A Hoveyda–Grubbs Metathesis Catalyst Bearing a Peri-Substituted

Article pubs.acs.org/Organometallics

A Missing Relative: A Hoveyda−Grubbs Metathesis Catalyst Bearing a Peri-Substituted Naphthalene Framework Michał Barbasiewicz,* Krzysztof Grudzień, and Maura Malinska Faculty of Chemistry, University of Warsaw, Pasteura 1, 02-093 Warsaw, Poland S Supporting Information *

ABSTRACT: Molecular scaffolds of polycyclic aromatic hydrocarbons can serve as unique tools to control the molecular and electronic structure of coordination compounds. Herein, we report the synthesis and properties of a Hoveyda−Grubbs metathesis catalyst bearing a chelating benzylidene ligand assembled on peri-substituted naphthalene. In contrast to other reported naphthalene-based complexes (Barbasiewicz, M.; Grela, K. et al. Chem. Eur. J. 2008, 14, 9330−9337), it exhibits a very fast initiation behavior, attributed to a distorted molecular structure and reduced π-electron delocalization within the chelate ring.

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metathesis reactions.13 Thus, factors influencing the initiation process control the rate of the main metathesis reaction14 and give a chance to improve the catalytic performance.15 Following these efforts,4b,c the nitro derivative 2, displaying fast initiation behavior, was introduced by Grela.10 The withdrawing properties of the nitro group present in the position para to the iPrO substituent were postulated to facilitate dissociation of the chelate ring by reducing electron density on the coordinating oxygen atom. In turn, an improved activity of the ortho-substituted complex 3 was explained by steric effects which cause an out-of-plane distortion of the isopropyl substituent and destabilization of the Ru···O bond.11 A different picture was presented by Barbasiewicz and Grela et al.,9 in systematic studies of the naphthalene-based complexes 4. Striking activity differences between the isomeric catalysts were observed in the model metathesis reactions of N,Ndiallyltosylamine, where the complex 4b initiated at 0 °C while 4a,c required refluxing toluene (111 °C) to give a similar activity profile. This observation inspired the synthesis of related polycyclic complexes and led to the conclusion that the effect can be attributed to the degree of cyclic electron delocalization in the chelate rings controlled by the topology of ligands. In a way similar to that for polyaromatic hydrocarbons, phenanthrene (5) and anthracene (6) differ with respect to their π-electron distribution; complexes 4 assembled on the naphthalene core in an angular (variants I and II) or linear (variant III) fashion, differ in the properties of the chelate rings (Chart 2). According to the Clar rule9 the electronic structure of phenanthrene consists of highly aromatic external rings with completed electron sextets and an “empty” character of the middle ring with a double-bond-like character of the C9−C10

ell-defined homogeneous metathesis catalysts are ubiquitous tools for the construction of carbon−carbon multiple bonds in modern synthetic chemistry.1 Evolving from phosphane complexes2 toward N-heterocyclic carbenes3 and benzylidene ligands bearing chelating arms of oxygen,4 sulfur,5 nitrogen,6 phosphorus,7 and selenium,8 the class of catalysts is still a subject for development by both conceptual and trial and error attempts. The parent o-isopropoxy-substituted benzylidene complex 1, introduced by Hoveyda,4a was developed in numerous ways, and structure−activity correlations effecting diminished activity (latent systems)9 and improved performance10,11 were discovered. Although the detailed mechanistic picture is complicated, it is believed that the family of etherchelated complexes (1 and 2; Chart 1)12 is characterized by an olefin-dependent rate-determining initiation process, where active 14-electron species are released and propagate in Chart 1. Selected Metathesis Catalysts Bearing a Chelating iPrO Group

Received: January 23, 2012 Published: March 26, 2012 © 2012 American Chemical Society

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alcohol 12 was oxidized with PCC and subjected to the Wittig reagent in two variants, as presented in Scheme 1.

Chart 2. (a) Isoelectronic Six-π-Electron Systems, Chelate Ring of Ruthenium Complexes, Furan, and Benzene, and (b) Complexes Assembled on the Naphthalene Core Inheriting the π-Electron Properties of the Corresponding Tricyclic Aromatic Hydrocarbons Phenanthrene (5), Anthracene (6), and Pleiadiene (7), in Which the Degree of Cyclic Electron Delocalization in Bold Fragments Decreases in a Series9 a

Scheme 1. Synthesis of Ligands 8a,ba

a

Legend: (a) NH2OH, TsCl, py; 10 (64%), ref 19; (b) NaOH, H2O, then NaNO2, HCl, heating, 11 (70%), ref 19; (c) DIBAL, THF, then HCl; (d) iPrBr, K2CO3, DMF, 12 (80%, in two steps from 11); (e) PCC, CH2Cl2, 13 (97%); (f) Ph3PCHRBr, t-PeOK/toluene, THF, 96% for R = H (8a), 88% for R = CH3 (8b). DIBAL = diisobutylaluminium hydride.

