Effects of electron-withdrawing group on the photoisomerization of tetraaryl-4H-thiopyran-1,1-dioxides

Article abstract of DOI:10.1002/hc.20455

Syntheses and photoisomerization of the new sulfone derivatives, 4,4-di (ptrifluoromethylphenyl)-2,6-diphenyl-4H-thiopyran-1, 1-dioxide and 4-(p-trifluoromethylphenyl)-2,4,6-triphenyl-4H-thiopyran-1,1-dioxide, have been investigated. The relative molar ra

Full text of DOI:10.1002/hc.20455

Heteroatom Chemistry
Volume 19, Number 6, 2008
Effects of Electron-Withdrawing Group
on the Photoisomerization of
Tetraaryl-4H-thiopyran-1,1-dioxides
Farnaz Jafarpour, Fatemeh Ramezani, and Hooshang Pirelahi
Department of Chemistry, College of Science, University of Tehran,
P.O. Box 13145-143, Tehran, Iran
Received 20 August 2007; revised 14 December 2007
photochemical transformations has been widely de-
ABSTRACT: Syntheses and photoisomeriza-
scribed in several heterocyclic substrates. Enormous
tion of the new sulfone derivatives, 4,4-di (p-
efforts have also been made on the photochemical
trifluoromethylphenyl)-2,6-diphenyl-4H-thiopyran-1,
behavior of tetrasubstituted 4H-thiopyrans and their
1-dioxide and 4-(p-trifluoromethylphenyl)-2,4,6-
sulfone derivatives in our laboratory as well as in
triphenyl-4H-thiopyran-1,1-dioxide, have been inves-
other laboratories [2–12]. In the course of our stud-
tigated. The relative molar ratios of the photoproducts
ies on photoisomerization of 2,4,4,6-tetrasubstituted
are compared with those of 2,4,4,6-tetraphenyl-4H-
4H-thiopyran-1,1-dioxides, recently we reported on
thiopyran-1,1-dioxide as well as electron-donating
the synthesis and photochemical transformations
substituted
4-methyl-2,4,6-triaryl-4H-thiopyran-
of 4-anisyl-4-methyl-2,6-diphenyl-4H-thiopyran-1,1-
dioxide 1a and 2,6-dianisyl-4-methyl-4-phenyl-4H-
thiopyran-1,1-dioxide 1b [13,14], which upon irra-
diation with low-pressure mercury lamp underwent
a di-π-methane rearrangement in high regioselectiv-
ity to form two syn-2a–2b and anti-2a–2b bicyclic
stereoisomers (Scheme 1). The electron-donating
p-methoxy group on the 4-phenyl ring of the sul-
fone increased the efficiency of formation of anti-
2a stereoisomer, whereas on the 2,6-diphenyls the
efficiencies increased for both syn-2b and anti-2b
stereoisomers.
To investigate the effects of electron-
withdrawing substituents on the photoisomer-
ization of 4H-thiopyran-1,1-dioxides and provide
further insight into mechanistic aspects of this reac-
tion, it seemed of considerable interest to investigate
some 4,4-diaryl derivatives of the system bearing
electron-withdrawing groups. Accordingly, two
new compounds, 4,4-di(p-trifluoromethylphenyl)-
1,1-dioxides as model compounds under identical
experimental conditions using 1H NMR spectroscopy.
The results observed are discussed on the basis
of a triplet excited state di-π-methane rearrange-
ꢀ
C
ment.
2008 Wiley Periodicals, Inc. Heteroatom Chem
19:557–561, 2008; Published online in Wiley InterScience
(www.interscience.wiley.com). DOI 10.1002/hc.20455
INTRODUCTION
The di-π-methane rearrangement has attracted
much attention from organic chemists because of
its importance in organic synthesis and its interest-
ing mechanistic aspects, which today still continues
to be an active field of research [1]. This type of
Correspondence to: Hooshang Pirelahi; e-mail: pirelahi@
khayam.ut.ac.ir.
Contract grant sponsor: Research Council, University of
Tehran, Iran.
2,6-diphenyl-4H-thiopyran-1,1-dioxide
1d
and
4-(p-trifluoromethylphenyl)-2,4,6-triphenyl-4H-thi-
opyran-1,1-dioxide 1e were selected for synthesis
Contract grant number: 559/4/514.
2008 Wiley Periodicals, Inc.
c
ꢀ
557

