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DFT Study on the Relative Stabilities of Substituted Ruthenacyclobutane Intermediates Involved in Olefin CrossMetathesis Reactions and Their Interconversion Pathways Katherine Paredes-Gil,† Xavier Solans-Monfort,*,‡ Luis Rodriguez-Santiago,‡ Mariona Sodupe,‡ and Pablo Jaque*,† †

Departamento de Ciencias Quı ́micas, Facultad de Ciencias Exactas, Universidad Andres Bello, Av. Republica 275, Santiago, Chile Departament de Quı ́mica, Universitat Autònoma de Barcelona, 08193 Bellaterra, Spain

‡

S Supporting Information *

ABSTRACT: DFT (M06-L) calculations have been used to determine the relative stabilities of the metallacyclobutane intermediates arising from the cross-metathesis reactions of terminal olefins as well as to get insights into the origin of the nondetection of the α,βsubstituted species. For that, we discuss the structures, NMR signatures, stabilities with respect to separated reactants, and experimentally proposed interconversion pathways of all potential metallacyclobutane intermediates arising from propene and styrene homocoupling. For the case of propene, the unsubstituted and monoand disubstituted metallacycles are lower in Gibbs energy than the separated reactants under the NMR experimental conditions. Moreover, for the same number of substituents, regardless of their nature, the metallacycles presenting substituents at the Cα carbons are always lower in energy than those presenting substituents at Cβ, the energy difference being between 1.7 and 8.8 kcal mol−1. The computed energy barriers associated with the olefin and carbene rotation processes, two of the experimentally proposed pathways for the metallacycle interconversion, are low and are in excellent agreement with the values previously determined through NMR studies. Cycloaddition and cycloreversion energy barriers are also low, and in fact, there is not a significant difference between the barrier heights of the processes leading to observed or nonobserved intermediates. Therefore, the nondetection of metallacyclobutane intermediates with substituents in Cβ seems to arise from their lower stability in comparison with the isomers with substituents in Cα, which makes their detection not feasible under thermodynamic equilibrium conditions. That is, for cross-metathesis processes involving small terminal alkenes and activated carbenes, the nature of the observed metallacycles is based on thermodynamic control. The preference of having the substituents in Cα is attributed to the formation of stronger M−C and C−C bonds during the cycloaddition when the substituents are in an α position due to higher charge transfer from the original alkene fragment to the metal carbene.

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relevant reactions for organic synthesis.4−6,12,13 The reaction only takes place in the presence of a suitable transition-metal catalyst. The existing molecular catalysts can be divided into the two following groups: the early-metal Mo- and W-based alkylidene complexes also known as Schrock type catalysts,5,10,14−17 and the catalysts generally denoted Grubbs type based on ruthenium carbenes.6,8,9,18−21 The general formula of Ru-based catalysts is Ru(CHR1)(L1)(L2)(X)(Y), and depending on the nature of L1 they are classified as firstgeneration18,19 (L1 = phosphine) or second-generation20,9 (L1 = N-heterocyclic carbene (NHC)) (Scheme 2). Species shown in Scheme 2 (A−F)15−24 are catalyst precursors.25−30 For the phosphine-containing Ru-based complexes, the active species are obtained by the dissociation of the L2 ligand and a cross-metathesis process that exchanges

INTRODUCTION Olefin metathesis is a redistribution of carbon−carbon double bonds that allows the conversion of the original reacting alkenes in new product olefins (Scheme 1).1−10 Currently, this reaction is widely used in the preparation of new polymeric materials,7,11 biologically active species, and relevant organic compounds with low energy cost, high yields, and significant selectivities.12,13 Therefore, it has become one of the most Scheme 1

Received: July 14, 2014 Published: October 15, 2014 © 2014 American Chemical Society

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dx.doi.org/10.1021/om500718a | Organometallics 2014, 33, 6065−6075

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isomerization of the final products. In this context, an understanding of the stability of the metallacyclobutane intermediates resulting from processes i−x is of interest. It is worth noting that the synthesis of the cationic carbenes, such as those named as complex G in Scheme 2,22 has led to highly active precursors due to their rapid initiation as a consequence of the absence of the phosphine dissociation step. This has allowed the detection of several metallacyclobutane intermediates by means of NMR spectroscopy.43−47 These species are usually observed at low temperatures (experiments are performed between −40 and −80 °C). In particular, Wenzel and Grubbs44,47 reported the detection of several metallacyclobutane intermediates arising from the reaction between [(NHC)Cl2RuCH(PCy3)]+[BF4]− and different terminal olefins. The reaction of propene and 1-butene with G leads after 3 h to the formation of three different metallacycles (see Scheme 5 for the nomenclature of metallacyclobutane intermediates used in this work): MC-0, MC-1α, and trans-MC-2αα with yields of 45 (60), 29 (39), and 2 (1)%, respectively, for propene (1-butene). Interestingly, at longer reaction times and for the case of 1-butene only, the metallacycle cis-MC-2αα was also detected in smaller quantities than MC-1α and the trans-MC-2αα. This was attributed to the ethylene removal. Therefore, for the reaction with the less volatile 1-hexene, even at short reaction times, the MC-0 metallacycle concentration was small (8%) and allowed the detection of MC-1α, trans-MC-2αα, and cis-MC-2αα intermediates in a ratio of 40:37:15, respectively. In addition to the identification of metallacyclobutanes, Wenzel and Grubbs reported exchange between the Cα and Cβ carbons and the hydrogen atoms of the Cβ carbon, and they indicated that these processes occur without significant alkene coordination/decoordination.47 The former was proposed to take place through an alkene rotation at the olefin complex intermediate (Scheme 6). The measured energy barrier (ΔG⧧) associated with the Cα−Cβ exchange in the reaction involving 1-butene is 12.2 kcal mol−1 for the MC-0 and 10.0 kcal mol−1 for the MC-1α. On the other hand, the Cβ substituent exchange was suggested to occur either through an olefin flip or, more likely, through carbene rotation (Scheme 6). The energy barrier for the conversion between trans-MC-2αα and cis-MC-2αα when R is ethyl was determined to be 10.8 ± 0.03 kcal mol−1.47 Finally, Romero and Piers45 found that, the metallacycle MC0 exchanges ethene molecules at high ethene concentrations and this takes place through an associative mechanism that involves the coordination of an alkene molecule to MC-0. The experimentally determined kinetic values are ΔH⧧ = 13.2 kcal mol−1 and ΔS⧧ = −15 cal mol−1 K−1. Overall, from an experimental point of view four mechanisms have been suggested to explain the dynamic behavior of the ruthenacyclobutanes: (i) alkene rotation, (ii) carbene rotation, (iii) olefin flip, and (iv) alkene exchange through an associative pathway (Scheme 6).

