Synthesis and Catalytic Properties of Sulfur-Chelated Ruthenium

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Letter

Synthesis and Catalytic Properties of Sulfur-Chelated Ruthenium Benzylidenes Bearing a CAAC Ligand Illya Rozenberg, Or Eivgi, Alexander Frenklah, Danielle Butilkov, Sebastian Kozuch, Israel Goldberg, and Norberto Gabriel Lemcoff ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.8b02122 • Publication Date (Web): 06 Aug 2018 Downloaded from http://pubs.acs.org on August 6, 2018

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ACS Catalysis

Synthesis and Catalytic Properties of SulfurChelated Ruthenium Benzylidenes Bearing a CAAC Ligand Illya Rozenberg,† Or Eivgi,† Alexander Frenklah,† Danielle Butilkov,† Sebastian Kozuch,† Israel Goldberg‡ and N. Gabriel Lemcoff†§* †

Chemistry Department, Ben-Gurion University of the Negev, Beer-Sheva, 8410501, Israel §

Ilse Katz Institute for Nanoscale Science and Technology, Beer-Sheva 8410501, Israel ‡

School of Chemistry, Tel-Aviv University, Tel-Aviv 69978, Israel

ABSTRACT

Sulfur-chelated

ruthenium

olefin

metathesis

precatalysts

that

possess

cyclic

(alkyl)(amino)carbenes (CAAC) can benefit from the synergetic effect of both ligands. Changing the steric bulk of the CAAC ligand by using different substitution patterns was shown to affect the geometry of the complexes produced and determined whether the complexes could be catalytically dormant. The cis-dichloro latent catalysts could be activated both by heat or light, even in the visible region, for representative ADMET and ROMP reactions, olefin crossmetathesis and RCM without isomerization byproducts. Thus, these complexes were shown to

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combine the uniqueness of CAAC containing Ru olefin metathesis catalysts with the advantage of the thermal and photolatency imposed by sulfur chelation of the benzylidene.

KEYWORDS Olefin metathesis, ruthenium, sulfur-chelated benzylidenes, photoinduced catalysis, CAAC ligand, latency, double-bond isomerization

INTRODUCTION The olefin metathesis reaction is amongst the most studied and important reactions for the formation of carbon-carbon bonds.1-2 The ample scope of metathesis procedures that may be used to construct complex organic compounds and materials has encouraged the development of a wide variety of complexes that catalyze this reaction and has received much attention both from academia and industry.3-10 The most popular precatalysts currently used are based on ruthenium complexes stabilized by Arduengo’s N-heterocyclic carbene (NHC) ligands, mainly due to their high activity and relative tolerance towards air, water and many common functional groups.11-13 The great versatility and modularity of these complexes allows different reactivity patterns,14 latency,15-18 stereoselectivity,19-23 asymmetric metathesis,24-25 reactions in aqueous media,26-27 recycling of catalysts28-29 and applications in photoinduced olefin metathesis reactions.30-31 One of the most significant improvements in stability and functional group tolerance of olefin metathesis catalysts was the introduction of an oxygen chelated benzylidene ligand by Hoveyda


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and Blechert.32-33 Replacement of the chelating ether functionality in the Hoveyda-Grubbs second generation (HG2) catalyst by various thioether groups yielded a family of complexes that appear in two isomeric forms: trans-Cl2 (1) and cis-Cl2 (2) (Scheme 1).18, 34-46 Complexes 1 were found to be active for olefin metathesis; however, they readily convert to the inactive complexes 2. The relative stability between both forms was attributed to the strong trans-influence of the NHC ligand, which deters having another good σ-donor ligand in the trans position, such as the sulfur atom.37 Notably, complexes 2 can be isomerized back to 1 by irradiation with UV-A light and recover their catalytic activity.47 The most efficient photoisomerizations from 2 to 1 were obtained when the R group attached to sulfur was either an aromatic ring36 or a trifluoromethyl moiety.40

