Chemical Fixation of CO2 to Ethylene Carbonate

中国科技论文在线 http://www.paper.edu.cn

Applied Catalysis A: General 275 (2004) 73–78 www.elsevier.com/locate/apcata

Chemical ﬁxation of CO2 to ethylene carbonate under supercritical conditions: continuous and selective

Xiao-Bing Lu*, Jing-Hai Xiu, Ren He, Kun Jin, Li-Mei Luo, Xiu-Juan Feng

State Key Laboratory of Fine Chemicals, Dalian University of Technology, Dalian 116012, PR China

Received 17 January 2004; received in revised form 5 July 2004; accepted 21 July 2004 Available online 1 September 2004

Abstract

Chemical fixation of CO2 to ethylene carbonate proceeds effectively under supercritical conditions by using as catalyst the system of an immobilized cobalt complex in conjunction with a quaternary ammonium salt in a flow apparatus, based on supercritical fluid used as both a solvent and a reactant for continuous synthesis of organic compound. A conversion of up to 85.6% for ethylene oxide was achieved at 110 8C and 12.5 MPa, which value is about 4.5 times that under subcritical condition (4.0 MPa). No byproduct such as polycarbonates or polyester was observed in the obtained products. The immobilized cobalt complex exhibited good stability and was subjected to utilization for 24 h with no loss of activity. The powder X-ray diffraction patterns indicate no change in the structure of the supported mesoporous silica during the reaction. # 2004 Elsevier B.V. All rights reserved.

Keywords: Supercritical carbon dioxide; Ethylene oxide; Ethylene carbonate; Cycloaddition; Salen–cobalt complex; Quaternary ammonium salt; MCM-41; Flow reactor.

1. Introduction mediates, and in many biomedical applications [4–6].In recent decades, numerous catalyst systems have been From the standpoint of environmental protection and developed for this transformation [7–22]. resource utilization, the development of a truly environmen- On the other hand, supercritical CO2 (SC-CO2) is an attrac- tally benign process utilizing CO2, which is the largest single tive substitute solvent, because it combines an environmen- source of greenhouse gas, has drawn current interest in tally benign character with favorable physico-chemical industrial chemistry and biotechnology. Chemical fixation properties for chemical synthesis [23–25]. Also, CO2 may of CO2 is one of the most attractive methods since there are be a particularly advantageous reaction medium when CO2 many possibilities for CO2 to be used as a safe and cheap C1 serves as both a reactant and a solvent. The improved rates for building block in organic synthesis [1,2]. One of the most catalytic hydrogenation of CO2 to formic acid and its promising methodologies in this area is the synthesis of five- derivatives in supercritical conditions provided support for membered cyclic carbonates via the cycloaddition reaction this approach [26–28]. The success of Noyori and co-workers of CO2 and epoxides [3]. These carbonates are valuable as suggests that investigation of other CO2 reactions in SC-CO2 precursors for polymeric materials such as polycarbonates, is worthwhile. In fact, the cycloaddition reaction of CO2 to aprotic polar solvents, pharmaceutical/fine chemical inter- epoxides is very suited for proceeding under supercritical conditions, because SC-CO2 possesses high solvating power towards various epoxides [29–31]. In previous paper [30], * Corresponding author. Tel.: +86 411 88993864; fax: +86 411 83633080. we reported that ethylene carbonate could be rapidly E-mail address: [email protected] (X.-B. Lu). synthesized from supercritical CO2/ethylene oxide mixture

转载中国科技论文在线 http://www.paper.edu.cn

74 X.-B. Lu et al. / Applied Catalysis A: General 275 (2004) 73–78

Scheme 1. Continuous cycloadditon of CO2 and ethylene oxide with an immobilized cobalt complex (the metal complex covalently bonded on both the inside and the outside of the MCM-41 pores) in conjunction with a quaternary ammonium salt.