Scheme 2

the initial carbene substituents by those involved in the productive reaction (from RuCHR to RuCHR′) (Scheme 3). Today’s accepted mechanism for the productive alkene metathesis reaction was initially proposed by Hérrinson and Chauvin.4,31 For the particular case of ruthenium catalysts, it involves four steps (alkene coordination (b), cycloaddition (c), cycloreversion (d), and alkene dissociation (e)) and, when X and Y are halide ligands, it takes place with the alkene coordination trans to L1 (Schemes 2 and 3).32−35 Regarding the nature of the initial reactants and/or the obtained product, many different derivative processes can be envisaged. Among them, cross metathesis (CM) (Scheme 1) is used in several processes of great importance such the Shell Higher Olefin Process (SHOP) or the production of 1-hexene and neohexene (3,3-dimethyl-1-butene).36 For most catalysts, this reaction leads to the major formation of the (E)-olefins as result of its higher thermodynamic stability in comparison with the (Z)-olefins. Catalysts able to form selectively the Z product have only very recently been synthesized.23,24,37−42 The E/Z selectivity arises from the energetic balance of all potential reactions that can take place during the catalytic process. This implies the reaction of all the alkenes potentially present in the reaction mixture, the different carbene species that can be formed, and the relative orientations of reactant and catalyst (Scheme 4). In this way, on consideration of a unique terminal olefin (RCHCH2) as reactant (homocoupling reaction) and identical X and Y ligands, the total number of reactions is 14 (Scheme 4). Note that the processes named vii− x imply two different reactant−catalyst relative orientations that translate into eight different processes. Within these 14 processes, only vii leads to the desired (Z) or (E)-alkenes and iii forms ethylene, the side product of the cross-metathesis process. All other reactions correspond to nonproductive steps or to the reverse processes of iii and vii or imply the Scheme 3

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Scheme 4

lacyclobutane intermediate has attracted the attention of several groups. For d0 metals two different isomers exist, one presenting a trigonal-bipyramidal coordination around the metal center and the other one having a square-basedpyramidal geometry.49,52,78,79 In the case of ruthenium metallacycles the trigonal-bipyramidal complex is the only one observed43−45,47 and calculations show that the squarebased-pyramidal species is strongly disfavored.80,81 Regardless of the metal center, the trigonal-bipyramidal isomer presents elongated Cα−Cβ bonds and a relatively short Ru···Cβ distance that has been related to the existence of an agostic interaction in the case of ruthenium.69 For molybdenum, it is more accepted that the short M···Cβ distance arises from the M−Cα orbital interactions.49

Scheme 5

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COMPUTATIONAL DETAILS

All calculations have been performed on the isolated molecules within the density functional theory (DFT) approach using the Gaussian 09 program.82 Molecular structures were fully optimized with the M06L83 exchange-correlation functional. This functional has been shown to provide good results in related studies on the olefin metathesis reaction.60,71,84−87 The ruthenium core electrons were described by quasi-relativistic effective core pseudopotentials (RECPs) developed by the Stuttgart group and the so-called MWB28,88−90 which replaces the 28 inner electrons by a nonlocal effective potential. The resting electrons were described with the associated (8s7p6d)/[6s5p3d] basis set, which was augmented with an f-type polarization function.91 All other elements (C, H, N, Cl) were represented by the 6-31+G(d,p) basis sets.92,93 We carried out the vibrational analysis of all located stationary points to ensure their nature (intermediate or transition structure). We verified the nature of the reactants and products connected through all located transition structures by using either an intrinsic reaction coordinate (IRC) or alternatively applying the eigenvector corresponding to the imaginary frequency to the located transition structure and performing the subsequent optimization of the resulting structure. Thermal contributions to enthalpy and Gibbs energy were evaluated using the harmonic oscillator and rigid rotor models as it is implemented in the Gaussian 09 package at a pressure of 1 atm and two different temperatures (298 and 186 K). For energy differences implying a change of molecularity, separated reactants were considered as the origin of energies, except for the case of the ethene exchange mechanism proposed by Romero and Piers.45 In this case,

