Article pubs.acs.org/Organometallics
Understanding the Reactivity Difference of Isocyanate and Isothiocyanate toward a Ruthenium Silylene Hydride Complex Hujun Xie†,‡ and Zhenyang Lin*,† †
Department of Chemistry, The Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong, People’s Republic of China ‡ Department of Applied Chemistry, Zhejiang Gongshang University, Hangzhou 310035, People’s Republic of China S Supporting Information *
ABSTRACT: The detailed reaction mechanisms of the CO hydrosilylation of isocyanate and the CS bond cleavage of isothiocyanate mediated by the neutral ruthenium silylene hydride complex Cp*(CO)(H)RuSi(H){C(SiMe3)3} have been investigated with the aid of density functional theory calculations. Through the investigation, the difference in reactivity between isocyanate and isothiocyanate toward the ruthenium silylene hydride complex has been examined and discussed. The different bond strengths and π-accepting abilities of CO and CS and the different degrees of affinity of O and S toward the Si center in the silylene ligand contribute to the different reactivities of the isocyanate and isothiocyanate substrates observed experimentally.
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INTRODUCTION Transition-metal silylene complexes, as silicon analogues of metal carbenes, have attracted considerable research interest over the past few years.1 These complexes play an important role in metal-catalyzed transformations of organosilicon substrates.2 Extensive studies have indicated that these complexes show many interesting reactivities and react with varieties of unsaturated organic substrates.3−5 For example, the cationic ruthenium silylene complex [Cp*(PMe3)2RuSiMe2]+ reacted with RNCO (R = Me, Ph) to give [2π + 2π] cycloaddition products between the RuSi and CO moieties.3a The cationic ruthenium silylene hydride complex [Cp*(PiPr3)2(H)2RuSi(H)Ph]+ was reported to catalyze hydrosilylation of alkenes, where the catalytic process was proposed to occur via a [2σ + 2π] addition between the Si−H σ bond of the silylene ligand and the alkene π bond.3b The neutral ruthenium silylene hydride complex Cp*(CO)(H)RuSi(H){C(SiMe3)3} reacted with RCN (R = Me, Ph) to give Cp*(CO)Ru{C(R) NSiH2{C(SiMe3)3}, which can be formally considered as a [2σ + 2π] addition product between the Ru−SiH2{C(SiMe3)3} σ bond (formed via a hydride migration from the metal center to the silylene Si center) and one RCN π bond.4a The tungsten silylene hydride complex Cp*(CO) 2 (H)WSi(H){C(SiMe3)3} reacted with MeCN to give a [2σ + 2π] addition between the W−H bond and one RCN π bond followed by coordination of the N lone pair of the resulting −NC(H)Me ligand to the silylene Si center.5a When it reacted with acetone, [2σ + 2π] addition between the Si−H σ bond of the silylene ligand instead and the acetone CO π bond was observed.5b However, when it reacted with α,β-unsaturated carbonyl compounds, there © XXXX American Chemical Society
was a formal [2σ + 4π] conjugate addition of the W− SiH2{C(SiMe3)3} σ bond (similar to what was mentioned above for the reaction of Cp*(CO)(H)RuSi(H){C(SiMe3)3} with RCN, formed via a hydride migration from the metal center to the silylene Si center) across the CCCO moiety.5c Computational studies were also carried out to understand some of the reactions mentioned above. For example, a few theoretical investigations examined and provided support to the proposed mechanism involving [2σ + 2π] addition between the Si−H σ bond of the silylene ligand and the alkene π bond for the hydrosilylation of alkenes catalyzed by the cationic ruthenium silylene complex [Cp*(PiPr3)2(H)2RuSi(H)Ph]+.3b,6 DFT studies were carried out to understand the reactivity difference between MeCN and MeC(O)Me toward the tungsten silylene hydride complex Cp*(CO)2(H)WSi(H){C(SiMe3)3}.7 It was found that MeCN is sterically less hindered and has a poorer electrophilic cyanide carbon center, and thus cyanide insertion into a tungsten−silyl bond (formed via hydride migration from the metal center to the silylene Si center) was experimentally observed. Theoretical calculations have also been performed to explore the reaction mechanism between the tungsten silylene hydride complex and α,β-unsaturated carbonyl compounds.8 The results indicated that the favorable reaction pathway corresponds to [2π + 4π] conjugate cycloaddition followed by Si−H reductive elimination. In this paper, we are interested in the recently reported reactions of the neutral ruthenium silylene hydride complex Received: October 18, 2013
