A Thorough DFT Study of the Mechanism of Homodimerization of

Oct 9, 2012 - Z -Selektive Kreuzmetathese mit Ruthenium-Katalysatoren: Anwendung ... Peng Liu , Buck L. H. Taylor , Jesus Garcia-Lopez , Kendall N. Ho...
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A Thorough DFT Study of the Mechanism of Homodimerization of Terminal Olefins through Metathesis with a Chelated Ruthenium Catalyst: From Initiation to Z Selectivity to Regeneration Yanfeng Dang,† Zhi-Xiang Wang,*,† and Xiaotai Wang*,‡ †

College of Chemistry and Chemical Engineering, Graduate University of the Chinese Academy of Sciences, Beijing 100049, People's Republic of China ‡ Department of Chemistry, University of Colorado Denver, Campus Box 194, P.O. Box 173364, Denver, Colorado 80217-3364, United States S Supporting Information *

ABSTRACT: Density functional theory (DFT) calculations (B3LYP, M06, and M06-L) have been performed to investigate the mechanism and origins of Z selectivity of the metathesis homodimerization of terminal olefins catalyzed by chelated ruthenium complexes. The chosen system is, without any simplification, the experimentally performed homocoupling reaction of 3-phenyl-1-propene with 1cat, a pivalate and N-heterocyclic carbene (NHC) chelated Ru precatalyst. The six-coordinate 1cat converts to a trigonal-bipyramidal intermediate (3) through initial dissociation and isomerization. The metathesis reaction of complex 3 with 3phenyl-1-propene occurs in a side-bound mechanism and generates the trigonal-bipyramidal Ru−benzylidene complex 6. Complex 6 is the active catalyst for the subsequent side-bound metathesis with 3-phenyl-1-propene, which forms metallacyclobutanes that lead to the (Z)- and (E)-olefin homodimers. The transition states of cycloreversion leading to the (Z)- and (E)-olefins differ in energy by 2.2 kcal/mol, which gives rise to a calculated Z selectivity that agrees with experimental results. The Z selectivity stems from reduced steric repulsion in the transition state. The regeneration of complex 6 occurs along with the formation of the gaseous byproduct ethylene, whose evolution drives the overall reaction. As our results indicate, the chelating ligands are crucial for this new class of Ru catalysts to achieve Z-selective olefin metathesis, because they direct olefin attack, differentiate energies of the transition states and intermediates, and support the complexes in certain coordination geometries.

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INTRODUCTION Olefin metathesis mediated by transition-metal complexes has become an important method for creating carbon−carbon double bonds.1 In particular, ruthenium-based catalysts, as exemplified by A−C (Figure 1), have been developed and shown increasing activity and tolerance of organic functionality, thereby finding great utility in organic and polymer syntheses.2−4 The mechanism of olefin metathesis with the Ru-based first- and second-generation Grubbs catalysts (A and B) has been well studied experimentally and computationally.5−22 Of the side-bound and bottom-bound pathways (Scheme 1), the latter is more generally accepted, in which after dissociation of one phosphine ligand the Ru−alkylidene complex coordinates with a substrate olefin molecule in the bottom position, forming a square-pyramidal complex. This complex converts through cycloaddition to a trigonalbipyramidal (tbp) metallacyclobutane, which in turn cycloreverts to give a new olefin-coordinated Ru−alkylidene complex (square pyramidal) where the RuC bond is inverted as compared with the original Ru−alkylidene complex. The Grubbs−Hoveyda catalyst (C) shows high stability toward oxygen, moisture, and a range of temperatures. Several recent experimental and computational studies have found that C © 2012 American Chemical Society

could initiate olefin metathesis reactions by a dissociative or interchange mechanism, depending on the olefin substrate.23−25 Despite the enormous versatility of olefin metatheses catalyzed by A−C or their analogues, such reactions are equilibrium processes yielding a mixture of products, including a combination of (Z)- and (E)-olefins where the E isomer mostly occurs in a higher proportion due to its favored thermodynamics. The development of catalysts for stereocontrolled olefin metathesis is significant, because such catalysts 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 that show high Z selectivity in the cross-metathesis and homocoupling of terminal olefins,26 the Grubbs group has recently developed a class of Ru-based catalysts with chelating Nheterocyclic carbene (NHC) ligands that effects Z-selective olefin metathesis, some examples (1cat−3cat) being given in Figure 1.27 Their excellent experimental work, which did not aim at in-depth mechanistic investigation, showed the broad Received: August 14, 2012 Published: October 9, 2012 7222

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When we were in the process of carrying out this work, Liu et al. reported a DFT study that involved the acetatesubstituted catalyst 2cat as well as truncated model substrates and other simplifications, as shown in Scheme 3.28 The authors Scheme 3. Some Results Regarding the DFT Study Involving 2cat and Truncated Model Substratesa

a

The drawings were rigorously based on structures created with the coordinates provided in the Supporting Information for ref 28. Original numbering: 21 for 1*, cpx1-B for 2*, and cpx1-D for 3*. The enantiomorph of 2cat shown in Figure 1 was apparently considered by the authors of ref 28 to begin structure derivation.