Scheme 1. Structures of trans-Cl2 (1) and cis-Cl2 (2) isomers of thioether chelated precatalysts for olefin metathesis. In 2005 Bertrand and co-workers introduced a new type of singlet carbene, called cyclic (alkyl)(amino)carbene (CAAC).48 This ligand was found to be an even stronger σ-donor and a better π-acceptor than common NHCs, and has been applied in the development of many improved catalysts, benefiting from the tunable electronic and steric properties of the CAAC ligands.49-50 The first Ru-CAAC complexes for olefin metathesis applications were introduced a decade ago jointly by Bertrand, Grubbs and co-workers (Figure 1, complexes 3a and 3b).51 Initially, these precatalysts showed relatively low activity in benchmark reactions, but judicious


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tuning of substituents on the CAAC mainframe52-53 led to the design of extremely efficient precatalysts. For example, complex 3c boasted impressive turnover numbers of over 300,000 for the industrially important ethenolysis reaction of methyl oleate. Further diversity to the family of CAAC catalysts for olefin metathesis was recently introduced by Skowerski and co-workers, by producing reactive bis-CAAC complexes, such as 4.54 These complexes did not just widen the scope of olefin metathesis reactions performed by CAAC-containing precatalysts, but also served as useful precursors for the synthesis of other precatalysts, e.g. 5, which could efficiently perform challenging macrocyclization reactions and cross-metathesis with acrylonitrile.55 Highlighting the special properties of this valuable ligand, Butilkov et al. have also recently shown that the CAAC-containing complex 3c significantly reduced side-reactions associated with isomerization processes commonly seen with commercial ruthenium olefin metathesis catalysts; particularly when olefin metathesis reactions are carried out at harsher conditions where detrimental ruthenium hydrides may be formed.56

Figure 1. Selected CAAC-bearing precatalysts for olefin metathesis reactions.


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Taking into account the improved catalytic activity of the Bertrand-Grubbs ruthenium benzylidenes and the ability of S-chelation to enforce a latent cis-Cl2 configuration, we envisioned that the combination of CAAC ligands with S-chelated benzylidenes could yield enhanced precatalysts for applications where latency can be beneficial. Herein, we report on the synthesis of a new family of thermo- and photoactive precatalysts containing both CAAC and chelating thioether ligands, including their characterization and reactivity studies for representative olefin metathesis reactions.

RESULTS AND DISCUSSION Synthesis and characterization of complexes with diisopropylphenyl-CAAC (DiPPCAAC). Given that the most active S-chelated complexes were obtained with CF3 and Ph substituents on the sulfur (vide supra), these same substitution patterns were targeted for the CAAC containing complexes. Thus, complexes 10a and 10b were produced by exchanging the ligands of 8 by styrenes 9a and 9b respectively (Scheme 2). An alternative synthetic pathway involving precatalyst 3a was unsuccessful. The complexes were assigned the trans-Cl2 configuration based on 1H-NMR analyses and this was endorsed by the solved crystal structures of the pair (Figure 2). The data obtained from the X-ray analysis determined that the Ru-CCAAC and Ru-Cbenzylidene bonds in both complexes 10 had the same length (1.99Å and 1.83Å respectively). Moreover, the Ru-CCAAC bonds were about 4 pm shorter than the Ru-CNHC bond in an S-chelated trans-Cl2 complex (2.03Å),36 consistent with the stronger σ-donor and π-acceptor abilities of the CAAC, and in line with the observation made by Bertrand and Grubbs for Ochelated trans-Cl2 complexes.51 The Ru-S bond in 10a was slightly longer than its counterpart in 10b, and this may be attributed to a stronger electron withdrawing effect of the CF3 group.


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Scheme 2. Synthesis of 10a and 10b.

Figure 2. ORTEP representation (50% probability ellipsoids) of complexes 10a (A) and 10b (B) with selected bond lengths. Hydrogen atoms were omitted for clarity. However, the isolation of complexes 10 in their trans-Cl2 configuration was somewhat unexpected, because the corresponding S-chelated ruthenium benzylidenes with DiPP-NHCs are much more stable in the alternative form.46 Moreover, it was previously shown that increased steric repulsion between the NHC and a bulky S-chelated benzylidene could be relieved to a