in the presence of homogeneously bifunctional catalyst. 2.2. Preparation and characterization of the anchored However, irreversible phase separation, resulting from the cobalt complexes formation of product during the reaction, was observed and thus reduces the rate significantly. A similar phase separation Ordered MCM-41 silica, which was prepared according was also reported for propylene carbonate synthesis under to a previously described method [36], was chosen as supercritical CO2 conditions [31]. Very recently, Kawanami support material for immobilizing Schiff-base cobalt and coworkers found that in the presence of 1-octyl-3- complexes. MCM-41 silica, discovered by Mobil scientists methylimidazolium tetrafluoroborate cyclic carbonates in 1992 [37,38], possesses regularly hexagonal arrays of could be rapidly synthesized in nearly 100% selectivity cylindrical mesopores and changeable pore diameter from corresponding epoxides and CO2 at 14 MPa and between 1.5 and 20 nm. These properties make MCM-41 100 8C [32]. materials to be the best candidates of catalyst or catalyst Indeed, the low viscosity and the high diffusivity inherent support, and the hosts for many guest materials [39,40].The to supercritical fluids are ideally suited for continuous flow cobalt complexes were grafted onto inorganic supports in reactors rather than batch reactors [33,34]. Furthermore, the three steps, as outlined in Scheme 2. use of an immobilized homogeneous catalyst in the flow In a typical synthesis process, activated MCM-41 silica 1 reactor not only provides a direct and quantitative separation (5 g, 300 8C, under vacuum) was first treated with a reflux- of the products from the catalyst and avoids any solubility ing anhydrous toluene solution (100 ml) of 3-chloropropyl- limitation of homogeneous catalysts [35], but also easily trimethoxysilane (3 ml) under N2 atmosphere followed by overcomes the influence of phase change during some washing with diethyl ether-dichloromethane in a Soxhlet reactions. Herein, we report the use of a flow reactor system apparatus yielding covalently anchored 3-chloropropylsi- À1 for continuous cycloaddition reaction from SC-CO2/ lane moieties 2 (Cl, 1.02 mmol g ). The later (3 g) was then ethylene oxide mixture fluid by using as catalyst the system treated with a refluxing toluene solution (60 ml) of 3-[N,N’- of an immobilized cobalt complex in conjunction with a bis-(2-salicylidenamino)ethyl]amine (SalenH2) or 3-[N,N’- quaternary ammonium salt (Scheme 1), based on super- bis-2-(5-tert-butylsalicylidenamino)ethyl]amine (t-BuSa- critical fluid used as both a solvent and a reactant for lenH2) (15 mmol) followed by washing with dichloro- continuous synthesis of organic compounds. methane in a Soxhlet apparatus leading to the formation of SalenH2-MCM-41 3a or t-BuSalenH2-MCM-41 3b. The anchored ligand 3a or 3b (2 g) was further reacted with 2. Experimental anhydrous CoCl2 (0.4 g) or Co(OAc)2Á4H2Oinrefluxing ethanol (50 ml) under N2 for 24 h followed by washing in a 2.1. Materials Soxhlet apparatus with ethanol in a N2 atmosphere and then drying at 100 8C in vacuum to yield SalenCo-MCM-41 4a The Schiff-base ligands 3-[N,N’-bis-(2-salicylidenami- (Co, 0.39 mmol gÀ1)ort-BuSalenCo-MCM-41 4b (Co, À1 no)ethyl]amine (SalenH2) and 3-[N,N’-bis-2-(5-tert-butylsa- 0.18 mmol g ). À1 licylidenamino)ethyl]amine (t-BuSalenH2) were synthesized SalenH2: FTIR (Neat, cm ): 2845 m, 1632 versus, from diethyltriamine and corresponding salicylaldehyde in 1581(s), 1497(s), 1461(s), 1279 versus, 757 versus; 1H- ethanol. Ethylene oxide was distilled after refluxing over NMR (CDCl3/TMS): d 2.94 (s, 4H, NCH2), 3.64 (s, 4H, a mixture of potassium hydroxide and calcium hydride. NCH2), 6.81 (t, 2H, PhH), 6.90 (m, 2H, PhH), 7.17 (m, 2H, Methanol and ethanol were distilled after refluxing over the PhH), 7.23 (t, 2H, PhH), 8.28 (s, 2H, PhCH); 13C-NMR corresponding magnesium alkoxides under nitrogen atmo- (CDCl3): d49.8 (NHCH2), 59.5 (CH2N=C), 116.9, 118.5, sphere. CO2 was purified by passing through a column 131.3, 132.1 (aromatic CH), 118.7, 161.1 (aromatic C), À1 packed with 4A molecular sieves before use. 165.9 (C=N). (t-Bu)SalenH2: FTIR (Neat, cm ): 2961 中国科技论文在线 http://www.paper.edu.cn