Figure 1. Ru-based catalysts for olefin metathesis reactions, where cy = cyclohexyl and mes = 2,4,6-trimethylphenyl (or mesityl). Also included for reference is the X-ray crystal structure of 1cat with H atoms omitted.27a

first considered the metathesis of ethylene (a truncated model substrate) with 2cat and viewed the resulting complex 1* as the active catalyst for the following degenerate ethylene metathesis (or transalkylidenation). However, the report provided no information on the initiation mechanism: that is, how 2cat would react with ethylene to generate 1*. The authors then explored various possible pathways for the ethylene metathesis with 1*, demonstrated the favorable side-bound pathway as opposed to the bottom-bound pathway favored by unchelated Ru catalysts, and explained the preference for the side-bound mechanism by a combination of electronic and steric effects. Furthermore, they considered the metathesis homodimerization of propene (another truncated model substrate designed to study the Z selectivity) and used 1* as the active intermediate to further react with propene. In the 2cat−propene system, complex 1* would have to result from the initial metathesis reaction(s) of propene with 2cat, but again the initiation mechanism is unclear. The authors reasoned that the continuing metathesis of 1* with propene through the sidebound mechanism could lead to 2* and 3*. Complexes 2* and 3* were then both considered for metathesis homodimerization reactions with another propene molecule, of which the transition states of cycloaddition and cycloreversion were computed that would lead to the (Z)- and (E)-CH3CH CHCH3 products. The key transition state leading to (Z)CH3CHCHCH3, which they identified and coded 32-TS in ref 28, was derived from complex 2*. The key transition state leading to (E)-CH3CHCHCH3, which they identified and coded 35-TS, was derived from complex 3*. They compared 35-TS and 32-TS in free energy and noticed the former being higher than the latter by 4.3 kcal/mol. This difference was used to calculate the Z selectivity, which turned out to be higher than

Scheme 1. Side- and Bottom-Bound Pathways of Olefin Metathesis with Grubbs I and II Catalysts

range of Z-selective olefin metathesis reactions with such Ru catalysts.27 We followed the published work27a,b and began a thorough DFT study of olefin metathesis with such new chelated Ru catalysts in an effort to elucidate the mechanism and origins of Z selectivity. We chose to study an actual reaction without using any truncated models or other simplifications, that is, the experimentally performed homocoupling of 3-phenyl-1-propene (phenylpropene hereafter) catalyzed by 1cat, as this pivalate-substituted catalyst was the first of its kind to be reported and displayed the best conversion (>95%) and Z selectivity (>95%) in this well-defined reaction (Scheme 2).27 Scheme 2. Homodimerization of Phenylpropene through Metathesis with 1cat

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Figure 2. Free energy profile for the steps that 1cat undergoes prior to interaction with olefin, with solvent-corrected free energy and enthalpy (similarly hereafter).

experimentally observed, as the authors noted themselves.28 In addition, their results suggested that the (Z)- and (E)CH3CHCHCH3 products would stem from different intermediates (Z from 2* and E from 3*). This computational work provides valuable insights into this new type of Rucatalyzed olefin metathesis, yet the complete reaction mechanism remains unclear, particularly in terms of how the six-coordinate and chelated Ru precatalysts initiate the reaction and what active Ru−alkylidene complex results from the initial metathesis event(s) that will catalyze the following homodimerization and give rise to the Z selectivity. In the present computational study, we present a thorough mechanistic account of the actual homodimerization reaction of phenylpropene through metathesis catalyzed by 1cat. We have considered various possible products and pathways from initiation to homocoupling to regeneration, identifying the favored ones and excluding the others. The findings allow us to gain a clear understanding of the reaction mechanism and origins of Z selectivity.

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ruthenium catalysts for olefin metathesis in that it is a sixcoordinate complex with three bulky chelating ligands. For 1cat to initiate a metathesis reaction, the ruthenium center must first interact with the olefin substrate, and this would be unlikely to occur without dissociation of at least one ligand donor atom from Ru to make room for the olefin. We reasoned that the Ru−O (isopropoxy) linkage is the weakest coordination bond in 1cat, because the oxygen atom, a hard donor atom, has no negative charge and binds to a soft Ru(II) center. On the basis of initiation rate measurements, Grubbs et al. have suggested that a simple associative or interchange mechanism would not occur with 1cat and that 1cat likely undergoes multiple steps (e.g., the dissociation of the chelated isopropoxy oxygen) prior to reaction with olefin.27c Indeed, we were able to locate the dissociation transition state TS1 and the resulting fivecoordination square-pyramidal intermediate 2, as shown in Figure 2. The Ru−O(isopropoxy) bond distance in TS1 is 3.51 Å, comparable to those found for the transition states arising from the rupture of the analogous Ru−O bond in the Grubbs− Hoveyda catalyst (C).24,25 We then logically considered phenylpropene binding to the open bottom site in 2, from which only one bottom-bound pathway was located leading to a new Ru−alkylidene complex (Supporting Information, Scheme S1). However, the highest energy transition state (TS3B) in this path, which would be required for cleaving a metallacyclobutane, is 42.4 kcal/mol relative to 1cat and phenylpropene. This large barrier prohibits the bottom-bound mechanism. This observation and the result obtained by Liu et al.28 from studying the 1*−ethylene model are consistent, both pointing to a conclusion that the NHCchelated Ru catalysts disfavor the bottom-bound mechanism. In the following sections, we will focus on discussing our investigation of the side-bound mechanism. We computed the isomerization of 2 to 3 after reasoning that the trigonal-bipyramidal (tbp) geometry of 3 (Figure 2) would facilitate the side-bound olefin attack. Complex 3 is essentially isoenergetic with complex 2, and the low activation free energy of 1.7 kcal/mol indicates a facile reaction. The bulky NHC chelator in 3 not only forms robust coordination bonds with ruthenium but also exerts considerable steric hindrance. Thus, it acts as a spectator ligand and also plays an important role in directing olefin interaction with complex 3. The incoming olefin would be trans to the NHC admantyl group to minimize steric repulsion; that is, it would approach the ruthenium center

