Photochemical electron-transfer generation of arylthiirane radical cations with tetranitromethane and chloranil - Some novel observations

Article abstract of DOI:10.1002/ejoc.200600400

The radical cations from 2,2,3,3-tetraphenylthiirane (1a), 2,2-bis(4-methoxyphenyl)-3,3-diphenylthiirane (1b), and trans-2,3- diphenylthiirane (1c) have been generated by photoinduced electron transfer (PET) reactions with tetranitromethane [C(NO₂)₄] and chloranil (CA). A charge-transfer complexe (CTC) absorption is observed by UV/Vis spectroscopy between thiiranes (1) and C(NO₂)₄. On the other hand, quenching studies with azulene suggest that the ET reaction occurs between thiiranes and the triplet CA (³CA). The photochemical reaction of the CTC between thiiranes 1 and C(NO₂)₄ yields mainly the corresponding alkenes from the fragmentation of the radical-cation intermediate 1^.+, together with the products derived from nitration on the phenyl rings. However, oxygen transfer to afford the sulfoxides is not found. A marked solvent effect is observed in this reaction, with cage coupling favored in CH₂Cl₂ (nitration derivatives as primary products) and non-cage coupling observed in CH₃CN (the alkene as the primary product). The PET reactions between 1a-b and CA, in the presence of CH₃OH (or another possible oxygen-centered nucleophile), give the ketone derivatives through ring opening, followed by oxidative cleavage. Conversely, under the same experimental conditions, the thiirane 1c affords only trans-stilbene 2c. This different behavior is ascribed to a different spin density in the corresponding singly occupied molecular orbital (SOMO) of the radical cation. For 1c^+., the spin density is concentrated at the sulfur atom, whereas for 1a^.+ and 1b^.+, the charge is distributed onto the aromatic rings. Wiley-VCH Verlag GmbH & Co. KGaA, 2006.

Full text of DOI:10.1002/ejoc.200600400

FULL PAPER
DOI: 10.1002/ejoc.200600400
Photochemical Electron-Transfer Generation of Arylthiirane Radical Cations
with Tetranitromethane and Chloranil − Some Novel Observations
Marcelo Puiatti,[a] Juan E. Argüello,[a] and Alicia B. Peñéñory*[a]
Keywords: Thiirane / Radicals / Photoinduced electron transfer / Nitration / Oxidative cleavage / Photochemistry
The radical cations from 2,2,3,3-tetraphenylthiirane (1a), 2,2-
bis(4-methoxyphenyl)-3,3-diphenylthiirane (1b), and trans-
2,3-diphenylthiirane (1c) have been generated by photoin-
duced electron transfer (PET) reactions with tetranitrometh-
ane [C(NO2)4] and chloranil (CA). A charge-transfer com-
plexe (CTC) absorption is observed by UV/Vis spectroscopy
between thiiranes (1) and C(NO2)4. On the other hand,
quenching studies with azulene suggest that the ET reaction
occurs between thiiranes and the triplet CA (3CA). The pho-
tochemical reaction of the CTC between thiiranes 1 and
C(NO2)4 yields mainly the corresponding alkenes from the
fragmentation of the radical-cation intermediate 1·+, together
with the products derived from nitration on the phenyl rings.
However, oxygen transfer to afford the sulfoxides is not
found. A marked solvent effect is observed in this reaction,
with cage coupling favored in CH2Cl2 (nitration derivatives
as primary products) and non-cage coupling observed in
CH3CN (the alkene as the primary product). The PET reac-
tions between 1a–b and CA, in the presence of CH3OH (or
another possible oxygen-centered nucleophile), give the
ketone derivatives through ring opening, followed by oxidat-
ive cleavage. Conversely, under the same experimental con-
ditions, the thiirane 1c affords only trans-stilbene 2c. This dif-
ferent behavior is ascribed to a different spin density in the
corresponding singly occupied molecular orbital (SOMO) of
the radical cation. For 1c+·, the spin density is concentrated
at the sulfur atom, whereas for 1a·+ and 1b·+, the charge is
distributed onto the aromatic rings.
