Article pubs.acs.org/Organometallics
Mechanism of Z‑Selective Olefin Metathesis Catalyzed by a Ruthenium Monothiolate Carbene Complex: A DFT Study John W. Nelson,†,§ Lara M. Grundy,†,§ Yanfeng Dang,‡ Zhi-Xiang Wang,*,‡ and Xiaotai Wang*,† †
Department of Chemistry, University of Colorado Denver, Campus Box 194, P.O. Box 173364, Denver, Colorado 80217-3364, United States ‡ College of Chemistry and Chemical Engineering, University of the Chinese Academy of Sciences, Beijing 100049, People’s Republic of China S Supporting Information *
ABSTRACT: A ruthenium monothiolate carbene complex (2cat) readily derived from the Grubbs−Hoveyda system is among the newly developed catalysts for Z-selective olefin metathesis reactions. We have performed density functional theory calculations (B3LYP and M06) to elucidate the detailed mechanism of 2cat-catalyzed homometathesis of terminal olefins. The five-coordinate 2cat dissociates to a tetrahedral intermediate, from which two consecutive metathesis events via the bottom-bound olefin attack mechanism lead to (Z)-olefins as major products. The Z selectivity stems from the bulky thiolate ligand, which sterically forces both olefinic substituents to the far side of the metallacyclobutane ring to achieve a Z geometry in the resulting olefin product.
■
INTRODUCTION lefin metathesis effected by molecularly defined ruthenium catalysts has been one of the most significant developments in organometallic catalysis and synthetic methodology over the last two decades.1−5 This powerful method for creating carbon−carbon bonds has had an enormous and lasting impact on the practice of synthetic chemistry and polymer science. A recent advance in this ongoing and active field has been the discovery of new ruthenium complexes capable of catalyzing Z-selective olefin metathesis, and such stereoselective catalysts could enable facile syntheses of complex natural products and stereoregular polymers. After the original work by Schrock, Hoveyda, and co-workers on Mo- and W-based catalysts for Z-selective olefin metathesis,6−9 the Grubbs group reported a class of ruthenium catalysts (1cat) involving an N-heterocyclic carbene (NHC)− admantyl chelator (Figure 1), which exhibits high Z selectivity in various olefin metathesis reactions.10−14 We and others have performed DFT computations to elucidate the mechanism and origin of Z selectivity for olefin metathesis reactions achieved by the 1cat-class catalysts.15−17 These studies show that the bulky N-substituted NHC chelator causes 1cat to favor the side-bound over the bottom-bound olefin attack mechanism for both the initiation and coupling metatheses, which ultimately leads to the Z selectivity. Jensen and co-workers later synthesized 2cat, the first of the Z-selective ruthenium catalysts with thiolate or dithiolate ligands,18,19 via a facile one-step substitution of 2,4,6-triphenylbenzenethiolate for one of the chloride ligands in the commercially available Grubbs− Hoveyda catalyst (A).20 The applications of 2cat have been in the Z-selective homometathesis of terminal alkenes, with the
O
© 2014 American Chemical Society
Figure 1. Ru-based catalysts 1cat and 2cat for Z-selective olefin metathesis reactions, where mes = 2,4,6-trimethylphenyl (or mesityl) and R = Me, i-Pr, etc. The standard Grubbs−Hoveyda catalyst (A) is also shown for reference.
