A Stable Ruthenium Catalyst for Productive Olefin Metathesis

4824 Organometallics 2004, 23, 4824-4827 A Stable Ruthenium Catalyst for Productive Olefin Metathesis Grant S. Forman,*,† Ann E. McConnell,† Martin J. Hanton,† Alexandra M. Z. Slawin,‡,§ Robert P. Tooze,† Werner Janse van Rensburg,| Wolfgang H. Meyer,|,⊥ Cathy Dwyer,| Megan M. Kirk,| and D. Wynand Serfontein| Sasol Technology Research Laboratory, St Andrews, Sasol Technology (UK) Limited, Purdie Building, North Haugh, St Andrews, Fife KY16 9ST, Scotland, U.K., School of Chemistry, University of St Andrews, Purdie Building, North Haugh, St Andrews, Fife KY16 9ST, Scotland, U.K., and Sasol Technology Research & Development, PO Box 1, Klasie Havenga Road, Sasolburg, 1947, South Africa Received August 13, 2004 Summary: New ruthenium carbene complexes 5 and 6, containing a rigid bicyclic phosphine moiety, have been prepared, and the structure of 5 has been unambiguously established by single-crystal X-ray diffraction studies. These ruthenium-based complexes show excellent stability to air and moisture, can be recycled by chromatography, and are available from simple precursors. They are efficient catalysts for various metathesis reactions, particularly for applications where high selectivity is required. Ruthenium alkylidene complexes have attracted significant attention in recent years as efficient catalysts for olefin metathesis and have rapidly evolved into versatile tools for organic and polymer chemistry.1 Attempts to improve the stability and overall performance of the ruthenium benzylidene complex [(PCy3)2(Cl2)RudCHPh] (1)2 have been mainly focused on the development of N-heterocyclic carbene-coordinated ruthenium complexes such as [(H2IMes)(PCy3)(Cl2)Rud CHPh] (2),3 which are reported to display dramatically * To whom correspondence should be addressed. E-mail: [email protected]. † Sasol Technology (UK) Limited. ‡ University of St. Andrews. § Fax: +44 (0) 1334 463 384. Tel: +44 (0) 1334 467 280. E-mail: [email protected]. | Sasol Technology Research and Development. ⊥ Fax: +27 11 5223593. Tel: +27 16 960 4443. E-mail: [email protected]. (1) For recent reviews on olefin metathesis, see: (a) Schuster, M.; Blechart, S. Angew Chem., Int. Ed. 1997, 109, 2037-2056. (b) Fürstner, A. Top. Catal. 1997, 4, 285-299. (c) Armstrong, S. K. J. Chem. Soc., Perkin Trans. 1 1998, 371-388. (d) Grubbs, R. H.; Chang, S. Tetrahedron 1998, 54, 4413-4450. (e) Phillips, A. J.; Abell, A. D. Aldrichim. Acta 1999, 32, 75-89. (f) Kingsbury, C. L.; Mehrman, S. J.; Takacs, J. M. Curr. Org. Chem. 1999, 3, 497-555. (g) Fürstner, A. Angew Chem., Int. Ed. 2000, 39, 3012-3043. (h) Buchmeiser, M. R. Chem. Rev. 2000, 100, 1565-1604. (i) Cook, G. R. Curr. Org. Chem. 2000, 4, 869-885. (j) Yet, L. Chem. Rev. 2000, 100, 2963-3007. (k) Roy, R.; Das, S. K. Chem. Commun. 