 � 10 mol/l in DMSO)
containing various concentrations of DBA and tert-
�
2
BuOK (2.0 � 10 mol/l in DMSO). The reciprocal of
each quantum yield against the reciprocal of each
concentration of DBA was plotted.
Autoxidation of 1a, 1b, 2, and 3
A solution of 1a (0.5 g, 2.07 mmol) and tert-BuOK
(0.4 g, 3.0 mmol) in DMSO (50 ml) was bubbled with
ACKNOWLEDGMENT
oxygen for 2 h at room temperature. After addition of
water (30 ml), the product was extracted with chlo-
The authors thank Associate Professor Dr.
Tomoshige Kobayashi (Faculty of Textile Science,
Shinshu University) for the measurement of mass
spectra.
roform and the extract was dried over Na
2
SO . Af-
4
ter evaporation of the solvent, the residue was chro-
matographed on silica gel, using chloroform as an
eluant to give NMA (0.25 g, 59%), which showed the
same chemical and physical properties as those of an
authentic sample. When excess methyl iodide was
added to the reaction mixture, thioanisole was de-
tected in 22% yield by gas chromatography.
Autoxidation of 1b, 2, and 3 was carried out
in a similar manner. Analysis of benzophenone and
diphenyl sulfide was done by gas chromatography.
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