Article pubs.acs.org/Organometallics
Mechanistic Insight into the Rhodium-Catalyzed O−H Insertion Reaction: A DFT Study Zhi-Zhong Xie,*,† Wen-Juan Liao,† Jun Cao,⊥ Li-Ping Guo,† Francis Verpoort,*,†,‡,§ and Weihai Fang*,⊥ †
Department of Chemistry, School of Science, Wuhan University of Technology, Wuhan 430070, People’s Republic of China State Key Laboratory of Advanced Technology for Materials Synthesis and Processing, Center for Chemical and Material Engineering, Wuhan University of Technology, Wuhan 430070, People’s Republic of China § Department of Inorganic and Physical Chemistry, Laboratory of Organometallic Chemistry and Catalysis, Ghent University, Krijgslaan 281 (S-3), 9000 Ghent, Belgium ⊥ College of Chemistry, Beijing Normal University, Beijing 100875, People’s Republic of China ‡
S Supporting Information *
ABSTRACT: A DFT study on the reaction of diazoacetate with primary allyl alcohol mediated by dirhodium catalyst has been carried out in detail. Calculations indicate that the major O−H insertion product can be obtained via either a [1,3]-proton shift of the free enol or a [1,2]-proton shift of the free oxonium ylide, which are regulated by the orientation of the ester group. In the case of a [1,3]-proton shift the reaction begins with the nucleophilic attack of the alcohol at the carbenoid, generating a metal-associated oxonium ylide followed by a [1,4]-proton shift to the adjacent carbonyl oxygen atom of the ester group, resulting in a metal-associated enol. Subsequently, its decomposition liberates a free enol intermediate. The whole process requires an overall barrier of 4.2 kcal/mol and is exergonic by 6.4 kcal/mol. The [1,3]-proton shift of the enol also readily provides the final O−H insertion product, which has a barrier of 11.7 kcal/mol using a three-alcohol cluster as catalyst. For the free oxonium ylide pathway, formation of an alternative metalassociated oxonium ylide is also straightforward, having an overall barrier of 4.5 kcal/mol. In the presence of extra alcohol molecules, the decomposition of the metal-associated oxonium ylide can generate an alcohol-stabilized free oxonium ylide (endergonic by only 4.1 kcal/mol). Afterward, it undergoes a [1,2]-proton shift, resulting in the O−H insertion product, which requires an energy barrier of 4.7 kcal/mol. In comparison, the competitive [2,3]-sigmatropic rearrangement for the metalassociated oxonium ylides is not sensitive to the orientation of the ester, which has a similar activation free energy around 14.0 kcal/mol. Accordingly, it is always disfavored over the O−H insertion, which kinetically agrees well with the experimental observations, in which traces of [2,3]-sigmatropic rearrangement product were obtained for the primary allyl alcohol.