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dx.doi.org/10.1021/om401020g | Organometallics XXXX, XXX, XXX−XXX
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To obtain solvation-corrected releative free energies, we employed a continuum medium to do single-point energy calculations for all of the optimized species, using UAHF radii on the conductor-like polarizable continuum model (CPCM).16 Hexane was used as the solvent, according to the experimental reaction conditions. We also applied a correction of −2.6 (or 2.6) kcal/mol in the estimation of relative free energies for 2:1 (or 1:2) transformations to correct the overestimation of entropy contribution from the gas-phase calculation results as many earlier theoretical studies did.17 To reduce the computational cost, the tris(trimethylsilyl)methyl (C(SiMe3)3) group was replaced by the trisilylmethyl (C(SiH3)3) group. This simplication was also tested for the rate-determining transition states of the three pathways studied, shown in Figure 1. Using the model group, TS23O, TS56O, and TS89O are respectively 18.1, 33.9, and 21.8 kcal/mol with respect to the energy reference point. Using the real tris(trimethylsilyl)methyl (C(SiMe3)3) group, TS23O, TS56O, and TS89O are respectively 23.4, 39.6, and 26.1 kcal/mol with respect to the energy reference point. As expected, the barriers calculated using the full tris(trimethylsilyl)methyl group are ca. 5 kcal/mol higher than those calculated using the small model group. However, the changes in the relative barriers, which are the most important and crucial numbers in our discussion, are almost negligible and have little effect on the conclusions we have made. In all of the figures that contain potential energy profiles, solvationand entropy-corrected relative energies (kcal/mol) (free energies and electronic energies in parentheses) are presented. In this paper, the corrected relative free energies are used to analyze the reaction mechanism. All calculations were performed with the Gaussian 03 software package.18
Cp*(CO)(H)RuSi(H){C(SiMe3)3} with heterocumulenes (MesNCO and MesNCS; Mes = 2,4,6-trimethylphenyl) (eqs 1
and 2).9 In the reaction with MesNCO, hydrosilylation of the carbonyl group was observed. However, in the reaction with MesNCS, cleavage of the CS double bond was found. Continuing our interest in the reactions of metal silylene complexes, we carried out DFT studies on the reaction mechanisms of these two reactions and hoped to provide theoretical insight into the reactivity difference of the two heterocumulenes toward the ruthenium silylene hydride complex.
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COMPUTATIONAL DETAILS
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The molecular geometries of the reactants, intermediates, transition states, and products were fully optimized via density functional theory (DFT) calculations at the Becke3LYP (B3LYP) level.10 The reliability of the chosen method has been verified by our previous work7 and other theoretical studies of Ru-catalyzed reactions.11 In the DFT calculations, the 6-31g(d,p) basis set was used for the C, O, N, H, and S atoms, while the effective core potentials (ECPs) of Hay and Wadt with double-ζ valence basis set (LanL2DZ)12 were chosen to describe the Ru and Si atoms. Polarization functions were also added: Ru(ζf) = 1.23513 and Si(ζd) = 0.262.14 Frequency analyses have been performed to obtain the Gibbs free energies and identify all of the stationary points as minima (zero imaginary frequency) or transition states (one imaginary frequency) on the potential energy surfaces (PES). Intrinsic reaction coordinate (IRC) calculations were also calculated for the transition states to confirm that such structures indeed connect two relevant minima.15
RESULTS AND DISCUSSION As mentioned in the Introduction, our main objective is to understand the reactivity difference of the two heterocumulenes MesNCO and MesNCS toward the ruthenium silylene hydride complex. Therefore, let us first look into the possible reaction pathways proposed by Tobita and co-workers in their paper (Scheme 1).9 In the proposed pathway for the reaction of MesNCO, MesNCO first coordinates through its oxygen atom to the silylene Si center to form an acid−base adduct. From the adduct, a hydride migration from the Ru center to the MesNCO carbonyl carbon, followed by a 180° bond rotation along the C− O bond and then N coordination to the metal center, gives the experimentally observed reaction product.
