Competitive photochemical pathways in the rearrangement of tetrasubstituted 4H-thiopyrans

Article abstract of DOI:10.1007/s00706-003-0142-x

The photoisomerization of 4,4-dialkyl-2,6-diphenyl-4H-thiopyrans and 4-benzyl-2,4,6-triphenyl-4H-thiopyran was investigated and compared with those of the 2,4,4,6-tetraaryl- and 4-alkyl-2,4,6-triaryl-4H-thiopyran analogues reported earlier. Obviously, the

Full text of DOI:10.1007/s00706-003-0142-x

Monatshefte f€ur Chemie 135, 973–978 (2004)
DOI 10.1007/s00706-003-0142-x
Competitive Photochemical Pathways
in the Rearrangement of Tetrasubstituted
4H-Thiopyrans
ꢀ
Seyyedmoossavi, and Hossein Daryanavardedargahani
Hooshang Pirelahi , Assieh Atarodiekashani, Seyyedmajid
Department of Chemistry, University of Tehran, P.O. Box 13145-143, Tehran, Iran
Received October 14, 2003; accepted October 17, 2003
Published online April 9, 2004 # Springer-Verlag 2004
Summary. The photoisomerization of 4,4-dialkyl-2,6-diphenyl-4H-thiopyrans and 4-benzyl-2,4,6-tri-
phenyl-4H-thiopyran was investigated and compared with those of the 2,4,4,6-tetraaryl- and 4-alkyl-
2,4,6-triaryl-4H-thiopyran analogues reported earlier. Obviously, the alkyl groups at the 4-positions of
the 4H-thiopyrans strongly diminish the efficiency of the di-ꢀ-methane rearrangement and, contrast-
ingly, a four-electron suprafacial [1,3]-sigmatropic rearrangement is found to be highly efficient.
Keywords. Di-ꢀ-methane rearrangement; Photoisomerization; [1,3]-Sigmatropic rearrangement; 2H-
Thiopyran; 4H-Thiopyran.
Introduction
The photochemistry of 4H-thiopyrans has been the subject of study in our
and other laboratories [1–4]. The previous investigations revealed that, under the
influence of ultraviolet light, 2,4,4,6-tetraaryl- or 4-alkyl-2,4,6-triaryl-4H-thio-
pyrans 1 undergo isomerization to give only aryl migrated 2H-thiopyrans 3 through
the bicyclic intermediates 2 (Scheme 1) with no alkyl group migration [2, 3].
Furthermore, in the latter work, the detailed mechanism of the photoisomerization
behavior was also discussed with respect to the aryl-vinyl bonding interactions via
di-ꢀ-methane rearrangement [5].
Considering the lack of alkyl migration in 4H-thiopyrans 1, it seemed of con-
siderable interest to study systems where the 4-substituents were both alkyl rather
than aryl or aryl-alkyl groups. Accordingly, new tetrasubstituted 4,4-dimethyl-
2,6-diphenyl-4H-thiopyran (4a), 4-tert-butyl-4-methyl-2,6-diphenyl-4H-thiopyran
(4b), and 4-benzyl-4-methyl-2,6-diphenyl-4H-thiopyran (4c) (Scheme 2) were
selected for synthesis and study.
ꢀ
Corresponding author. E-mail: pirelahi@khayam.ut.ac.ir