1. INTRODUCTION
where they should be considered in the specified X−H reactions for a detailed mechanistic understanding. Although the detailed mechanism for the O−H insertion remains controversial, it is generally accepted that the initial processes should involve the decomposition of diazoacetate to afford the carbenoid (CB) followed by a nucleophilic attack to give the C-bound oxonium ylide A (having a carbon as the coordinative atom) (see Scheme 1). 20−22 Likewise, a subsequent [1,2]-proton shift of either the free oxonium ylide B or the O-bound oxonium ylide C (with oxygen as the coordinative atom) has been postulated corresponding to the formation of the O−H insertion product D.1,22,23 On the basis of the mechanistic study on the reaction of diazoacetate with water catalyzed by a dirhodium compound, Yu et al.9 pointed out that the free oxonium ylide (E) was the most likely intermediate responsible for the O−H insertion. In addition, formation of the free enol intermediate F as proposed
Rhodium-catalyzed insertion of α-diazocarbonyl compounds into the X−H (X = N, O, S, Si, etc.) bond is an attractive method to generate a C−X bond1−8 and has been widely employed in organic synthesis. On the other hand, it has been also considered as one promising candidate compared to its copper counterpart in preparing optically pure organic molecules for the synthesis of natural products and pharmaceutical targets.1,4,9−14 However, although many chiral dirhodium catalysts have been investigated, no significant advance has been achieved so far.15−19 A possible explanation for this status quo could be found in its stepwise mechanism, in which certain reactive intermediate(s) should depart from the chiral center of the dirhodium catalyst. However, until now the actual reactive intermediate(s) responsible for the X−H insertion remains unclear. In the present work we attempted to elucidate this issue by a detailed mechanistic study on the Rh-catalyzed O−H insertion. Noteworthily, the reactive intermediates proposed herein might not correspond to other kinds of X−H insertion completely, © 2014 American Chemical Society
Received: November 14, 2013 Published: May 15, 2014 2448
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Scheme 1
Scheme 4
by Wood24−26 was also favored. However, its intramolecular [1,3]-proton shift is difficult. Although this process could be accelerated significantly using a water molecule as the proton transport catalyst, similar to their findings on phosphinecatalyzed [3+2] cycloaddition reactions,27−29 the overall barrier was still as high as 27.4 kcal/mol. Thus, it was reasonable to rule it out as the reactive precursor (see Scheme 2).
2. COMPUTATIONAL METHODS All calculations have been performed using the DFT method implemented in the commercial Gaussian 09 program package.31 Molecular geometries of the model complexes were optimized without constraints applying the B3LYP functional.32,33 The effective core potential (ECP) of LanL2DZ34,35 was used to describe Rh. For all the other atoms the 6-31G* Pople basis set36 was used. For convenience, such a combination will be referred to as BSI. As soon as the convergences of optimizations were obtained, the frequency calculations37 at the same level have been performed to identify all the stationary points as minima or transition states. The intrinsic reaction coordinate (IRC)38,39 calculations have been carried out to confirm that the transition structures can indeed connect the related reactant and product. Additionally, the single-point energies for all stationary points have also been calculated at the M06/BSI level with the SMD solvation model and pentane as the solvent on the B3LYP-optimized geometries.40−42 For convenience, such a combination will be referred to as M06/BSI/SMD//B3LYP/BSI.
Scheme 2
3. RESULTS AND DISCUSSIONS In this section, a detailed computational study of the reaction as depicted in Scheme 3 is presented. However, as the formation of rhodium carbenoid has been well documented, this will not be discussed.9,43 In addition, a widely used procedure to simplify the dirhodium catalyst Rh2(S-DOSP)4 to Rh2(OAc)4 (Rh2L4) has been conducted to reduce the computational cost (see Figure 1).9,30,43
16
Nonetheless, Davies et al. found that enol could be obtained through the C-bound oxonium ylide directly, requiring an activation free energy of 4.6 kcal/mol, which was lower than that of 7.3 kcal/mol for the formation of E (highlighted in Scheme 2 with red arrow). Consequently, this makes the free enol F rather than the free oxonium ylide E more possible with regard to the O−H insertion. Aiming to elucidate the Rh-catalyzed O−H insertion reaction mechanism in detail, extensive calculations on the reaction as depicted in Scheme 3 have been conducted.30 According to our Scheme 3
calculations, we find that, as proposed by Wood24−26 and Davies,30 the free enol is indeed one of the reactive intermediates responsible for the O−H insertion. Its [1,3]proton shift can be facilitated efficiently by an alcohol cluster (see Scheme 4). In addition, the alcohol-stabilized free oxonium ylide G rather than E as proposed by Yu9 is an alternative intermediate, which can also give the O−H insertion product via [1,2]-proton shift.
Figure 1. Optimized structures for reactants and catalyst adopted in the present work at the B3LYP/BSI level with distances in angstroms.
