[Au(NHC)(OH)] Complexes for Silver-Free Protocols - ACS Publications

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New [Au(NHC)(OH)] Complexes for Silver-Free Protocols Scott R. Patrick, Adrián Gómez-Suárez, Alexandra M. Z. Slawin, and Steven P. Nolan* EaStCHEM School of Chemistry, University of St Andrews, St Andrews, KY16 9ST, U.K. S Supporting Information *

ABSTRACT: Novel (NHC)gold(I) hydroxides are reported. Their differing tolerances to air and moisture are rationalized by the varying abilities of the NHC ligands to sterically protect the reactive metal center. A number of gold hydroxides were used in the silver-free preparation of Gagosz-type catalysts. This general procedure led to analytically pure products while avoiding the use of expensive silver reagents.

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free gold-catalyzed protocols,17 and many other transformations.18 Avoiding the use of silver salts decreases costs and makes handling easier, as they can be sensitive to light and moisture. They may also persist as impurities in the catalyst and, therefore, cause confusion about the identity of the active catalytic species when the ubiquitous cationic gold(I) species has been generated by halide abstraction using silver.19 This problem was encountered by Sheppard and co-workers, who observed side-reactions when using [Au(PPh3)(NTf2)] that was contaminated with AgNTf2 (NTf2 = bis(trifluoromethanesulfonyl)imidate).20 In order to combat this problem, we envisaged a new synthetic route to prepare Gagosz-type [Au(NHC)(NTf2)] complexes21 while forgoing the use of silver salts. The simple, silver-free synthesis of [Au(NHC)Cl] complexes under mild conditions was recently independently reported by our group and that of Gimeno.22 It was previously necessary to prepare these complexes either from the copper or silver congeners or via free carbenes. Additionally, we recently demonstrated that [Au(IPr)(OH)] could be reacted with HNTf2 in benzene to produce [Au(IPr)(NTf2)] in a 91% yield.7a Therefore, if a broad range of gold(I) hydroxides could be prepared, then it could be combined with the two aforementioned procedures to open a general route to gold catalysts with minimal (or, better yet, no) silver impurities (Scheme 1). It was with this goal in mind that work began on synthesizing new (NHC)gold(I) hydroxides. The synthetic procedure to form [Au(IPr)(OH)] was recently updated by our group to also permit the formation of the SIPr analogue in good yields.7b The new method involves 10 equiv of KOH to force the reaction forward, while the temperature was lowered to 30 °C to avoid decomposition due to a suspected thermally sensitive intermediate. This milder methodology has now been applied to form new [Au(NHC)(OH)] complexes (Figure 1). It has proven successful for NHC ligands with a variety of backbone

he use of gold in catalysis has grown dramatically over the past decade due to its broad reactivity.1 Organogold chloride complexes bearing monodentate ancillary ligands represent the state of the art among the Au(I) catalysts. Early gold complexes predominantly featured phosphine ligands such as PPh32 and were more recently joined by N-heterocyclic carbenes (NHCs).3 In the case of Au(I) complexes, NHCs have become very popular, as they possess unprecedented σdonation and steric bulk that enhance the stability of catalytic intermediates.4 Further advantages offered by NHC ligands, in practical terms, are their ease of synthesis and modification, providing straightforward access to a wide number of Au(I) complexes.5 In recent years, transition metal hydroxide complexes have been synthesized as an elegant and environmentally friendly alternative to the use of external bases in metal-catalyzed organic transformations, in addition to their versatility as synthetic reagents.6 Our group has been very active in this field, using NHC ligands to stabilize the hydroxide moiety. To date, we have reported the synthesis of metal hydroxide species featuring gold,7 copper,8 rhodium,9 ruthenium,10 palladium,11 and iridium.12 Pursuing the synthesis of new gold complexes, we recently reported the synthesis of the first mononuclear Au-NHC hydroxide complex, [Au(IPr)(OH)] (1).7a This was obtained by reacting [Au(IPr)Cl] with 2 equiv of KOH at 60 °C in a 1:1 THF/toluene mixture. Maier et al. later explored alternative synthetic procedures involving either mechanical grinding or aqueous KOH solutions.13 Using these techniques, they were able to quickly prepare gold(I) hydroxides featuring ligands such as JohnPhos and IPr. However, none of the above methods could be successfully applied to other NHC ligands. The development of complex 1 has allowed us to access a wide variety of novel Au(I)-IPr species while avoiding the use of inert atmospheres and external bases. This has been due to its outstanding stability and basicity (pKa 30.3 in DMSO).7a,14 For example, complex 1 has been used successfully for the synthesis of gold-acetylene complexes,15 carboxylation and decarboxylation reactions,14,16 the development of new silver© 2013 American Chemical Society

