Photochemical deprotection of nitro-substituted benzenesulfenates via photoinduced electron transfer

Article abstract of DOI:10.1016/j.tetlet.2004.07.007

The photochemical deprotection of alkyl 2,4-dinitrobenzenesulfenate or alkyl 2-nitrobenzenesulfenate was successfully achieved by addition of triethylamine, while it was unsuccessful without triethylamine. The sulfur-oxygen bond cleavage is thought to occ

Full text of DOI:10.1016/j.tetlet.2004.07.007

Tetrahedron
Letters
Tetrahedron Letters 45 (2004) 6395–6398
Photochemical deprotection of nitro-substituted
benzenesulfenates via photoinduced electron transfer
*
Kan Wakamatsu, Mitsuru Kouda, Kazuyoshi Shimaoka and Haruo Yamada
Department of Chemistry, Faculty of Science, Okayama University of Science, Ridai-cho, Okayama 700-0005, Japan
Received 13 May 2004;revised 30 June 2004;accepted 5 July 2004
Available online 19 July 2004
Abstract—The photochemical deprotection of alkyl 2,4-dinitrobenzenesulfenate or alkyl 2-nitrobenzenesulfenate was successfully
achieved by addition of triethylamine, while it was unsuccessful without triethylamine. The sulfur–oxygen bond cleavage is thought
to occur heterolytically in the sulfenate anion radical produced by photoinduced electron transfer with triethylamine.
ꢀ
2004 Elsevier Ltd. All rights reserved.
Selective protection and deprotection of specific func-
tional groups are fundamental procedures in multi-step
organic syntheses. Among various deprotection meth-
ods, photochemical deprotection is attractive because
it minimizes the use of additional reagents, can be con-
trolled simply by switching on a light, and can be
applied to photofunctional materials. The 2,4-
dinitrobenzenesulfenyl group is known to be useful in
also reacts photochemically but is moderately unstable
5
thermally.
With the aim of avoiding generation of the alkoxyl rad-
ical, we have investigated sulfur–oxygen bond cleavage
in the photochemically generated anion radical, which
can transform to a sulfenyl radical and an alkoxide
anion. Several applications of photoinduced electron
transfer (PET) have been reported for deprotection,
for example reductive cleavage of sulfonates and sulfon-
1
the protection of hydroxy groups. In addition to depro-
tection by conventional procedures, this protecting
group can also be removed photochemically, absorbing
in the near-ultraviolet to visible light region (<450nm).
As early as the 1960s, Barton and his co-workers re-
ported efficient deprotection of 2,4-dinitrobenzenesulf-
enyl carboxylate by photochemical cleavage of the
sulfur–oxygen bond to obtain the corresponding carb-
6
,7
amides.
donor to enable photoreduction of carbonyl compounds
Triethylamine can be used as an electron
as well as for carbon–oxygen bond cleavage of a-oxy-
8
–11
genated ketones.
We report herein the efficient
photochemical deprotection of alkyl 2,4-dinitrobenzene-
sulfenate by the addition of triethylamine.
2
–4
oxylic acid.
They suggested that cleavage occurred
by an ionic mechanism. The equivalent protected alco-
hols, that is, alkyl 2,4-dinitrobenzenesulfenates, however,
are less reactive under the same photochemical condi-
tions except for the benzyl derivative. The 4-nitro-
benzenesulfenyl group is more sensitive to light than
for the dinitro group, including in the case of alcohols.
However, the sulfur–oxygen bond in this substrate tends
to cleave homolytically, and thus generates b-fission
products (a carbonyl compound and an alkyl radical)
besides the alcohols. Unsubstituted benzenesulfenate
Sulfenates 1 and 2 (Scheme 1) were prepared by reaction
of the corresponding alcohol with (di)nitrobenzenesulf-
1
2
enyl chloride by a literature method. Characteristic
absorption bands of 2,4-dinitrobenzenesulfenyl and
2-nitrobenzenesulfenyl groups were observed in
near-ultraviolet to visible region in dichloromethane:
for example, for 1d, kmax = 339nm (e = 9.5 ·
3
À1
3
À1
À1
10 cm dm mol ), 390nm (shoulder);for 2d, 400nm
3
3
À1
(e = 3.5 · 10 cm dm mol ). When
methane solution containing 1a (0.01moldm ) in a
Pyrex tube was irradiated (k > 280nm, 12h) using a
a
dichloro-
À3
500-W super-high-pressure mercury lamp under argon
atmosphere, the alcohol was not obtained and 1a was
Keywords: Sulfenate;Photochemistry;Deprotection;Electron transfer;
Anion radical.
almost completely recovered (86%), as has been previ-
2
–4
*
Corresponding author. Tel.: +81-86-256-9663;fax: +81-86-256-8460/
81-86-254-2891;e-mail: waka@chem.ous.ac.jp
ously reported. Under these conditions, the major
product was a rearranged product, the sulfoxide 4a
+
0
040-4039/$ - see front matter ꢀ 2004 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2004.07.007