Next, the synthesis of complex 14 was attempted under ligand exchange conditions with the Grubbs second-generation catalyst 15 and CuCl as a phosphine scavenger (Scheme 2).20 Scheme 2. Synthesis of the Complex 14

a

NHC = 1,3-bis(2,4,6-trimethylphenyl)imidazolin-2-ylidene.

bond. Thus, the angular orientation of the chelate assembled on the naphthalene framework in complexes 4a,c, similar to that found in 5 (Chart 2, variants I and II), results in increased aromatic properties and stabilization of the chelate. In turn, the linear isomer 4b, related to anthracene (variant III), displays activity similar to that of the parent Hoveyda−Grubbs complex 1. This is attributed to the properties of 6, for which only one electron sextet can be completed, and is considered as “migrating” among three positions, providing only a fraction of aromatic stabilization per ring. However, a simple analysis of the naphthalene molecule allows us to distinguish another assembly of coordinating sites required to form the chelate. While complexes 4, related to variants I−III, were described earlier,9 we wondered about a missing isomer16,17 featuring a peri substitution 18 of the naphthalene core with the coordinating sites placed at different rings (variant IV). The synthesis and properties of an isomeric Hoveyda−Grubbs type ruthenium complex bearing a 1,8-substituted naphthalene ligand (14) are presented in this report. At the beginning, we synthesized ligand 8 in five steps starting from desymmetrization of the commercially available 1,8-naphthalic anhydride 9 via a Beckmann rearrangement to form lactam 10.19 The product was hydrolyzed and diazotized with sodium nitrite solution to afford lactone 11.19 Then the lactone was reduced with DIBAL, and the phenolic oxygen atom was alkylated with isopropyl bromide in a one-pot procedure to afford 1-hydroxymethyl-8-isopropoxynaphthalene (12) in 80% yield after chromatographic purification. Then the

In both cases, when starting from styrenes 8a,b, the ruthenium complex 14 was obtained in practically the same yield: 47 and 44%, respectively. Surprisingly, an alternative protocol17 with the indenylidene catalyst 1621 at higher temperature, followed by flash chromatography and crystallization, gave 14 as a green microcrystalline powder in very good yield: 70%. The complex was stable as a solid but decomposed slowly in solution at room temperature in air. Complex 14 was characterized by 1H and 13C NMR spectroscopy, IR, and HRMS (ESI).22 NMR data suggested differences from the parent Hoveyda−Grubbs complex 1, manifested23 by an abnormal deshielding of the benzylidene proton and carbon atoms appearing at 18.66 and 313.8 ppm (CD2Cl2), respectively, as compared to 16.56 and 296.8 ppm in 1 (CDCl3).4a,24 Further structural insights were available from X-ray studies.25 The complex 14 crystallized in the monoclinic C2 space group with two molecules in the asymmetric unit. More detailed information on the refinement of this structure can be 3172

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Table 1. Selected Bond Lengths (Å) of Complexes 1,27 2,9 4a,9 and 14a

found in the Supporting Information. In general the molecules have similar structural parameters, and the final conclusions apply to both of them. The Ortep representation26 of one arbitrarily chosen molecule in two projections is presented in Figure 1. The side view (presented on the left) depicts the

bond Ru−O RuC C−CAr iPrO−CAr

14 2.253(3)/ 2.237(3) 1.820(4)/ 1.815(4) 1.457(7)/ 1.454(7) 1.406(6)/ 1.396(6)

29 2.287(1)/ 2.258(1) 1.825(2)/ 1.828(2) 1.448(3)/ 1.457(3) 1.368(2)/ 1.363(2)

127

4a9

2.256(1)

2.228(1)

1.829(1)

1.833(2)

1.448(2)

1.439(2)

1.370(2)

1.377(2)

a

Values separated with a slant correspond to parameters of two molecules present in the asymmetric unit. The RuC bond length increases and CH−C bond length decreases in the order 14 → 2 → 1 → 4a.

cyclic electron delocalization rather by the simple strength of the Ru···O interaction.14 Thus, complex 14 was expected to display a fast initiation behavior associated with reduced electron delocalization of the benzylidene chelate. To verify this hypothesis, we tested complex 14 in two benchmark RCM reactions and compared its activity with that of complexes 1, 2, and 4a,b (Figure 2). In accordance with the data reported previously, complex 4a, bearing a chelate ring assembled on naphthalene in an angular fashion, remained inactive at 25 °C (the activity of complex 4c is expectedly the same).9 In turn, complexes 1, 2,10 and 4b initiated easily, with the nitro catalyst (2) being superior to both the parent Hoveyda complex (1) and the linear naphthalene derivative (4b). Expectedly, the complex 14 displayed an excellent activity and at the beginning of reactions initiated even more quickly than the nitro complex 2. Thus, the observed initiation rates of the three isomeric naphthalenebased complexes 4a (blue triangles), 4b (yellow squares), and 14 (red squares) spanned a very wide range from latency attributed to aromatic stabilization of the chelate in 4a to overactivity of the complex 14, which suffered from diminished stability and deactivated at ca. 75% of the substrate conversion in both cases. Despite the limited stability displayed by catalyst 14, which makes it unsuitable for regular applications,29 the results of activity studies may give valuable conclusions about electronic effects present in the chelates. To correlate structural parameters of 14 with the observed activity, we focused on two effects: (1) distortion of the molecular structure of the chelate which possibly weakens the Ru···O coordination and suppresses cyclic π-electron conjugation as a consequence of less efficient overlapping of orbitals and (2) suppression of the cyclic electron delocalization in the chelate controlled by the structure of the peri-substituted naphthalene ligand, following the trends demonstrated earlier for complexes 4a−c.9,14 However, the first issue cannot be simply quantified; we assume that the curvature of the chelate ring, the twist of the benzylidene RuCH bond, and disturbance of the ruthenium geometry may facilitate olefin approach and accelerate the initiation step.13,30 To discuss the latter effect, a short introduction on the properties of peri-substituted naphthalenes is required. The peri substitution of the naphthalene core acts as a clamping framework in which substituents are forced into close proximity ideally at a distance of ca. 2.5 Å, much shorter than the separation present in ortho-substituted benzenes (3.1 Å).18 In contrast to chelates represented by variants I−III (Chart 2) which display strong or moderate π-electron delocalization, positions 1 and 8 of the naphthalene molecule

