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Organometallics 2009, 28, 2880–2887
Cross-Metathesis of Vinyl Halides. Scope and Limitations of Ruthenium-Based Catalysts Marisa L. Macnaughtan, J. Brannon Gary, Deidra L. Gerlach, Marc J. A. Johnson,* and Jeff W. Kampf Department of Chemistry, UniVersity of Michigan, 930 North UniVersity AVenue, Ann Arbor, Michigan 48109-1055 ReceiVed May 21, 2008
The phosphine-free “second-generation” Blechert/Hoveyda-Grubbs catalyst Ru(dC(H)C6H4-o-O-iPr)(H2IMes)Cl2 (H2IMes ) 4,5-dihydro-1,3-dimesitylimidazol-2-ylidene) and Piers catalyst [Ru(dCHPCy3)(H2IMes)Cl2]BF4 (Cy ) cyclo-C6H11) for olefin metathesis effected cross-metathesis (CM) of vinyl chloride and 1,2-dichloroethene with several unhindered terminal and internal alkenes in up to 95% yield (5 mol % catalyst). In most cases, 1,2-dichloroethene was more successful than vinyl chloride. Ring-opening CM of cyclooctene provided greater yields than CM: with vinyl chloride, 93% yield; with 1,2dichloroethene, >95%. Other common Ru-based catalysts failed to effect CM under similar conditions, but instead underwent rapid decomposition. The dimeric ruthenium-monochloromethylidene complex [Ru(dCHCl)(H2IMes)Cl(µ-Cl)]2 was isolated as a thermally unstable intermediate. CM reactions with 1,2-dibromoethene afforded 22% CM product in the best case; halide exchange with the catalyst was significant. CM reactions involving vinyl fluoride typically led to Cl . F), the ruthenium monohalomethylidene intermediate becomes more sensitive to decomposition.3-5,9 Computational studies of the mechanism of carbide formation10 and the factors that control the stability of the carbide unit9 indicate that with strong σ-donating PCy3 ligands the formation of the terminal carbide complex 4 via expulsion of acetic acid (Scheme 1, top; X ) O2CMe, L ) PCy3) is barely favorable. Furthermore, the ancillary ligands are bonded more strongly to the Ru center in the terminal carbide complexes than in their precursors (Scheme 3).9 The greater metal-ligand bond strengths in the carbide complexes compared to their precursors is reflected experimentally by the much greater difficulty in substituting the ancillary ligands following formation of the (7) Fomine, S.; Ortega, J. V.; Tlenkopatchev, M. A. J. Mol. Catal. A 2007, 263, 121. (8) Trnka, T. M.; Day, M. W.; Grubbs, R. H. Angew. Chem., Int. Ed. 2001, 40, 3441. (9) Gary, J. B.; Buda, C.; Johnson, M. J. A.; Dunietz, B. D. Organometallics 2008, 27, 814. (10) Buda, C.; Caskey, S. R.; Johnson, M. J. A.; Dunietz, B. D. Organometallics 2006, 25, 4756.
Scheme 3. Ancillary Ligand Effect on Carbide Formation
carbide ligand.3,11 Together, these data suggest that terminal carbide formation from monohalomethylidene complexes could be avoided by the use of catalysts in which the labile ligand is very weak or absent, i.e., by making the ruthenium center more electron-deficient. Additionally, the labile PCy3 ligands in compounds such as 6 are implicated in catalyst decomposition.12 Furthermore, avoiding the presence of PCy3 ligands prevents the formation of the phosphoniomethylidene complex 5 (Scheme 1, bottom). Therefore, we examined the reactions of vinyl halides with reactive terminal and internal olefins in the presence of catalysts 7-9 (Chart 1). Grela and co-workers reached similar conclusions. Using catalyst 8 and closely related compounds, they describe CM in neat 1,2-dichloroethene in good yields for four substrates and low to moderate yields in a few other cases.13 Herein we report the successful CM reactions and ring-opening CM reactions of vinyl halides in more conventional solvents as well as the identification of ruthenium monohalomethyidene dimers that act as intermediates in these CM reactions.