558 Jafarpour, Ramezani, and Pirelahi
1
R2
R1
R3
In the H NMR spectra, the singlet signals of
R1
R2
R1
S
R2
S
1d and 1e at δ 6.63 and 6.67 (H-3, H-5) ppm grad-
ually decreased, whereas three new sets of signals
appeared at 4.10 (H-5), and 6.96 (H-4) ppm for 2d,
3.94 (H-5), and 6.92 (H-4) ppm for syn-2e, and 3.98
(H-5), and 6.90 (H-4) ppm for anti-2e (Table 1).
Under prolonged irradiation, the signals of the bi-
cyclic photoproducts 2d and 2e began to decrease. In
the model compounds 1a–1c [13,14], the singlets of
1a–1c gradually decreased, whereas five new charac-
teristic sets of signals appeared for syn- and anti-2a–
2c. The molar ratios of the compounds at different
time intervals were obtained from the intensities of
the relative signals in the 1H NMR spectra. The varia-
tions of 4H-thiopyran-1,1-dioxides and bicyclic pho-
toproducts during 480 min of irradiation under iden-
tical experimental conditions are summarized in
Table 2.
hv
254 nm
+
R3
R3 R3
R3
S
R3
O
O
O
O
O
O
1a–1e
anti-2a–2e
syn-2a–2e
a: R1 = Me, R2 = C6H4OMe-p, R3 = Ph
b: R1 = Me, R2 = Ph, R3 = C6H4OMe-p
c: R1 = R2 = R3 = Ph
d: R1 = R2 = C6H4CF3-p, R3 = Ph
e: R1 = Ph, R2 = C6H4CF3-p, R3 = Ph
SCHEME 1
and photochemical studies (Scheme 1). The results
were compared with those of the model compounds
2,4,4,6-tetraphenyl-4H-thiopyran-1,1-dioxide
and electron-donating substituted 4-methyl-2,4,6-
triaryl-4H-thiopyran-1,1-dioxides 1a and 1b under
the same experimental conditions. The molar
ratios of the compounds during the photochemical
transformations are determined using 1H NMR
spectroscopy.
1c
Because of the stability of the photoproducts at
room temperature, we could isolate and identify the
bicyclic photoproducts in pure forms. Distinction of
the syn- and anti-stereoisomers established on the
1
basis of H NMR spectroscopy. In the bicyclic pho-
toproducts anti- and syn-2e, H-5 and H-4 protons
appear as doublets with J = 3.5 Hz, where H-5 for
syn-2e is shifted upfield relative to that of anti-2e
(Table 1) due to less anisotropic shielding effect of
the phenyl ring with an electron-withdrawing CF3-
substituted group. Moreover, the H-4 in syn-2e is
more shifted downfield relative to that of anti-2e due
to anisotropic effect of two phenyl groups at 3- and
6-positions.
As shown in Table 2, the presence of electron-
withdrawing groups at C-4 of the sulfone ring
(1d and 1e) diminishes the yields of the bicyclic
photoproducts. The lower yields might be due to
the phenyl migration side reactions that result in
RESULTS AND DISCUSSION
To study the photochemical transformation behav-
ior of compounds 1d and 1e in comparison with the
model compounds 1a–1c, 3 × 10−2 M solutions of
these compounds in chloroform-d1 were irradiated
in sealed NMR tubes with a low-pressure mercury
lamp at 254 nm (85% transmission of 254 nm and
15% transmission of light from 254 to 579 nm) un-
der an argon atmosphere at room temperature. The
time dependence of the transformations was studied
by 1H NMR spectroscopy.
TABLE 1 The Characteristic Chemical Shifts (δ/ppm) and Coupling Constants (J/Hz) for 4H-thiopyran-1,1-dioxides 1a–1e and
Photoproducts 2a–2e in Chloroform-d1 as the Solvent
4H-Thiopyran-1,1-dioxide
Photoproducts
δ(ppm)
δ(ppm)
Compound
Me-4
1.84
1.85
MeO
3.82
3.81
H-3, H-5
6.30
Compound
Me-6
MeO
H-4
H-5
J4,5(Hz)
1a
1b
syn-2a
anti-2a
syn-2b
1.83
1.26
1.61
3.68
3.76
3.68
3.82
3.73
3.85
7.07
6.82
6.88
3.58
3.02
3.53
3.9
3.5
3.9
6.25
anti-2b
1.31
6.71
2.95
3.4
1 c
1 d
1e
6.71
6.63
6.67
2c
2d
syn-2e
anti-2e
6.91
6.96
6.92
6.90
3.96
4.10
3.94
3.98
3.8
3.4
3.5
3.5
Heteroatom Chemistry DOI 10.1002/hc

