Photoactivation of Ruthenium Olefin Metathesis Initiators

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Organometallics 2009, 28, 4652–4655 DOI: 10.1021/om9004302

Photoactivation of Ruthenium Olefin Metathesis Initiators )

Amos Ben-Asuly,†,‡ Anna Aharoni,† Charles E. Diesendruck,† Yuval Vidavsky,† Israel Goldberg,§ Bernd F. Straub, and N. Gabriel Lemcoff*,† Chemistry Department, Ben-Gurion University of the Negev, Beer-Sheva 84105, Israel, ‡Achva Academic College, Shikmim 79800, Israel, §School of Chemistry, Tel-Aviv University, Tel Aviv 69978, Israel, and Organisch-Chemisches Institut der Universit€ at Heidelberg, D-69120 Heidelberg, Germany )

†

Received May 23, 2009 Summary: UV irradiation of latent sulfur-chelated ruthenium olefin metathesis precatalysts produced a cis-dichloro-transdichloro photoisomerization. The enhanced activity of the trans isomer, when aromatic substituents were attached to the sulfur chelating atom, induced several metathesis reactions, including ROMP, at room temperature. Living systems rely on enzymatic catalysis for almost every biological process. This type of catalysis is exceptionally well regulated by differing incentives that trigger enzyme activity on and off depending on the organism’s need. The ability to regulate artificial catalysis is accordingly an appealing goal for chemists, for it may lead to advanced systems with unique properties. Control over catalyst activity has been achieved by judicious ligand design,1 and many examples have been put forward which demonstrate the use of chemical2 and physical3 stimuli in order to start chemical reactions. For instance, a notable recent study by Hecht4 reveals the use of a bulky azobenzene coupled to a nitrogen base to afford a photoswitchable general base catalyst. We herein report on latent5 metathesis precatalysts that may be purposely *To whom correspondence should be addressed. Tel: +97286461196. Fax: +972-86461740. E-mail: [email protected]. (1) (a) Matsugi, T.; Fujita, T. Chem. Soc. Rev. 2008, 37, 1264–1277. (b) Deshmukh, P. H.; Blechert, S. Dalton Trans. 2007, 2479–2491. (c) DiezGonzalez, S.; Nolan, S. P. Coord. Chem. Rev. 2007, 251, 874–883. (d) Ouchi, M.; Terashima, T.; Sawamoto, M. Acc. Chem. Res. 2008, 41, 1120–1132. (e) Masahiro, M. Angew. Chem., Int. Ed. 2004, 43, 2201–2203. (f) Monfette, S.; Camm, K. D.; Gorelsky, S. I.; Fogg, D. E. Organometallics 2009, 28, 944–946. (2) (a) P’Pool, S. J.; Schanz, H.-J. J. Am. Chem. Soc. 2007, 129, 14200–14212. (b) Zuccaccia, C.; Macchioni, A.; Busico, V.; Cipullo, R.; Talarico, G.; Alfano, F.; Boone, H. W.; Frazier, K. A.; Hustad, P. D.; Stevens, J. C.; Vosejpka, P. C.; Abboud, K. A. J. Am. Chem. Soc. 2008, 130, 10354– 10368. (c) Ledoux, N.; Allaert, B.; Schaubroeck, D.; Monsaert, S.; Drozdzak, R.; Van Der Voort, P.; Verpoort, F. J. Organomet. Chem. 2006, 691, 5482– 5486. (d) Haukka, M.; Ven€al€ainen, T.; Kallinen, M.; Pakkanen, T. A. J. Mol. Catal. A: Chem. 1998, 136, 127–134. (3) (a) Rau, S.; Walther, D.; Vos, J. G. Dalton Trans. 2007, 915–919. (b) Diez-Gonzalez, S.; Stevens, E. D.; Nolan, S. P. Chem. Commun. 2008, 4747–4749. (c) Kishi, K.; Ishimaru, T.; Ozono, M.; Tomita, I.; Endo, T. Macromolecules 1998, 31, 9392–9394. (d) Heijl, A.; Day, M. W.; Grubbs, R. H. Organometallics 2006, 25, 6149–6154. (e) Slugovc, C.; Burtscher, D.; Stelzer, F.; Mereiter, K. Organometallics 2005, 24, 2255–2258. (4) Peters, M. V.; Stoll, R. S.; Kuhn, A.; Hecht, S.. Angew. Chem., Int. Ed. 2008, 47, 5968–5972. (5) The term “latent” herein is used to describe a situation in which at normal room temperatures (15-35 °C) no appreciable reaction is observed for a given time period (i.e., 24 h) and noticeable product formation may be measured after external stimuli are applied (in our case UV-vis light). Naturally, as stated, latency will depend on many reaction parameters, such as substrates and solvents. Thus, a catalyst may display latency for various specific substrates, reactions, and solvent conditions and be reactive (not latent) for others. See also reference 14. pubs.acs.org/Organometallics