Results and Discussion CM of vinyl halides was tested with “phosphine-free” catalysts to determine if decomposition of the intermediates could be suppressed in order to allow for productive CM. Precatalyst 714 has weakly donating 3-bromopyridine ligands. Weakly donating neutral ligands greatly enhance the initiation rate of the metathesis catalyst. The presence of neutral ligands in the reaction mixture increases the longevity of the ruthenium alkylidene intermediates through reassociation to form more stable 16-electron ruthenium species. However, the presence of these neutral ligands may contribute to faster decomposition of the ruthenium monohalomethylidene intermediates as discussed earlier.15 The chelating ether group in 816,17 renders 8 slow to initiate, but this donor is lost in the first metathesis cycle, generating the 14-electron active catalyst (Scheme 4, boxed). Moreover, the liberated isopropoxystryene may undergo metathesis with a ruthenium intermediate to regenerate precata(11) Stewart, M. H.; Johnson, M. J. A.; Kampf, J. W. Organometallics 2007, 26, 5102. (12) Hong, S. H.; Wenzel, A. G.; Salguero, T. T.; Day, M. W.; Grubbs, R. H. J. Am. Chem. Soc. 2007, 129, 7961. (13) Sashuk, V.; Samojlowicz, C.; Szadkowska, A.; Grela, K. Chem. Commun. 2008, 2468. (14) Love, J. A.; Morgan, J. P.; Trnka, T. M.; Grubbs, R. H. Angew. Chem., Int. Ed. 2002, 41, 4035. (15) Courchay, F. C.; Sworen, J. C.; Wagener, K. B. Macromolecules 2003, 36, 8231. (16) Garber, S. B.; Kingsbury, J. S.; Gray, B. L.; Hoveyda, A. H. J. Am. Chem. Soc. 2000, 122, 8168. (17) Gessler, S.; Randl, S.; Blechert, S. Tetrahedron Lett. 2000, 41, 9973.
2882 Organometallics, Vol. 28, No. 9, 2009 Scheme 4. Initiation of Precatalysts 7, 8, and 9
lyst 8, thereby extending the lifetime of the catalyst without directly jeopardizing the monohalomethylidene intermediates. The same active species forms when the Piers complex 918 irreversibly loses vinyltricyclohexylphosphonium ion in the first metathesis cycle (Scheme 4). Overall, monohalomethylidene complexes formed from catalysts 7, 8, and 9 would be less likely to undergo conversion into terminal carbide species when compared with catalysts containing PCy3 ligands. Because RudCHF complexes such as 10 and 14 are more persistent than their RudCHCl (12) counterparts with respect to ruthenium carbide formation,3-5 we initially examined metathesis reactions with vinyl fluoride in the presence of 1, 7, and 8. In the CM reaction of vinyl fluoride with 5-decene (eq 2), only 2.6 turnovers (TON) were observed. The only new carbene intermediate observed was the monofluoromethylidene complex (10), which decomposed to the corresponding carbide (3). Precatalyst 7 failed to afford the desired product. Reactions with 8 (Table 1, entry 1) gave 95% yield. Ring-opening CM is even more favorable, giving close to quantitative conversion to the chlorinated alkene products (>95%, 3% catalyst loading) and 55% yield with vinyl fluoride. Yields in CM reactions of vinyl fluoride and 1,2-dibromoethene are low, but catalysis occurs for these substrates also. At present, only quite reactive olefins participate successfully in these CM reactions. Rapid catalyst decomposition precludes reactions with more challenging substrates for vinyl chlorides and bromides; with vinyl fluoride, a key RudCHF intermediate is too stable with respect to continued CM. More active or more robust catalysts may be needed in order to reduce the catalyst loading with sensitive substrates. The rational design of more robust catalysts will rely on the identification of the catalyst decomposition products under these conditions, an area we are now exploring.
