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Organometallics 2009, 28, 944–946
Electronic Effects of the Anionic Ligand in Ruthenium-Catalyzed Olefin Metathesis Sebastien Monfette, Kenneth D. Camm, Serge I. Gorelsky, and Deryn E. Fogg* Department of Chemistry and Center for Catalysis Research & InnoVation, UniVersity of Ottawa, Ottawa, Ontario, Canada K1N 6N5 ReceiVed January 5, 2009 Summary: Synthesis of a family of Ru-catecholate catalysts, in which catechol acidity spans 4.5 pKa units, enables eValuation of electronic effects in Ru-catalyzed olefin metathesis. The metathesis actiVity drops as the anionic ligands become more electron-deficient, in contrast to the trend established for the Schrock group 6 catalysts. The advent of well-defined catalysts for olefin metathesis, particularly easily handled ruthenium catalysts, has had a significant impact on organic synthesis.1 Much study has clarified the optimal electronic features of the neutral L-donor ligands in the Grubbs-class Ru catalysts (e.g., 1 and 2; Chart 1).2 Activity is maximized when one ligand is a strongly σ-donating phosphine or N-heterocyclic carbene (NHC) and the other a weakly donating, readily dissociated ligand such as pyridine (3)3a or an inductively weakened ether donor (e.g., 4).3b In the extreme, this ligand is absent (5).3c The optimum electronic properties of the anionic ligands, in contrast, remain little studied and poorly understood. While the metathesis activity of Schrock’s group 6 catalysts can be increased by orders of magnitude by reducing the pKa of the alkoxide ligand (this serving to enhance rates of olefin binding),1c no such unequivocal relationship has been established for the Ru systems. In a recent QSAR study, Jensen and co-workers suggested that catalyst productivity may be maximized when the anionic ligands absorb more electron density donated by L, thus lowering the barrier to formation of the Ru(IV) intermediate.4 The trend in activity for halide derivatives (Cl > Br > I) originally reported by the Grubbs group5 could reflect inductive effects consistent with this, but the influence of the simultaneous increase in steric demand cannot be discounted. A contrasting trend (Cl < Br ≈ I), moreover, was reported for derivatives of * To whom correspondence should be addressed. E-mail: dfogg@ uottawa.ca. (1) For selected reviews on olefin metathesis, see: (a) Handbook of Metathesis; Grubbs, R. H., Ed.; Wiley-VCH: Weinheim, Germany, 2003. (b) Deshmukh, P. H.; Blechert, S. Dalton Trans. 2007, 2479–2491. (c) Schrock, R. R. Angew. Chem., Int. Ed. 2006, 45, 3748–3759. (d) Grubbs, R. H. Angew. Chem., Int. Ed. 2006, 45, 3760–3765. (e) Astruc, D. New J. Chem. 2005, 29, 42–56. (f) Fu¨rstner, A. Angew. Chem., Int. Ed. 2000, 39, 3012–3043. (2) For detailed discussions, see: (a) Sanford, M. S.; Love, J. A. In ref 1a, Vol. 1, pp 112-131. (b) Conrad, J. C.; Fogg, D. E. Curr. Org. Chem. 2006, 10, 185–202. (3) (a) Sanford, M. S.; Love, J. A.; Grubbs, R. H. J. Am. Chem. Soc. 2001, 123, 6543–6554. (b) Michrowska, A.; Bujok, R.; Harutyunyan, S.; Sashuk, V.; Dolgonos, G.; Grela, K. J. Am. Chem. Soc. 2004, 126, 9318– 9325. (c) Romero, P. E.; Piers, W. E.; McDonald, R. Angew. Chem., Int. Ed. 2004, 43, 6161–6165. (4) Occhipinti, G.; Bjorsvik, H.-R.; Jensen, V. R. J. Am. Chem. Soc. 2006, 128, 6952–6964. (5) The relative rates reported for the RCM of diethyldiallyl malonate by RuX2(PCy3)2(dCHCHdCPh2) are ca. 19 (X ) Cl), 15 (X ) Br), and 1 (X ) I). See: Dias, E. L.; Nguyen, S. T.; Grubbs, R. H. J. Am. Chem. Soc. 1997, 119, 3887–3897.
