DFT Study on the Mechanism of PtCl2-Catalyzed

Article pubs.acs.org/Organometallics

DFT Study on the Mechanism of PtCl2-Catalyzed Rearrangement of Cyclopropenes to Allenes Ran Fang,* Lizi Yang, and Qiang Wang Key Laboratory of Nonferrous Metals Chemistry and Resources Utilization of Gansu Province and College of Chemistry and Chemical Engineering, Lanzhou University, Lanzhou 730000, People's Republic of China S Supporting Information *

ABSTRACT: The mechanisms of PtCl2-catalyzed rearrangement of cyclopropenes to allenes have been investigated using density functional theory calculations carried out at the B3LYP/6-31G(d,p) (LANL2TZ(f) for Pt) level of theory. The solvent effect was taken into account by B3LYP/6-311++G(d,p) (LANL2TZ(f) for Pt) single-point calculations with the integral equation formalism polarizable continuum model (IEFPCM) in dichloromethane. The radii and nonelectrostatic terms were taken from Truhlar and co-workers’ universal solvation model (SMD). Three pathways which lead to the formation of allene via a [1,2]-C−C bond migration (regioselectivity) and C−Si bond migration were performed. Our calculations suggest the following. (1) The major pathway of the cycle involves an initial [1,2]-silyl shift leading to a platinum intermediate. A subsequent [1,2]-C−C bond shift of this intermediate then provides the corresponding allenes. (2) Due to the consequence of an interaction between the Lewis acidic platinum and the allene, the rearrangement of α-alkoxy-substituted cyclopropenes does not give the allene as the final product. (3) The calculations suggest that the electronic effect of a silyl substituent on the cyclopropene is essential for this reaction. Furthermore, the silyl groups provide not only the β-cation-stabilizing effect but also a facile migration group for the whole reaction. Scheme 1

1. INTRODUCTION Cyclopropenes are relevant structures in physical organic, natural products, and medicinal chemistry.1 As a result, this three-membered motif has inspired various methods for its synthesis.1,2 Cyclopropenes represent versatile synthetic building blocks and exhibit special reactivity because of their inherent ring strain.3 These properties have already been widely exploited in metal-catalyzed processes.4 The conformationally rigid scaffold of the cyclopropene represents an ideal model for mechanistic investigations and the design of novel stereoselective transformations.4c While cyclopropenes with their π-rich double bond and highly strained character would appear to be ideal substrates for activation by π-philic transition metals such as gold, there has been relatively little investigation of this aspect of their chemistry. To date, Lee and co-workers have described the gold-catalyzed solvolysis of simple alkylcyclopropenes,5 while Shi and Zhu6 as well as Wang and co-workers7 have demonstrated that cyclopropenes are capable of cycloisomerizing to afford indene derivatives. Furthermore, an experimental and theoretical investigation into the gold-catalyzed reactivity of cyclopropenylmethyl acetates also has been performed by Hyland et al.8 As we know, the rearrangement of cyclopropenes to allenes under thermal and photochemical conditions has been documented extensively, yet the corresponding metal-catalyzed reaction is virtually unprecedented.9 Recently, PtCl2-catalyzed rearrangements of silylated cyclopropenes were found to give efficient access to the corresponding allenes under mild conditions by Lee et al. (Scheme 1).10 According to the experimental results, © 2012 American Chemical Society

three general mechanisms were postulated to explain the formation of allenes from silylated cyclopropenes. As depicted in Scheme 2, the interaction of the C−C double bond of cyclopropenes with the platinum atom would favor the formation of intermediate A due to the β-cation-stabilizing effect of a silyl group. A would provide the platinum carbene intermediate B by a [1,2]-C−C bond shift. A subsequent [1,2]-alkyl shift leads to the observed allene products. Alternatively, C−Si bond migration leading to C followed by a [1,2]-C−C bond shift would also be quite plausible. In another pathway, the formation of regioisomeric intermediate D via the cycloreversion of A followed by a [1,2]-silyl shift would be equally possible. Furthermore, the insertion of the metal catalyst into one of the C−C bonds to form the metallacyclobutene E or F followed by its electrocyclic ring opening would be another pathway leading to B and D. To our knowledge, there are no detailed theoretical studies available in the literature for the novel platinum(II)-catalyzed Received: March 22, 2012 Published: May 11, 2012 4020