2000, 519-529. (l) Maier, M. E. Angew. Chem., Int. Ed. 2000, 39, 2073-2077. (m) Coates, G. W. Dalton 2002, 467-475. (n) Mecking, S.; Held, A.; Bauers, F. M. Angew. Chem., Int. Ed. 2002, 41, 544-561. (o) Connon, S. J.; Blechert, S. Angew. Chem., Int. Ed. 2003, 42, 1900-1923. (2) (a) Nguyen, S. T.; Grubbs, R. H.; Ziller, J. W. J. Am. Chem. Soc. 1993, 115, 9858-9859. (b) Schwab, P.; France, M. B.; Ziller, J. W.; Grubbs, R. H. Angew. Chem., Int. Ed. 1995, 34, 2039-2041. (c) Schwab, P.; Grubbs, R. H.; Ziller, J. W. J. Am. Chem. Soc. 1996, 118, 100-110. (d) Dias, E. L.; Nguyen, S. T.; Grubbs, R. H. J. Am. Chem. Soc. 1997, 119, 3887-3897. (e) Ulman, M.; Grubbs, R. H. J. Org. Chem. 1999, 64, 7202-7207. (f) Blackwell, H. E.; O’Leary, D. J.; Chatterjee, A. K.; Washenfelder, R. A.; Bussmann, D. A.; Grubbs, R. H. J. Am. Chem. Soc. 2000, 122, 58-71. increased reactivity with a wide variety of olefin substrates.4 The high activity and broad application profile of ruthenium alkylidene metathesis catalysts can be offset by the propensity toward decomposition at low catalyst loadings,5 particularly for ring-closing metathesis (RCM) and cross-metathesis (CM) reactions. To successfully apply homogeneous metathesis to the industrial-scale production of commodity olefins, both high turnover numbers and selectivities are required.6 In this context, the Grubbs first-generation catalyst 1 is generally incompatible with olefin feedstocks derived from Fischer-Tropsch7 conversion of synthesis (3) (a) Weskamp, T.; Schattenmann, W. C.; Speigler, M.; Herrmann, W. A. Angew. Chem., Int. Ed. 1998, 37, 2490-2493. (b) Weskamp, T.; Kohl, F. J.; Hieringer, W.; Gleich, D.; Herrmann, W. A. Angew. Chem., Int. Ed. 1999, 38, 2416-2419. (c) Ackermann, L.; Fürstner, A.; Weskamp, T.; Kohl, F. J.; Herrmann, W. A. Tetrahedron Lett. 1999, 40, 4787-4790. (d) Weskamp, T.; Kohl, F. J.; Herrmann, W. A. J. Organomet. Chem., 1999, 582, 362-365. (e) Frenzel, U.; Weskamp, T.; Kohl, F. J.; Schattenmann, W. C.; Nuyken, O.; Herrmann, W. A. J. Organomet. Chem. 1999, 586, 263-265. (f) Scholl, M.; Trnka, T. M.; Morgan, J. P.; Grubbs, R. H. Tetrahedron Lett. 1999, 40, 2247-2250. (g) Scholl, M.; Ding, S.; Lee, C. W.; Grubbs, R. H. Org. Lett. 1999, 1, 953-956. (h) Huang, J.; Stevens, E. D.; Nolan, S. P.; Petersen, J. L. J. Am. Chem. Soc. 1999, 121, 2674-2678. (i) Jafarpour, L.; Huang, J.; Stevens, E. D.; Nolan, S. P. Organometallics 1999, 18, 3760-3763. (4) For selected examples, see: (a) Chatterjee, A. K.; Grubbs, R. H. Org. Lett. 1999, 1, 1751-1753. (b) Chatterjee, A. K.; Morgan, J. P.; Scholl, M.; Grubbs, R. H. J. Am. Chem. Soc. 2000, 122, 3783-3784. (c) Choi, T.-L.; Lee, C. W.; Chatterjee, A. K.; Grubbs, R. H. J. Am. Chem. Soc. 2001, 123, 10417-10418. (d) Chatterjee, A. K.; Choi, T. L.; Grubbs, R. H. Synlett 2001, 1034-1037. (e) Choi, T. L.; Chatterjee, A. K.; Grubbs, R. H. Angew. Chem., Int. Ed. 2001, 40, 1277-1279. (f) Grela, K. Bieniek, M. Tetrahedron Lett. 2001, 42, 6425-6428. (g) Morgan, J. P.; Morrill, C.; Grubbs, R. H. Org. Lett. 2002, 4, 67-73. (h) Chatterjee, A. K.; Sanders, D. P.; Grubbs, R. H. Org. Lett. 2002, 4, 1939-1942. (i) Van Nguyen, T.; Debenedetti, S.; De Kimpe, N. Tetrahedron Lett. 2003, 44, 4199-4201. (5) (a) Ulman, M.; Grubbs, R. H. J. Org. Chem. 1999, 64, 72027207. (b) Sanford, M. S.; Love, J. A.; Grubbs, R. H. J. Am. Chem. Soc. 2001, 123, 6543-6554. (c) Bourgeois, D.; Pancrazi, A.; Nolan, S. P.; Prunet, J. J. Organomet. Chem. 2002, 643-644, 247-252. (d) Lehman, S. E., Jr.; Schwendeman, J. E.; O’Donnell, P. M.; Wagener, K. B. Inorg. Chim. Acta 2003, 345, 190-198. (6) Burdett, K. A.; Harris, L. D.; Margl, P.; Maughon, B. R.; MokhtarZadeh, T.; Saucier, P. C.; Wasserman, E. P. Organometallics 2004, 23, 2027-2047. (7) Dry, M. E., In Encyclopedia of Catalysis; Horváth, I. T., Ed.; Wiley-Interscience: New York, 2003; Vol. 3, p 347. 