6396
K. Wakamatsu et al. / Tetrahedron Letters 45 (2004) 6395–6398
R
S
N
hν
O
R
OH
Et
3
2 2
N / CH Cl
O
2
X
1
(X = NO
2
), 2 (X = H)
3
a: R = PhCH
2
CH -, b: R = PhCH
2
2
CH CH -, c: R = PhSCH CH -, d: R = CH =CH-(CH ) -
2 2 2 2 2 2 9
Scheme 1.
(14%). Previously, it was reported that the unsubstituted
benzenesulfenate did not rearrange to the sulfoxide pho-
tochemically. The details of this rearrangement will be
discussed in another paper. On the other hand, under
the same conditions except for addition of triethylamine
Me
S
N
Me
Me
S
O
O
1
3
O
2
NO
2
O
2
N
1
e
2e
(
1% vs. dichloromethane), the deprotected alcohol (3)
Me
S
was afforded in high yield (Table 1) and no b-fission
products were obtained. Highly polar materials were de-
tected, probably compounds related to sulfonic or sulfi-
nic acid;the fate of the dinitrobenzenesulfenyl part
remains unclarified. Since the excess amount of triethyl-
amine could be removed by evaporation, the alcohol 3
was easily isolated by subsequent chromatography.
In the dark, 1a–d were unchanged after 10h in triethyl-
amine solvent. Furthermore, 1a–d were completely
recovered after heating in acetic acid at 100ꢁC for
O
S
O
Me
6
7
First, the possibility of the electron transfer was esti-
mated as follows. Calculated (B3LYP/6-31+G*) elec-
tron affinities of the excited triplet states of 1e, 2e, and
nitrobenzene were 4.21, 3.57, and 3.64eV, respectively;
the experimental literature value for nitrobenzene is
3
0min.
1
5–17
3
.52eV.
These values indicate that sulfenates 1e
O
S
and 2e are comparable electron acceptors to nitrobenz-
ene. Electron transfer from triethylamine to the excited
triplet of nitrobenzene is estimated to be exothermic
by the Weller equation when inputting experimental val-
ues, thus electron transfer to 1e or 2e is also con-
sidered feasible.
R
2
O N
X
4
(X = NO
2
), 5 (X = H)
16,18,19ꢀ
The 2-nitro derivative (2), when irradiated in the presence
of triethylamine, also yielded similar results but was less
reactive than the dinitro derivative 1 (Table 1). At pre-
sent, we consider this to result from the weaker light
absorptivity and electron-accepting ability of 2 compared
to 1. Irradiation of 2a without triethylamine afforded
sulfoxide 5a (16% after irradiation for 12h) but the
recovery of 2a (30%) was reduced compared to that of
Next, geometries and electronic properties of the anion
radicals were investigated. The LUMOs of 6 and 7 are
assigned as antibonding r-orbitals on the sulfur–oxygen
bond, and as expected the sulfur–oxygen bond in those
anion radicals is extremely elongated compared to the
neutral molecules (Table 2). Thus bond dissociation
may occur simultaneously with electron attachment.
For the anion radicals of 6 and 7 both the sulfur atom
and methoxy group are calculated to have moderate
spin density as well as negative charge.
1a. Similarly to 1, compound 2 was stable both in triethyl-
at room temperature and in hot acetic acid.
To gain insight into the electronic properties and ener-
gies of the anion radicals of 1 and 2, we performed the
density functional theoretical (DFT) calculations.
Methyl sulfenates (1e, 2e) were selected as model mole-
cules, and unsubstituted benzenesulfenate (6) and meth-
anesulfenate (7) were also calculated for comparison.
For the anion radicals of 1e and 2e, elongation of the
sulfur–oxygen bond occurred to a lesser extent. The spin
densities on both the sulfur atom and the methoxy
group are calculated to be nearly zero, thus the spin is
localized on the (di)nitrophenyl group in 1e and 2e.
Additionally, the 2-nitro group in the anion radical of
1e is more negatively charged and has more spin density
than the 4-nitro group: the charges are calculated to be
À0.74 (2-NO ) and À0.39 (4-NO ), and the spin densi-
1
4
a
Table 1. Photodeprotection of the sulfenates (1, 2) with triethylamine
b
Substrate
Irradiation time (h)
Yield of alcohol 3 (%)
2
2
1
1
1
1
2
2
2
2
a
b
c
7
9
94
99
93
97
95
99
93
99
ties are 0.68 (2-NO ) and 0.06 (4-NO ). The charges of
2
2
5.5
7
d
a
b
c
ꢀ
16
18
16
When the experimental values of +1.15, À1.10, and 2.52eV are
input as the oxidation potential of triethylamine (Eox vs. SCE in
acetonitrile), the reduction potential of nitrobenzene (Ered vs. SCE in
21
28
14
23
acetonitrile) and the triplet energy (E
the free energy difference of the electron transfer (DG = Eox
T
) of nitrobenzene, respectively,
d
À
a
19
Ered À E + C) is calculated to be À0.11eV for the Coulombic
Reaction conditions described in text.
Determined by isolation.
T
b
correction term (C) in dichloromethane of +0.16eV.

