Photoisomerization of 2,4,4,6-tetraaryl-4H-selenopyrans: A new heterocyclic ring contraction

Article abstract of DOI:10.1070/MC2001v011n03ABEH001430

The UV illumination of title compounds 3a,b in acetonitrile solutions leads to corresponding five-membered ring isomers 4a,b, probably, via open-ring intermediates, whereas the photocolouration was observed in the solid state.

Full text of DOI:10.1070/MC2001v011n03ABEH001430

, 2001, 11(3), 90–91
Photoisomerization of 2,4,4,6-tetraaryl-4H-selenopyrans:
a new heterocyclic ring contraction
Jirí Kroulík,*a Jan Cejka,b Pavel Šebek,a Petr Sedmera,c Petr Halada,c Vladimír Havlícek,c Stanislav Nešpurek,d
Bohumil Kratochvílb and Josef Kuthana
a Department of Organic Chemistry, Prague Institute of Chemical Technology, 166 28 Prague 6, Czech Republic.
Fax: +42 022 431 1082; e-mail: Jiri.Kroulik@vscht.cz
b Department of Solid State Chemistry, Prague Institute of Chemical Technology, 166 28 Prague 6, Czech Republic
c Institute of Microbiology, Academy of Sciences of the Czech Republic, 141 20 Prague 4, Czech Republic. Fax: +42 022 475 2749
d Institute of Macromolecular Chemistry, Academy of Sciences of the Czech Republic, 162 06 Prague 6, Czech Republic.
Fax: +42 022 251 6969
10.1070/MC2001v011n03ABEH001430
The UV illumination of title compounds 3a,b in acetonitrile solutions leads to corresponding five-membered ring isomers 4a,b,
probably, via open-ring intermediates, whereas the photocolouration was observed in the solid state.
The photochromism has recently highlighted a considerable ap-
plication potential for data storage technologies.1 The increas-
ing activity focused on the development and features of new
compounds with photochromic properties1 prompted us to inves-
tigate possible consequences of the integration of selenium with
a favourable photochromic system. 2,4,4,6-Tetraaryl-4H-thio-
pyrans 1 are known to exhibit UV photocolouration2 followed
by multi-step transformations to final 2,3,4,6-tetraaryl-2H-thio-
pyrans 2.2(c),(d) Contrary, the properties and photochemistry of
analogous 2,4,4,6-tetraaryl-4H-selenopyrans 3 remain almost
unknown.3 Hitherto reports are mostly intent on similar 2,4,6-
triphenyl-4H-selenopyrans.3 Of the considered 2,4,4,6-tetraphenyl
derivatives, only parent 2,4,4,6-tetraphenyl-4H-selenopyran 3a,
prepared by the reaction of a 2,4,6-triphenylselenopyrylium salt
with phenylmagnesium bromide, and its reaction with bromine
have been reported.4 Hence, here we report that the replacement
of the sulfur 1-heteroatom by selenium dramatically changes the
photoisomerization. Thus, acetonitrile solutions of 2,4,4,6-tetra-
phenyl-4H-selenopyran 3a or 2,6-bis(4-fluorophenyl)-4,4-diphenyl-
4H-selenopyran 3b, prepared by the cyclocondensations of cor-
responding 1,5-diones (ArCOCH2)2CPh2 with the H2Se–HCl
reagent, have been directly irradiated with a high-pressure 125 W
mercury UV lamp in a quartz photoreactor at 12 °C for 1 h
under argon† to afford approximately 2:3 equilibrium mixtures
of (E)-3,3,5-triphenyl-2-(phenylmethylidene)-2,3-dihydroseleno-
phene 4a or (E)-5-(4-fluorophenyl)-2-[(4-fluorophenyl)methyli-
dene]-3,3-diphenyl-2,3-dihydroselenophene 4b and starting 4H-
selenopyran 3a or 3b. Prolonged UV exposures lead to irre-
versible degradation of photoisomers 3a,b and 4a,b to complex
mixtures of unidentified compounds. In the solid state, a photo-
colouration was observed. A sample of the polycrystalline powder
of 3a in MgO showed a green photocolouration after irradiation
(300 s) with a high-pressure 200 W mercury discharge lamp.
