Conversion of Phenyl-Substituted Cyclopentadienes to Pyrylium Cations Gui Ling Ning,*,† Xin Cheng Li,† Megumu Munakata,‡ Wei Tao Gong,† Masahiko Maekawa,‡ and Tadao Kamikawa‡ School of Chemical Engineering, Dalian University of Technology, Dalian 116012, P. R. China, and Department of Chemistry, Kinki University, Higashi-Osaka 577-8502, Japan
[email protected] Received July 28, 2003
Abstract: Phenyl-substituted cyclopentadienes are proved to form phenylated pyrylium cations in the presence of silver(I) perchlorate by insertion of an oxygen atom into the cyclopentadiene-ring. Three phenylated pyrylium compounds, [(Ph5C5O+)(ClO4-)]2(CH2Cl2) (1), Ag(ClO4)(H2O)(Ph4HC5O+) (ClO4-) (2), and (Ph3H2C5O+)(ClO4-) (3) have been synthesized and characterized. A possible reaction pathway and formation mechanism of the pyrylium cation are proposed and discussed.
Pyrylium salts have opened up broad prospects in practical applications1-3 such as Q-switchers, laser dyes, organic luminophores, secondary nonaqueous-electrolyle batteries, and the like.1,4 Because the peculiarities in their photophysical and electrochemical properties are attributable to the substitution patterns of the pyrylium ring,1,4-6 a large number of differently substituted pyrylium salts have been synthesized and studied.4-8 Very often, pyrylium cations are obtained by a lengthy stepwise manner hinging on the formation and cyclization of a 1,5-diketone.7 In contrast, the preparation from a ring oxidation is a new approach. In our continuing studies on the silver(I) complexes with polycyclic aromatic * To whom correspondence should be addressed. † Dalian University of Technology. ‡ Kinki University. (1) (a) Tamamura, T.; Yokoyama, M.; Kusabyashi, S.; Mikawa, H. Bull. Chem. Soc. Jpn. 1974, 47, 442. (b) Miranda, M. A.; Garcia, H. Chem. Rev. 1994, 1063. (c) Akihiko, K.; Toru, S. JP 2001223022. (d) Czerney, P.; Grummt, U.-W. J. Chem. Res., Synop. 1996, 173. (2) (a) Tully, W.; Main, L.; Nicholson, B. K. J. Org. Chem. 1996, 61, 103. (b) Katritzky, A. R.; Czerney, P.; Levell, J. R. J. Org. Chem. 1997, 62, 8198. (3) (a) Jayanthi, S. S.; Ramamurthy, P. Phys. Chem. A 1998, 102, 511. (b) Carvalho, M.; Gozzo, F. C.; Mendes, M. A.; Sparrapan, R.; Kascheres, C.; Eberlin, M. N. Chem.sEur. J. 1998, 4, 1161. (4) (a) Nikolov, P.; Metzov, S. J. Photochem. Photobiol. A: Chem. 2000, 135, 13. (b) Tripathi, S.; Simalty, M.; Kossanyi, J. Tetrahedron Lett. 1985, 26, 1995. (5) (a) Lampre, I.; Markovitsi, D.; Fiorini, C.; Charra, F.; Veber, M. J. Phys. Chem. 1996, 100, 10701. (b) Lampre, I.; Markovitsi, D.; Millie, P. J. Phys. Chem. 1997, 101, 90 (6) Vollmer, F.; Rettig, W.; Birckner, E.; Haucke, G.; Czemey, P. J. Inf. Rec. Mater. 1994, 21, 497. (7) (a) Staunton, J. In Comprehensive Organic Chemistry; Sammes, P. J., Ed.; Pergamon Press: Elmsford, NY, 1979; Vol. 4, pp 607-627. (b) Kharchenko, V. G.; Pchelintseve, N. V.; Markova, L. L.; Fedotova, O. V. Chemistry of Heterocyclic Compounds 2000, 36, 1007. (8) (a) Caro, B.; Robin-Le Guen, F.; Salmain, M.; Jaouen, G. Tetrahedron 2000, 56, 257. (b) Caro, B.; Robin-Le Guen, F.; SenechalTocquer, M.-C.; Prat, V.; Vaissermann, J. J. Organomet. Chem. 1997, 543, 87.