Received: October 14, 2013 Published: December 11, 2013 421

dx.doi.org/10.1021/om4010065 | Organometallics 2014, 33, 421−424

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small differences from the chlorides (IPr: 43.2% vs 44.5%; SIPr: 46.6% vs 47.0%),24 and therefore the [Au(NHC)Cl] complexes will be used for the purpose of comparison. As expected, the complexes with lower percent buried volumes were found to be more sensitive when reacting to form hydroxides. Gold(I) chlorides featuring the ligands IPr and SIPr were considerably better protected than those of IMes and SIMes (%VBur = 36.5 and 36.9, respectively). The lowest percent buried volume was observed for [Au(ICy)Cl] (%VBur = 27.4), which may explain the particularly high sensitivity when forming its hydroxide derivative despite the use of a glovebox. The electronics of the NHC ligands did not appear to play a role in the sensitivities of their complexes. The Tolman Electronic Parameter (TEP) of SIMes is very similar to that of IPr,24 and yet only SIMes requires anaerobic and anhydrous conditions. Additionally, the TEP values of IAd and ICy are very similar, but [Au(IAd)(OH)] does not display the same intolerance to minor impurities as the ICy analogue. With these new gold(I) hydroxides in hand, we then employed a number of them in the silver-free protocol to generate Gagosz-type [Au(NHC)(NTf2)] complexes. They were reacted with HNTf2 in benzene for 6 h (Table 1).

Scheme 1. Silver-Free Synthesis of Gold Catalysts

substitutions (IPrCl, IPrMe), steric bulks (IPr*, IPr*Tol), flexible N-substituents (ICy, IDD), and electronic properties (IMes, SIMes, ItBu, IAd). The lower temperature used in this method is the key to its success, as decomposition is observed at 60 °C. Additionally, the large excess of base is required to obtain full conversion to the desired product, since the hydroxide cannot be easily isolated from the chloride. While the complexes featuring IPr-based ligands were stable under ambient conditions, the others were more sensitive and needed to be prepared under inert atmosphere conditions using Schlenk or glovebox techniques. The use of polar solvents, such as THF and DCM, occasionally led to decomposition, and toluene gave incomplete conversions. Therefore, benzene was used under these conditions despite inferior solubility of the starting materials. Cesium hydroxide was used as the base for these reactions, as it demonstrated better solubility properties. Great care must be taken to optimize mixing during the synthesis of the gold hydroxides. The base must be finely ground and relatively free of moisture to ensure the maximum surface area. Stirring should also be vigorous so that the base is a dispersed suspension rather than an agglomerate at the bottom of the reaction vessel. Maier et al. were unsuccessful in generating [Au(PPh3)(OH)] (2), likely due to insufficient steric protection and σdonation to the metal center by the ligand. Attempts to prepare 2 by using the new inert atmosphere procedure described herein also failed, instead resulting in decomposition to a purple suspension. The increased sensitivity of some of the complexes may be due to relatively poor shielding of the metal center by the NHC ligand. This hypothesis can be explored computationally by obtaining percent buried volumes (%VBur) from crystal structures. The buried volume is defined as the percentage of a metal center covered by a ligand at a given radius.23 Unfortunately, despite our repeated best efforts, it was not possible to grow crystals of the new complexes that were suitable for X-ray diffraction. However, the percent buried volumes of previously reported gold(I) hydroxides showed only

Table 1. Synthesis of [Au(NHC)(NTf2)] Complexes

NHC

[AuOH] yield

[AuNTf2] yield

two-step yield

silver route yield

IPr SIPr IPrCl IPr*

927a 757b 72 93

917a 94 87 79

84 71 62 73

6921b 7321b 8725 9326

The yields of the two-step procedure from gold chlorides to Gagosz complexes are good, and analytically pure products are afforded in all cases. When compared to the one-step silver route, the overall yields are competitive and expensive silver reagents (and their corresponding impurities) may be completely avoided. In conclusion, the methodology for synthesizing [Au(NHC)(OH)] complexes has been extended to include a wide range of NHC ligands, including a number of highly sensitive species. As well as being versatile reactive species in their own right, they have been incorporated as part of a scheme to develop silverfree synthetic protocols. The yields for this process are good and highlight a promising ability to remove costly silver additives from future reactions.