The correct structures of 3a,b of the starting 4H-selenopyrans
photoproduct from difluoro derivative 3b was analysed by X-ray
diffraction,‡ which confirmed the structure of 4b (Figure 1).
Then, the analogous structure of 4a can be assigned to the
photoisomer of 3a by comparison of the corresponding spectral
data.§
The results indicate that the photochemically induced isomeri-
zation 3 ® 4 in the 4H-selenopyran series surprisingly differs
from the isomerization 1 ® 2 in the 4H-thiopyran series and
probably proceeds via a labile acetylenic intermediate like
PhCºC–CPh2–CH=C(SeH)Ph, which may undergo two parallel
intramolecular ring-closures to either 2,3-dihydroselenophenes
4 or to 4H-selenopyrans 3.
For the sake of completeness, the UV–VIS absorption spec-
trum of an acetonitrile solution consists of a main maximum
at 238 (log e = 3.73 dm3 mol–1 cm–1) or 273 nm (log e =
†
The photochemical reactions were monitored by HPLC with an UV
detector (254 nm) on Separon SGX C18 (Tessek, Czech Republic), par-
ticle size of 5 µm, eluent MeOH. The photoisomer mixtures were sepa-
rated by preparative TLC. NMR spectra were measured on a Varian
VXR-400 or INOVA-400 (399.90 and 100.57 MHz for 1H and 13C,
respectively) instrument in CDCl3 solutions at 25 to 30 °C. The assign-
ments were based on COSY, HOM2DJ, HMQC, HMBC, 1D-TOCSY,
and 1D-NOE experiments; the methine (exocyclic) carbon of compounds
4 is indicated with the number 6; i- ipso, o- ortho, m- meta, p- para.
In fluorine-containing compounds, capital letters denote multiplicity due
to protons, lowercase letters are used for fluorine-related multiplicity.
Positive-ion mass spectra were recorded on a Finnigan MAT 95 double-
focusing instrument of BE geometry equipped with an EI ion source
[ionization energy of 70 eV, source temperature of 250 °C, emission cur-
rent of 0.5 mA, accelerating voltage of 5 kV, direct inlet (150–160 °C)].
For high resolution experiments, the instrument was tuned to a resolution
of about 8000 (10% valley definition), and the measurements were carried
out by the peak-matching method against the Ultramark 1600F (PCR
Inc. Gainesville, USA) as an internal standard.
1
For 3a: mp 138–140 °C (lit.,4 mp 138–139 °C). H NMR (CDCl3) d:
1
follow from their H and 13C NMR and EI mass spectra.† On
6.453 (s, 2H, 3,5-H), 7.240 (m, 4H, m-2,6-Ph), 7.293–7.380 (m, 12H,
aromatic), 7.560 (m, 4H, o-2,6-Ph). 13C NMR (CDCl3) d: 56.11 (s, 1C,
4-C), 126.37 (d, 2CH, p-4,4-Ph), 126.87 (d, 4CH, o-2,6-Ph), 127.49 (d,
2CH, 3,5-CH), 128.24 (d, 4CH, m-4,4-Ph), 128.36 (d, 4CH, o-4,4-Ph),
128.41 (d, 2CH, p-2,6-Ph), 128.61 (d, 4CH, m-2,6-Ph), 130.71 (s, 2C,
i-2,6-Ph), 139.74 (s, 2C, i-4,4-Ph), 147.56 (s, 2C, 2,6-C). MS, m/z (%):
450.0884 (100, M+, C29H22Se), 373.0495 (85), 291.1174 (56), 267.1174
(19), 215.0861 (25), 191.0861 (16), 165.0704 (22).
