Novel “turn-on” fluorescent chemodosimeters based on thioxanthen-9-thione for the selective detection of mercuric ions in aqueous media

Abstract

Two Hg²⁺-selective chemodosimeters based on thioxanthen-9-thione (DP-TXT and BDPA-TXT) were synthesized and structurally characterized. The highly sensitive and selective chemodosimetric behaviors of thioxanthen-9-thione towards Hg²⁺ ions were investigated. Significant chromogenic and fluorogenic signaling occurred from the transformation of thioxanthen-9-thione to thioxanthen-9-one by Hg²⁺-induced selective desulfurization. Both of the two probes exhibit excellent Hg²⁺-specific fluorescence enhancement over various competitive cations in aqueous media.

---

Introduction

To date, fluorescence detection has become the dominant strategy for Hg²⁺ sensing owing to its operational simplicity, low cost, real time monitoring and high sensitivity.^1–6 Our increasing awareness of the deleterious effects of mercury exposure has sparked interest in the development of novel fluorescent probes for detecting Hg²⁺. As known, there are a number of sophisticated systems for the optical detection of Hg²⁺ ions,^7–20 and many of the small molecule-derived fluorescence sensors for Hg²⁺ reported so far compose of two components: a recognition moiety containing nitrogen/sulfur atoms and a fluorophore that signals the change of absorption/fluorescence upon exposure to Hg²⁺.^21–28 As a result, Hg²⁺-triggered desulfurization and transformation of thioamide,^29,30 thiourea^31–33 and thione^34,35 have been successfully utilized. For instance, Czarnik et al. reported that a chemodosimeter based on thioamide substituted anthracene could give 56-fold fluorescence enhancement by Hg²⁺ addition.³⁶ However, in some cases interference from other metal ions can affect the selectivity, and only a handful of fluorescent sensors displayed good water solubility, so the search for readily accessible fluorescent Hg²⁺ probes with good water solubility as well as high sensitivity and selectivity is still a challenging task. Herein, we report two thioxanthen-9-thione derivatives, DP-TXT and BDPA-TXT (Scheme 1), can be used as highly sensitive and selective chemodosimeters for Hg²⁺ ions in aqueous media.

Scheme 1 Fluorescence changes in the presence of Hg²⁺ and proposed sensing mechanism: desulfurization of DP-TXT and BDPA-TXT promoted by Hg²⁺.

When the carbonyl group in thioxanthen-9-one was replaced by a thiocarbonyl group, there existed obvious fluorescence quenching which was attributed to the heavy atom effect and photo-induced electron transfer (PET) properties of S atoms.³⁵ Hence, we designed and synthesized two thioxanthen-9-thione based chemodosimeters. Addition of Hg²⁺ resulted in significant fluorescence enhancement by desulfurization of thione. For modification of the optical properties of compound DP-TXO, compound BDPA-TXO possessing diphenylamine group was synthesized via Buchwald–Hartwig amination (Scheme 2). As expected, compound BDPA-TXO exhibited a bathochromic-shift both in UV-vis and fluorescence spectra due to the enhanced intramolecular charge transfer (ICT) process.

Scheme 2 Synthesis of chemodosimeters DP-TXT and BDPA-TXT.

Experimental

Materials

Toluene and tetrahydrofuran (THF) were pre-dried over 4 Å molecular sieves and distilled under an argon atmosphere from sodium and benzophenone immediately prior to use. Unless otherwise stated, all reagents were obtained from commercial suppliers and were used without further purification.

Apparatus

¹H NMR and ¹³C NMR spectra were recorded on Brucker AM-400 spectrometers. Chemical shifts were reported in ppm relative to a tetramethylsilane (TMS) standard in CDCl₃ or DMSO-d₆. Mass spectra (MS) were obtained on a Waters LCT Premier XE spectrometer. UV-vis absorption spectra were performed on a Varian Cray 500 spectrophotometer and fluorescence spectra were recorded on a Varian Cray Eclipse. Quantum yields of samples (1.0 × 10⁻⁵ M in aqueous media) were recorded on a Horiba Fluoromax-4 with a calibrated intergrating sphere system. The melting points were measured on an X4 Micro-melting point apparatus.