Z selectivity deriving from the bulkiness of the arenethiolate ligand according to some DFT calculations done on the coupling segment of the metathesis reaction.21 This readily available catalyst system warrants further development, and for that reason, it is important to elucidate the detailed mechanism of 2cat-catalyzed olefin metathesis. While this Article was in preparation, a report appeared in the literature that contains a simplified DFT treatment for the coupling segment of the Zselective homodimerization of allylbenzene with 2cat.22 The authors postulated a ruthenium benzylidene intermediate as the active species without investigating the initiation of 2cat. Then they used this complex in coupling metathesis with allylbenzene via the bottom-bound pathway and located the selectivityReceived: June 9, 2014 Published: August 1, 2014 4290
dx.doi.org/10.1021/om500612r | Organometallics 2014, 33, 4290−4294
Organometallics
Article
Paths I1 and I2 lead to ruthenium ethylidene complexes 5 and 9 (Figure 3). Path I1 begins with bottom-bound propene attack on 1, forming a five-coordinate π complex (2), which cycloadds rapidly via TS2 to afford the trigonal-bipyramidal metallacyclobutane 3. The structure of 2 already resembles that of TS2, which accounts for the small activation free energy for the cycloaddition step (0.2 kcal/mol). Cycloreversion of 3 via TS3 is also facile with a small activation free energy of 1.3 kcal/mol, affording the new ruthenium ethylidene complex 4 with πbound metathesized olefin R1CHCH2, which progresses to 5 after dissociation of R1CHCH2. Path I2 is analogous to path I1 and leads to complex 9. The highest kinetic barriers in path I1 (TS2) and path I2 (TS4) are both transition states of cycloaddition, with TS2 being lower than TS4 by 1.7 kcal/mol. This energy gap originates from the different orientations of the propene methyl substituent toward the bulky 2,4,6-triphenylbenzene group (R2) of the arenethiolate ligand. In TS2 the methyl points away from R2, whereas in TS4 it points closer toward R2, causing stronger steric repulsion that destabilizes TS4. This steric bias first occurs to 2 and 6, the precursors to TS2 and TS4, and continues to differentiate the energies of the ensuing complexes (3 vs 7, 4 vs 8, and 5 vs 9) and transition states (TS3 vs TS5). As a result, path I1 is more favorable than path I2 by 1.7 kcal/mol kinetically and by 4.9 kcal/mol thermodynamically. Of the two pathways (paths I3 and I4) leading to a ruthenium methylidene complex, path I3 is more favorable (Figure S1, Supporting Information). However, the overall kinetic barrier of path I3 (TS20) is higher than that of path I1 (TS2) by 1.3 kcal/mol, because in TS20 the propene methyl substituent is adjacent to the bulky o-isopropoxyphenyl group (R1), which induces significant steric hindrance. Furthermore, path I1 has a thermodynamic preference of 5.6 kcal/mol over path I3. Thus, on the basis of analyzing the free energy profiles for the pathways of the initial metathesis event, we consider the ethylidene complex 5 to be the plausible ruthenium carbene product from the initiation of 2cat, which would function as the active catalytic species in the following coupling metathesis.42 This result can also be extended to support the hypothesis of the aforementioned DFT study on the 2cat−allylbenzene system that a ruthenium benzylidene complex would arise from the initiation metathesis.22 Furthermore, similarities can be drawn with the initiation metathesis of 1cat with terminal olefins (CH2CHR), which favors the RuCHR carbene over the methylidene (RuCH2) product notwithstanding occurring by the side-bound olefin attack mechanism, as shown by our previous studies.15,16 It is also worth noting that the overall kinetic barrier TS2 (ΔG⧧ = 26.6 kcal/mol) for the initiation of 2cat is higher than that for the 1cat system (ΔG⧧ ≤ 24 kcal/mol).15,16 An insight into this is the destabilization of TS2 by the steric repulsion between the bulky 2,4,6triphenylbenzenethiolate and o-isopropoxybenzylidene ligands. Experimentally, the metathesis reactions effected by 2cat require somewhat higher temperatures.18 This steric effect can also explain why the initiation barrier of 2cat is 8 kcal/mol higher in comparison with that of the dichloride A,40 as there is much less steric hindrance in A than in 2cat. The continuing coupling metathesis of complex 5 with propene could lead to (Z)- and (E)-2-butene, for which we have considered the possibilities of both bottom-bound and side-bound olefin attack on the ruthenium center (Figure 4 and Figure S3 (Supporting Information)). The favored bottombound pathways are shown here by the free energy profiles in
