Theoretical Insight into PtCl2-Catalyzed Isomerization of

Jun 19, 2012 - Theoretical Insight into PtCl2-Catalyzed Isomerization of ... the d−π interaction between the metal center and the substrate to form...
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Theoretical Insight into PtCl2-Catalyzed Isomerization of Cyclopropenes to Allenes Yuxia Liu,† Dongju Zhang,*,† and Siwei Bi‡ †

Key Lab of Colloid and Interface Chemistry, Ministry of Education, Institute of Theoretical Chemistry, Shandong University, Jinan, 250100, People's Republic of China ‡ College of Chemistry and Chemical Engineering, Qufu Normal University, Qufu, 273165, People's Republic of China S Supporting Information *

ABSTRACT: To understand the mechanism of allene formation through the rearrangement of cyclopropenes catalyzed by PtCl2, we have performed a detailed density functional theory calculation study on a representative substrate, 1-(trimethylsilyl)-2-(phenylethyl)cyclopropene. Three reaction pathways proposed in the original study have been examined; however the calculated results seem not to completely rationalize the experimental findings. Alternatively, by performing an exhaustive search on the potential energy surface, we present a novel mechanism of PtCl2, which is fixed appropriately on the cyclopropene/allene to form the linear Cl−Pt−Cl disposition, a vital configuration for catalyzing the rearrangement of cyclopropene. The newly proposed mechanism involves an SN2-type C−C bond activation of the cyclopropene by PtCl2 fixed on a cyclopropene molecule via the d−π interaction between the metal center and the substrate to form the product precursor PtCl2-allene with the metal center coordinated to the external CC bond in the allene framework. Once formed, the PtCl2-allene immediately serves as a new active center to catalyze the rearrangement reaction rather than directly dissociating into the allene product and the PtCl2 catalyst due to its high stability. During the catalytic cycle, an allene-PtCl2-allene sandwich compound is identified as the most stable structure on the potential energy surface, and its direct dissociation results in the formation of the product allene and the regeneration of the catalytically active center PtCl2-allene with an energy demand of 24.4 kcal/mol. This process is found to be the rate-determining step of the catalytic cycle. In addition, to understand the experimental finding that the H-substituted cyclopropenes do not provide any allenes, we have also performed calculations on the H-substituted cyclopropene system and found that the highest barrier to be overcome during the catalytic cycle amounts to 35.2 kcal/mol. This high energy barrier can be attributed to the fact that the C−H bond activation is more difficult than the C−Si bond activation. The theoretical results not only rationalize well the experimental observations but provide new insight into the mechanism of the important rearrangement reaction.

1. INTRODUCTION Cyclopropenes are highly strained but readily accessible carbocyclic molecules and therefore exhibit remarkable activities that extend far beyond typical reactions of simple olefins, alkynes, and allenes. They are extensively used to synthesize a variety of novel organic molecules. In the past several decades, the isomerization of cyclopropenes to allenes under thermal and photochemical conditions1−5 has attracted much attention. In contrast, the corresponding metal-catalyzed rearrangement under mild reaction conditions, an inherently atom-economical, operationally simple, and safe synthesis, is rarely investigated. Recently, Lee and co-workers6 developed the PtCl2-catalyzed rearrangement of silylated cyclopropenes to allenes in dichloromethane at 50 °C in high yields (Scheme 1, where 1-(trimethylsilyl)2-(phenylethyl)cyclopropene, denoted as R, is considered a representative of silylated cyclopropenes, leading to the corresponding product, denoted as P, with a yield of 95%, and © 2012 American Chemical Society

Scheme 1. PtCl2-Catalyzed Rearrangement of 1(Trimethylsilyl)-2-(phenylethyl)cyclopropene to the Corresponding Allene Reported by Lee and Co-workers6

similarly for Scheme 3 below), which presents a new methodology for the synthesis of allenes by readily accessible cyclopropenes under mild conditions. It is known that the reaction of cyclopropenes with transition metals generally Received: April 23, 2012 Published: June 19, 2012 4769

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results, we expect to uncover (i) whether the mechanism proposed by Lee and co-workers6 is reasonable for explaining all experimental results, (ii) if not, what the available mechanism is, and (iii) what the role of the trimethylsilyl substituent in R is. Answers to these would aid us in understanding the molecular mechanism of the important rearrangement reaction.