COMPUTATIONAL METHODS

Geometry optimizations were performed at the B3LYP/BS1 level in the gas phase,29 BS1 designating a mixed basis set of SDD30 for Ru and 6-31G(d) for other atoms. The optimized geometry of 1cat agrees well with its crystal structure.27a When necessary, IRC (intrinsic reaction coordinate) calculations were performed to verify the connections between a transition state and its forward and backward minima.31 The B3LYP/BS1-optimized geometries were used for solvent-corrected single-point energy calculations at the M0632,33/BS2 level with solvation effects modeled by SMD34 in THF (the solvent used for the reaction), BS2 denoting a mixed basis set of SDD for Ru and 6311++G(d,p) for other atoms. This kind of combined use of B3LYP and M06 has been demonstrated by numerous studies to successfully produce energy profiles of reactions involving transition-metal systems.10,28,35−39 The gas-phase B3LYP/BS1 harmonic frequencies were used for thermal and entropic corrections to obtain the enthalpies and free energies at 298.15 K and 1 atm. Free energies (kcal/mol) were discussed and enthalpies (kcal/mol) given for reference. For comparison purposes, the key transition states obtained with the B3LYP/BS1-M06 (SMD)/BS2 calculations were also subjected to calculations with M06-L/BS1-M06-L (SMD)/BS2. All calculations were performed with Gaussian 0340 and Gaussian 09.41

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RESULTS AND DISCUSSION Initiation and Ensuing Olefin Metathesis. Catalyst 1cat is structurally distinct from all previous five-coordinate 7224

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Scheme 4. Overview of Metathesis Reactions of 3 with Phenylpropene Leading to New Ru−Alkylidene Complexes

Scheme 5. Possible Pathways for Reaction of 3 with Phenylpropene Leading to Complex 6

monodentate pivalate coordination. On the basis of these considerations, we envisage that the reaction of complex 3 with phenylpropene (an unsymmetrical olefin) could result in three active Ru−alkylidene complexes via four possible metallacycles having the substituents R and R′ in different positions and orientations, as shown in Scheme 4. We characterized four possible pathways going through 1cyc and leading to complex 6 (Scheme 5). In path I1, although a stable six-coordinate phenylpropene complex could not be

through the space between the equatorial Ru−alkylidene and Ru−O bonds, forming a six-coordinate complex. An interchange mechanism is also possible with complex 3; that is, as olefin binds to Ru, one of the Ru−O bonds breaks simultaneously, as has been proposed for the Grubbs−Hoveyda catalyst (C).24 Complex 3 would be unlikely to undergo pivalate dissociation, because this chelator has stable bidentate coordination with Ru. Not surprisingly, we did not locate any reasonable four-coordinate isomers of complex 3 with 7225

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Scheme 6. The Most Favorable Pathway Leading to Complex 6′

Figure 3. Free energy profiles of the pathways leading to the Ru−alkylidene complexes 6 (in red) and 6′ (in blue).

metallacyclobutane (path I3). The cleavage of 4″ is different from that of 4 or 4′, as an olefin complex product was not located probably due to the steric repulsion between the bulky R group and the pivalate ligand (see above). Path I4 would begin with an interchange mechanism via TS10, a transition state which involves simultaneous phenylpropene binding and dissociation of one pivalate oxygen atom and results in a fivecoordinate tbp complex (8). Complex 8 could converge to 4′ by passing through the cycloaddition barrier TS11. The aforementioned dissociation of complex 7 could also be viewed as the reverse of an interchange coordination reaction on the basis of the principle of microreversibility. Path I4 can be ruled out because it would have to pass two high-energy barriers (TS10 and TS11) to reach complex 4′. Of the other three pathways, the most favorable is path I2, whose overall kinetic barrier (TS6) is lower than those of path I1 and path I3 by 1.3 and 1.5 kcal/mol, respectively, as revealed by comparing TS6 with TS4 and TS9. The primary structural difference among TS4, TS6, and TS9 is in the conformation of the pivalate ligands with respect to the Ru−O coordination bond. In TS6, the pivalate is in such a position that it should experience the least steric repulsion by R and R′, which makes TS6 the lowest energy barrier of the three. In addition, the activation free energy for the elementary step corresponding to TS6 is 10.3 kcal, more than 3.5 kcal/mol lower than that of TS4 or TS9. Similarly, the four possible pathways going through 2cyc and leading to complex 6′ were characterized (Supporting Information, Scheme S2), the most favorable one being shown in Scheme 6. This pathway is analogous to path I2