Figure 1. ORTEP26 drawings of the complex 14 molecule, represented by thermal ellipsoids drawn at the 50% probability level (one arbitrary molecule present in asymmetric unit is shown). Selected bond lengths (Å) are presented in Table 1.

complex geometry, which is similar to that of the related ruthenium complexes 1, 2, and 4. Some interesting structural properties are noticeable in a front view projection (presented on the right), oriented along the axis of the benzylidene Ru CH bond. The chelating ligand is evidently tilted away from the plane constituted by atoms coordinated directly to the ruthenium atom: imidazolin-2-ylidene carbon C41, benzylidene carbon C62, and oxygen O2. The torsion angle C41−Ru− C62−C63 is reduced from 170.3(1)° in 127 to 153.9(4)° (−142.4(4)° in the second molecule). The oxygen atom is slightly out of plane, giving a sum of valence angles around it of 354.3° (356.6°), and the ruthenium atom is pushed out of the chelate ring plane (C62−C63−C71−C70−O2) by about 0.607 Å (0.738 Å). Breaking of the Cs symmetry is also evident from the diverse bond lengths to chlorine atoms, ranging from 2.380(1) Å for Ru(1)−Cl(1) to 2.342(1) Å for Ru(1)−Cl(2) (2.394(1) and 2.350(1) Å for the second molecule). The observed structural oddities most probably arise from the unique properties of the peri substitution of the naphthalene core, substituents of which are forced into close proximity and oriented in a parallel manner.18 Thus, the six-membered chelate in complex 1428 must adopt an envelope conformation to minimize the strain of valence angles (RuCH−CAr and CAr− iPrO···Ru), and in addition the naphthalene core deplanarizes by about 12° (C62−C63−C70−O2, torsion angle in both molecules), opening the gap between coordination sites of the ligand. However, even more interesting conclusions can be drawn from an analysis of the structure of the chelate ring of the complex 14 and comparison with other Hoveyda-type complexes. Interesting trends concerning the RuC and  C−CAr bond lengths were observed within the series 14, 2, 1, 4a (Table 1). The benzylidene RuCH bond elongates, while the adjacent CH−CAr bond gradually contracts, thus indicating an increase of the π-electron delocalization and concomitant equalization of the bond orders within the chelate.9 As demonstrated by recent theoretical treatments by Solans-Monfort et al., the key parameter responsible for the initiation rate of the chelated systems is the length of the  CH−CAr bond. This bond length is controlled more by the 3173

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Figure 2. Reaction profiles of the ring-closing metathesis determined by 1H NMR: (left) diethyl diallylmalonate (0.5 mol % of catalyst, 25 °C, CD2Cl2, 0.2 M concentration of substrate); (right) diethyl allylmethallylmalonate (1.0 mol % of catalyst, 25 °C, CD2Cl2, 0.2 M concentration of substrate).

Figure 3. HOMA34 values calculated for the structures of phenanthrene (5), anthracene (6), pleiadiene (cyclohepta[d,e]naphthalene, 7), isomeric naphthofurans (17−19), and naphtho[1,8-b,c]pyran (20) optimized with B3LYP/6-311+G(d,p) and nuclear independent chemical shift (NICS)35 values calculated with the GIAO method at the same level.36 X-ray structural data of 7 were taken from ref 32.

catalysts 4a,b and 14, corresponding to a series of tricyclic hydrocarbons (5−7; Chart 2), electronic delocalization within the conjugated chelates gradually decreases. In complex 14 it is even lower than in the parent Hoveyda complex (1), which is manifested in a higher rate of initiation of the (pre)catalyst. To support our hypothesis and better assess the π-electron properties of the chelate rings, we calculated structural and magnetic properties of isoelectronic model molecules: hydro-

seem to be electronically isolated. In the model aromatic hydrocarbon pleiadiene (7), related to variant IV, the unsaturated carbon linker (−CHCH−CHCH−), connecting peri positions, displays a large bond length alternations and weakly disturbs the electronic structure of naphthalene. The isolated character of the diene fragment in 7 was further supported by theoretical,31 structural,32 and reactivity studies, in which a Diels−Alder reaction of the diene fragment with maleic anhydride was demonstrated.33 Thus, in the series of the 3174