Figure 1 depicts that the primary allyl alcohol (PA) can attack the C1 atom of CB either from the re-face or si-face.44 In view of the symmetric nature of Rh2L4, only the reaction occurring at the re-face has been considered. Additionally, the C1−C2 has a length of 1.49 Å, featured as a C−C single bond. It is also reasonable to expect that the rotation of the ester group 2449
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Figure 2. DFT-computed free energy surface for the formation of an enol at the M06/BSI/SMD//B3LYP/BSI level and the relative free energies calculated at the B3LYP/BSI level in the gas phase are given in brackets.
Figure 3. Optimized stationary points for the free enol and [2,3]-sigmatropic rearrangement at the B3LYP/BSI level with distances in angstroms.
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Figure 4. DFT-computed free energy surface for the alcohol molecule catalyzed conversion of free enol into the O−H insertion product at the M06/ BSI/SMD//B3LYP/BSI level. The relative free energies calculated at the B3LYP/BSI level are in brackets.
along the C1−C2 axis has low energy barrier. Thus, the ester group can orient toward the approaching PA either with its carbonyl or methoxide moiety. For a detailed mechanistic understanding of the O−H insertion it is crucial to take this factor into account, as demonstrated in the postulated mechanism depicted in Scheme 4. Hence, for clarity, the results will be discussed as follows: (1) formation of free enol; (2) the conversion of enol into an O−H insertion product; (3) formation of free oxonium ylide and its conversion into an O− H insertion product. For comparison, the minor [2,3]sigmatropic rearrangement has also been considered in the enol and free oxonium ylide pathways, respectively. 3.1. DFT Investigations of the Energy Profile for the Formations of Enol and [2,3]-Sigmatropic Rearrangement Using PA as the Substrate. The computed free energy profiles in the gas phase for the reaction between CB and PA resulting in the free enol as well as the [2,3]-sigmatropic rearrangement product are given in Figure 2. The optimized structures along this reaction pathway are collected in Figure 3. In this work the relative free energy, ΔGsol, calculated at the M06/BSI/SMD//B3LYP/BSI level is used to analyze the reaction mechanism,40 and the sum of the relative free energies of PA and CB is used as the energy reference. The enol pathway begins with the complexation of CB and PA, producing intermediate A1. At A1, the PA moiety is stabilized by two weak interactions with distances of C1···O1 and H1···O3 of 2.92 and 1.92 Å, respectively (see Figure 3). This process is slightly endergonic, by 2.3 kcal/mol. Calculations indicate that A1 is very active owing to the subsequent nucleophilic attack of O1 on C1 requiring an activation free energy of 1.9 kcal/mol to give the metalassociated oxonium ylide, A3. The low energy barrier can be attributed to the minor geometrical variation of ATS2 with respect to A1. For instance, the elongation of the Rh1C1 bond is only 0.06 Å, and the Rh1C1 bond has a slight deviation of 14.6° from the plane. For intermediate A3, the formed C1−O1 bond (1.52 Å) is relatively weak in comparison with the shorter C3−O1 bond (1.42 Å) in PA. Meanwhile, a considerable elongation for both C3−O1 and Rh1C1 bonds
has been observed, with bond lengths of 1.51 and 2.25 Å, respectively. It should be noted that the elongated Rh1C1 bond will weaken significantly the catalyst to stabilize the developed negative charges on the C1 center. Consequently, although one new C−O bond is formed in A3, the relative energy does not drop dramatically (exergonic only by 5.9 kcal/ mol). Calculations indicate that the metal-assisted conversion of A3 to enol intermediate A6 is a two-step process. It begins with a [1,4]-proton shift of H1 from O1 to O2, resulting in a metalassociated enol A5 followed by a direct decomposition to liberate A6 from the dirhodium catalyst. The located transition state ATS4 involves a coupling of the rotation of O1−H1 along the C1−O1 axis, resulting in a