Novel [2 + 1] Concerted Reaction Path for Disilacyclobutenes with

Jan 28, 2014 - Novel [2 + 1] Concerted Reaction Path for Disilacyclobutenes with Acetylene ... Institute for Innovative Science and Technology, Nagasa...
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Novel [2 + 1] Concerted Reaction Path for Disilacyclobutenes with Acetylene Yoshihiro Hayashi,† Takafumi Natsumeda,† Shun Otsu,† Ryo Yamada,† Akinobu Naka,‡ Mitsuo Ishikawa,‡ Tokio Yamabe,§ and Susumu Kawauchi*,† †

Department of Organic and Polymeric Materials, Tokyo Institute of Technology, 2-12-1 O-okayama, Meguro-ku, Tokyo 152-8552, Japan ‡ Department of Life Science, Kurashiki University of Science and the Arts, 2640 Nishinoura, Tsurajima-cho, Kurashiki, Okayama 712-8505, Japan § Institute for Innovative Science and Technology, Nagasaki Institute of Applied Science, 3-1 Shikumachi, Nagasaki 851-0121, Japan S Supporting Information *

ABSTRACT: Thermal reactions of benzodisilacyclobutene (1) and disilacyclobutene (2) with acetylene were investigated theoretically. The reactions are thought to proceed via the conventional Diels− Alder reaction of disilabutadiene, the conrotatory ring-opening product of disilacyclobutene, with acetylene. However, this mechanism is incompatible with the observed similar reactivities of 1 and 2 with acetylene and the retention of stereochemistry during the reaction. In our previous paper, we proposed an alternative [2 + 1] cycloaddition pathway that involved the direct addition of acetylene to the Si−Si σ−bond of 1 without ring opening. In this study, we extensively investigated the reaction pathways for both 1 and 2 on a theoretical basis. We found that charge transfer (CT) played a key role in the [2 + 1] cycloaddition pathway. On the basis of natural bond orbital (NBO) analysis, the interaction of the Si−Si σ-bond orbital (donor) with the π* orbital (acceptor) of the acetylene was attributed mainly to the CT process. Finally, an experiment to verify the [2 + 1] cycloaddition mechanism was proposed, in which the use of triacetylene as a terminal alkyne would allow the key intermediate in the pathway to be trapped.

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INTRODUCTION This paper describes a theoretical mechanistic investigation of the reactions of benzodisilacyclobutenes (1) and disilacyclobutenes (2) with acetylene, which lead to benzodisilacyclohexadienes (3) and disilacyclohexadienes (4), respectively. Recently, we proposed a new [2 + 1] addition mechanism for the reaction,1 although a two-step mechanism is generally recognized for the reactions of 1 and 2 with terminal alkynes.2 In the two-step process, conrotatory ring openings of 1 and 2 would lead to o-quinodisilane (1′) and 1,4-disilabutadiene (2′) intermediates, respectively; these would undergo [4 + 2] Diels−Alder reactions with terminal alkynes to produce 3 and 4 (Scheme 1). However, the two-step mechanism does not account for the experimental observation in which the reactivity of 1 with terminal alkynes was similar to that of 2,3−6 although the calculated activation energy difference between 1 and 2 was significantly large (over 17 kcal mol−1).2 In addition, the calculated activation energy for 1 in the two-step mechanism seemed too high (over 64 kcal mol−1) for the reaction to proceed thermally. Recently, we investigated experimentally the stereochemistry for the reactions of benzodisilacyclobutenes with terminal alkynes.1 cis-3,4-Benzo-1,2-diisopropyl-1,2-dimethyl-1,2-disilacyclobut-3-ene reacted with terminal alkynes © 2014 American Chemical Society

Scheme 1

stereospecifically to give cis-5,6-benzo-1,4-diisopropyl-1,4-dimethyl-1,4-disilacyclohexa-2,5-diene derivatives. Similarly, the reaction of the trans isomer led to the trans product. These experimental results were also in conflict with the two-step mechanism because of the chirality inversion at Si (Scheme 2). Therefore, we performed extensive theoretical studies to search for alternative reaction pathways. As a result, we found a novel [2 + 1] cycloaddition pathway in which terminal alkynes added directly to the Si−Si bond in 1 without ring opening.1 The Received: November 27, 2013 Published: January 28, 2014 763