located probably due to the large steric repulsion between the bulky R′ (benzyl) group and the pivalate ligand, the transition state (TS3) for the [2 + 2] cycloaddition leading to a metallacyclobutane (4) was located successfully. On the basis of previous computational studies of the Grubbs−Hoveyda-type catalysts,25a olefin complexes with significant steric repulsions between the olefin and the other ligands may not be located as minima of the potential energy surface at the level of theory used, and in such cases, olefin coordination/dissociation may be viewed as occurring simultaneously with cycloaddition/cycloreversion. If phenylpropene and 3 are viewed as undergoing a simultaneous coordination and [2 + 2] cycloaddition via TS3, this concerted reaction would have a reasonable activation free energy (10.4 kcal/mol). In the six-coordinate TS3, one Ru−O bond (marked by the dashed line) is lengthened to 2.36 from 2.19 Å in complex 3, and the emerging Ru−C and C−C bonds (marked by the single dashed lines) of the ring are at 2.17 and 1.96 Å, respectively. The resulting complex 4 has a fivecoordinate tbp geometry. In fact, most of the metallacyclobutanes in this study are five-coordinate tbp rather than six-coordinate octahedral complexes, and this is attributed to the steric interactions between the bulky ligands. Direct cleavage of 4 via TS4 would produce the intermediate 5, which leads to 6 after dissociation of the bound olefin RCHCH2.42 Alternatively, 4 could isomerize to 4′ through rotating the Ru− O(carboxylate) bond via TS5 (not shown), opening a different metallacycle-cleaving pathway (path I2). The metallacyclobutane 4′ cleaves via TS6 to produce 7, which then passes through TS7 to yield 6 after dissociation of RCHCH2. Complex 4′ could still isomerize to 4″, a third isomer of the 7226

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Scheme 7. The Most Favorable Pathway Leading to Complex 17

Figure 4. Optimized structures of TS6, TS17, and TS33, with selected atoms labeled, bond distances listed, and H···H interactions marked by dashed lines. H atoms are omitted for clarity except those in question. All distances are in Å (similarly hereafter).

probably because the orientation of R′CHCH2 reduces its steric repulsion from the other ligands. Complex 19 cycloadds via TS32 to form the metallacyclobutane 20, which must overcome a high barrier (TS33) to cyclorevert to 21. Note that TS33 is 5.2 kcal/mol higher than TS6, the highest barrier which is for the cycloreversion of 4′in the aformentioned most favorable path I2 leading to complex 6. Intermediate 21, the resulting ruthenium−alkylidene complex with coordinated olefin RCCR′, undergoes dissociation via TS34 to form 16, a four-coordinate tetrahedral complex, as indicated by the monodentate coordination of pivalate with the longer Ru−O separation at 3.31 Å. Complex 16 easily isomerizes via TS26 to the more stable tbp complex 17, but the formation of 17 is still thermodynamically less favorable than the formation of 6 by 3.6 kcal/mol (14.8 − 11.2). Thus, both the thermodynamic and kinetic factors are overwhelmingly against the formation of complex 17. In view of the structure−property relationship, we have examined the optimized structures of TS6, TS17, and TS33 in an effort to explain their differences in energy. As shown in Figure 4, TS6 and TS17 have the same kinds of donor atoms in the tbp geometry around the Ru centers. In addition, the partially broken metallacycles in TS6 and TS17 have closely similar corresponding Ru−C and C−C bond distances. Thus, the electronic effects on the energy difference between TS6 and

discussed above, involving [2 + 2] cycloaddition to form the metallacyclobutane 9, which then isomerizes to 9′, where the pivalate has the greatest separation from R and R′. As a result of the reduced steric repulsion, the overall kinetic barrier of this path (TS17) is lower by at least 1.1 kcal/mol than that of any of the other three pathways. We have compared the most favorable pathways leading to 6 and 6′ in the form of free energy profiles shown in Figure 3. Every intermediate or transition state in the pathway to 6 (shown in red) is energetically favored over its counterpart in the pathway to 6′ (shown in blue). The two pathways first differentiate energetically in the cycloaddition step, with TS3 lower than TS12 by 2.6 kcal/mol. On proceeding to the cycloreversion step that determines the overall kinetic barrier for both these pathways, the gap between TS6 and TS17 is 2.2 kcal/mol in favor of TS6. In addition, the formation of 6 is more favorable by 1.3 kcal/mol in overall thermodynamics. Thus, complex 6 appears to be favored over 6′, but we cannot leave out 6′ as yet because both complexes can enter the subsequent metathesis homodimerization with phenylpropene, whose energetics are yet to be worked out. There are a total of seven pathways going through 3cyc or 4cyc and leading to complex 17 (Supporting Information, Schemes S3 and S4), the most favorable of which is shown in Scheme 7. The six-coordinate olefin complex 19 was located 7227

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Scheme 8. Possible Reactions Involving Complex 6 or 6′a

a

[Z] = (Z)-R′CHCHR′; [E] = (E)-R′CHCHR′.