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organic phases were washed with brine and dried with MgSO4. The mixture was filtered and evaporated in a flask. To the residue were added 2-bromopropane (6.179 g; 50.2 mmol), K2CO3 (6.91 g; 50.0 mmol), and DMF (25 mL), and the mixture was stirred at 60 °C for 48 h in air. The mixture was poured into water (150 mL) and extracted with ethyl acetate (3 × 80 mL). The combined organic phases were washed with brine (100 mL) and dried with MgSO4. The mixture was filtered, evaporated, and separated with column chromatography (300 mL of silica; cyclohexane/ethyl acetate 4/1 to 2/1) to give 12 (1.739 g; 8.04 mmol; 80%) as a yellowish oil. Data for 12 are as follows. 1H NMR (200 MHz, CDCl3): δ 7.69− 7.79 (m, 1H), 7.46 (dd, J = 8.0, 1.5 Hz, 1H), 7.32−7.43 (m, 3H), 6.90−6.96 (m, 1H), 5.06 (s, 2H), 4.87 (sept, J = 2.0 Hz, 1H), 3.38 (s br, 1H), 1.50 (d, J = 2.0 Hz, 6H). 13C NMR (50 MHz, CDCl3): δ 153.5, 136.8, 136.5, 128.7, 128.0, 125.8, 125.5, 124.5, 121.8, 107.9, 70.8, 67.1, 21.8. MS (EI, m/z, relative intensity): 216 (M•+, 28), 174 (9), 156 (100), 128 (37), 127 (34), 115 (19). Anal. Calcd for C14H16O2: C, 77.75; H, 7.46. Found: C, 77.73; H, 7.27. Synthesis of 13. A flask was charged with 12 (1.739 g; 8.04 mmol) and CH2Cl2 (150 mL). To the solution was slowly added PCC (2.085 g; 9.67 mmol). The mixture was stirred under air overnight and then filtered through a pad of silica and eluted with CH2Cl2. After evaporation 13 was obtained (1.665 g; 7.77 mmol; 97%) as a pale orange oil, which slowly solidified. Data for 13 are as follows. Mp: 59−61 °C. 1H NMR (200 MHz, CDCl3): δ 11.12 (s, 1H), 7.94 (dd, J = 8.0, 1.2 Hz, 1H), 7.85 (dd, J = 1.4, 7.2 Hz, 1H), 7.36−7.55 (m, 3H), 6.97 (dd, J = 1.8, 6.8 Hz, 1H), 4.78 (sept, J = 6.0 Hz, 1H), 1.44 (d, J = 6.0 Hz, 6H). 13C NMR (50 MHz, CDCl3): δ 195.6, 154.1, 135.5, 135.4, 132.9, 127.0, 126.5, 125.6, 124.1, 121.0, 108.6, 71.4, 22.0. MS (EI, m/z, relative intensity): 214 (M•+, 22), 172 (100), 155 (14), 144 (10), 115 (56). Anal. Calcd for C14H14O2: C, 78.48; H, 6.59. Found: C, 78.25; H, 6.50. Synthesis of 8a. A flask was charged with methyltriphenylphosphonium bromide (0.977 g; 2.73 mmol) and THF (25 mL), and a solution of potassium tert-amylate (2 mL; 3.4 mmol; 1.7 M solution in toluene) was added dropwise with stirring. After 15 min the resulting yellowish suspension was cooled with an ice−water bath and a solution of 13 (0.484 g; 2.26 mmol) in THF (5 mL) was added dropwise. The cooling bath was removed, and after 1 h an aqueous solution of NH4Cl (50 mL, 10% w/w) was added. The mixture was extracted with ethyl acetate (3 × 50 mL), and the combined organic phases were washed with brine (50 mL) and dried with MgSO4. The mixture was filtered and evaporated, and the residue was separated with column chromatography (150 mL of silica; cyclohexane/ethyl acetate 10/1) to give 8a (0.460 g; 2.17 mmol; 96%) as a colorless oil. Data for 8a are as follows. 1H NMR (200 MHz, CDCl3): δ 8.06 (dd, J = 10.8, 17.4 Hz, 1H), 7.79 (dd, J = 2.0, 7.8 Hz, 1H), 7.36−7.56 (m, 3H), 6.92 (dd, J = 1.6, 7.2 Hz, 1H), 5.48 (dd, J = 2.0, 17.4 Hz, 1H), 5.28 (dd, J = 2.0, 10.8 Hz, 1H), 4.75 (sept, J = 6.0 Hz, 1H), 1.50 (d, J = 2.0 Hz, 6H). 13C NMR (50 MHz, CDCl3): δ 155.2, 141.8, 137.2, 135.9, 128.0, 126.0, 125.8, 125.6, 124.2, 120.8, 112.7, 108.2, 70.9, 21.9. MS (EI, m/z, relative intensity): 212 (M•+, 29), 170 (21), 155 (100), 139 (8), 127 (9), 115 (6). Anal. Calcd for C15H16O: C, 84.87; H, 7.60. Found: C, 84.95; H, 7.73. Synthesis of 8b. A flask was charged with ethyltriphenylphosphonium bromide (1.788 g; 4.82 mmol) and THF (30 mL), and a solution of potassium tert-amylate (3.6 mL; 6.1 mmol; 1.7 M solution in toluene) was added dropwise with stirring (cooling with a water bath was applied to avoid warming). After 20 min the resulting orange suspension was cooled with an ice−water bath and a solution of 13 (0.858 g; 4.00 mmol) in THF (6 mL) was added dropwise. The cooling bath was removed, and after 1 h an aqueous solution of NH4Cl (25 mL; 10% w/w) was added. The mixture was extracted with ethyl acetate (3 × 75 mL), and the combined organic phases were washed with brine (100 mL) and dried with MgSO4. The mixture was filtered and evaporated, and the residue was separated with column chromatography (250 mL of silica; cyclohexane/ethyl acetate 10/1) to give 8b (0.797 g; 3.52 mmol; 88%) as a pale yellow oil. Data for 8b, as a mixture of E/Z isomers (ca. 1/1), are as follows. 1 H NMR (200 MHz, CDCl3): δ 7.66−7.75 (m, 2H), 7.58−7.63 (m,