10.1021/om049370c CCC: $27.50 © 2004 American Chemical Society Publication on Web 09/16/2004 Communications gas, since it displays poor thermal stability and short lifetimes at low catalyst loadings. For example, complex 1 has a lifetime of only ca. 15 min for the self-metathesis (SM) of 1-octene at 50 °C (S/C ) 20 000:1). While the N-heterocyclic carbene (NHC) complex 2 displays significantly enhanced activity relative to 1, olefin isomerization5d,8,9 is a competitive and complicated process, often resulting in poor overall selectivity. On the basis of the foregoing, we have focused our attention on developing improved first-generation olefin metathesis catalysts that are more likely to give selective reactions. Phosphine substitution has already been studied extensively by Grubbs and co-workers,2d and tricyclohexylphosphine (PCy3) was determined to be an optimal ligand. However, with one notable exception,10 since the advent of NHC-containing ruthenium olefin catalysts, relatively little effort has been devoted toward further developing ruthenium-based catalysts containing two phosphine donor ligands.11 We recently undertook a study to investigate the effectiveness of relatively inexpensive phosphabicyclononane12 (Phoban) ligands as scaffolds for various catalytic metathesis reactions. Phoban ligand mixtures are currently used industrially for the cobalt-catalyzed hydroformylation reaction13 and have found use for other catalytic processes.14 9-Cyclohexyl-9-phospha-9Hbicyclononane (3) was conveniently prepared as a ca. 3:1 mixture of [3.3.1]- and [4.2.1]-bridged isomers (3a,b) via radical addition of 1,5-cyclooctadiene to cyclohexylphosphine.15 Alternatively, the isomerically pure [3.3.1] isomer 3a was prepared by initial separation of isomeric (8) Hong, S. H.; Day, M. W.; Grubbs, R. H. J. Am. Chem. Soc. 2004, 126, 7414-7415. (9) (a) Huang, J.; Stevens, E. D.; Nolan, S. P.; Peterson, J. L. J. Am. Chem. Soc. 1999, 121, 2674-2678. (b) Scholl, M.; Trnka, T. M.; Morgan, J. P.; Grubbs. R. H. Tetrahedron Lett. 1999, 40, 2247-2250. (c) Fürstner, A.; Thiel, O. R.; Ackermann, L.; Schanz, H.-J.; Nolan, S. P. J. Org. Chem. 2000, 65, 2204-2207. (d) Wagner, J.; Martin Cabrejas, L. M.; Grosssmith, C. E.; Papageorgiou, C.; Senia, F.; Wagner, D.; France, J.; Nolan, S. P. J. Org. Chem. 2000, 65, 9255-9260. (e) Kinderman, S. S.; van Maarseveen, J. H.; Schoemaker, H. E.; Hiemstra, H.; Rutjes, F. P. J. T. Org. Lett. 2001, 13, 2045-2048. (f) Bourgeois, D.; Pancrazi, A.; Nolan, S. P.; Prunet, J. J. Organomet. Chem. 2002, 643-644, 247-252. (g) Lehman, S. E., Jr.; Schwendeman, J. E.; O’Donnell, P. M.; Wagener, K. B. Inorg. Chim. Acta 2003, 345, 190198. (10) (a) Hansen, S. M.; Rominger, F.; Eisenträger, F.; Metz, M.; Hofmann, P. Chem. Eur. J. 1999, 5, 557-566. (b) Hansen, S. M.; Volland, M. A. O.; Rominger, F.; Eisenträger, F.; Hofmann, P. Angew. Chem., Int. Ed. 1999, 38, 1273-1276. (c) Volland, M. A. O.; Hansen, S. M.; Eisenträger, F.; Gross, J. H.; Stengel, K.; Hofmann, P. J. Organomet. Chem. 2000, 606, 88-92. (11) (a) Buchowicz, W.; Mol, J. C.; Lutz, M.; Spek, A. L. J. Organomet. Chem. 1999, 588, 205-210. (b) Leung, W.