(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim,
Germany, 2006)
tuted thiiranes, catalyzed by aminium radical-cation salts,
as an efficient preparation of aryl-substituted olefins.[7]
Lately, this reaction has been extended to allyl and diallyl
thiiranes.[8] In the reaction of cis and trans diphenyl thiir-
anes, it has been observed that disubstituted 1,2,3,4-tetra-
thianes were formed as minor products, thus giving further
evidence for the dimerization of the thiirane radical cat-
ion.[9] Furthermore, the CTCs of tetraaryl thiiranes and tet-
racyanoethylene (TCNE) give [3+2] cycloaddition products
upon irradiation.[10]
Introduction
Thiiranes or episulfides are three-membered-ring sulfur
heterocycles and constitute versatile synthetic intermediates
in organic synthesis. New and convenient methods for their
preparation have been developed and reviewed.[1] Analo-
gously to oxiranes[2] and aziridines,[3] thiiranes react with
nucleophiles under catalytic conditions to give products
from ring opening.[4] Thiiranes have also been used advan-
tageously in the pharmaceutical, polymer, pesticide, and
herbicide industries.[4] For example, thiirane ring opening
by methanesulfenyl bromide results in a halo disulfide
which is easily transformed into 2-thioglyceraldehyde, a re-
ported antineoplasic agent.[5]
Studies of thiirane radical cations by EPR spec-
troscopy,[6a] mass spectrometry,[6b] and theoretical calcula-
tions[6c] have provided evidence for dimer formation. The
reactivity of the cyclopropane radical cations and their ni-
trogen and oxygen analogues has been extensively studied;
however, there have been only a few studies on the reactivity
of thiirane radical cations in solution. Kamata and co-
workers have reported the desulfurization of aryl-substi-
Iranpoor and co-workers have studied the alcoholysis
and acetolysis of thiiranes with cerium(IV)[11] and iodide[12]
,
which yielded the corresponding β-substituted disulfide de-
rivatives [Equation (1)]. However, to date, there appear to
be no examples in the literature of the reaction of photo-
chemically generated thiirane radical cations with nucleo-
philes.
We have previously studied the generation and reactivity
of sulfide radical cations in photochemical reactions with
C(NO2)4.[13] These reactions proceed through the initial for-
mation of a CTC followed by light-induced dissociative ET,
leading to a triad of reactive species consisting of a sulfur-
[a] INFIQC, Departamento de Química Orgánica, Facultad de
Ciencias Químicas, Universidad Nacional de Córdoba, Ciudad
Universitaria,
5000 Córdoba, Argentina
Fax: +54-351-4334170
E-mail: penenory@fcq.unc.edu.ar
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Photochemical Generation of Arylthiirane Radical Cations
FULL PAPER
centered radical cation, the radical nitrogen dioxide, and
the nitroform anion (Scheme 1). Coupling of the two radi-
cals and subsequent loss of nitrosyl cation give the corre-
sponding sulfoxide (sulfoxidation). Alternatively, C–S cleav-
age occurs to produce a thiyl radical and a carbocation
(fragmentation); the radical dimerizes to the corresponding
disulfide, and the cation is trapped by a nucleophile. Com-
petition between sulfoxidation and/or fragmentation de-
pends markedly on the substrate structure (Scheme 1).
Figure 2. UV/Vis Spectra of Thiirane 1b and C(NO2)4 in CH3CN.
Scheme 1.
CA is a well-established ET photosensitizer, and the 3CA
(Ered = 2.15 V vs. SCE, in the triplet state) can react at near-
diffusion-limited rates with a variety of electron donors.[14]
This compound has been used in PET reactions with small-
ring hydrocarbons,[15] aziridines,[16] oxiranes,[17] and oxet-
anes.[18]
This account makes it clear that the study of the chemis-
try of such novel thiirane (1a–c, Figure 1) radical cations in
solution should be of great interest. In this work we exam-
ine the reactivity of thiirane radical cations generated by
photochemical ET with C(NO2)4 and CA, the latter being
studied in the presence of CH3OH. Although the expected
β-methoxy disulfide derivatives from ring opening did not
occur, a novel fragmentation was observed to afford the
ketone derivatives. In comparison, chemical ET (CET) with
cerium(IV) ammonium nitrate (CAN) has also been per-
formed.