the other hand, the molecular structures of their photoisomers
could not be positively derived in the same way. Therefore, the
Ar2
Ar2 Ar 2
Ar1
H
Ar2
1
Ar
Ar1
Ar1
S
1
S
For 3b: mp 113–115 °C. H NMR (CDCl3) d: 6.368 (s, 2H, 3,5-H),
7.041 (m, 4H, o-2,6-C6H4F, 2×0.5 of AA'BB'X spectrum, JHF 8.5 Hz,
1
2
J 8.9 Hz), 7.242 (m, 2H, p-4,4-Ph), 7.285 (m, 4H, o-4,4-Ph), 7.338
Σ
(m, 4H, m-4,4-Ph), 7.506 (m, 4H, m-2,6-C6H4F, 2×0.5 of AA'BB'X
Ph Ph
spectrum, JHF 5.3 Hz, J 8.9 Hz). 13C NMR (CDCl3) d: 56.17 (S, 1C,
Ph
Σ
4
Ph
4
4-C), 115.56 (Dd, 4CH, m-2,6-C6H4F, JCF 21.7 Hz), 126.51 (D, 2CH,
p-4,4-Ph), 127.68 (D, 2CH, 3,5-CH), 128.18 (D, 4CH, m-4,4-Ph), 128.44
(D, 4CH, o-4,4-Ph), 128.63 (Dd, 4CH, o-2,6-C6H4F, JCF 8.2 Hz), 129.69
(S, 2C, i-4,4-Ph), 135.78 (Sd, 2C, i-2,6-C6H4F, JCF 3.2 Hz), 147.48 (S,
2C, 2,6-C), 162.85 (Sd, 2CF, p-2,6-C6H4F, JCF 248.2 Hz). MS, m/z (%):
486.0692 (100, M+, C29H20F2Se), 409 (78), 391 (5), 327 (27), 309 (25),
285 (22), 251 (7), 233 (15), 209 (13), 165 (17).
5
3
2
3
Ar
5
6
1
1
Se
2
Ar
Ar
Se Ar
3
4
a Ar = Ph
b Ar = 4-FC6H4
– 90–

, 2001, 11(3), 90–91
unexplored area.6 Laser-induced photolysis of selenophene seems
to be a formally similar process.7 Note that a topologically analo-
gous isomerization has been only observed8 after lithiation of
2,6-di-tert-butylseleno-4-pyron, where the acetylenic interme-
diate ButCºC–CO–CH=C(SeMe)But was evidently trapped
with methyl triflate. The selenopyran derivatives and their reac-
tivity, including the solid-state UV photocolouration of 3-like
4H-selenopyrans, will be considered in detail elsewhere.
C(17)
C(18)
C(21)
C(16)
C(22)
C(23)
C(20)
F(1)
C(13)
C(19)
C(15)
C(14)
C(4)
C(26)
C(11)
C(10)
C(3)
C(12)
C(24)
C(27)
C(5)
C(9)
C(8)
C(7)
C(2)
C(6)
F(2)
C(28)
C(29)
C(25)
C(30)
References
1 Chem. Rev., Photochromism: Memories and Switches, ed. M. Irie, 2000,
100 (5).
Se(1)
2 (a) Y. Mori and K. Maeda, J. Chem. Soc., Perkin Trans. 2, 1991, 2061;
(b) H. Pirelahi, I. Parchamazad, M. S. Abaii and S. Sheikhebrahimi,
Phosphorus Sulfur Silicon, 1991, 59, 251; (c) P. Šebek, S. Nešpurek,
R. Hrabal, M. Adamec and J. Kuthan, J. Chem. Soc., Perkin Trans. 2,
1992, 1301; (d) S. Böhm, P. Šebek, S. Nešpurek and J. Kuthan, Collect.
Czech. Chem. Commun., 1994, 59, 1115; (e) J. Kroulík, M. Chadim,
M. Polášek, S. Nešpurek and J. Kuthan, Collect. Czech. Chem. Commun.,
1998, 63, 662.
3 J. Kuthan, P. Šebek and S. Böhm, Adv. Heterocycl. Chem., 1994, 59, 179.
4 B. I. Drevko, M. I. Smushkin and V. G. Kharchenko, Khim. Geterotsikl.
Soedin., 1997, 604 [Chem. Heterocycl. Compd. (Engl. Transl.), 1997, 33,
520].
