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Theoretical Study on the Formation of Silacyclopropene from Acylsilane and Acetylene via Silene-to-Silylene Rearrangement Hiromasa Tanaka,† Yoshiyuki Kondo,† Yoshihito Shiota,† Akinobu Naka,‡ Mitsuo Ishikawa,‡ and Kazunari Yoshizawa*,† †

Institute for Materials Chemistry and Engineering and International Research Center for Molecular System, Kyushu University, Fukuoka 819-0395, Japan ‡ Department of Chemistry and Bioscience, Kurashiki University of Science and the Arts, Kurashiki, Okayama 712-8505, Japan

bS Supporting Information ABSTRACT: Density functional theory calculations have been performed for a proposal of possible mechanisms of the thermal reaction of an acylsilane, pivaloyltris(trimethylsilyl)silane, and bis(trimethylsilyl)acetylene yielding a silacyclopropene, 1-[(tertbutyl)bis(trimethylsilyl)methyl]-1-trimethylsiloxy-2,3-bis(trimethylsilyl)-1-silacycloprop-2-ene. Two reaction pathways in which two different silyl species play a role were considered based on analogous reactions of acylsilane and alkyne: (i) A silene (SidC) intermediate derived from the acylsilane reacts with the acetylene to yield a silacyclobutene intermediate as a result of a stepwise [2 þ 2] cycloaddition, and a ring-opening reaction of the silacyclobutene triggers the formation of the silacyclopropene. (ii) The silene intermediate is rearranged to a silylene intermediate, and then [2 þ 1] cycloaddition of the acetylene and the silylene gives the silacyclopropene. The high activation energy calculated for the [2 þ 2] cycloaddition indicates that the silene would not react with the acetylene, which is consistent with the experimental result that no silacyclobutene intermediate was observed. On the other hand, the second reaction pathway involving the silene-to-silylene rearrangement and the [2 þ 1] cycloaddition is more realistic from thermodynamic and kinetic points of view. All the calculated results strongly suggest that the silyl species reacting with bis(trimethylsilyl)acetylene is not silene but silylene.

1. INTRODUCTION Unsaturated silicon compounds such as silene (SidC),1
7 disilene (SidSi),4c,8 and silylene (divalent Si)9 are known to be highly reactive and used as reagents and key reaction intermediates in the synthesis of various organosilicon compounds.10 Although a great deal of experimental and theoretical efforts have been devoted to the chemistry of unsaturated silicon compounds, a systematic understanding is still lacking, particularly on mechanistic aspects of their diverse chemical reactivities. Ishikawa and co-workers have been focusing attention on unique chemical behavior of silene intermediates in thermal reactions of acylsilanes with silyl-substituted alkynes.6,7 Reactive silene intermediates generated from the thermolysis of acylsilanes were trapped with various alkynes via [2 þ 2] cycloaddition, leading to the formation of silacyclobutenes (Scheme 1). The generated silacyclobutene derivatives, isolated as stable intermediates in many cases, were prepared for a wide variety of thermal and photochemical reactions, strongly depending on the substituents on the silicon and carbon atoms in the ring (Scheme 2).4f,6c,6e,6f The formation of silacyclobutenes through [2 þ 2] cycloaddition is of particular interest because the thermal [2s þ 2s] cycloaddition is symmetrically forbidden in carbon systems according to the Woodward
Hoffmann rules.11 The thermally allowed [2s þ 2a] process requires a highly distorted transition-state structure, and thus it should be unfavorable. Apeloig and co-workers theoretically investigated a stepwise mechanism on the [2 þ 2] cycloaddition of acetylene and silene.12 Recently Baines and r 2011 American Chemical Society

co-workers exhibited experimental evidence for a biradical intermediate in the addition of cyclopropyl alkynes to silene 3 derived from acylsilane 1.5 Yoshizawa and co-workers computationally investigated the mechanism of the transformation of acylsilane into silene, as well as the [2 þ 2] cycloaddition between acetylene and silene that gives silacyclobutene.7 On the formation of the silacyclobutene, they demonstrated that the bond formation between the Si atom of the silene and the diagonal C atom of the acetylene, which occurs earlier than the formation of the proper Si
C bond in the silacyclobutene, plays an important role in the initial stage of ring closure. Our interest in this study is the reaction of pivaloyltris(trimethylsilyl)silane 1 with bis(trimethylsilyl)acetylene yielding silacyclopropene 2, 1-[(tert-butyl)bis(trimethylsilyl)methyl]1-trimethylsiloxy-2,3-bis(trimethylsilyl)-1-silacycloprop-2-ene, reported by Naka and Ishikawa (Scheme 3).6d Although thermal reactions of acylsilane and alkyne are commonly understood to form silacyclobutene intermediates,2,4e,6c,6e
6g there is no experimental evidence for the presence of silacyclobutene intermediates leading to 2. Naka and Ishikawa also reported that treatment of 2 with 2,3-dimethyl-1,3-butadiene at 200 °C gave bis(trimethylsilyl)acetylene and 1-silacyclopent-3-ene (Scheme 3).6d This experimental result suggests that a silylene intermediate was generated by the thermolysis of 2 and the 1,4-addition of the silylene to the butadiene afforded the silacyclopentene. If a silene Received: March 19, 2011 Published: May 17, 2011 3160