carbons (5−7) and heterocycles represented by naphthofurans (17−19) and naphtho[1,8-bc]pyran (20) (Figure 3). The π-electron delocalization in the model compounds was studied with magnetic and structural aromaticity indexes and compared with the energetic properties of ruthenium chelates attributed to the catalytic activity. The structural data of the model compounds were analyzed with the HOMA (harmonic oscillator model of aromaticity)34 parameter, which is a measure of bond length alternation in conjugated structures. The HOMA values are defined as 0 for the hypothetical cyclohexatriene molecule (Kekulé resonance form) and 1 for the equalized structure of benzene. In the hydrocarbon series (5−7) the HOMA values of external rings (which correspond to chelate rings in ruthenium complexes 4 and 14) decrease in the series +0.87, +0.63, and +0.09, suggesting a large alternation of bond lengths in the seven-membered ring of 7. Similar results apply to the heterocyclic series (17−20) and only the last two structures (19 and 20) display a slightly reversed trend (+0.14/+0.12; −0.06; +0.05).37 In turn, magnetic properties expressed by the nucleus independent chemical shift35 values (NICS, calculated at and 1 Å above the molecular plane of rings, abbreviated as NICS(0) and NICS(1), respectively) suggest a gradual change in both series, from the aromatic properties of external rings of phenanthrene (5; −8.5, −10.7), naphtho[2,1-b]furan (17; −10.0, −8.6) and naphtho[1,2b]furan (18; −9.9, −8.7) toward antiaromatic properties in pleiadiene (7; +13.9, +8.7) and naphtho[1,8-b,c]pyran (20; +2.6, +6.5). In general, the aromaticity indexes indicate that both unsaturated 4π linkers (−CHCH−CHCH− and −O−CHCH−) binding the peri positions of naphthalene form rings with suppressed π-electron delocalization and destabilizing (antiaromatic) properties.38 The same properties are thus expected for the chelate ring of the catalyst 14, and in fact the observed activity of complexes 4 and 14 correlate with properties of the calculated structures. According to our aromaticity-controlled initiation concept9 the rate-determining initiation process of the Hoveyda-type catalysts includes opening of the chelate ring; thus, factors related to stabilization of the chelates can control the catalyst activity over a broad range, as was demonstrated for the family of naphthalene-based complexes. In conclusion, we synthesized a missing member of the naphthalene-based family of the Grubbs−Hoveyda complexes. In contrast to the naphthalene-based complexes 4, it displays a very fast initiation behavior, attributed to distorted geometry around the ruthenium atom and suppressed π-electron delocalization of the chelate ring. On the basis of the aromaticity-controlled activity concept,9 the peri-substituted complex 14 represents a weakly stabilized chelate and extends the scale of initiation rates controlled by ligand topology toward systems more active than the parent Hoveyda−Grubbs complex.

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EXPERIMENTAL SECTION

General methods are detailed in the Supporting Information. Synthesis of 12. A Schlenk flask was charged with lactone 11 (1.703 g; 10.01 mmol) and THF (20 mL) and cooled with an ice− water bath. A solution of DIBAL (21 mL; 21 mmol; 1.0 M solution in toluene) was added dropwise with stirring. After 15 min the cooling bath was removed, and after 30 min MeOH (5 mL) was added slowly and the cooling bath was applied again (slight exothermic effect). The mixture had gelated. Ethyl acetate (50 mL) and aqueous HCl (30 mL; 1/5 v/v) were added, and the phases were separated. The aqueous phase was extracted with ethyl acetate (100 mL), and the combined 3175