Figure 1. Synthesis of various [Au(NHC)(OH)] complexes. 422


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(s, 1H) ppm. 13C{1H} NMR (100 MHz, C6D6): δ 172.5 (s, Ccarbene), 139.3 (s, CAr), 135.6 (s, CAr), 134.9 (s, CAr), 129.6 (s, CHAr), 121.2 (s, CHimid), 21.1 (s, CH3), 17.8 (s, CH3) ppm. Anal. Calcd for C21H25AuN2O: C, 48.65; H, 4.86; N, 5.40. Found: C, 48.72; H, 4.79; N, 5.31. [Au(SIMes)(OH)]. Procedure B in a 0.060 mmol scale afforded a white, microcrystalline solid (19.2 mg, 62%). 1H NMR (300 MHz, C6D6): δ 6.71 (s, 4H), 2.97 (s, 4H), 2.14 (s, 12H), 2.08 (s, 6H), −0.22 (s, 1H) ppm. 13C{1H} NMR (100 MHz, C6D6): δ 194.4 (s, Ccarbene), 138.4 (s, CAr), 135.8 (s, CAr), 129.9 (s, CHAr), 128.6 (s, CAr), 50.1 (s, CH2imid), 21.1 (s, CH3), 18.0 (s, CH3) ppm. Anal. Calcd for C21H27AuN2O: C, 48.47; H, 5.23; N, 5.38. Found: C, 48.59; H, 5.36; N, 5.34. [Au(IAd)(OH)]. Procedure B in a 0.040 mmol scale afforded a white, microcrystalline solid (17.6 mg, 80%). 1H NMR (400 MHz, C6D6): δ 6.54 (s, 2H), 2.46 (d, J = 2.6 Hz, 12H), 1.90 (s, 6H), 1.59−1.38 (m, 12H), 0.19 (s, 1H) ppm. 13C{1H} NMR (100 MHz, C6D6): δ 166.8 (s, Ccarbene), 115.0 (s, CHimid), 59.1 (s, NCadam), 44.0 (s, CH2), 36.0 (s, CH2), 30.2 (s, CH2) ppm. Anal. Calcd for C23H33AuN2O: C, 50.18; H, 6.04; N, 5.09. Found: C, 50.29; H, 5.97; N, 4.99. [Au(ICy)(OH)]. Procedure B in a 0.040 mmol scale afforded a white, microcrystalline solid (16.6 mg, 93%). 1H NMR (400 MHz, C6D6): δ 6.47 (s, 2H), 4.66 (tt, J = 12.0 Hz, 3.9 Hz, 2H), 1.85−1.77 (m, 4H), 1.48−1.40 (m, 4H), 1.39−1.32 (m, 2H), 1.22 (qd, J = 12.2 Hz, 3.9 Hz, 4H), 1.01 (qt, J = 12.8 Hz, 3.5 Hz, 4H), 0.85 (qt, J = 12.8 Hz, 3.9 Hz, 2H), 0.42 (s, 1H) ppm. 13C{1H} NMR (100 MHz, C6D6): δ 168.6 (s, Ccarbene), 116.4 (s, CHimid), 60.5 (s, NCH), 33.9 (s CH2), 25.3 (s, CH2), 25.3 (s, CH2) ppm. Anal. Calcd for C15H25AuN2O: C, 40.36; H, 5.65; N, 6.28. Found: C, 40.55; H, 5.49; N, 6.13. [Au(IDD)(OH)]. Procedure B in a 0.040 mmol scale afforded a white, microcrystalline solid (21.2 mg, 86%). 1H NMR (400 MHz, C6D6): δ 6.29 (s, 2H), 5.11 (quin, J = 6.7 Hz, 2H), 1.86−1.59 (m, 10H), 1.39− 1.09 (m, 34H), 0.31 (s, 1H, OH) ppm. 13C{1H} NMR (100 MHz, C6D6): δ 170.0 (s, Ccarbene), 116.9 (s, CHimid), 57.2 (s, CH), 31.4 (s, CH2), 24.3 (s, CH2), 23.9 (s, CH2), 23.6 (s, CH2), 23.3 (s, CH2), 22.0 (s, CH2) ppm. Anal. Calcd for C27H49AuN2O: C, 51.22; H, 7.64; N, 4.42. Found: C, 51.36; H, 7.60; N, 4.46. [Au(ItBu)(OH)]. Procedure B in a 0.038 mmol scale afforded a white, microcrystalline solid (14.1 mg, 94%). 1H NMR (400 MHz, C6D6): δ 6.42 (s, 2H), 1.52 (s, 18H), 0.17 (s, 1H) ppm. 13C{1H} NMR (100 MHz, C6D6): δ 168.3 (s, Ccarbene), 115.6 (s, CHimid), 58.5 (s, NCMe3), 31.3 (s, CH3) ppm. Anal. Calcd for C11H21AuN2O: C, 33.51; H, 5.37; N, 7.11. Found: C, 33.59; H, 5.26; N, 7.21. Characterization data for [Au(IPr)(NTf 2 )], 21b [Au(SIPr)(NTf2)],21b [Au(IPrCl)(NTf2)],25,27 and [Au(IPr*)(NTf2)]26 can be found in the references.