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certain extent by partial rotation around the CNHC-Ru axis and not by isomerization to the transCl2 isomer.39 An analogous stabilization of this sort is not plausible in the cis-Cl2 isomer with the DiPP-CAAC ligand, as partial rotation in this case does not alleviate steric strain; on the contrary it increases the hindrance by placing the benzylidene aromatic ring directly beneath one of the CAAC-isopropyl groups. This reasoning was supported by DFT calculations that show that rotation around the CCAAC-Ru axis leads to a strong destabilization with a high energy barrier (Figure S57, Supporting Information). Attempts to isomerize 10-trans to 10-cis in dichloromethane (DCM), a solvent that normally facilitates the isomerization process, were unsuccessful even after 16 hours at 35°C. Replacing the solvent by 1,1,2,2-tetrachloroethane (TCE) and heating to 75°C afforded only marginally affected 10a. However, heating 10b in TCE did produce a visible change (see Figure S63 in Supporting Information). In any case, it is clear that the strong steric repulsion from the bulky DiPP-CAAC ligand destabilizes the cis complex, reminiscent of the effect seen in Grubbs’ second generation complex which is also more stable as trans-Cl2 (even though the P atom in the PCy3 ligand is a strong sigma electron donor).37, 57 The isomerization behavior of 10a and 10b under UV-irradiation was also studied. Again, 10a was quite stable under UV-A irradiation. However, in accordance to what was observed by the thermal stimulus, 10b isomerized from trans-Cl2 to cis-Cl2, reaching a ratio of 1.2:1 after 20 hours irradiation with only minor decomposition. The assignment of 10b-cis was supported by NOESY NMR experiments. Particularly, in 10b-trans a correlation was observed between the benzylidene proton at 16.86ppm and two of the methyl signals attached to the isopropyl groups at 0.65ppm (Figure 3A). However, the NOESY spectrum of the reaction mixture after irradiation revealed a new correlation between the incipient benzylidene proton (16.79ppm) and just one of the isopropyl methyl peaks (1.80ppm); moreover, also no correlations were observed between


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the benzylidene proton with the alkyl methyl groups placing the aromatic group of the CAAC on top of the aromatic ring of the benzylidene (Figure 3B and Figures S70 and S71 in Supporting Information). This is thus the first observation where irradiation is used to achieve isomerization at room temperature from the trans form to the cis form in S-chelated Ru precatalysts.

Figure 3. Partial NOESY spectra of 10b with observed correlations: A. trans-Cl2 complex before irradiation, B. Mixture of trans- and cis-Cl2 isomers after 24h of irradiation at 350nm.

Synthesis and characterization of complexes with diethylphenyl-CAAC (DEtP-CAAC). A relief of the steric hindrance was achieved by exchanging the isopropyl groups on the aromatic rings in the CAAC ligand by ethyl groups. Thus, complexes 13a and 13b were prepared following the Skowerski protocol by ligand exchange from complex 12 (Scheme 3),55 as the methodology followed for the synthesis of 10 failed in our hands. Complexes 13 could be obtained as mixtures of both cis-Cl2 and trans-Cl2 isomers, which were readily separated by column chromatography. As previously observed for S-chelated NHC complexes, the ratio of cis/trans-Cl2 isomers could be readily tuned by solvent polarity and temperature.39


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Scheme 3. Synthesis of 13a and 13b. Figure 4 details the crystal structures of cis-13a and cis-13b. Compared to the previously reported Mes-NHC analogues, cis-14a40 and cis-14b,35 it can be observed that the CAAC ligand is closer to the metal center by about 6 pm. Moreover, the strong sigma donation and corresponding trans influence of the CAAC ligand correlates well with the observed increase in the Ru-Cltrans bond lengths.


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Figure 4. ORTEP representation (50% probability ellipsoids) of complexes cis-13a (A) and cis13b (B) with selected bond lengths. Hydrogen atoms were omitted for clarity. Both cis-13a and cis-13b could also be transformed into their trans counterparts by means of UV-A irradiation. Complex cis-13a reached its photostationary state with a cis:trans ratio of 1:2 after irradiation of the complex at 350nm for 5 hours. A similar ratio was obtained when trans13a was irradiated for 3 hours. Cis-13b isomerizes to its trans counterpart reaching a cis:trans ratio of 4:1 after 5 hours. Computed relative energies of trans- and cis-Cl2 isomers of 10 and 13 show the stronger preference of 13 for the cis isomer compared to 10, correlating well with the experimental results (see Table S10 in Supporting Information). Latency of the complexes in typical OM polymerization reactions. The main application of latent catalysis is decidedly in polymerization reactions.58 Thus, complexes of 10 and 13 were screened for five typical ROMP and ADMET reactions (Scheme 4) at 35°C and 75°C. Monomer conversion was monitored relative to an internal standard (mesitylene) after specified time periods and the results are presented in Figure 5 (see Table S12 in Supporting Info for additional details).