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(2) Schwab, P.; Grubbs, R. H.; Ziller, J. W. J. Am. Chem. Soc. 1996, 118, 100−110. (3) Samojłowicz, C.; Bieniek, M.; Grela, K. Chem. Rev. 2009, 109, 3708−3742. (4) (a) Garber, S. B.; Kingsbury, J. S.; Gray, B. L.; Hoveyda, A. H. J. Am. Chem. Soc. 2000, 122, 8168−8179. (b) Vougioukalakis, G. C.; Grubbs, R. H. Chem. Rev. 2010, 110, 1746−1787. (c) Vidavsky, Y.; Anaby, A.; Lemcoff, N. G. Dalton Trans. 2012, 41, 32−43. (5) (a) Kost, T.; Sigalov, M.; Goldberg, I.; Ben-Asuly, A.; Lemcoff, N. G. J. Organomet. Chem. 2008, 693, 2200−2203. (b) Ben-Asuly, A.; Tzur, E.; Diesendruck, C. E.; Sigalov, M.; Goldberg, I.; Lemcoff, N. G. Organometallics 2008, 27, 811−813. (c) Aharoni, A.; Vidavsky, Y.; Diesendruck, C. E.; Ben-Asuly, A.; Goldberg, I.; Lemcoff, N. G. Organometallics 2011, 30, 1607−1615. (d) Ben-Asuly, A.; Aharoni, A.; Diesendruck, C. E.; Vidavsky, Y.; Goldberg, I.; Straub, B. F.; Lemcoff, N. G. Organometallics 2009, 28, 4652−4655. (e) Ginzburg, Y.; Anaby, A.; Vidavsky, Y.; Diesendruck, C. E.; Ben-Asuly, A.; Goldberg, I.; Lemcoff, N. G. Organometallics 2011, 30, 3430−3437. (f) Szadkowska, A.; Makal, A.; Woźniak, K.; Kadyrov, R.; Grela, K. Organometallics 2009, 28, 2693−2700. (g) Szadkowska, A.; Ż ukowska, K.; Pazio, A. E.; Woźniak, K.; Kadyrov, R.; Grela, K. Organometallics 2011, 30, 1130− 1138. (6) (a) Barbasiewicz, M.; Szadkowska, A.; Bujok, R.; Grela, K. Organometallics 2006, 25, 3599−3604. (b) Tzur, E.; Szadkowska, A.; Ben-Asuly, A.; Makal, A.; Goldberg, I.; Woźniak, K.; Grela, K.; Lemcoff, N. G. Chem. Eur. J. 2010, 16, 8726−8737. (c) Ż ukowska, K.; Szadkowska, A.; Pazio, A. E.; Woźniak, K.; Grela, K. Organometallics 2012, 31, 462−469. (7) Lexer, C.; Burtscher, D.; Perner, B.; Tzur, E.; Lemcoff, N. G.; Slugovc, C. J. Organomet. Chem. 2011, 696, 2466−2470. (8) Diesendruck, C. E.; Tzur, E.; Ben-Asuly, A.; Goldberg, I.; Straub, B. F.; Lemcoff, N. G. Inorg. Chem. 2009, 48, 10819−10825. (9) Barbasiewicz, M.; Szadkowska, A.; Makal, A.; Jarzembska, K.; Woźniak, K.; Grela, K. Chem. Eur. J. 2008, 14, 9330−9337. (10) (a) Grela, K.; Harutyunyan, S.; Michrowska, A. Angew. Chem., Int. Ed. 2002, 41, 4038−4040. (b) Michrowska, A.; Bujok, R.; Harutyunyan, S.; Sashuk, V.; Dolgonos, G.; Grela, K. J. Am. Chem. Soc. 2004, 126, 9318−9325. (11) Wakamatsu, H.; Blechert, S. Angew. Chem., Int. Ed. 2002, 41, 2403−2405. (12) Vorfalt, T.; Wannowius, K.-J.; Plenio, H. Angew. Chem., Int. Ed. 2010, 49, 5533−5536. (13) Thiel, V.; Hendann, M.; Wannowius, K.-J.; Plenio, H. J. Am. Chem. Soc. 2012, 134, 1104−1114. (14) Solans-Monfort, X.; Pleixats, R.; Sodupe, M. Chem. Eur. J. 2010, 16, 7331−7343. (15) The assumption is valid until structure of substrates and reaction conditions do not cause a change of the rate-determining step. For structural variations of NHC ligands, which influence properties of propagating species, see ref 3. (16) Prof. Christian Slugovc (Graz University of Technology) called the isomeric complexes 4a−c “unequal siblings” to emphasize their activity differences in ring-opening metathesis polymerization (ROMP).17 Obviously, our isomeric complex 14 represents still another relative of the family, missing until now. (17) Leitgeb, A.; Szadkowska, A.; Michalak, M.; Barbasiewicz, M.; Grela, K.; Slugovc, Ch. J. Pol. Sci. A: Polym. Chem. 2011, 49, 3448− 3454. (18) (a) Kilian, P.; Knight, F. R.; Woollins, J. D. Chem. Eur. J. 2011, 17, 2302−2328. (b) Kilian, P.; Knight, F. R.; Woollins, J. D. Coord. Chem. Rev. 2011, 255, 1387−1413. (19) Cammidge, A. N.; Ö ztürk, O. J. Org. Chem. 2002, 67, 7457− 7464. (20) Zaja, M.; Connon, S. J.; Dunne, A. M.; Rivard, M.; Buschmann, N.; Jiricek, J.; Blechert, S. Tetrahedron 2003, 59, 6545−6558. (21) For the synthesis of complex 16, see: (a) Monsaert, S.; Drozdzak, R.; Dragutan, V.; Dragutan, I.; Verpoort, F. Eur. J. Inorg. Chem. 2008, 432−440. (b) Monsaert, S.; De Canck, E.; Drozdzak, R.;