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H. Pirelahi et al.
Scheme 1
Scheme 2
Results and Discussion
To study the effects of 4,4-dialkyl substituents on the photoisomerization of tetra-
substituted 4H-thiopyrans, irradiation of degassed solutions of 4-alkyl-4-methyl-
2,6-diphenyl-4H-thiopyrans 4a–4c in benzene-d6 was performed in sealed pyrex or
quartz NMR tubes by an unfiltered low-pressure mercury lamp with a transmission
maximum at 254 nm under an argon atmosphere at room temperature. The reac-
1
tions were followed by H NMR, and the spectra were recorded at different time
intervals.
Irradiation through pyrex of 4a in bezene-d6 under experimental conditions iden-
tical to those used with the previously reported 2,4,4,6-tetraaryl- or 4-alkyl-2,4,6-
triaryl-4H-thiopyrans 1 [2, 4–5] failed to give any photoproduct, even on prolonged
time of 7 h, whereas under vigorous conditions irradiation through quartz of 4a in
benzene-d6 after 1 h gave 13% of the photoisomer 5a, 43% of unchanged 4a, and
44% of unidentifiable decomposition products. However, on irradiation of 4b–4c
(Scheme 3) in pyrex NMR tubes in benzene-d6 under identical experimental condi-
tions as those of 4a singlets at ꢁ ¼ 0.99 (CMe3), 1.10 (Me), and 5.79 (H-3, H-5) ppm
for 4b and 1.18 (Me), 2.73 (CH2), and 5.64 (H-3, H-5) ppm for 4c gradually
decreased, while two new sets of signals appeared at ꢁ ¼ 1.09 (s, CMe3), 1.80 (d,
Me), 5.92 (br q, H-3), and 6.09 (s, H-5) ppm for 5b and 1.77 (d, Me), 3.42 (s, CH2),
6.21 (br q, H-3), and 6.92 (s, H-5) ppm for 5c. At the end of irradiations only the

Photochemical Pathways in the Rearrangement of 4H-Thiopyrans
975
Scheme 3
signals assigned to the photoproducts 5b–5c are observable in the 1H NMR spectra
with no signs of byproducts, which indicated a selective and nearly quantitative
transformation of 4H-thiopyrans 4b–4c to 2H-thiopyrans 5b–5c. The structural dif-
ference between the photoproducts 5a–5c with those of a di-ꢀ-methane one are
pretty evident from the allylic coupling between H-3 and the CH3 groups.
In another experiment, irradiation of 4-benzyl-2,4,6-triphenyl-4H-thiopyran (6)
was performed in benzene-d6 under conditions identical to those of 4a using a
1
pyrex NMR tube. In the H NMR spectrum of 6, the singlets at ꢁ ¼ 3.19 (CH2)
and 5.96 (H-3, H-5) ppm gradually decreased, while three new sets of signals
appeared at ꢁ ¼ 3.49 (s, CH2), 6.14 (d, H-3), and 6.78 (d, H-5) ppm for 7, at
ꢁ ¼ 2.18 (s, H-6), 3.46 (s, CH2), and 5.44 (s, H-4) ppm for intermediate 8, and
at ꢁ ¼ 3.72 (s, CH2), 4.64 (s, H-2), and 6.6 (s, H-5) ppm for 9 (Scheme 4). The
signals characteristic of intermediate 8 appear in the spectrum at an early stage of
photoisomerization, then the signals of product 9 become detectable on progress of
Scheme 4