controlling transition states of cycloreversion of the metallacyclobutanes. The work did not treat the regeneration of the active catalyst, nor did it explore any side-bound olefin attack pathways. In this Article, we present a detailed DFT study of the complete mechanism of the homometathesis of terminal olefins with 2cat beginning with precatalyst initiation, using propene as a model for the substrate octene (Scheme 1). As with our Scheme 1. Homodimerization of Propene with 2cat
studies on the 1cat system,15,16 B3LYP23 and a mixed basis set of SDD24 for Ru and 6-31G(d) for other atoms were used in geometry optimizations, and single-point energies in THF solution were calculated with M0625/SDD-6-311++G(d,p)/ SMD.26 This B3LYP/M06 combined method gives facile geometric optimizations and refined energies for organotransition metal complexes, and it has been applied successfully by numerous studies.15−17,27−33 All calculations were performed with Gaussian 09.34
■
RESULTS AND DISCUSSION The second-generation Grubbs catalyst, to which the Grubbs− Hoveyda catalyst (A) is related, prefers the bottom-bound over the side-bound olefin attack pathway on the basis of several computational studies.35−38 The Grubbs−Hoveyda catalyst (A) has been shown by experimental and computational studies to initiate olefin metathesis by the dissociative or interchange mechanism, in which the olefin substrate attacks ruthenium from the bottom site of coordination.39−41 Although 2cat is structurally analogous to A, for the initiation metathesis of 2cat with propene, we did not find a six-coordinate associative or interchange transition state because the large arenethiolate ligand prevents propene from binding to ruthenium while the other large isopropoxy ligand is still attached. We explored the dissociative pathway beginning with the rupture of the relatively weak Ru−O(isopropoxy) bond of 2cat via C(carbene)− C(ipso) bond rotation and located the transition state (TS1) which proceeds to the four-coordinate tetrahedral intermediate 1 (Figure 2). We have considered the four possible π coordinations of propene (an unsymmetrical olefin) with the ruthenium center of 1 via bottom-bound attack, from which four pathways arise.
Figure 2. Free energy profile for the dissociation of 2cat. In this and subsequent figures, bond distances are given in Å. 4291
dx.doi.org/10.1021/om500612r | Organometallics 2014, 33, 4290−4294
Organometallics
Article
Figure 3. Free energy profiles for the initiation pathways leading to the Ru−ethylidene complexes 5 (in red) and 9 (in blue).
Figure 4. Free energy profiles for the homocoupling pathways leading to (Z)-2-butene (in red) and (E)-2-butene (in blue).
metallacyclobutane, with the barrier TS9 (ΔG⧧ = 25.3 kcal/ mol). Thus, TS7 and TS9 are the selectivity-determining energy barriers, and the difference (TS9 − TS7 = 1.2 kcal/mol) gives a calculated Z selectivity of 88%, which agrees qualitatively with the experimental values (75−86%) for octene.18,43 These results are consistent with those computed with the 2cat− allylbenzene system, for which cycloreversion of the metallacyclobutanes is also the selectivity-determining step and the difference between the Z/E-selective barriers is 1.0 kcal/mol.22 It is also worth noting that the relative energy of TS7 (24.1 kcal/mol) is comparable to that of the corresponding turnoverlimiting transition state (24.0 kcal/mol) for the Z-selective coupling metathesis performed by the 1cat-type catalyst with a bidentate nitrate ligand (see Figure 1).16 Thus, the bulky arenethiolate ligand does not appear to reduce the activity of the real catalyst 5 following the departure of the other bulky ligand o-isopropoxybenzylidene in the initiation phase. This
Figure 4. The Z-selective path C1 begins with propene coordination to 5, forming a five-coordinate complex (10) in which the two methyl groups are in a parallel orientation and project out of the plane of the paper. Complex 10 cycloadds via TS6 to give the metallacyclobutane 11, which has both methyl substituents on the same side of the ring. Cycloreversion of 11 via TS7 leads to the five-coordinate Ru−methylidene complex 12 with the product (Z)-olefin π-bound to ruthenium, which then releases (Z)-2-butene and converts to the four-coordinate complex 13. The transition state of the cycloreversion TS7 (ΔG⧧ = 24.1 kcal/mol) is the turnover-limiting barrier of path C1. The E-selective path C2 is analogous to path C1, going through metallacyclic transition states and intermediates (TS8, 15, TS9, and 16) that each have the two methyl groups on opposite sides of the metallacyclobutane ring, and this configuration results in the (E)-olefin product. The turnoverlimiting step of path C2 is also the cycloreversion of 4292
dx.doi.org/10.1021/om500612r | Organometallics 2014, 33, 4290−4294
Organometallics
Article
with electronic energies. This material is available free of charge via the Internet at http://pubs.acs.org.