1H), 7.22−7.48 (m, 9H), 6.83−6.91 (m, 2H), 5.63−5.91 (m, 2H), 4.60−4.80 (m, 2H), 1.97 (dd, J = 1.8, 6.6 Hz, 3H), 1.77 (dd, J = 1.8, 7.0 Hz, 3H), 1.47 (d, J = 6.0 Hz, 6H), 1.43 (d, J = 6.0 Hz, 6H). 13C NMR (50 MHz, CDCl3): δ 155.4 (ovl), 137.0, 136.1, 136.0, 135.7, 134.5, 134.3, 128.1, 127.2, 127.0, 125.9, 125.8, 125.53, 125.48, 125.3 (ovl), 124.4, 123.8, 121.2, 120.9, 120.8, 108.3, 108.2, 70.9, 70.8, 22.1, 21.9, 18.5, 14.0. MS (EI, m/z, relative intensity): 226 (M•+, 28), 184 (24), 168 (11), 155 (100), 139 (7), 127 (11). Anal. Calcd for C16H18O: C, 84.91; H, 8.02. Found: C, 84.71; H, 7.85. Synthesis of 14. A Schlenk flask was charged with 8b (0.274 g, 1.21 mmol), toluene (20 mL), CuCl (0.149 g; 1.51 mmol), and 16 (0.948 g, 1.00 mmol) and placed in a preheated oil bath at 80 °C with stirring. After 20 min the heating bath was removed, most of the solvent was removed in vacuo, and the residue was placed as a suspension in a small volume of ethyl acetate on the top of a chromatographic column (300 mL of silica) and eluted with gradually changing mixture of cyclohexane and ethyl acetate (6/1 to 1/2). The first yellowish red fractions were removed, and a green band was collected (the green product slowly dissolved and was desorbed from silica). Solvent was evaporated, the residue was dissolved in CH2Cl2 (9 mL), and MeOH (9 mL) was added. Most of the solvents were slowly removed by evaporation at 220 mbar.39 The resulting suspension was filtered on a small Schott filter, washed with MeOH (3 × 3 mL), and dried in vacuo to give 14 (0.477 g; 0.70 mmol; 70%), as a green microcrystalline solid. Data for 14 are as follows. 1H NMR (500 MHz, CH2Cl2): δ 18.66 (s, 1H), 8.11 (d, J = 7.6 Hz, 1H), 7.55 (d, J = 8.0 Hz, 1H), 7.33 (dd, J = 8.0, 8.0 Hz, 1H), 7.29 (dd, J = 7.6, 7.6 Hz, 1H), 7.21 (d, J = 8.1 Hz, 1H), 7.08 (s, 4H), 6.67 (d, J = 7.1 Hz, 1H), 5.09 (sept, J = 6.3 Hz, 1H), 3.97−4.18 (m, 4H), 2.63 (s, 6H), 2.49 (s, 3H), 2.38 (s, 3H), 2.31 (s, 6H), 2.16 (d, J = 6.3 Hz, 6H). 13C NMR (125 MHz, CD2Cl2): δ 313.8, 210.1, 152.2, 144.9, 140.4, 139.5, 138.9, 138.1, 137.9, 137.8, 134.9, 131.4, 130.1, 129.8, 126.85, 126.83, 124.3, 122.5, 120.1, 112.5, 79.5, 52.7, 50.9, 21.1, 20.5, 19.9, 18.3. IR (KBr, cm−1): 2912, 1606, 1555, 1503, 1479, 1416, 1379, 1264, 1236, 1221, 1171, 1129, 1089, 1031, 906, 852, 834, 740, 579, 422. HRMS (ESI, m/z): calcd for C35H40ClN2ORu 641.1873, found 641.1877 (M − Cl)•+. Anal. Calcd for C35H40Cl2N2ORu: C, 62.12; H, 5.96; N, 4.14. Found:22 C, 60.40; H, 5.85; N, 3.78.

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ASSOCIATED CONTENT

S Supporting Information *

Text, figures, tables, and CIF files giving NMR spectra for compounds 8a,b and 12−14, experimental procedures for the synthesis of 10 and 11, a complete modeling program citation, and crystallographic data for compounds 14 and 21. This material is available free of charge via the Internet at http:// pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Web: http://www. aromaticity.pl/. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financed by the Polish Ministry of Science and Higher Education (Grant No. N N204 152436). M.B. thanks Prof. Karol Grela for support and the opportunity to perform an independent research program in the field.