When a solution of the CTC was kept in the dark under
an argon atmosphere, no products were observed. After ir-
radiation of the CTC with a filtered, mercury, high-pressure
lamp (150 W, short cutoff filter, λ Ͼ 400 nm) and aqueous
work-up, the products from desulfurization [Equation (3)]
were isolated by silica gel chromatography and identified by
spectral analysis (Table 1). The alkene 2a and the mononi-
tro derivative 3a were found in comparable yields upon irra-
diation of the CTC of 1a with C(NO2)4. Nevertheless, the
photochemical reaction of 1b with C(NO2)4 afforded
mainly the mononitro alkene 3b, and as minor products, 2b,
4b, and the ketones 5 and 6 (Table 1, entries 1 and 2). In
both reactions, elemental sulfur was isolated as a yellow
solid and its identity confirmed by mass spectrometry.
Figure 1. Structure of the thiiranes employed.
Results
1. Photochemical Reactions of the CTCs between Thiiranes
1 with C(NO2)4
It is known that the desulfurization of thiiranes takes
When C(NO2)4 was added to a solution of the thiiranes place under irradiation at λ = 230–280 nm.[9] As a control
1a and 1b in CH3CN, a yellow color immediately devel- experiment, a 0.1  solution of 1b was irradiated at λ Ͼ
oped, due to the formation of a CTC, which showed ab- 400 nm, and the thiirane was recovered without decomposi-
sorption bands as a shoulder between 340–350 nm [Figure 2 tion.
and Equation (2)]. These results are in agreement with pre-
When the reaction of 1b was performed in CH2Cl2, only
viously reported data for the CTC between C(NO2)4 and the nitrated alkenes 3b and 4b were produced in 33% and
sulfides.[13]
27% yield, respectively, associated with 75% conversion.
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Table 1. Photoinduced reaction of thiirane 1 with C(NO2)4 and control experiments.[a]
Entry
Reactant
Reaction conditions
solvent, temp., time
Conversion [%]
Mb [%]
Product yield [%]
2
3
4
5[b]
1[c]
2[c]
3
1a
1b
1b
1b
1b
2b
2b
CH3CN, –5 °C, 3 h
CH3CN, –5 °C, 3 h
CH2Cl2, 20 °C, 2 h
CH3CN, 20 °C, 2 h
CH3CN, 20 °C, 2 h
CH3CN, 20 °C, 2 h
CH2Cl2, 20 °C, 2 h
64
100
75
51
43
47
64
86
78
100
98
99
99
30
4
–
16
17
53[f]
36[f]
20
57
33
16
15
46
60
–
17
27
–
–
–
–
[d]
15
17
10
–
4
5[e]
6
7
96
–
–
[a] A solution of 1 (0.1 mmol) and C(NO2)4 (0.1 mmol) in 5 mL of the solvent was irradiated at λ Ͼ 400 nm under a nitrogen atmosphere;
the ketones were quantified by GC and the thiiranes and alkenes by HPLC and 1H NMR spectroscopy analysis with an internal standard
(error 5%). Conversion was determined by quantification of the unreacted thiirane. Mass balance (Mb). [b] Together with an equimolar
amount of benzophenone (6). [c] The product yields were determined through isolation by silica gel chromatography. Elemental sulfur
was also isolated in 70–80% yield. [d] Not quantified. [e] Reaction with a C(NO2)4/1 of 25:75. [f] Unreacted recovered alkene 2b.
Also, the p-methoxyphenyl ketone (5) and benzophenone mononitroalkene 3b, and the ketones 5 and 6 were obtained
(6) were observed in 15% yield (Table 1, entry 3). A very (Table 1, entry 4). The desulfurization of 1b also proceeded
different product distribution was found when the reaction with substoichiometric amounts of C(NO2)4, revealing
was conducted in CH3CN at 20 °C. Thus, the alkene 2b, the some chain reaction character for these PET reactions
(Table 1, entry 5).
In order to assess whether 3 is a secondary product de-
rived from 2, some control experiments were performed.