dx.doi.org/10.1021/om300240n | Organometallics 2012, 31, 4020−4030

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Scheme 2

transformation reported by Lee et al.10 Here, we present a detailed density functional theory (DFT) computational investigation of the mechanism and regioselectivity of the PtCl2catalyzed rearrangement of silylated cyclopropenes to give the corresponding allenes on the basis of the experimental evidence reported by Lee et al.10 The present DFT study located the transition states for the reactions of interest and performed a vibrational analysis at these stationary points. From the results presented here, we hope to learn more about the factors that control the activation barriers of this important reaction and also further investigate the effects of solvent on the thermodynamic and kinetic properties of these reactions.

cyclopropenes), intermediates, transition states and products of the reactions are depicted schematically in Figure 2 along with selected key geometrical parameters (e.g., bond lengths). Their relative energies in the gas and solution phases, together with the activation barriers corresponding to the relevant transition structures, are given in Table 1. Unless otherwise noted, the relative energies discussed in subsequent sections refer to the values in CH2Cl2 solvent. The detailed structural parameters and energies for the structures determined here are collected in the Supporting Information. 3.1. Reaction Pathways. 3.1.1. Pathway a. As we know, the C−C double bond functional group is characterized by one π bond, which is higher in energy and easily interacts with the d orbitals in transition metals (electrophiles). Thus, the interaction of the C−C double bond of cyclopropenes with the platinum atom would give the preliminary intermediate 1. Furthermore, the insertion of the metal catalyst into the proximal and distal bonds would also provide preliminary intermediate metallacyclobutenes 1a,b. If we consider PtCl2 as the “active” species of the catalyst, 1 is 59.1 kcal/mol lower in energy than the reactants (PtCl2 (R1) + cyclopropenes (R2)) but remains 20.9 and 19.8 kcal/mol higher than 1a,b, respectively. This indicates that an intramolecular conversion would be take place between 1 and 1a or 1b. In 1, the lengths of the two Pt−C bonds are 2.113 and 2.110 Å, respectively. This indicates that a nearly symmetrical coordination reaction between the C−C double bond and the platinum complex was involved for 1. The C1−C2 bond has lost a small amount of its double-bond character and is now 1.418 Å (1.308 Å in R2). Meanwhile, the C2−C3 and C1−C3 bond lengths are 1.493 and 1.529 Å, respectively. Due to the strain of the three-membered ring, structure 1 is then converted to the platinum carbene intermediate 2a via the C1−C3 bond shift transition structure TSa1 (TSa1 has only one imaginary frequency of 95i cm−1, and IRC calculations confirmed that this TS connects the corresponding reactants and intermediate). The transition vector obtained from the frequency computations on TSa1 is dominated by the C1−C3 distances. Inspection of Figure 2

2. COMPUTATIONAL METHODS All calculations were carried out with the Gaussian 09 programs.11 The geometries of all the species were fully optimized by using density functional theory (DFT)12 of the B3LYP method13,14 with the 6-31G(d,p) basis set for all atoms except for Pt, for which the smallcore Los Alamos (LANL2TZ(f)) pseudopotentials and basis sets that include the Dunning−Huzinaga full TZ and Los Alamos ECPs plus TZ have been employed with an extra f polarization function.15 This computational method was successfully applied in the mechanistic studies of transition-metal- and non-transition-metal-catalyzed reactions.16−23 Vibrational frequency calculations done at the B3LYP/ 6-31G(d,p) level of theory were used to characterize all of the stationary points as either minima (the number of imaginary frequencies (NIMAG=0) or transition states (NIMAG=1)). The relative energies are thus corrected for the vibrational zero-point energies (ZPE, not scaled). In several significant cases, intrinsic reaction coordinate (IRC)24 calculations were performed to unambiguously connect the transition states with the reactants and the products. The solvent effect was taken into account by B3LYP/6-311++G(d,p) single-point calculations with the integral equation formalism polarizable continuum model (IEFPCM) in dichloromethane (ε = 8.93). The radii and nonelectrostatic terms were taken from Truhlar and coworkers’ universal solvation model (SMD).25