Synthesis of 2,7-dibromothioxanthen-9-one (DB-TXO)

To a solution of thioxanthone (5.0 g, 23.6 mmol) in glacial acetic acid (45 mL) was added bromine (10 mL) dropwise over 20 min. The solution was stirred continuously and heated at reflux temperature for 20 h, after which half of the solvent volume was removed by distillation. The reaction mixture was then allowed to cool then poured over crushed ice. The resulting yellow precipitate was collected by filtration and washed with saturated sodium bicarbonate, a 20% aqueous solution of sodium bisulfite, and finally with water. The solid was then dried under vacuum and recrystallized twice from toluene to afford a bright yellow solid (5.1 g, 13.9 mmol, yield: 59%). m.p. 263–266 °C. ¹H NMR (400 MHz, CDCl₃), δ (ppm): 7.47 (d, J = 8.4 Hz, 2H), 7.74 (d, J = 8.4 Hz, 2H), 8.74 (s, 2H). HR-MS (ESI): calculated for C₁₃H₆Br₂OS [M + H]⁺ 368.8584, found 368.8580.

Synthesis of 2,7-diphenylthioxanthen-9-one (DP-TXO)

Phenylboronic acid (0.54 g, 4.4 mmol) and compound DB-TXO (0.736 g, 2.0 mmol) were mixed in THF (15 mL). 10 mL of 2 M aqueous potassium carbonate solution was added, and the mixture was stirred with magnetic stirring. After added tetrakis(triphenylphsosphine)palladium (0.022 g, 0.02 mmol), the reaction solution was heated at reflux temperature for 12 h under the atmosphere of nitrogen. Then the mixture was cooled to room temperature, evaporated under reduced pressure and extracted with dichloromethane (3 × 10 mL). The organic portion was washed with brine and water, and then dried by anhydrous MgSO₄. The solvent was evaporated, and the residue was purified by column chromatography with petroleum ether–dichloromethane (2 : 1, v/v) as the eluent to afford a yellow solid (0.61 g, 1.7 mmol, yield: 85%). m.p. 200–203 °C. ¹H NMR (400 MHz, CDCl₃), δ (ppm): 7.41 (t, J = 7.4 Hz, 2H), 7.50 (t, J = 7.4 Hz, 4H), 7.68 (d, J = 8.4 Hz, 2H), 7.73 (d, J = 7.8 Hz, 4H), 7.90 (dd, J = 8.4 Hz, 2H), 8.89 (d, J = 2.1 Hz, 2H). ¹³C NMR (100 MHz, CDCl₃) δ (ppm): 126.67, 127.14, 127.94, 129.04, 129.43, 131.10, 135.99, 139.41, 139.51, 180.02. HR-MS (ESI): calculated for C₂₅H₁₆OS [M + H]⁺ 365.1000, found 365.1000.

Synthesis of 2,7-diphenylthioxanthen-9-thione (DP-TXT)

Compound DP-TXO (0.364 g, 1.0 mmol) and Lawesson's reagent (0.405 g, 1.0 mmol) were dissolved in toluene (15 mL) in a 50 mL round-bottom flask. Then the reaction mixture was refluxed for 3 h under the atmosphere of nitrogen. The mixture was concentrated under vacuum, purified by silica gel column chromatography with petroleum ether/dichloromethane (3 : 1, v/v) as eluent to afford a tan solid (0.3 g, 0.8 mmol, yield: 80%). m.p. 210–212 °C. ¹H NMR (400 MHz, CDCl₃), δ (ppm): 7.40 (t, J = 7.3 Hz, 2H), 7.50 (t, J = 7.6 Hz, 4H), 7.67 (d, J = 8.3 Hz, 2H), 7.71 (d, J = 7.3 Hz, 4H), 7.88 (dd, J = 8.3 Hz, 2H), 9.29 (d, J = 1.8 Hz, 2H). ¹³C NMR (100 MHz, CDCl₃) δ (ppm): 126.60, 127.21, 127.93, 129.03, 130.57, 130.77, 131.61, 137.70, 139.60,140.04, 210.73. HR-MS (ESI): calculated for C₂₅H₁₆S₂ [M + H]⁺ 381.0772, found 381.0773.