Scheme 1. The Possible Reaction Pathways Proposed by Tobita and Co-Workers for the Reactions of Isocyanate and Isothiocyanate with the Neutral Ruthenium Silylene Hydride Complex
B
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Figure 1. Free energy profiles calculated for the reaction of isocyanate with the neutral ruthenium silylene hydride complex Cp*(CO)(H)Ru Si(H){C(SiH3)3} (1). The relative free energies and relative electronic energies (in parentheses) are given in kcal/mol.
silyl intermediate, a subsequent silyl migration to the sulfur atom of the η2-coordinated unit gives the Ru−S−C three-memberedring intermediate. Finally, oxidative addition of the C−S bond to the metal center cleaves the C−S bond to form the experimentally observed isocyanide product. In path 2, the first
Two possible reaction pathways (paths 1 and 2 in Scheme 1) were proposed for the reaction of MesNCS. In path 1, a 1,2hydride migration from the Ru center to the silylene Si center accompanied by the coordination of MesNCS to the metal center in an η2-CS fashion gives a silyl intermediate. From the C
dx.doi.org/10.1021/om401020g | Organometallics XXXX, XXX, XXX−XXX
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Figure 2. Free energy profiles calculated for the reaction of isothiocyanate with the neutral ruthenium silylene hydride complex Cp*(CO)(H)Ru Si(H){C(SiH3)3} (1). The relative free energies and relative electronic energies (in parentheses) are given in kcal/mol.
step also involves the coordination of MesNCS to the silylene Si center to form an acid−base adduct. From the adduct, instead of a hydride migration, a [2π + 2π] cycloaddition between the Ru Si and CS moieties occurs to give a four-membered-ring intermediate, from which a reductive elimination results in extrusion of the SiH2C(SiMe3)3 group, coordination of S to the
metal center, and formation of the Ru−S−C three-memberedring intermediate (an intermediate appeared in path 1 also). The final step (C−S oxidative addition) to the experimentally observed product is the same as that in path 1. Mechanism for the Reaction of MesNCO. We first examined the mechanism for the reaction of MesNCO with the D
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Si−O, we found that 1 + MesCNS → 4S is less exergonic by 6.8 kcal/mol than 1 + MesCNO → 4O (−13.9 kcal/mol versus −20.7 kcal/mol). The overall reaction barrier calculated for the former is higher by 3.9 kcal/mol than that calculated for the latter (22.0 kcal/mol versus 18.1 kcal/mol). In path B (Figure 2b), formation of the η2-CS intermediate 5S is endergonic by 5.3 kcal/mol and requires overcoming a small free energy barrier of 13.3 kcal/mol via TS15S. This process is much easier, however, in comparison with formation of the corresponding η2-CO intermediate 5O in the reaction of MesNCO (Figure 1b). The results are consistent with the notion that the CS moiety in MesNCS is a much stronger πacceptor than the CO moiety in MesNCO, giving rise to a stronger bonding interaction of Ru−η2-CS versus Ru−η2-CO. The step followed immediately after the formation of the η2-CS intermediate 5S is the [2σ + 2π] addition. This [2σ + 2π] addition leads to complete CS bond cleavage to give 7S, which is a model product observed in the experiments. In theory, the conversion from 5S via the transition state TS57S should give a four-membered-ring intermediate before formation of 7S through C−S oxidative addition. However, the four-membered-ring species does not correspond to a stationary point in the potential energy surface, due to its instability. In the reaction of MesNCO (Figure 1b), formation of 7O from 5O is a two-step process: [2σ + 2π] addition to give 6O followed by C−O cleavage to give 7O. The difference here is clearly related to the fact that the CO bond is much stronger than the CS bond. Because of the differences in the bond strengths and the π-accepting properties between CS and CO, 1 + MesNCS → 7S is much more exergonic by 11.8 kcal/mol than 1 + MesNCO → 7O (−27.4 kcal/mol versus −15.6 kcal/mol). The overall reaction barrier for the former is much smaller (16.2 kcal/mol) than that for the latter (33.9 kcal/mol). The calculation results show that path B for the reaction of isothiocyanate is both thermodynamically and kinetically more favorable than path A. In path C (Figure 2c), the [2π + 2π] addition leading to the formation of the four-membered-ring intermediate 9S is a onestep process. This [2π + 2π] addition requires overcoming a barrier of 24.9 kcal/mol, which is greater than the barrier (21.8 kcal/mol) calculated for the corresponding [2π + 2π] addition in the reaction of MesNCO. The difference is again related to the fact that silicon is oxophilic, but not chalcophilic. The step followed corresponds to the C−S bond cleavage, which requires a very small barrier. In this pathway, the rate-determining step is the [2π + 2π] addition. Therefore, path C is less favored for the reaction of MesNCS than for the reaction of MesNCO.
