Ligand Isomerization in Sulfur-Chelated Ruthenium Benzylidenes

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Ligand Isomerization in Sulfur-Chelated Ruthenium Benzylidenes Anna Aharoni,† Yuval Vidavsky,† Charles E. Diesendruck,† Amos Ben-Asuly,†,‡ Israel Goldberg,§ 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 ‡

bS Supporting Information ABSTRACT: cis-Dichloro-trans-dichloro isomerization was studied in sulfur-chelated ruthenium benzylidenes. The effect of solvents and additives on the isomerization process was analyzed. Notably, π-acid ligand molecules, dichloromethane, and polar solvents were found to significantly accelerate the isomerization rate. As expected, the cis-trans isomer equilibria had a strong effect on the olefin metathesis reactivity; the bulkier cis-dichloro catalysts, which show a larger amount of trans isomer in the equilibrium, were more reactive than their less bulky counterparts. This tendency was generally reversed in the isolated trans isomer series, where both steric factors and faster isomerization play a role in dictating the olefin metathesis reactivity. A better understanding of the trans-cis isomerization and the mechanism that governs reactivity in strongly chelated cis-dichloro ruthenium benzylidenes was obtained.

’ INTRODUCTION Well-defined ruthenium carbene catalysts provide a powerful methodology for carbon-carbon bond formation and show a broad range of applications in metathesis, such as, ring-opening metathesis polymerization (ROMP), acyclic diene metathesis polymerization (ADMET), cross-metathesis (CM), ring-closing metathesis (RCM), enyne metathesis (EM), equilibrium ringclosing metathesis (ERCM), ring-expansion metathesis polymerization (REMP), ring-rearrangement metathesis (RRM), and dimer ring-closing metathesis (DRCM) reactions.1-4 Commonly used ruthenium olefin metathesis precatalysts 1a-c (Figure 1) possess a carbene ligand, two additional neutral donor ligands, and two anionic ligands.5 This family of complexes habitually share a common trans geometry of the X-type, as well as the n-type ligands, with the carbene in the apical position of a square-pyramidal structure. Some examples that escape this stereochemical description are shown in Figure 2. For instance, complex 2a, first described by Hofmann, was the first Grubbs-type complex with a cis-dichloro configuration, enforced by the chelating bisphosphine ligand.6 Later, F€urstner et al. presented the first trans-cis isomerization of precatalyst 2b by action of silica gel.7 Following this, works by Grubbs, Fogg, Slugovc, and Grela described cis-dichloro or cis-X2 complexes 2c-e, and the equilibrium between these species was discussed.8-11 The first Hoveyda-Grubbs-type complex with a sulfur-chelating atom (3) was recently disclosed by us.12 This complex also displayed the less common cis-dichloro configuration and exhibited latency toward RCM and other metathesis reactions at room temperature.13 More recently Cazin and coworkers14 have described a cis-dichloro system (2f) with an indenylidene carbene and a phosphite ligand. r 2011 American Chemical Society

Figure 1. Commercial Grubbs-type ruthenium olefin metathesis catalysts.

Even though structural and activity reports on these cis-type complexes may be found in the literature, studies on the factors affecting the equilibrium between the species are less common. Grubbs et al. have shown8 that the trans species feature enhanced reactivity relative to the cis counterparts. Recently, the isomerization process between the cis and trans species in nitrogen-chelated benzylidenes was detailed by Cavallo et al.15 In this regard, we have also shown that a cis-trans photoisomerization process promotes the activity of several S-chelated latent precatalysts, leading to photoactivated olefin metathesis.16 This photoinduced isomerization process affords a more energetic isomer and has been recently mentioned as a possible way to store solar energy.17 Further study of the features that influence the relative position of the ligands in ruthenium alkylidenes and a better understanding of the mechanisms at play should have repercussions on the properties, and maybe uses, of the catalysts.

Received: December 3, 2010 Published: February 23, 2011 1607

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Figure 2. Some examples of known cis-dichloro ruthenium II alkylidene complexes.

Scheme 1. Preparation of Olefin Metathesis Catalysts 4-6a

Conditions: (a) propane-2-thiol, K2CO3, DMF, 60 °C, 24 h; (b) CH3PPh3I, KOtBu, ether, 2 h; (c) phenyl/2,6-dimethylphenylboronic acid, Pd(PPh3)4, K2CO3, H2O, toluene, 110 °C, 24 h; (d) 1b, CuCl, CH2Cl2, 45 °C, 24 h; (e) 2,4,6-triisopropylphenylboronic acid, Pd(OAc)2, S-Phos, K3PO4, H2O, toluene, 110 °C, 24 h. a

Both electronic and steric factors may influence the complexes’ geometry. Our own studies have shown, by both DFT calculations and experimental results, that simple Hoveyda-type sulfur-, selenium-, and phosphorus-chelated ruthenium benzylidenes prefer the cis geometry in solution, while oxygen- and sp3 nitrogen-chelated complexes are more stable in the trans-dichloro arrangement.18 We now report on the syntheses, structures, and properties of new sulfur-chelated complexes 4-6 (Scheme 1), designed to exert an increasing steric interaction on the cis-dichloro configuration in order to reveal new facets of their isomerization behavior and catalytic activity.

’ RESULTS AND DISCUSSION Complex Syntheses and Characterizations. Styrene isopropyl o-sulfides with gradually larger aromatic substitutents in the

para position to the sulfur atom were synthesized as ligand precursors. 5-Bromo-2-fluorobenzaldehyde was used as starting material, and styrenes 9 and 10 were produced in around 70% overall yield by aromatic nucleophilic substitutions followed by Wittig olefinations and Suzuki-Miyaura coupling reactions using Pd(PPh3)4 to couple the phenyl and m-xylyl moieties. This pathway afforded low yields for 13; thus an alternative strategy was applied: Pd(OAc)2 and 2-dicyclohexylphosphino20 ,60 -dimethoxybiphenyl (S-Phos) were used to produce a catalytic system that allowed coupling of the extremely hindered substrates.19 Under these conditions 5-bromo-2-fluorobenzaldehyde and 2,4,6-triisopropylphenylboronic acid provided the desired product 11 in 80% yield. Subsequent aromatic nucleophilic substitution and Wittig olefination readily produced compound 13. Finally, reactions of 1b with 9, 10, and 13 afforded new complexes 4-6, respectively (Scheme 1). 1608

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Figure 3. X-ray structures of complexes 4-6 with thermal ellipsoids drawn at the 50% probability level. Solvent molecules and hydrogen atoms are removed for clarity.