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Scheme 1

Scheme 2 Figure 1. The first reaction mechanism of the formation of 2 based on the proposal of Naka and Ishikawa.6d

Scheme 3

intermediate can be thermally rearranged to a silylene intermediate in the formation of 2, we should consider the possibility that 2 was formed by [2 þ 1] cycloaddition of the acetylene and the silylene. This reaction mechanism is likely to be reasonable because alkynes are widely employed as trapping reagents of silylenes13 and several examples of the silene-to-silylene rearrangement were reported by Barton and Jacobi,14 West and coworkers,15 and Conlin and Wood.16 Photoinduced silene-to-silene rearrangements were also reported by Brook and co-workers.17 In the present study, we have investigated the thermal reaction of acylsilane 1 and bis(trimethylsilyl)acetylene by density functional theory (DFT) calculations for a proposal of possible reaction mechanisms yielding silacyclopropene 2. A question on the active species in the reaction will be answered by comparing the reactivities of silene and silylene intermediates with the acetylene.

2. METHOD OF CALCULATION We searched local minima and saddle points on potential energy surfaces using the hybrid density functional B3LYP method18
20 combined with the 6-31G* basis set.21
23 Vibrational analyses were performed to characterize the obtained stationary point structures.

Figure 2. The second reaction mechanism of the formation of 2 proposed in the present study. Intrinsic reaction coordinate (IRC)24 analyses were performed at the B3LYP/6-31G* level of theory in order to check appropriate connections between a reactant (product) and a transition structure. Bond formations and cleavages in the reaction steps were analyzed with the Mayer-bond-order calculation.25 All the optimized intermediates and transition structures have a total charge of zero, and their electronic ground states are singlet except for intermediate 9, whose ground state is triplet. To determine energy profiles of the examined reaction mechanisms, single-point calculations were carried out at the optimized geometries at the B3LYP/6-311þG** level of theory. The energy profiles were described using Gibbs free energy changes (ΔG) at the reaction temperature (160 °C). All calculations were carried out by using the Gaussian 03 program.26

3. RESULTS AND DISCUSSION We have considered two reaction mechanisms shown in Figures 1 and 2. The first reaction mechanism proposed by Naka and Ishikawa (Figure 1) involves [2 þ 2] cycloaddition of bis(trimethylsilyl)acetylene and silene 3, leading to silacyclobutene 4.6d Silene 3 is formed via the migration of one of the SiMe3 groups to the acyl oxygen atom in acylsilane 1. Our group previously investigated a mechanism of the formation of silene from acylsilane.7a A ring-opening reaction of 4 leading to silabutadiene 5 triggers the formation of silacyclopropene 2. In the second mechanism, described in Figure 2, silene 3 is rearranged to silylene 6, and then [2 þ 1] cycloaddition of the acetylene to 6 gives 2.

[2 þ 2] Cycloaddition of Acetylene and Silene Intermediate. According to the Woodward
Hoffmann rules,11 the

[2s þ 2s] cycloaddition is thermally forbidden in carbon systems such as ethylene and acetylene. Actually, for the [2 þ 2]

3161

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Organometallics cycloaddition of bis(trimethylsilyl)acetylene and silene 3, the concerted [2s þ 2a] cycloaddition requires a highly distorted transition structure, and the reaction would not proceed even at 160 °C due to a high activation energy of 61.1 kcal/mol (see Supporting Information). We thus considered a stepwise mechanism involving the formation of a biradical intermediate (Scheme 4). First, the acetylene and silene 3 form a biradical intermediate, 7. The CdC bond in 7 rotates along the Si
C bond to prepare intermediate 8 for ring closure. Finally silacyclobutene 4 is generated by the C
C bond formation in 8. Figure 3 shows an energy diagram for the addition of silene 3 to bis(trimethylsilyl)acetylene that proceeds in a stepwise manScheme 4