X.-B. Lu et al. / Applied Catalysis A: General 275 (2004) 73–78 75

Scheme 2. Synthesis of SalenCo(II) or (t-Bu)SalenCo(II) complexes covalently anchored to MCM-41 silica.

versus 2866(s), 1635 versus 1590(s), 1493 versus 1463(s), MCM-41: FTIR (KBr, cmÀ1): 2900–3100, 1631, 1571, 1 1267 versus 829(s); H-NMR (CDCl3): d1.25 (s, 18H, 1458, 814. 2 À1 C(CH3)3), 2.99 (s, 4H, NCH2), 3.69 (s, 4H, NCH2), 6.73 (m, N2 sorption: 1; surface area (S) = 868 m g , mesoporous 3 À1 2H, PhH), 6.88 (t, 2H, PhH), 7.22 (t, 2H, PhH), 7.33 (m, 2H, volume (Vm) = 0.881 cm g , BJH pore diameter (DBJH)= 13 2 À1 3 PhH), 8.36 (s, 2H, PhCH); C-NMR (CDCl3): d31.7 (CH3), 2.81 nm. For 4a; S = 655 m g , Vm = 0.512 cm , DBJH = 34.2 (C(CH3)3), 50.1 (NHCH2), 59.7 (CH2N=C), 115.2, 2.33 nm. XRD (d100 value): for 1, 3.97 nm; 2, 3.97 nm; 3, 127.8, 129.7 (aromatic CH), 116.9, 141.4, 158.9 (aromatic 3.98 nm; 4, 4.01 nm. 13 C), 166.5 (C=N). C MAS NMR of SalenH2-MCM-41: d10.5 (CH2Si), 26.4, 30.4 (CH2CH2N), 46.0, 46.8 2.3. Measurement (NCH2CH2N=), 114.3, 118.1, 126.2, 127.1, 132.4, 151.2 (aromatic C), 156.2 (C=N). SalenCo-MCM-41: FTIR (KBr, The MCM-41 samples were characterized by powder cmÀ1): 2900-2960, 1629, 1560, 1457, 801. t-BuSalenCo- X-ray diffraction (XRD, RIGAKU D/Max 3400, Cu Ka

Fig. 1. Schematic view of the continuous-ﬂow reaction apparatus. The parts are labeled as follows: P, HPLC pump; F, one-way valve; M, mixer; PH, preheater; R, relief valve; T, heating oven; S, pressure regulator; W, switch valve; D, analytic system for ethylene oxide (see ref. [30]); G, gas ﬂow gauging device. 中国科技论文在线 http://www.paper.edu.cn

76 X.-B. Lu et al. / Applied Catalysis A: General 275 (2004) 73–78

radiation). The pore diameter, pore volume and surface areas 3. Results and discussion were calculated from adsorption and desorption isotherms of N2 at 77 K according to the Barrett-Joyner-Halenda (BJH) Although, SC-CO2 is a good solvent for most nonpolar method using a micromeritics instrument from Quanta- and some polar organic compounds with low molecular chrome Corporation. NMR spectra were recorded by a Varian weight, it dissolves quaternary salts such as tetrabutylam- INOVA-400 type spectrometer (400 MHz). Infrared spectra monium bromide (n-Bu4NBr) hardly at all. On the contrary, were recorded under ambient condition with a Nicolet 50X n-Bu4NBr was found to dissolve in SC-CO2/ethylene oxide FTIR spectrophotometer. Cobalt analyse was obtained on a mixture, perhaps resulting from its high solubility in Perkin-Elmer 4000 atomic absorption spectrometer after ethylene oxide. digestion of the immobilized cobalt complexes. The catalyst used is cobalt complexes of 3-[N,N’-bis-(2- salicylidenamino)ethyl]amine (Salen) or 3-[N,N’-bis-2-(5- 2.4. Cycloaddition procedure tert-butylsalicylidenamino)ethyl]amine (t-BuSalen), immobilized onto ordered mesoporous MCM-41 silica. A A schematic view of the continuous flow apparatus is quaternary ammonium salt such as n-Bu4NBr was dissolved shown in Fig. 1. The reactor made of stainless steel tubing of in ethylene oxide and used as co-catalyst. 6 mm in inner diameter and 300 mm in length, was used for The catalyst performed well in the cycloaddition of CO2 holding the anchored cobalt complexes of 4 g (about 8 cm3) and ethylene oxide to continuously produce corresponding during the reaction. A frit (glass fiber) at the bottom of the cyclic carbonate. A conversion of up to 85.6% for ethylene tube keeps the immobilized catalyst in place. In a typical oxide was achieved at 110 8C and 12.5 MPa with a flow rate À1 À1 procedure, liquid CO2 and ethylene oxide together with n- of 10 ml h for ethylene oxide and that of 20 ml h for Bu4NBr were pumped with HPLC pumps and allowed to liquid CO2 (Table 1, entry 1). No byproduct such as flow through the system at constant pressure. They meet at polycarbonates or polyester was observed in the obtained the mixing area (M) and then pass through the preheater products. However, the cycloaddition reaction proceeded (PH). The CO2/ethylene oxide mixture, after being warmed under subcritical condition (4 MPa), the conversion of only in the preheater area, was introduced into the reactor. The 18.4%, which value is less than one-quarter of that at effluent from the reactor was then introduced the collecting 12.5 MPa, was obtained at the same temperature and flow vessel, where the pressure was reduced to atmospheric. rates of reactants. We have reported that with homogeneous Notes: the pressure regulator (S) should be maintained above SalenAlCl/n-Bu4NBr binary catalyst [30], the formation rate 40 8C for avoiding the block of the produced ethylene of ethylene carbonate under supercritical conditions is about carbonate towards the regulator and the introducing tube. In two times that under 4 MPa pressure in the autoclave. The the (D) area, the unreacted ethylene oxide was quantitatively higher rate for the cycloaddition reaction under supercritical ring-opened with sulfuric acid dissolved in the saturated conditions could be ascribed to the increasing residence time solution of MgCl2 at 0 8C, and CO2 were further allowed to of the reactants in the reactor, as well as the enhanced flow the (G) area, in which extra CO2 was measured. desorption and transfer of product by internal and external

