Note pubs.acs.org/Organometallics
Does the Ruthenium Nitrato Catalyst Work Differently in Z‑Selective Olefin Metathesis? A DFT Study 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: In the new class of N-heterocyclic carbene (NHC) chelated ruthenium catalysts for Z-selective olefin metathesis, the nitrato-supported complex 3cat appears distinct from all the other carboxylato-supported analogues. We have performed DFT calculations (B3LYP and M06) to elucidate the mechanism of 3cat-catalyzed metathesis homodimerization of 3-phenyl-1propene. The six-coordinate 3cat transforms via initial dissociation and isomerization into a trigonal-bipyramidal intermediate (5), from which two consecutive metathesis reactions via the side-bound mechanism lead to (Z)PhCH2CHCHCH2Ph (major) and (E)-PhCH2CHCHCH2Ph (minor). In the overall mechanism, 3cat functions similarly to the pivalate-supported analogue 1cat. The substitution of a smaller nitrato group does not change the side-bound olefin attack mechanism for either the initiation or homocoupling metathesis. The chelation of the NHC ligand causes this class of Ru catalysts to favor the side-bound over the bottom-bound mechanism. The calculated energetics corroborate the experimental observation that 3cat is somewhat more active than 1cat in catalyzing the homodimerization of 3-phenyl-1-propene.
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INTRODUCTION The Z-selective olefin metathesis catalyzed by NHC-chelated ruthenium−carbene complexes represents a significant advancement in this active field.1−3 The new ruthenium catalysts, as exemplified by 1cat−3cat (Figure 1), have shown high
Scheme 1. Reaction Sequence of 1cat-Catalyzed Phenylpropene Homodimerization
Figure 1. Ru-based catalysts for Z-selective olefin metathesis reactions, where Mes = 2,4,6-trimethylbenzene.
activity and Z selectivity in a broad range of metathesis reactions. We have recently performed a thorough DFT study of the mechanism and origins of Z selectivity for the metathesis homodimerization of 3-phenyl-1-propene (phenylpropene hereafter) with 1cat (Scheme 1).4 The six-coordinate 1cat undergoes initial Ru−O(isopropoxy) bond dissociation, followed by geometric isomerization, affording a five-coordinate trigonal-bipyramidal (tbp) intermediate (1). Two consecutive metathesis reactions via the side-bound mechanism lead to the (Z)- and (E)-olefin homodimers. An earlier DFT study by Houk and co-workers involving 2cat and truncated model substrates also established the side-bound mechanism.5 3cat appears to be different in this class of catalysts, in that the nitrato ligand is weaker and smaller in comparison with the © 2012 American Chemical Society
carboxylates. Experimentally, the use of 3cat leads to improved activity for some substrates.3 More recently, 3cat has been found to catalyze Z-selective ring-opening metathesis polymerization.6 With these qualities, 3cat has the potential to become Received: October 18, 2012 Published: December 4, 2012 8654
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a common catalyst for Z-selective olefin metathesis. Thus, it is worthwhile to elucidate the mechanism of 3cat-catalyzed olefin metathesis reactions. Does 3cat work similarly to or differently from 1cat? Particularly, does the substitution of nitrato for carboxylato alter the side-bound mechanism? This Note addresses these questions on the basis of a DFT study of the mechanism of the experimentally performed homodimerization of phenylpropene with 3cat (Scheme 2). As with the study of