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ner in the open-shell singlet state. In the first step, the formation 0 of the SiA
CA bond via TS7 is calculated to be endergonic by 32.1 kcal/mol, and its activation barrier is 40.8 kcal/mol.0 Mulliken spin densities calculated for 7 are polarized on CB (þ0.83) and CB (
0.82), indicating a biradical nature of 7. As a A A0
C bond, the dihedral result of the rotation about the Si 0 0 CB
CA
SiA
CB angle is varied from 140.5° in 7 to 57.8° in 8. The rotation barrier is estimated to be about 7 kcal/mol in the SCF energy (see Supporting Information). The ring closure of 8 via TS8/4 that leads to silacyclobutene 4 requires an activation energy of 3.9 kcal/mol. For the [2 þ 2] cycloaddition of bis(trimethylsilyl)acetylene and 3, the rate-determining step is the Si
C bond formation, with an activation energy of 40.8 kcal/ mol. If silacyclopropene 2 is formed via silacyclobutene 4, the SiA
CB bond will be cleaved to give silabutadiene 5.7b The activation energy for the SiA
CB bond cleavage in 4 is calculated to be 39.5 kcal/mol, which is comparable to that for the stepwise [2 þ 2] cycloaddition yielding 2 (see Supporting Information). Previously we reported that a silyl-substituted cyclopropene can be formed after the Si
C bond cleavage in a silacyclobutene, and the highest activation energy in this process is 36.5 kcal/mol in the SCF energy.7b As described later, these values are very large relative to the highest activation energy calculated for the other mechanism involving a silene-to-silylene rearrangement (28.5 kcal/mol), and thus we do not discuss here the formation

Figure 3. Energy profile and optimized structures for the stepwise [2 þ 2] cycloaddition of bis(trimethylsilyl)acetylene and silene 3. Interatomic distances and Gibbs free energy changes (ΔG) at 160 °C are presented in Å and kcal/mol, respectively. Mayer bond orders are shown in parentheses. 3162

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Figure 4. Two possible reaction pathways for the rearrangement of silene 3 to silylene 6.

Figure 5. Energy profile and optimized intermediates (transition structures) for the rearrangement of silene 3 to silylene 6 along path A. Interatomic distances and Gibbs free energy changes (ΔG) at 160 °C are presented in Å and kcal/mol, respectively. Mayer bond orders are shown in parentheses.

of silacyclopropene 2 triggered by the cleavage of the SiA
CB bond in 4.

Silene-to-Silylene Rearrangement and [2 þ 1] Cycloaddition. We have considered two possible mechanisms on the 3163

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Figure 6. Energy profile and optimized intermediates (transition structures) for the rearrangement of silene 3 to silylene 6 along path B. Interatomic distances and Gibbs free energy changes (ΔG) at 160 °C are presented in Å and kcal/mol, respectively. Mayer bond orders are shown in parentheses.

rearrangement of silene 3 to silylene 6 as shown in Figure 4. In path A, the migration of the OSiMe3 group to the silene Si atom in 3 leads to carbene intermediate 9. The carbene C atom in 9 accepts a SiMe3 group on the neighboring Si atom to form silene intermediate 10. In path B, on the other hand, a SiMe3 group on the silene Si atom in 3 migrates to the adjacent C atom to form silylene intermediate 11. The OSiMe3 group in 11 moves onto the silylene Si atom to form intermediate 12, having a threemembered-ring structure. The C
O bond in 12 is then cleaved to give silene intermediate 10. In both paths, silylene 6 is formed by the migration of the SiMe3 group on the silene Si atom in 10 to the adjacent C atom. As shown in Figure 2, the [2 þ 1] cycloaddition of bis(trimethylsilyl)acetylene and silylene 6 yields the final product, silacyclopropene 2. Figure 5 shows energy diagrams for the silene-to-silylene rearrangement along path A, together with optimized intermediates and transition structures. We considered the singlet and triplet states for this reaction path because the carbene intermediate 9 adopts a triplet as the ground state (39 in Figure 5). The triplet carbene 39 is 18.4 kcal/mol more stable than the singlet carbene 19, while the other intermediates adopt a singlet as the ground state. On the formation of 9 (3 f TS3/9 f 9), the activation energy calculated for the triplet state is 35.7 kcal/mol, which is much higher than that calculated for the singlet state (27.1 kcal/mol). Thus, the reaction along path A will proceed in the singlet state. On the way to 19 and 39, we did not find any three-membered-ring intermediate corresponding to 12 in path B. The migration of a SiMe3 group onto the carbene C atom in 1 9 gives silene 110 with a low activation barrier (2.5 kcal/mol). Silene 110 is energetically more stable than its isomer 13 by 37.9 kcal/mol, probably due to a very strong Si
O bond in 110. The final step toward silylene 6 from 10 requires an activation