Thus, the alkene 2b gave the nitro derivative 3b by a photo-
reaction with C(NO2)4 in CH2Cl2 or in CH3CN (Table 1,
entries 6 and 7). We have also monitored the reactions of
1b with C(NO2)4 in both solvents by HPLC analysis (Fig-
ure 3 and Figure 4). While the mononitro alkene 3b is
formed in CH3CN after 30 min of reaction, in CH2Cl2 3b
is observed almost at the beginning of the irradiation.
2. Photochemical Reactions of Thiiranes with CA in the
Presence of CH3OH
The photochemical reaction of thiirane 1a with CA as a
sensitizer was performed in CH3CN in the presence of
CH3OH as nucleophile. The main product was benzophe-
none (6), with tetraphenylethylene (2a) and 9,10-diphenyl-
phenanthrene (7) as minor products [Equation (4)].
Figure 3. Time profile for the reaction of 1b with C(NO2)4 in
CH3CN at 20 °C.
The phenanthrene 7, isolated from the reaction mixture
in Ͻ5% yield, was prepared independently by photocycliza-
tion of alkene 2a according to the known procedure.[19]
The results of the photochemically induced reactions of
thiirane 1b with CA are presented in Table 2. There was no
reaction between thiirane 1b and CA in the dark in 2 h,
since the substrate was quantitatively recovered. When thiir-
Figure 4. Time profile for the reaction of 1b with C(NO2)4 in
CH2Cl2 at 20 °C.
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Photochemical Generation of Arylthiirane Radical Cations
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Table 2. Photoinduced reaction of thiirane 1b.[a]
Entry
Reaction conditions
λ [nm], solvent, atm., oxidant, additive
Conversion [%]
Mb [%]
Product yield [%]
2b
5
6
1
2
3
4
5
6
7
8
9
350, CH3CN/CH3OH, N2, CA
350, CH3CN/CH3OH, N2, CA, azulene[b]
350, CH3CN/CH3OH, O2, CA
350, CH3CN/CH3OH, Ar, CA
350, CH3CN/CH3OH, N2, –
419, CH3CN/CH3OH, N2, –
419, CH3CN/CH3OH, N2, CA
419, CH3CN, N2, CA
100
8
93
90
7
99
100
80
50
3
23
20
7
55
43
5.4
46
44
–
[c]
55
49
–
77
100
100
91
99
99
97
95
100
0
–
–
–
74
61
86
88
100
41
18
37
24
24
50
12
50
23
61
61
50
29
44
[c]
[d]
[d]
419, CH2Cl2/CH3OH, N2, CA
419, CH2Cl2, N2, CA
–, CH3CN/CH3OH, N2, CAN
–, CH3CN, N2, CAN
10
11
12
40
[c]
[a] A solution of 1b (0.4 mmol), oxidant (0.4 mmol) and CH3OH (80 mmol) in 10 mL of CH3CN was irradiated with a medium-pressure
mercury lamp (maximal emission at λ = 350 nm) or a Rayonet reactor (λ = 419 nm) for 2 h under a nitrogen atmosphere. Ketones were
quantified by GC and thiiranes and alkenes by HPLC and 1H NMR spectroscopy analysis with an internal standard (error 5%). Conver-
sion was determined by quantification of the unreacted thiirane. The average of 5 and 6 was used to evaluate the mass balance (Mb).
[b] 0.2 mmol. [c] Not quantified. [d] A comparable amount of thiobenzophenone and benzophenone was observed by GC-MS.
ane 1b was irradiated for 2 h with CA in CH3CN and in the ketones 5 and 6 considerably decreased, while the formation
presence of CH3OH [Equation (5)], ketones 5 and 6 were of the alkene 2b increased (Table 2, entry 8).
obtained, together with the alkene 2b (Table 2, entry 1).
In CH2Cl2 as solvent, in the presence or absence of
CH3OH, the reaction proceeded with higher conversion. In
these reactions, a significant amount of thiobenzophenone
was observed by GC-MS analysis before work-up (Table 2,
entries 9 and 10).
The generation of the radical cation of 1b was unequivo-
cally proven by its CET reaction in CH3CN/CH3OH with
CAN, a well-known one-electron oxidant (Ered Ϸ 1.6 eV vs.