hydrocarbons,9 we found, unexpectedly, that the reaction of tetraphenyl cyclopentadiene (Ph4H2C5) with silver(I) perchlorate in a mixed solvent gave the phenylated pyrylium salt, Ag(ClO4)(H2O)(Ph4HC5O+) (ClO4-) (2), rather than the normal coordination complex. This motivated us to investigate the generality of this reaction. In our study, pentaphenylcyclopentadiene (Ph5HC5) and triphenylcyclopentadiene (Ph3H3C5) have been selected as precursor ligands, and their pyrylium salts, [(Ph5C5O+)(ClO4-)]2(CH2Cl2) (1) and (Ph3H2C5O+)(ClO4-) (3), were generated and isolated. We report here the synthesis, structural characterizations, and formation mechanism of these compounds. Treatment of (Ph5HC5), (Ph4H2C5), or (Ph3H3C5) with AgClO4‚H2O in a mixed solvent of dichloromethane/ toluene gave 1, 2, or 3, respectively, and a gray-white precipitate of silver metal (Scheme 1). All three compounds are soluble in common polar organic solvents such as CH3OH or CH2Cl2. They are stable in the air, although compound 2 exhibits slight moisture-sensitivity. Compounds 1 and 3 show up as green needle single crystals, whereas 2 gives its crystals in a brown column appearance. These compounds underwent the characterizations without further purification. The molecular structures of 1-3 determined by single-crystal X-ray analysis11 are illustrated in Figure 1. The crystallographic studies revealed that the original central five-membered ring of each precursor ligand was converted to a six-membered ring by insertion of an oxygen atom. The C-C and C-O bond lengths together with the bond angles in the central rings are within the limits of those in the reported pyrylium cations.2a,12 In particular, compound 1 contains two crystallographically independent pyrylium cations between which one perchlorate anion is sandwiched, while 2 involves one three-coordinate Ag(I) ion (taking the CdC group as one ligand)9,10 interacting with two adjacent phenyl rings and one water molecule at normal bond lengths. Characterization of the compounds was primarily based on X-ray diffraction studies. Elemental analysis and spectroscopic studies of these compounds further confirmed the formation of pyrylium cations and the reactions in Scheme 1. Taking compound 2 as an example, the IR spectrum showed strong absorptions at 1610 and 1112 cm-1, suggesting the presence of a (9) (a) Ning, G. L.; Munakata, M.; Wu, L. P.; Maekawa, M.; Suenaga, Y.; Kuroda-Sowa, T.; Sugimoto, K. Inorg. Chem. 1999, 38, 5669. (b) Ning, G. L.; Munakata, M.; Wu, L. P.; Maekawa, M.; Suenaga, Y.; Kuroda-Sowa, T.; Suenaga, Y.; Sugimoto, K. Inorg. Chem. 1999, 38, 1376. (10) (a) Munakata, M.; Ning, G. L.; Suenaga, Y.; Kuroda-Sowa, T.; Maekawa, M.; Ohta, T. Angew. Chem., Int. Ed. 2000, 39, 4555. (b) Ning, G. L.; Wu, L. P.; Sugimoto, K.; Munakata, M.; Kuroda-Sowa, T.; Suenaga, Y.; Maekawa, M. J. Chem. Soc., Dalton Trans. 1999, 2529. (11) For details data, see Supporting Information. Selected crystal data for 1: orthorhombic, P212121, a ) 12.948(2), b ) 17.7599(4), c ) 28. 2209(3) Å, V ) 6489.7(5) Å3, Z ) 4, R1 ) 0.087, Rw ) 0.108. For 2: monoclinic, P21/n, a ) 12.030(3), b ) 13.764(4), c ) 17.662(6) Å, V ) 2919(1) Å3, β ) 93.25(3)°, Z ) 4, R1 ) 0.063, Rw ) 0.085. For 3: monoclinic, P21/n, a ) 6.9904(10), b ) 14.747(2), c ) 18. 830(3)Å, V ) 1917.9(5) Å3, β ) 98.874(5)°, Z ) 4, R1 ) 0.0470, wR ) 0.1278. (12) (a) Schroth, W.; Spitzner, R. Tetrahedron Lett. 1985, 26, 3963. (c) Caro, B.; Se´ne´chal-Tocquer, M. C.; Se´ne´chal, D.; Marrec, P. Tetrahedron Lett. 1993, 34, 7259. 10.1021/jo030243k CCC: $27.50 © 2004 American Chemical Society
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Published on Web 01/23/2004
FIGURE 1. Crystal structure with labeling of pyrylium cations synthesized according to Scheme 1: (a) one of the two independent pyrylium cations in 1; (b) 2; (c) 3. (ClO4- and CH2Cl2 omitted.)