Published on Web 07/24/2009

activated by the use of light irradiation and may be returned to the off state by heat. Ruthenium olefin metathesis has erupted throughout the past decade to become one of the most widely used carboncarbon bond forming methodologies.6 The reaction mechanism is well understood and usually implies dissociation of a labile ligand as the initiation step.7 Ligand photodissociation and photoisomerization of ruthenium compounds is well-known.8 Thus, it is not surprising that there are some examples in the literature, most prominently the works of Hafner,9 Dixneuf,10 Grubbs,11 and Buchmeiser,12 which have used light to influence the behavior of ruthenium metathesis catalysts. However, reversible control of catalyst activation-deactivation has scarcely been explored. We have recently put forward a series of dormant sulfurchelated precatalysts.13,14 These complexes are excellent targets for photoactivation, since they are completely inert at room temperature. Thus, we hypothesized that if we could generate a measured photodissociation of the chelating sulfur, a very delicate control over the activity of the catalyst may be achieved. For this purpose we synthesized and characterized additional new complexes 1b,c (see the (6) (a) Hoveyda, A. H.; Zhugralin, A. R. Nature 2007, 450, 243–250. (b) Malcolmson, S. J.; Meek, S. J.; Sattely, E. S.; Schrock, R. R.; Hoveyda, A. H. Nature 2008, 456, 933–937. (c) F€urstner, A. Angew. Chem., Int. Ed. 2000, 39, 3012–3043. (d) Angew. Chem. 2000, 112, 3140-3172; (e) Schrock, R. R.; Czekelius, C. Adv. Synth. Catal. 2007, 349, 55–77. (f) Bieniek, M.; Michrowska, A.; Usanov, D. L.; Grela, K. Chem. Eur. J. 2008, 14, 806–818. (g) Bieniek, M.; Bujok, R.; Cabaj, M.; Lugan, N.; Lavigne, G.; Artl, D.; Grela, K. J. Am. Chem. Soc. 2006, 128, 13652–13653. (h) Trnka, T. M.; Grubbs, R. H. Acc. Chem. Res. 2001, 34, 18–29. (7) (a) Sanford, M. S.; Ulman, M.; Grubbs, R. H. J. Am. Chem. Soc. 2001, 123, 749–750. (b) Straub, B. F. Adv. Synth. Catal. 2007, 349, 204–214. (8) (a) Bonnet, S.; Collin, J.-P.; Sauvage, J.-P. Inorg. Chem. 2006, 45, 4024–4034. (b) Salassa, L.; Garino, C.; Salassa, G.; Gobetto, R.; Nervi, C. J. Am. Chem. Soc. 2008, 130, 9590–9597. (c) Yamashita, K.-I.; Kawano, M.; Fujita, M. J. Am. Chem. Soc. 2007, 129, 1850–1851. (d) Alessio, E.; Mestroni, G.; Nardin, G.; Attia, W. M.; Calligaris, M.; Sava, G.; Zorzet, S. Inorg. Chem. 1988, 27, 4099–4106. (9) Hafner, A.; Muhlebach, A.; Van Der Schaaf, P. A. Angew. Chem., Int. Ed. 1997, 36, 2121–2124. (10) Lo, C.; Cariou, R.; Fischmeister, C.; Dixneuf, P. H. Adv. Synth. Catal. 2007, 349, 546–550. (11) Keitz, B. K.; Grubbs, R. H. J. Am. Chem. Soc. 2009, 131, 2038–2039. (12) Wang, D.; Wurst, K.; Knolle, W.; Decker, U.; Prager, L.; Naumov, S.; Buchmeiser, M. R. Angew. Chem., Int. Ed. 2008, 47, 3267–3270. (13) (a) Ben-Asuly, A.; Tzur, E.; Diesendruck, C. E.; Sigalov, M.; Goldberg, I.; Lemcoff, N. G. Organometallics 2008, 27, 811–813. (b) Kost, T.; Sigalov, M.; Goldberg, I.; Ben-Asuly, A.; Lemcoff, N. G. J. Organomet. Chem. 2008, 693, 2200–2203. (14) For a recent review on latent olefin metathesis initiators see: Szadkowska, A.; Grela, K. Curr. Org. Chem. 2008, 12, 1631–1647. r 2009 American Chemical Society