3. RESULTS AND DISCUSSION Energy profiles for reaction pathways a−c are shown in Figure 1. The optimized geometries for the reactants (R1, PtCl2; R2, 4021


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Figure 1. Energy profiles for paths a−c. Relative energies are given in kcal/mol.

process is exothermic, −79.1 kcal/mol lower than the reactants. The higher barrier found for this step can be mainly attributed to the steric hindrance effect of the migration group. This step also is the rate-determining step for the whole catalytic process. 3.1.2. Pathway b. Due to the ring strain of the threemembered ring, all of the strained C1−C3 and C2−C3 carbon− carbon single bonds can be further activated in principle. This fact indicates that C2−C3 bond migration would give rise to another possible reaction pathway. Similar to pathway a, the platinum carbene intermediate 2b also should be formed from a C2−C3 bond shift (TSb1) of 1 or electrocyclic ring opening (TSb2) of 1b. Figure 2 shows that the platinum atom is completely connected to the C2 atom of the cyclopropenes (the Pt−C2 bond distance is 1.989 Å) in TSb1. The C1−C3 bond has lost a small amount of its single-bond character and is now 1.427 Å. Meanwhile, the C2−C3 bond length changes from 1.493 to 1.723 Å. As the reaction goes from the transition state (TSb1) to 2b, it is evident that the C1−C3 bond completes its change from a single bond to a double bond (1.355 Å) and the C2−C3 bond becomes completely broken. The C1−C2 bond also has some single-bond character and is now 1.454 Å. Inspection of Table 1 shows that the activation free energy of the C2−C3 bond migration is 17.8 kcal/mol and the reaction free energy is −20.4 kcal/mol with respect to 1. The higher barriers found for TSb1 in comparison to those for TSa1 can be mainly attributed to the following reasons. First, the NBO charges for the C1, C2, and C3 atoms of 1 are −0.157, 0.022, and −0.198 au, respectively. A positive charge found for the C2

shows that the platinum atom is completely connected with the C1 atom (the bond distance Pt−C1 is 1.988 Å) in TSa1. Furthermore, the bonds of the C1−C3 and C2−C3 atoms change from 1.529 to 1.649 Å and from 1.493 to 1.434 Å. From these changes in the bond distances, we can find that the transition states have similar structures close to those of the reactants. As will be discussed below, this step reaction is exothermic. According to the Hammond postulate,26 the reaction should have an early transition state close to the reactants, and our results are consistent with this premise. Inspection of Table 1 shows that the energy of activation for this step is calculated to be 3.9 kcal/mol for TSa1 and that for 2a is −17.4 kcal/mol with respect to 1. On the other hand, 2a could also be formed through electrocyclic ring opening of 1a. Table 1 shows that the energy of activation for this step is calculated to be 18.1 kcal/mol for TSa2 and the energy of reaction for 2a is 3.5 kcal/mol with respect to 1a. In 2a, it is evident that the C2−C3 bond completes its change from a single bond to a double bond (1.356 Å) and the C1−C3 bond becomes completely broken. In order to accomplish a rearrangement, a subsequent step for 2a undergoing a [1,2]-alkyl shift resulted in formation of the final product (3a) and regeneration of the catalyst (R1) through TSa3. Figure 2 shows that the C1− C4 and C2−C4 bond distances are 1.780 and 2.064 Å in TSa3, respectively. Furthermore, the C1−C2 bond has lost its singlebond character and is now 1.356 Å. The free activation energy of the second step is 34.2 kcal/mol, the formation of 3a is an exothermic process (the free energy of reaction for 3a was −2.6 kcal/mol with respect to 2a), and the whole catalytic 4022