Scheme 6

In this paper we study the relative stabilities of all potential metallacyclobutanes arising from the reactivity of two terminal olefins (propene and styrene) with a second-generation Grubbs type catalyst as well as their corresponding NMR signatures. Moreover, we evaluate several pathways for the metallacyclobutane substituent exchange. At this point, it is worth mentioning that many theoretical contributions devoted to the alkene metathesis and its derivative processes either with d0 metal Schrock type catalysts48−56 or Ru-based carbenes can be found in the literature.30,32−35,57−77 In particular, the metal6067


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both separated reactants and a “supermolecule” approach in which the second ethene molecule is in the vicinity of the metallacycle were considered as an origin of energies. The former probably overestimates the entropic cost of putting the two reactants together, especially when modeling high reactant concentrations, whereas the latter fully neglects this entropic cost. Therefore, the two approximations correspond to the two limiting situations. Solvent effects were included by performing single-point calculations at the gas-phase optimized geometries using the SMD continuum solvation model,94 the same basis sets, and dichloromethane as solvent. The NMR chemical shifts were computed using the GIAO method95 with the same M06-L functional and the IGLO-II basis set.96,97 Similar methodologies for computing NMR responses have been used by some of us,98,99 giving reasonably accurate results. The reported atomic charges are obtained with the natural population analysis of Weinhold et al.100

Supporting Information. The metallacyclobutane fragment is essentially planar. The M−Cα−Cβ−Cα′ dihedral angle varies from 0.01 to 10.94°. The Cα−Cβ bonds are usually slightly elongated (between 1.547 and 1.667 Å) with respect to the typical C−C simple bonds, and the M···Cβ distances are rather short and vary from 2.210 to 2.273 Å. The main geometrical features are not affected by the nature and number of substituents in the metallacyclobutane fragment and are equivalent to the values published before.68,69,81 In particular, the M−Cα bond becomes generally longer when Cα has a substituent (CH3 or Ph). Moreover, the Cα−Cβ bonds also generally elongate with the addition of substituents in either an α or β position. A subtler trend is found for the Ru··Cβ distance. The presence of methyl or phenyl substituents in the Cα position generally produces a shortening of the Ru···Cβ distance. In contrast, the presence of substituents in the β position has the opposite effect with elongations of about 0.04 Å for the case of R = CH3. This leads to the fact that there is not an inverse correlation between the length of the Cα−Cβ donor bonds and the M···Cβ distances as should be expected in agostic interactions. That is, for R = CH3, the longest M···Cβ distance (MC-1β) is not associated with the shortest Cα−Cβ bonds (MC-0) and the shortest M···Cβ distance (cis-MC-2αα) is not associated with the longest Cα−Cβ bond (cis-MC-2αβ and the trisubstituted species). 13 C and 1H NMR Chemical Shifts. We also computed the 1 H and 13C NMR chemical shifts. Values for the metallacyclobutanes associated with the propene homocoupling metathesis are summarized in Table 1. The NMR signatures for species related with the reaction of styrene can be found in the Supporting Information (Table S1). The computed values are reasonably similar to those obtained experimentally.43,44,47 Although direct comparison can only be made in four cases, our methodology reproduces the large difference between the 13C NMR chemical shifts of Cα and Cβ as well as the deshielding of protons in an α position in comparison to the shift of hydrogens at a β position. The mean absolute errors are around 3.3 ppm for the shifts of the Cβ carbon, around 7 ppm for those of the Cα carbons, and less than 1.7 ppm for the hydrogen atoms. More interestingly, calculations are able to reproduce the effect in the 13C and 1H chemical shifts originated by the presence of a methyl in the Cα position (compare MC-0 with MC-1α and/or trans-MC-2αα): (i) the chemical shift of the Cα bearing the methyl increases about 25 ppm, (ii) this is accompanied by a smaller increase of the chemical shift of Cβ of between 6 and 13 ppm, and (iii) the shift of the remaining Hα also increases by more than 1.0 ppm. That is, the addition of methyl groups produces a general deshielding of carbon and hydrogen atoms of the Cα−Cβ−Cα′ fragment as expected. Similar trends are predicted for the nondetected MC-2αβ and trisubstituted metallacycles (MC-3). It is also remarkable that calculations are even able to reproduce the subtle different chemical shifts of the Hβ hydrogens of cis-MC-2αα. The computed chemical shift of the Hβ (−1.1 ppm) trans to the methyl substituents is more negative than the chemical shift of the Hβ (−0.3 ppm) cis to the CH3. Finally, it is worth mentioning that the same general features are observed for the 13 C and 1H protons of the metallacycles involved in the reaction with styrene. The Cα carbons present computed NMR chemical shifts of about 120−150 ppm, the signals of the Cβ carbons appear between 10 and 30 ppm, and the addition of phenyl groups in the ring produces a general increase of all 1H and 13C chemical shifts.