Effects of Electron-Withdrawing Group on the Photoisomerization of Tetraaryl-4H-thiopyran-1,1-dioxides 559
TABLE 2 The variations (%) of 4H-thiopyran-1,1-dioxides 1a–1e and bicyclic photoproducts 2a–2e during 480 min of irradiation
under identical experimental conditions
Time (min)
Compound
15
30
45
60
120
180
240
360
480
1a
1b
1c
1d
85.6
77.4
84.3
79.0
90.7
1.4
2.0
10.4
5.9
3.8
4.3
0.6
0.8
74.8
56.3
76.5
61.6
77.2
3.5
67.4
44.7
61.4
48.1
76.9
4.9
59.9
29.3
55.6
35.8
64.9
5.9
32.5
0.0
21.6
0.0
43.0
8.6
35.9
17.9
22.7
47.0
28.4
27.9
20.8
5.9
12.1
0.0
22.8
4.8
31.1
21.2
26.5
45.6
27.5
35.0
19.6
7.1
4.0
0.0
19.6
0.0
22.7
18.0
24.6
39.4
23.4
26.6
15.1
8.1
2.1
0.0
13.7
0.0
21.5
16.2
22.4
30.3
18.4
19.5
10.5
7.8
48.0
16.3
52.9
14.7
17.5
42.6
25.2
22.6
17.6
4.3
1e
syn-2a
anti-2a
syn-2b
anti-2b
2c
2d
syn-2e
anti-2e
4.6
5.8
7.1
16.3
10.2
7.2
8.2
1.2
22.5
13.6
10.5
9.2
1.6
2.6
31.0
19.4
13.2
13.0
2.0
1.7
3.0
6.0
8.2
10.1
12.0
10.0
formation of 1,2,4,5-tetrasubstituted cyclopenta-1,3-
dienes through SO2 extrusion. However, no signs
of SO2 extruded by-products were observable in the
model compounds 1a and 1b with electron-donating
substituents at C4 and C2,6 positions, respectively.
One can predict this on the basis of the fact that the
electron-donating substituents diminish the energy
content of the transition state in the vinyl–vinyl di-π-
methane rearrangement pathway relative to that of
phenyl–vinyl di-π-methane rearrangement. Further-
more, the inductive effect of electron-withdrawing
groups results in instability of a triplet state interme-
diate I-2 in vinyl-vinyl di-π-methane rearrangement
pathway (Scheme 2), which in turn gives the syn- and
anti-2e isomers as well as the bicyclic photoproduct
2d in lower efficiency relative to model compounds
1a–1c.
tion of antiisomer in vinyl–vinyl DPM rearrangement
should proceed by rotation around the C C bond of
the intermediate I-2 via a triplet state. The C C bond
rotation followed by bond formation produces the
most stable isomer, anti-2e, more selectively which
has been confirmed theoretically to be 0.1 kcal/mol
more stable than the syn-2e isomer.
To gain a better picture of the conformational
profile of the given compounds and to determine the
most stable conformation of the photoproducts, ab
initio calculations performed at the Hartree–Fock
level using the 6-31G(d) basis set with full geom-
etry optimization. We used the GAMESS program
in the present ab initio calculations [15]. The cal-
culated heats of formation revealed that the anti-2e
isomer is 0.1 kcal/mol more stable than the syn-2e
isomer. This result is consistent with the results of
experimental section. The full geometry optimized
structures of these compounds are presented in
Fig 1.
It should also be noted that anti-2e is formed
in higher yield than the syn isomer, which contrasts
with that of the model compound 2b. The forma-
R1
R1
S
6
R2
R3
R2
6
R2
R3
R1
R3
4
5
4
5
1
O
3 S
O
R3
R3
3
1
S
2
O
O
O
O
2
R3
R1
R2
R1
R2
R1
R2
syn-2
hv
ISC
254 nm
R3
S
R3
R3
S
R3
R3
S
R3
R2
R1
R3
R2
R1
O
O
O
O
O
O
R2
S
R1
R3
R3
1
1-S1
1-T1
O
R3
S
R3
S
O
O
O
O
O
R3
I-1
anti-2
I-2
SCHEME 2 Proposed mechanism for the photochemical rearrangement of compounds 1a–1e.
Heteroatom Chemistry DOI 10.1002/hc