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REFERENCES

(1) For a recent review of metathesis applications, see: Kotha, S.; Dipak, M. K. Tetrahedron 2012, 68, 397−421. 3176

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Van Der Voort, P.; Hendrickx, P. M. S.; Martins, J. C.; Verpoort, F. NATO Secur. Sci., Ser. A 2010, I, 31−38. (22) Unfortunately, we were unable to obtain a satisfactory elementary analysis for complex 14. (23) In complex 14 the presence of four methyl resonances (in a 2:1:1:2 ratio) of the NHC ligand was observed. For a detailed discussion of the conformational landscape of similar complexes, see: Kotyk, M. W.; Gorelsky, S. I.; Conrad, J. C.; Carra, C.; Fogg, D. E. Organometallics 2009, 28, 5424−5431 and references cited therein. (24) The strong deshielding of the RuCH− fragment can be rationalized by reduced overlapping of ruthenium and carbon p-type orbitals caused by a distortion (twist) of the benzylidene double bond and close proximity to the aromatic ring of the NHC ligand, in comparison with the case for complex 1. Reduced π-electron delocalization between peri positions of naphthalene in complex 14 should cause the opposite effect, however. (25) Crystals suitable for diffraction experiments were obtained by layering 0.2 mL of a CH2Cl2 solution of complex 14 (0.015 g) with MeOH (0.6 mL) in an NMR tube at low temperature (−20 °C). (26) We wish to acknowledge the use of a free version of Ortep-3 for Windows program: Farrugia, L. J. J. Appl. Crystallogr. 1997, 30, 565− 565. (27) Barbasiewicz, M.; Bieniek, M.; Michrowska, A.; Szadkowska, A.; Makal, A.; Woźniak, K.; Grela, K. Adv. Synth. Catal. 2007, 349, 193− 203. (28) For other six-membered oxygen chelates, see: Fürstner, A.; Thiel, O. R.; Lehmann, C. W. Organometallics 2002, 21, 331−335. (29) However, complex 14 deactivated within minutes during the metathesis reaction when tested in nondegassed CD2Cl2 solutions; the same solution without added olefin was much more stable, and 14 was detected by TLC after 5 days, on storage at room temperature in air. See the Supporting Information for details. (30) Ashworth, I. W.; Hillier, I. H.; Nelson, D. J.; Percy, J. M.; Vincent, M. A. Chem. Commun. 2011, 47, 5428−5430. (31) Michl, J. J. Am. Chem. Soc. 1976, 98, 4546−4549. (32) Hazell, A.; Grønbæk Hazell, R.; Larsen, F. K. Acta Crystallogr. 1986, B42, 621−626. (33) Boekelheide, V.; Vick, G. K. J. Am. Chem. Soc. 1956, 78, 653− 658. (34) (a) Krygowski, T. M. J. Chem. Inf. Comput. Sci. 1993, 33, 70−78. (b) Sobczyk, L.; Grabowski, J. S.; Krygowski, T. M. Chem. Rev. 2005, 105, 3513−3560. (c) Cyrański, M. K. Chem. Rev. 2005, 105, 3773− 3811. (35) (a) Fallah-Bagher-Shaidaei, H.; Wannere, C. S.; Corminboeuf, C.; Puchta, R.; Schleyer, P. v. R. Org. Lett. 2006, 8, 863−866. (b) Chen, Z.; Wannere, C. S.; Corminboeuf, C.; Puchta, R.; Schleyer, P. v. R. Chem. Rev. 2005, 105, 3842−3888. (36) All the calculations were performed using Gaussian 03 on a PC/ Linux workstation: Gaussian 03, Revision E.01; Gaussian, Inc., Wallingford, CT; 2004 (for the complete citation see the Supporting Information). Structures of compounds 5−7 and 17−19 were optimized using B3LYP with the 6-311+G(d,p) basis set. Only real values of the analytical harmonic vibrational frequencies confirmed that the geometries under study correspond to the minimum-energy structures. (37) HOMA and NICS values calculated at the same level of theory for benzene and furan molecules are as follows: benzene, HOMA = +0.99, NICS(0) = −8.0, NICS(1) = −10.2; furan, HOMA = +0.19, NICS(0) = −11.9, NICS(1) = −9.4. In the pyran molecule cyclic electron conjugation is lacking (it is fully conjugated in annulated system 20 only). Negative values of the HOMA parameter correspond to systems in which bond alternation is larger than in the model cyclohexatriene. Negative NICS values correspond to diatropic ring currents (aromatic), while positive values correspond to paratropic ring currents (antiaromatic). (38) Structural distortion of the chelate ring of complex 14 may arise from its destabilizing (antiaromatic) properties. From the other side formation of the ring should cause structural changes of the naphthalene framework. To examine how the formation of the chelate

influences the structure of the naphthalene core, we determined the structure of 8-hydroxymethyl-1-naphthol (21; intermediate of synthesis of compound 9) by X-ray methods and compared it with that of complex 14. See the Supporting Information for details. (39) The evaporation process is a trigger for crystallization; thus, rotation on a rotary evaporator should be very slow and no heating bath should be applied. The whole crystallization process takes ca. 2 h, when most the solvent slowly evaporates and the flask is cold.

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