-H.; Lau, K.-K.; Zhang, Q.-F.; Wong, W.-T.; Tang, B. Organometallics 2000, 19, 20842089. (c) Six, C.; Beck, K.; Wegner, A.; Leitner, W. Organometallics 2000, 19, 4639-4642. (d) Nieczypor, P.; van Leeuwen, P. W. N. M.; Mol, J. C.; Lutz, M.; Spek, A. L. J. Organomet. Chem. 2001, 625, 5866. (e) Rölle, T.; Grubbs, R. H. Chem. Commun. 2002, 1070-1071. (f) Stüer, W.; Wolf, J.; Werner, H. J. Organomet. Chem. 2002, 641, 203207. (g) Ferrando-Miguel, G.; Coalter, J. N., III; Gerard, H.; Huffman, J. C.; Eisenstein, O.; Caulton, K. G. New J. Chem. 2002, 26, 687-700. (12) (a) Mason, R. F.; Van Winkle, J. L. (Shell Oil Co.) U.S. Patent 3 400 163, 1968. (b) Abbenhuis, H. C. L.; Burckhardt, U.; Gramlich, V.; Köllner, C.; Pregosin, P. S.; Salzmann, R.; Togni, A. Organometallics 1995, 14, 759-766. (13) (a) Wattimena, F. (Shell Oil Co.) Netherlands Patent 6 604 094, 1966; Chem. Abstr. 1967, 66, 65101r. (b) Jpn. Kokai Tokkyo Koho (Mitsubishi Petrochemical Co. Ltd.) JP 80 113 731, 1980; Chem. Abstr. 1981, 95, 186627z. (14) (a) Drent, E.; Pello, D. H. L.; Suykerbuyk, J. C. L. J.; Van Gogh, J. (Shell International Research) World Patent 5354, 1995. (b) Tooze, R. P. (ICI) World Patent 15 938, 1995. (c) Howard, S. T.; Foreman, J. P.; Edwards, P. G. Inorg. Chem. 1996, 35, 5805-5812. (d) Abbenhuis, H. C. L.; Burckhardt, U.; Gramlich, V.; Togni, A.; Albinati, A.; Müller, B. Organometallics 1994, 13, 2563-65. Organometallics, Vol. 23, No. 21, 2004 4825 Scheme 1. Preparation of Cyclohexyl-phoban Derivatives 5 and 6 bicyclic secondary phosphabicyclononanes,16 followed by treatment of the desired [3.3.1] isomer with nbutylithium and the phosphide thus formed was treated with bromocyclohexane. When 1 and 417 were treated with a ca. 3:1 mixture of 3a,b, 31P NMR spectroscopy revealed that the symmetrical [3.3.1] isomer 3a preferentially underwent ligand exchange reaction to afford exclusively 518 and 6 in 72% and 75% yield, respectively (Scheme 1).19 Alternatively, ligand exchange of 1 with isomerically pure 3a provided 6 in 75% yield.20 Formation of 5 and 6 can be monitored by 31P NMR spectroscopy as the appearance of very broad singlets at δ 24.5 and 23.3 ppm, respectively.21 The alkylidene proton (HR) appears as a doublet (3JHH ) 11.52 Hz) at δ 19.1 ppm for 5 and as a singlet at δ 20.2 ppm for 6. The isolated Ru-alkylidene complexes 5 and 6 are surprisingly air and moisture stable compared to 1 and can be stored in an ordinary vial for long periods. In solution 6 was stable in acetonitrile for at least 3 h, whereas decomposition was observed for complex 1 after only 3 min. Complex 5 remained unchanged after treatment with a 2 M HCl solution, while 1 instantly decomposed. Complex 6 is stable to column chromatography, can be recovered as a solid residue, and maintains its catalytic activity in subsequent metathesis reactions upon addition of new substrate (see below). In addition, complexes 5 