Synthesis of 2,7-bis(diphenylamino)thioxanthen-9-one (BDPA-TXO)^37,38

Tris(dibenzylideneacetone)dipalladium(0) (Pd₂dba₃) (0.055 g, 0.06 mmol) and 1,1′-bis(diphenylphosphino)ferrocene (DPPF) (0.066 g, 0.12 mmol) were dissolved in toluene under argon and degassed by two freeze-pump-thaw cycles. This activated catalyst was then added to a degassed mixture of compound DB-TXO (0.5 g, 1.36 mmol), diphenylamine (0.506 g, 2.99 mmol) and sodium tert-butoxide (NaO^tBu, 0.366 g, 3.81 mmol) in toluene. The mixture was stirred for 15 h at 90 °C. After the mixture was cooled to room temperature, the reaction was quenched by adding 5 mL of methanol, diluted with diethyl ether, and washed twice with brine and water, respectively. The combined aqueous phase was extracted twice with diethyl ether. The combined organic phase was then dried with MgSO₄, filtered, and the solvent evaporated under reduced pressure. The residue was purified by column chromatography on silica gel with petroleum ether–dichloromethane (2 : 1, v/v) as the eluent to afford a yellow solid (0.360 g, 0.66 mmol, yield: 49%). m.p. 250–251 °C. ¹H NMR (400 MHz, DMSO-d₆), δ (ppm): 7.10 (d, J = 8.1 Hz, 8H), 7.14 (d, J = 7.4 Hz, 4H), 7.34–7.38 (m, 8H), 7.39 (d, J = 2.7 Hz, 2H), 7.75 (d, J = 8.8 Hz, 2H), 7.89 (d, J = 2.6 Hz, 2H). ¹³C NMR (100 MHz, CDCl₃) δ (ppm): 122.57, 123.59, 124.60, 126.89, 128.19, 129.52, 129.82, 130.24, 146.55, 147.16, 179.07. HR-MS (ESI): calculated for C₃₇H₂₆N₂OS [M + H]⁺ 547.1844, found 547.1847.

Synthesis of 2,7-bis(diphenylamino)thioxanthen-9-thione (BDPA-TXT)

Compound BDPA-TXO (0.109 g, 0.2 mmol) and Lawesson's reagent (0.081 g, 0.2 mmol) were dissolved in toluene (10 mL) in a 50 mL round-bottom flask. Then the reaction mixture was refluxed for 3 h under the atmosphere of nitrogen. The mixture was concentrated under vacuum, purified by silica gel column chromatography with petroleum ether–dichloromethane (1 : 1, v/v) as the eluent to afford a dark purple solid (0.086 g, 0.15 mmol, yield: 76%). m.p. 238–239 °C. ¹H NMR (400 MHz, DMSO-d₆), δ (ppm): 7.10 (d, J = 7.5 Hz, 8H), 7.13 (d, J = 7.3 Hz, 4H), 7.35 (t, J = 8.2 Hz, 8H), 7.41 (d, J = 8.8 Hz, 2H), 7.81 (d, J = 8.8 Hz, 2H), 8.52 (d, J = 2.6 Hz, 2H). ¹³C NMR (100 MHz, CDCl₃) δ (ppm): 123.59, 124.56, 125.29, 125.83, 126.82, 127.80, 129.50, 138.34, 147.07, 208.16. HR-MS (ESI): calculated for C₃₇H₂₆N₂S₂ [M + H]⁺ 563.1616, found 563.1615.