SCE).[21] Ketones 5 and 6 were formed in 50% and 40%
yield, respectively, in the presence of CH3OH, and in its
absence, a lower conversion and yield of the products were
observed (Table 2, entries 11 and 12). The reaction of 1b
with CAN in CH2Cl2 proceeded with less than 5% conver-
sion, due to the very low solubility of the cerium salt in this
solvent.
On the other hand, the CET of thiirane 1b with the
tris(4-bromophenyl) hexachloroantimonate aminium salt
rendered quantitatively only the alkene 2b in CH3CN or in
CH2Cl2, without any traces of ketones 5 and 6. Similar re-
sults have been previously reported as a convenient method
for the desulfurization of thiiranes.[7,8]
The formation of a CTC between the thiirane and CA
was not observed by UV/Vis spectroscopy. In the presence
of azulene, a known energy-transfer quencher of (3CA),[20]
the reaction between 1b and CA was strongly inhibited
(Table 2, entry 2). Furthermore, a control experiment estab-
lished that no reaction between the thiirane and the azulene
took place.
The photochemical reaction of 1b with CA in the pres-
ence of CH3OH was monitored by GC-MS analysis every
30 min. Ketals 8 and 9 were observed as well as minor
amounts of the thioketones 10 and 11 (Figure 5). The latter
products were hydrolyzed after work-up to the respective
ketones 6 and 5.
Alkene 2b was proven to be stable under irradiation in
the presence of CA and CH3OH (the same experimental
conditions as those used for the photoreactions with thiir-
anes) and in the presence of CAN. After 2 h of irradiation,
alkene 2b was quantitatively recovered.
The photoinduced reaction of the thiirane 1b in the pres-
ence of O2 afforded ketones 5 and 6 in 55% and 46% yield,
respectively (Table 2, entry 3). When the reaction was per-
formed under an argon atmosphere (Table 2, entry 4), no
significant differences were observed between the results of
this reaction and those of the reaction performed under a
nitrogen or oxygen atmosphere (Table 2, entries 1 and 3).
In the absence of the photosensitizer CA, only desulfur-
ization was observed, with the alkene 2b being formed in a
7% yield (Table 2, entry 5). This photochemical desulfuriza-
tion was avoided by irradiation at λ = 419 nm. Thus, the
reaction of 1b with CA at λ = 419 nm in the presence of
Finally, the reaction of the thiirane of trans-stilbene 1c
CH3OH gave a lower yield of 2b (Table 2, entries 6 and 7). with CA in the presence of CH3OH gave only trans-stilbene
When the photoinduced reaction of the thiirane 1b with (2c) in 51% yield without cleavage or dimerization products
CA was performed in the absence of CH3OH, the yields of being observed [Equation (6)].
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M. Puiatti, J. E. Argüello, A. B. Peñéñory
FULL PAPER
nate coupling with the radical 12 and deprotonation. In
pathway (b), fragmentation and desulfurization after escape
from the solvent cage occur with the formation of the al-
kene radical cation 2·+, which by ET[22] with the thiirane 1,
yields the alkene 2. The newly generated radical cation 1·+
engages in a chain reaction. This accounts for the catalytic
behavior partly observed in this reaction, since the nitro
derivative 3b required two moles of C(NO2)4 (pathways b–
c). Ketones 5 and 6 are probably generated by ring opening
of the radical cation 1·+ followed by reaction with water
present in trace amounts in CH3CN or the nitroform anion
in CH2Cl2, (for further discussion of this reaction pathway,
see below). The nitroalkenes 3 and 4 [Equation (3)] can also
be formed by further oxidation of the alkene 2 by
C(NO2)4 [Scheme 2, pathway (c) and (d)].
Figure 5. Products from the reaction of 1b with CA.