and 6 display an improved thermal stability at elevated temperatures relative to 1,5a allowing efficient metathesis reactions to be performed routinely above 50 °C. The X-ray analysis22 of 5 (Figure 1) indicated a distorted-square-pyramidal geometry about the metal center in which the carbene ligand occupies the apical (15) On the industrial scale phosphine (PH3) is added to cycloocta1,5-diene to give a mixture of isomeric [3.3.1]- and [4.2.1]-bridged 9-phospha-9H-bicyclononane secondary phoban phosphines, which are subsequently treated with an alkene to afford the desired mixture of tertiary phosphines.12 (16) Downing, J. H.; Gee, V.; Pringle, P. G. Chem. Commun. 1997, 1527-1528. (17) Amoroso, D.; Snelgrove, J. L.; Conrad, J. C.; Drouin, S. D.; Yap, G. P. A.; Fogg, D. E. Adv. Synth. Catal. 2002, 344, 757-763. (18) 31P NMR (121 MHz, C7D8) analysis of the reaction mixture indicated quantitative conversion of 4 to 5. The reduced yield is largely due to the partial solubility of 5 in hexanes mixtures. (19) When isomerically pure 3b was reacted with 4, no identifiable products were recovered. (20) Full details of these studies will be disclosed in due course. (21) Extensive NMR studies indicate that 6 is resolved into a number of rotational isomers at low temperature. Full details of these studies will be disclosed in due course. 4826 Organometallics, Vol. 23, No. 21, 2004 Communications Figure 1. Perspective drawing of [(PhobCy)2Cl2RudCHCd C(Me)2] (5).22 The dimethylvinyl carbene system contained disorder at C(2), which was refined in two orientations (60/ 40 split), the higher occupancy orientation is illustrated. Table 1. Metathesis Reactions via Ru-Alkylidene Catalysts entry substratea process cat. (%) conversn (%)b selectivity (%)b productive TONc 1 1-decened SM 1 (0.01) 2 (0.01) 6 (0.01) 8 52 77 94 63 98 752 3276 7546 2 methyl oleated SM 1 (0.0025) 2 (0.0025) 6 (0.0025) 7 51 47 95 93 97 2660 18972 18236 3 methyl oleatee ethenolysis 1 (0.003) 2 (0.003) 6 (0.003) 14 37 43 91 58 98 4247 7153 14047 4 diethyl diallyl malonatef RCM 1 (0.06) 2 (0.06) 6 (0.06) 59 98 85 95 92 96 934 1502 1360 a Purified by passing through alumina before use. b Determined by GC. c Productive TON ) TON × (selectivity to required product). d Conditions: 60 °C, neat, 4 h. e Conditions: 10 bar of ethylene, 60 °C, neat, 2 h. f Conditions: 50 °C, 0.125 mM in toluene, 4 h. position and the phosphine ligands in the basal plane occupy a trans position. The Ru-C1 bond distance (1.795(7) Å) is significantly shorter compared to the related Ph-substituted butylidene complex2a [(PCy3)2Cl2RudCHCHdCPh2] (7; 1.851(21) Å) and Cl-substituted benzylidene complex2b [(PCy3)2Cl2RudCH-p-C6H4Cl] (8; 1.839(3) Å). These data suggest a stronger Rud C bond for 5 compared to that of other first-generation systems. The isolated Ru alkylidene complexes are efficient catalysts for various metathesis reactions, particularly for applications where high selectivity is required. As depicted in Table 1, complex 6 is an efficient catalyst for self-metathesis (SM, entries 1 and 2), ethenolysis (entry 3), and RCM (entry 4) reactions. In all cases the catalytic activity of 6 is significantly greater