Complexes 4-6 were characterized by FAB-MS and NMR spectroscopy. The NMR analyses unequivocally showed that all complexes retained the cis-dichloro arrangement in solution, notwithstanding the highly sterically hindered substituents. In addition, good-quality single crystals could be obtained for these compounds, supporting the same cis geometry in the solid state (Figure 3). The solid-state structures revealed that steric repulsions are alleviated by rotation of the mesitylene group away from the bulky benzylidene ligand. This could be visibly noted by comparing the overlapping of aromatic rings in the crystal structures for unhindered complex 3 and hindered complex 5 (Figure 4). This relaxation mechanism could also be clearly observed at room temperature in solution by 1H NMR analysis. The anisotropic effect observed for the corresponding aromatic mesitylene proton was increasingly hampered in complexes 3-6, the proton’s chemical shift being altered from 5.96 ppm observed in 3 all the way to 6.41 ppm in complex 6 (Figure 5). trans-Dichloro complexes 3-6 are usually observed in low quantities as kinetic products in the preparation of cis-dichloro 3-6 and may be separated by column chromatography.12 To allow the study of the activity and isomerization rates for the trans structures, the yield of the trans-dichloro isomer was raised by lowering the temperature and shortening the reaction time of the styrene derivative with 1b, thus preventing its full conversion to the more stable cis isomer (Scheme 2). In this fashion the trans isomers could be obtained in ∼35% yield. However, during this study we discovered that by changing the solvent from methylene chloride to benzene the yield of the trans isomer could be raised to 85% (vide infra). Trans-Cis Isomerization. Having isolated the pure trans isomers, the rate of the trans-cis isomerization for complexes 3-6 was measured by 1H NMR at 20 °C in dichloromethane. Given that steric hindrance is more prominent in the cis isomer, it was somewhat surprising to observe that the least hindered complexes, 3trans and 4trans, were the slowest to isomerize, with half-lives of approximately 30 h, while isomerization t1/2 for the most sterically hindered, 6trans, was just 7 h (Figure 6).

According to previous studies, it was expected that solvent polarity may influence the isomerization process.8,20,21 Therefore, the trans to cis isomerization rates with complex 4 in additional solvents such as DMSO-d6, acetone-d6, THF-d8, and benzene-d6 were measured (Figure 7). In line with its low polarity, the rate of isomerization in benzene-d6 was very slow, with just 45% conversion to the cis isomer after 15 days. Having noticed the very slow rate of isomerization in benzene, all subsequent syntheses of trans isomers were carried out in this solvent with a significant increase to 85% yield (see the Experimental Section). The isomerization rate in THF-d8 was faster than in benzene, reaching 50% cis isomer after 12 days, while in acetone-d6 the measured t1/2 was less than 5 days. Finally, in DMSO-d6 almost immediate isomerization was observed (no trans isomer could be detected in DMSO-d6 solution). Surprisingly, the process in CD2Cl2 was much faster than in the more polar acetone solvent, taking just around 30 h to reach 50% isomerization. At the moment we do not have an explanation for this abnormal behavior; one possibility may be that dichloromethane interactions with the metal influence the isomerization.22 In any case, our results hinted that other parameters besides solvent polarity were influencing the rates of isomerization. This finding led us to investigate whether a coordinating solvent (or other molecules) could have an effect on the isomerization process, either by accelerating a pseudorotation process or by facilitating dissociation of the sulfur-ruthenium bond.23 Thus, 10 equivalents of DMSO (relative to the complex) was added to a solution of complex 4trans in benzene-d6. Indeed, an increase in the rate of the isomerization was observed, resulting in 50% conversion of the trans-dichloro isomer to the cis-dichloro isomer after just 2 h, a more than 200-fold acceleration effect (Figure 8). After 48 h full conversion to the stable isomer could be observed. This effect is less pronounced when the procedure is repeated in CD2Cl2, in accordance with our previous explanation. To determine the generality of this observation, the rate of isomerization of 5trans in CD2Cl2 was systematically studied by adding molecules that can interact with the ruthenium atom (Table 1). 1609


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Figure 6. Trans-cis isomerization (monitored by 1H NMR) in CD2Cl2 at 20 °C.

Scheme 2. trans-Dichloro-cis-Dichloro Complex Equilibrium Figure 4. Overlap of aromatic rings in complexes 312 and 5.

Figure 5. Diminishing anisotropic effect (1H NMR, CD2Cl2) observed for the aromatic mesitylene hydrogen atom in the series of complexes 3 (a), 4 (b), 5 (c), and 6 (d).

For example, the addition of tricyclohexylphosphine (PCy3), water, and DMSO (Table 1, entries 4-9) had a relatively small, but nonetheless important, acceleration effect on the isomerization rate. On the other hand, the addition of just 10 equivalents of pyridine (Table 1, entry 3) or 3-chloropyridine (Table 1, entry 2) resulted in the immediate formation of new trans-dichloro sixcoordinated 18-electron complexes (readily observed by 1H