hydrogen bond exchange between O1−H1···O3 and O1−H1···O2 and shifting of H1 to O2 simultaneously (see Figure 3). At ATS4, the shifted proton is stabilized well by two strong hydrogen-bond interactions with the O1···H1 and H1···O2 distances of 1.15 and 1.34 Å, respectively. In addition, it is also observed that this [1,4]proton shift occurs locally, where the variation of the Rh1−C1 (2.27 Å) bond is negligible relative to the value of 2.25 Å in A3. The computed low energy barrier of 1.2 kcal/mol is comparable to the value of 4.6 kcal/mol calculated at the B3LYP level for the reaction of CB with propargylic alcohol.16 The resulting metal-associated enol A5 has a C1C2 double bond of 1.40 Å along with a significant reduction of the C1−O1 and C3−O1 bond lengths to 1.40 and 1.45 Å, respectively. Meanwhile, a considerable elongation for the Rh1−C1 bond to 2.51 Å is also observed. This can be explained by the weak intrinsic interaction between the soft base (C1C2) and hard acid (Rh(II)). Calculations indicate that formation of A6 is endergonic by only 3.1 kcal/mol. Hence, relative to the initial state, formation of A6 bears an overall barrier of 4.2 kcal/mol and is exergonic by 6.6 kcal/mol. For comparison, the unconstrained optimization for the free oxonium ylide as proposed by Yu has been performed at the B3LYP/6-31G* and MP2/6-311++G** level, respectively. Both indicate that the proposed intermediate is unstable, resulting in enol A6 automatically. The different results can be 2451
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Figure 5. Optimized stationary points for the alcohol molecule catalyzed conversion of free enol into the O−H insertion product at the B3LYP/BSI level calculations (distances in Å).
and the remaining part in ATS7 are charged by −0.32, +0.20, and +0.12 e, respectively, demonstrating that despite the long Rh1···C1 distance, the electrostatic interaction between them is still significant. Calculations indicate that the [2,3]-sigmatropic rearrangement also has an accessible energy barrier of 14.1 kcal/mol at room temperature. However, it is disfavored by 12.9 kcal/mol compared with ATS4, indicating that this process cannot compete with the formation of A6. 3.2. The [1,3]-H Shift of Enol. As discussed above, one would expect that the enol intermediate A6 serves as the actual species responsible for the O−H insertion. To explore the energetically feasible pathway for the [1,3]-proton shift of enol in detail, three pathways catalyzed by the alcohol molecule have been taken into account first. The energy profiles are illustrated in Figure 4, and the optimized geometries are presented in Figure 5. Figure 4 shows that A6 and PA can form a hydrogen-bond cluster, A9, in which PA acts as a proton acceptor and donor concurrently with two kinds of hydrogen bonds, namely, O2−
attributed to the partial optimization conducted by Yu,9 in which the free oxonium ylide is obtained by fixing the O−H bond length at 1.01 Å. Therefore, formation of A6 is inevitable in this pathway. In comparison with the formation of A6, the competitive [2,3]-sigmatropic rearrangement for A3 has also been studied. Calculations indicate that the [2,3]-sigmatropic rearrangement proceeds via a concerted mechanism instead of the stepwise one in the reaction of CB with propargylic alcohol.16 B3LYP/ BSI level calculations, concerning the located [2,3]-sigmatropic rearrangement transition state, ATS7, confirmed that ATS7 is a first-order saddle point with the imaginary frequency of 105.8i cm−1. IRC calculations show that ATS7 can indeed connect A3 with A8. As illustrated in Figure 3, ATS7 splits into three parts owing to the simultaneously breaking of Rh1−C1 (2.65 Å) and O1−C3 (2.33 Å) bonds. Meanwhile, the forming C1−C5 bond has a long distance of 3.00 Å, indicating that the O1−C3 bond cleavage and the C1−C5 bond formation are asynchronous. The Mulliken charge analysis shows that Rh2L4, the allyl moiety, 2452