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functional which includes empirical atom−atom dispersion corrections24 and many optimized parameters. This exchange functional percentage attains 22.2036% HF exchange at short range and 100% HF exchange at long range. These two LC functionals, CAM-B3LYP and ωB97X-D, show improvement in various benchmarks such as the polarizability of long chains,32,33 excitations using TD-DFT,34−37 and reaction barrier heights.38−43 In particular, the ωB97X-D functional gives results close to those of CCSD(T)44 for the reactions of strained alkynes with alkenes.45 In this study, single-point calculations at the CCSD(T) level of theory were performed using ωB97X-D-optimized structures as highly precise reference energies, denoted as CCSD(T)// ωB97X-D. Total electronic energies were corrected for the zero-point vibrational energy. The 6-311G(d,p) basis set46,47 was used throughout the calculations. The optimized molecular structures were verified by vibrational analysis; equilibrium structures did not have imaginary frequencies, and transition state structures had only one imaginary frequency corresponding to the reaction coordinate. Additionally, the stability of the wave function was tested. If the wave function of a closed-shell singlet had instability, the molecular structure was reoptimized at the open-shell singlet state with broken-symmetry initial guesses.48 NBO9,10 and AIM analyses11,12 were carried out to study the bonding properties. The IRC7,8 was followed from the transition state toward both reactants and products. All calculations were carried out using the Gaussian 09 program.49

Scheme 2

calculated activation energy was 33 kcal mol−1, which was sufficiently low for a thermal reaction. In addition, the reaction proceeded with retention of stereochemistry. Therefore, the [2 + 1] cycloaddition pathway alone can exclusively explain the thermal reactions of disilacyclobutenes with terminal alkynes. In this paper, we investigated the [2 + 1] cycloaddition pathway in detail using the intrinsic reaction coordinate (IRC),7,8 natural bond orbital (NBO),9,10 and atoms in molecules (AIM) computational methods.11,12 Additionally, we proposed an experiment which would verify the pathway. Finally, we discussed an analogy between the reactivities of Si−Si σ bonds and C−C π bonds.

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Chart 1

RESULTS AND DISCUSSION We carried out extensive theoretical studies to explore reaction pathways distinct from the conventional Diels−Alder reaction. We found two alternative reaction pathways: the same [2 + 1] cycloaddition pathway described in our previous paper and a distinct biradical pathway. In the [2 + 1] cycloaddition pathway, one acetylenic carbon atom adds directly to the Si−Si bond of the starting compounds (5 and 6) without ring opening. In the second pathway, a biradical intermediate is formed by the SN2like reaction of an acetylenic carbon atom with a silicon atom in the Si−Si bond of the starting compounds. Energy Profile for the [2 + 1] Cycloaddition Pathway. Figure 1 displays the energy profiles of the [2 + 1] cycloaddition pathways for the reactions of 5 and 6 with acetylene at the CCSD(T)//ωB97X-D level of theory. Figure 2

We employed three functionals commonly used in density functional theory (DFT) calculations: B3LYP, CAM-B3LYP, and ωB97X-D. Becke’s three-parameter hybrid functional, B3LYP, is the most popular DFT functional and incorporates 20% Hartree−Fock (HF) exchange plus 80% Becke 1988 (B88) exchange.13−15 However, B3LYP is unsuccessful in several crucial applications: the polarizability of long chains,16 excitations using time-dependent DFT (TDDFT),17−21 and van der Waals interactions.22−24 The most important issue relevant to this paper is the underestimation of reaction barrier heights.25−28 Hirao, Tsuneda, and co-workers pointed out the significance of the long-range correction (LC) to exchange functionals for improving the many problems with DFT and developed the LC scheme on the basis of an error function.29 CAM-B3LYP, developed by Handy and co-workers,30 is an LC version of B3LYP, which switches from 19% HF and 81% B88 exchanges at short range to 65% HF and 35% B88 at long range. The medium-range smooth switching function in this case was also based on a standard error function. ωB97X-D was developed by Chai and Head-Gordon,31 with an LC

Figure 1. Energy profiles of the [2 + 1] cycloaddition pathway for 5 (top) and 6 (bottom) with acetylene in kcal mol−1 at the CCSD(T)/ 6-311G(d,p)//ωB97X-D/6-311G(d,p) level of theory.

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COMPUTATIONAL METHODS

To explore the reaction pathways for the thermal reaction of a disilacyclobutene with an alkyne, the reactions of 1,1,2,2-tetramethyl1,2-disilacyclobut-3-ene (5) and 3,4-benzo-1,1,2,2-tetramethyl-1,2disilacyclobut-3-ene (6) (see Chart 1) with acetylene were considered.