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Scheme 4. Polymerization scope for complexes 10 and cis-13. All precatalysts at 35°C were latent for all but the most reactive ROMP monomer, M5 (norbornene). 10a was somewhat more active (less latent) than the others and also promoted ROMP of M3 and ADMET of M1 at room temperature; albeit only giving low conversions after 24 hours. Cis-13a also polymerized M3 in low yields after 24 hours. M2 (cyclooctadiene) was also checked after 10 days at 35°C with precatalysts cis-13a and cis-13b to assess the ability of the complexes to remain with the substrate for extended time periods. Indeed, 71% of starting material was still present with cis-13a and 96% of M2 was found for cis-13b; highlighting the prolonged latency with this monomer. It is notable that the benzylidene signal in the 1H-NMR was still prominent after the 10-day period, indicating the presence of intact precatalyst (see Figures S82-S85 in Supporting Information). Heating to 75°C, afforded full polymerization of M5 after just one hour with all precatalysts. As expected, the other monomers were less prone to polymerization. M1 and M3 were polymerized at 75°C by all the precatalysts after one day; but only cis-13a could efficiently polymerize M3 after just one hour. 10a and cis-13a also showed some catalytic activity with M4; however, the only precatalyst that could polymerize monomer M2 under thermal stimulus was cis-13a. In summary, M5 cannot be considered a latent monomer for these catalysts, as it reacts at 35°C; and the most active precatalyst after thermal activation was cis-13a; which was able to fully polymerize M3 after one hour and could promote polymerizations of M1, M2 and M4 to some extent after 24 hours.


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Figure 5. Results of testing polymerization reactions activated by 35°C (A) and 75°C (B). The photoactivation screening at 350nm was quite revealing given that only cis-13a could be activated by this wavelength (See Table S12 in Supporting Information). Thus, UV-A irradiation of the reaction mixture containing 0.1 mol% cis-13a led to efficient polymerizations of M5 and M3 and to some polymerization of M1 and M2, reminiscent of the thermal activation behavior observed for this complex (Figure 6A). Notably, complex 10a, that performed well under thermal stimulus, was completely oblivious to activation with light. Additional wavelengths for precatalyst activation (254nm and 419nm) were also screened (See Table S13 in Supporting Information). To our satisfaction, irradiation of cis-13a with 419nm light gave excellent results, especially with monomer M2. Figure 6 details the photochemical activation of cis-13a, showing


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ACS Catalysis

that visible light can also be efficiently used to initiate ROMP. The activation of cis-13a with UV-A and visible light marks the first CAAC containing olefin metathesis catalyst that may be stimulated by this approach.

Figure 6. Results of testing polymerization reactions with cis-13a activated by 350nm (A) and 419nm (B) irradiation. An additional monomer of significant industrial importance is dicyclopentadiene (M6).59 This monomer is together with norbornene (M5), one of the most reactive ROMP substrates, and latency with M6 is rarely achieved. Nevertheless, cis-13a showed only 7% conversion after mixing it with M6 in tetrachloroethane-d2 for 30 minutes (still flowing liquid) at 35°C in the dark; while irradiation for the same period of time at 419nm or heating to 80°C led to full polymerization (completely gelated, see Figures S88-S90 in Supporting Information). These results show the viability of using this type of complex for advanced polymerization techniques even with reactive DCPD. The pM3 polymers obtained with 13 were analyzed both by 1H-NMR and GPC (Table 1). As previously discussed, cis-13a was found to be much more reactive than cis-13b, reaching full conversion by both thermal and photo activation methods. Polymerizations by thermal activation of cis-13a afforded pM3 polymers with slightly wider molecular weights distributions (PDI)