TS17 should not be significant. Sterically, the most important difference between TS6 and TS17 is in the orientation of the benzyl (R′) group toward the NHC mesityl substituent. The benzyl (R′) group points closer toward the mesityl in TS17 than in TS6, and as such, stronger steric repulsions occur in TS17 than in TS6, which is indicated by comparing the two sets of shortest nonbonding H···H distances. The H···H repulsion at 2.19 Å in TS17 is the most significant, because this distance is less than 2 times the van der Waals radius of hydrogen (1.20 Å). Thus, it is mostly the steric effects that destabilize TS17 relative to TS6. TS33 is similar to TS6 and TS17 in terms of the Ru coordination environment, but the metallacycle in TS33 is more twisted, apparently due to the stronger steric repulsion between the benzyl (R′) and NHC mesityl groups. As a result, the C(94)−C(95) bond is lengthened to 2.48 Å, 0.12 Å longer than the corresponding C−C bond in TS6. The importance of steric repulsions in TS33 is further indicated by the short H···H distances at 2.10, 2.44, and 2.29 Å involving two H atoms on the metallacyclic ring and one on the i-Pr group of R. A combination of these factors causes TS33 to be at a significantly higher stationary point on the free energy surface. Homocoupling and Z Selectivity. As outlined in Scheme 8, when complex 6 or 6′ continues metathesizing with phenylpropene, the reaction could be productive, leading to the (Z)- and (E)-olefin homodimers and complex 27, or it could be due to the unsymmetrical nature of the chelating NHC ligand, transalkylidenation yielding the new complex 35

or 35′ which could then undergo homocoupling with phenylpropene to produce the (Z)- and (E)-olefin homodimers and complex 17. We have considered all these possibilities and determined the favored and disfavored pathways as indicated in Scheme 8. To be clear and succinct, we will first discuss the favored homocoupling reaction involving complex 6, including explaining the Z selectivity. We will then summarize the results that allow us to disregard the transalkylidenations and the disfavored homodimerizations. Thermodynamically, the metatheses of complex 6 with phenylpropene to form 27 along with [Z] and [E] are reasonable, with ΔG = 0.2 kcal/mol for the former and ΔG = −1.4 kcal/mol for the latter. In treating the reaction mechanism, we considered the role of the bulky NHC chelator of complex 6 in directing olefin attack, using the same reasoning as with complex 3. Thus, we explored seven possible pathways, as shown in Scheme 9. Paths C1−C3 all go through a metallacyclobutane with the two benzyl (R′) groups on the same side of the ring and hence lead to the (Z)-olefin product. Paths C1 and C2 begin with phenylpropene coordination to 6, forming a six-coordinate complex (24) that in turn cycloadds via TS42 to form the metallacyclobutane 25, which diverges into two possible pathways. Direct cleavage of 25 via TS43 (path C1) would generate 26, a new Ru−alkylidene complex with the product (Z)-olefin coordinated. The dissociation of 26 via TS44 would lead to (Z)-R′CHCHR′ and 27, a step that can be viewed as the reverse of an interchange reaction (see above). Alternatively, 25 could isomerize to 25′ (path C2). 7228

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Scheme 9. Possible Pathways for Phenylpropene Homodimerization

corresponding to TS43 is the largest of the three, thereby causing the cleavage of 25 to be the rate-determining step. Paths C4−C7 all go through a metallacyclobutane with the two benzyl (R′) groups on opposite sides of the ring, and this orientation leads to the (E)-olefin product. Paths C4, C6, and C7 are respectively analogous to paths C1, C2, and C3. The additional pathway C5 arises because complex 30′ could cyclorevert via TS54, whereas 25′ itself could not cyclorevert. We have found path C4 to be the most favorable due to its lowest overall barrier TS51 at 24.5 kcal/mol. In contrast, paths C5−C7 have the higher overall barriers TS54 (26.2 kcal/mol), TS57 (25.3 kcal/mol), and TS58 (27.2 kcal/mol). It is worth noting that complexes 31 and 32 are similar structurally with two small differences: (1) the pivalate ligand as a whole in 31 is slightly more turned toward the admantyl group and (2) the pivalate ligands in 31 and 32 have different conformations with respect to rotation around the Me3C−COO bond. The bulky R′ groups in 31 and 32 appear to constrain the Ru−O(pivalate) and Me3C−COO bonds, which otherwise might be able to rotate more easily. This steric impact probably led to 31 and 32 being located as two distinct minima. The same rationale also applies to TS52 and TS55 being located separately.

However, the transition state for the cycloreversion of 25′ appears nonexistent, as the attempted optimizations all converged to TS43, which is for the cycloreversion of 25. The pivalate ligand situated between the two bulky R′ groups in 25′ causes significant steric repulsion; therefore, a transition state with that configuration would be too high in energy to be attainable. Nonetheless, 25′ could further isomerize to 25″, which could cleave through a six-coordinate transition state (TS47) to afford (Z)-R′CHCHR′ and 27. Although the highest stationary points for paths C1 and C2 are the same at 22.9 kcal/mol (TS44 and TS47), the latter is considered less favorable because the cleavage of 25″ must overcome a large activation free energy barrier of 11.9 kcal/mol, which is 3.2 kcal/mol more than the cleavage of 25. In path C3, we have located a five-coordinate tbp phenylpropene complex (28) that could result from an interchange mechanism. Complex 28 could cycloadd to converge to 25′, but the high barrier (TS48) practically rules out this path. Thus, we have established path C1 as the most favorable pathway leading to the (Z)-olefin product of homodimerization. In path C1, the three highest stationary points (TS42, TS43, and TS44) have comparable energy, and the activation free energy for the elementary step 7229

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Figure 5. Free energy profiles of the homocoupling pathways leading to (Z)-R′CHCHR′ (in red) and (E)-R′CHCHR′ (in blue).