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RESULTS AND DISCUSSION This section is divided into four parts. In the first two parts we summarize the main geometrical features and the general NMR signatures of all metallacyclobutane intermediates (Scheme 5). In the third part, we focus our attention on the relative stabilities of the different intermediates with respect to separated reactants. Finally, in the last part we present the energetics of the different processes proposed for the metallacyclobutane interconversion. Geometrical Features of Unsubstituted and Mono-, Di-, and Trisubstituted Ruthenacyclobutanes. Figure 1 shows the coordination sphere of the metal center as well as the main distances of the metallacyclobutane fragment for the intermediates arising from the homocoupling metathesis of propene. The analogous metallacycles resulting from the reaction with styrene can be found in Figure S1 of the

Figure 1. Optimized geometries of all considered metallacyclobutane intermediates resulting from propene homocoupling (distances in Å). 6068


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Table 1. 13C and 1H NMR Chemical Shifts (ppm) for All Considered Metallacyclobutanes Arising from the Homocoupling Metathesis Reaction of Propenea metallacycle MC-0 MC-1α MC-1β trans-MC-2αβ cis-MC-2αβ trans-MC-2αα cis-MC-2αα trans,trans-MC-3 trans,cis-MC-3 cis,cis-MC-3

Cα 103 131 112 141 132 127 124 132 138 127

(94.9)b (120.2)b

(123.0)c (121.1)c

Cβ 5 (2.37)b 11 (9.0)b 12 19 15 18 (14.0)c 17 (12.6)c 24 20 16

Cα′ 104 100 112 107 110 127 125 133 125 128

Hα

(93.7)b (89.3)b

4.8 (6.8)b 5.9 (7.8)b 4.3, 5.3 4.0 4.8 6.3 (7.8)c 5.9 (7.1)c 5.7 5.7 5.9

(123.0)c (121.1)c

Hβ −1.0 −1.0 −0.4 −0.6 −0.6 −0.8 −1.1 −0.7 −0.7 −0.7

(−2.7)b (−2.8),b −0.6 (−2.1)b

(−2.4)c (−3.2),c −0.30 (−1.7)c

Hα′ 4.7 (6.6)b 4.7 (6.6),b 4.7 (6.1)b 4.6, 4.9 6.1 5.3 4.6 6.2 6.3 (7.8)c 5.5 (7.1)c 5.3 5.7 5.5

a See Scheme 5 for metallacyclobutane labeling. Experimental values are given in parentheses. bTaken from refs 43 and 44. cTaken from ref 47 for the reaction with 1-butene.

between these two processes is very small (0.3 kcal mol−1) and suggests that both species should be detected experimentally, if methylidene, ethene, and propene are present in similar amounts in the reaction mixture and equilibrium conditions are reached. Moreover, the significant Gibbs energy difference (ΔΔG298 K = 4.5) between the MC-1α and MC-1β metallacycle intermediates evidences that the position of the methyl (α or β) has an important effect on the stabilization of the metallacycle. The reaction between ethylidene and the four potentially present olefins during the propene homocoupling leads to eight different metallacycles whose computed Gibbs energies at 298 K are between −6.1 and 2.7 kcal mol−1 (Table 2). Again, the most favorable reaction leads to the formation of MC-1α, and it involves ethene as the reacting olefin. The reaction Gibbs energy at 298 K is −6.1 kcal mol−1. All other reactions present significantly less favorable reaction Gibbs energies, the ΔG298 K values for the other processes ranging from −2.5 to 2.7 kcal mol−1. The reaction of propene with ethylidene can lead to four metallacycles. The most favorable ones are those having the two methyl substituents at the Cα carbon. The energy differences between MC-2αα and MC-2αβ metallacycles range from 1.7 to 3.0 kcal mol−1 depending on the isomer, and this suggests a MC-2αα/MC-2αβ ratio larger than 90/10 under equilibrium conditions. Within the MC-2αα metallacycles, the isomer presenting the methyl substituents in trans positions is more stable than the isomer with the methyl groups in cis positions. The energy difference between trans-MC-2αα and cis-MC-2αα is 0.5 kcal mol−1, which suggests a trans/cis ratio of around 70/30 under equilibrium conditions. With regard to the less stable MC-2αβ ruthenacycles, calculations suggest that cis-MC-2αβ is marginally more stable than transMC-2αβ, at variance to what is found for the respective α,βdisubstituted metallacyclobutanes involved in ring-closing metathesis processes of small- and medium-sized rings. In these cases, the preference for cis-MC-2αβ is more pronounced due to the chain constraints of the ring.76,77,81 Trisubstituted metallacycles are even less stable with respect to the separated reactants than the mono- or disubstituted intermediates. The three potential species (trans,trans-MC-3, trans,cis-MC-3, and cis,cis-MC-3) have quite similar energies, the trans,trans-MC-3 intermediate being the most stable one, while the cis,cis-MC-3 complex is the trisubstituted metallacycle having the highest Gibbs energy (see the absolute energies reported in the coordinate file of the Supporting Information). It is worth noting that these relative stabilities of the trisubstituted

Relative Stabilities of Unsubstituted and Mono-, Di-, and Trisubstituted Ruthenacyclobutanes. Here we discuss the relative stabilities of the different metallacyclobutanes with respect to separated reactants. Since it is not easy to fix a unique origin of energies, we have divided the metallacyclobutanes into two groups, as established in Scheme 4. Table 2 reports all Table 2. Relative Gibbs Energies (Ggp + ΔGsolv) with Respect to Separated Reactants of the Metallacyclobutane Intermediates Formed during Propene Homocouplinga

a

See Scheme 5 for metallacyclobutane labeling. Values are given in kcal mol−1.