5 L. E. E. Christiaens, in Comprehensive Heterocyclic Chemistry, ed.
A. McKillop, Pergamon, Oxford, 1996, vol. 5, ch. 5.11, p. 619.
6 A. A. Leone and P. S. Mariano, Rev. Chem. Intermed., 1981, 4, 81.
7 J. Pola and A. Ouchi, J. Org. Chem., 2000, 65, 2759.
8 M. R. Detty and L. W. McGarry, J. Org. Chem., 1988, 53, 1203.
9 (a) G. M. Sheldrick, SHELXS-86, Program for Crystal Structure
Solution, University of Göttingen, Göttingen, Germany, 1986; (b) D. J.
Watkin, R. J. Carruthers and P. Betteridge, CRYSTALS, Chemical Crys-
tallography Laboratory, Oxford, UK, 1998, issue 10; (c) J. R. Carruthers
and D. J. Watkin, Acta Crystallogr., Sect. A, 1979, 35, 698; (d) L. Zsolnai
and G. Huttner, XPMA, ZORTEP, University of Heidelberg, 1994.
Figure 1 Molecular structure of compound 4b. Selected bond lengths (Å):
Se(1)–C(2) 1.920(3), Se(1)–C(5) 1.907(3), C(2)–C(6) 1.331(4), C(2)–C(3)
1.531(3), C(3)–C(4) 1.519(3), C(4)–C(5) 1.329(4); selected bond angles (°):
C(2)–Se(1)–C(5) 87.43(9), Se(1)–C(2)–C(3) 110.8(2), C(2)–C(3)–C(4)
105.7(2), C(3)–C(4)–C(5) 120.2(2), Se(1)–C(5)–C(4) 112.1(2), Se(1)–
C(2)–C(6) 119.2(2); selected torsion angles (°): C(2)–Se(1)–C(5)–C(4)
7.8(2), C(5)–Se(1)–C(2)–C(3) –16.4(2), Se(1)–C(2)–C(3)–C(4) 20.1(2),
Se(1)–C(2)–C(6)–C(7) –179.9(2).
= 3.79 dm3 mol–1 cm–1) and a shoulder at about 287 or 283 nm
for compound 3a or 3b, respectively. A similarity between the
UV–VIS spectra of selenopyrans and appropriate derivatives of
thiopyrans2(c) is evident, and it can be attributed to identical
quantal transitions.
To our knowledge, the described conversion 3 ® 4 is a unique
example of photochemical ring conversion among six-membered
selenium heterocycles5 and, contrary to other 2,4,4,6-tetraaryl-
4H-(hetero)pyrans,2 no photochemical di-π-methane rearrange-
ment of one of the 4,4-phenyl groups has been observed. The
photochemistry of such di-π-selenide systems belongs to an
‡
Crystal data for 4b: C29H20F2Se, M = 485.43, monoclinic, space
group P21/c, a = 11.495(2) Å, b = 11.909(4) Å, c = 16.353(1) Å, b =
90.50(1)°, V = 2238.7 Å3, Z = 4, dcalc = 1.44 g cm–3, F(000) = 982.88,
m = 2.52 mm–1. 8525 reflections measured with an Enraf Nonius CAD4
diffractometer (293 K, graphite-monochromated CuKα radiation, l =
= 1.54184 Å, w/2q scan mode, 2q range of 5–134°). The structure was
solved by direct methods and anisotropically refined by full-matrix least
squares.9 Hydrogen atoms were located from a ∆–r map, positions and
isotropical thermal motion were refined. The final agreement factors are
R = 4.11% and Rw = 4.11%. Atomic coordinates, bond lengths, bond
angles and thermal parameters have been deposited at the Cambridge
Crystallographic Data Centre (CCDC). For details, see ‘Notice to
Authors’, Mendeleev Commun., Issue 1, 2001. Any request to the CCDC
for data should quote the full literature citation and the reference number
1135/85.