A marked solvent effect was observed in the PET reac-
tion of 1b with C(NO2)4, by monitoring the reaction in both
CH3CN and CH2Cl2 at 20 °C. In CH3CN, the reaction pro-
ceeded with formation of the alkene 2b as a primary prod-
uct and nitro derivative 3b as a secondary product from
further reaction with C(NO2)4 [Scheme 2, pathways (b) and
(c)]. In contrast, the reaction in the non-polar CH2Cl2
yielded the mononitro derivative 3b as a primary product
by coupling of the nitrogen dioxide radical with 2b·+ within
the solvent cage and further deprotonation [Scheme 2, path-
way (a)]. The experimental results show that the nitration of
the phenyl rings is more effective in CH2Cl2 than in CH3CN
under the same conditions (Table 1, entries 3 and 4). Tem-
perature also affects the mechanism of this reaction by
changing the viscosity of the solvent. When the reaction
was conducted in CH3CN at –5 °C, a cage reaction was
Mechanistic Discussion
Photochemical ET of Thiiranes with C(NO2)4
The products observed in the photochemical reaction of
the CTC between thiiranes 1 and C(NO2)4 (Table 1) may be
readily rationalized in terms of the thiirane radical cation
1·+, generated by a PET process together with nitrite radical
(12) and the nitroform anion 13 (Scheme 2). The radical
cation 1·+ has two reaction possibilities. Pathway (a) in-
volves C–S fragmentation and desulfurization in the solvent
cage to yield the alkene radical cation 2·+. This radical cat- favored, due to a higher viscosity of the solvent, and the
ion finally renders the nitroalkene derivative 3 after gemi- ratio of products was more similar to that found in CH2Cl2,
Scheme 2.
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Photochemical Generation of Arylthiirane Radical Cations
FULL PAPER
(Table 1 entries 2 and 3). On the other hand, in CH3CN at
20 °C, out-of-cage products were observed (Table 1, entry
4).
Photochemical Ring Opening Induced by CA in the
Presence of CH3OH
The photochemical reaction between 1 and CA pro-
3
ceeded by an ET pathway between CA and the thiirane
Scheme 3.
with concomitant generation of 1·+, as confirmed by the
inhibition experiments with azulene (triplet energy
quencher).[20] Surprisingly, these reactions afforded the di-
The mechanism outlined in Scheme 4 is proposed to ac-
aryl ketones as the main products instead of the expected count for the results herein obtained. After ring opening of
2-methoxythiol or disulfide derivatives as products ring the thiirane radical cation, two possibilities can be envis-
opened by CH3OH.[11,12] The formation of ketones by the aged: a) nucleophilic substitution or b) desulfurization. In
reaction of the radical cation 1·+ through a photo-oxygena- view of the observed products 8–11, reaction with CH3OH
tion mechanism[23] can be ruled out, based on the following may take place on both carbon atoms (pathway a); β-frag-
facts. Practically the same results were obtained when the mentation of the thiyl radical (C–C cleavage) affords thioke-
photoreaction was performed under either a nitrogen, ar- tones and new carbon-centered radicals. Further oxidation
gon, or oxygen atmosphere. Ketals and thioketones 8–11 of the latter and reaction with CH3OH yield dimethyl ketals
were detected.
8 and 9.[29] Photoinduced deprotection of ketals by CA[30]
While the ring opening of alkyl-substituted thiirane radi- and a water work-up affords the observed ketones 5 and
cal cations by alcohols has been reported to afford the di- 6. In the absence of CH3OH or another oxygen-centered
sulfides from the 2-alkoxythiol derivatives [C–S fragmenta- nucleophilic species such as the residual water present in
tion, Equation (1)],[11,12] evidence for C–C cleavage for tet- the CH3CN solvent, CA·– or NO3– should also be responsi-
raarylthiirane radical cations was obtained from the cy- ble for the formation of the ketones.[31] This statement is
cloadducts formed in the photochemical reaction with clearly supported by the absence of the ketones in the reac-
TCNE.[24] In that reaction, a C–C cleavage afforded a radi- tion of thiirane 1b with the aminium salt in CH2Cl2. Under
cal-ion pair consisting of the thiocarbonyl ylide radical cat- these conditions, there is no nucleophilic species able to af-
ion and the TCNE radical anion. Back ET occurred, and ford the ketones. Finally, the stability of the alkene 2b under
the thiocarbonyl ylide formed was captured by the TCNE irradiation in the presence of CA and CH3OH or in the
inside the solvent cage to yield a cycloaddition product.