Communication

Figure 1. Crystal structures of (a) trans-1b and (b) cis-1b (ellipsoids displayed at the 30% probability level, solvent and hydrogens removed for clarity). Scheme 1. UV-Induced RCM of Diethyl Diallylmalonate

Supporting Information). Then, precatalysts 1a-d were irradiated with a 100 W UV lamp (365 nm) for 5 h in the presence of diethyl diallylmalonate (DDM) as a benchmark substrate to check for metathesis activity. To our satisfaction, UV irradiation elicited ring-closing metathesis (RCM) activity of 1a-c and high conversions could be obtained after 24 h at room temperature (Scheme 1). Note that complexes 1a-d are completely inert at room temperature without irradiation and only a very small DDM conversion could be obtained by irradiating complex 1d. However, in addition to the induced photoactivation, a revealing cis- to trans-dichloro photoisomerization process was also observed. All sulfur pentacoordinated chelated complexes we have secured to date,13 along with the novel 1b,c, are more stable in solution in the cis-dichloro configuration. The trans-dichloro kinetic product compounds readily isomerize in solution to the cis-dichloro configuration, even at room temperature, as detailed in our previous publication for 1d.13a We have now succeeded, by shortening reaction times and lowering temperatures to isolate the trans forms, and the single-crystal structures of both cis- and trans1b are depicted in Figure 1.15 Comparable to the observation made by Calligaris for the chemotherapeutic RuCl2(dmso)4 complex,8d UV irradiation of the cis-dichloro complexes afforded the less thermodynamically (15) Crystal data: cis-1b, C38H38Cl2N2RuS 3 CH2Cl2, M = 811.66, monoclinic, space group P21/n, a = 13.8353(2) A˚, b = 15.5902(2) A˚, c = 16.9225(3) A˚, β = 90.2912(6)°, V = 3650.05(10) A˚3, Z = 4, T = 110(2) K, Dc = 1.477 g cm-3, μ(Mo KR) = 0.811 mm-1, 30 275 reflections measured, 8678 unique (Rint = 0.035), final R = 0.044 (Rw = 0.109) for 6565 reflections with I > 2σ(I) and R = 0.064 (Rw = 0.120) for all data, CCDC 729278; trans-1b, C38H38Cl2N2RuS (excluding the disordered hexanes solvent), M = 726.73, monoclinic, space group C2/c, a = 38.9862(9) A˚, b = 8.5771(3) A˚, c = 28.1811(8) A˚, β = 125.811(2)°, V = 7641.9(4) A˚3, Z = 8, T = 110(2) K, Dc = 1.263 g cm-3, μ(Mo KR) = 0.631 mm-1, 27 710 reflections measured, 9057 unique (Rint = 0.072), final R = 0.069 (Rw = 0.160) for 6613 reflections with I > 2σ(I) and R = 0.097 (Rw = 0.169) for all data, CCDC 729279.