Scheme 2. Two Typical Activation Patterns for the Reaction of Cyclopropenes with Transition Metals

2. COMPUTATIONAL DETAILS The calculations presented in this work were carried out in the framework of DFT using the B3LYP functional,14−17 which has been shown to describe static Pt-catalyzed organometallic systems18,19 reasonably well. The standard 6-311+G(d,p) basis set was used for all atoms except for Pt, which was described by the effective core potentials of Hay and Wadt combined with double-ζ valence basis sets (LanL2DZ),20−22 and also a polarization function was added with ζf = 0.18. Such combined basis sets are more flexible than those previously used in studying Pt-catalyzed systems.19,23−25 Therefore, we expect that the conclusions drawn from the present calculations are reliable enough. Full geometry optimizations of minima and transition states were performed at the chosen level of theory, and the intrinsic reaction coordinates26,27 from the transition states have been followed to confirm that such structures actually connected the two desired minima. Frequency calculations at the same level of theory have also been carried out to identify all stationary points as minima (zero imaginary frequencies) or first-order saddle points (one imaginary frequency) and to provide free energies at 298.15 K, which include entropic contributions by considering the vibrations, rotations, and translations of the species. Solvent effects have been considered at the same DFT level by calculating the single-point energies of the geometries obtained in the gas phase employing the simple self-consistent reaction field method28−30 based on the polarizable continuum model31,32 with UAKS cavities.33 Here, the dielectric constant ε = 8.93 was used to simulate dichloromethane as the solvent, which corresponds to the experimental conditions. All the calculations were performed with the Gaussian-03 software package.34

involves two typical patterns (Scheme 2):7−11 either the oxidative addition of the C−C single bond of cyclopropene to the metal or the metalation of the CC double bond. However, for the significant Pt-catalyzed rearrangement of silylated cyclopropenes, Lee et al.6 surmised that the reaction could proceed through a different mechanism, which starts from the coordination of the metal center to the CC bond in R to form the zwitterionic intermediate 1 (Scheme 3), which can be effectively stabilized by the silyl group via the β-cation-stabilizing effect of silicon.12,13 As indicated in Scheme 3, Lee et al.6 believed that the reaction starts from the coordination of the CC bond to the metal center, leading to the formation of zwitterionic intermediate 1. After that, the reaction evolves into three possible pathways, I, II, and III. On the basis of the isotopic labeling experiments, they excluded the possibility of the phenylethyl shift via pathway III and believed that the reaction evolves via pathway I or pathway II. In addition, they found that the H-substituted cyclopropenes do not provide any allenes, which is in strong contrast to the formation of allenes in excellent yield from the silylated cyclopropenes under the same conditions. To obtain detailed mechanisms for the important rearrangement reaction and to better understand the experimental observations, theoretical studies based on the first-principle calculations are highly desired, which have become one of the most popular strategies for understanding the intriguing experimental findings and revealing the hidden nature behind them. Herein, we carry out a detailed density functional theory (DFT) study on the mechanisms of PtCl2-catalyzed rearrangements of the representative silylated cyclopropene, R, and its H-substituted counterpart, HR. On the basis of the calculated

3. RESULTS AND DISCUSSION 3.1. Mechanisms Proposed by Lee and Co-workers. In this section, we examine the three pathways proposed by Lee and co-workers.6 As shown in Scheme 3, the proposed mechanisms involve the initial Pt coordination to the double bond in R to give the zwitterionic intermediate 1. Unfortunately, after trying many times we have failed to locate such a structure. Alternatively, the Pt π-coordinated complex 2 (the reactant precursor) (Figure 1) with a trans chlorine ligand disposition was

Scheme 3. Three Pathways Proposed by Lee and Co-worker6 for the PtCl2-Catalyzed Rearrangement of 1-(Trimethylsilyl)-2(phenylethyl)cyclopropene

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Figure 1. Calculated free energy profiles in dichloromethane solution for the formation of the product precursor IM4 along the pathways proposed by Lee et al.6 The relative free energies are given in kcal/mol.