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H1···O4 and O4−H2···C1, generating a ring configuration. Formation of A9 is favored energetically (exergonic by 15.2 kcal/mol). Calculations indicate that the conversion of A9 into the O− H insertion product is disfavored whether via the stepwise or concerted pathways. For the stepwise one, A9 first isomerizes to an unstable PA-stabilized ylide A10 (endergonic by 16.7 kcal/mol), and the subsequent [1,2]-proton shift via the transition state of ATS11 requires an activation free energy of 11.3 kcal/mol (see Figure 4). Relative to A9, the computed overall barrier of 28.0 kcal/mol is in good agreement with the value of 29.4 kcal/mol reported by Yu et al.9 For the concerted one, the [1,3]-proton shift A9 proceeds through the transition state ATS12. At ATS12, both H1 and H2 atoms are located at O4 with similar bond distances (∼1.11 Å) accompanied with an orientation change of O4−H2 from O1 to C1 in order to facilitate the formation of the C1−H2 bond (see Figure 5). Although ATS12 is 3.5 kcal/mol lower in energy than ATS11, the activation free energy of 24.5 kcal/mol for this process also implies that the PA-catalyzed [1,3]-proton shift of free enol is very difficult at room temperature. The third case corresponds to a metal-assisted process. Starting with the complexation of A6 and Rh2L4 with O2 as the coordinative atom, A14 is produced, of which the hydrogenbonding interaction with an incoming PA yields a larger cluster. Actually, the resulting intermediate A15 can be also viewed as the coordination of A9 with Rh2L4. Calculations indicate that the [1,3]-proton shift of enol cooperatively catalyzed by Rh2L4 and PA is also disfavored, possessing a similar energy barrier of 24.8 kcal/mol to the metal-free process, caused by Rh2L4 having a slight effect on stabilizing the developing negative charges populated in the enoate moiety, as evidenced by the minor variation of the Rh1−O2 bond length. As show in Figure 5, in comparison with the value of 2.33 Å in A15, the Rh1−O2 bond length is shortened slightly by 0.05 Å to 2.28 Å in ATS16. In addition, the distances of O2···H1, C2···H2, and C1−C2 are 1.39, 1.64, and 1.43 Å, respectively, in ATS12 and are close to the corresponding values of 1.45, 1.70, and 1.42 Å in ATS16, respectively. This also suggests that the effect of Rh2L4 in this step can be ignored. Applying the above calculations, further structural analysis for ATS12 has been conducted. Figure 5 displays that PA stabilizes the shifting proton and the enoate tends to form a threecentered (C1, C2, and O2) four-electron (3c-4e) bond to maximize the electron delocalization among C1−C2−O2 in ATS12. Ideally, the sp3 hybridization of the O4 atom should have a bond angle of ∠H1O4H2 close to 109°28′ and the dihedral angle of ∠O1C1C2O2 should tend to be planar to facilitate the electron delocalization. However, the computed bond angles of ∠H1O4H2 and ∠O1C1C2O2 are 88.4° and 35.0°, respectively (see Supporting Information), indicating that both are heavily distorted. One could attribute this to the short O2··· O4···C1 molecule chain resulting in the twist of the enoate to facilitate the interaction between C1 and H2 in ATS12. To confirm this hypothesis, the [1,3]-proton shift of A6 catalyzed by two-PA and three-PA alcohol clusters has been studied, respectively. Figure 6 gives the optimized geometries along with the relative free energies. Furthermore, the average bond angle of ∠HOH and the dihedral angle of ∠O1C1C2O2 are 100.0° and 16.2° in ATS18 and 106.1° and 11.8° in ATS21, respectively. This indicates that, with the elongation of the molecule chain, both angles become more and more normal. Meanwhile, their computed activation free energies drop
Figure 6. Optimized stationary points for alcohol cluster catalyzed conversion of free enol into the O−H insertion product at the B3LYP/ BSI level with distances in angstroms and angles in degrees. The relative free energies calculated at the M06/BSI/SMD//B3LYP/BSI level are given in kcal/mol, and those at the B3LYP/BSI level are in brackets. aAveraged values of ∠H1O4H2 and ∠H2O5H3; baveraged values of ∠H1O4H2, ∠H2O5H3, and ∠H3O6H4. For clarity, the hydrogen atoms on the allyl groups are omitted.