NMR, Figure 9), which isomerized 4 to 8 times faster than the parent complex (Figure 10). Interestingly, the cis products did not bind the pyridines and returned to the 16-electron configuration. Finally, we surmised that by adding either chloride ions or silver ions the rates should be significantly altered if the anionic chloride ligands dissociate during the isomerization. Naturally, it was expected that chloride salts should strongly decelerate the process and silver salts accelerate it. However, the addition of either tetrabutylammonium chloride or silver tetrafluoroborate had only a small retarding effect on the isomerization rate (Table 1, entries 10 and 11), strongly disfavoring the participation of cationic species in the isomerization process.24 The differences in thermodynamic stability between the cis and trans isomers in nonpolar solvents were obtained by heating the cis isomers in deuterated toluene at different temperatures and determining the equilibrium concentrations by NMR. As expected, the complexes possessing a bulkier ligand displayed a higher percentage of trans isomer in the equilibrium state when compared to the less bulky complexes. The measured Gibbs free energy differences for 5 and 6 (Table 2) were 2.84 and 1.66 kcal/mol, respectively, in favor of the cis isomers at room temperature (see the Supporting Information for full details). Accordingly, approximately 5 and 25 mol % respectively of active trans isomers of 5 and 6 may be found in toluene solution at 80 °C. Noteworthy, only complexes 5 and 6 produced measurable quantities of the trans isomer in equilibria, meaning that the difference in energy for complexes 3 and 4 must be over 3 kcal/mol. Olefin Metathesis Activity. Having obtained information about the isomerization rates, cis-trans equilibria, and the factors that influence them, their relationship with metathesis activity was studied. The catalytic activity of new precatalysts 4-6 was examined by olefin metathesis reactions with various substrates. 1610


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As previously observed,16 the trans isomers were moderately active at room temperature toward RCM reactions, while the cisdichloro isomers showed metathesis activity only at elevated temperatures (Table 3). Following the same trends as for the RCM reactions, at room temperature the cis-dichloro initiators did not exhibit any ROMP catalytic activity; however, when heated to 80 °C with reactive monomers such as cyclooctene and cyclooctadiene, polymers

could be produced. As expected,16 the trans-dichloro isomers were found to be reactive ROMP initiators also at room temperature (Table 4). Clearly, the differences in steric volumes of the ligands in complexes 4-6 had a major impact on their olefin metathesis reactivity. This effect, however, was strongly dependent on the isomer used as the starting catalyst.25 Thus, the progress of metathesis reactions catalyzed by the trans-dichloro complexes at room temperature was found to be inversely proportional to the bulk of the benzylidene ligand. Accordingly, less bulky complex 4trans produced a significantly higher conversion percentage for all metathesis reactions than complex 6trans under the same reaction conditions. This trend may be attributed to two factors; the first is a less intrinsically active trans isomer due to a greater steric hindrance to the olefin coordination site. After dissociation of the chelating sulfur atom, the bulky aromatic group can obstruct the access of the olefin to the metal. The second factor may be a steadily decreasing amount of active trans isomer as the reaction progresses; that is, complexes that isomerize faster to the inactive isomer are overall less active. On the other hand, the precatalyst reactivity is reversed for the cis-dichloro isomers (at high temperatures); for example, complex 6cis was significantly more active than 4cis. Judging from the results obtained, the reactivity observed for dormant cis isomers seems to be dependent on the amount of trans isomer present in the equilibrium, strongly suggesting that the reaction proceeds principally through the trans isomer. However, the different behavior observed in the ROMP reactions could advocate a more complicated picture. In the cis isomer series a higher E/Z ratio is observed as the ligand bulk is increased; however, this is not observed in the trans isomer series. Contrary to what is usually observed when complex 1b or 1c is used to catalyze ROMP,26 the E/Z ratio in ROMP polymers catalyzed by sulfur-chelated catalysts remains invariant throughout the reaction (see Supporting Information); this is probably due to the fact that no active catalytic species remain in solution to promote secondary metathesis processes.13 We argue that if metathesis occurs always through the accepted trans isomer mechanism, finding different trends for E/Z ratios in reactions carried out by means of the same catalytic species is unlikely.27 Further studies with complexes where the cis-trans isomerism in the complex is arrested may assist in elucidating this issue.

Figure 7. Isomerization (according to 1H NMR) of 4trans to 4cis at 20 °C in various solvents (dielectric constant).

Figure 8. Accelerated isomerization (according to 1H NMR) of 4trans to 4cis in benzene-d6 with addition of 10 equivalents of DMSO-d6.

Table 1. Rate Constants (k) for Trans-Cis Isomerization of 5 entrya

additive

isomerization t1/2 (h)

k (10-2/h)

acceleration (k/k0)

1 2

none 10 equiv 3-chloropyridine

8.8 2.1

7.85 32.98

1.00 4.20

3

10 equiv pyridine

1.1

66.04

8.41

4

10 equiv PCy3

5.8

11.99

1.53

5

1 equiv DMSO-d6

8.1

8.53

1.09

6

5 equiv DMSO-d6

6.7

10.40

1.32

7

10 equiv DMSO-d6

5.9

11.68

1.49

8

20 equiv DMSO-d6

5.7

12.24

1.56

9 10

D2Ob 10 equiv TBACl

5.4 10.2

12.78 6.79

1.63 0.86

11

10 equiv AgBF4

10.0

6.93

0.88

Conditions: 5trans (10-3 M) in 0.5 mL of CD2Cl2 at 298 K. k determined by 1H NMR integration of the benzylidene signal (see the Supporting Information). b A drop of D2O was added to 0.5 mL of the CD2Cl2 solution of 5trans. a

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Table 3. RCM Activity of cis- and trans-Dichloro Isomers of Complexes 4-6

Figure 9. Hexacoordinated trans-dichloro ruthenium complex (trans 18e) observed by addition of 3-chloropyridine to 5trans.

Figure 10. 1H NMR spectra of 5: (a) 5trans in CD2Cl2; (b) 5trans in CD2Cl2 3 min after addition of 10 equiv of 3-chloropyridine (hexacoordinated complex); (c) 5trans in CD2Cl2 3 h after addition of 10 equiv of 3-chloropyridine; (d) 5cis in CD2Cl2.