presence of CAN suggests that 2b is not an intermediate in
On the basis of the previously reported data, the possibil- the generation of ketones 5 and 6. Thus, after desulfuriza-
ity of an initial C–S or C–C cleavage should be considered tion, the tight ion pair 2b·+ and CA·– affords alkene 2b and
for the reaction of thiiranes 1a and 1b with CH3OH CA by ET. This is consistent with the fact that trans-4-
(Scheme 3). The most likely process for the radical cations methoxystilbene[32] and trans-anethole[18] were stable in the
1a·+ and 1b·+ is an initial C–S cleavage to afford the distonic presence of CA upon irradiation.
radical cation 15,[25] taking into consideration the lower
The reaction of the thiirane of trans-stilbene (1c) with
bond dissociation energy of a C–S versus that of a C–C, CA in the presence of CH3OH gave only trans-stilbene 2c in
the instability of the thiocarbonyl ylide radical cation 14, 51% yield, without cleavage products being observed. This
which rapidly collapses to a thiirane in the absence of a desulfurization reaction seems to be faster than the nucleo-
dipolarophile[10] and the fact that desulfurization is ob- philic substitution by CH3OH. In order to rationalize these
served as a competitive pathway. Thus, for the ring opening results, we have computed the SOMO coefficients of the
of the tetraaryl-substituted thiiranes under study, a uni- thiirane radical cations of 1b and 1c by the semiempirical
molecular pathway is favored, leading to the intermediate AM1/UHF[33] method.
15, which bears a stabilized diaryl-substituted carbocation
Figure 6 shows that the SOMO of the radical cation of
and a thiyl radical. The nucleophilic substitution reaction the trans-2,3-diphenylthiirane (1c) has the higher spin den-
of an arylcyclopropane radical cation is recognized as a sity on the sulfur atom. In contrast, for the tetrasubstituted
three-electron SN2 reaction,[26] and a similar CH3OH-as- thiirane 1b, the SOMO of its radical cation is localized on
sisted ring opening of an oxirane radical cation has been the anisyl ring, in agreement with previous data from the
assumed.[27] Yet, previous results of the reaction of thiirane EPR spectrum of 1b·+. This suggests that it is not a sulfur-
radical cations generated by CAN have shown that substitu- centered radical cation but a p-anisyl π-radical cation.[34]
tion with CH3OH occurs at the more substituted carbon These preliminary results account for the exclusive forma-
atom, suggesting a non-concerted pathway.[11,12] Further- tion of the trans-stilbene 2c from 1c·+, whereas 1b·+ af-
more, stereochemical evidence for a unimolecular pathway forded products from desulfurization (2b) and β-fragmenta-
has been provided in the fragmentation of 1-phenylethyl tion (5 and 6). Presumably, 1c·+ desulfurizes directly to af-
phenyl sulfide radical cation.[28]
ford 2c·+ without the formation of the distonic radical cat-
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M. Puiatti, J. E. Argüello, A. B. Peñéñory
FULL PAPER
Scheme 4.
ion 15 as an intermediate, as suggested by the localization whereas quenching studies with azulene suggest that the ET
3
of spin density mainly on the sulfur atom.[35] For the radical reaction occurs between CA and thiiranes.
cation 1b·+, the spin density distribution on the anisyl moi-
The PET reaction between thiirane 1 and C(NO2)4 yields
ety strongly supports the formation of a distonic radical products from the radical-cation intermediate through de-
cation intermediate 15, which can account for the competi- sulfurization, together with products from the nitration of
tion observed between desulfurization and nucleophilic at- the phenyl rings; sulfoxidation was not observed. Two dif-
tack by CH3OH.
ferent pathways may account for the nitrated products, de-
pending on the solvent employed (Scheme 2). For pathway
(a) in CH2Cl2, nitration within the solvent cage occurs with-
out the alkene as intermediate. However, for pathways (b)
and (c) in CH3CN, the alkene is formed as a primary prod-
uct. Nitration of the alkene intermediate then takes place
as a secondary reaction.
A novel oxidative fragmentation of thiirane radical cat-
ions to afford the ketone derivatives is presented herein.
These reactions proceed by PET between thiiranes 1b–c and
CA. Ketone products are obtained by ring opening fol-
lowed by reaction with CH3OH as nucleophile (or another
oxygen-centered nucleophilic species), β-fragmentation of
the thiyl radical, and further oxidation.