Figure 6. Optimized structures of TS43 and TS51, with selected H···H interactions marked by dashed lines. H atoms are omitted for clarity, except for those in question.

Figure 6 shows the optimized structures of TS43 and TS51. Although the two have the same coordination environment around the Ru centers, there is a striking difference in the geometry and conformation of the metallacycles that are most important to these transition states. While TS43 has a nearly planar metallacyclobutane with both benzyl (R′) groups below the ring and pointing away from the bulky NHC mesityl substituent, TS51 has a twisted metallacycle resulting from the strong steric repulsion between the benzyl (R′) above the ring and the NHC mesityl. The substantial difference in the H···H distances involving analogous H atoms on TS43 and TS51 clearly demonstrates stronger steric repulsions in TS51, which translate into a higher stationary point on the free energy surface. Now let us return to Scheme 8. We have identified the most favorable pathways for the transalkylidenations of 6 to 35 and 35′, as well as the subsequent homocoupling reactions involving 35 and 35′ and yielding complex 17 and [Z] and [E] (Supporting Information, Figure S3 and Schemes S5 and S6). Table 1 gives the ΔG values and the highest kinetic barriers (ΔG⧧) of these reactions in comparison with those for the metathesis homocoupling of 6 with phenylpropene to form complex 27 and [Z] and [E]. In each of entries 3−6, the reaction is endergonic by 3.6 or 5.2 kcal/mol, and the ΔG⧧ occurs in the homodimerization stage involving 35 or 35′. ΔG⧧ of entry 3 is that of the most favorable pathway that could produce [Z] through 35 or 35′, but it is still higher than ΔG⧧ of entry 1 by 2.8 kcal/mol. In addition, ΔG of entry 3 is larger

Figure 5 shows the free energies and enthalpies of all the TSs and intermediates in the most favorable pathways leading to the (Z)- and (E)-olefin products. The overall barrier for path C1 is lower than that for path C4 (TS44 vs TS51). The ratedetermining steps for both pathways are cycloreversions (via TS43 and TS51), as each of them corresponds to the highest activation free energy of any elementary step in the specific path. TS51 is also the highest stationary point for path C4, and TS43 is only 0.6 kcal/mol below TS44, the highest stationary point for path C1. In addition, the subsequent and final olefin dissociation steps have much smaller and closely similar activation free energies: 3.8 kcal/mol (TS44 − 26) for path C1 and 3.6 kcal/mol (TS52 − 31) for path C4. Thus, we conclude that the Z selectivity originates from the difference of 2.2 kcal/mol between TS43 and TS51, which gives a calculated value of 97%43 that agrees with experimental observations (>95%). The gas-phase energy profiles also support our conclusions qualitatively (Supporting Information, Figure S1). For comparison purposes, we also calculated the key transition states of TS42, TS43, TS50, and TS51 with M06-L; that is, we optimized their geometries and calculated the frequencies in the gas phase with M06-L/BS1 and then computed the solution-phase single-point energies based on these geometries with M06-L (SMD)/BS2. The geometries are in good agreement with those of B3LYP, and the energetics are also consistent with the above conclusion that TS43 and TS51 are rate- and selectivity-determining and the (Z)-olefin is the major product (Supporting Information, Figure S2). 7230

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In summary, our results demonstrate that the (Z)- and (E)R′CHCHR′ products essentially originate from the same active complex (6) and that the predicted Z selectivity agrees with experiment. As discussed earlier, in the work on the 2cat− propene model system, the results presented by the authors suggested that (Z)-CH3CHCHCH3 would stem from 2* and (E)-CH3CHCHCH3 would stem from 3*.28 Intrigued by this inconsistency, we explored the possibility of more transition states that could be derived from 2* and 3* and would lead to (Z)- and (E)-CH3CHCHCH3, using exactly the same computational methods as the authors had employed. After noticing that the transition states leading to (Z)- and (E)CH3CHCHCH3 in ref 28 are all six-coordinate with the acetate ligand in the bidentate bonding mode, we sought to locate additional five-coordinate TSs with monodentate acetate and identified one of them (TSnew) as crucial (Figure 7).