Table 2. Gibbs Free Energy Differences for Complexes 5 and 6 entry

complex

1

5

2 3 4

6

temp (K)

ΔG (kcal/mol)

20

2.84

80

2.13

20

1.66

80

0.98

’ CONCLUSIONS We have shown that sulfur-chelated ruthenium benzylidenes are more stable in their cis-dichloro conformation, even when severe steric strain is introduced, although, as expected, as the steric hindrance increases the difference in energy between the isomers is reduced. Surprisingly, trans to cis isomerism is faster with the bulkier ligands and was also found to be significantly solvent dependent, being faster in more polar solvents or in a halogen-containing solvent. Moreover, the addition of small π-acidic molecules that have the ability to ligate the empty coordination site of the 16e- ruthenium precatalysts significantly lowered the barrier for isomerization in apolar solvents.28 These results shed new light on the possible isomerization mechanism for chelated ruthenium benzylidenes. The previous theoretical reports on the isomerization of this type of complexes did not take into account a possible additional coordination on the

a

Conditions: 1 mol % cat.; substrate 0.1 M, in toluene. Yields were determined by GC-MS after 24 h. Reactions with trans isomers were conducted at 25 °C; reactions with cis isomers were conducted at 80 °C.

ruthenium as part of the mechanism.15,21 Our results also suggest that metathesis reactions in ruthenium cis-dichloro-chelated benzylidenes occur mainly through the trans-dichloro isomer present in the equilibrium, although the reason for the difference in E/Z selectivity observed in ROMP remains an open question.29 It would seem that the use of strongly chelating benzylidene ligands influences the selectivity of the catalysts by remaining in the ligand sphere even after the first cycle of metathesis.30 A better understanding of the trans-cis isomerism and the mechanisms that govern reactivity in strongly chelated ruthenium olefin metathesis catalysts may lead to new applications and to the development of selective reactions not achievable with the classic ruthenium carbenes regularly used in academia and industry.

’ EXPERIMENTAL SECTION General Procedures. All reagents were of reagent-grade quality, purchased commercially from Sigma, Aldrich, or Fluka, and used without further purification. All solvents were dried and distilled prior to use. Purification by column chromatography was performed on Davisil chromatographic silica media (40-6 μm). TLC analyses were performed using Merck precoated silica gel (0.2 mm) aluminum [backed] sheets. NMR spectra were recorded on Bruker DPX200 or DMX500 instruments; chemical shifts, given in ppm, are relative to Me4Si as the internal standard, or using the residual solvent peak. Gas chromatography and MS data were obtained using an Agilent 6850 GC equipped with an Agilent 5973 MSD working under standard conditions and an Agilent 1612


Organometallics Table 4. ROMP Activity of cis-Dichloro and trans-Dichloro Isomers of Complexes 4-6

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 toluene; [monomer]/[cat.] = 300. Reactions with trans isomers were conducted at 25 °C; reactions with cis isomers were conducted at 80 °C. Mw and PDI values were determined by triple-detector SEC; E/Z values were determined by 1H NMR. HP5-MS column. FAB-MS of organometallic compounds were obtained using a Bruker Daltonics Ion Trap MS Esquire 3000 Plus using 3-nitrobenzyl alcohol as the matrix. GPC analyses were obtained using an Agilent 1200 HPLC equipped with two Agilent PLgel 5 μm Mixed-C columns and a Phenomenex Phenogel 5 μm 103A column. A Wyatt’s Mini-Dawn Tristar laser light-scattering instrument was used for detection. Wyatt’s Viscostar viscometer and Optilab Rex refractometer were also used. Astra 5.3.12.3 was used for calibration and calculation of polymer polydispersities and molecular weights. 5-Bromo-2-(isopropylthio)benzaldehyde (7). The corresponding 5-bromo-2-fluorobenzaldehyde (8 mmol), potassium carbonate (8.5 mmol), and 2-propanethiol (8.5 mmol) were dissolved in 10.0 mL of DMF in a 50 mL round-bottomed flask under dry nitrogen, topped with a reflux condenser. The reaction mixture was heated to 60 °C for 24 h. After cooling, the mixture was added to 50 mL of saturated potassium carbonate solution, and the mixture was extracted with 3 50 mL portions of ether. The extracts were dried with magnesium sulfate and evaporated. The obtained yellow-brown oil residue was further purified by chromatography on silica gel using hexane as eluent to afford 7 as a yellow oil (1.88 g, 91%). 1H NMR (200 MHz, CDCl3, ppm): δ 1.30 (d, J = 6.5 Hz, 6H), 3.36 (sept, J = 6.5 Hz, 1H), 7.37 (d, J = 8.4 Hz, 1H), 7.61 (dd, J = 8.4 Hz, J = 2.2 Hz, 1H), 7.96 (d, J = 2.2 Hz, 1H) 10.45 (s, 1H). 13 C NMR (50 MHz, CDCl3, ppm): δ 22.8, 39.1, 121.1, 132.5, 134.2, 136.5, 137.0, 140.2, 190.5. C10H11BrOS MS (EI): m/z found 258.0 (Mþ), 214.9 (Mþ - iPr), calcd 258.0. (4-Bromo-2-vinylphenyl)(isopropyl)sulfide (8). Methyl triphenylphosphonium iodide (0.78 g, 1.94 mmol) was dissolved in 15 mL of ether in a 50 mL round-bottomed flask at 0 °C under dry nitrogen. To the mixture was added in one portion potassium tert-butoxide (0.24 g, 2.08 mmol), and it was stirred for 10 min at room temperature. 7 (1.4 mmol) was added in one portion at 0 °C, and the reaction mixture was stirred for an additional 2 h at room temperature. The mixture was added to 100 mL of saturated sodium bicarbonate solution and then was extracted with 3 50 mL portions of ether. The extracts were dried with magnesium sulfate and evaporated. The crude product was purified by flash chromatography using 95:5 hexane and ether as an eluent to afford