EXPERIMENTAL SECTION

General Information. All reagents were used as received. Deuterochloroform was filtered through a plug of basic alumina to remove trace HCl from the NMR solvent. 1H and 13C spectra were recorded on a Bruker Avance 300, a Bruker Avance II 400 Ultrashield, or a Bruker Avance III 500 spectrometer. Elemental analyses were performed by the London Metropolitan University Elemental Analysis Service. Procedure A for Synthesis of [Au(NHC)(OH)] (ref 7b). Potassium hydroxide (10 equiv) was added to a stirred solution of [Au(NHC)Cl] (1 equiv) in THF (0.1 mol L−1). It was stirred at 30 °C for 20 h and then filtered through Celite. It was concentrated, precipitated by addition of pentane, and collected by filtration. Procedure B for Synthesis of [Au(NHC)(OH)]. In a glovebox, cesium hydroxide (10 equiv) was added to a stirred solution of [Au(NHC)Cl] (1 equiv) in benzene (0.01 mol L−1). It was stirred at 30 °C for 20 h and then filtered through Celite. It was concentrated, precipitated by addition of diethyl ether, and collected by filtration. Synthesis of [Au(NHC)(NTf2)] (ref 7a). A vial was charged with [Au(NHC)(OH)] (0.04 mmol), HNTf2 (excess), and benzene (3 mL). The reaction was stirred at room temperature for 6 h. It was filtered through a silica plug using DCM (3 mL). Solvent was removed under vacuum, and pentane (3 mL) was added. The resulting precipitate was collected on a frit and washed with pentane (3 × 3 mL). The solid was dried under vacuum to afford the product as a microcrystalline, white solid. Characterization Data. [Au(IPrMe)(OH)]. Procedure A in a 0.385 mmol scale afforded a white, microcrystalline solid (115 mg, 47%). 1H NMR (300 MHz, C6D6): δ 7.23 (t, J = 7.5 Hz, 2H), 7.07 (d, J = 7.5 Hz, 4H), 2.54 (sept, J = 7.0 Hz, 4H), 1.49 (d, J = 7.0 Hz, 12H), 1.41 (s, 6H), 1.06 (d, J = 7.0 Hz, 12H), −0.32 (br, 1H) ppm. 13C{1H} NMR (100 MHz, C6D6): δ 170.4 (s, Ccarbene), 146.2 (s, CAr), 133.3 (s, CAr), 130.7 (s, CHAr), 125.5 (s, Cimid), 124.5 (s, CHAr), 29.0 (s, CH), 25.1 (s, CH3), 23.5 (s, CH3), 9.3 (s, CH3) ppm. Anal. Calcd for C29H41AuN2O: C, 55.23; H, 6.55; N, 4.44. Found: C, 55.31; H, 6.65; N, 4.43. [Au(IPrCl)(OH)]. Procedure A in a 1.45 mmol scale afforded a white, microcrystalline solid (700 mg, 72%). 