agrees with the observation that 2cat is comparable to some 1cat-class catalysts in activity.18 The optimized structures of TS7 and TS9 reveal steric factors that make TS7 energetically more favorable than TS9 (Figure S4, Supporting Information). In TS7, the two methyl substituents are positioned on the same side of the metallacyclobutane ring and point away from the bulky arenethiolate ligand, thereby reducing steric repulsion. This is reflected by the nearly planar metallacyclobutane ring, whose greatest dihedral angle is found at only 1.6°. In contrast, the two methyl substituents in TS9 are on opposite sides of the metallacyclobutane ring, with one of them pointing closer toward the bulky arenethiolate ligand and thereby causing stronger steric repulsion. This is shown by the H(methyl)···C(arenethiolate) interaction at distance c′ (2.60 Å) in TS9, which is less than the sum of the van der Waals radii (H, 1.20 Å; C, 1.70 Å). This steric effect is also indicated by the puckering of the metallacyclobutane ring of TS9, which creates a maximum dihedral angle at 15°. Similar steric effects, however, are not as important in the cycloaddition steps, whose barriers TS6 and TS8 are essentially isoenergetic. A possible explanation for this is that the metallacycle in TS6/TS8 is somewhat more separated in space from the thiolate ligand than it is in TS7/ TS9, thereby making TS6/TS8 less sensitive to the steric bias that sets apart TS7 and TS9. Another noteworthy structural feature of TS7 and TS9 is that the three phenyl substituents on the benzene ring of the thiolate are distant from the metallacyclobutanes and thereby do not appear to contribute much to the steric bias. Thus, substitution of a smaller substituent for phenyl might improve the initiation rate (see above) without sacrificing the stereoselectivity in the coupling metathesis of a small substrate such as propene. A catalytic cycle requires the Ru−methylidene complex 13 to continue metathesizing with propene to regenerate the active catalyst 5. We have located the pathways of such regeneration (Figure S5, Supporting Information), which indicate that complex 5 is indeed the favored product due to the same kinds of steric factors as discussed above for the initiation metathesis. On another note, the byproduct ethylene is produced in the regeneration phase of the catalytic cycle.
■
*E-mail for Z.-X.W.:
[email protected]. *E-mail for X.W.:
[email protected]. Author Contributions §
These authors contributed equally to this work.
Notes
The authors declare no competing financial interest.
■
ACKNOWLEDGMENTS We acknowledge support for this work from the University of Colorado Denver and the Chinese Academy of Sciences. This work used the Extreme Science and Engineering Discovery Environment (XSEDE), which is supported by National Science Foundation Grant No. ACI-1053575.