Table 1. Selected Energetics (in kcal/mol) of Reactions Involving Complex 6 entry 1 2 3 4 5 6

reaction 6 6 6 6 6 6

+ + + + + +

olefin olefin olefin olefin olefin olefin

→ → → → → →

27 + [Z] 27 + [E] 35 + olefin → 17 + [Z] 35 + olefin → 17 + [E] 35′ + olefin → 17 + [Z] 35′ + olefin → 17 + [E]

ΔG⧧

ΔG

22.9 24.5 25.7 26.0 30.1 29.5

0.20 −1.4 5.2 3.6 5.2 3.6

than ΔG of entry 1 by 5.0 kcal/mol. Similarly, ΔG⧧ of entry 4 is that of the most favorable pathway that could produce [E] through 35 or 35′, but it is still higher than ΔG⧧ of entry 2 by 1.5 kcal/mol, and ΔG of entry 4 is larger than ΔG of entry 2 by 5.0 kcal/mol. Furthermore, either of entries 1 and 2 is more favorable both thermodynamically and kinetically than any of entries 3−6. Thus, the homodimerization reactions involving 35 or 35′ are clearly disfavored both thermodynamically and kinetically. We have also computed the most favorable pathways for the metathesis homocoupling of 6′ with phenylpropene to form complex 27 and [Z] and [E] (Supporting Information, Scheme S7). Because complexes 6 and 6′ both stem from 3, we consider the two-stage 3 → 6/6′ → 27 + [Z]/[E] paths shown in Scheme 8 for a comparison of the kinetic barriers and summarize the highest kinetic barriers (ΔG⧧) for the eight stages in the four most favorable pathways (Scheme 10). Of the Scheme 10. Highest Kinetic Barriers (ΔG⧧ in kcal/mol)

Figure 7. Structure of TSnew optimized in the gas phase with B3LYP/ LANL2DZ-6-31G(d), with H atoms omitted for clarity. The singlepoint energy in THF solution was calculated with M06/SDD-6311+G(d,p)/SMD.

TSnew derives from 2*, concerns metallacyclobutane cleavage, and leads to (E)-CH3CHCHCH3, as indicated by our computations (Supporting Information, Scheme S8). TSnew is actually analogous to TS51 in our system (see Scheme 9). Most importantly, TSnew is lower by 1.3 kcal/mol than 35-TS, the key transition state indentified by the authors which stems from 3* and concerns cycloreversion that would lead to (E)CH3CHCHCH3.28 Replacement of 35-TS with TSnew would give rise to the following: (1) both (Z)- and (E)CH3CHCHCH3 would stem from 2*, which is consistent with our conclusion drawn from the actual 1cat−phenylpropene system, and (2) the energy difference used for predicting the Z selectivity would be lowered from 4.3 to 3.0 kcal/mol, thereby giving a calculated Z selectivity that is closer to the experimental value. Such are the new outcomes for the 2cat−propene model system that seem more plausible. Regeneration. A complete catalytic cycle requires the Ru− alkylidene complex 27, a product from the homocoupling steps, to continue reacting with phenylpropene to regenerate the active catalyst 6 favorably. Complex 27 is structurally analogous to complex 3 in the initial metathesis. As with complex 3, we have studied all eight possible pathways for the reaction of 27 with phenylpropene that would lead to 6 or 6′ (Supporting Information, Schemes S9 and S10), and the most favorable pathways to 6 and 6′, along with the free energies and enthalpies of all TSs and intermediates, are shown in the energy profiles in Figure 8. At the beginning of path R3, we located the transition state (TS101) leading to a five-coordinate tbp metallacyclobutane

two pathways leading to [Z], that going though complex 6 is overall more favorable by 5.7 kcal/mol (TS100 − TS6) than that going through 6′. Thus, complex 6′ practically has no contribution to the formation of Z-R′CHCHR′. Of the two pathways leading to [E], that going through 6 is overall more favorable only by 0.7 kcal/mol (TS17 − TS51); therefore, a closer examination is necessary to better compare and contrast the two pathways. Because the 6 → [E] and 6′ → [E] phases have comparable overall kinetic barriers (TS51 vs TS96), the 3 → 6/6′ stages appear to determine the kinetics of the pathways. The 3 → 6 stage is overall more favorable kinetically by 2.2 kcal/mol than the 3 → 6′ stage, and as discussed above using the 3 → 6/6′ energy profiles (Figure 3), the formation of 6 is more favorable in every elementary step both thermodynamically and kinetically than the formation of 6′. Thus, as to how the (E)-olefin homodimer is produced, the pathway going through 6 is much favored over the pathway going through 6′. On another note, complex 6′ could theoretically undergo transalkylidenation with phenylpropene to form 35 or 35′ (Scheme 8), but the kinetics are unfavorable for 35 or 35′ to enter the subsequent metathesis homocoupling, as discussed above. In addition, the 6′ + olefin → 35/35′ + olefin → 17 + [Z]/[E] pathways are overall thermodynamically unfavorable, with the ΔG values being 3.9 kcal/mol for the [Z]-forming route and 2.3 kcal/mol for the [E]-forming route. 7231

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Organometallics

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Figure 8. Free energy profiles of the regeneration pathways leading to 6 and 6′.

Scheme 11. Transalkylidenation of Complex 27

or 35′, but as discussed earlier, neither 35 nor 35′ would enter the metathesis homocoupling reaction. Finally, it should be noted that the continuous evolution of the gaseous ethylene byproduct in the regeneration stage keeps shifting the equilibrium away from the reactants and toward the products in the overall reaction, thereby driving the reaction to a high conversion rate.