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a colorless oil (0.27 g, 75%). 1H NMR (500 MHz, CDCl3, ppm): δ 1.27 (d, J = 6.5 Hz, 6H), 3.27 (sept, J = 6.5 Hz, 1H), 5.38 (dd, J = 11.0 Hz, J = 0.9 Hz, 1H), 5.70 (dd, J = 17.4 Hz, J = 0.9 Hz, 1H), 7.27 (dd, J = 17.4 Hz, J = 11 Hz, 1H), 7.33 (m, 1H), 7.68 (m, 1H). 13C NMR (125 MHz, CDCl3, ppm): δ 22.9, 38.8, 116.6, 121.6, 128.8, 130.7, 132.9, 134.1, 135.1, 141.9. C11H13BrS MS (EI): m/z found 256.0 (Mþ), 212.9 (Mþ iPr), calcd 256.0. Isopropyl(vinylbiphenyl-4-yl)sulfide (9). A round-bottomed flask under dry nitrogen was charged with 8 (2.5 mmol, 1 equiv), phenylboronic acid (5.0 mmol, 2 equiv), and K2CO3 (5.0 mmol, 2 equiv) in 1.6 mL of water. The flask was then evacuated and backfilled with nitrogen five times. DMF (2 mL) and Pd(PPh3)4 (70 mg, 5 mol %) were added to the flask, and the resulting mixture was evacuated and backfilled with nitrogen again five times. The reaction mixture was heated at 80 °C for 24 h, cooled to room temperature, diluted with diethyl ether, filtered through a thin pad of silica gel, and concentrated under reduced pressure. The crude product was purified by flash chromatography using 95:5 hexane and ether as eluent to afford 9 as a colorless oil (0.56 g, 88%). 1H NMR (500 MHz, CDCl3, ppm): δ 1.31 (d, J = 6.6 Hz, 6H), 3.35 (sept, J = 6.6 Hz, 1H), 5.38 (dd, J = 11.0 Hz, J = 1.2 Hz, 1H), 5.77 (dd, J = 17.6 Hz, J = 1.2 Hz, 1H), 7.34-7.65 (m, 8H), 7.78 (d, J = 2 Hz, 1H). 13C NMR (125 MHz, CDCl3, ppm): δ 23.1, 38.7, 115.6, 124.6, 126.9, 127.0, 127.4, 128.6, 128.7, 132.9, 133.9, 135.2, 140.3, 140.5. C17H18S MS (EI): m/z found 254.1 (Mþ), 211.1 (Mþ-iPr), calcd 254.1. (20 ,60 -Dimethyl-3-vinylbiphenyl-4-yl)(isopropyl)sulfide (10). 10 was prepared by the same procedure as 9, using 2,6-dimethylphenylboronic acid (0.6 g, 86%). 1H NMR (500 MHz, CDCl3, ppm): δ 1.34 (d, J = 6.6 Hz, 6H), 2.1 (s, 6H), 3.38 (sept, J = 6.6 Hz, 1H), 5.34 (dd, J = 11.0 Hz, J = 1.2 Hz, 1H), 5.68 (dd, J = 17.6 Hz, J = 1.2 Hz, 1H), 6.95-7.67 (m, 7H). 13C NMR (125 MHz, CDCl3, ppm): δ 20.8, 23.1, 38.6, 115.3, 126.4, 127.1, 127.3, 128.6, 132.1, 133.8, 135.2, 136.0, 140.0, 140.3, 141.1. C19H22S MS (EI): m/z found 282.2 (Mþ), 239.1 (Mþ-iPr), calcd 282.4. 4-Fluoro-20 ,40 ,60 -triisopropylbiphenyl-3-carbaldehyde(11).A round-bottomed flask under dry nitrogen was charged with 2-fluoro-5-bromobenzaldehyde (0.25 mmol, 1 equiv), 2-dicyclohexylphosphino-20 ,60 -dimethoxybiphenyl (10 mg, 10 mol %), 2,4,6-triisopropylphenylboronic acid (0.37 mmol, 1.5 equiv), and K3PO4 (0.74 mmol, 3 equiv). The flask was then evacuated and backfilled with nitrogen five times. Toluene (2 mL) and Pd(OAc)2 (3 mg, 5 mol %) were added to the flask, and the resulting mixture was evacuated and backfilled with nitrogen again five times. The reaction mixture was heated at 110 °C for 24 h, then was allowed to cool to room temperature, diluted with diethyl ether, filtered through a thin pad of silica gel, and concentrated under reduced pressure. The crude product was purified by flash chromatography on silica gel using 98:2 hexane and ether as eluent to afford 11 as a white solid (0.7 g, 77%). 1H NMR (500 MHz, CDCl3, ppm): δ 1.07 (d, J = 7.0 Hz, 12H), 1.30 (d, J = 7.0 Hz, 6H), 2.49 (sept, J = 7.0 Hz, 2H), 2.94 (sept, J = 7.0 Hz, 1H), 7.06 (s, 1H), 7.22-7.27 (m, 1H), 7.39-7.47 (m, 1H), 7.69 (dd, J = 6.6 Hz, J = 2.2 Hz, 1H), 10.42 (s, 1H). 13C NMR (125 MHz, CDCl3, ppm): δ 24.0, 30.3, 34.3, 115.9, 116.4, 120.7, 123.7, 129.6, 134.5, 137.6, 146.5, 148.7, 162.5, 164.5, 187.3. C22H27FO MS (EI): m/z found 326.2 (Mþ), 283.2 (Mþ - iPr), calcd 326.2. 20 ,40 ,60 -Triisopropyl-4-(isopropylthio)biphenyl-3-carbaldehyde (12). 12 was prepared by the same procedure as 7, starting from 20 ,40 ,60 triisopropyl-4-fluorobiphenyl-3-carbaldehyde (11) (yield 2.65 g, 87%). 1 H NMR (200 MHz, CDCl3, ppm): δ 1.08 (d, J = 6.8 Hz, 12H), 1.31 (d, J = 6.8 Hz, 6H), 1.41 (d, J = 6.8 Hz, 6H), 2.54 (sept, J = 6.8 Hz, 2H), 2.95 (sept, J = 6.8 Hz, 1H), 3.50 (sept, J = 6.8 Hz, 1H), 7.07 (s, 2H), 7.37 (dd, J = 7.8 Hz, J = 2.1 Hz, 1H), 7.54 (d, J = 7.8 Hz, 1H), 7.71 (d, J = 2.1 Hz, 1H), 10.52 (s, 1H). 13C NMR (50 MHz, CDCl3, ppm): δ 23.0, 24.0, 24.1, 30.3, 34.2, 38.2, 120.6, 131.2, 131.9, 135.0, 135.3, 138.6, 139.3, 146.4, 148.5, 192.0. C25H34OS MS (EI): m/z found 382.3 (Mþ), 337.3 (Mþ - iPr), calcd 382.2. 1613