Figure 6. AM1 SOMOs spin density of the thiirane radical cations
1b·+ and 1c·+.
Experimental Section
Conclusions
Methods: The general methods and procedures for the photoin-
duced reaction are the same as those published before.[13] The reac-
tion products were quantified by GC, HPLC, or 1H NMR with the
The electron acceptors C(NO2)4 and CA generate thiir-
ane radical cations by PET reactions. CTCs between thiir-
anes 1 and C(NO2)4 are confirmed by UV/Vis spectroscopy, internal standard method.
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Photochemical Generation of Arylthiirane Radical Cations
FULL PAPER
Materials: Commercially available compounds were used as re-
ceived. CH3CN, CH2Cl2, and CH3OH were distilled under vacuum
and stored over molecular sieves (4 Å). The substituted thiiranes
were prepared by standard procedures from the thioketones and
diphenylcarbene, generated in situ from the diazo derivative[36] or
from the reaction of the epoxide with thiourea for the thiirane of
the trans-stilbene (1c).[37] The tris(4-bromophenyl)aminium
hexachloroantimonate salt was generated from the triarylamine
and SbCl5, according to the literature procedure.[38] All products
were characterized by 1H and 13C NMR spectroscopy and mass
spectrometry and exhibited physical properties identical to those
reported in the literature.
Acknowledgments
This research work was conducted in cooperation with Professor
W. Adam, University of Würzburg, with a joint research grant from
the Volkswagen Foundation.This work was also supported in part
by the Third World Academy of Sciences (TWAS) and SECYT-
Universidad Nacional de Córdoba, Argentina. M. P. gratefully ac-
knowledges receipt of fellowships from CONICET.
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emission at λ = 350 nm). The reaction mixture was treated with
water (10 mL) and diethyl ether (30 mL). The ethereal layer was
separated and submitted to GC, GC-MS, HPLC, and NMR analy-
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Compound 3b was isolated by silica gel chromatography with pe-
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(200 MHz, CDCl3, 30 °C): δ = 3.74 (s, 3 H, OCH3), 3.76 (s, 3 H,
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127.01, 128.38, 131.58, 132.36, 132.87, 132.95, 135.48, 135.56,
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petroleum ether/diethyl ether (80:20) as the eluent from the ethereal
layer of the reaction mixture of 1b with C(NO2)4. 1H NMR
(200 MHz, CDCl3): δ = 3.77 (s, 6 H, OCH3), 6.67 (d, J = 8.8 Hz,
4 H, Ar-H), 6.91 (d, J = 8.8 Hz, 4 H, Ar-H), 7.14 (d, J = 8.8 Hz,
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(50 MHz, CDCl3, 30 °C): δ = 55.43, 113.81, 123.69, 132.38, 132.97,
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Eur. J. Org. Chem. 2006, 4528–4536
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4535

M. Puiatti, J. E. Argüello, A. B. Peñéñory
FULL PAPER
try and Photobiology (Eds.: W. Horspool, F. Lenci), 2nd ed.,
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tion which, is the main pathway with alkylthiiranes [Equa-
tion (1)].This different behavior is probably due to the C–C
bond dissociation energy, which is lower in the arylthiirane
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[20] Handbook of Photochemistry, S. L. Murov, I. Carmichael, G. L.
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[23] In the photoinduced reaction of 1b with TCNE, oxygenation
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oxygen, affording tetrahydrofurans and 1,2,4-trioxolanes,
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radical cations, which rearrange to carbonyl compounds or re-
act with nucleophiles to give addition products. See ref.[17b]
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[32] J.-H. Xu, L.-C. Wang, J.-W. Xu, B.-Z. Yan, H.-C. Yuan, J.
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[34] R. S. Glass, W. Jung, Phosphorus Sulfur Silicon Relat. Elem.
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[35] In the alkylthiirane radical cations, the spin density should also
be located on the sulfur atom. Nevertheless, these species do
not afford the alkene [Equation (1)], probably due to the higher
enthalpy change in the formation of the alkene radical cations.
[36] H. Staudinger, J. Siegwart, Helv. Chim. Acta 1920, 3, 833–840.
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Received: May 5, 2006
Published Online: August 3, 2006
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