560 Jafarpour, Ramezani, and Pirelahi
FIGURE 1 The full geometry optimized structures of the bicyclic photoproducts syn- and anti-2e.
CONCLUSION
4H-thiopyran-1-oxides and starting materials was
apparent by TLC, the reaction mixtures were cooled
and poured on ice (20 g). White precipitates were
collected and recrystallized from ethanol to give 1d
(85%) and 1e (90%).
In conclusion, on the contrary to electron-donating
groups at C4 position of thiopyran rings, the oppo-
site observed role of the electron-withdrawing sub-
stituents in decreasing the efficiencies of photoprod-
ucts is consistent with operation of an unfavorable
electronic effect of these groups on the intermedi-
ates leading to photoproducts. The relative ratios of
anti- and syn-bicyclic photoproducts are consistent
with their relative stability predicted by theoretical
calculations.
4,4-di(p-Trifluoromethylphenyl)-2,6-diphenyl-4H-
thiopyran-1,1-dioxide (1d). Colorless crystals, yield
1
85%; mp 220–221◦C; H NMR (500 MHz, CDCl3):
δ = 6.63 (s, H-3, H-5), 7.38–7.76 (m, 18H, Ar-H)
ppm; EI-MS (m/z, %) = 570 [M+, 12], 506 [M+–SO2,
100]; UV (EtOH): λmax (log ε) = 242 (4.6) nm; IR:
1128, 1288 (SO2) cm−1; Anal. Calcd for C31H20F6O2
S. (570.54): C, 65.26; H, 3.53; S, 5.62%. Found: C,
65.35; H, 3.48; S, 5.51%.
EXPERIMENTAL
Melting points were determined on a Gallenkamp
melting point apparatus and are uncorrected. UV ab-
sorption spectra were taken on a Shimadzu 265-FW
spectrophotometer. IR spectra were obtained on a
Shimadzu FT-IR 4300 spectrophotometer in KBr
disks. 1H NMR spectra were recorded on Bruker
Ac-80 or 500 MHz FT-NMR spectrometers relative
to internal standard tetramethylsilane. Mass spec-
tra were determined with a Finningan MAT-TSQ 70
mass spectrometer. Photolysis was performed using
a low-pressure mercury lamp with a transmission
maximum at λ = 254 nm (85% transmission) and
transmitted light from 254 to 579 nm (15% trans-
mission).
4-(p-Trifluoromethylphenyl)-2,4,6-triphenyl-4H-
thiopyran-1,1-dioxide (1e). Colorless crystals, yield
1
90%; mp 171–172◦C; H NMR (500 MHz, CDCl3):
δ = 6.67 (s, H-3, H-5), 7.32–7.75 (m, 19H, Ar-H)
ppm; EI-MS (m/z, %) = 502 [M+, 23], 438 [M+–SO2,
100]; UV (EtOH): λmax (log ε) = 233 (4.5) nm; IR:
1130, 1294 (SO2) cm−1; Anal. Calcd for C30H21F3O2
S. (502.55):C, 71.70; H, 4.21; S, 6.38%. Found: C,
71.61; H, 4.28; S, 6.45%.
General Procedure for Photolysis
3 × 10−2 M solutions of 4H-thiopyran-1,1-dioxides
1d and 1e were prepared in chloroform-d1 in NMR
tubes and irradiated with a low-pressure mercury
lamp at 254 nm under an argon atmosphere at room
temperature. The progress of the photochemical re-
Syntheses
The new 4H-thiopyran-1,1-dioxides 1d and 1e were
synthesized from the reaction of the corresponding
4H-thiopyrans (0.5 mmol) with hydrogen peroxide
30% (0.8 mL) in acetic acid (10 mL) at 60◦C. Once
the complete disappearance of transitionally formed
1
actions was followed by H NMR at different time
intervals.
Heteroatom Chemistry DOI 10.1002/hc

Effects of Electron-Withdrawing Group on the Photoisomerization of Tetraaryl-4H-thiopyran-1,1-dioxides 561
(m, 19H, Ar–H) ppm; EI-MS (m/z, %) = 502 [M+, 2],
438 [M+–SO2, 100]; UV (EtOH): λmax (log ε) = 220
Photoproducts
Irradiations in chloroform gave bicyclic photoprod-
uct 2d for 1d and a mixture of two photoprod-
ucts anti-2e and syn-2e for 1e accompanied by the
corresponding starting materials as evidenced by
1H NMR. The photoproducts were isolated by PLC
on neutral alumina with petroleum ether:diethyl
ether (35:65) as eluent and purified by recrystalliza-
tion from ethanol.
(4.1), 263 (3.1) nm; IR: 1136, 1296 (SO2) cm−1; Anal.
Calcd for C30H21F3O2 S. (502.55): C, 71.70; H, 4.21;
S, 6.38%. Found: C, 71.79; H, 4.27; S, 6.48%.
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1
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Heteroatom Chemistry DOI 10.1002/hc