Results and discussion

Synthesis and characterization

The synthetic route of DP-TXT and BDPA-TXT is shown in Scheme 2. Bromination of thioxanthen-9-one was carried out according to literature procedures³⁹ to afford a bright yellow solid DB-TXO in 59% yield through recrystallization twice from toluene. It was necessary that one of the reagent bromines should be in large excess for a reaction time of 20 h. Buchwald–Hartwig amination proceeded under the conditions of Pd₂dba₃ (catalyst), DPPF (ligand), NaO^tBu (base) and toluene solutions, and the catalyst and ligand were activated under argon and degassed by two freeze-pump-thaw cycles prior to the addition of other reagents. As for the thionation procedure, it was performed in refluxing toluene with Lawesson's reagent under an atmosphere of nitrogen. The structures of DP-TXT, BDPA-TXT and other intermediate products were confirmed by ¹H NMR, ¹³C NMR, and ESI-MS (Fig. S1–S14 in the ESI†). As for the solvent choosing principles of DP-TXT and BDPA-TXT for the optical detection of Hg²⁺, acetonitrile (CH₃CN) and dimethyl sulfoxide (DMSO) are both water-miscible and are low-toxicity organic solvents, so they provide opportunities for optical detection in aqueous media. What's more, DP-TXT and BDPA-TXT show good solubility in CH₃CN and DMSO, respectively.

Colorimetric response of chemodosimeters DP-TXT and BDPA-TXT

As shown in Fig. 1a and Fig. 2, among the various metal ions tested, only Hg²⁺ significantly affected the absorption behavior of DP-TXT. The absorption band was blue shifted from 487 nm to 397 nm with the addition of Hg²⁺ in CH₃CN–H₂O (5 : 5, v/v). The solution color changed from orange to colorless, which allowed a naked-eye detection of Hg²⁺. As for BDPA-TXT, the absorption band also exhibited a dramatic blue shift upon addition of Hg²⁺ in DMSO–H₂O (9 : 1, v/v), and it displayed an obvious colorimetric response from purple to yellow (Fig. 1b and Fig. 3). All these results above suggest that chemodosimeters DP-TXT and BDPA-TXT can serve as “naked-eye” indicators for Hg²⁺.

Fig. 1 (a) UV-vis spectra of DP-TXT in the absence (black line) and presence (red line) of Hg²⁺ in CH₃CN–H₂O (5 : 5, v/v). Inset: Color change of DP-TXT upon addition of Hg²⁺. (b) UV-vis spectra of BDPA-TXT in the absence (black line) and presence (red line) of Hg²⁺ in DMSO–H₂O (9 : 1, v/v). Inset: Color change of BDPA-TXT upon addition of Hg²⁺.

Fig. 2 Color changes of DP-TXT towards various metal ions.

Fig. 3 Color changes of BDPA-TXT towards various metal ions.

Fluorescence spectra of chemodosimeter DP-TXT in the presence of Hg²⁺

DP-TXT exhibited almost no emission centered on 460 nm in 50% aqueous acetonitrile. Upon interaction with Hg²⁺ ions, however, the fluorescence intensity at 460 nm increased 40-fold and the solution color under illumination with a UV lamp changed from dark to bright blue (Fig. 4), and the absolute fluorescence quantum yield varied dramatically from 1.3% to 21.7%. Furthermore, the fluorescence intensity as a function of Hg²⁺ concentration shows a near-linear relationship (Fig. S15 in the ESI†). As a consequence, the detection limit reached 21 nM.

Fig. 4 Fluorescence spectra of DP-TXT (1.0 × 10⁻⁵ M) in the presence of Hg²⁺ (0–3.0 × 10⁻⁵ M) in CH₃CN–H₂O (5 : 5, v/v), λ_ex = 309 nm. Inset: Fluorescence change of DP-TXT upon addition of Hg²⁺.