energy of 27.6 kcal/mol, which is the highest in all the reaction steps in path A. The silene-to-silylene rearrangement along path B undergoes four reaction steps, presented in Figure 4. First, one of the SiMe3 groups on the silene Si atom in 3 migrates to the adjacent carbon atom to form silylene 11. The oxygen atom of the OSiMe3 group attacks the silylene Si atom to form 12, having a three-membered Si
C
O ring structure. The C
O bond in 12 is cleaved to form silene 10, which is a structural isomer of 3. Finally the remaining SiMe3 group on the silene Si atom migrates to the adjacent carbon atom to finalize the silene-to-silylene rearrangement. The overall energy profile of the rearrangement is summarized in Figure 6. In the first step, the SiDMe3 group on SiA in 3 migrates to CB to form silylene 11 via TS3/11. Silylene 11 is 28.7 kcal/mol less stable than silene 3, and the migration of the first SiMe3 group requires an activation energy of 31.2 kcal/mol. The electronic ground state of silylene 11 is singlet, and the singlet
triplet energy gap is calculated to be 5.5 kcal/mol. In the second step, the OSiMe3 group migrates to SiA to form silene 10. IRC calculations to characterize the optimized transition structure leading to 10 demonstrated that the OSiMe3 migration occurs via an additional intermediate 12, which is 4.8 kcal/mol lower in energy than 11. A computed short SiA
O distance (1.964 Å) in 12 implies that this intermediate is an intramolecularly basestabilized silylene. Intermediate 11 is 1.3 kcal/mol less stable than TS11/12 after thermal corrections at 160 °C. At the B3LYP/ 6-31G* level of theory employed for optimization, 11 is more stable than TS11/12 by 0.7 kcal/mol in the SCF energy. In the third step, the C
O bond in the three-membered ring of 11 is cleaved to give silene 10 via TS12/10. Silene 10 is 37.9 kcal/mol more stable than its isomer (3) due to the replacement of a Si
Si 3164

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Figure 7. Energy profile and optimized intermediates (transition structure) for [2 þ 1] cycloaddition of bis(trimethylsilyl)acetylene and silylene 6. Interatomic distances and Gibbs free energy changes (ΔG) at 160 °C are presented in Å and kcal/mol, respectively. Mayer bond orders are shown in parentheses.

bond by a stronger Si
O bond. The activation energy for the C
O bond cleavage is very low (4.0 kcal/mol). Silene 10 is finally converted into silyene 6 in the same manner as described for path A. The highest activation energy calculated for path B is 31.2 kcal/mol, and therefore the silene-to-silylene rearrangement is likely to occur along path A, in which a carbene intermediate plays a role. We also examined a dyotropic rearrangement from 3 to 10, which is a simultaneous intramolecular migration of the OSiMe3 group and a SiMe3 group. When the OSiMe3 group on CB in 13 was moved to SiA by partial optimizations (the SiA
O distance as a parameter), a simultaneous migration of the SiCMe3 group was observed. In spite of our thorough search on the potential energy surface, only 1TS3/9 was found instead of a transition state corresponding to the dyotropic rearrangement. Previously Ishikawa and co-workers theoretically demonstrated that a dyotropic rearrangement in a simplified model of 1-silacyclohex-4-ene requires an activation energy of 43.4 kcal/mol.7h,27 While the isomerization of silene 3 into silene 10 should proceed in a stepwise manner, the free energy changes in optimized intermediates and transition states between 3 and 10 seem very subtle in both path A (19 f TS9/10) and path B (11 f TS12/10) when the reaction temperature (160 °C) is considered. Therefore, the isomerization of 3 to 10 at 160 °C would be substantially a dyotropic rearrangement. After the silene-to-silylene rearrangement, silylene 6 reacts with bis(trimethylsilyl)acetylene to yield the final product, silacyclopropene 2 (Figure 7). The silylene approaches the acetylene in a similar manner to that predicted for addition of carbene to acetylene, which is a non-least-motion approach.28 To maximize bonding interactions between the frontier orbitals of the silylene and the acetylene, the molecular plane of the silylene formed by SiA and CB and O is not perpendicular to the CtC bond of the acetylene during the approach; the CA
SiA
CB angles in 6
acetylene and TS2 are 131.4° and 126.3°, respectively. Figure 8 depicts orbital interactions between the silylene and the acetylene in the HOMO and LUMO of TS2. In the HOMO of TS2 the nonbonding orbital of the silylene interacts with the π* orbital of the acetylene, while in the LUMO the