Scheme 4. Calculated Schematic Structures and Relative Free Energies (in kcal/mol) in Dichloromethane Solution for Species Involved in the Formation of Reactant Precursor 2a

a

The imaginary frequencies (in parentheses) characterizing transition states are given in cm−1.

identified as true first-order saddle points (with one and only one imaginary frequency). Thus, we believe that the interaction of R with PtCl2 directly leads to the formation of complex 2. Starting from complex 2, we have calculated the three pathways (I, II, and III) shown in Scheme 5, which collects the schematic intermediates and transition states located, and the calculated potential energy profiles in dichloromethane solution are given in Figure 1. According to the suggestion of Lee and coworkers,6 pathway I proceeds via the cleavage of the C2−C3 bond in 2 to form open-chained zwitterionic intermediate 3, which then undertakes the 1,2-silyl shift, giving the final allene product P. Our calculations indicate that the C2−C3 bond cleavage of 2 takes place via ring-opening transition state TS3 with an energy requirement of 18.8 kcal/mol. The optimized geometry of 3 shows that the Pt−C2 bond distance is 1.858 Ǻ , which is remarkably shorter than that in 2 (2.133 Ǻ ). Clearly, 3 is characteristic of a Pt-carbenoid intermediate. Then, 3 evolves by the 1,2-silyl shift to energy-rich intermediate IM3 through TS4 with a barrier of 33.3 kcal/mol. Note that in TS4 the Pt−Cl and Cl−C1 bond distances are much longer and shorter, respectively, than the corresponding ones in 3, implying that the Pt−Cl bond is breaking and the Cl−C1 bond is forming. Subsequently, the formation of the product precursor IM4 takes place via TS5 with a barrier of 11.6 kcal/mol, and this process is calculated to be exothermic by 42.5 kcal/mol relative to the separate reactants. The rate-determining step along pathway I corresponds to the silyl-migration process (3 → IM3) with a barrier of 33.3 kcal/mol.

always obtained, which may be closely related to the strong carbophilic Lewis acidity of PtCl2.16−19,35−37 We find that 2 can be formed via the coordination of the metal center to one of the C3−H bonds in R from either the phenylethyl substituent side or the trimethylsilyl substituent side. Scheme 4 shows the detailed formation mechanism of 2 in the latter situation, which involves the more stable catalyst-cyclopropene adduct than that in the former situation, whose results are collected into Figures S7 and S8 in the Supporting Information. As shown in Scheme 4, the Pt attacks one of the C3−H bonds in R, giving the slightly more stable Pt σ-coordinated complex IM1, from which the metal center migrates via TS1 to yield the Pt-π-coordinated intermediate IM2 with a cis chlorine ligand disposition. IM2 is 30.1 kcal/mol lower in energy than the separated materials (R + PtCl2). The subsequent step involves a metal fragment rearrangement via TS2 to afford relatively more stable Pt-π-coordinated complex 2 with a trans chlorine ligand disposition. In TS2, the Cl−Pt−Cl angle has increased to 123.5° from 99.7° in IM2, implying the arrangement change of the chlorine ligands from an incipient cis disposition to the trans disposition. Scheme 4 shows that TS1 and TS2 involve the imaginary frequencies of 42 and 67 cm−1, respectively, implying that the very small geometric distortions must be reached. These results are also consistent with the low activation barriers calculated for the two elementary steps (1.2 kcal/mol for the transition IM1 → IM2 and 1.0 kcal/mol for the transition IM2 → 2). The facts above indicate that the formations of IM1 and IM2 are not important steps, although TS1 and TS2 have been 4771

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Scheme 5. Calculated Schematic Structures for the Formation of Product Precursor IM4 Starting from the Intermediate 2 According to Three Pathways Proposed by Lee and Co-worker6a

a

The imaginary frequencies (in parentheses) characterizing transition states are given in cm−1.

