J. Am. Chem. SOC. zyxwvu 1994,116, zyxwvut 2163-2164 2163 Generation of Cyclopentadienyl Ligands via the Pauson-nand and Retro-Diels-Alder Reactions Bun Yeoul Lee, Hyungjoon Moon, and Young Keun Chung' Department of Chemistry, College of Natural Sciences Seoul National University, Seoul 151 -742, Korea Nakcheol Jeong Hanhyo Institutes of Technology San 6, Daeyadong, Shiheungshi, Kyunggido, Korea Received December 6, 1993 The development of efficient approaches to the generation of highly substituted or functionalized cyclopentadienyl ligands is of continuing importance.' One of the strategies to synthesize cyclopentadienylmetal compounds is to use cyclopentenone as a starting material.2 The Pauson-Khand reaction is one of the most effective methods in the syntheses of cyclopentenone~.~ Herein we report a powerful new method for the preparation of 1,2-disubstituted or 1,2,3-trisubstituted cyclopentadienyl ligands via the retro-Diels-Alder reaction of the intermolecular Pauson- Khand reaction products. Stille reported4 the use of the retro- Diels-Alder reaction to generate cyclopentadienyl anions and to prepare a polymer containing cyclopentadiene units. However, to the best of our knowledge, this is the first example of the general method of preparation 1 ,Zdisubstituted and 1,2,3- trisubstituted cyclopentadienyl ligands (Scheme 1). The substrates zyxwvutsrq 2 for the retro-Diels-Alder reaction could be efficiently prepared in two steps. The Pauson-Khand products 1, obtained in high yields by using DMSO as a promoter,5 were transformed into compounds 2 in high yields (Table 1). The retro-Diels-Alder reaction commonly requires high tem- peratures for convenient reaction.6 However, a number of retro- Diels-Alder reactions involving anionic intermediates take place at more moderate temperatures.' Grutzner* reported the retro- Diels-Alder reaction of zyxwvutsr 7-phenyl-7-methoxynorbornene at room temperature using excess sodium-potassium alloy. Karpfg and Ichihara'o groups demonstrated the generation of multiply substituted cyclohexenes by treatment with potassium hydride at room temperature. Instead of using sodium-potassium alloy or potassium hydride, we used potassium and n-BuLi." The retro- Diels-Alder reaction of 2 with potassium and n-BuLi was straightforward at room temperature within 2-3 h. The scope of the retro-Diels-Alder reaction was examined with a number of substrates (Table 1). We have observed an apparent require- (1) Halterman, R. L. Chem. Reu. 1992, 92, 965. Okuda, J. Top. Curr. Chem. 1991, 160, 97. Janiak, C.; Schumann. H. Adu. Organomet. Chem. 1991, 33, 291. Siemeling, U. J. Chem. SOC., Chem. Commun. 1992, 1335. Venier, C. G.; Casserly, E. W. J. Am. Chem. SOC. 1990, 112, 2808. (2) Halterman, R. L.; Vollhardt, K. P. C. Organometallics 1988, 7, 883. Halterman, R. L.; Vollhart, K. P. C. Tetrahedron Lett. 1986, 27, 1461. Paquette, L. A.; Moriaty, K. J.; McKinney, J. A,; Rogers, R. D. Organo- metallics 1989,8, 1707. Gibson, C. P.; Bem, D. S.; Falloon, S. B.; Hitchens, T. K.; Cortopassi, J. E. Organometallics 1992,11, 1742. Bunel, E. E.; Valle, L.; Jones, N. L.; Carroll, P. J.; Gonzalez, M.; Munoz, N.; Manriquez, J. M. Organometallics 1988, 7, 789. (3) Pauson. P. L.; Khand. I. U. Ann. N. zyxwvutsrq Y. Acad.Sci. 1977,295,2. Pauson, P. L..Tetrahedron 1985, 41, 5855. (4) Sekiya, A.; Stille, J. K. J. Am. Chem. SOC. 1981, 103, 5096. (5) Lee, B.,Y.; Chung, Y. K.; Jeong, N.; Hudecek, M.; Pauson, P. L. Organometallics 1993, 12, 220. (6) Ripoll, J.-L.; Rouessac, A.; Rouessac, F. Tetruhedron 1978, 34, 19. Lasne, M.-C.; Ripoll, J.