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Figure 2. Optimized structures for paths a−c shown in Figure 1, with selected structural parameters (bond lengths in Å).

atom makes the C−C bond shift of the C1−C3 bond more feasible than that of the C2−C3 bond. Second, the DFT calculations show that the breaking C−C bonds in the TSa1 and TSb1 structures are longer by 7.5% and 13.2%, respectively, than those in the corresponding intermediates

(see Figure 1). These structural features reveal that the former transition structures take on more reactant-like character than the latter structures. According to the Hammond postulate,26 the former should have lower activation barriers, and the latter, higher activation barriers. Table 1 shows that the activation 4023


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Table 1. Thermodynamic Properties (Relative Free Energies and Activation Free Energies in the Gas Phase and in Solution) of the Structures in Figures 1 and 2a system

ΔErelgas

ΔGrelgas

ΔE⧧gas

ΔG⧧gas

ΔErelsol

ΔGrelsol

ΔE⧧sol

ΔG⧧sol

1 1a TSa1 TSa2 2a TSa3 3a 1b TSb1 TSb2 2b TSb3 3b TSc1 2c TSc2 3b

0 −18.4 8.0 −1.2 −18.0 23.0 −16.4 −18.2 −4.4 21.3 −19.7 8.9 −16.4 16.2 −3.5 4.2 −16.4

0 −18.8 7.8 −1.6 −17.7 23.5 −17.3 −18.0 −4.5 20.6 −20.7 9.7 −17.3 16.3 −4.1 3.9 −17.3

0

0

0

7.8 17.2

4.1 18.1

3.9 18.1

40.9

41.3

33.9

34.2

13.8 21.3

13.6 20.6

14.3 18.5

14.0 17.8

28.6

30.4

25.3

27.2

16.2

16.3

11.0

11.2

7.7

8.0

0 −20.9 3.9 −2.8 −17.4 16.8 −20.0 −19.8 −5.8 17.8 −20.4 6.7 −20.0 11.2 −3.4 −0.6 −20.0

0

8.0 17.2

0 −20.5 4.1 −2.4 −17.7 16.2 −19.2 −19.9 −5.7 18.5 −19.4 5.9 −19.2 11.0 −2.8 −0.3 −19.2

2.5

2.8

a

These values, in kcal/mol, were calculated at the B3LYP/6-31G(d,p) (LANL2TZ(f) for Pt) level of theory and include a zero-point energy correction using single-point integral equation formalism polarizable continuum model (IEFPCM) calculations at the B3LYP/6-311++G(d,p) (LANL2TZ(f) for Pt) level of theory to model the effect of the solvent (CH2Cl2).

Figure 3. Energy profiles for paths d−f. Relative energies are given in kcal/mol.

energy is calculated to be 14.0 kcal/mol for TSb2 and energy of reaction for 2b is −0.6 kcal/mol with respect to The subsequent step for [1,2]-silyl migration results in formation of the final product 3b and regeneration of

catalyst (R1) through TSb3. Inspection of Figure 2 shows that the Si−C1 and Si−C2 bond lengths are 2.661 and 2.064 Å for TSb3, respectively. Furthermore, the C1−C2 bond almost completes its change from a single bond to a double bond

the 1b. the the 4024


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Figure 4. Optimized structures for paths d−f shown in Figure 3, with selected structural parameters (bond lengths in Å).

(1.376 Å). The final barrier of 27.2 kcal/mol is required to release the product and regenerate the catalyst. These final steps are endothermic by 0.4 kcal/mol, and the whole catalytic

process is exothermic, −79.1 kcal/mol lower than the reactants. It is important to point out that this step also is the ratedetermining step. 4025


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ΔErelgas

ΔGrelgas

ΔE⧧gas

ΔG⧧gas

ΔErelsol

ΔGrelsol

ΔE⧧sol

ΔG⧧sol

11 1d TSd1 TSd2 2d TSd3 3d 1e TSe1 TSe2 2e TSe3 3e TSf1 2f TSf2 3e TSf3 4f

0 −21.1 10.0 −3.1 −22.0 18.7 −20.2 −19.6 21.2 −4.4 −19.5 7.6 −20.2 17.0 −7.3 4.8 −20.2 −6.7 −11.3