ruthenium silylene hydride complex 1 according to the proposal shown in Scheme 1. Figure 1a shows the free energy profile calculated (path A). The coordination of isocyanate to the silylene Si center followed by the hydride migration to the CO carbon center gives an agostic intermediate (3O), which requires overcoming a free energy barrier of 18.1 kcal/mol. Finally, a very small barrier was calculated for the 180° bond rotation along the C−O bond to give 4O, a model for the experimentally observed reaction product. The overall reaction is highly exergonic with a reaction free energy of −20.7 kcal/mol. The calculation results clearly support the proposed mechanism shown in Scheme 1 for the reaction of MesNCO. It should be noted here that the same mechanism has also been proposed and calculated for the reaction of a tungsten germylene hydride complex with ArNCO (Ar = Mes, Ph).19 As mentioned in the Introduction, the reaction of MesNCS gives a product in which the CS bond is cleaved (eq 2). Therefore, for the reaction of MesNCO, we also calculated a possible pathway similar to path 1 proposed for the reaction of MesNCS shown in Scheme 1 to examine the energetic aspect related to the experimentally unobserved CO cleavage of MesNCO. Figure 1b shows the free energy profile calculated for this possible path. Here, we label this as path B. The hydride migrates to the silylene Si center accompanying an η 2 coordination of MeNCO through the CO unit to give the silyl intermediate 5O. Then a [2σ + 2π] addition gives 6O. The overall free energy barrier for these two steps is 33.9 kcal/mol. From 6O, oxidative addition with a very small barrier cleaves the C−O bond to give the CO cleaved product 7O. The instability of the η2-coordinated intermediate 5O and the fact that the C O π bond is strong makes this pathway inaccessible. According to Scheme 1, there is another pathway (path 2, Scheme 1) proposed for the reaction of MesNCS. For the reaction of MesNCO, we also calculated this possible pathway (labeled as path C). Figure 1c shows the energy profile calculated. Similar to path A, the MesNCO oxygen atom is initially coordinated to the electron-deficient silylene silicon atom but from a direction opposite to the hydride ligand to afford the base-stabilized adduct 8O, and this step is endergonic by 10.5 kcal/mol. From 8O, a [2π + 2π] cycloaddition between the Ru Si and CO moieties becomes possible, which takes place to form the four-membered-ring intermediate 9O. An free energy barrier of 21.8 kcal/mol was calculated for the overall [2π + 2π] cycloaddition step from 1 + MesNCO. Finally, a hydride migration from the Ru atom to the Si atom, which accompanies the C−O cleavage, forms the CO cleaved product 7O. This hydride migration step is facile. The calculation results indicate that path C is kinetically less favorable in comparison with path A. Mechanism for the Reaction of MesNCS. To understand the difference in the reactivity of MesNCS in comparison to that of MesNCO, we also calculated the three pathways discussed above (paths A−C) for the reaction of isothiocyanate with the ruthenium silylene hydride complex 1. Figure 2 shows the corresponding energy profiles calculated. In path A (Figure 2a), formation of the corresponding agostic species 3S, which corresponds to a [3 + 2] addition, is a one-step process. No corresponding adduct intermediate (2S) was found, unlike in the reaction of MesNCO (path A, Figure 1a), where an adduct intermediate structure (2O) was located. This result is understandable because silicon is oxophilic, but not chalcophilic. Similar to what we found in the reaction of MesCNO, from 3S with a very small barrier, a 180° bond rotation along the C−S bond gives 4S. Because of the weaker interaction of Si−S versus