-L. Synthesis 1985, 121. Karpf, M. Angew. Chem., Int. Ed. Engl. 1986, 25,414. De Keyser, J.-L.; De Cock, C. J. C.; Poupaert, J. H.; Dumont, P. J. Org. Chem. 1988, 53, 4859. (7) Rajanbaubu, T. V.; Eaton, D. F.; Fukunaga, T. J. Org. Chem. 1983, 48, 652. Bunnelli, W. H.; Shangraw, W. R. Tetrahedron 1987, 43, 2005. (8) Bowman, E. S.; Hughes, G. B.; Grutzner, J. B. J. Am. Chem. SOC. 1976, 98, 8273. (9) Karpf, M. Angew. Chem., Inr. Ed. Engl. 1986, 25, 414. (10) Ichihara, A. Synthesis 1987, 207. Scheme 1 1) R'MgBr 01 R'CeCil, THF dRL 1) NIH, MeI, DMF 1) Cq1CO)r RL+R' Z)DMSO,50'C c 2 3 4 Table 1. yield (W)" entry RL RSR' 1 2 3 4 1 Ph Ph Ph 84 78 90 80 2 Ph Me Me 97 87 63 546 3 Ph Ph Bu 84 79 86 58 4 Ph Ph Me 84 80 76 64 5 Bu H Ph 86 92 91 516 6 (CH2)30Me H Ph 90 90 68 75 7 (CH2)40Me H Ph 72 85 95 63 8 (CH2)2CH=CH2 H Ph 80 75 65 47 H Ph 75 87 62 62 The reaction condition was not optimized. Isolated yields. The monodealkylated product was obtained in ca. 10% yield. ment for the presence of one phenyl group. Workup furnished an isomeric mixture of cyclopentadienes in 61-95%. Even with a methoxyalkyl substituent (entries 6 and 7), the reaction proceeded to give a high yield of cyclopentadienes. Furthermore, the reaction of substrates with an alkenyl substituent (entries 8 and 9) gave reasonable yields of cyclopentadienes. Compounds 3 are moderately air-stable and can be stored in hexane solution for 4-5 days. The isomeric mixture was reacted further. Metalation of 3 was carried out by treatment with Mn2(CO)lo in refluxing xylene for 0.5-2 days. After reaction, compounds 4 were obtained in reasonable to high yields.12 However, for entries 2 and 5, the reaction proceeded to give two kinds of products; one was 4 and the other was monodealkylated. Due to the dealkylation, the yields of 4 for entries 2 and 5 were rather low. Dealkylations via competitive activation of the carbon- carbon bond of the diene have been reported for other metal complexes.13 In summary, we have demonstrated that the retro-Diels-Alder reaction of the Pauson-Khand reaction product generates high (1 1) A typical procedure is as follows. A single piece of potassium (0.17 g, 4.4 mmol) and n-BuLi (1.3 mL, 2.0 mmol) were added to the THF solution (10 mL) of compound 2 (RL = RS = R' = Ph, 0.41 g, 1.04 mmol). After being stirred for 2-3 h, the reaction solution was transferred via syringe to a flask which contained 20 mL of aqueous, cold, saturated NaHCO, and 20 mL of hexane. The product was extracted with hexane. The product was purified by silica gel short-column chromatography eluting with hexane/ethyl acetate (v/v, 30:l). The yield was 90%. 3 (R = R = R = Ph): IH NMR (CDC1,) 67.186.95(m,Ph,lSH),6.46(t,J= 1.7Hz,CH=C,lH),3.55(d,J- (12) A typical procedure is as follows. Compound 3 (R = R = R = Ph), (0.15 g, 0.5 mmol), MnZ(CO)lo (0.214 g, 0.55 mmol), and xylene (25 mL) were placed in a Schlenk flask. After the mixture was refluxed for 2 days, the solvent was removed by rotary evaporation. The crude reaction product was purified by flashcolumnchromatographyelutingwith hexane. The product was obtained as an oily compound, yield of 80%. 4 (R = R = R = Ph): IR (NaCI) uco 2010, 1923 cm-1; LH NMR (C6Ds) 6 7.26-6.85 (m, Ph, 15 H), 129.96, 129.46, 129.17, 128.72, 128.43 (Ph), 106.27, 103.94, 80.54 (Cp); HRMS M+ obsd 432.0560, calcd 432.0558. (13) Hemond, R. C.; Hughes, R. P.; Locker, H. B. Orguometullics 1986, 5, 2391. Filbracht, P.; Dahler, P. Chem. Be?. 1980, 113, 542. King, R. B.; Efraty, A. J. Am. Chem. SOC. 1972, 94, 3773. 1.7 Hz, CHz,,2 H). 4.51 (s, Cp, 2 H); I3CNMR (C&) 6 225.71 (CO), 133.08, 132.86, 132.59, 0 1994 American Chemical Society