0 −20.4 9.6 −3.7 −23.5 18.7 −20.9 −19.5 21.0 −4.1 −20.6 8.6 −20.9 17.0 −8.0 4.5 −20.9 −6.6 −9.9

0

0

0

9.6 16.7

6.3 17.0

5.9 15.6

40.7

42.2

35.2

36.7

21.2 15.2

21.0 15.4

19.1 15.2

18.9 15.3

27.1

29.2

21.8

23.9

17.0

17.0

12.5

12.4

12.1

12.6

5.6

6.0

13.5

14.4

0 −19.7 5.9 −4.0 −22.1 14.6 −23.0 −20.9 18.9 −5.5 −20.6 3.3 −23.0 12.4 −5.7 0.2 −23.0 −9.8 −14.9

0

10.0 18.0

0 −20.4 6.3 −3.4 −20.7 14.6 −22.3 −21.0 19.1 −5.8 −19.6 2.2 −22.3 12.5 −5.1 0.5 −22.3 −9.9 −16.3

12.4

13.3

a

These values, in kcal/mol, were calculated at the B3LYP/6-31G(d,p) (LANL2TZ(f) for Pt) level of theory and included a zero-point energy correction using single-point integral equation formalism polarizable continuum model (IEFPCM) calculations at the B3LYP/6-311++G(d,p) (LANL2TZ(f) for Pt) level of theory to model the effect of the solvent (CH2Cl2).


ΔErelgas

ΔGrelgas

ΔE⧧gas

ΔG⧧gas

ΔErelsol

ΔGrelsol

ΔE⧧sol

ΔG⧧sol

1h 1ah TSa1h TSa2h 2ah TSa3h 3ah 1bh TSb1h TSb2h 2bh TSb3h 3bh TSc1h 2ch TSc2h 3bh

0 −16.3 11.4 −2.5 −24.1 19.7 −21.0 −19.7 23.7 −7.4 −27.5 15.6 −21.0 25.4 −6.3 −0.7 −21.0

0 −16.9 10.4 −2.7 −25.3 18.7 −21.9 −19.2 23.2 −7.2 −28.2 14.3 −21.9 23.2 −7.1 −1.9 −21.9

0

0

0

10.4 14.2

5.7 14.1

4.7 14.6

43.9

44.1

35.4

35.6

23.7 12.3

23.2 12.1

20.5 12.4

20.0 12.2

43.1

42.5

33.1

32.5

25.4

23.2

19.8

18.9

5.6

5.3

0 −19.1 4.7 −4.5 −25.5 10.1 −25.3 −20.4 20.0 −8.2 −28.3 4.2 −25.3 18.9 −7.7 −6.1 −25.3

0

11.4 13.8

0 −18.5 5.7 −4.4 −24.3 11.1 −24.3 −20.9 20.5 −8.5 −27.7 5.4 −24.3 19.8 −6.5 −5.3 −24.3

1.2

1.6

a

These values, in kcal/mol, were calculated at the B3LYP/6-31G(d,p) (LANL2TZ(f) for Pt) level of theory and included a zero-point energy correction using single-point integral equation formalism polarizable continuum model (IEFPCM) calculations at the B3LYP/6-311++G(d,p) (LANL2TZ(f) for Pt) level of theory to model the effect of the solvent (CH2Cl2).