Gibbs energy values associated with propene homocoupling. The values for the styrene homocoupling can be found in the Supporting Information (Table S2). It is worth mentioning that, since the mesityl groups of the N-heterocyclic carbene ligand are not completely perpendicular to the metallacycle,65 several different conformations are possible, as illustrated in the Supporting Information (Scheme S1). We have computed all these conformations, but in Table 2 we only report the energetics of the most stable species. The results for the remaining conformers are given in Table S3 in the Supporting Information. The reaction of methylidene with ethene and propene can lead to three different metallacyclobutanes whose relative Gibbs energies at 298 K with respect to separated reactants lie between −10.9 and −6.4 kcal mol−1. According to the present calculations, the most favorable reactions are that between methylidene and propene leading to the metallacyclobutane MC-1α (ΔG298 K = −10.9 kcal mol−1) and that between methylidene and ethene leading to the parent metallacycle MC0 (ΔG298 K = −10.6 kcal mol−1). The Gibbs energy difference 6069


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propene and the carbene complex at the geometry of the reactants), a reorganization of the two fragments in its triplet state, and finally Ru−C and C−C bond formation from the fragments at the geometry of the intermediate that leads to the metallacyclobutane. As detailed in Table 3, the largest differences between MC-1α and MC-1β as well as between trans- or cis-MC-2αα and trans- or cis-MC-2αβ arise from the Ru−C + C−C bond formation. This correlates with a stronger electron donation from the organic moiety to the metal center when the substituent is in an α position. This is evidenced by the larger positive charge of the original propene fragment in the MC-1α and trans- and cis-MC-2αα metallacycle intermediates in comparison to MC-1β and trans- and cisMC-2αβ, respectively (Table 3). Therefore, it seems that placing the substituent at Cα favors electron donation from the original alkene fragment to the metal−carbene species, favoring the M-C and C−C bonding, which translates in a larger preference for the metallacycles with substituents at the α position. Energetics for the Metallacyclobutane Interconversion. We have studied the metallacyclobutane interconversion pathways that imply the unsubstituted (MC-0), the monosubstituted (MC-1α and MC-1β), and disubstituted (MC-2αα, MC-2αβ) ruthenacyclobutanes (Scheme 5) having a methyl as R group. Therefore, we have not limited our exploration to the experimentally observed processes. We also considered the processes leading to MC-1β and MC-2αβ to get further insights into the origin of their lack of observation. Here we only report the interconversion processes arising from the most stable metallacyclobutane conformers already reported in the previous section. However, we have verified by computing the energy barriers associated with some of the less stable conformers that the ΔG⧧ values are only slightly affected by the relative orientation of the alkene and/or carbene substituents with respect to the mesityl groups (Table S4 of the Supporting Information). Moreover, we have explored the ethene exchange through an associative pathway for the unsubstituted (MC-0) system only, as was observed by Romero and Piers45 and already computed by Webster.101 The results for the ethene exchange are reported in Figure 2, and they are in qualitative agreement with those previously reported by Webster.101 Our results and those by Webster highlight that the process does not occur at the metallacyclobutane intermediate (the computed energy barrier is 34.4 kcal mol−1 with respect to MC-0) but more likely at the alkene complex (ACp-0).101 The data reported here show that an alkene complex containing two ethene molecules is found to be a minimum of the potential energy surface. This complex (ACp-0···∥) lies 15.8 kcal mol−1 above AC-0 (the alkene complex containing one ethene molecule only) in terms of Gibbs energies and 25.9 kcal mol−1 with respect to MC-0. Its formation and the AC-0 regeneration require overcoming transition structures that are only 0.1 and 1.4 kcal mol−1 above the two olefin-containing intermediate in terms of potential energy. Addition of the thermal corrections makes these transition structures become marginally lower in Gibbs energy than the two olefin-containing intermediates, and thus, we assume 25.9 kcal mol−1 to be the energy barrier for the ethene exchange. It is worth noting that the computed values are significantly higher than the values determined experimentally45 and also they are higher than those reported by Webster.101 Part of this disagreement seems to be an overestimation of the entropic contribution. As mentioned in Computational Details,