976
H. Pirelahi et al.
the reaction. At the end of irradiation only the signals assigned to photoproducts 7
and 9 are observable in a ratio of 5.5 to 1.0 with no signs of byproducts.
In conclusion, in dramatic contrast to the photoisomerization of 2,4,
4,6-tetraaryl- and 4-alkyl-2,4,6-triaryl-4H-thiopyrans 1, photolysis of the 4,4-
dialkyl-2,6-diphenyl-4H-thiopyrans 4a–4c failed to produce any di-ꢀ-methane
photorearrangement products but, instead, led to efficient four-electron suprafa-
cial [1,3]-sigmatropic isomerization. Moreover, in 4-benzyl-2,4,6-triphenyl-4H-
thiopyran (6) the benzyl-phenyl competition at the 4-position is highly in favor
of the [1,3]-sigmatropic photorearrangement product 7, which may indicate the
operation of favorable interactions of the benzyl group in the transition state that
leads to the photorearranged product.
Experimental
Melting points were measured on a Gallenkamp apparatus and are uncorrected. 1H NMR sepectra were
recorded on a Bruker AC-80MHz FT-NMR spectrometer using TMS as internal strandard. UV spectra
were taken on a Shimadzu 265-FW spectrometer. Mass spectra were determined with a Finningan
MAT-TSQ 70 mass spectrometer. Photolyses were performed using a low-pressure mercury lamp with
a transmission maximum at ꢂ ¼ 254 nm (85% transmission) and transmitted light from 254–579 nm
(15% transmission).
Syntheses
The new tetrasubstituted 4H-thiopyrans 4a–4c were synthesized by reactions of the corresponding
organomagnesium salts (RMgX; R ¼ Me, tBu, CH2Ph; X ¼ Br, Cl) with 4-methyl-2,6-diphenylthio-
pyrylium perchlorate [6] according to the reported method [7]. The products were isolated by PLC on
neutral alumina (petroleum ether 40–60ꢁC:diethyl ether ¼ 95:5) and purified by recrystallization from
ethanol. Alternatively, 4a was also synthesized by heating 3,3-dimethyl-1,5-diphenyl-1,5-pentanedione
[8] with P4S10 in xylene at a bath temperature of 135–140ꢁC [4c]. The 4H-thiopyran 6 was synthesized
by reaction of benzyl magnesium bromide with 2,4,6-triphenylthiopyrylium perchlorate according to
the method reported earlier [9].
4,4-Dimethyl-2,6-diphenyl-4H-thiopyran (4a, C19H18S)
Colorless crystals, mp 90ꢁC; MS: m=z (%) ¼ 278 (Mꢂþ, 22), 264 (M–CH2, 64), 262 (M–CH4, 100);
UV (MeOH): ꢂmax (log ") ¼ 234 (4.27), 275 (sh) nm; 1H NMR (C6D6): ꢁ ¼ 1.16 (s, 2Me), 5.70 (s, H-3,
H-5), 7.02–7.65 (m, 10H, Ph) ppm.
4-tert-Butyl-4-methyl-2,6-diphenyl-4H-thiopyran (4b, C22H24S)
Colorless crystals, mp 72–73ꢁC; MS: m=z (100) ¼ 320 (Mꢂþ, 5), 262 (M–C4H10, 100), 246 (35); UV
1
(EtOH): ꢂmax (log ") ¼ 255 (4.04), 275 (sh) nm; H NMR (C6O6): ꢁ ¼ 0.99 (s, CMe3), 1.10 (s, Me),
5.79 (s, H-3, H-5), 7.07–7.65 (m, 10H, Ph) ppm.
4-Benzyl-4-methyl-2,6-diphenyl-4H-thiopyran (4c, C25H22S)
Colorless crystals, mp 82.5–83.5ꢁC; MS: m=z (%) ¼ 354 (Mꢂþ, 19), 263 (M–CH2Ph, 100), 262
1
(M–MePh, 20); UV (EtOH): ꢂmax (log ") ¼ 237 (4.32), 275 (sh) nm; H NMR (C6D6): ꢁ ¼ 1.18
(s, Me), 2.73 (s, CH2), 5.64 (s, H-3, H-5), 7.03–7.63 (m, 15H, Ph) ppm.