§
1
For 4a: mp 150–152 °C, preparative yield 11%. H NMR (CDCl3) d:
6.588 (s, 1H, 6-H), 6.701 (s, 1H, 4-H), 7.218–7.368 (m, 14H, aromatic),
7.382 (m, 4H, o-3,3-Ph), 7.480 (m, 2H, o-5-Ph). 13C NMR (CDCl3) d:
74.73 (s, 1C, 3-C), 126.83 (d, 2CH, o-5-Ph), 126.92 (d, 2CH, o-6-Ph),
127.08 (d, CH, p-6-Ph), 127.89 (d, 2CH, p-3,3-Ph), 128.32 (d, 4CH,
m-3,3-Ph), 128.45 (d, 2CH, m-5-Ph), 128.49 (d, 1CH, p-5-Ph), 128.53
(d, 4CH, o-3,3-Ph), 128.60 (d, 2CH, m-6-Ph), 129.85 (d, 1CH, 6-CH),
129.91 (d, 1CH, 4-CH), 135.11 (s, 1C, i-5-Ph), 136.22 (s, 1C, 5-C),
137.59 (s, 1C, i-6-Ph), 144.42 (s, 1C, 2-C), 145.26 (s, 2C, i-3,3-Ph).
MS, m/z (%): 450.0885 (100, M+, C29H22Se), 373 (47), 291 (52), 278
(9), 215 (35), 191 (23), 165 (11).
1
For 4b: mp 159–161 °C, preparative yield 34%. H NMR (CDCl3) d:
6.535 (s, 1H, 6-H), 6.619 (s, 1H, 4-H), 7.034 (m, 2H, m-5-C6H4F, 0.5 of
AA'BB'X spectrum, JHF 8.4 Hz, J 8.9 Hz), 7.060 (m, 2H, m-6-C H F,
Σ
6
4
0.5 of AA'BB'X spectrum, JHF 8.6 Hz, J 8.7 Hz), 7.265 (m, 2H, o-6-
Σ
C6H4F, half of AA'BB'X spectrum, JHF 5.3 Hz, J 8.7 Hz), 7.290 (m,
2H, p-3,3-Ph), 7.337–7.367 (m, 8H, o-3,3-Ph and m-3,3-Ph), 7.442 (m,
2H, o-5-C6H4F, 0.5 of AA'BB'X spectrum, JHF 5.2 Hz, J 8.9 Hz). 13
Σ
C
Σ
NMR (CDCl3) d: 74.66 (S, 1C, 3-C), 115.41 (Dd, 2CH, m-5-C6H4F,
JCF 21.5 Hz), 115.59 (Dd, 2CH, m-6-C6H4F, JCF 22.0 Hz), 126.94 (D,
2CH, p-3,3-Ph), 128.38 (D, 4CH, m-3,3-Ph), 128.48 (D, 4CH, o-3,3-
Ph), 128.59 (Dd, 2CH, o-5-C6H4F, JCF 8.3 Hz), 128.87 (Dd, 1CH, 6-CH,
JCF 1.5 Hz), 129.50 (Dd, 2CH, o-6-C6H4F, JCF 8.3 Hz), 129.93 (Dd,
1CH, 4-CH, JCF 1.5 Hz), 131.29 (Sd, 1C, i-5-C6H4F, JCF 2.9 Hz), 133.77
(Sd, 1C, i-6-C6H4F, JCF 2.9 Hz), 134.89 (S, 1C, 5-C), 143.31 (S, 1C, 2-C),
145.05 (S, 2C, i-3,3-Ph), 161.73 (Sd, 1CF, p-6-C6H4F, JCF 247.6 Hz),
162.77 (Sd, 1CF, p-5-C6H4F, JCF 249.0 Hz). MS, m/z (%): 486.0698
(100, M+, C29H20F2Se), 409 (37), 405 (58), 391 (11), 327 (29), 309 (29),
285 (9), 233 (32), 209 (24), 183 (13), 165 (6).
Received: 25th January 2001; Com. 01/1756
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