atomic numbers in Scheme 5, the phenylethyl group in the product moiety now connects to the C1 atom, which is incompatible with the Lee group’s 13C-labeled experiment,6 which showed a product with unchanged connection of the phenylethyl group to the initial C2 atom. Therefore, as proposed by Lee et al.,6 pathway III must be excluded. On the basis of the results above, it is clear that pathway II is energetically more favorable and can result in the experimentally observed product. However, from the energy profile shown in Figure 1, we find that at the reaction entrance, complex 2 prefers to convert to IM6 via TS9 rather than to structure 4 via TS6 due to the relatively lower barrier and larger energy release involved in the former. Thus the formation of structure 4 actually must overcome a barrier of as high as 30.0 kcal/mol (the energy difference between TS6 and IM6), meaning a kinetically unfavorable conversion. In addition, as is seen in Figure 1, the product precursor IM4 is significantly less stable than the Ptcarbenoid intermediate IM6. In this case, the conversion from IM6 to IM4 is also a thermodynamically unfavorable process. In other words, according to our calculations based on the mechanisms proposed by Lee and co-workers,6 the reaction might lead to the sole carbenoid IM6 instead of the product precursor IM4. Thus we have to perform additional calculations to search for new and available mechanisms, which can rationalize the experimental observation well. We note that in the product precursor IM4 displayed in Figure 2, the Pt π-coordinates to the internal C−C double bond of the allene via the d−π interaction between the metal center and the substrate. Alternatively, we located another product precursor, denoted as IM7, where the metal coordinates to the external C− C double bond of the allene. IM7 is calculated to be more stable by 11.2 kcal/mol than IM4. This can be understood by comparing the highest occupied molecular orbitals (HOMOs) of the two product precursors (Figure 2). Clearly, in IM7, the π-orbital of the internal C−C double bond interacts with the σ-orbital of the adjacent C−C single bond, generating π-electron delocalization, which contributes significantly to the stability of IM7, while in IM4 there is no such delocalization effect. Therefore, we conjecture that the final formed product precursor

Along pathway II, the allene product P is obtained via a 1,2silyl migration with the subsequent 1,2-C−C bond shift. Our calculations suggest that the silyl group in 2 transfers from the C1 to C2 atom via transition state TS6, leading to slightly more stable cyclopropene intermediate 4, in which the threemembered strained ring stays intact. The barrier of this silylmigration step is calculated to be 13.7 kcal/mol, which is lower than that (18.8 kcal/mol) of the C2−C3 bond cleavage process in pathway I. Then, the C2−C3 bond of 4 is broken (i.e., 1,2-C− C bond shift process) via TS7 with a barrier of 7.1 kcal/mol to give intermediate IM5. Note that in IM5 Pt binds to the C1 atom and the chlorine atom connects with C2, in sharp contrast to the bonding of Pt with C2 and the bonding of the chlorine atom with the C1 atom in IM3. In the last step, the chlorine attached to the C2 atom is rebonded to the metal with a barrier of 7.5 kcal/mol, forming the product precursor IM4. As displayed in Figure 1, the conversion from 2 to 4 via TS6, corresponding to the 1,2-silyl migration, is the rate-controlling step along pathway II with an energy requirement of 13.7 kcal/mol. Clearly, pathway II is energetically more favorable than pathway I discussed above. Next, we give our attention to pathway III, which involves the C1−C3 bond cleavage, i.e., 1,2-C−C bond shift. Along this pathway, the reaction first experiences the breaking of the C1− C3 bond and then the 1,2-phenylethyl migration occurs to form the product precursor IM4. The present calculations show that by breaking the C1−C3 bond via TS9 with a barrier of 7.4 kcal/mol the relatively more stable chained Pt-carbenoid intermediate IM6 can be obtained, which lies 50.5 kcal/mol in free energy below the initial materials. IM6 then isomerizes into the energetically comparable isomer 5 by rotating the Pt−C1−Si plane along the C1−C2 bond. Subsequently, the phenylethyl in 5 migrates from C2 to the C1 atom with a barrier of 34.5 kcal/mol to furnish IM3, which is considered to be the rate-determining step along this pathway. Finally, the reaction evolves by the recoordination of the chlorine ligand initially connected to the C2 atom via TS5 to form product precursor IM4 with an energy requirement of 11.6 kcal/mol. It should be noted that the atomic arrangement in IM4 obtained from pathway III is different from that obtained from pathways I and II. As indicated by green 4772