Table 1 a Cycloaddition of CO2 and ethylene oxide in a continuous ﬂow system b c d Entry Temperature (8C) Flow rate of CO2 (ml/hr) Flow rate of EO (ml/hr) Conversion (%) Activity (ml/hr) 1 110 20 10 85.6 111 2 110 40 20 62.1 161 3 110 60 30 42.8 166 4 110 18 12 79.8 124 5 110 15 15 61.5 119 6 100 20 10 56.3 73 7 120 40 20 81.5 211 8e 110 20 10 63.9 179 9f 110 20 10 18.4 24 10g 110 20 10 4.1 – 11h 110 20 10 0 0 12i 110 ––56.7 945 a n-Bu4NBr/EO = 1/1000 (molar ratio); pressure, 12.5 MPa; SalenCo(II)-MCM-41 was used. b EO = ethylene oxide. c Based on ethylene oxide. d Mole of product (ethylene carbonate)/mol of Co catalyst per hour. e (t-Bu)SalenCo(II)-MCM-41 was used. f The reaction was carried out at low pressure of 4 MPa. g SalenCo(II)-MCM-41 was replaced with non-modiﬁed MCM-41 silica. h In the absence of n-Bu4NBr. i The reaction was carried out in 75 ml autoclave at 12–14 MPa, (t-Bu)SalenCo(II)/n-Bu4NBr/EO/CO2 = 1/1/5000/10000 (molar ratio), 3 h. 中国科技论文在线 http://www.paper.edu.cn

X.-B. Lu et al. / Applied Catalysis A: General 275 (2004) 73–78 77

Fig. 2. A plot of conversion of ethylene oxide towards ethylene carbonate versus time in the continuous-flow reactor. Condition: n-Bu NBr/ethylene 4 Fig. 4. Powder X-ray diffraction patterns of SalenCo-MCM-41: (a) before oxide = 1/1000 (molar ratio); temperature, 110 8C; pressure, 12.5 MPa; flow reaction; (b) after reaction in the continuous-flow reactor for 24 h. rate of CO2 was 40 ml/h, flow rate of ethylene oxide was 20 ml/h.