1H NMR (400 MHz, C6D6): δ 7.20 (t, J = 8.0 Hz, 2H), 7.03 (d, J = 8.0 Hz, 4H), 2.58 (sept, J = 6.6 Hz, 4H), 1.42 (d, J = 6.8 Hz, 12H), 1.09 (d, J = 6.8 Hz, 12H), −0.23 (br, 1H) ppm. 13C{1H} NMR (100 MHz, C6D6): δ 175.0 (s, Ccarbene), 146.4 (s, CAr), 131.9 (s, CAr), 131.6 (s, CHAr), 124.7 (s, CHAr), 118.3 (s, CClimid), 29.5 (s, CH), 24.6 (s, CH3), 23.4 (s, CH3) ppm. Anal. Calcd for C27H35AuN2O: C, 48.30; H, 5.25; N, 4.17. Found: C, 48.47; H, 5.37; N, 4.26. [Au(IPr*)(OH)]. Procedure A in a 0.436 mmol scale afforded a white, microcrystalline solid (459 mg, 93%). 1H NMR (500 MHz, CD2Cl2): δ 7.24−7.14 (m, 32H), 6.93 (s, 4H), 6.87 (dd, J = 6.6, 2.9 Hz, 8H), 5.75 (s, 2H), 5.32 (s, 4H), 2.26 (s, 6H). 13C{1H} NMR (126 MHz, CD2Cl2): δ 172.3 (s, Ccarbene), 143.2 (s, CAr), 143.1 (s, CAr), 141.3 (s, CAr), 140.5 (s, NCAr), 134.4 (s, CHAr), 130.5 (s, CHAr), 130.3 (s, CHAr), 130.1 (s, CHAr), 129.6 (s, CHAr), 128.8 (s, CHAr), 127.0 (s, CHAr), 127.0 (s, CHAr), 123.3 (s, CHimid), 51.6 (s, CH), 21.9 (s, CH3) ppm. Anal. Calcd for C69H57AuN2O: C, 73.52; H, 5.10; N, 2.49. Found: C, 73.39; H, 4.97; N, 2.34. [Au(IPr*Tol)(OH)]. Procedure A in a 0.160 mmol scale afforded a white, microcrystalline solid (158 mg, 80%). 1H NMR (500 MHz, CD2Cl2): δ 7.07−7.01 (m, 16H), 6.97 (d, J = 7.9 Hz, 8H), 6.92 (s, 4H), 6.74 (d, J = 7.9 Hz, 8H), 5.79 (s, 2H), 5.22 (s, 4H), 2.30 (s, 12H), 2.28 (s, 12H), 2.26 (s, 6H). 13C{1H} NMR (100 MHz; CD2Cl2): δ 171.8 (s, Ccarbene), 141.6 (s, CAr), 140.4 (s, CAr), 140.4 (s, CAr), 140.2 (s, CAr), 136.5 (s, CAr), 136.5 (s, CAr), 134.3 (s, CAr), 130.2 (s, CHAr), 129.9 (s, CHAr), 129.5 (s, CHAr), 129.3 (s, CHAr), 123.3 (s, CHimid), 50.8 (s, CH), 21.9 (s, CH3), 21.1 (s, CH3), 21.0 (s, CH3) ppm. Anal. Calcd for C77H73AuN2O: C, 74.62; H, 5.94; N, 2.26. Found: C, 74.53; H, 6.02; N, 2.33. [Au(IMes)(OH)]. Procedure B in a 0.060 mmol scale afforded a white, microcrystalline solid (30.6 mg, 98%). 1H NMR (400 MHz, C6D6): δ 6.69 (s, 4H), 5.97 (s, 2H), 2.08 (s, 6H), 1.96 (s, 12H), −0.15

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ASSOCIATED CONTENT

S Supporting Information *

NMR spectra and crystallographic data. 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]. Fax: +44 1334 463 808. Tel: +44 1334 463763. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The ERC (Advanced Investigator Award-FUNCAT), the EPSRC, and Syngenta are gratefully acknowledged for support of this work. Umicore AG is acknowledged for their generous gift of auric acid. S.P.N. is a Royal Society Wolfson Research Merit Award holder. 423