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This structure features octahedral coordination of the ligands with trans disposition of the chlorine ligands. According to the experiment by Lee et al.,6 for the allene skeleton with the connection of the phenylethyl group to the original C2 atom to remain unchanged, IM8 has to undergo C3 migration from the C2 atom to the C1 atom via TS12 with an energy demand of 36.3 kcal/mol, leading to IM9, a formal five-membered-ring configuration. It is noted from Figure 3 that IM9 is a metastable structure and thus may be of transient nature. So it cannot be an experimentally identifiable intermediate (a similar comment also applies to IM17 shown in Figure 4). From IM9, the silyl group migrates to the C2 atom from C1 with the simultaneous π-coordination of the metal to the external C−C double bond in the allene, giving product precursor IM7 with a barrier of 11.1 kcal/mol. It is found that the rate-determining step along this pathway is the C3 migration, with an energy requirement of 36.3 kcal/mol. Pathway B presented in Figure 3 starts from IM1 (Scheme 6), obtained through the combination of the metal with the C3−H bond of the substrate. It performs a direct metal insertion into the C3−H bond of R via TS14 to afford remarkably more stable platinahydride species IM10, which lies below the entrance of the reaction by 37.1 kcal/mol. From IM10, the hydrogen atom initially bonded to the metal now remigrates back to the C3 atom, accompanied by the simultaneous C2−C3 bond cleavage, providing the formal five-membered cyclization intermediate IM9, which then follows the silyl-migration step via TS13 to furnish IM7. As shown in Figure 3, the step from IM10 to IM9 requires overcoming a barrier of 24.1 kcal/mol and thus is the rate-determining step along this pathway. The calculated pathway C also originates from IM1, as shown in Scheme 6. It is found that the Pt insertion into the C1−C3 bond of IM1, through transition state TS16 with a barrier of

Figure 2. Diagrams of the HOMOs of two product precursors, IM4 and IM7, where the catalyst coordinates to the internal and external C−C double bond in the allene, respectively. The relative free energies (in parentheses) are given in kcal/mol.

should be IM7. On the basis of this more stable product precursor, we have designed and calculated another four pathways, and the detailed results are described in the next section. 3.2. Newly Proposed Mechanisms. The four newly designed pathways are denoted as pathways A, B, C, and D. Scheme 6 shows the calculated schematic structures for the species involved in these four pathways, and the corresponding energy profiles are collected in Figure 3. As is seen in Scheme 6, pathway A originates from carbenoid 5 discussed above, which through an oxidative cyclometalation process via TS11 with a barrier of 15.7 kcal/mol evolves into the slightly more stable PtIV complex IM8, a chelated structure analogous to that previously reported by the Soriano group.38

Scheme 6. Calculated Schematic Structures for the Silyl-Substituted Cyclopropene System According to the Four Pathways (A, B, C, and D) Established in the Present Worka

a

The imaginary frequencies (in parentheses) characterizing transition states are given in cm−1. 4773

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Figure 3. Calculated free energy profiles in dichloromethane solution along newly proposed pathways A, B, C, and D for forming the silyl-substituted catalyst-allene product precursor IM7. The relative free energies are given in kcal/mol.

Figure 4. Calculated free energy profiles in dichloromethane solution for the release of allene product P from the product precursor IM7 along pathways (a), (b), and (c) with the schematic structures for the intermediates involved. The relative free energies are given in kcal/mol. The imaginary frequencies (in parentheses) characterizing transition states are given in cm−1.