than that (22) Crystal structure data for 5: C39H64Cl2P2Ru, Mr ) 766.81, orthorhombic, space group P212121, a ) 9.0223(14) Å. b ) 9.8916(16) Å, c ) 42.043(7) Å, V ) 3752.1(10) Å3, T ) 125(2) K, Z ) 4, Fcalcd ) 1.357 g cm-3, SMART diffractometer, 19 470 reflections collected, 6761 unique reflections (Rint ) 0.1270), R1 ) 0.0520, wR2 ) 0.0549, Flack parameter 0.01(4). The file CCDC-239497 contains supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/conts/retrieving.html (or from the Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge, CB21EZ, U.K.; fax, (+44) 1223-336-033; e-mail, [email protected]). of the first-generation catalyst 1, while selectivity is maintained above 95%. Complex 6 is also an efficient catalyst for ring-opening metathesis polymerization (ROMP) reactions. For instance, ROMP of cyclooctadiene at 50 °C with 0.008 mol % of 6 gave 98% conversion to the corresponding polymer after 10 min, while under identical conditions 20% conversion was observed with 1. Although complex 6 readily catalyzes the crossmetathesis (CM) of various functionalized olefins (Table 2), including 4° allylic carbon-containing olefins (entry 3), it shows no significant activity toward more difficult substrates such as acrylates and 1,1-disubstituted olefins. Complex 6 can be recovered by column chromatography from metathesis reactions using concentrated catalyst solutions (>0.1 mol %) and used for subsequent metathesis reactions. For example, using 6 (2 mol %), the CM of styrene with 1-dodecene produced the crossproduct in 87% conversion, and the catalyst was subsequently recovered and used for the self-metathesis of methyl oleate (46% conversion). Additional catalyst recovery by column chromatography provided further material that was used for the self-metathesis of 1-decene (70% conversion). When the present protocol is used, 1H NMR indicates that the recovered catalyst is [(PhobCy)2Cl2RudCHR], where R is predominantly the alkylidene component(s) originating from the previous metathesis reaction. Ruthenium methylidenes are key propagating species present within RCM, CM, and ethenolysis reactions and represent the thermally least stable intermediate species present within the catalytic metathesis cycle.5a Increasing understanding of the methylidene stability and decomposition pathways is critical for developing catalysts with extended lifetimes.8 Thus, in an effort to gain a better insight into methylidene decomposition, the thermal decomposition of the deuterated carbene complex (PhobCy)2Cl2RudCD2 (9) in benzene at 50 °C was studied by 2H NMR. Under these conditions, after 20 min a sharp resonance at 5.25 ppm, corresponding to D2CdCD2, and a broad signal between δ 1.0 and 2.3 ppm, corresponding to saturated aliphatic resonances, were observed, along with the original carbene signal (δ 19.2 ppm).23-25 Given that bimolecular decomposition products (ethylene) have not been observed previously for other bis-phosphine methylidene complexes,5a these observations suggest that the phosphabicyclononane ligand present within 9 is altering the relative rates of bimolecular and unimolecular decomposition pathways relative to those of other first-generation