The Hg²⁺-selective signaling process of compound DP-TXT was also examined by ¹H NMR titration experiment (Fig. S16 in the ESI†). Upon treatment of DP-TXT with 3 equiv. of Hg²⁺, the resonances of DP-TXT were completely transformed to those of DP-TXO. As shown in Fig. S16,† the resonances of the thioxanthen-9-thione moiety were upfield shifted (from 9.29 to 8.89 ppm for the protons at the ortho position of the thiocarbonyl of DP-TXT), while the resonances for other protons did not significantly change. Analogous to DP-TXT, the peak of the protons at the ortho position of the thiocarbonyl of BDPA-TXT also upfield shifted from 8.52 to 7.89 ppm (Fig. S17 in the ESI†).

Fluorescence spectra of chemodosimeter BDPA-TXT in the presence of Hg²⁺

The fluorescent changes of BDPA-TXT towards different concentrations of Hg²⁺ were also examined in DMSO–H₂O (9 : 1, v/v). Compared to DP-TXT, the addition of Hg²⁺ to BDPA-TXT revealed an emission band at 570 nm which bathochromically shifted 110 nm owing to the two diphenylamine moieties introducing as donor groups. A 43-fold fluorescent enhancement was obtained at 570 nm and the solution color varied from nearly dark to bright yellow by using hand-held UV light irradiation (Fig. 5). The absolute fluorescence quantum yields in the absence and presence of Hg²⁺ were measured to be 0.8% and 9.6%, respectively. The detection limit for Hg²⁺ with chemodosimeter BDPA-TXT was determined to be 75 nM (Fig. S18 in the ESI†).

Fig. 5 Fluorescence spectra of BDPA-TXT (1.0 × 10⁻⁵ M) in the presence of Hg²⁺ (0–4.0 × 10⁻⁵ M) in DMSO–H₂O (9 : 1, v/v), λ_ex = 357 nm. Inset: Fluorescence change of BDPA-TXT upon addition of Hg²⁺.

Moreover, mass spectra demonstrated the proposed Hg²⁺-induced conversion process by revealing a diagnostic peak at m/z = 547.1847 of BDPA-TXO, instead of the peak at m/z = 563.1615 that would be expected for the molecular ion of BDPA-TXT (Fig. S19 in the ESI†). HRMS (ESI) experiments for DP-TXT also confirmed the Hg²⁺-triggered desulfurization process (Fig. S20 in the ESI†).

Selective fluorescence response of chemodosimeters DP-TXT and BDPA-TXT

The fluorescence responses of chemodosimeters DP-TXT and BDPA-TXT towards various other metal ions including alkali (Na⁺, K⁺), alkaline earth (Mg²⁺, Ca²⁺, Ba²⁺), and transition-metal ions (Mn²⁺, Co²⁺, Ni²⁺, Cu²⁺, Ag⁺, Zn²⁺, Cd²⁺, Pb²⁺) were examined, and there is no significant influence on the emission color of DP-TXT except for Hg²⁺ (Fig. 6a and Fig. 7). Enhancing efficiency, defined as I/I₀ (I₀ and I represent the fluorescence intensity of DP-TXT in the absence and in the presence of metal ions at 460 nm, respectively), for Hg²⁺ was calculated to be more than 40, whereas the ratio of I/I₀ varied in a limited range from 0.97 (Cu²⁺) to 1.29 (Ag⁺) for the other metal ions (Fig. S21a in the ESI†).

Fig. 6 (a) Fluorescence spectra of chemodosimeter DP-TXT towards various metal ions in CH₃CN–H₂O (5 : 5, v/v). [DP-TXT] = 1.0 × 10⁻⁵ M, [Hg²⁺] = 3.0 × 10⁻⁵ M, [Mⁿ⁺] = 1.0 × 10⁻⁴ M. (b) Fluorescence spectra of chemodosimeter BDPA-TXT towards various metal ions in DMSO–H₂O (9 : 1, v/v). [BDPA-TXT] = 1.0 × 10⁻⁵ M, [Hg²⁺] = 4.0 × 10⁻⁵ M, [Mⁿ⁺] = 1.0 × 10⁻⁴ M.