3.8 kcal/mol, directly evolves into the chelated PtIV complex IM8, from which the C3 migration occurs to give IM9, followed by silyl migration via TS13, resulting in IM7. It is worth emphasizing that the Pt−H(C3) and Pt−C1 bonds in TS16 are much longer and shorter, respectively, than the corresponding bond distances in IM1, suggesting that the Pt is migrating from the C−H bond to the C−C bond. Thus we conjecture that C−H activation is the driving force of the subsequent C−C activation. The rate-controlling barrier along this pathway is calculated to be 36.3 kcal/mol (same as that along pathway A), corresponding to the conversion from IM8 to IM9. It is noteworthy that the PtCl2 unit in complex 2, significantly different from the isolated PtCl2 molecule with V-shaped configuration, adopts a Cl−Pt−Cl linear disposition. In this case, the Pt center in 2 may be able to use its dπ-orbital to interact with the π*-orbital of the C2 atom in a second reactant, thereby promoting the cleavage of the C2−C3 bond of the second

reactant molecule. Inspired by this, we designed pathway D. As seen from Scheme 6, after 2 a second reactant molecule is introduced into the reaction, and the subsequent step involves the C2−C3 bond activation of the second reactant molecule and the simultaneous expulsion of the initial reactant molecule coordinated to the catalyst. This process is very similar to the well-known SN2 substitution reaction. In detail, the Pt-π complex 2 combines with the second reactant molecule R to afford the adduct IM11, which lies below the entrance of the reaction by 27.8 kcal/mol, less stable than complex 2. Next, the Pt center of IM11 inserts into the C2−C3 bond of the second reactant molecule, through TS17, with a barrier of only 0.8 kcal/mol, to form IM12, from which the reactant molecule initially coordinated with the PtCl2 unit is expelled, resulting in IM13, a PtIV four-membered chelate structure. Note that IM13, where the Pt center is attached to the C2 atom, is geometrically very different from the energetically comparable species IM8, where 4774

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Scheme 7. Sketch of the Catalytic Cycle for the PtCl2-Catalyzed Rearrangement of 1-(Trimethylsilyl)-2(phenylethyl)cyclopropene (R) Based on the Present Calculations

disposition to catalyze the conversion of other reactant molecules via an SN2-like mechanism. 3.3. Product Formation Mechanism from the Precursor. According to the general understanding, through the direct dissociation of the product precursor IM7 we can obtain the desired allene product P with the release of the catalyst PtCl2, which then participates in the next catalytic cycle. However, for the present system, we find that the direct dissociation process requires an energy of 35.7 kcal/mol (pathway (a) in Figure 4), which appears to be too high to explain the observed fact that the reaction was carried out under mild conditions.6 So we want to know what the reasonable product formation mechanism is. Here, we propose two potential pathways (pathways (b) and (c) in Figure 4) based on the geometry of IM7 (Figure 2), where the PtCl2 unit still keeps the Cl−Pt−Cl linear disposition like that in the Pt-π-coordinated complex 2. One (pathway (b) in Figure 4) is imagined as follows: when the product allene dissociates from IM7, the PtCl2 unit maintains its linear configuration and immediately π-coordinates to a second reactant molecule to form intermediate 2, which severs as the starting point of the next catalytic cycle. The energy demand from IM7 to 2 is calculated to be 19.5 kcal/mol, which is in contrast to the high one (35.7 kcal/mol) in the direct dissociation process. Another assumed mechanism (pathway (c) in Figure 4) is based on the fact that the linear PtCl2 unit can effectively activate the reactant. We attach a second reactant molecule around the linear PtCl2 unit in IM7 and expect that the product may be easily removed as the PtCl2 unit activates the second reactant molecule. On the basis of such an idea, we preformed the relevant calculations on such a reactant-assisted product dissociation process, and corresponding results are summarized in Figure 4. In detail, the combination