diffusion in the mesopores of MCM-41. Compared to observed in the system of the immobilized complex being supercritical homogeneous catalysis proceeded in the loaded in the reactor (entry 8), but its catalytic activity is autoclave, the influence of phase change during the reaction about 1.6 times that of SalenCo-MCM-41. For a comparation, on the rate can be avoided or greatly decreased, because of a homogeneous catalyst system consisted of (t-Bu)SalenCo the formed ethylene carbonate being promptly removed in and n-Bu4NBr was also examined for this reaction in a 75 ml the continuous-flow reactor. autoclave (entry 12). Decreasing the residence time of the reactants in the The immobilized cobalt complex exhibited good stability reactor by increasing the flow rates of liquid CO2 and and was subjected to utilization for 24 h with no loss of ethylene oxide results in a decrease in conversion, but the activity (Fig. 2). Fig. 3, based on cobalt leaching from the formation rate of ethylene carbonate increases to a certain reactor during the reaction, further confirmed the excellent extent (entry 1–3). For a given total flow, decreasing the stability of the immobilized catalyst. Slight cobalt leaching molar ratio of CO2 to ethylene oxide causes a decrease in mainly occurred in the first 12 h, which probably resulted only conversion rather than the formation rate of ethylene from non-covalently coordinated cobalt complexes. The carbonate (entry 1, 4, 5). The cycloaddition reaction appea- powder X-ray diffraction patterns (Fig. 4) of the SalenCo- red to be dependent of the temperature. The conversion of MCM-41 sample having been used for 24 h at 12.5 MPa are 62.1% at 110 8C with a flow rate of 20 ml hÀ1 for ethylene similar to that of the fresh SalenCo-MCM-41, indicating no À1 oxide and 40 ml h for liquid CO2 increased to 81.5% with change in the structure of the supported mesoporous silica the catalyst bed at 120 8C. Because of the low loading for during the reaction. (t-Bu)SalenCo-MCM-41, a relative low conversion was Surprisingly, the immobilized cobalt complex alone shows any catalytic activity hardly at all (entry 11). With a quaternary salt, such as n-Bu4NBr as co-catalyst, the cobalt complex exhibits high catalytic property towards the cycloaddition reaction, whereas n-Bu4NBr alone only shows low catalytic activity under employed conditions (entry 10). The results indicate that a synergistic effect exists in the formation of ethylene carbonate from CO2 and ethylene oxide by using as catalyst the system of the immobilized cobalt complex in conjunction with the quaternary salt, but the mechanism is not clear.

4. Conclusion

In summary, chemical ﬁxation of CO2 to ethylene carbonate proceeds effectively under supercritical condi- Fig. 3. Cobalt leaching from the reactor during the reaction. The reaction tions by using as catalyst the system of an immobilized conditions are the same as Fig. 2. cobalt complex in conjunction with a quaternary ammonium 中国科技论文在线 http://www.paper.edu.cn