Figure 8. Orbital interactions between bis(trimethylsilyl)acetylene and silylene 6 in the HOMO and LUMO of TS2. Hydrogen atoms are omitted for clarity.

vacant 3p orbital of the silylene Si atom perpendicular to the SiA
CB
O plane interacts with the π* orbital of the acetylene.0 The Si
C distances (b.o.) in TS0 2 are 2.184 Å (0.49) for Si
CA and 2.689 Å (0.39) for Si
CB , respectively, indicating a concerted formation of the Si
C
C ring structure. The formation of silacyclopropene 2 is an exergonic reaction (ΔG =
6.4 kcal/ mol), and its activation energy is 28.5 kcal/mol. After all, the [2 þ 1] cycloaddition is the rate-determining step in the formation of 2 via the silene-to-silylene arrangement, and the calculated activation energy (28.5 kcal/mol) is much smaller than that for a [2 þ 2] cycloaddition of bis(trimethylsilyl)acetylene and silene 3 (40.8 kcal/mol). These calculated results reasonably explain the experimental fact that no silacyclobutene intermediate was observed at 160 °C.6d Moreover, the ringopening reaction of 2 to liberate silylene 6 requires an activation energy of 34.9 kcal/mol, which is higher than that for the [2 þ 1] cycloaddition of bis(trimethylsilyl)acetylene and 6. This also supports that a silylene adduct derived from 2 was prepared at 200 °C (see Scheme 3).6d All of the calculated results strongly suggest that the silyl species reacting with the acetylene is not 3165

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Organometallics silene but silylene, and the reaction at 160 °C undergoes sileneto-silylene rearrangement, followed by the [2 þ 1] cycloaddition of bis(trimethylsilyl)acetylene and silylene 6.

4. CONCLUSIONS We have performed DFT calculations to propose a plausible mechanism of the thermal reaction of acylsilane 1 and bis(trimethylsilyl)acetylene yielding silacyclopropene 2, which was reported by Naka and Ishikawa.6d With an assumption of silene 3 as the initial intermediate, we discussed two reaction mechanisms, in which two different silyl species play a role: (1) Silacyclobutene 4 is generated by the [2 þ 2] cycloaddition of the acetylene and silene 3 in a stepwise manner. A ring-opening reaction of 4 initiates the formation of 2. Silacyclobutene is commonly observed as a major intermediate in thermal reactions of acylsilane and alkyne.2,4f,6c,6e
6g (2) Silene 3 is first rearranged to silylene 6, and then silacyclopropene 2 is formed by the [2 þ 1] cycloaddition of the acetylene and 6. The formation of silacyclobutene 4 requires 40.8 kcal/mol of activation energy and is slightly endergonic (ΔG = þ4.8 kcal/mol) at 160 °C. Moreover, the activation energy for the ring-opening reaction of 4 is calculated to be 39.5 kcal/mol. On the other hand, the [2 þ 1] cycloaddition via the silene-to-silylene rearrangement proceeds in an exergonic way, and the highest activation energy in this pathway is 28.5 kcal/mol. In the silene-to-silylene rearrangement, the isomerization of silene 3 to silene 10 proceeds in a stepwise manner, but it would be virtually a dyotropic rearrangement when the reaction temperature is considered. We conclude that the silyl species reacting with bis(trimethylsilyl)acetylene is silylene and a silene intermediate would not play a key role in the formation of 2 at 160 °C. Silacyclopropene 2 is formed by the [2 þ 1] cycloaddition of the acetylene and silylene 6. The calculated results are fully consistent with the experimental fact that no silacyclobutene intermediate was observed in the formation of 2. ’ ASSOCIATED CONTENT

bS

Supporting Information. Details on energetics of the concerted reaction mechanism of the formation of silacyclobutene 4, the rotation barrier between 7 and 8, an energy profile of the formation of 4 calculated by using a multireference theory, Cartesian coordinates for all optimized intermediates and transition structures, and the complete author list of ref 26. This material is available free of charge via the Internet at http://pubs. acs.org.

’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected].

’ ACKNOWLEDGMENT We thank Grants-in-Aid for Scientific Research (Nos. 18GS0207, 21750063, and 2245028) from Japan Society for the Promotion of Science (JSPS) and the Ministry of Education, Culture, Sports, Science and Technology of Japan (MEXT), the Nanotechnology Support Project of MEXT, the MEXT Project of Integrated Research on Chemical Synthesis, and the Kyushu University Global COE Project for their support of this work.

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dx.doi.org/10.1021/om2002393 |Organometallics 2011, 30, 3160–3167