Chart 1. Key Ru Metathesis Catalystsa
a For 2 and 3, NHC ) IMes (a), H2IMes (b). IMes ) N,N′-bis(mesityl)imidazol-2-ylidene, and py ) pyridine.
Scheme 1. Synthesis of Catecholate Catalysts from 3a
2b.3a Within a series of carboxylate derivatives of 1, the Mol group found no correlation between activity and proligand pKa,6 but, as Buchmeiser has pointed out, variations in ligand size mean that the stereoelectronic ambiguity remains.7 Our recent synthesis of catecholate catalyst 68 opens the way to modulating the electronic properties of the anionic donors without perturbing steric parameters. The catecholate platform thus provides the first opportunity in Ru-catalyzed olefin metathesis to directly assess the electronic effect of the anionic ligands on catalyst activity. While some propertiessincluding initiation efficiencysmay be specific to cis-anionic species9,10 (and it remains unclear to what extent cis-Cl2 intermediates participate in metathesis via 2 and related species,11,12 despite their occasional observation13), a generalized effect on Ru (6) Buchowicz, W.; Ingold, F.; Mol, J. C.; Lutz, M.; Spek, A. L. Chem. Eur. J. 2001, 7, 2842–2847. (7) Halbach, T. S.; Mix, S.; Fischer, D.; Maechling, S.; Krause, J. O.; Sievers, C.; Blechert, S.; Nuyken, O.; Buchmeiser, M. R. J. Org. Chem. 2005, 70, 4687–4694. (8) Monfette, S.; Fogg, D. E. Organometallics 2006, 25, 1940–1944. (9) Recent studies suggest that related cis-Cl2 species initiate more slowly than their trans-Cl2 isomers. See: (a) Barbasiewicz, M.; Szadkowska, A.; Bujok, R.; Grela, K. Organometallics 2006, 25, 3599–3604. (b) Ung, T.; Hejl, A.; Grubbs, R. H.; Schrodi, Y. Organometallics 2004, 23, 5399–5401. (c) Benitez, D.; Goddard, W. A. J. Am. Chem. Soc. 2005, 127, 12218– 12219. (10) Related studies likewise suggest that cis-Cl2 geometries may be associated with low activity. See: (a) Slugovc, C.; Perner, B.; Stelzer, F.; Mereiter, K. Organometallics 2004, 23, 3622–3626. (b) Ben-Asuly, A.; Tzur, E.; Diesendruck, C. E.; Sigalov, M.; Goldberg, I.; Lemcoff, N. G. Organometallics 2008, 27, 811–813. (11) Correa, A.; Cavallo, L. J. Am. Chem. Soc. 2006, 128, 13352–13353. (12) Straub, B. F. AdV. Synth. Catal. 2007, 349, 204–214. (13) Anderson, D. R.; Hickstein, D. D.; O’Leary, D. J.; Grubbs, R. H. J. Am. Chem. Soc. 2006, 128, 8386–8387.