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SUMMARY With the aid of DFT calculations, the difference in reactivity between MesNCO and MesNCS toward the ruthenium silylene hydride complex Cp*(CO)(H)RuSi(H){C(SiMe3)3} has been investigated. In the two organic substrates, the CO bond of MesNCO is significantly stronger than the CS bond of MesNCS, and as a result, the η2-CO group is a much weaker πacceptor than the η2-CS group. In addition, the oxygen atom of the CO moiety in MesNCO shows a much greater affinity toward the silicon center than the sulfur atom of the CS moiety in MesNCS. In the reaction of MesNCO, coordination of MesNCO through the oxygen atom to the silylene Si center followed by hydride migration breaks the CO π bond. Then a simple rotation along the resulting C−O single bond gives the experimentally observed product (4O). This reaction pathway E
dx.doi.org/10.1021/om401020g | Organometallics XXXX, XXX, XXX−XXX
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(8) Bi, S. W.; Liu, Y. X.; Chen, G.; Zhang, L. S.; Sun, M.; Li, P. J. Organomet. Chem. 2009, 694, 3456. (9) Ochiai, M.; Hashimoto, H.; Tobita, H. Organometallics 2012, 31, 527. (10) (a) Becke, A. D. J. Chem. Phys. 1993, 98, 5648. (b) Lee, C.; Yang, W.; Parr, R. G. Phys. Rev. B 1988, 37, 785. (c) Becke, A. D. J. Chem. Phys. 1993, 98, 1372. (d) Becke, A. D. Phys. Rev. B 1988, 38, 3098. (11) (a) Chung, L. A.; Wu, Y.; Trost, B. M.; Ball, Z. T. J. Am. Chem. Soc. 2003, 125, 11578. (b) Sakaki, S.; Sumimoto, M.; Fukuhara, M.; Sugimoto, M.; Fujimoto, H.; Matsuzaki, S. Organometallics 2002, 21, 3788. (c) Liu, P.; Xu, X. F.; Dong, X. F.; Keitz, B. K.; Herbert, M. B.; Grubbs, R. H.; Houk, K. N. J. Am. Chem. Soc. 2012, 134, 1464. (d) Nunez-Zarur, F.; Solans-Monfort, X.; Rodriguez-Santiago, L.; Pleixats, R.; Sodupe, M. Chem. Eur. J. 2011, 17, 7506. (e) Stewart, B.; Nyhlen, J.; Martin-Matute, B.; Backvall, J. E.; Privalov, T. Dalton Trans. 2013, 42, 927. (12) (a) Check, C. E.; Faust, T. O.; Bailey, J. M.; Wright, B. J.; Gilbert, T. M.; Sunderlin, L. S. J. Phys. Chem. A 2001, 105, 8111. (b) Hay, P. J.; Wadt, W. R. J. Chem. Phys. 1985, 82, 299. (13) Ehlers, A. W.; Böhme, M.; Dapprich, S.; Gobbi, A.; Höllwarth, A.; Jonas, V.; Köhler, K. F.; Stegmann, R.; Veldkamp, A.; Frenking, G. Chem. Phys. Lett. 1993, 208, 111. (14) Huzinaga, S. Gaussian Basis Sets for Molecular Calculations; Elsevier Science: Amsterdam, 1984. (15) (a) Fukui, K. J. Phys. Chem. 1970, 74, 4161. (b) Fukui, K. Acc. Chem. Res. 1981, 14, 363. (16) (a) Barone, V.; Cossi, M. J. Phys. Chem. A 1998, 102, 1995. (b) Cossi, M.; Rega, N.; Scalmani, G.; Barone, V. J. Comput. Chem. 2003, 24, 669. (17) (a) Benson, S. W. The Foundations of Chemical Kinetics; Krieger: Malabar, FL, 1982. (b) Okuno, Y. Chem. Eur. J. 1997, 3, 212. (c) Ardura, D.; López, R.; Sordo, T. L. J. Phys. Chem. B 2005, 109, 23618. (d) Schoenebeck, F.; Houk, K. N. J. Am. Chem. Soc. 2010, 132, 2496. (e) Liu, Q.; Lan, Y.; Liu, J.; Li, G.; Wu, Y. D.; Lei, A. J. Am. Chem. Soc. 2009, 131, 10201. (f) Ariafard, A.; Brookes, N. J.; Stanger, R.; Yates, B. F. Organometallics 2011, 30, 1340. (g) Yu, H.; Lu, Q.; Dang, Z.; Fu, Y. Chem. Asian J. 2013, 8, 8. (h) Ariafard, A.; Ghohe, N. M.; Abbasi, K. K.; Canty, A. J.; Yates, B. F. Inorg. Chem. 2013, 52, 707−717. (18) Frisch, M. J., et al. Gaussian 03, revision E.01; Gaussian, Inc., Pittsburgh, PA, 2003. (19) Hashimoto, H.; Fukuda, T.; Tobita, H.; Ray, M.; Sakaki, S. Angew. Chem., Int. Ed. 2012, 51, 2930.