monotonically to 13.5 and 11.7 kcal/mol for ATS18 and ATS21, respectively, and thus the [1,3]-proton shift of A6 can occur smoothly at room temperature. As a consequence thereof, a carful conclusion can be made that enol is one of the exact intermediates corresponding to the O−H insertion. 3.3. Formation of Free Oxonium Ylide and Its [1,2]Proton Shift. As the methoxide moiety of the ester group of CB is directed toward the approaching PA, an alternative free oxonium pathway is also energetically feasible. The computed free energy profile and optimized stationary points are presented in Figure 7 and Figure 8, respectively. As show in Figure 7, the free oxonium ylide pathway is similar to the free enol one initially, giving a metal-associated free oxonium ylide B3. Calculations indicate that formation of B3 has an overall barrier of 4.5 kcal/mol and is exergonic for 3.3 kcal/mol, which are close to the corresponding values of 4.2 and 3.6 kcal/mol for the formation of A3. Therefore, the reactions in this pathway should also be taken into account. As shown in Figure 7, the direct decomposition of B3 to liberate the free oxonium ylide B4 is rather difficult. The estimated dissociation energy at the M06/BSI/SMD//B3LYP/ BSI level is as high as 25.3 kcal/mol. On the other hand, the Obound oxonium ylide B5 can be formed through the 2453
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Figure 7. DFT-computed free energy surface for the [1,2]-proton shift in the free ylide pathway at the M06/BSI/SMD//B3LYP/BSI level with the relative free energies calculated at the B3LYP/BSI level in brackets.
recombination of B4 and Rh2L4 with the O2 as the coordinative atom. Thus, the dissociation energy for B3 can be viewed as the energy barrier corresponding to the formation of B5, indicating that formation of B5 is also problematic at room temperature. Calculations demonstrate that the decomposition of B3 should be an alcohol-promoted process because the dissociation energy for the formation of PA-stabilized oxonium ylide B6 decreases dramatically to 4.1 kcal/mol. In comparison with the direct process, it is favored in energy by 21.2 kcal/mol. Such a preference can be attributed to the extra PA molecule playing a crucial role in stabilizing the positive charges of the oxonium ylide. As we know, the oxonium ylide accommodates both the cation and anion centers in close proximity. If both can be stabilized, it is favored in energy. At B4, the positively charged H1 atom tends to form a hydrogen bond with the ester to stabilize the charge populating it. However, this results in a distortion of the plane consisting of the C2O2, C1C2, and phenyl groups by 15.0°. It damages the delocalization of the negative charges in B4. While at B6, the H1 atom does not coordinate to the ester directly. In turn, all the atoms of the C2O2, C1C2, and phenyl groups in B6 can almost reside at the same plane, maximizing the delocalization of the negative charges among the large conjugated system. In addition, PA can form two kinds of hydrogen bonds in B6, which are also favored in energy. Calculations indicate that the conversion of B6 into an alternative O−H insertion product B8 is also easy. At the located transition state BTS7, the shifting proton (H1) is shared by the O1 and O4 atoms with distances of O1···H1 and O4···H1 of 1.26 and 1.18 Å, respectively, indicating that the shifting H1 can be stabilized well by the O1 and O4 atoms. In turn, a low energy barrier of 4.7 kcal/mol is achieved. For comparison, the [2,3]-sigmatropic rearrangement in this pathway for B3 has also been studied. The optimized transition state BTS9 also features a loose structure, which has similar bond distances for O1···C3 (2.30 Å), Rh1···C1 (2.63 Å), and C1···C5 (3.02 Å) to those in ATS7. A similar energy barrier of 13.5 kcal/mol (14.1 kcal/mol for ATS7) is obtained,
demonstrating that the orientation of the ester has a slight influence on the [2,3]-sigmatropic rearrangement. In light of these calculations, the energy barrier for the [2,3]sigmatropic rearrangement of B3 can also be overcome at room temperature. However, it is disfavored by 9.4 kcal/mol over the formation of B6, signifying that in this pathway the O−H insertion is also exclusive.