Organometallics Isopropyl(20 ,40 ,60 -triisopropyl-3-vinylbiphenyl-4-yl)sulfide (13). 13 was prepared by the same procedure as 8, starting from 12 (yield 0.43 g, 81%). 1H NMR (200 MHz, CDCl3, ppm): δ 1.11 (d, J = 7.0 Hz, 12H), 1.33 (d, J = 7.0 Hz, 6H), 1.34 (d, J = 6.8 Hz, 6H), 2.65 (sept, J = 7.0 Hz, 2H), 2.96 (sept, J = 7.0 Hz, 1H), 3.38 (sept, J = 6.8 Hz, 1H), 5.31 (dd, J = 11.0 Hz, J = 1.2 Hz, 1H), 5.62 (dd, J = 17.4 Hz, J = 1.2 Hz, 1H), 7.047.08 (m, 3H), 7.33-7.49 (m, 3H). 13C NMR (50 MHz, CDCl3, ppm): δ 23.2, 24.0, 24.2, 30.2, 34.2, 38.5, 115.3, 120.5, 127.2, 129.4, 131.9, 132.8, 135.2, 136.3, 139.3, 139.9, 146.4, 148.0. C26H36S MS (EI): m/z found 380.3 (Mþ), 337.3 (Mþ - iPr), calcd 380.2. General Procedure for cis-Dichloro Catalysts (4-6). Styrene (9, 10, 13) (0.14 mmol), cuprous chloride (0.17 mmol), and secondgeneration Grubbs catalyst 1b (0.14 mmol) were dissolved in 6 mL of dichloromethane in a 10 mL round-bottomed flask under dry nitrogen topped with a reflux condenser. The reaction mixture was refluxed for 24 h. The resulting mixture was concentrated to 2-3 mL of DCM. The crude product was purified by chromatography on silica gel using 7:3 nhexane/acetone as eluent. 4cis: dark green solid (55.0 mg, 54%). Crystals suitable for X-ray analysis were obtained by laying hexanes over a solution of 4cis in dichloromethane and hexanes for few days at -18 °C. 1H NMR (500 MHz, CD2Cl2, ppm): δ 0.84 (d, J = 7.0 Hz, 3H), 1.43 (d, J = 7.0 Hz, 3H), 1.55 (s, 1H), 1.69 (s, 1H), 2.37 (s, 1H), 2.46 (s, 1H), 2.58 (s, 1H), 2.65 (s, 1H), 3.6 (sept, J = 7.0 Hz, 1H), 3.75-3.88 (m, 2H), 3.97-4.04 (m, 1H), 4.08-4.14 (m, 1H), 6.02 (s, 1H), 6.83 (s, 1H), 7.03-7.04 (m, 2H), 7.13 (s, 1H), 7.38-7.41 (m, 1H), 7.49 (t, J = 8.0 Hz, 2H), 7.54 (d, J = 8.0 Hz, 1H), 7.59-7.61 (m, 2H), 7.76 (dd, J = 2.0 Hz, J = 8.0 Hz, 1H), 17.24 (s, 1H). 13C NMR (125 MHz, CD2Cl2, ppm): δ 17.7, 18.9, 19.8, 20.4, 20.6, 21.3, 24.3, 26.3, 39.3, 51.4, 51.8, 122.2, 126.7, 127.4, 128.5, 129.2, 129.4, 129.7, 130.0, 130.8, 131.2, 131.7, 133.9, 135.5, 136.0, 137.4, 137.9, 138.8, 139.3, 139.4, 140.3, 140.6, 143.3, 157.3, 213.5, 285.6. FABMS: calcd for C37H42Cl2NRuS [M]þ 718.1; found 718.1. C46H60Cl2N2RuS 3 CH2Cl2, M = 929.93, monoclinic, space group P21/c, a = 13.2232(2) Å, b = 7.9543(1) Å, c = 32.1113(6) Å, β = 101.5323(7)°, V = 3309.32(9) Å3, Z = 4, T = 110(2) K, Fcalcd = 1.443 g cm-3, μ(Mo KR) = 0.728 mm-1, 16 584 reflections measured (2θmax = 51.0°), 7741 unique (Rint = 0.038), final R = 0.0495 (wR = 0.1239) for 5551 reflections with I > 2σ(I) and R = 0.079 (wR = 0.141) for all data, Δ|Fmax| = 2.09 e/Å3. 5cis: dark green solid (51.0 mg, 49%). Crystals suitable for X-ray analysis were obtained by laying hexanes over a solution of 5cis in dichloromethane and hexanes for few days at -18 °C. 1H NMR (500 MHz, CD2Cl2, ppm): δ 0.88 (d, J = 6.5 Hz, 3H), 1.41 (s, 3H), 1.87 (s, 3H), 1.99 (s, 3H), 2.36 (s, 3H), 2.38 (s, 3H), 2.54 (s, 3H), 2.57 (s, 3H), 2.75 (s, 3H), 3.48 (sept, J = 6.5 Hz, 1H), 3.78-3.85 (m, 1H), 3.91-4.04 (m, 2H), 4.13-4.19 (m, 1H), 6.32 (s, 1H), 6.74 (s, 1H), 6.90 (s, 1H), 7.06-7.18 (m, 5H), 7.33-7.35 (m, 1H), 7.56 (d, J = 8.0 Hz, 1H), 16.94 (s, 1H). 13C NMR (125 MHz, CD2Cl2, ppm): δ 17.4, 19.3, 19.7, 20.2, 20.8, 21.0, 21.2, 21.3, 21.5, 24.5, 39.3, 51.4, 51.5, 125.5, 127.9, 128.1, 128.9, 129.9, 130.1, 130.4, 130.9, 132.3, 134.8, 135.6, 135.8, 135.9, 136.8, 137.0, 138.4, 139.9, 140.5, 143.2, 156.8, 213.5, 285.4. FAB-MS: calcd for C39H46Cl2NRuS [M]þ 746.1; found 746.1. C39H46Cl2N2RuS, M = 746.81, monoclinic, space group P21/n, a = 15.7063(7) Å, b = 15.3923(9) Å, c = 19.8339(12) Å, β = 112.494(3)°, V = 4430.2(4) Å3, Z = 4, T = 110(2) K, Fcalcd = 1.120 g cm-3, μ(Mo KR) = 0.546 mm-1, 25 141 reflections measured (2θmax = 51.0°), 8043 unique (Rint = 0.093), final R = 0.0633 (wR = 0.1561) for 4191 reflections with I > 2σ(I) and R = 0.125 (wR = 0.181) for all data, Δ|Fmax| = 0.67 e/Å3. The asymmetric unit contains two additional molecules of a severely disordered dichloromethane solvent, which could not be modeled reliably by discrete atoms and were excluded from the final crystallographic refinement. 6cis: dark green solid (75.0 mg, 63%). Crystals suitable for X-ray analysis were obtained by laying hexanes over a solution of 6cis in dichloromethane and hexanes for few days at -18 °C. 1H NMR (500 MHz, CD2Cl2, ppm): δ 0.83-0.86 (m, 6H), 1.00 (d, J = 7.5 Hz, 3H),