Fig. 7 Fluorescence images of DP-TXT towards various metal ions.

Analogous to DP-TXT, no significant fluorescence changes of BDPA-TXT were observed in the presence of other metal ions in excess (Fig. 6b and Fig. 8). Addition of Ag⁺ resulted in weak fluorescent enhancement at 570 nm because of its weak thiophile ability. The enhancing efficiency of BDPA-TXT remained constant (≈1) after addition of other surveyed metal ions except Ag⁺ (6.28) and Hg²⁺ (43.41) (Fig. S21b in the ESI†).

Fig. 8 Fluorescence images of BDPA-TXT towards various metal ions.

Therefore, compounds DP-TXT and BDPA-TXT exhibited specific selectivity for Hg²⁺ over other examined metal ions in aqueous media.

Conclusions

In conclusion, two new fluorescent chemodosimeters based on thioxanthen-9-thione were developed for the selective detection of mercuric ions in aqueous media. Pronounced Hg²⁺-selective fluorogenic signaling behaviors were observed: dark to bright blue, dark to bright yellow under illumination with a UV lamp, respectively. Between the two chemodosimeters, DP-TXT exhibits the better sensitivity and selectivity properties with a lower detection limit and less interference from other metal ions. The recognition mechanism is attributed to Hg²⁺-triggered transformation of thioxanthen-9-thione to a thioxanthen-9-one derivative, and this kind of transformation could potentially be used for the design of other supramolecular systems that show sensitive and selective signaling behaviors for Hg²⁺ ions in aqueous environments. Furthermore, efforts will be focused on the sensing system's actual applicability, such as the feasibility of detecting real environmental samples in future research.