3.1.3. Pathway c. Apart from a [1,2]-C−C bond shift, It has been suggested that C−Si bond migration can also account for the formation of 3b. In order to investigate this assertion, pathway c, [1,2]-C−Si bond migration, was investigated. The silyl group is known to undergo facile [1,2]-migration to electron-deficient centers. Furthermore, [1,2]-Si migration to cationic centers27,28 and free29 or metal-stabilized30 carbenes has also been reported. Examination of Figure 1 suggests that the first step for pathway c also involves the preliminary intermediate 1. The next step for migration of the C−Si bond gives the new and stable structure 2c through the threemembered-ring transition structure TSc1. Inspection of Figure

2 shows that the Si−C1 and Si−C2 bond lengths are 2.201 and 2.443 Å, respectively. As the reaction goes from TSc1 to 2c, the Si−C1 bond is completely broken in 2c and the Si−C2 bond is completely formed (1.951 Å). Table 1 shows that the free energy of activation for TSc1 is calculated to be 11.2 kcal/mol and the formation of 2c is an exothermic process (the free energy of reaction for 2c is −3.4 kcal/mol with respect to 1). The lower barrier found for TSc1 in comparison to that for TSb3 indicates that the C−Si bond migration is favorable for the cyclopropene structure. Furthermore, the lower barriers found for TSc1 can be mainly attributed to the following reasons. The NBO charges for the C1, C2, and C3 atoms of 1 4026


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Figure 5. Energy profiles for paths a−c. Relative energies are given in kcal/mol.

are −0.157, 0.022, and −0.198 au, respectively. The NBO charges for the C1, C2, and C3 atoms of 2b are −0.093, −0.071, and −0.199 au, respectively. A positive charge found for the C2 atom of 1 makes the C−Si bond shift for cyclopropene structure 1 more feasible than the alkene structure 2b. It is important to point out that this step also is the rate-determining step. The subsequent [1,2]-C−C bond shift results in the formation of the final product 3b and regenerates the catalyst (R1). Inspection of Figure 2 shows that the C2−C3 bond of TSc2 is 2.030 Å. The final barrier of 2.8 kcal/mol is required to release the product and regenerate the catalyst. These final steps are exothermic by 16.6 kcal/mol, and the whole catalytic process is exothermic, −79.1 kcal/mol lower than the reactants. The lower barrier found for TSc1 for the whole catalytic process renders the pathway for TSc1 more feasible than those of TSa3 and TSb3. Our calculations suggest that the major pathway of the cycle should be an initial [1,2]-silyl shift leading to an intermediate. A subsequent [1,2]-C−C bond shift of this intermediate then provides the corresponding allenes. According to our calculation results, the Pt2+ ion plays a double role of activating the C−C double bond of cyclopropenes in 1 and stabilizing the transition state TSc1. 31 Numerous experimental and theoretical investigations support the intuitive view that a positive charge on the metal center results in enhanced electrophilicity of the bound π ligand relative to the neutral analogues.32 That is to say, catalysts imparting higher charge density onto the ligated π system lead to a C−Si bond that migrates more easily. The calculated NBO charges for the Pt atoms of PtCl2 and cis-PtCl2(PPh3)2 (ligand PPh3 replaced with PH3) are 0.198 and −0.119 au, respectively, which might explain

why other metal complexes such as Rh2(O2CCF3)4, PtCl2(PPh3)2, and RuCl2(PPh3)3 are virtually inert toward the substrate. 3.2. [1,2]-C−C Bond Shift vs [1,2]-Alkyl Shift. According to experimental results, three general mechanisms have been postulated to explain the formation of allenes from cyclopropenes (Scheme 2). To gain more insight into the reaction mechanism, isotopically labeled cyclopropene was prepared and subjected to the reaction conditions for allene formation. This exclusively afforded 13C-labeled allene in which the alkyl group remained connected to the original 13C-labeled carbon and also clearly rules out the possibility of a [1,2]-alkyl shift involving intermediate B and supports the other pathways with intermediates C and D. Unfortunately, the current experiment with 13C-labeled cyclopropene could not distinguish the remaining two pathways.10 As we know, the actual pathway that is followed to reach 3a or 3b is determined by the energy difference between the transition states of highest energy along the different pathways. According to our calculated results, TSa3, TSb3, and TSc1 play vital roles in the title reaction. The higher activation energy found for TSa3 indicates that the formation of product 3a from 1 via a C−C bond migration should be unfavorable. Our calculated results are in good agreement with the experimental observations of Lee et al.10 Furthermore, the current experiment with 13C-labeled cyclopropene could not distinguish the remaining two pathways. However, the relative energy difference between the activation energies of TSb2 and TSc1 was 16.0 kcal/mol, which suggests that the major pathway of the cycle causes an initial [1,2]-silyl shift leading to an intermediate. A subsequent [1,2]-C−C bond shift of this intermediate then provides the corresponding allenes. 4027