propene, which would lead to the Ru−alkylidene complex 8 or 14 (Figure 3) that could enter the subsequent dimerization stage. We have computed the transition states (TSs) and intermediates for all eight possible pathways leading to 8 or 14 (Supporting Information, Schemes S2 and S3), and the free energies and enthalpies for all TSs and intermediates in the most favorable pathways are summarized in the energy profiles shown in Figure 3. In path I1, TS3 was located successfully for what could be a concerted [2 + 2] cycloaddition reaction leading to the metallacyclobutane 6, which is similar to the 1cat−phenylpropene system.4 Complex 6 isomerizes to intermediate 7 via TS4 by rotating the nitrato ligand counterclockwise, passing through the NHC adamantyl group, and this is different from the 1cat−phenylpropene system, where the bulkier pivalate could only rotate clockwise to steer clear of the adamantyl group. Cycloreversion of 7 via TS5 requires an activation free energy of 10.7 kcal/mol and leads to the 16-electron Ru−alkylidene complex 8. TS5 has a putative six-coordinate geometry around Ru with the emerging Ru−O (marked by the dashed line) bond at 2.32 Å, whereas the corresponding TS for the 1cat system is five-coordinate and trigonal bipyramidal.4 The higher coordination number of TS5 helps stabilize TS5 as compared with the five-coordinate TS6 and TS8 in the less favorable pathways (Supporting Information, Scheme S2). Path I5, the most favorable pathway to complex 14, is analogous to path I1, with metallacyclic TSs and intermediates where the R and R′ groups are on opposite sides of the metallacyclobutane ring. The highest activation barriers for path I1 (TS5) and path I5 (TS13) are isoenergetic.20 The continuing metathesis reaction of complex 8 or 14 with phenylpropene could be productive, leading to the (Z)- and (E)-olefin homodimers and complex 21, or it could be transalkylidenation, generating two new Ru−alkylidene complexes which could then undergo homocoupling with phenylpropene. We have explored all these possibilities (Supporting Information, Schemes S4−S8). Here we discuss the most favored homocoupling pathways to the (Z)- and (E)-olefin products, both of them involving complex 8. As shown by the free energy profiles in Figure 4, the Z-selective path C2 begins with phenylpropene coordination to 8, forming a six-coordinate complex (18). With the two R (benzyl) groups in a parallel orientation, complex 18 cycloadds via TS18 to give the metallacyclobutane 19, which has both R groups on the same side of the ring. Complex 19 easily isomerizes to intermediate 22 via TS21 by rotating the nitrato ligand clockwise, passing
Scheme 2. Metathesis Homodimerization of Phenylpropene with 3cat
the 1cat−phenylpropene system, B3LYP and a mixed basis set of SDD for Ru and 6-31G(d) for other atoms were used in geometry optimizations, and single-point energies in THF solution were calculated with M06/SDD-6-311++G(d,p)/SMD because the M06 functional was found to give more accurate energies for Ru complexes in olefin metathesis.4,5,7−12 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.13−18 All calculations were performed with Gaussian 0919 (see the Supporting Information for more references and details regarding the computational methods).
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RESULTS AND DISCUSSION Similar to the case for 1cat, 3cat undergoes initial Ru− O(isopropoxy) bond dissociation via TS1, forming the fivecoordinate square-pyramidal intermediate 4 (Figure 2). In relative free energy ordering, TS1 (18.0 kcal/mol) is comparable to the dissociation transition state for 1cat (17.2 kcal/mol), and this agrees with the observation that the two catalysts give similar initiation rates.3,4 Despite the smaller size of the nitrato group, the bottom-bound mechanism, which begins with phenylpropene attacking the open bottom coordination site in 4, would be disfavored due to a high activation barrier in the pathway (Supporting Information, Scheme S1). This is similar to the case for the 1cat− phenylpropene system.4 Complex 4 can isomerize via TS2 to the trigonal-bipyramidal (tbp) complex 5 (Figure 2). The tbp geometry of complex 5 would facilitate the ensuing side-bound attack by phenyl-
Figure 2. Free energy profile for the steps that 1cat undergoes prior to reaction with olefin, with solvent-corrected free energy and enthalpy (similarly hereafter). 8655
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Figure 3. Free energy profiles for the most favorable pathways leading to the Ru−alkylidene complexes 8 (in red) and 14 (in blue).
Figure 4. Free energy profiles for the homocoupling pathways leading to [Z] (in red) and [E] (in blue): [Z] = (Z)-RCHCHR and [E] = (E)RCHCHR.