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compared to the results obtained when the complex was activated by light, possibly due to chaintransfer reactions that may happen during prolonged heating.60 Notably, a much lower polydispersity was obtained by thermally activating initiator cis-13b. This may be the result of the lower conversions, that diminish also side reactions which increase the PDI. Moreover, photoinduced polymerization of M3 with cis-13b failed, and only the presence of short oligomers in low conversions could be detected after 24h UV-A irradiation. Another interesting aspect of this reaction was the lower content of E-double bonds in polymers obtained by cis-13b compared to those obtained with cis-13a, probably also implying a reduction also in secondary metathesis reactions (the polymers obtained by the initiator cis-13a give a Z/E ratio consistent with a thermodynamic equilibration of the system). The Z/E isomerization ability of complex cis13a was further tested on commercial all-cis polybutadiene (98% Z, Mw = 250 kDa, PDI = 2.3); where, after equilibration with 13a for 24h at 80°C, 50% E double bonds was observed by 1HNMR (See Figure S86 in Supporting Information). Table 1. Polymerization of M3 by cis-13.

Activation method

Conversion (%)a

Z/E ratioa

Theoretical Mn (kDa)

Mw (kDa)b

Mn (kDa)b

PDIb

cis-13a

75°C

99

1/2.1

109

29

16

1.8

2

cis-13a

350nm

98

1/1.9

108

102

68

1.5

3

cis-13b

75°C

40

1/1.0

44

150

120

1.2

4

cis-13b

350nm

13

1/1.0

14

NAc

NAc

NAc

Entry

Catalyst

1


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a. Elucidated from 1H-NMR and averaged for two experiments after 24h of reaction. b. Determined by triple detector GPC analysis after 24h of reaction. c. The polymer could not be isolated.

Selectivity of the complexes in thermally activated reactions. One of the main drawbacks of carrying out olefin metathesis reactions at elevated temperatures is the upsurge of harmful sidereactions, such as double bond migration.61-70 It was recently reported that ruthenium olefin metathesis catalysts containing CAAC ligands produced much lower amounts of isomerized products, in comparison to the NHC-bearing complexes.56 Thus, the performance of 10 and 13 was screened in the self-metathesis reaction of methyl oleate (MO) at 100°C and compared to the performance of latent NHC complex 14. Self-metathesis of MO can produce two products: octadec-9-ene (C18) and the diester (DE) (Scheme 5). Double bond migration gives rise to the corresponding homologues; therefore, this is a useful reaction to analyze whether non-metathetic reactions are at play. Consequently, neat MO was heated at 100°C for 1 hour in the presence of 0.1 mol% precatalyst. 10a catalyzed MO self-metathesis to form the desired products, in conjunction with approximately 10% isomerization, after 1 hour of heating. In contrast, 10b gave very low conversions after 24h; suggesting that this precatalyst decomposes at elevated temperatures. Both cis-13a and cis-13b reacted with MO to yield the expected products without any noticeable isomerization sideproducts. Having in hand also the corresponding trans isomers, trans-13a and trans-13b, we decided to probe whether the initial configuration of the ligands can affect the reaction results (even though these complexes readily isomerize to cis-13 at high temperatures). As expected, both trans precatalysts gave exactly the same results as their cis counterparts. Finally, 14a and 14b were also screened. Surprisingly both 14a and 14b also gave no isomerization products. It may well be that at this temperature there is a only a very small amount of active trans complex


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that quickly reacts with the substrate, thus it cannot generate the ruthenium hydride required for double bond migration71 or decompose through a dimeric species72 (as suggested in the literature). So, the experiments with complexes 10, cis-13, 14 and HG2 were repeated at 120°C to analyze how this would affect the catalyst performance. Except for 10b that quickly decomposed, all precatalysts produced the equilibrium distribution expected for the selfmetathesis of MO. The general trend observed was that SCF3 based complexes gave higher isomerization degree than SPh analogues. However, the most salient discovery was that cis-13b was the only complex that did not isomerize the substrate, making it the most selective latent precatalyst to date under these harsh conditions (Figure 7).

Scheme 5. Self-metathesis of methyl oleate.