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Figure 6. Optimized structures for paths a−c shown in Figure 5, with selected structural parameters (bond lengths in Å).

moiety and examined their rearrangements. While α-alkoxysubstituted cyclopropenes did not produce the corresponding allene (α-alkoxy-substituted cyclopropenes were completely consumed, but allene was not observed), the other alkoxy-substituted substrates afforded allenes in good yields. These experiments indicate that the different substitutions have important effects on the reaction mechanism. In order to further account for the

3.3. Effect of the Substitution. In the original experimental paper,10 the authors surmised that a putative interaction between Lewis acidic platinum and the allene takes place during the reaction. If this is indeed the case, then other basic functionalities (e.g., ether) should interfere with the process. To verify this hypothesis, they prepared cyclopropenes containing ether linkages at various distances from the cyclopropene 4028


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the activation free energy of the rate-determining step (TSb3h) is 32.5 kcal/mol. The higher activation free energy found for this step in comparison to that for TSb3 indicates that [1,2]-Si migration is easier than [1,2]-H migration. A similar phenomenon was also found for pathway c. For example, the activation free energy for the rate-determining step (TSc1h) of pathway c is 18.9 kcal/mol and the activation free energy for TSc2h is 1.6 kcal/mol. These values clearly indicate that the silyl substituent provides a facile migration group for the whole reaction. Moreover, TSc1h is lower in energy than TSa3h and TSb3h by 16.7 and 13.6 kcal/mol, respectively, suggesting that cyclopropenes without silyl groups should also be caused by an initial [1,2]-silyl shift leading to a platinum intermediate. A subsequent [1,2]-C−C bond shift of this intermediate provides the corresponding allenes. The calculations suggest that the electronic effects of a silyl substituent on the cyclopropene are essential for this reaction. Furthermore, the silyl groups provide not only the β-cation-stabilizing effect but also a facile migration group for the whole reaction.

effect of the substitution, reaction pathways d−f for the α-alkoxy-substituted cyclopropenes were calculated and the energy profile for this process is represented in Figure 3. The optimized geometries for the reactants (R1, PtCl2; R3, α-alkoxysubstituted cyclopropenes), intermediates, transition states, and products of the reactions are depicted schematically in Figure 4 along with selected key geometrical parameters (e.g., bond lengths). Table 2 shows that the activation free energies for TSd1 and TSd2 are 5.9 and 15.6 kcal/mol. The activation free energy of the rate-determining step (TSd3) for path d is 36.7 kcal/mol, which indicates that the substitution has little effect on this pathway. A similar phenomenon was also found for pathway e. For example, the activation free energies for TSe1 and TSe2 are 18.9 and 15.3 kcal/mol and the activation free energy of the ratedetermining step (TSe3) for path e is 32.5 kcal/mol. Examination of Table 3 shows that the activation free energy for the ratedetermining step (TSf1) of pathway c is 12.4 kcal/mol. It is obvious that a favorable process for the formation of allenes was also found for α-alkoxy-substituted cyclopropenes. However, the coordination of the Lewis acidic platinum and the allene enhances the electrophilicity of the double bond, which induces a cyclization of the alkoxy oxygen onto the double bond. The new intermediate 4f is formed through TSf3. Table 2 shows that the free energy of activation is calculated to be 13.4 kcal/mol for TSf3 and the change in free energy for this step is 8.1 kcal/mol. The lower barriers found for this step indicate that the allene appears not to be a product on the potential energy surface for α-alkoxysubstituted cyclopropenes. Owing to the strain of the fivemembered ring and thermodynamic instability, 4f could be easily transformed into other products. This might account for the experimental fact that α-alkoxy-substituted cyclopropenes are completely consumed but allene is not observed. Moreover, TSf1 is lower in energy than TSd3 and TSe3 by 24.3 and 11.1 kcal/mol, respectively, suggesting that the first step of the cycle for the α-alkoxy-substituted cyclopropenes should also be caused by an initial [1,2]-silyl shift leading to a platinum intermediate. A subsequent [1,2]-C−C bond shift of this intermediate provides the corresponding allenes. Finally, a cyclization of the alkoxy oxygen onto the double bond would give the new intermediate 4f. These results suggest that a functional group near the reaction center for the substrate would interfere with the reaction. Our calculated results are in good agreement with the experimental observations of Lee et al.10 Furthermore, we have made calculations on two other substrates and the same reactivity trends have been found.33 3.4. Role of the Silyl Groups. According to experimental results, the corresponding unsubstituted cyclopropenes provided only intractable material without any sign of allenes, which clearly indicates that the silyl substituent should play an important role in the rearrangement of cyclopropenes to the corresponding allenes. Thus, DFT calculations were performed to evaluate the effects and establish overall reaction pathways without silyl groups in DCM solvents. The energy profile for this process is represented in Figure 5. The optimized geometries for the reactants, intermediates, transition states, and products of the reactions are depicted schematically in Figure 6 along with selected key geometrical parameters (e.g., bond lengths). Table 3 shows that the activation free energies for TSa1h and TSa2h are 4.7 and 14.6 kcal/mol. The activation free energy of the rate-determining step (TSa3h) for path a is 35.6 kcal/mol, which indicates that the silyl groups have little effect on this pathway. For pathway b, the activation free energies for TSb1h and TSb2h are 20.0 and 12.2 kcal/mol and