■
REFERENCES
(1) Fürstner, A. Angew. Chem., Int. Ed. 2000, 39, 3012. (2) Trnka, T. M.; Grubbs, R. H. Acc. Chem. Res. 2001, 34, 18. (3) Samojłowicz, C.; Bieniek, M.; Grela, K. Chem. Rev. 2009, 109, 3708. (4) Vougioukalakis, G. C.; Grubbs, R. H. Chem. Rev. 2010, 110, 1746. (5) Fürstner, A. Science 2013, 341, 1357. (6) Jiang, A. J.; Zhao, Y.; Schrock, R. R.; Hoveyda, A. H. J. Am. Chem. Soc. 2009, 131, 16630. (7) Meek, S. J.; O’Brien, R. V.; Llaveria, J.; Schrock, R. R.; Hoveyda, A. H. Nature 2011, 471, 461. (8) Marinescu, S. C.; Levine, D. S.; Zhao, Y.; Schrock, R. R.; Hoveyda, A. H. J. Am. Chem. Soc. 2011, 133, 11512. (9) Peryshkov, D. V.; Schrock, R. R.; Takase, M. K.; Müller, P.; Hoveyda, A. H. J. Am. Chem. Soc. 2011, 133, 20754. (10) Endo, K.; Grubbs, R. H. J. Am. Chem. Soc. 2011, 133, 8525. (11) Keitz, B. K.; Endo, K.; Herbert, M. B.; Grubbs, R. H. J. Am. Chem. Soc. 2011, 133, 9686. (12) Keitz, B. K.; Endo, K.; Patel, P. R.; Herbert, M. B.; Grubbs, R. H. J. Am. Chem. Soc. 2012, 134, 693. (13) Rosebrugh, L. E.; Herbert, M. B.; Marx, V. M.; Keitz, B. K.; Grubbs, R. H. J. Am. Chem. Soc. 2013, 135, 1276. (14) Hartung, J.; Grubbs, R. H. J. Am. Chem. Soc. 2013, 135, 10183. (15) Dang, Y.; Wang, Z.-X.; Wang, X. Organometallics 2012, 31, 7222. (16) Dang, Y.; Wang, Z.-X.; Wang, X. Organometallics 2012, 31, 8654. (17) Liu, P.; Xu, X.; Dong, X.; Keitz, B. K.; Herbert, M. B.; Grubbs, R. H.; Houk, K. N. J. Am. Chem. Soc. 2012, 134, 1464. (18) Occhipinti, G.; Hansen, F. R.; Törnroos, K. W.; Jensen, V. R. J. Am. Chem. Soc. 2013, 135, 3331. (19) (a) Khan, R. K. M.; Torker, S.; Hoveyda, A. H. J. Am. Chem. Soc. 2013, 135, 10258. (b) Torker, S.; Khan, R. K. M.; Hoveyda, A. H. J. Am. Chem. Soc. 2014, 136, 3439. (20) Garber, S. B.; Kingsbury, J. S.; Gray, B. L.; Hoveyda, A. H. J. Am. Chem. Soc. 2000, 122, 8168. (21) Jensen, V. R.; Occhipinti, G.; Hansen, F. Novel Olefin Metathesis Catalysts. Int. Patent Appl. WO 2012032131, 2012. (22) Occhipinti, G.; Koudriavtsev, V.; Törnroos, K. W.; Jensen, V. R. Dalton Trans. 2014, 43, 11106. (23) (a) Lee, C. T.; Yang, W. T.; Parr, R. G. Phys. Rev. B 1988, 37, 785. (b) Becke, A. D. J. Chem. Phys. 1993, 98, 5648. (24) (a) Andrae, D.; Häussermann, U.; Dolg, M.; Stoll, H.; Preuss, H. Theor. Chim. Acta 1990, 77, 123. (b) Roy, L. E.; Hay, P. J.; Martin, R. L. J. Chem. Theory Comput. 2008, 4, 1029. (25) (a) Zhao, Y.; Truhlar, D. G. Acc. Chem. Res. 2008, 41, 157. (b) Zhao, Y.; Truhlar, D. G. J. Chem. Theory Comput. 2009, 5, 324.