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1.17 (s, 3H), 1.22 (d, J = 7.5 Hz, 3H), 1.3 (d, J = 6.5 Hz, 6H), 1.37 (d, J = 7.5 Hz, 3H), 1.42 (d, J = 6.5 Hz, 3H), 2.15 (s, 3H), 2.38 (s, 3H), 2.51 (s, 3H), 2.61 (s, 3H), 2.84 (s, 3H), 2.91-2.98 (m, 2H), 3.26 (sept, J = 7.5 Hz, 1H), 3.78-3.86 (m, 1H), 3.93-4.03 (m, 2H), 4.14-4.21 (m, 1H), 6.42 (s, 1H), 6.93 (s, 1H), 6.98 (s, 1H), 7.05-7.06 (m, 2H), 7.14 (m, 2H), 7.34 (dd, J = 2.0 Hz, J = 7.0 Hz, 1H), 7.53 (d, J = 7.0 Hz, 1H), 16.18 (s, 1H). 13C NMR (125 MHz, CD2Cl2, ppm): δ 16.9, 19.3, 19.4, 19.7, 20.7, 20.8, 20.9, 23.5, 23.7, 23.8, 23.9, 24.0, 25.2, 30.0, 31.0, 31.5, 34.4, 38.9, 51.2, 51.5, 120.6, 120.9, 125.3, 128.2, 129.7, 129.8, 129.9, 130.3, 130.7, 132.7, 134.7, 135.1, 135.2, 135.4, 136.1, 138.0, 138.1, 138.9, 139.9, 142.9, 146.0, 147.2, 148.7, 156.5, 212.0, 285.2. FAB-MS: calcd for C46H60Cl2NRuS [M]þ 844.29; found 844.3. C46H60Cl2N2RuS 3 2CH2Cl2, M = 929.93, orthorhombic, space group Pbca, a = 16.3458(2) Å, b = 16.3996(2) Å, c = 34.2389(5) Å, β = 90.0(0)°, V = 9178.2(2) Å3, Z = 8, T = 110(2) K, Fcalcd = 1.346 g cm-3, μ(Mo KR) = 0.654 mm-1, 24 631 reflections measured (2θmax = 51.0°), 10 455 unique (Rint = 0.052), final R = 0.0506 (wR = 0.1224) for 7200 reflections with I > 2σ(I) and R = 0.086 (wR = 0.140) for all data, Δ|Fmax| = 1.39 e/Å3. The asymmetric unit contains three additional molecules of a dichloromethane solvent, two ordered and one that could not be modeled reliably by discrete atoms and was excluded from the final crystallographic refinement. General Procedure for trans-Dichloro Catalysts (4-6). Trans isomers were synthesized according to the same procedure as the cis isomers by shortening reaction time (2 h) and lowering temperature (room temperature). 4trans: green solid (30 mg, 30%). 1H NMR (500 MHz, CD2Cl2): δ 0.95 (d, J = 7.0 Hz, 6H), 2.39 (s, 6H), 2.47 (s, 12H), 3.08 (sept, J = 7.0 Hz, 1H), 4.14 (s, 4H), 6.98 (s, 1H), 7.06 (s, 4H), 7.39 (t, J = 7.5 Hz, 1H), 7.47-7.50 (m, 3H), 7.53-7.55 (m, 2H), 7.77-7.79 (m, 1H), 17.24 (s, 1H). 5trans: green solid (40 mg, 39%). 1H NMR (500 MHz, CD2Cl2, ppm): δ 1.00 (d, J = 7.0 Hz, 6H), 1.26 (s, 12H), 2.25 (s, 6H), 2.46 (s, 12H), 3.10 (sept, J = 7.0 Hz, 1H), 4.12 (s, 4H), 6.54 (s, 1H), 6.99 (4H), 7.11-7.18 (m, 3H), 5.35 (dd, J = 2.0 Hz, J = 7.5 Hz, 1H), 7.53 (d, J = 7.5 Hz, 1H), 17.20 (s, 1H). 6trans: green solid (43 mg, 37%). 1H NMR (500 MHz, CD2Cl2, ppm): δ 1.00-1.05 (m, 12H), 1.29 (d, J = 7.0 Hz, 12H), 2.22 (s, 6H), 2.45-2.58 (m, 14H), 2.94 (sept, J = 7.0 Hz, 1H), 3.14 (sept, J = 7.0 Hz, 1H), 4.11 (s, 4H), 6.55 (d, J = 2.0 Hz, 1H), 6.97 (s, 4H), 7.07 (s, 2H), 7.33 (dd, J = 2.0 Hz, J = 8.0 Hz, 1H), 7.50 (d, (d, J = 8.0 Hz, 1H), 17.22 (s, 1H).

Alternative Procedure for trans-Dichloro Complexes.