Photochemical Pathways in the Rearrangement of 4H-Thiopyrans
4-Benzyl-2,4,6-triphenyl-4H-thiopyran (6, C30H24S)
977
Colorless crystals, mp 111–112ꢁC; MS: m=z (%) ¼ 416 (Mꢂþ, 17), 325 (M–CH2Ph, 100); UV (EtOH):
1
ꢂmax (log ") ¼ 234 (4.58) nm; H NMR (C6D6): ꢁ ¼ 3.19 (s, CH2), 5.96 (s, H-3, H-5), 6.84–7.53
(m, 20H, Ph) ppm.
General Procedure for Photolysis
Sample solutions were prepared by dissolving 2ꢃ10ꢄ5 mol of 4a–4c and 6 in 0.4 cm3 of benzene-d6 in
a pyrex or quartz NMR tube, and then they were degassed and sealed under Ar. Irradiations were
carried out by a low-pressure mercury lamp with a transmission maximum of ꢂ ¼ 254 nm at room
temperature. The progress of the photochemical reactions was followed by 1H NMR spectroscopy and
TLC at different time intervals. The variations of 4H-thiopyrans 4a–4c and 6 mol fractions during the
photolysis procedure were measured by integration of the characteristic H-3 and H-5 signals in the 1H
NMR spectra of the reaction mixtures.
Photoproducts
At the end of irradiations in benzene-d6 solutions, the 4H-thiopyrans 4b–4c and 6 were converted
1
selectively to their isomeric 2H-thiopyrans with over 90% yield as evidenced by H NMR spectra.
However, in transformation of 4a to 5a only about 13% of photoproduct 5a could be detected. The
photoproducts were isolated by PLC on neutral alumina with petroleum ether 40–60ꢁC as eluent and
purified by recrystallization from ethanol.
2,4-Dimethyl-2,6-diphenyl-2H-thiopyran (5a, C19H18S)
Pale yellow crystals, mp 70–71ꢁC; MS: m=z (%) ¼ 278 (Mꢂþ, 17), 263 (M–Me, 78), 262 (M–CH4,
1
100); UV (MeOH): ꢂmax (log ") ¼ 243 (4.29), 328 (4.48) nm; H NMR (CDCl3): ꢁ ¼ 1.79 (s, Me-2),
2.02 (d, J ¼ 1.45 Hz, Me-4), 5.50 (br q, J ¼ 1.45 Hz, H-3), 6.4 (s, H-5), 7.21–7.70 (m, 10H, Ph) ppm.
2-tert-Butyl-4-methyl-2,6-diphenyl-2H-thiopyran (5b, C22H24S)
Pale yellow crystals, mp 109–110ꢁC; MS: m=z (%) ¼ 320 (Mꢂþ, 7), 262 (M–C4H10, 100); UV (EtOH):
ꢂ
max (log ") ¼ 238 (3.76), 253 (4.24), 336 (3.32) nm; 1H NMR (CDCl3): ꢁ ¼ 1.07 (s, CMe3), 2.02 (d,
J ¼ 1.45Hz, Me), 5.98 (br q, J ¼ 1.45Hz, H-3), 6.24 (s, H-5), 7.10–7.70 (m, 10H, Ph) ppm.
2-Benzyl-4-methyl-2,6-diphenyl-2H-thiopyran (5c, C25H22S)
Pale yellow crystals, mp 107–108ꢁC; MS: m=z (%) ¼ 354 (Mꢂþ, 7), 263 (M–CH2Ph, 100) 262 (M–
MePh, 33); UV (EtOH): ꢂmax (log ") ¼ 250 (3.91), 265 (sh), 340 (3.73) nm; 1H NMR (CDCl3): ꢁ ¼ 2.0
(d, J ¼ 1.8 Hz, Me), 3.41 (s, CH2), 5.66 (br q, J ¼ 1.8 Hz, H-3), 6.35 (s, H-5), 6.80–7.58 (m, 15H, Ph)
ppm.
2-Benzyl-2,4,6-triphenyl-2H-thiopyran (7, C30H24S)
Pale yellow crystals, mp 156.5–157.5ꢁC; MS: m=z ¼ 416 (Mꢂþ, 3), 325 (M–CH2Ph, 100), 248
(M–CH2Ph–Ph, 53); UV (EtOH): ꢂmax (log ") ¼ 259 (4.34), 348 (3.74) nm; 1H NMR (CDCl3):
ꢁ ¼ 3.53 (s, CH2), 6.09 (d, J ¼ 1 Hz, H-3), 6.81 (d, J ¼ 1 Hz, H-5), 7.10–7.59 (m, 20H, Ph) ppm.
The structures of photoproducts 5a–5c and 7 were also confirmed by authentic syntheses [7a].
Attempts failed to isolate photoproduct 9 from the reaction mixture.

978
H. Pirelahi et al.: Photochemical Pathways in the Rearrangement of 4H-Thiopyrans
Acknowledgements
The support of this investigation by the Research Council at the University of Tehran through Grant
No. 559=1=514 is gratefully acknowledged.
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