metallacyclobutanes are equivalent to the results reported by Goddard and co-workers.35 Overall, in our opinion, the most remarkable finding is that, the symmetric (degenerate) pathways lead to metallacyclobutanes lower in energy than those in the pathway leading to either the internal (Z)- or (E)2-butene (MC-2αα vs MC-2αβ). The metallacyclobutanes involved in the homocoupling reaction of styrene are, in general, less stable with respect to separated reactants than those of the propene homocoupling. The computed values are between −10.6 and 20.7 kcal mol−1, and all but MC-0, MC-1α, and MC-1β are above the separated reactants at 298 K. The reaction leading to MC-1α is thermodynamically preferred to that leading to MC-1β, showing again the preference for placing the substituents at the α carbon. The reactions involving the substituted metal− carbene complex are even less favorable than those involving methylidene. As was found for propene, the formation of disubstituted intermediates (MC-2αα and MC-2αβ) is favored with respect to the formation of trisubstituted metallacycles (MC-3). Moreover, the disubstituted MC-2αα species, which have the substituents as far away as possible, are predicted to be more stable than the metallacycles presenting the substituents in contiguous carbons (MC-2αβ). Finally, same general features are observed for the relative Gibbs energies of all the metallacycles at 186 K with a Gibbs energy difference (ΔΔG298−186 K) between these two temperatures of ∼5−6 kcal mol−1. Overall, the computed energies are in good agreement with the experimental data.43−45,47 Calculations predict that, for the reaction of propene, the potential unsubstituted and mono- and disubstituted metallacyclobutane intermediates are more stable than the separated reactants under the conditions in which NMR experiments were performed and, thus, they are potentially observed. However, the energy difference between MC-1β and MC-1α as well as that between trans- or cis-MC2αα and trans- or cis-MC-2αβ is large enough to prevent the detection of the less stable MC-1β and the trans- and cis-MC2αβ isomers under equilibrium conditions. Therefore, the fact that MC-1β and MC-2αβ isomers are not observed does not come from the energy difference with respect to separated reactants but from an important preference for placing the substituents in an α position. With the aim of getting further insights into the preference for MC-1α and MC-2αα isomers with respect to MC-1β and MC-2αβ isomers, we performed an energy partitioning scheme similar to that introduced by Poater et al. for the formation of Mo and W metallacyclobutanes (Scheme 7 and Table 3).52 In this partitioning scheme, we assume that the process implies the breaking of the RuC and CC π bonds (evaluated as the energy difference between the singlet and triplet states of Scheme 7

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Table 3. Energies (kcal mol−1) Associated with the Energy Partitioning Analysis Described in Scheme 7a

a

metallacycle

ΔE

BDERuC

BDECC

ΔEdist

BDEcycle

Qalkeneb

MC-1α MC-1β trans-MC-2αα cis-MC-2αα trans-MC-2αβ cis-MC-2αβ

−27.8 −25.2 −22.6 −22.7 −20.3 −20.0

42.6 42.6 42.2 42.2 42.2 42.2

98.6 98.6 98.6 98.6 98.6 98.6

−20.3 −21.9 −19.5 −20.1 −22.1 −19.3

−148.7 −144.5 −143.9 −143.4 −139.0 −141.5

+0.26 +0.22 +0.26 +0.26 +0.21 +0.22

See Scheme 5 for metallacyclobutane labeling. bSum of the natural atomic charges for the alkene fragment.

Figure 2. Relative Gibbs (Ggp + ΔGsolv) energies with respect to separated reactants of intermediates and transition structures associated with the alkene exchange through an associative pathway. Energies are given in kcal mol−1. The superscript “a” indicates that these transition structures are higher in potential energy than ACp-0···∥.

Figure 3. Gibbs energy profile (Ggp + ΔGsolv) associated with the metallacyclobutane interconversions starting from trans-MC-2αα through alkene rotation (AR), carbene rotation (CR), and double-bond rotation (DBR). Energies are given in kcal mol−1.

lacyclobutane interconversion (see below), indicating that the former is less favorable. With regard explicitly to metallacyclobutane interconversion, we have explored the alkene rotation around the Ru···NHC axis (AR), the carbene rotation (CR), and the olefin flip (Scheme 6). These are the three pathways proposed by Wenzel and Grubbs.47 It is worth noting that we were not able to find any transition structure for the olefin flip and all our trials evolved either to the alkene complex or to the decoordination of the olefin. We explore also the possibility of a CC double-bond

we are considering separated reactants as the origin of energies and this may overestimate the entropic cost of getting ethene and the metallacyclobutane together. Webster considered a “supermolecule” in which ethene is already close to the metallacyclobutane complex as the origin of energies. When we take the same origin of energies, we find that ACp-0···∥ lies 19.0 kcal mol−1 above the metallacyclobutane intermediate, only 0.5 kcal mol−1 higher than the energy barrier reported by Webster. Overall, the values associated with the ethene exchange (see Figure 2) are higher than those proposed for the metal6071


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Figure 4. Optimized structures of the alkene complex intermediates (AC) and the transition structures associated with the metallacyclobutane interconversions starting from MC-trans-2αα. AR stands for alkene rotation, CR stands for carbene rotation, and DBR stands for double-bond rotation. Distances are given in Å.

Table 4. Relative Gibbs Energies with Respect to the Metallacyclobutane Intermediates of All Intermediates and Transition Structures Associated with the Metallacycle Exchange through Alkene Rotation (AR)a isomer

reactant

MC

TS(MCF)

AC

TS(AR1)

ACp

TS(AR2)

AC′

TS(MCF)′

MC′

isomer′

0 1α 1α 1β trans-2αα cis-2αα trans-2αβ trans-2αβ cis-2αβ cis-2αβ

10.6 10.9 6.1 6.4 2.5 2.0 −0.5 5.3 0.3 6.8

0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0

10.6 14.7 9.7 10.2 8.9 8.5 6.9 7.1 8.0 11.1

10.1 11.0 10.0 5.4 5.7 4.4 3.0 2.9 2.7 7.3

13.4 (12.2)b 17.0 10.5 (10.0)b 9.9 10.4 10.5 5.4c N.A.d 7.2 10.8

10.3 12.5 9.0 8.1 6.2 8.7 6.0 6.5 4.0 8.8

13.4 (12.2)b 14.3 10.9 (10.0)b 12.6 9.4 8.1c 7.8 15.1 8.2 11.0

10.1 9.8 10.0 6.6 4.9 5.7 2.3 8.1 1.0 7.7

10.6 14.6 10.1 10.3 10.2 9.6 6.3 16.2 4.3 11.5

0.0 4.4 0.0 −4.4 2.2 2.7 −2.5 0.0 −2.2 0.0

0 1β 1α 1α cis-2αβ trans-2αβ cis-2αα trans-2αβ trans-2αα cis-2αβ

See Scheme 5 for labeling. The experimental values are reported in parentheses. Energies are given in kcal mol−1. bValues taken from refs 44 and 47. These transition structures are higher in terms of potential energy than the intermediates they interconnect. dWe were not able to localize this transition structure. a c