78 X.-B. Lu et al. / Applied Catalysis A: General 275 (2004) 73–78

salt in a flow apparatus, based on supercritical fluid used as [14] T. Yano, H. Matsui, T. Koike, H. Ishiguro, H. Fujihara, M. Yoshihara, both solvent and reactant. No byproduct such as poly- T. Maeshima, Chem. Commun. (1997) 1129. [15] K. Yamaguchi, K. Ebitani, T. Yoshida, H. Yoshida, K. Kaneda, J. Am. carbonates or polyester was observed in the obtained Chem. Soc. 121 (1999) 4526. products. The immobilized cobalt complex exhibited good [16] H.S. Kim, J.J. Kim, B.G. Lee, O.S. Jung, H.G. Jang, S.O. Kang, stability and was subjected to utilization for 24 h with no Angew. Chem. Int. Ed. 39 (2000) 4096. loss of activity. Furthermore, this work demonstrates that the [17] R.L. Paddock, S.T. Nguyen, J. Am. Chem. Soc. 123 (2001) 11498. cycloaddition of CO with ethylene oxide, and other CO [18] H.Z. Yang, Y.L. Gu, Y.Q. Deng, F. Shi, Chem. Commun. (2002) 274. 2 2 [19] X.B. Lu, H. Wang, R. He, J. Mol. Catal. A: Chem. 186 (2002) 33. reactions in SC-CO2 are ideally suited for proceeding in flow [20] H. Yasuda, L.N. He, T. Sakakura, J. Catal. 209 (2002) 547. reactors. It is clear that the methodology presented here [21] S.I. Fujita, B.M. Bhanage, Y. Ikushima, M. Shirai, K. Torii, M. Arai, could be highly relevant for the further development of truly Catal. Lett. 79 (2002) 95. environmentally benign process based on SC-CO2 serving as [22] D.W. Park, D.H. Shin, S.W. Park, I. Kim, J.W. Lee, Stud. Surf. Sci. both solvent and cheap C component. Catal. 145 (2003) 259. 1 [23] P.E. Savage, S. Gopalan, T.I. Mizan, C.J. Martino, E.E. Brock, AICHE J. 41 (1995) 1723. [24] P.G. Jessop, T. Ikariya, R. Noyori, Chem. Rev. 99 (1999) 475. Acknowledgment [25] B. Subramaniam, C.J. Lyon, V. Arunajatesan, Appl. Catal. B 3 (7) (2002) 279. Gratitude is expressed to the National Science Founda- [26] P.G. Jessop, T. Ikariya, R. Noyori, Nature 368 (1994) 231. [27] P.G. Jessop, Y. Hsiao, T. Ikariya, R. Noyori, J. Am. Chem. Soc. 116 tion of China (NSFC) program (Grant 20204002) and the (1994) 8851. Science Foundation of Liaoning Province (Grant 20031074) [28] P.G. Jessop, Y. Hsiao, T. Ikariya, R. Noyori, J. Am. Chem. Soc. 118 for financial support. (1996) 344. [29] X.B. Lu, Y.Z. Pan, D.F. Ji, R. He, Chin. Chem. Lett. 11 (2000) 589. [30] X.B. Lu, R. He, C.X. Bai, J. Mol. Catal. A: Chem. 186 (2002) 1. [31] L.N. He, H. Yasuda, Sakakura, Green Chem. 5 (2003) 92. References [32] H. Kawanami, A. Sasaki, K. Matsui, Y. Ikushima, Chem. Commun. (2003) 896. [1] W. Leitner, Coord. Chem. Rev. 153 (1996) 257. [33] M.G. Hitzler, M. Poliakoff, Chem. Commun. (1997) 1667. [2] X.L. Yin, J.R. Moss, Coord. Chem. Rev. 181 (1999) 27. [34] W.K. Gray, F.R. Smail, M.G. Hitzler, S.K. Ross, M. Poliakoff, J. Am. [3] D.J. Darensbourg, M.W. Holtcamp, Coord. Chem. Rev. 153 (1996) Chem. Soc. 121 (1999) 10711. 155. [35] N.J. Meehan, A.J. Sandee, J.N.H. Reek, P.C.J. Kamer, P.W.N. van [4] A.A.G. Shaikh, S. Sivaram, Chem. Rev. 96 (1996) 951. Leeuwen, M. Poliakoff, Chem. Commun. (2000) 1497. [5] K.C. Nicolaou, E.A. Couladouros, P.G. Nantermet, J. Renaud, R.K. [36] X.B. Lu, W.H. Zhang, J.H. Xiu, R. He, L.G. Chen, X. Li, Ind. Eng. Guy, W. Wrasidlo, Angew. Chem. Int. Ed. 33 (1994) 1581. Chem. Res. 42 (2003) 653. [6] J.H. Clements, Ind. Eng. Chem. Res. 42 (2003) 663. [37] C.T. Kresge, M.E. Leonowicz, W.J. Roth, J.C. Vartuli, J.S. Beck, [7] R.L. De Pasquale, Chem. Commun. (1973) 157. Nature 359 (1992) 710. [8] T. Tsuda, Y. Chujo, T. Saegusa, Chem. Commun. (1976) 415. [38] J.S. Beck, J.C. Vartuli, W.J. Roth, M.E. Leonowicz, C.T. Kresge, K.D. [9] M. Ratzenhofer, H. Kisch, Angew. Chem. Int. Ed. 19 (1980) 317. Schmitt, C.T-W. Chu, D.H. Olson, E.W. Sheppard, S.B. McCullen, [10] T. Aida, S. Inoue, J. Am. Chem. Soc. 105 (1983) 1304. J.B. Higgins, J.L. Schlenker, J. Am. Chem. Soc. 114 (1992) 10834. [11] R. Nomura, A. Ninagawa, H. Matsuda, J. Org. Chem. 45 (1980) 3735. [39] J.Y. Ying, C.P. Mehnert, M.S. Wong, Angew. Chem. Int. Ed. 38 (1999) [12] K. Kasuga, S. Nagao, T. Fukumoto, M. Handa, Polyhedron 15 (1996) 56. 69. [40] X.G. Zhou, X.Q. Yu, J.S. Huang, S.G. Li, L.S. Li, C.M. Che, Chem. [13] W.J. Kruper, D.V. Dellar, J. Org. Chem. 60 (1995) 725. Commun. (1999) 1789.