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Figure 2. DFT calculations on cis-trans isomerism for 1d. Scheme 2. Proposed Photoisomerization Mechanism of 1

stable trans-dichloro isomers in about 30% yields for all cases (visible light irradiation promoted a similar isomerization, although it was less pronounced and longer times were needed; see the Supporting Information). To explain the configuration change, we propose photodissociation of the sulfur-ruthenium bond, followed by a fast rearrangement of the complex, affording a 14e ruthenium species with both chlorine ligands trans to each other (trans-1 14e, Scheme 2). Subsequent ligation of the sulfur to the ruthenium may occur in the alternate available site, trans to the N-heterocyclic carbene (NHC) ligand, generating trans-1 (Scheme 2). The calculated cis-dichloro-trans-dichloro isomerization pathway recently put forward by Goddard15 and our own DFT predictions of the mechanistic cycle involving dissociation of the chelating ligand (Figure 2) firmly support the mechanism proposed in Scheme 2 (see the Supporting Information for alternative nonpreferred pathways). The trans-dichloro geometry has been shown in the literature to be more active than its cis counterpart.17 Thus, we decided to isolate trans-1 by chromatography (as the kinetic products) and probe their activity. Unmistakably, the isolated trans complexes were found to efficiently promote RCM of DDM in CH2Cl2, with the exception of trans-1d. Thus, the photoisomerization process was demonstrated to be responsible for the activation of the metathesis (16) (a) Benitez, D.; Goddard, W. A.III J. Am. Chem. Soc. 2005, 127, 12218–12219. (b) Benitez, D.; Tkatchouk, E.; Goddard, W. A.III Chem. Commun. 2008, 6194–6196. (17) Barbasiewicz, M.; Szadkowska, A.; Bujok, R.; Grela, K. Organometallics 2006, 25, 3599–3604.

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Table 1. Photoisomerization Ratio and RCM Conversion cis:transa UV irradiation time, h 0 0.5 1 5 10

1a

1b

100:0 47:1 10:1 3:1 2.6:1

100:0 157:1 60:1 5.5:1 6:1

Table 3. UV-Activated ROMP

conversn of DDM, %b 1a 0 21 39 85 86

1b 0 6 13 74 65

a Measured by 1H NMR. b DDM was added after irradiation; conversion measurements by GC-MS were taken 24 h after DDM addition.

Table 2. UV-Activated RCM

a

Conversions were calculated by the reduction in monomer concentration by GC-MS after 24 h using mesitylene as internal standard. Conditions: initial monomer concentration 0.5 M in CH2Cl2; [Monomer]/[Cat.] = 300. Mn and PDI values were determined by triple-detector SEC; Z/E values were determined by 1H NMR.

a Conditions: 5 mol % cat.; substrate 0.1 M in CH2Cl2; UV irradiation at 365 nm for 5 h; thermostated at 28 °C. Yields were determined by GC-MS after 24 h using mesitylene as internal standard. 0% yields were obtained without UV irradiation in all entries. b Yield includes isomerization products.

precatalyst. Table 1 illustrates the relationship between the cis-trans ratio and RCM conversion. As shown, the maximum amount of trans-1a isomer obtained is about one-third after 5 h of UV irradiation at 365 nm. In addition, the higher the trans ratio, the higher the metathesis conversion. Additional ring-closing metathesis reactions were successfully carried out using UV light as the activating stimulus, and the results are summarized in Table 2. Due to the importance of latency in ROMP,18 we decided to analyze three typical ROMP monomers, as detailed in Table 3. To our satisfaction, the monomers were completely inert when mixed with the catalysts in methylene chloride solution at 30 °C for 3 days, but efficient polymerization could be observed when the solutions were irradiated by UV light. However, two other reactive monomers, 2-norbornene and (18) For thermally activated ROMP with sulfur-chelated Hoveyda type catalysts see: Diesendruck, C. E.; Vidavsky, Y.; Ben-Asuly, A.; Lemcoff, N. G. J. Polym. Sci., Part A: Polym. Chem. 2009, 47, 4209–4213.