rotation (DBR), in which one CH2 or CHR rotates 90°, while maintaining the other end fixed, as an alternative pathway. As expected, this possibility lies very high in energy. Therefore, hereafter we are going to describe only in detail the alkene (AR) and carbene (CR) rotation processes. As a representative example, Figure 3 presents the energy profile associated with the metallacyclobutane interconversion starting from transMC-2αα and Figure 4 shows the optimized structures of all the stationary points associated with these interconvertions. It is worth mentioning that the alkene rotation process interconverts trans-MC-2αα to cis-MC-2αβ while the carbene rotation connects trans-MC-2αα to cis-MC-2αα. The obtained relative energies with respect to the metallacyclobutane intermediate for all other isomers are given in Tables 4 and 5. In these tables, we have added for comparison the energetics of the alkene

complex (AC) and those of the separated fragments as well as those of the cycloaddition/cycloreversion (MCF) transition structures. Noteworthy, the cycloaddition and cycloreversion steps are found to present energy barriers between 7.1 and 16.2 kcal mol−1 above the metallacyclobutane intermediate, indicating that these processes are relatively easy, at least when small terminal alkene molecules such as those involved here are present. In general, the alkene rotation is the process interconverting two different alkene complexes presenting the transition structures that are lowest in energy. This process implies two steps as an alkene complex with the olefin parallel to the Cl− Ru−Cl axis similar to that computed by Straub58 is also a minimum of the potential energy surface. The Gibbs energy of this second alkene complex intermediate relative to the 6072


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Table 5. Relative Gibbs Energies with Respect to the Metallacyclobutane Intermediates of All Intermediates and Transition Structures Associated with the Metallacycle Exchange through Carbene (CR)a isomer 0 1α 1β trans-2αα cis-2αα trans-2αβ cis-2αβ

reactant

MC

TS(MCF)

AC

TS(CR1)

10.6 10.9 6.1 6.4 2.5 2.0 −0.5 5.3 0.3 6.8

0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0

10.6 14.7 9.7 10.2 9.0 8.8 7.1 16.2 8.1 11.5

10.1 11.0 10.0 5.4 5.7 4.8 2.9 8.1 2.8 7.7

16.0 18.8 15.2 11.4 11.5 (10.8)b 10.8 9.2 13.8 9.4 13.3

TS(CR2)

24.3 18.2 17.5 14.7 17.5

AC″

TS(MCF)″

MC″

10.1 11.0 9.2 5.4 5.3 4.9 2.0 8.1 4.0 7.7

10.6 14.7 10.1 10.2 9.3 9.0 7.3 16.2 7.9 11.5

0.0 2.7 2.7 0.0 2.0 2.6 −0.8 −1.5 0.8 1.5

isomer′ 0 1α′ 1β cis-2αα trans-2αα cis-2αβ trans-2αβ

a See Scheme 5 for labeling. The experimental values are reported in parentheses. Energies are given in kcal mol−1. bValue taken from ref 47 for the reaction of 1-butene.

The computed energy barriers agree well with the values obtained from NMR experiments.43−45,47 Alkene rotation is found to be 12.2 and 10.0 kcal mol−1 for MC-0 and MC-1α (R = CH2CH3) by means of NMR spectroscopy, and it is computed to be 13.4 and 10.9 kcal mol−1 for the same two metallacycles (R = CH3). Similar agreement is found for the carbene rotation: 10.8 νs. 11.5 kcal mol−1 for the trans/cis isomerization of MC-2αα (R = CH2CH3 in the experiments and CH3 in the computational model). This excellent agreement corroborates the viability of the proposed pathways. Moreover, the results reported here for the metallacycles that are not observed experimentally show that the exchange pathways leading to their formation as well as their cycloaddition and cycloreversion processes present energy barriers similar to and sometimes lower than those reported for the experimentally observed intermediates. Therefore, this suggests that they should be easily formed also and, thus, equilibrium conditions are rapidly reached. Overall, the lack of observation of metallacycles having substituents at Cβ does not arise from a highly energetically demanding formation but more likely because their higher Gibbs energy in comparison with the analogous species with substituents at Cα is sufficient to prevent their detection under equilibrium conditions. That is, in those cases involving small terminal alkenes and in which the precursor has a fast activation, it is expected that the observed metallacyclobutanes are determined by thermodynamic control. It is worth noting that this suggests that during the catalytic cycle of terminal olefin cross-metathesis reactions most of the processes are degenerate, with the substituents being placed as far away as possible at the α-carbons of the metallacyclobutane.