Trans isomers can also be synthesized according to the same procedure as the cis isomers by replacing the reaction solvent with benzene and lowering the temperature (room temperature). 3trans. 1-(Isopropylsulfanyl)-2-vinylbenzene (0.065 mmol), cuprous chloride (0.065 mmol), and second-generation Grubbs catalyst 1b (0.06 mmol) were dissolved in 6 mL of benzene in a 10 mL round-bottomed flask under dry nitrogen. The reaction mixture was stirred at room temperature for 24 h. The resulting mixture was purified by chromatography on silica gel using 7:2 n-hexane/acetone as eluent. 3trans: green solid (31.5 mg, 85%). General Procedure for Isomerization Rate Study. A 0.5 μmol amount of complex was weighed in a vial. In a glovebox, deuterated solvent was added (0.5 mL) followed by the additive (if necessary). The solution was transferred to a screw-cap NMR tube and closed tight. Reaction progress was monitored by 1H NMR. General Procedure for RCM Reactions. A round-bottom flask was charged with precatalyst (2 μmol), olefin (0.2 mmol, 0.1 M), and toluene (2 mL). The reaction mixture was stirred at room temperature (for trans catalysts) or 80 °C (cis catalysts). Reaction progress was monitored by GC-MS. General Procedure for ROMP Reactions. A round-bottom flask was charged with initiator (3.33 μmol, 0.3%), olefin (1 mmol), mesitylene (internal standard, 200 mg), and toluene (2 mL). The reaction mixture was stirred at room temperature (for trans catalysts) 1614


Organometallics or 80 °C (cis catalysts). Monomer consumption was monitored by GCMS. The polymer was isolated by pouring the reaction mixture in methanol, filtering, and washing the white, gummy product with methanol (until all color is removed), followed by 12 h drying under high vacuum. The resulting polymer was characterized by GPC analysis and the E/Z ratio determined by 1H NMR.

’ ASSOCIATED CONTENT

bS

Supporting Information. NMR and MS details for 4-6, isomerization rate measurements, and E/Z measurements of polymers over time. This material is available free of charge via the Internet at http://pubs.acs.org.

’ AUTHOR INFORMATION Corresponding Author

*E-mail: lemcoff@bgu.ac.il.

’ ACKNOWLEDGMENT We would like to thank Prof. Dr. Bernd F. Straub for stimulating discussions. The Edmond J. Safra Foundation and the Israel Science Foundation are gratefully acknowledged for financial support. ’ REFERENCES (1) (a) F€urstner, A. Angew. Chem., Int. Ed. 2000, 39, 3012–3043. (b) Buchmeiser, M. R. Chem. Rev. 2000, 100, 1565–1604. (c) Vougioukalakis, G. C.; Grubbs, R. H. Chem. Rev. 2010, 110, 1746–1787. (2) (a) Samojlowicz, C.; Bieniek, M.; Grela, K. Chem. Rev. 2009, 109, 3708–3742. (b) Boeda, F.; Clavier, H.; Nolan, S. P. Chem. Commun. 2008, 2726–2740. (c) Diesendruck, C. E.; Tzur, E.; Lemcoff, N. G. Eur. J. Inorg. Chem. 2009, 28, 4185–4203. (d) Monfette, S.; Fogg, D. E. Chem. Rev. 2009, 109, 3783–3816. (e) Leitao, E. M.; van der Eide, E. F.; Romero, P. E.; Piers, W. E.; McDonald, R. J. Am. Chem. Soc. 2009, 132, 2784–2794. (f) Dunbar, M. A.; Balof, S. L.; LaBeaud, L. J.; Yu, B.; Lowe, A. B.; Valente, E. J.; Schanz, H.-J. Chem.—Eur. J. 2009, 15, 12435–12446. (g) Lozano-Vila, A. M.; Monsaert, S.; Bajek, A.; Verpoort, F. Chem. Rev. 2010, 110, 4865–4909. (3) (a) Bielawski, C. W.; Benitez, D.; Grubbs, R. H. Science 2002, 297, 2041. (b) Xia, Y.; Boydston, A. J.; Yao, Y.; Kornfield, J. A.; Gorodetskaya, I. A.; Spiess, H. W.; Grubbs, R. H. J. Am. Chem. Soc. 2009, 131, 2670–2677. (c) Zuercher, W. J.; Hashimoto, M.; Grubbs, R. H. J. Am. Chem. Soc. 1996, 118 (28), 6634–6640. (4) (a) Tzur, E.; Ben-Asuly, A.; Diesendruck, C. E.; Goldberg, I.; Lemcoff, N. G. Angew. Chem., Int. Ed. 2008, 47, 6422–6425. (b) Diesendruck, C. E.; Ben-Asuly, A.; Goldberg, I.; Lemcoff, N. G. Chim. Oggi 2010, 28, 15–18. (5) (a) Nguyen, S. T.; Johnson, L. K.; Grubbs, R. H.; Ziler, J. W. J. Am. Chem. Soc. 1992, 114, 3974–3975. (b) Scholl, M.; Ding, S.; Lee, C. W.; Grubbs, R. H. Org. Lett. 1999, 1, 953–956. (c) Garber, S. B.; Kingsbury, J. S.; Gray, B. L.; Hoveyda, A. H. J. Am. Chem. Soc. 2000, 122, 8168–8179. (6) (a) Hansen, S. M.; Rominger, F.; Metz, M.; Hofmann, P. Chem.— Eur. J. 1995, 5, 557–566. (b) Hansen, S. M.; Volland, M. A. O.; Rominger, F.; Eisentr€ager, F.; Hofmann, P. Angew. Chem., Int. Ed. 1999, 38, 1273–1276. (7) Pruhs, S.; Lehmann, C. W.; F€urstner, A. Organometallics 2004, 23, 280–287. (8) Ung, T.; Hejl, A.; Grubbs, R. H.; Schrodi, Y. Organometallics 2004, 23, 5399–5401. (9) For another example of forcing a cis conformation by chelation (as in ref 6), see: Amoroso, D.; Jabri, A.; Yap, G. P. A.; Gusev, D. G.; dos Santos, E. N.; Fogg, D. E. Organometallics 2004, 23, 4047–4054.

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Ligand Isomerization in Sulfur-Chelated Ruthenium Benzylidenes

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