catalysts. Experimentally, these observations are evidenced by increased catalyst stability, particularly for ethenolysis reactions, in which the ruthenium methylidene is a key propagating species in the catalytic metathesis cycle. For example, ethenolysis of methyl oleate6 using 6 allows reactions to be performed at higher temperatures relative to 1, a natural consequence of which is higher activity and shorter reaction times.27 (23) When the identical decomposition reaction was performed at 70 °C, no ethylene formation was observed. (24) The methylidene complex (PhobCy)2Cl2RudCH2 (10) decomposed with a half-life of 36 min at 70 °C. See the Supporting Information for more details. (25) Grubbs et al. has reported that decomposition of the methylidene complex (PCy3)2Cl2RudCH2 (11) is primarily first order.5a Organometallics, Vol. 23, No. 21, 2004 4827 Communications Table 2. Cross-Metathesis Reactions via Ru-Alkylidene Catalysts selectivity (%)b cross-partnera cat. (%) conversn (%)b styrene 1-dodecene 1 (0.25) 2 (0.25) 6 (0.25) 81 87 87 93 41 95 2c styrene 5-hexene-2-one 1 (0.50) 2 (0.50) 6 (0.50) 36 99 61 95 90 95 3d 4-hexen-1-yl acetate 3,3-dimethyl-1butene 1 (1.0) 2 (1.0) 6 (1.0) 0 96 53 93 96 entry 1c substratea a Purified by passing through alumina before use. b Determined by GC. c Conditions: substrate:cross-partner = 3:1, 50 °C, 4 h. d Conditions: excess cross-partner, 40 °C, 8 h. In summary, the ruthenium carbene complexes 5 and 6 bearing phosphabicyclononane ligands are robust and recyclable catalysts for a variety of metathesis reactions. (26) (a) Six, C.; Gabor, B.; Görls, H.; Mynott, R.; Philipps, P.; Leitner, W. Organometallics 1999, 18, 3316-3326. (b) Jafarpour, L.; Schanz, H.-J.; Stevens, E. D.; Nolan, S. P. Organometallics 1999, 18, 54165419 and references therein. (c) Amoroso, D.; Yap, G. P. A.; Fogg, D. E. Can. J. Chem. 2001, 79, 958-963. (27) See the Supporting Information for more details. These complexes are more stable and active than the first-generation catalyst 1, can be accessed from simple precursors,17 and although they do not share the same broad application profile of second-generation catalysts, they are very useful in situations where high selectivity is required.28 A full DFT comparison of complexes 5 and 6 with classical Grubbs carbenes 1 and 2, along with NMR studies devoted to the study of rotational isomerization within cyclohexylphoban-containing Grubbs catalysts, will be reported shortly. Acknowledgment. We gratefully acknowledge the assistance of Mrs. Melanja Smith of the University of St Andrews Chemistry Department NMR Service and Dr. Ronan Bellabarba (Sasol, St Andrews) for stimulating discussions. Supporting Information Available: Text and tables giving experimental data for the synthesis of all new complexes, metathesis experiments, and crystallographic data (atomic coordinates, all bond distances and angles, and anisotropic displacement parameters) for complex 5. This material is available free of charge via the Internet at https://pubs.acs.org. OM049370C (28) Forman, G. S.; McConnell, A. E.; Tooze, R. P.; Dwyer, C.; Serfontein, D. W. (Sasol Technology UK) World Patent ZA03/00087, 2003.