intermediate involved in the catalytic pathway is between 0.2 and 4.3 kcal mol−1. Therefore, it is slightly higher in energy than the alkene complex in which the carbene and the olefin are in the same plane. At the alkene rotation transition structures the substituents (R or H) of the alkene are eclipsed with either the carbene or the chlorine atoms justifying their higher energy. The computed Gibbs energy barriers with respect to the alkene complex are always small, the highest value being 8.6 kcal mol−1, and consequently, the energy barrier with respect to the metallacyclobutane involved in the process ranges between 17.0 and 7.8 kcal mol−1. The computed highest Gibbs energy barrier with respect to the metallacyclobutane intermediate corresponds to that of the monosubstituted MC-1α in which the substituted Cα carbon exchanges with the Cβ carbon (+17.0 kcal mol−1), thus forming MC-1β. The lowest Gibbs energy barriers correspond to the interconversions of the Cα and Cβ carbons between trans-MC-2αα and cis-MC-2αβ and between cis-MC-2αα and trans-MC-2αβ. Carbene rotation can occur through two different transition structures when the alkylidene group presents a methyl substituent, (i) the methyl group points away from the mesityl groups (CR1) or (ii) the methyl group points toward the mesityl ring (CR2), and thus, we explored the two potential pathways. The preferred rotation occurs with the methyl group anti to the N-heterocyclic carbene (NHC) ligand (CR1). Rotation with the methyl pointing toward the NHC (CR2) requires overcoming transition structures that are between 14.7 and 24.3 kcal mol−1 above the metallacyclobutane intermediate, whereas the rotation with the methyl group being anti to the NHC exhibits Gibbs energy barriers that are always lower than 18.8 kcal mol−1. In both cases the Ru···Calkene distances, which become around 3 Å, increase significantly and this elongation is more pronounced for the carbene rotation, implying a transition structure with the methyl group pointing toward the alkene molecule. This is related to the steric repulsion between the carbene groups and the coordinated alkene, and thus, the Ru···Calkene distances become slightly larger with the number of substituents. Overall, within the most favorable processes for each metallacycle, the highest Gibbs energy barriers with respect to the corresponding intermediate are computed to be those of the MC-1α and MC-0 metallacycles (+18.8 and +16.0 kcal mol−1, respectively) and they decrease with the addition of other methyl groups until +9.2 kcal mol−1, due to mainly the destabilization of the metallacycle intermediate.

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CONCLUSIONS DFT (M06-L) calculations have been used to analyze the structure, NMR signatures, stabilities, and interconversion pathways of all potential metallacyclobutane intermediates arising from the homocoupling reaction of propene and styrene. The aim of the study is to get insights into the origin of the lack of detection of the α,β-substituted metallacyclobutanes as well as the viability of the experimentally proposed metallacyclobutane exchange pathways. For the case of propene, the unsubstituted and mono- and disubstituted metallacycles in terms of Gibbs energies at 298 K are lower than or essentially degenerate with the separated reactants and only the α,β,α′-trisubstituted intermediates lie significantly above the reactant asymptote. On the other hand, the metallacyclobutane intermediates resulting from the homocou6073


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pling reaction of styrene are in general less stable. This in turn shows that all di- and trisubstituted species lie above the separated reactants at room temperature and only MC-0, MC1α, and MC-1β should in principle be potentially observed. With regard to the metallacyclobutane interconversion, it is found that both the alkene rotation and carbene rotation calculated energy barriers are low and are in excellent agreement with the experimentally determined energy barriers. This suggests that these two pathways are applicable to the metallacyclobutane exchanges. Moreover, results for the experimentally unobserved metallacycles show that the exchange between trans- and cis-MC-2αα species and transand cis-MC-2αβ species are easy and, thus, a rapid equilibrium should also be reached. Consequently, the lack of detection of MC-1β and MC-2αβ metallacyclobutane intermediates seems to arise mainly from thermodynamic control. That is, isomers MC-1α and MC-2αα are low enough in Gibbs energy (2.0 kcal mol −1 ) in comparison to the MC-1β and MC-2αβ intermediates to prevent the detection of the latter under equilibrium conditions. The preference for metallacyclobutanes containing the substituents in an α position is attributed to the stronger M-C and C−C bonds formed during the cycloaddition. This implies a larger electron transfer from the alkene to the metal fragment that is favored by the presence of substituents at an α position.

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ASSOCIATED CONTENT

S Supporting Information *

Tables S1 and S2, reporting respectively the NMR signatures and the relative stabilities of all metallacyclobutane intermediates related with the styrene homocoupling, Table S3, containing the relative stabilities of the less stable metallacyclobutane conformers, Table S4, showing the Gibbs energy barriers for the metallacyclobutane interconversion of several less stable conformers, Scheme 1, illustrating the less stable conformers, Figures S1−S9, showing the optimized geometries of all stationary points reported in the paper, and a full list of all computed molecule Cartesian coordinates with the associated absolute E, G, and E + ΔGsolv energies presented in an xyz file format. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Notes

The authors declare no competing financial interest.

ACKNOWLEDGMENTS Financial support from FONDECYT through the project number 1140340, the Millenium Nucleus CPC grant (NC120082), the Spanish MINECO (CTQ2011-24847/ BQU), and the Generalitat de Catalunya (SGR2009-638) is gratefully acknowledged. M.S. gratefully acknowledges support from ICREA Academia. K.P-G. acknowledges the MECESUP for a Ph.D. fellowship and the Universidad Andres Bello for the DI-08-09-11/I grant. We thank the Catalan Supercomputing Center (CESCA) for computational time.

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REFERENCES

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