Organometallics 2010, 29, 6045–6050 DOI: 10.1021/om1007924
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Facile Synthesis of Effcient and Selective Ruthenium Olefin Metathesis Catalysts with Sulfonate and Phosphate Ligands Peili Teo and Robert H. Grubbs* Arnold and Mabel Beckman Laboratory of Chemical Synthesis, Division of Chemistry and Chemical Engineering, California Institute of Technology, Pasadena, California 91125, United States Received August 14, 2010
A series of novel, air-stable ruthenium NHC catalysts with sulfonate and phosphate anions have been prepared easily in one pot at high yields using commercially available precursors. The catalysts were found to be effective for ring-opening metathesis polymerization, ring-closing metathesis, and cross-metathesis. The catalysts showed higher cis-selectivity in olefin cross-metathesis reactions as compared to earlier known ruthenium-based olefin metathesis catalysts, with allylbenzene and cis-1,4-diacetoxybutene as substrates. Introduction Olefin metathesis has become a standard method in the formation of C-C double bonds.1,2 In particular, olefin crossmetathesis (CM) is a convenient route to functionalize higher olefins from simple precursors. CM has gained prominence due to the availability of catalysts of varying activities such as the phosphine-based3-6 and the N-heterocyclic carbene (NHC)-based Ru catalysts7,8 and the molybdenum/tungstenbased catalysts.9-11 However, application of cross-metathesis has been relatively limited when compared to ring-opening metathesis polymerizations (ROMP) and ring-closing metathesis (RCM).12-14 This is a result of poor stereoselectivity of the olefin products at high conversions. Z-Selectivity is usually compromised at high conversions, where current ruthenium *To whom correspondence should be addressed. E-mail: rhg@ caltech.edu. (1) Handbook of Metathesis; Grubbs, R. H., Ed.; Wiley VCH: Weinheim, Germany, 2003; Vols. 1-3. (2) Olefin Metathesis and Metathesis Polymerization; Ivin, K. J.; Mol, J. C., Eds.; Academic Press: San Diego, CA, 1997. (3) Schwab, P.; France, M. B.; Ziller, J. W.; Grubbs, R. H. Angew. Chem., Int. Ed. 1995, 34, 2039. (4) Schwab, P.; Grubbs, R. H.; Ziller, J. W. J. Am. Chem. Soc. 1996, 118, 100. (5) Schwab, P.; France, M. B.; Ziller, J. W.; Grubbs, R. H. Angew. Chem. 1995, 107, 2179. (6) Belderrain, T. R.; Grubbs, R. H. Organometallics 1997, 1997, 4001. (7) Scholl, S.; Ding, S.; Lee, C. W.; Grubbs, R. H. Org. Lett. 1999, 1, 953. (8) Sanford, M. S.; Love, J. A.; Grubbs, R. H. J. Am. Chem. Soc. 2001, 123, 6543. (9) Schrock, R. R.; Murdzek, J. S.; Bazan, G. C.; Robbins, J.; Dimare, M.; O’Regan, M. B. J. Am. Chem. Soc. 1990, 112, 3875. (10) Bazan, G. C.; Kosravi, E.; Schrock, R. R.; Feast, W. J.; Gibson, V. C.; O’Regan, M. B.; Thomas, J. K.; Davis, W. M. J. Am. Chem. Soc. 1990, 112, 8378. (11) Bazan, G. C.; Oskam, J. H.; Cho, H. N.; Park, L. Y.; Schrock, R. R. J. Am. Chem. Soc. 1991, 113, 6899. (12) Furstner, A. Angew. Chem. 2000, 112, 3013. (13) Trnka, T. M.; Grubbs, R. H. Acc. Chem. Res. 2004, 34, 18. (14) Kotha, S.; Screenicasachary, N. Ind. J. Chem. B 2001, 40, 763. (15) Ritter, T.; Hejl, A.; Wenzel, A. G.; Funk, T. W.; Grubbs, R. H. Organometallics 2006, 25, 5740. (16) Anderson, D. R.; Ung, T.; Mkrtumyan, G.; Bertrand, G.; Grubbs, R. H.; Schrodi, Y. Organometallics 2008, 27, 563. (17) Vehlow, K.; Maechling, S.; Blechert, S. Organometallics 2006, 25, 25. r 2010 American Chemical Society
catalysts has a tendency to give high E/Z ratios (>6) at conversions >75%.15-17 Although phosphine-based catalysts give lower E/Z ratios than their NHC counterparts, they tend to decompose at conversions no more than 60%.15 A highly Z-selective molybdenum catalyst has been recently developed by Hoveyda and Schrock.18-20 These very important catalysts should have numerous applications.18-21 However, as has been seen in the past, ruthenium-based catalysts can be more applicable in many situations. Although there are some ruthenium-based catalysts in recent literature that give better E/Z ratios than the more traditional catalysts, they are generally much less active.22 Other catalysts tend to have poor control over selectivity at high conversions as a result of secondary metathesis.16 The precatalysts themselves are also difficult to prepare with multistep synthesis and very low yields, making them less commercially viable. Z-Olefins are commonly found in natural products, and the ability to form Z-olefins from a single catalytic process remains important to synthetic organic chemistry.1 Hence, there remains an urgent need to develop tolerant catalysts that enable high conversions while delivering low E/Z ratios for production of Z-olefins. Many metathesis catalysts based on the [L2X2RudCHR] scaffold have been synthesized in an effort to increase catalyst stability, activity, and selectivity.3-5,7,8,12-17,22-24 However, most efforts were concentrated on the modifications of the L2 units or R group.23,25-28 Chen et al. has very recently reported the use (18) Flook, M. M.; Jiang, A. J.; Schrock, R. R.; Muller, P.; Hoveyda, A. H. J. Am. Chem. Soc. 2009, 131, 7962. (19) Ibrahem, I.; Yu, M.; Schrock, R. R.; Hoveyda, A. H. J. Am. Chem. Soc. 2009, 131, 