is the most favorable, manifesting the strong affinity of oxygen toward the silyene Si center and the strong CO bond of the substrate. In the reaction of MesNCS, a pathway similar to that for the reaction of MesNCO (initiated through coordination to the silylene Si center via the S atom) is also possible, although the reaction barrier is relatively higher (in comparison to that calculated for the reaction of MesNCO), which is expected because the affinity of sulfur toward the silylene Si center is poorer. However, the calculation results show that the pathway, which starts with a simultaneous process of both η2-CS coordination to the Ru center and hydride migration from the Ru center to the silylene center and then a [2σ + 2π] addition leading to complete CS bond cleavage, is the most favorable. This result is closely related to the fact that an η2-CS group is a strong π acceptor and the CS bond is significantly weak.
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ASSOCIATED CONTENT
* Supporting Information S
Text giving the complete ref 18, tables giving Cartesian coordinates and electronic energies for all of the calculated structures, and a .xyz file giving one of the calculated structures. 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 for Z.L.:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by the Research Grants Council of Hong Kong (HKUST603711) and the National Natural Science Foundation of China (21203166).
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REFERENCES
(1) (a) Pannell, K. H.; Cervantes, J.; Hernandez, C.; Cassias, J.; Vincenti, S. Organometallics 1986, 5, 1056. (b) Tilley, T. D. Comments Inorg. Chem. 1990, 10, 37. (c) Yamashita, H.; Tanaka, M.; Goto, M. Organometallics 1992, 11, 3227. (d) Pestana, D. C.; Koloski, T. S.; Berry, D. H. Organometallics 1994, 13, 4173. (e) Tanaka, Y.; Yamashita, H.; Tanaka, M. Organometallics 1995, 14, 530. (f) Okazaki, M.; Tobita, H.; Ogino, H. Dalton Trans. 2003, 493. (g) Tilley, T. D. Acc. Chem. Res. 2007, 40, 712. (2) (a) Sharma, H. K.; Pannell, K. H. Chem. Rev. 1995, 95, 1351. (b) Klei, S. R.; Tilley, T. D.; Bergman, R. G. Organometallics 2002, 21, 4648. (c) Waterman, R.; Hayes, P. G.; Tilley, T. D. Acc. Chem. Res. 2007, 40, 712. (3) (a) Mitchell, G. P.; Tilley, T. D. J. Am. Chem. Soc. 1997, 119, 11236. (b) Glaser, P. B.; Tilley, T. D. J. Am. Chem. Soc. 2003, 125, 13640. (4) (a) Ochiai, M.; Hashimoto, H.; Tobita, H. Angew. Chem., Int. Ed. 2007, 46, 8192. (b) Ochiai, M.; Hashimoto, H.; Tobita, H. Dalton Trans. 2009, 1812. (5) (a) Watanabe, T.; Hashimoto, H.; Tobita, H. J. Am. Chem. Soc. 2006, 128, 2176. (b) Watanabe, T.; Hashimoto, H.; Tobita, H. Chem. Asian. J. 2012, 7, 1408. (c) Watanabe, T.; Hashimoto, H.; Tobita, H. J. Am. Chem. Soc. 2007, 129, 11338. (6) (a) Beddie, C.; Hall, M. B. J. Am. Chem. Soc. 2004, 126, 13564. (b) Hayes, P. G.; Beddie, C.; Hall, M. B.; Waterman, R.; Tilley, T. D. J. Am. Chem. Soc. 2006, 128, 428. (c) Beddie, C.; Hall, M. B. J. Phys. Chem. A 2006, 110, 1416. (d) Böhme, U. J. Organomet. Chem. 2006, 691, 4400. (7) Zhang, X. H.; Chung, L. W.; Lin, Z. Y.; Wu, Y. D. J. Org. Chem. 2008, 73, 820. F
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