SCHEME 1
pyrylium ring skeleton4a,14 and a ClO4- anion,9,10 respectively. The mass spectrum proved the existence of the intact pyrylium cation of Ph4HC5O+ at m/z 385. The 1H NMR spectrum showed one strong signal of the pyryliumring proton at 9.89 ppm,7a,13,14 which did not exist in that of its precursor ligand. The chemical shifts of the pyrylium ring in the 13C NMR spectrum were 209.97, 206.20, 173.61, 170.55, and 164.03 ppm. For compounds 1 and 3, similar information on their pyrylium cations was recognized and listed in the Experimental Section. Apart from the identification of these pyrylium salts, the precipitate of the gray-white powder was isolated and proved to be silver metal by XRD. It was also found that a protic acid was generated during the reaction, which was detected in a water-extracted aqueous solution using a pH meter by extracting the acid into water. The formation of protic acid and the isolation of silver metal precipitate during the synthesis of 3 have been reported in our previous communication.15 Although the synthetic chemistry of pyrylium salts has been well established in the literature, the reactions outlined in Scheme 1 are significant. Extensive Chemical Abstracts Service (CAS) and Cambridge Structural Database searches showed that the synthesis of most pyrylium cations started from the open-chain organics,7 (13) Balaban, A. T.; Toma, C. Tetrahedron 1966, Supplement 7, 1. (14) Balaban, A. T.; Dinculescu, A.; Dorofeenko, G. N.; Fischer, G. W.; Koblik, A. V.; Mezheritskii, V. V.; Schroth, W. Adv. Heterocycl. Chem. 1982 Supplement 2, 178-186. (15) Li, X. C.; Ning, G. L.; Wu, L. P.; Lin, Y. Chin Chem. Lett. 2002, 13, 1141.
not from a ring oxidation. The most striking feature of this study is the novel oxidation reaction of the cyclopentadiene ring promoted by Ag(I) ion. As noted, the metal-promoted C-C bond cleavage under mild conditions has been a challenging subject of organic synthesis because of the weak coordinating ability and strong bond energy of the C-C bond.16 Although Ag(I) ion has been showed to be a powerful catalyst in promoting a variety of rearrangements of organic compounds,17 the C-C bond oxidation with the formation of silver metal and protic acid has only been observed in the strained hydrocarbons.17b The present work displayed a facile and novel synthesis of phenylated pyrylium cations from unstrained cyclopentadiene-ring by the C-C bond oxidation in solution. It has been found that without silver(I) ions, the reaction in Scheme 1 cannot take place. The question to be answered here is where the O atom comes from for the oxygen insertion reaction. Noticeably, AgClO4 is moisture-sensitive and these phenyl-substituted cyclopentadienes are moisture-adsorbing chemicals, too. It was observed that the pyrylium salts could not be obtained with freshly dried reactants. This implies that H2O might provide the inserted oxygen atom and contribute to formation of the pyrylium ring. To confirm this assumption, the preparation of 2 was carried out under an Ar atmosphere using standard Schlenk techniques to (16) Kando, T.; Kodoi, K.; Nishinaga, E.; Okata, T.; Morigaki, Y.; Watanabe, Y.; Mitsudo, T. J. Am. Chem. Soc. 1998, 120, 5587. (17) (a) Padwa, A.; Blacklock, T. J.; Loza, R. J. Am. Chem. Soc. 1981, 103, 2404. (b) Filippo, J. S.; Romano, L. J., Jr.; Chern, C.-I.; Valentine, J. S. J. Org. Chem. 1976, 41, 583.