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Figure 7. GC-MS chromatograms of self-metathesis of MO product mixtures after reactions with various precatalysts at 100ºC (A) and 120ºC (B). The percent value left to chromatograms indicates isomerization percent (See Supporting Information for calculation method). NR – no reaction occurred. Having investigated the thermally activated self-metathesis reaction of MO, we decided to take advantage of the alternative methodology to activate the latent complex, light. All light induced olefin metathesis reactions studied to date include photo-induced RCM, ROMP and a few cross-metathesis (CM) reactions of terminal alkenes.30-31 Photo-induced CM of internal alkenes has not been reported in the literature to date. Thus, to test whether the new complex 13 may put forth this more difficult photo-CM reaction, neat MO and 0.1 mol% cis-13a were irradiated at 350nm. To our delight, the reaction proceeded smoothly, without any isomerization (as expected for a reaction at room temperature), to afford the equilibrium mixture promoted by light. For the sake of completeness, 14a was also tested in this challenging photoinduced selfmetathesis reaction and gave excellent results as well. No product formation was observed in the dark control reactions; nicely highlighting, also in this case, the necessity of light for the reaction to proceed (See Figures S108-S111 in Supporting Information for further details). Finally, the complexes were also tested under more strenuous conditions through the ringclosing metathesis (RCM) of N-allyl-N-(pent-4-en-1-yl)tosylsulfonamide (15) at 150°C (Scheme 6). RCM of tosylsulfonamide 15 affords cyclic product 16. At higher temperatures, additional products 17 and 18 appear due to non-metathetic processes after the ring-closing step.69 To determine whether the CAAC bearing complex could avoid the isomerization in this case, the reaction was carried out for 1 hour in boiling deuterated TCE and monitored by 1H-NMR. Indeed, precatalysts 14a and 14b afforded a very high percentage of isomerized products.


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However, complexes containing the CAAC ligand gave very low amounts of byproduct 17 (Figure 8), once again demonstrating the important characteristics of these novel latent complexes.

Scheme 6. RCM of 15.

Figure 8. Product distribution after RCM of tosylsulfonamide 15. [15]=0.1M in TCE-d2, [Ru]=1mol%, 150°C, 1h. The yield was determined by integration of 1H-NMR spectra. CONCLUSIONS


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Latency in olefin metathesis is an important property that allows specialty applications, ranging from specific orthogonal chemistries73 to well-controlled polymerizations.58 The introduction of CAAC ligands to the family of S-chelated benzylidenes can open new possibilities for achieving olefin metathesis reactions under harsh conditions without pernicious side reactions. We learned that the use of more hindered CAAC ligands does not produce efficient latent catalysts; however, relieving the steric stress, by using ethyl groups instead of isopropyl groups, afforded optimal S-chelated complexes that efficiently catalyze several olefin metathesis reactions. The metathesis reactions include ROMP and ADMET polymerization reactions and difficult cross-metathesis of internal alkenes, both by light and heat activation, without the pernicious side reactions observed with the more ubiquitous NHC containing complexes. The realization of more selective, light sensitive, latent olefin metathesis catalysts is of great need for developing practical applications in a field that is starting to combine welldefined metathesis materials and photochemistry. ASSOCIATED CONTENT Supporting Information. The Supporting Information is available free of charge on the ACS Publications website. Synthetic procedures for complexes and their characterization (1H-, 13C- and 19F-NMR, mass and UV-Vis spectra), crystallographic data summary for complexes, NMR spectra of thermal and photo-activated isomerization, GPC chromatograms, GC-MS analyses, NMR spectra of catalysis products. (PDF) X-ray crystallographic data for 10a (CIF) X-ray crystallographic data for 10b (CIF)


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X-ray crystallographic data for cis-13a (CIF) X-ray crystallographic data for cis-13b (CIF)

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]; Fax: (+972) 8647-1740; Tel: (+972) 8646-1196. ORCID Illya Rozenberg: 0000-0002-3873-8321 Or Eivgi: 0000-0003-0997-1711 Alexander Frenklah: 0000-0002-4575-1292 Sebastian Kozuch: 0000-0003-3070-8141 Israel Goldberg: 0000-0002-3117-0534 N. Gabriel Lemcoff: 0000-0003-1254-1149 Funding Sources The Israel Science Foundation is gratefully acknowledged for funding this research Notes The authors declare no competing financial interest. ACKNOWLEDGMENT The authors dedicate this paper to the memory of our collaborator Prof. Israel Goldberg.


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ABBREVIATIONS CAAC, cyclic (alkyl)(amino) carbene; NHC, N-heterocyclic carbene; ROMP, ring-opening metathesis polymerization; ADMET, acyclic diene metathesis; RCM, ring-closing metathesis. REFERENCES (1)

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Synthesis and Catalytic Properties of Sulfur-Chelated Ruthenium

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