Experimental Section General Procedures. All reactions were set up in a nitrogenfilled MBRAUN Labmaster 130 glovebox, unless otherwise
General Preparative Procedure for Reactions Involving Only Liquid/Solid Substrates. Internal standard (1,3,5-trimethoxybenzene or 1-bromo-3,5-bis(trifluoromethyl)benzene; 0.05 mmol, from a 0.10 M stock solution, 0.50 mL), alkene A (nonhalogenated olefin), and alkene B (halogenated olefin) were dissolved in 0.8 mL of C6D6 (and/or CD2Cl2 depending on reaction conditions; see Table 2), and a standard NMR spectrum was acquired. The appropriate amount of 7, 8, or 9 (0.03 M stock solution) was added to the reaction mixture at 23 °C (5% or 10% relative to limiting reagent; see Table 2 for quantities of starting materials). The reaction mixture was then heated to 40-50 °C or kept at room temperature (24) Gottlieb, H. E.; Kotlyar, V.; Nudelman, A. J. Org. Chem. 1997, 62, 7512. (25) Petasis, N. A.; Yudin, A. K.; Zavialov, I. A.; Prakash, G. K. S.; Olah, G. A. Synlett 1997, 606.
2886 Organometallics, Vol. 28, No. 9, 2009 (23 °C) for up to 24 h. NMR spectra were acquired at 1, 2, 3, 5, 16, and 24 h. Most reactions required less than 3 h to reach the end point. The reaction mixture was then passed through alumina and washed through with either pentane or C6D6. Finally, GC-MS and NMR data of the resulting catalyst-free solution were acquired to distinguish all products. General Preparative Procedure for Reactions with Gaseous Reagents. In a J. Young tube was placed the appropriate amount of stock solution of the catalyst (7, 8, or 9) (0.03 M stock solution) and C6D6 (and/or CD2Cl2 depending on reaction conditions), and the solution was frozen in the cold well of the glovebox. Internal standard (0.05 mmol) and alkene A (nonhalogenated olefin) were dissolved in 0.8 mL of C6D6, and the solution was added to the J. Young tube. The reaction mixture was again frozen, the head space in the J. Young tube was evacuated, and vinyl halide was added either while the reaction mixture was submerged in liquid N2 or the tube was simply refilled while the solution remained frozen without submergence. Amounts of vinyl halide in the reaction mixture were then determined by integration against the internal standard in the 1H NMR spectrum. The reaction mixture was heated to 40-50 °C or kept at room temperature (23 °C) for up to 24 h (with the exception of entry 1, Table 2, which was monitored for 48 h). NMR spectra were acquired after reaction times of 1, 2, 3, 5, 16, and 24 h. Most reactions required less than 3 h to reach the end point. The reaction mixture was then passed through alumina and washed through with either pentane or C6D6. Finally, GC-MS and NMR data of the resulting catalyst-free solution were acquired. Specific conditions for these reactions are collected in Table 2. Characterization data for alkenyl halide CM products (Chart 2) were in agreement with reported data: 24,26 32,27 43,28,29 23,30 34,31 25,32 35,33 26,34 36,35 37,36 27,25 38,33,37 31,38 39.13 Multiple CM reactions were attempted using the general preparative procedures detailed above with several combinations of reagents in which no catalytic reaction was seen. The following substrates proved unreactive in reactions with vinyl fluoride, vinyl chloride, and 1,2-dichloroethylene: stilbene, allyl alcohol, allyl acetate, ethyl vinyl ketone, methyl acrylate, vinyl boronic acid n-butyl ester. Multiple CM reactions of R-fluorostyrene, β-fluorostyrene, and 1,1difluoroethylene (separately) with 5-decene or 1-hexene yielded