(50), with an accessible activation free energy (11.4 kcal/mol) for what could be a concerted [2 + 2] cycloaddition step (see above). Complex 50 undergoes two stepwise isomerizations to pass through 50′ and attain 50″. Note that TS107 is higher than the intermediate 50′ in the gas phase but becomes slightly lower after solvent correction, and this could mean a facile isomerization of 50′ to 50″. The isomerizations enable complex 50″ to cleave at a lower barrier (TS108) than is required for 50 and 50′ by 3.1 and 3.4 kcal/mol, respectively. This is different from path I3 discussed above beginning with complex 3 (see Scheme 5), where the metallacyclobutane cleaves most favorably through 4′, which is analogous to 50′. TS108 has a six-coordinate geometry around Ru, with the emerging Ru−O (pivalate) bond being as short as 2.31 Å, as opposed to the fivecoordinate TSs for the cleavage of 50 and 50′ (Supporting Information, Scheme S9). The six-coordination, an electronic effect, helps stabilize TS108. Furthermore, unlike the sixcoordinate TS9 in path I3 that has a bulky substituent on the leaving olefin that repels the pivalate ligand (see Scheme 5), there is no substituent on the outgoing ethylene in TS108. Thus, the favorable electronic and steric effects jointly lead the metallacycle to cleave via TS108. Path R7 leading to complex 6′ is completely analogous to path R3, with every intermediate or transition state in a higher energy position than its counterpart in path R3. After numerous differentiating barriers, the amount of complex 6′ produced from the regeneration should be negligibly small. In theory, complex 27 could also undergo transalkylidenation with phenylpropene to form 17 (Scheme 11). In practice, this is not a matter of concern. First, this reaction would be highly unfavorable with ΔG = 5.0 kcal/mol. Second, should 17 be produced, it would either revert to 27 through transalkylidenation or metathesize with phenylpropene to form 35

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CONCLUSION We have presented a thorough computational study of the mechanism of the homodimerization of 3-phenyl-1-propene through metathesis with the pivalate- and NHC-chelated precatalyst 1cat. Scheme 12 summarizes the reaction sequence and depicts the catalytic cycle. The six-coordinate 1cat undergoes the initial dissociation and isomerization steps, affording a five-coordinate trigonalbipyramidal intermediate (3). Complex 3 undergoes the first metathesis reaction with olefin via a side-bound mechanism, giving the trigonal-bipyramidal Ru−benzylidene complex 6 as the thermodynamically and kinetically favored active species, where the benzyl group points away from the NHC mesityl substituent to minimize steric repulsion. The active complex 6 reacts with another phenylpropene molecule via a side-bound pathway, forming metallacyclobutanes that can lead to the final (Z)- and (E)-olefin products. The transition states of cycloreversion leading to the (Z)- and (E)-olefins differentiate in free energy by 2.2 kcal, which gives a predicted Z selectivity of 97% that agrees with experimental results. Optimized structures indicate that the Z selectivity arises from the reduced steric repulsion in the (Z)-olefin-forming transition state that has both benzyl groups on the ring pointing away from the NHC mesityl group, as compared with the (E)-olefin-forming transition state, where one benzyl group has a closer orientation 7232

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the 2cat−propene system, the (Z)- and (E)-olefin products stem from different intermediates, which was found to be inaccurate. (4) The calculated Z selectivity is closer to the experimental value.

Scheme 12. Metathesis Homodimerization of Phenylpropene with 1cat

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ASSOCIATED CONTENT

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AUTHOR INFORMATION

* Supporting Information S

Text, figures, and tables giving additional computational results and the complete refs 35 and 36. This material is available free of charge via the Internet at http://pubs.acs.org. Corresponding Author

*E-mail: [email protected] (Z.-X.W.); xiaotai.wang@ ucdenver.edu. (X.W.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We acknowledge support for this work from the Chinese Academy of Science, the National Science Foundation of China (Grant Nos. 20973197 and 21173263), and the University of Colorado Denver.

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toward the mesityl group. The regeneration completes the catalytic cycle, affording complex 6 favorably, as well as the byproduct ethylene whose evolution as a gas drives the overall reaction. Taken together, our results provide insights into the critical roles of the bidentate chelating ligands that define this new class of Ru catalysts for Z-selective olefin metathesis. First, the bulky N-substituted NHC chelator directs olefin interaction with the active Ru−alkylidene complexes and differentiates by steric repulsion the energies of the key transition states in which the metallacycles have substituents in different orientations. Second, the carboxylate acts as a versatile ligand that can shift between monodentate and bidentate coordination modes as necessary in supporting the intermediates and transition states; note that replacement of carboxylate with a monodentate ligand such as iodide would make the catalyst metathesis inactive.27c In conclusion, by studying the full mechanism of the real 1cat−phenylpropene reaction, we have acquired significant new knowledge about the ruthenium-catalyzed Z-selective olefin metathesis. We wish to recapitulate the new findings in contrast to the work of ref 28: (1) We have shown how the six-coordinate and chelated Ru precatalyst transforms itself to initiate the metathesis reaction. In addition, we have studied the regeneration of the active catalyst. (2) From studying the initiation part, we have shown that the resulting Ru−benzylidene complex 6 is the active catalyst. In contrast, without considering the initiation part, ref 28 did not identify exactly what the active catalyst is in the 2cat−propene system, as several active complexes could result from the initial metathesis event(s). (3) We have shown that the active complex 6 leads to both the (Z)- and (E)-olefin products and that the Z selectivity arises from different kinetic barriers in the key transition states. In contrast, ref 28 suggested that, in

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