References

1. D. T. Quang and J. S. Kim, Chem. Rev., 2010, 110, 6280.
2. X. L. Feng, L. B. Liu, S. Wang and D. B. Zhu, Chem. Soc. Rev., 2010, 39, 2411.
3. E. M. Nolan and S. J. Lippard, J. Am. Chem. Soc., 2003, 125, 14270.
4. M. Kumar, R. Kumar and V. Bhalla, RSC Adv., 2011, 1, 1045.
5. Y. Q. Wu, H. Jing, Z. S. Dong, Q. Zhao, H. Z. Wu and F. Y. Li, Inorg. Chem., 2011, 50, 7412.
6. L. L. An, Y. L. Tang, F. D. Feng, F. He and S. Wang, J. Mater. Chem., 2007, 17, 4147.
7. R. K. Mahajan, R. Kaur, V. Bhalla, M. Kumar, T. Hattori and S. Miyano, Sens. Actuators, B, 2008, 130, 290.
8. C. Chen, R. Y. Wang, L. Q. Guo, N. Y. Fu, H. J. Dong and Y. F. Yuan, Org. Lett., 2011, 13, 1162.
9. H. Yang, J. J. Qian, L. T. Li, Z. G. Zhou, D. R. Li, H. X. Wu and S. P. Yang, Inorg. Chim. Acta, 2010, 363, 1755.
10. Y. Liu, M. Y. Li, Q. Zhao, H. Z. Wu, K. W. Huang and F. Y. Li, Inorg. Chem., 2011, 50, 5969.
11. D. N. Lee, G. J. Kim and H. J. Kim, Tetrahedron Lett., 2009, 50, 4766.
12. J. W. Liu and Y. Lu, Angew. Chem., Int. Ed., 2007, 46, 7587.
13. X. F. Guo, X. H. Qian and L. H. Jia, J. Am. Chem. Soc., 2004, 126, 2272.
14. M. Kumar, R. Kumar and V. Bhalla, Chem. Commun., 2009, 45, 7384.
15. M. Kadarkaraisamy, S. Thammavongkeo, P. N. Basa, G. Caple and A. G. Sykes, Org. Lett., 2011, 13, 2364.
16. L. Q. Guo, H. Hu, R. Q. Sun and G. N. Chen, Talanta, 2009, 79, 775.
17. J. S. Wu, W. M. Liu, J. C. Ge, H. Y. Zhang and P. F. Wang, Chem. Soc. Rev., 2011, 40, 3483.
18. Q. Liu, J. J. Peng, L. N. Sun and F. Y. Li, ACS Nano, 2011, 5, 8040.
19. M. H. Yang, P. Thirupathi and K. H. Lee, Org. Lett., 2011, 13, 5028.
20. Q. Zhao, F. Y. Li and C. H. Huang, Chem. Soc. Rev., 2010, 39, 3007.
21. W. Jiang and W. Wang, Chem. Commun., 2009, 45, 3913.
22. W. Huang, D. Y. Wu and C. Y. Duan, Inorg. Chem. Commun., 2010, 13, 294.
23. H. G. Wang, Y. P. Li, S. F. Xu, Y. C. Li, C. Zhou, X. L. Fei, L. Sun, C. Q. Zhang, Y. X. Li, Q. B. Yang and X. Y. Xu, Org. Biomol. Chem., 2011, 9, 2850.
24. N. Aksuner, B. Basaran, E. Henden, I. Yilmaz and A. Cukurovali, Dyes Pigm., 2011, 88, 143.
25. Y. K. Yang, K. J. Yook and J. Tae, J. Am. Chem. Soc., 2005, 127, 16760.
26. Z. K. Wu, Y. F. Zhang, J. S. Ma and G. Q. Yang, Inorg. Chem., 2006, 45, 3140.
27. L. Liu, G. X. Zhang, J. F. Xiang, D. Q. Zhang and D. B. Zhu, Org. Lett., 2008, 10, 4581.
28. H. N. Kim, M. H. Lee, H. J. Kim, J. S. Kim and J. Yoon, Chem. Soc. Rev., 2008, 37, 1465.
29. K. C. Song, J. S. Kim, S. M. Park, K. C. Chung, S. Ahn and S. K. Chang, Org. Lett., 2006, 8, 3413.
30. W. M. Liu, L. W. Xu, H. Y. Zhang, J. J. You, X. L. Zhang, R. L. Sheng, H. P. Li, S. K. Wu and P. F. Wang, Org. Biomol. Chem., 2009, 7, 660.
31. B. Liu and H. Tian, Chem. Commun., 2005, 41, 3156.
32. J. S. Wu, I. C. Hwang, K. S. Kim and J. S. Kim, Org. Lett., 2007, 9, 907.
33. M. H. Lee, B. K. Cho, J. Yoon and J. S. Kim, Org. Lett., 2007, 9, 4515.
34. M. G. Choi, Y. H. Kim, J. E. Namgoong and S. K. Chang, Chem. Commun., 2009, 45, 3560.
35. G. X. Zhang, D. Q. Zhang, S. W. Yin, X. D. Yang, Z. G. Shuai and D. B. Zhu, Chem. Commun., 2005, 41, 2161.
36. M. Y. Chae and A. W. Czarnik, J. Am. Chem. Soc., 1992, 114, 9704.
37. N. Haberkorn, S. A. L. Weber, R. Berger and P. Theato, ACS Appl. Mater. Interfaces, 2010, 2, 1573.
38. Y. Monguchi, K. Kitamoto, T. Ikawa, T. Maegawa and H. Sajiki, Adv. Synth. Catal., 2008, 350, 2767.
39. M. P. Coleman and M. K. Boyd, J. Org. Chem., 2002, 67, 7641.

---

Footnote

† Electronic Supplementary Information (ESI) available. See DOI: 10.1039/c2ra00048b/

---