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(18) (a) Gaillard, S.; Nun, P.; Slawin, A. M. Z.; Nolan, S. P. Organometallics 2010, 29, 5402. (b) Nun, P.; Gaillard, S.; Slawin, A. M. Z.; Nolan, S. P. Chem. Commun. 2010, 46, 9113. (c) Brulé, E.; Gaillard, S.; Rager, M.-N.; Roisnel, T.; Guérineau, V.; Nolan, S. P.; Thomas, C. M. Organometallics 2011, 2650. (d) Konkolewicz, D.; Gaillard, S.; West, A. G.; Cheng, Y. Y.; Gray-Weale, A.; Schmidt, T. W.; Nolan, S. P.; Perrier, S. Organometallics 2011, 30, 1315. (e) Nun, P.; Dupuy, S.; Gaillard, S.; Poater, A.; Cavallo, L.; Nolan, S. P. Catal. Sci. Technol. 2011, 1, 58. (f) Nun, P.; Gaillard, S.; Poater, A.; Cavallo, L.; Nolan, S. P. Org. Biomol. Chem. 2011, 9, 101. (g) Patrick, S. R.; Boogaerts, I. I. F.; Gaillard, S.; Slawin, A. M. Z.; Nolan, S. P. Beilstein J. Org. Chem. 2011, 7, 892. (19) Wang, D.; Cai, R.; Sharma, S.; Jirak, J.; Thummanapelli, S. K.; Akhmedov, N. G.; Zhang, H.; Liu, X.; Petersen, J. L.; Shi, X. J. Am. Chem. Soc. 2012, 134, 9012. (20) Pennell, M. N.; Turner, P. G.; Sheppard, T. D. Chem.Eur. J. 2012, 18, 4748. (21) (a) Mézailles, N.; Ricard, L.; Gagosz, F. Org. Lett. 2005, 7, 4133. (b) Ricard, L.; Gagosz, F. Organometallics 2007, 26, 4704. (22) (a) Nolan, S. P.; Slawin, A. M. Z.; Gómez-Suárez, A.; Collado, A.; Martin, A. R. Chem. Commun. 2013, 49, 5541. (b) Visbal, R.; Laguna, A.; Gimeno, M. C. Chem. Commun. 2013, 49, 5642. (23) Parameters used for SambVca calculations: (a) 3.50 Å was selected as the value for the sphere radius, (b) 2.00 Å was considered to be the distance of the metal−ligand bond, (c) hydrogen atoms were omitted, and (d) Bondi radii scaled by 1.17 were used. (24) Dröge, T.; Glorius, F. Angew. Chem., Int. Ed. 2010, 49, 6940. (25) Ramón, R. S. Ph.D. Dissertation, University of St Andrews, 2011. (26) Gómez-Suárez, A.; Ramón, R. S.; Songis, O.; Slawin, A. M. Z.; Cazin, C. S. J.; Nolan, S. P. Organometallics 2011, 30, 5463. (27) Crystallographic data available free from CCDC, ref no. 966017.

ABBREVIATIONS IPr, 1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene; SIPr, 1,3-bis(2,6-diisopropylphenyl)imidazolin-2-ylidene; IPrMe, 4,5dimethyl-1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene; IPrCl, 4,5-dichloro-1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene; IPr*, 1,3-bis(2,6-bis(diphenylmethyl)-4-methylphenyl)imidazol-2-ylidene); IPr*Tol, 1,3-bis(2,6-bis(di-p-tolylmethyl)4-methylphenyl)imidazol-2-ylidene); IMes, 1,3-bis(2,4,6trimethylphenyl)imidazol-2-ylidene; SIMes, 1,3-bis(2,4,6trimethylphenyl)imidazolin-2-ylidene; ItBu, 1,3-bis(tert-butyl)imidazol-2-ylidene; IAd, 1,3-bis(adamantyl)imidazol-2-ylidene; ICy, 1,3-bis(cyclohexyl)imidazol-2-ylidene; IDD, 1,3-bis(cyclododecyl)imidazol-2-ylidene

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REFERENCES

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NOTE ADDED AFTER ASAP PUBLICATION Due to a production error, an uncorrected version of the manuscript was published on December 11, 2013. The corrected version reposted on Dec 16, 2013.

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