■
CONCLUSION In summary, the ruthenium monothiolate carbene complex 2cat follows the bottom-bound olefin attack mechanism for both the initiation and coupling metatheses. The bulkiness of the 2,4,6-triphenylbenzenethiolate ligand raises the energy barrier for the precatalyst initiation, but it does not appear to diminish the activity of the resulting active catalyst 5. In addition, the arenethiolate ligand places the rigid sulfur-bonded benzene ring parallel and close to the metallacyclobutane ring of the intermediates, thereby sterically forcing both olefinic substituents on the metallacycle to the far side of the ring to achieve Z selectivity for the olefin product. However, the extra phenyl substituents on the benzene ring do not contribute significantly to the steric bias for the 2cat−propene system. These findings and insights hopefully will be useful for the further study and improvement of the 2cat system.
■
AUTHOR INFORMATION
Corresponding Authors
ASSOCIATED CONTENT
S Supporting Information *
Text, figures, and an xyz file giving computational methods, additional free energy profiles, optimized structures of TS7 and TS9, full citation for ref 34, and Cartesian coordinates along 4293
dx.doi.org/10.1021/om500612r | Organometallics 2014, 33, 4290−4294
Organometallics
Article
(c) Kulkarni, A. D.; Truhlar, D. G. J. Chem. Theory Comput. 2011, 7, 2325. (26) Marenich, A. V.; Cramer, C. J.; Truhlar, D. G. J. Phys. Chem. B 2009, 113, 6378. (27) Giri, R.; Lan, Y.; Liu, P.; Houk, K. N.; Yu, J.-Q. J. Am. Chem. Soc. 2012, 134, 14118. (28) Hong, X.; Trost, B. M.; Houk, K. N. J. Am. Chem. Soc. 2013, 135, 6588. (29) Xu, X.; Liu, P.; Shu, X.-z.; Tang, W.; Houk, K. N. J. Am. Chem. Soc. 2013, 135, 9271. (30) Tang, S.-Y.; Guo, Q.-X.; Fu, Y. Chem. Eur. J. 2011, 17, 13866. (31) Ariafard, A.; Asadollah, E.; Ostadebrahim, M.; Rajabi, N. A.; Yates, B. F. J. Am. Chem. Soc. 2012, 134, 16882. (32) Lin, M.; Kang, C.-Y.; Guo, Y.-A.; Yu, Z.-X. J. Am. Chem. Soc. 2012, 134, 398. (33) Yeom, H.-S.; Koo, J.; Park, H.-S.; Wang, Y.; Liang, Y.; Yu, Z.-X.; Shin, S. J. Am. Chem. Soc. 2012, 134, 208. (34) Frisch, M. J., et al. Gaussian09; revision C.01 ed.; Gaussian, Inc., Wallingford, CT, 2010. (35) Cavallo, L. J. Am. Chem. Soc. 2002, 124, 8965. (36) Cavallo, L.; Correa, A. J. Am. Chem. Soc. 2006, 128, 13352. (37) Cavallo, L.; Bahri-Laleh, N.; Credendino, R.; Beilstein. J. Org. Chem. 2011, 7, 40. (38) Benitez, D.; Tkatchouk, E.; Goddard, W. A., III. Chem. Commun. 2008, 6194. (39) Vorfalt, T.; Wannowius, K.-J.; Plenio, H. Angew. Chem., Int. Ed. 2010, 49, 5533. (40) Ashworth, I. W.; Hillier, I. H.; Nelson, D. J.; Percy, J. M.; Vincent, M. A. Chem. Commun. 2011, 47, 5428. (41) Thiel, V.; Hendann, M.; Wannowius, K.-J.; Plenio, H. J. Am. Chem. Soc. 2012, 134, 1104. (42) We also considered side-bound olefin attack pathways that would lead to ruthenium ethylidene complexes, but these proved to be much less favorable energetically (Figure S2, Supporting Information). (43) Lower Z selectivity is typically observed at higher conversion, indicating a competing Z/E isomerization process.
4294
dx.doi.org/10.1021/om500612r | Organometallics 2014, 33, 4290−4294