dicyclopentadiene, were polymerized under these conditions without irradiation; thus, catalysts 1a,b did not show latent behavior for the more active 2-norbornene and dicylopentadiene compounds. Furthermore, we also studied activation of complexes 1a-c with visible light instead of UV light. The results achieved were similar to those obtained with UV, although lower yields were obtained, as expected for a less efficient photoisomerization (for details see the Supporting Information). Having demonstrated the feasibility of catalyst activation by light, we sought an efficient deactivation protocol to obtain a switchable catalytic system. Since the latent cis form is also thermodynamically more stable, the most straightforward method to achieve the off state should be a raise in temperature. Deuterated 1,1,2,2-tetrachloroethane was used instead of dichloromethane to attain the desired temperature of 80 °C. UV irradiation for 15 min of a solution of cis-1b and DDM followed by 5 min at 80 °C produced about 15% conversion.19 As desired, after 5 min of heating, full conversion of all trans to cis species was obtained, effectively stopping the reaction after this period. Additional irradiation sequences formed new trans-1b and resumed reaction conversion. This allowed for precise quantities of DDM to be ring-closed at every activation step. Each irradiation cycle isomerized a small fraction of the precatalyst, preserving the remaining complex to be activated in a future round. Thus, RCM could be photoactivated in a controlled manner several times (Figure 3), analogous to the thermally switchable behavior observed in other sulfur-chelated ruthenium benzylidene complexes;13 however, in this case the activation stimulus was light, while the deactivation factor was a short heating phase. In conclusion, we show latent metathesis precatalysts that can be activated by an optical stimulus due to a phototriggered isomerization process. In light of the importance of the metathesis reaction, we find that the control of its activation is highly significant, especially in ROMP reactions with active monomers. Multiple cycles of irradiation-heating lead to activation-deactivation sequences generating a (19) A 5 min heating period at this temperature without previous UV irradiation in the presence of cis-1b and DDM produced negligible conversion (see the Supporting Information).

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photoswitchable behavior, or it may be also advantageous when safety concerns dictate halting a reaction in progress. Moreover, we have found that light-activated RCM is much more significant if aromatic substituents are present at the sulfur chelating atom due to enhanced activity of the trans isomer. Future studies are ongoing to investigate our ability to further control and understand the factors that govern the cis-trans photoisomerization process by steric and electronic effects. In addition, we are pursuing the development of new light-triggered metathesis catalysts and studying the influence of additional substituents on the chelating heteroatom to obtain higher efficiencies and defined wavelengths for light activation.

Figure 3. Controlled UV-activated RCM of 0.1 M DDM in C2D2Cl4 with 5% molar 1b. Rest periods: 4 h at room temperature. Activation: 15 min of UV irradiation followed by 5 min of heating to 80 °C. The conversion was measured by NMR.

switchable RCM system without the addition of further reagents. This intimate control over reaction progress allows for the development of novel one-pot multicomponent reactions where the order of reactions may be controlled by the

Acknowledgment. The Edmond Safra Foundation is gratefully acknowledged for funding. C.E.D. thanks the Minerva Foundation for funding a short-term fellowship in Heidelberg. Supporting Information Available: Text, figures and tables giving synthetic and characterization details for 1b,c, full details for irradiation experiments, and DFT calculations and CIF files giving crystallographic data for trans-1b and cis-1b. This material is available free of charge via the Internet at http://pubs. acs.org.