4. CONCLUSION The mechanism of the Rh(II)-catalyzed O−H insertion reaction of primary allyl alcohol with diazoacetate has been studied at the M06/BSI/SMD//B3LYP/BSI level. Calculations indicate that the orientation of the ester of the in situ generated carbenoid plays a critical role in the determination of the reaction progress either via the enol pathway or the free oxonium ylide pathway. As the carbonyl moiety is directed toward the approaching alcohol, the reaction of the carbenoid with alcohol gives a metal-associated oxonium ylide first with an energy barrier of 1.9 kcal/mol. The subsequent [1,4]-proton shift of the metal-associated oxonium ylide is also fast to generate the enol intermediate requiring an activation free energy of 1.2 kcal/mol. The final process is the [1,3]-proton shift of enol to give the O−H insertion product. Calculations indicate that an alcohol cluster rather than an alcohol molecule can facilitate the transformation of enol efficiently. A threealcohol cluster adopted as proton transport catalyst bears a moderate energy barrier of 11.7 kcal/mol. On the contrary, the reaction of the carbenoid with alcohol can also afford an alternative metal-associated oxonium ylide rapidly with an energy barrier of 2.6 kcal/mol. In this case, the direct decomposition of the metal-associated oxonium ylide is difficult since the dissociation energy amounts to 25.3 kcal/mol. Calculations indicate that this process requires an extra alcohol molecule to stabilize the yielded unstable free oxonium ylide. By this means, the decomposition of the metal-associated oxonium ylide becomes much easier (endergonic by 4.1 kcal/ mol). The succeeding [1,2]-proton shift of the alcoholstabilized oxonium ylide has a low energy barrier of only 4.7 kcal/mol. Accordingly, both the enol and the oxonium ylide pathway are possible relevant to the O−H insertion at room 2454
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Figure 8. Optimized stationary points for the O−H insertion proceeding via the free ylide pathway at the B3LYP/BSI level with the distances in angstroms.
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temperature. In addition, for comparison, the [2,3]-sigmatropic rearrangements in both pathways have been investigated individually. Calculations indicate that the [2,3]-sigmatropic rearrangements are not sensitive to the orientation of the ester; besides both have similar activation free energies of about 14.0 kcal/mol, making them also accessible at room temperature, although they are disfavored over the O−H insertion. These results are in good agreement with the experimental observations that the O−H insertion product is exclusive for the primary allyl alcohol. The outcomes also suggested that to achieve asymmetric [2,3]-sigmatropic rearrangement products, formation of the achiral enol and free oxonium ylide both should be compressed as proposed previously.9,30
ASSOCIATED CONTENT
S Supporting Information *
A text file of all computed molecule Cartesian coordinates in .xyz format for convenient visualization of Cartesian coordinates and energies for all optimized stationary points. This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Authors
*E-mail:
[email protected]. *E-mail:
[email protected]. *E-mail:
[email protected]. Notes
The authors declare no competing financial interest. 2455
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ACKNOWLEDGMENTS The authors acknowledge generous financial support by the Natural Science Foundation of China (21103130, 21172027, and 61274135) and the Fundamental Research Funds for the Central Universities (WUT:2014-Ia-016, 2012-Ia-019, and 2010-IV-004). F.V. is grateful to Wuhan University of Technology and the FWO-Flanders (fund for Scientific Research-Flanders) project grant (3G022912) for financial support. F.V. acknowledges the Chinese Central Government for an “Expert of the State” position in the program of “Thousand Talents” and the support of the Natural Science Foundation of China (No. 21172027). All calculations were performed on the cluster of Key Laboratory of Theoretical and Computational Photochemistry, Ministry of Education.
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dx.doi.org/10.1021/om401092h | Organometallics 2014, 33, 2448−2456