the metal is connected to the C1 atom. Subsequently, one of the chlorine ligands transfers to the C2 atom from Pt via transition state TS18 with an energy demand of 21.3 kcal/mol, affording the formal five-membered cyclization intermediate IM9, which follows the silyl-migration step via TS13 to furnish the product precursor IM7. From Figure 3, it is clear that the transformation from IM13 to IM9 (the chlorine-migration step) is the rate-controlling step along pathway D and has a barrier of 21.3 kcal/mol, which is the lowest among all pathways studied in this work. Therefore, this pathway is considered to be the optimal one. Here, it should be emphasized that structure 2 is a common species involved in pathways A and D, and it evolves into either the very stable intermediate IM8 in pathway A or almost the same stable intermediate IM13 in pathway D. However, the former involves a relatively higher barrier (16.5 kcal/mol, corresponding to the conversion from IM6 to IM8) and therefore is considered kinetically less favorable than the latter, involving a barrier of 7.2 kcal/mol, corresponding to the energy difference between TS17 and 2. Furthermore, because of the high energy requirement (36.3 kcal/mol) for the subsequent evolution, IM8, potentially formed along pathway A, cannot carry out further development to IM7 but rather reversibly returns to 2 and then evolves along pathway D. The results above demonstrate that the linear disposition of the PtCl2 unit in 2 makes the Pt center effectively activate the C2−C3 bond of the second reactant molecule, significantly reducing the energy barrier forming the product precursor IM7. In 2, the reactant molecule serves as a support of the PtCl2 catalyst, fixing the two chlorine ligands in the proper spatial 4775

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Figure 5. Calculated free energy profiles for the formation of the product precursor HIM7 from the reaction of the H-substituted cyclopropene along four pathways corresponding to pathways A−D shown in Figure 3. The relative free energies are given in kcal/mol.

Scheme 8. Calculated Schematic Structures for the H-Substituted Cyclopropene System According to Four Pathways (Pathways H A, HB, HC, and HD)a

a

The imaginary frequencies (in parentheses) characterizing transition states are given in cm−1.

favorable than the other two mechanisms (pathways (a) and (b) in Figure 4). On the basis of the calculated results above, we schematically present the entire catalytic cycle for the cycloisomerization of R in Scheme 7, which provides a detailed and consistent view of the mechanism details of the significant catalytic rearrangement of silyl-substituted cyclopropene. As seen from Scheme 7, the reaction under study involves an unusual catalytic mechanism, where both the reactant precursor 2 and the product precursor IM7 play key roles in the catalytic reaction. In these two precursors the reactant and product moieties serve as supports to fix the catalyst, giving the catalyst PtCl2 a catalytically effective

of IM7 with a second reactant molecule generates slightly unstable complex IM15, which evolves into IM16 via a C2−C3 bond cleavage process (TS19) with a barrier of 3.5 kcal/mol. After that, IM16 undertakes chlorine migration with a barrier of 6.2 kcal/mol, affording intermediate IM17. Subsequently, the reaction follows a silyl-shift process with a barrier of 9.7 kcal/mol to form substantially stable allene-catalyst-allene adduct IM18, which dissociates into product P and IM7 with an energy demand of 24.4 kcal/mol. From the energy profiles shown in Figure 4, it is clear that the reactant-assisted product dissociation pathway (pathway (c) in Figure 4) is energetically much more 4776

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findings. Alternatively, based on the calculated results, a novel catalytic mechanism by PtCl2, which is fixed on the cyclopropene/allene to form a linear Cl−Pt−Cl disposition, provides the most efficient access to the significant catalytic rearrangement. In the newly proposed mechanism, the PtCl2 fixed on a cyclopropene molecule activates a C−C single bond of a second cyclopropene molecule via an SN2-tye mechanism to afford the product precursor PtCl2-allene with the metal center coordinated to the external CC bond in the allene framework. Once formed, the PtCl2-allene immediately serves as a new active center to catalyze the rearrangement reaction rather than directly dissociating into the allene product and the PtCl2 catalyst due to its high stability. During the catalytic cycle, an allene-PtCl2-allene sandwich compound is identified as the most stable structure on the potential energy surface, and its direct dissociation results in the formation of the product allene and the regeneration of the catalytically active center Pt-allene with an energy demand of 24.4 kcal/mol. This process is found to be the rate-determining step of the catalytic cycle. In addition, we also have performed calculations on the H-substituted cyclopropene system to understand the experimental finding that the H-substituted cyclopropenes do not provide any allenes. It is found that the highest barrier to be overcome in the reaction amounts to 35.2 kcal/mol. The stronger C−H bond than the C−Si bond is likely to be mainly responsible for the high energy barrier involved. The present theoretical results provide a new insight into the mechanistic details of the significant rearrangement reaction, which is expected to be informative for the efficient design of new cyclopropene catalytic rearrangements.