3844. (20) Schrock, R. R. Chem. Rev. 2009, 109, 3211. (21) Courchay, F. C.; Sworen, J. C.; Wagener, K. B. Macromolecules 2003, 36, 8231. (22) Rosen, E. L.; Sung, D. H.; Chen, Z.; Lynch, V. M.; Bielawski, C. W. Organometallics 2010, 29, 250. (23) Szadkowska, A.; Makal, A.; Wozniak, K.; Kadyrov, R.; Grela, K. Organometallics 2009, 28, 2693. (24) Bornand, M.; Torker, S.; Chen, P. Organometallics 2007, 26, 3585. (25) Keitz, B. K.; Grubbs, R. H. Organometallics 2010, 29, 403. (26) Vougioukalakis, G. C.; Grubbs, R. H. Chem. Rev. 2010, 110, 1746. (27) Keitz, B. K.; Grubbs, R. H. J. Am. Chem. Soc. 2009, 131, 2038. (28) Lichtenheldt, M.; Wang, D.; Vehlow, K.; Reinhardt, I.; Kuhnel, C.; Decker, U.; Blechert, S.; Buchmeiser, M. R. Chem.;Eur. J. 2009, 15, 9451. Published on Web 10/25/2010
pubs.acs.org/Organometallics
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Organometallics, Vol. 29, No. 22, 2010
Teo and Grubbs
Scheme 1. Standard Olefin Cross-Metathesis Reaction Using [Ru] Catalysts
Scheme 2. Synthesis of Catalysts Initiators 4-14 from 1-3
of phosphinephenoxide and sulfoxide anionic ligands to control stereoselectivity in ROMP of norbornene/cyclooctene.29 Buchmeiser et al. has also reported the use of triflate and trifluoroacetate anions as ligands for Ru complexes for RCM and enyne metathesis reactions.30 To the best of our knowledge, there have been no other reports on the successful modification of the anions (X2) for improved E/Z selectivities in olefin metathesis reactions, especially in CM reactions.26,27,31-33 Anion modification is mostly done by replacing a chloride anion by an aryloxide attached to the NHC or PR3 itself. These NHCs or phosphine ligands, however, are often difficult to synthesize, resulting in poor yields of the ligand itself and the final precatalysts prepared.26 Herein, we report a series of air-stable and easy-toprepare ruthenium olefin metathesis catalysts bearing bulky sulfonate or phosphate ligands with significantly improved Z-selectivity in CM reactions (eq 1) over other known Ru-based olefin metathesis catalysts.
Results and Discussion It is envisioned that the installation of bulky groups on the anionic ligands on the Ru catalysts would force the resultant metallacyclobutane transition state to adopt a cis-conformation, thereby resulting in the preferential production of cisolefins (Figure 1).29 Standard ligand exchange reactions using various silver sulfonate or phosphate salts in a benzene solution of the corresponding Ru precursors result in the formation of the desired metal complexes. The products can be isolated in high yields (>75%) by simple filtration to remove the AgCl byproduct and unreacted silver salts followed by subsequent removal of solvents (eq 2). Only one chloride ligand is replaced (29) Torker, S.; Muller, A.; Chen, P. Angew. Chem., Int. Ed. 2010, 49, 3762. (30) Krause, J. O.; Nuyken, O.; Wurst, K.; Buchmeiser, M. R. Chem.;Eur. J. 2004, 10, 777. (31) Giudici, R. E.; Hoveyda, A. H. J. Am. Chem. Soc. 2007, 129, 3824. (32) Veldhuizen, J. J. v.; Campbell, J. E.; Giudici, R. E.; Hoveyda, A. H. J. Am. Chem. Soc. 2005, 127, 6877. (33) Occhipinti, G.; Bjorsvik, H.-R.; Tornroos, K. W.; Furstner, A.; Jensen, V. R. Organometallics 2007, 26, 4383.
Figure 1. Schematic drawing of metallacycle formation for formation of (a) Z-olefin and (b) E-olefin.
by the bulky sulfonate or phosphate ligand to improve catalytic activity, as substitution of both chlorides by bulky anionic ligands would slow the reaction dramatically (vide infra). Formation of a small amount of dianion exchanged product is inevitable, and their presence in small quantities (70% (78%) can be attained. This is considerably lower than any of the Ru precursors or even that attainable using the phosphine-based catalysts.15 Although catalysts with similar E/Z selectivities have been reported prior to this work, higher catalyst loadings and harsher reaction conditions are required.17 6 also displays a relatively linear relationship between conversion and E/Z ratio, which is mostly not observed in all other NHC-based Ru catalysts due to secondary metathesis reactions by the catalysts under high olefin product concentrations. When olefin 17 is reacted with 1 mol % of 5, the E/Z ratio of 17 changed by 2σ(F2) is used only for calculating R-factors(gt), etc., and is not relevant to the choice of reflections for refinement. RFactors based on F2 are statistically about twice as large as those based on F, and R-factors based on ALL data will be even larger. All esd’s (except the esd in the dihedral angle between two leastsquares planes) are estimated using the full covariance matrix. The cell esd’s are taken into account individually in the estimation of esd’s in distances, angles, and torsion angles; correlations between esd’s in cell parameters are used only when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell esd’s is used for estimating esd’s involving leastsquares planes.
Acknowledgment. We thank K. Endo for helpful discussions. P.T. is grateful to A*STAR Singapore for a postdoctoral fellowship and NSF for a research grant. Supporting Information Available: This material is available free of charge via the Internet at http://pubs.acs.org.
