Unexpected Cis,Cis to Trans,Trans Isomerization of a Disilanyl Analogue of 1,5-Cyclooctadiene

We report the synthesis and unanticipated cis,cis to trans,trans isomerization of a unique disilanyl analogue of 1,5-cyclooctadiene. Scheme 1 shows the synthesis of cis,cis1,1,2,2,5,5,6,6-octamethyl-1,2,5,6-tetrasilacycloocta-3,7-diene (1) and its Ru-catalyzed isomerization to trans,trans1,1,2,2,5,5,6,6-octamethyl-1,2,5,6-tetrasilacycloocta-3,7diene (2).1,2 The isomerization proceeds slowly in methylene chloride upon exposure of 1 to a catalytic portion of the recently described dimeric ruthenium(II) alkylidene complex (Cy3P)RuCl2(dCHPh)Ru(p-cymene)Cl2. Compounds 1 and 2 were characterized by single-crystal X-ray diffraction (Figure 1). Both compounds exist as chair conformers in the solid state. Table 1 shows selected bond lengths and angles of the chair conformers. We probed the mechanism of the isomerization with a variety of experiments. First, dissolution of 1 in CD2Cl2 in the absence of the Ru(II) complex produced no trace of 2, even upon heating at 40 °C for 20 days. Second, the rate of isomerization increased when we increased the concentration of the Ru(II) complex from 2.5 mol % to 5.0 mol % to 10 mol %. Third, exposure of 2 to the Ru(II) complex in CD2Cl2 produced no trace of 1, even upon heating at 40 °C for 20 days. These results suggest that the isomerization is catalyzed by Ru and that the trans,trans conformer 2 is more stable than the cis,cis conformer 1 by at least 1.8 kcal/mol (assuming a ratio of products g95:5 based on a conservative estimate of our ability to detect trace amounts of 1 by 1H NMR spectroscopy).4 We explored several alternative methods known to catalyze the cis to trans isomerization of olefins. Iodine, which isomerizes cis,trans-1,5-cyclooctadiene to cis,cis-1,5-cyclooctadiene in 65% yield,5 failed to isomerize 1 to 2, even upon heating at 40 °C for 10 days. Transition-metal-based isomerization catalysts such as cobalt(I) hydrocarbonyl (HCo(CO)4) and tris(triphenylphosphine)rhodium(I) chloride (RhCl(PPh3)3) also failed to effect the isomerization.6-8 Ruthenium(II) hydrides have been implicated as catalysts in the isomerization of olefins, even when the source of the hydride remains obscure.9 These considerations prompted us to examine the reactivity of 1 with the commercially available ruthenium(II) hydride complexes chlorohydridotris(triphenylphosphine)ruthenium(II) ([Ph3P]3Ru(Cl)H) and dihydridotetrakis(triphenylphosphine)ruthenium(II) ([Ph3P]4RuH2). Indeed, exposure of 1 to these ruthenium(II) hydrides in CD2Cl2 at 40 °C leads to isomer 2. We also found that bis(cyclopentadienyl)zirconium(II) chloride hydride (Cp2Zr(Cl)H) effects the isomerization, although this catalyst appears to be less efficient than the ruthenium(II) hydrides. Taken together, these results suggest that a ruthenium(II) hydride is the active catalyst in the isomerization promoted by (Cy3P)RuCl2(dCHPh)Ru(p-cymene)Cl2. In considering possible reaction pathways, it is likely that the isomerization proceeds via a hydride addition/elimination mechanism.9 A probable intermediate in this pathway would be the cis,trans isomer. Although we have been unable to identify or isolate this intermediate, we undertook a series of calculations to evaluate the energy of this isomer relative to isomers 1 and 2.10-12 In our calculations, we employed the MM2 force field and two semiempirical methods (based on the PM3 and AM1 parametrizations, respectively).13 Table 2 shows the results of the calculations. The methods suggest a similar trend in stabilities: cis,cis > trans,trans > cis,trans. We note, however, that the * To whom correspondence should be addressed. E-mail: trlee@uh.edu. † Department of Chemistry, Harvard University, Cambridge, MA. ‡ Division of Chemistry and Chemical Engineering, California Institute of Technology, Pasadena, CA. (1) Sakurai, H.; Nakadaira, Y.; Hosomi, A.; Eriyama, Y.; Kabuto, C. J. Am. Chem. Soc. 1983, 105, 3359. Gleiter, R.; Schafer, W.; Sakurai, H. J. Am. Chem. Soc. 1983, 105, 3046. Iwahara, T.; West, R. J. Chem. Soc., Chem. Commun. 1988, 954. Ishikawa, M.; Hatano, T.; Hasegawa, Y.; Horio, T.; Funai, A.; Miyai, A.; Ishita, T.; Tsukihara, T.; Yamanaka, T.; Koike, T.; Shioya, J. Organometallics 1992, 11, 1604. Sakurai, H.; Tominaga, K.; Watanabe, K.; Kumada, M. Tetrahedron Lett. 1966, 45, 5493. (2) Eisch, J. J.; Foxton, M. W. J. Org. Chem. 1971, 36, 3520. (3) Dias, E. L.; Grubbs, R. H. Organometallics 1998, 17, 2758. (4) In kinetics studies of the isomerization of 1 to 2 using ferrocene as an internal standard for integration by 1H NMR spectroscopy, we have found that 2 is more labile than 1 under the isomerization conditions. Furthermore, at the latter stages of the isomerization, the rate of loss of 2 becomes competitive with the rate of isomerization of 1 to 2. Consequently, we feel that the inability to detect trace amounts of 1 in the attempted isomerization of 2 to 1 provides strong (albeit negative) evidence that 2 is thermodynamically more stable than 1 within the limits described in the text. (5) Cope, A. C.; Howell, C. F.; Bowers, J.; Lord, R. C.; Whitesides, G. M. J. Am. Chem. Soc. 1967, 89, 4024. (6) Roos, L.; Orchin, M. J. Am. Chem. Soc. 1965, 87, 5502. (7) Hendrix, W. T.; Rosenberg, J. L. J. Am. Chem. Soc. 1976, 98, 4850. (8) Corey, E. J.; Suggs, J. W. J. Org. Chem. 1973, 38, 3224. (9) See, for example: McGrath, D. V.; Grubbs, R. H. Organometallics 1994, 13, 224. (10) Molecular mechanics calculations (MM2) were performed using PC Model (version 5.13). Semiempirical calculations were performed with Spartan 5.0 using the PM3 and AM1 parametrizations.11,12 In all cases, optimal geometries were determined without the use of additional symmetrization of the Hamiltonian (NOSYMTRY option). The absence of imaginary normal-mode frequencies confirmed that the structures correspond to local energy minima. (11) Spartan version 5.0, Wavefunction, Inc., Irvine, CA 1997. (12) For a review of semiempirical methods, see: Clark, T. A Handbook of Computational Chemistry; Wiley: New York, 1986. (13) We note that minimization with the PM3 Hamiltonian appears to provide the closest qualitative resemblance to the X-ray structures of 1 and 2 in terms of bond lengths and angles. Figure 1. ORTEP drawings (40% probability level) for 1 (left) and 2 (right).