through the NHC adamantyl group.21 Cycloreversion of 22 via TS22 requires an activation free energy of 10.8 kcal/mol and leads to the (Z)-olefin product and the 16-electron Ru− methylidene complex 21. Path C3 goes through metallacyclic TSs and intermediates (TS23, 24, and TS24) that each have the two R (benzyl) groups on opposite sides of the metallacyclobutane ring, and this configuration leads to the (E)-olefin product. Direct cleavage of 24 via TS24 generates 25, a Ru−methylidene complex with the (E)-olefin product coordinated. The dissociation of 25 via TS25 leads to 21 and (E)-RCHCHR, a step that can be viewed as the reverse of an interchange coordination reaction.4 The overall activation barrier for the formation of (E)-RCHCHR is TS24, which is higher than that (TS18) for forming (Z)-RCHCHR, and the difference (TS24 − TS18 = 1.7 kcal/mol) gives a calculated Z selectivity (95%) consistent with experimental results.3,22 The steric factors cause TS18 to be lower than TS23 and TS22 to be lower than TS24, which leads to the order for the highest barriers TS18 and TS24 that gives rise to the Z selectivity. In the (Z)-olefin-forming TS18 and TS22, the two R (benzyl) groups are on the same side of the metallacyclobutane ring and point away from the bulky NHC mesityl substituent, thereby
reducing steric repulsion. In contrast, in the (E)-olefin-forming TS23 and TS24, the R (benzyl) groups are on opposite sides of the metallacyclobutane ring, with one of them pointing closer toward the NHC mesityl substituent. The origins of Z selectivity are similar to those found for the 1cat−phenylpropene system. Furthermore, we considered the possibility that complex 8 could isomerize to a square-pyramidal structure, thereby opening up the distal coordination site for a bottom-bound mechanism for the metathesis homocoupling. We explored the possible rate- and selectivity-determining transition states of cycloaddition and cycloreversion that would lead to (Z)- and (E)-olefin products (Supporting Information, Scheme S9). The key TSs in the most favorable Z- and E-selective pathways are respectively TSB4 (ΔG⧧ = 34.3 kcal/mol) and TSB6 (ΔG⧧ = 33.4 kcal/mol), both of which are much less stable than the corresponding TSs (TS18 and TS24, Figure 4) in the sidebound mechanism. Thus, despite the smaller size of the nitrato group, its substitution does not change the side-bound olefin attack mechanism for either initiation or homocoupling metathesis. The findings support the observation by the model study that 8656
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ref 19. This material is available free of charge via the Internet at http://pubs.acs.org.
the chelation of the NHC ligand leads this class of Ru catalysts to favor the side-bound over the bottom-bound mechanism due to a combination of electronic and steric effects.5,23,24 A full catalytic cycle requires the Ru−methylidene complex 21 to continue reacting with phenylpropene to regenerate the active catalyst 8 favorably. We have considered all possible pathways of regeneration and shown that complex 8 is indeed the most favored product (Supporting Information, Figure S2). The regeneration also gives ethylene gas as a byproduct whose continuous evolution drives the overall reaction to high conversion. We have discussed the similarity in the initiation rates of 1cat and 3cat (see above). Here we compare 1cat and 3cat in terms of activity and Z selectivity for the metathesis homodimerization of phenylpropene in connection with the experimental results. The Z selectivities observed for this reaction are 95% for 1cat and 92% for 3cat,3 and the calculated values are 97% and 95%, respectively. Although there is an acceptable error between calculation and experiment, the trend is consistent regardless; that is, 1cat gives a slightly higher Z selectivity than 3cat both experimentally and computationally. Experimental runs indicate 3cat to be more active than 1cat for the homocoupling of phenylpropene, with a 90% versus 79% conversion in 3 h under the same conditions.3 For the 3catpromoted reaction, the active catalytic species after initiation is complex 8, the highest barrier of turnover-limiting cycloaddition/cycloreversion for the Z-selective path is TS18 (Figure 4), and therefore the relative overall energy barrier (mass balanced) from the reference level of 8 is 10.8 kcal/mol. In comparison, the corresponding energy barrier for the 1catpromoted Z-selective path is 12.5 kcal/mol,4 which is 1.7 kcal/ mol higher. This difference correlates qualitatively with the higher activity observed for 3cat. The energy barriers for the minor (E)-olefin-forming paths effected by 3cat and 1cat also indicate the former to be more active than the latter. In conclusion, DFT calculations have been performed to investigate the mechanism and origins of Z selectivity of olefin metathesis catalyzed by the NHC- and nitrato-chelated ruthenium complex 3cat. In general, 3cat works by a mechanism similar to that for 1cat, the carboxylato analogue. One major difference is in the manner in which the nitrato and pivalate groups rotate in the isomerization processes of the intermediates, which is attributed to their different sizes. The smaller size of the nitrato group also leads to higher coordination numbers for several intermediates and transition states. Nonetheless, substitution of the nitrato group does not alter the side-bound olefin attack mechanism that results from the chelation of the NHC ligand. The nitrato and carboxylato ligands both play an important role in supporting the intermediates and transition states by shifting as necessary between monodentate and bidentate coordination modes. The calculated free energy surfaces allow for a qualitative explanation of the experimental finding that 3cat is somewhat more active than 1cat for the homodimerization of phenylpropene. Hopefully, these findings will be helpful to the further study and utilization of 3cat-catalyzed olefin metathesis reactions.
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AUTHOR INFORMATION
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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REFERENCES
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ASSOCIATED CONTENT
S Supporting Information *
Text, figures, and tables giving details of computational methods, additional computational results, and the complete 8657
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