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SCHEME 2
maintain oxygen-free conditions. Likewise, AgCF3SO3 was employed in place of AgClO4‚H2O in the synthesis of 2 and 3 to eliminate possible oxygen from ClO4-, where the moisture trapped in the precursor ligands and/or AgCF3SO3 provides the water needed. In both cases, the normal 2 and 3 formed. Thus, the inserted oxygen atom most likely originated from water. All these observations provide insight into the possible mechanism of the reaction. A reasonable explanation for the formation of 1-3 involves a sequential process (Scheme 2), starting from the action of silver ion with the double bond of cyclopentadiene-ring to the addition of water molecule, ring opening, rearrangement, cyclization, and oxidation to generate the final six-membered pyrylium ring. Here the reactions from I to III are based on the strained ring system,17 with H2O (instead of CH3OH) added to the ring by a diene addition reaction.18 From IV to V is the rearrangement and chain cyclization reaction. Its rationalization and possibility have been demonstrated in the other substituted pyrylium cations.7 The last step, VI, is an oxidation reaction that is evident and has been well documented,7 although the oxidant is different. In our reaction, Ag(I) ion most likely functions as both a Lewis acid catalyst and an oxidant. Studies on other substituted cyclopentadiene systems are ongoing. Experimental Section [(Ph5C5O+)(ClO4-)]2(CH2Cl2) (1). To a solution of Ph5C5H (18 mg, 0.04 mmol) in a mixed solvent of CH2Cl2/toluene (3/1 in volume, 4 mL) was added AgClO4‚H2O (46 mg, 0.2 mmol). After being stirred for about 20 min, the resultant colorless solution was transferred to a 7 mm diameter glass tube and layered with n-hexane as a diffusion solvent. The glass tube was sealed under (18) Keenan, M. J. Cyclopentadiene and Dicyclopentadiene. In ECT, 4th ed.; 1993; Vol. 7, pp 859-876.
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Ar, wrapped with aluminum foil, and kept in the dark at room temperature. X-ray-quality green needle crystals of 1 were obtained after 3 weeks, 33% in yield. IR (KBr), ν (cm-1): 1597s, 1095vs. 1H NMR [23 °C, (CD3)2CO, 270 MHz, δ in ppm]: no pyrylium proton; for precursor ligand, Ph5HC5: 5.028(s, H; C5 ring). Anal. Calcd for C71H52Cl4O10, 1: C, 70.59; H, 4.31. Found: C, 70.18; H, 4.23. [Ag(ClO4)(H2O)(Ph4HC5O+)](ClO4-) (2). The brown column crystals of 2 were prepared in the same way as 1, using Ph4H2C5 (15 mg, 0.04 mmol) instead of Ph4HC5. X-ray-quality crystals of 2 were obtained after 2 weeks’ standing of the glass tube, 41% in yield. IR (KBr), ν (cm-1): 1610s, 1112vs. 1H NMR [23 °C, (CD3)2CO, 270 MHz, δ in ppm]: 9.89 (s, 1 H), for precursor ligand Ph4H2C5: 4.14 (s, 2H; C5 ring). 13C NMR [23 °C, (CD3)2CO, 400 MHz, δ in ppm]: 209.97, 206.20, 173.61, 170.55, 164.03 (5 signals). MS, m/z: 385. Anal. Calcd for C29H23AgCl2O10: C, 49.04; H, 3.26. Found: C, 48.36; H, 3.25. (Ph3H2C5O+)(ClO4-) (3). X-ray-quality green needle crystals of 3 were obtained in the same way as 1, by mixing Ph3H3C5 (120 mg, 0.4 mmol) and AgClO4‚H2O (10 mg, 0.045 mmol) in the mixed solvent (4 mL), 57% in yield. IR (KBr), ν (cm-1): 1610s, 1092vs. 1H NMR [23 °C, (CD3)2CO, 400 MHz, δ in ppm]: 9.73, 9.10 (s, s, H, H), for precursor ligand Ph3H3C5: 3.89, 7.0 (s, s, 2H, H; in C5 ring). 13C NMR [23 °C, (CD3)2CO, 400 MHz, δ in ppm]: 209.68, 205.88, 172.61, 168.85, 163.80 (5 signals). MS, m/z: 309.1. Anal. Calcd for C23H17ClO5, 3: C, 67.6; H, 4.16. Found: C, 67.98; H, 4.18.
Acknowledgment. This work was partially supported by the National Nature Science Foundation of China (No. 20072003) and a Grant-in-Aid for Science Research (No. 10016743) from Japan. We sincerely thank Buruk Company for its assistance in the X-ray diffraction analysis for 3. Supporting Information Available: Crystallographic data of 1-3, cell packing of 1 and 2, and IR, MS, 1H NMR, 13C NMR, and analytical details. This material is available free of charge via the Internet at http://pubs.acs.org. JO030243K