K. Wakamatsu et al. / Tetrahedron Letters 45 (2004) 6395–6398
6397
a
Table 2. Optimized S–O bond length, charge, and spin
˚
r (S–O)(A)
Charge
Spin
S
OMe
S
OMe
1
2
6
7
e
Anion radical
Neutral
1.777
1.692
1.783
1.698
2.536
1.692
2.508
1.695
+0.64
+0.73
+0.58
+0.69
À0.13
+0.56
À0.27
+0.48
À0.48
À0.38
À0.49
À0.39
À0.47
À0.37
À0.56
À0.38
0.01
0.00
0.00
0.52
0.44
e
Anion radical
Neutral
0.03
0.48
0.55
Anion radical
Neutral
Anion radical
Neutral
a
Calculations were performed using B3LYP functional and 6-31+G* basis set. Charge and spin were obtained by natural population analysis.
R
S
R
S
O
hν
O
Et N
+
3
Et N
3
O N
X
O N
X
2
2
Electron
Transfer
1
(X = NO ), 2 (X = H)
1 , 2
2
R
O
S
Et N
3
R
O
R
OH
O
N
O
X
3
S
Et N CH CH3
2
O
N
O
X
highly polar materials
presumably
sulfonic or sulfinic acid derivatives)
(
Scheme 2.
the both nitro groups in the anion radical are more neg-
ative than those in the neutral 1e (À0.26 for 2-NO and
R
O
S
S
2
O
H
O
4
-NO ). In contrast to 6 and 7, the sulfur atom is posi-
2
N
O
N
O
X
R
OH
+
X
tively charged as in the neutral molecules. The lower
unoccupied orbitals of neutral 1e and 2e are assigned
to p-orbitals spread over the benzene ring and the nitro
group(s), and the antibonding r-orbital on the sulfur–
oxygen bond appears at a higher lying MO: LUMO+3
Et N CH
2
Et N CH
2
CH3
CH3
8
3
Scheme 3.
(
1e) and LUMO+2 (2e). Therefore, cleavage of the an-
ion radicals 1 and 2 is expected to occur heterolytically
as in the neutral molecules rather than homolytically.
This is consistent with b-fission products not being pro-
duced.
fur–oxygen bond cleavage in the coupling product 8
from the resulting radical pair (Scheme 3) may need
more activation energy than that in the anion radical
ÆÀ
ÆÀ
or 2 .
1
A possible mechanism of this reaction is presented in
Scheme 2. The excited triplet state of 1 or 2 is rapidly
quenched by triethylamine to afford the anion radical of
In summary, photochemical deprotection of alkyl 2,4-
dinitrobenzenesulfenate was successfully achieved by
addition of triethylamine. The sulfur–oxygen bond
cleavage is thought to occur heterolytically in the sulf-
enate anion radical produced by photoinduced electron
transfer with triethylamine.
1
or 2 and the triethylamine cation radical. The nucleo-
philicity of the oxygen atom of the 2-nitro group in
the anion radical is increasing compared to the neutral
molecule. Therefore, intramolecular nucleophilic attack
of the oxygen atom of the 2-nitro group may assist the
cleavage of the sulfur–oxygen bond to produce the alk-
oxide anion, followed by a-proton transfer from the tri-
ethylamine cation radical to the alkoxide anion.
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ÆÀ
ÆÀ
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