10.1021/om900006f CCC: $40.75 2009 American Chemical Society Publication on Web 01/21/2009
Communications
Organometallics, Vol. 28, No. 4, 2009 945
Figure 1. RCM of substrates S1-S7 via catecholates 6-8. Conditions: CH2Cl2, reflux, 3 h, GC-FID analysis; (3% in replicate runs. 0.5 mol % Ru, 50 mM diene.14
ionization energy is also expected. By maintaining a constant ligand geometry while tuning inductive effects, systematic patterns of reactivity can be extracted. Here we report a strong inVerse correlation between metathesis activity and the inductive effect of the anionic ligands, in contrast to the trend established for the group 6 catalysts. Six new catecholate catalysts (7a-c and 8a-c; Scheme 1) were prepared by treating a THF solution of 3a with the appropriate catecholate.14 Reactions using sodium catecholates are incomplete after 24 h. For the dithallium salts, conversion is quantitative within 2 h, as judged by loss of the NMR singlet for the alkylidene proton in 3a (20.02 ppm) and emergence of a new singlet at ca. 17 ppm. The green solution also turns greenbrown, and TlCl deposits. Isolated yields approach 90%, except for 8b,c (ca. 50 and 70%, respectively), which are poorly soluble in common organic solvents. IR analysis supports formulation as catecholates, rather than quinolates: ν(C-O) 1269-1238 cm-1 for 6-8 vs 1505-1530 cm-1 for Ru-benzoquinones.15 MALDI-MS analysis16 reveals a well-defined isotope pattern for [M]•+ or [M - py]•+ as the highest mass peak, consistent with replacement of both chlorides by catecholate. NMR and elemental analysis support the structures shown. To elicit distinctions in catalyst performance across a range of substrates, we undertook the RCM of S1-S7 at a catalyst loading of 0.5 mol % Ru. Higher activity correlates with higher catechol pKa (Figure 1: catalysts in order of pKa), with the electron-rich dimethylcatecholate catalyst 7a effecting complete RCM of S1-S5 within 3 h. The histograms for S1-S5 establish a clear demarcation between the tetrahalocatecholates 8a-c and the more active catalysts 6 and 7a-c. Maximum discrimination within catalysts 8a-c emerges for substrates S2 and S4, which reveal a systematic decrease in conversions as the catecholate becomes more electron deficient. Substrates S5 and S6 enable discrimination between the more active catalysts. For ether S5, catalysts 6 and 7a,b effect quantitative RCM; cf. 80% for the dibromo catalyst 7c (and 15-20% for 8a-c). None of these catalysts effect complete RCM of S6 under the conditions used, but the activity of 7a is double that of the next most active catalyst, catecholate 6. All catalysts show low activity for diallyl sulfide S7, an established poison for other Ru catalysts.17 In a more detailed examination, we measured rate curves for the RCM of S1 and S2 and the ROMP of norbornene monomer S9, using all seven catalysts (Figure 2). The rate curves substantiate the trend in catalyst activity qualitatively evident from Figure 1. The corresponding TOF values (Table 1) reveal that dimethylcatecholate 7a is again almost twice as active as (14) For details, including numerical data where relevant, see the Supporting Information. (15) Bohle, D. S.; Carron, K. T.; Christensen, A. N.; Goodson, P. A.; Powell, A. K. Organometallics 1994, 13, 1355–1373. (16) Eelman, M. D.; Blacquiere, J. M.; Moriarty, M. M.; Fogg, D. E. Angew. Chem., Int. Ed. 2008, 47, 303–306. (17) Armstrong, S. K. J. Chem. Soc., Perkin Trans. 1998, 371–388.
Figure 2. Rate curves for the RCM of (a) S1 and (b) S2 (5 mM diene, 0.5 mol % Ru, C7H8, 60 °C) and for the ROMP of (c) S9 (100 mM, 1 mol % Ru, CDCl3, 23 °C); (3% in replicate runs. Curves in declining order of catalyst activity: 3a (purple 9, shown for S9 only); 7a (blue ]); 6 (red b); 7b (green ×); 7c (red ]); 8a (purple /); 8b (black 2); 8c(blue O).14 Table 1. Relative Activity and Initiation Efficiencies for Catalysts 6-8a TOF (h-1)
TOFrelb
cat.
S1
S2
S9
S1
S2
S9
IE (IErel) (%)b
7a 6 7b 7c 8a 8b 8c
100 54 36 21 10 6 2
156 83 48 34 11 5 0
350 135 83 64 56 5 4
50 27 18 10 5 3 1
31 16 9 7 2 1
87 34 21 16 14 1.2 1
1.06 (6.6) 0.92 (5.8) 0.58 (3.6) 0.48 (3.0) 0.43 (2.7) 0.20 (1.3) 0.16 (1)
a Conditions: for RCM, 60 °C, 5 mM, toluene, 0.5 mol % Ru; for ROMP, 23 °C, 100 mM, CDCl3. RCM TOFs at 5 h (identical trends found in TOF at 10% conversion for 6 and 7 (8, conversion