linear Cl−Pt−Cl disposition. Through the assistance of a second reactant molecule, the reactant precursor 2 is converted to the product precursor IM7, which is actually the starting point and end point of the catalytic recycle. 3.4. Role of Trimethylsilyl Substituent. In Lee’s experiment,6 an intriguing observation is presented, that is, upon replacement of the trimethylsilyl substituent by a hydrogen atom, the cyclopropene does not provide any allene, which is in strong contrast to the formation of allene in excellent yield from the substrate with the trimethylsilyl substituent under the same conditions. So, we also performed detailed calculations for allene formation from the H-substituted cyclopropene, from which we expect to reveal the role of the trimethylsilyl substituent in the reaction. The calculated free energy profiles along the four pathways, denoted as HA, HB, HC, and HD, corresponding to pathways A, B, C, and D in Figure 3, in dichloromethane solution involved in the H-substituted-cyclopropene reaction are illustrated in Figure 5, where the intermediates and transition states are marked with the capital letter “H” in the left superscript in order to differentiate them from those with the silyl substituent. The optimized schematic structures for the species involved in Figure 5 are given in Scheme 8. All these are based on Pt attack from the H substituent side of HR; the corresponding ones based on Pt attack from the phenylethyl substituent side are given in Figures S9 and S10 in the Supporting Information. We find that the H-substituted-cyclopropene system involves similar mechanisms to the silyl-substituted-cyclopropene system discussed above. Thus, the relevant mechanism details are not discussed again for simplification. Our attention is mainly focused on the optimal energy profiles of the two catalytic reactions. As shown in Figure 5, pathway HD is energetically most favorable, and the calculated energy profile before HIM9 is analogous to that of the silyl-substituted system shown in Figure 3. However, the final conversion from HIM9 to HIM7, corresponding to the direct H-shift process, is found to involve a barrier of 32.8 kcal/mol. Furthermore, according to the energetic span model developed by Shaik et al.,39,40 the overall barrier along this pathway is calculated to be as high as 35.2 kcal/mol (the energy difference between HTS13 and HIM13). In contrast, for the silylsubstituted system (Figure 3), the conversation from IM9 to IM7, corresponding to the silyl-migration event, has a barrier of only 11.1 kcal/mol, and the overall barrier of the reaction along pathway D (the energy difference between TS18 and IM13) is 21.3 kcal/mol. Clearly, the preference for the silyl-migration over the H-shift process is mainly attributed to the fact that C−H bond activation is more difficult than C−Si bond activation. Therefore, the silyl substituent is of key importance for the formation of the allene product. Additionally, from Figure 5, H IM9 and HIM13 are the two most stable intermediates along the most optimal pathway. We conjecture that the intractable materials experimentally obtained from the H-substituted cyclopropene reaction might be a mixture of these two stable intermediates.

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ASSOCIATED CONTENT

S Supporting Information *

Optimized geometries with selected structural parameters and Cartesian coordinates for all the species involved in the reaction, calculated free energy profiles for the formation of the product precursors IM7 and HIM7 based on the Pt attack from the phenylethyl substituent side of the substrate, and the complete author list for ref 34. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (Nos. 20873076 and 21173126) and the Graduate Independent Innovation Foundation of Shandong University (No. yzc10044).

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REFERENCES

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4. CONCLUDING REMARKS The detailed mechanisms for the formation of the allene complex from the PtCl2-catalyzed rearrangement reaction of 1-(trimethylsilyl)-2-(phenylethyl)cyclopropene have been systematically investigated with the aid of DFT calculations. The computational results indicate that the mechanisms proposed in previous literature cannot completely rationalize the experimental 4777

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