4. CONCLUSIONS In summary, this work has provided a theoretical study of the PtCl2-catalyzed rearrangement of cyclopropenes to allenes. Our calculations suggest the following. (1) The calculated results of the mechanisms are consistent with the experimental results with 13C-labeled cyclopropene. Furthermore, the current experiment with 13C-labeled cyclopropene only rules out the possibility of a [1,2]-alkyl shift and could not distinguish the remaining two pathways. Our calculated results clearly indicate that the major pathway of the cycle involves an initial [1,2]-silyl shift leading to the platinum-complexed intermediate 2c. A subsequent [1,2]-C−C bond shift of this intermediate then provides the corresponding allene. (2) The α-alkoxy-substituted cyclopropenes should also be formed by an initial [1,2]-silyl shift leading to a platinum intermediate. A subsequent [1,2]-C−C bond shift of this intermediate then provides the corresponding allene. Finally, cyclization of the alkoxy oxygen onto the double bond resulted in the formation of new intermediates 4f. Due to the ring strain of a five-membered ring and thermodynamic instability, 4f could be easily transformed into other products. In other words, α-alkoxy-substituted cyclopropenes would not give the allene as the final product. This accounts for the experimental fact that α-alkoxy-substituted cyclopropenes were completely consumed but allene was not observed. (3) The electronic effect of a silyl substituent on the cyclopropene has been shown by the calculations to be essential for this reaction. Furthermore, the silyl groups provide not only a β-cation-stabilizing effect but also a facile migration group for the whole reaction.

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ASSOCIATED CONTENT

S Supporting Information *

Text giving the complete citation for ref 11, tables giving Cartesian coordinates for the calculated stationary structures and the sum of the electronic and zero-point energies for the transition and ground states obtained from the DFT calculations, and figures giving additional structures as noted in the text. 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]. 4029


Organometallics

Article

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Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research has been supported by the Specialized Research Fund for the Doctoral Program of Higher Education (Grant No. 20110211120012) and the Fundamental Research Funds for the Central Universities (Grant Nos. lzujbky-2010-37 and lzujbky-2010-36). The high-performance computing facility at the Gansu Computing Center is also acknowledged. We are grateful to the reviewers for their invaluable suggestions.

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DFT Study on the Mechanism of PtCl2-Catalyzed

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