Mendeleev Communications Electronic Version, Issue 3, 2001 (pp. 85–124) Photoisomerization of 2,4,4,6-tetraaryl-4H-selenopyrans: a new heterocyclic ring contraction Ji í Kroulík,*a Jan ejka,b Pavel Šebek,a Petr Sedmera,c Petr Halada,c Vladimír Havlí ek,c Stanislav Nešp rek,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 application potential for data storage technologies.1 The increasing activity focused on the development and features of new compounds with photochromic properties1 prompted us to investigate possible consequences of the integration of selenium with a favourable photochromic system. 2,4,4,6-Tetraaryl-4H-thiopyrans 1 are known to exhibit UV photocolouration2 followed by multi-step transformations to final 2,3,4,6-tetraaryl-2H-thiopyrans 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-tetraphenyl- 4H-selenopyran 3a or 2,6-bis(4-fluorophenyl)-4,4-diphenyl- 4H-selenopyran 3b, prepared by the cyclocondensations of corresponding 1,5-diones (ArCOCH2)2CPh2 with the H2Se–HCl reagent, have been directly irradiated with a high-pressure 125W 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-dihydroselenophene 4a or (E)-5-(4-fluorophenyl)-2-[(4-fluorophenyl)methylidene]- 3,3-diphenyl-2,3-dihydroselenophene 4b and starting 4Hselenopyran 3a or 3b.Prolonged UV exposures lead to irreversible degradation of photoisomers 3a,b and 4a,b to complex mixtures of unidentified compounds. In the solid state, a photocolouration 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 follow from their 1H and 13C NMR and EI mass spectra.† On the other hand, the molecular structures of their photoisomers could not be positively derived in the same way. Therefore, the 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 isomerization 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 spectrum 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 = r C c u S Ar Ar Ar Ar Se Ar Ar Ph Ph 1 S Ar Ar 2 3 Ar Ar H Se Ar Ph Ph Ar 4 a Ar = Ph b Ar = 4-FC6H4 1 2 3 4 5 6 1 2 2 2 1 1 2 1 2 3 4 5 1 † The photochemical reactions were monitored by HPLC with an UV detector (254 nm) on Separon SGX C18 (Tessek, Czech Republic), particle size of 5 µm, eluent MeOH. The photoisomer mixtures were separated 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 assignments 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 doublefocusing instrument of BE geometry equipped with an EI ion source [ionization energy of 70 eV, source temperature of 250 °C, emission current 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. For 3a: mp 138–140 °C (lit.,4 mp 138–139 °C). 1H NMR (CDCl3) d: 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).For 3b: mp 113–115 °C. 1H 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, S J 8.9 Hz), 7.242 (m, 2H, p-4,4-Ph), 7.285 (m, 4H, o-4,4-Ph), 7.338 (m, 4H, m-4,4-Ph), 7.506 (m, 4H, m-2,6-C6H4F, 2×0.5 of AA'BB'X spectrum, JHF 5.3 Hz, S J 8.9 Hz). 13C NMR (CDCl3) d: 56.17 (S, 1C, 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).Mendeleev Communications Electronic Version, Issue 3, 2001 (pp. 85.124) = 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 ¢ç 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-¥�-methane rearrangement of one of the 4,4-phenyl groups has been observed.The photochemistry of such di-¥�-selenide systems belongs to an unexplored area.6 Laser-induced photolysis of selenophene seems to be a formally similar process.7 Note that a topologically analogous isomerization has been only observed8 after lithiation of 2,6-di-tert-butyeno-4-pyron, where the acetylenic intermediate ButC��C.CO.CH=C(SeMe)But was evidently trapped with methyl triflate.The selenopyran derivatives and their reactivity, including the solid-state UV photocolouration of 3-like 4H-selenopyrans, will be considered in detail elsewhere. References 1 Chem. Rev., Photochromism: Memories and Switches, ed. M. Irie, 2000, 100 (5). 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. .ebek, S. Ne.p rek, R. Hrabal, M. Adamec and J. Kuthan, J. Chem. Soc., Perkin Trans. 2, 1992, 1301; (d) S. Bohm, P. .ebek, S. Ne.p rek and J. Kuthan, Collect. Czech. Chem. Commun., 1994, 59, 1115; (e) J. Kroulik, M. Chadim, M. Pola.ek, S. Ne.p rek and J. Kuthan, Collect. Czech. Chem. Commun., 1998, 63, 662. 3 J.Kuthan, P. .ebek and S. Bohm, 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 Gottingen, Gottingen, Germany, 1986; (b) D. J. Watkin, R. J. Carruthers and P. Betteridge, CRYSTALS, Chemical Crystallography 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. ¢Ô Crystal data for 4b: C29H20F2Se, M = 485.43, monoclinic, space group P21/c, a = 11.495(2) A, b = 11.909(4) A, c = 16.353(1) A, b = 90.50(1)¡Æ, V = 2238.7 A3, Z = 4, dcalc = 1.44 g cm.3, F(000) = 982.88, m = 2.52mm.1. 8525 reflections measured with an Enraf Nonius CAD4 diffractometer (293 K, graphite-monochromated CuK¥á radiation, l = = 1.54184 A, w/2q scan mode, 2q range of 5.134¡Æ). 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.¡× For 4a: mp 150.152 ¡ÆC, preparative yield 11%. 1H 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). For 4b: mp 159.161 ¡ÆC, preparative yield 34%. 1H 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, ¥Ò J 8.9 Hz), 7.060 (m, 2H, m-6-C6H4F, 0.5 of AA'BB'X spectrum, JHF 8.6 Hz, ¥Ò J 8.7 Hz), 7.265 (m, 2H, o-6- C6H4F, half of AA'BB'X spectrum, JHF 5.3 Hz, ¥Ò 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, ¥Ò J 8.9 Hz). 13C 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). F(2) C(28) C(27) C(29) C(30) C(25) C(26) C(5) Se(1) C(2) C(6) C(7) C(8) C(9) C(10) F(1) C(11) C(12) C(3) C(19) C(20) C(21) C(22) C(23) C(24) C(4) C(14) C(13) C(15) C(16) C(17) C(18) Figure 1 Molecular structure of compound 4b. Selected bond lengths (A): 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 (¡Æ): 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 (¡Æ): 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). u u u Received: 25th January 2001; Com. 0