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(0.027e), and the divalent carbon atom of the acetylene moiety had singlet carbene character. For the 1,4-disilacyclohexa-2,5dienes 5-IM2 and 6-IM2, both energies were nearly the same (−34 kcal mol−1 ) and the acetylene moieties in the intermediates had trans double bonds. The two carbon atoms of the trans double bond in 6-IM2 were pyramidalized, with pyramidalization angles of 32°, where the pyramidalization angle is the angle between the plane containing one of the double-bonded carbons and the two substituents attached to it and the extension of the double bond.53 Finally, IM2 led to the final products (P1) through the transition state (TS3) in which trans−cis isomerization occurred. The activation energies of the rate-determining stepthose of 5-TS1 and 6-TS1were similar (33 kcal mol−1). These energies were sufficiently low in comparison with the conventional two-step mechanism involving ring opening and Diels−Alder reactions and conformed to the experimental results in which 1 and 2 showed similar reactivities. The stereochemistry of benzodisilacyclobutenes having chiral silicon atoms is retained before and after reaction with alkynes, as reported by Ishikawa and coworkers.1 This was consistent with the stereospecificity of the [2 + 1] cycloaddition pathway, whereas the stereochemistry would not be maintained if the reaction progressed via conrotatory ring opening. Hence, we will discuss TS1 in the [2 + 1] cycloaddition pathway, which is a convincing mechanism, in the next section. We will focus mainly on 6, because 5 and 6 showed approximately the same results along the calculated reaction pathway. In addition, because ωB97X-D yielded energy differences fairly close to CCSD(T) (see Table S1 of the Supporting Information), we hereafter will use ωB97X-D. Structure and Electronic Properties of the Transition State (TS1) for the [2 + 1] Cycloaddition Pathway. The optimized structure of the transition state 6-TS1 is shown in Figure 2. The Cs symmetry for the moiety of 6 in 6-TS1 was maintained, and the Si−Si bond was partially broken (2.547 Å in comparison to 2.348 Å in 6). The C−Si and C−C bond lengths were practically unchanged in comparison to 6. For the acetylene moiety of 6-TS1, the C−C bond length (1.281 Å) was longer than that in acetylene itself (1.196 Å), but the C−H bonds (1.082 Å) were nearly unchanged. Note that the acetylene moiety of 6-TS1 has a trans-bent structure (with a C−C−H angle of 129.3°) perpendicular to the benzodisilacyclobutene plane, with a highly negative natural charge (−0.474e). Such a TS for the Si−Si bond is unprecedented, to our knowledge. Charge transfer (CT) was additionally confirmed by NBO analysis. The NBO interaction energy between the Si−Si σ-bond orbital (donor) and the π* orbital (acceptor) for acetylene was significantly large, 175.49 kcal mol−1, as shown in Figure 3a,b. The electron occupancy for the Si−Si σ-bond orbital was 1.345, and that for the acetylene π* orbital was 0.512. On the basis of these results, the formation of the Si−Cacetylene bond was mainly attributed to CT from the Si− Si σ-bond orbital to the acetylene π* orbital. It is noteworthy that the [2 + 1] cycloaddition reaction can be interpreted by the Fukui frontier orbital theory.54 The Si−Si σ-bond orbital (HOMO) interacts with the acetylene π* antibonding orbital (LUMO), as illustrated in Figure 3c. Additionally, the Si−Si σbond orbital of 6 protrudes significantly, as displayed in Figure 3d, which may be caused by ring strain. The protrusion of the Si−Si σ-bond orbital would facilitate orbital interactions with the acetylene.

Figure 2. Optimized structures of the reactant (6), transition states (6TS1, 6-TS2, and 6-TS3), intermediate (6-IM2), and product (6−P1) of the [2 + 1] cycloaddition pathway for 6 with acetylene at the ωB97X-D/6-311G(d,p) level of theory. Selected bond lengths are given in Å. The blue arrows in transition states correspond to the reaction coordinate.

shows optimized structures for the reactant (6), transition states, intermediates, and product resulting from the reaction of 6 with acetylene at the ωB97X-D level of theory. In transition state TS1, an acetylene moiety aligned perpendicularly to the Si−Si bond of the starting compound and one carbon atom of the acetylene moiety and the Si−Si bond formed a threemembered ring with Cs symmetry. In addition, the acetylene moiety adopted a trans-bent structure.50 TS1 is the ratedetermining step (33 kcal mol−1) throughout the [2 + 1] cycloaddition pathway. The successive intermediates (IM1) of the five-membered ring with Cs symmetry are not true equilibrium structures because they have one imaginary frequency (65i cm−1).51 The imaginary frequency corresponds to the nuclear motion leading to the six-membered-ring structures (TS2). For this reason, either IM1 will not exist between TS1 and TS2 or, if it exists, the structure and energy of IM1 should be very close to that of TS2. The transition state TS2 had slight singlet biradical character (⟨S2⟩ = 0.200).52 The electron spin density was mainly localized on the Si atoms 765

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started to change. The Si−Si bond length was 2.374 Å, which was slightly longer than in the initial structure (2.348 Å), and the H−C−C angle of the acetylene moiety was 176.1°. The AIM analysis showed T-shaped bond paths between the Si−Si bond and the acetylene, and the electron densities on the critical points of the Si−Si bond and between the Si−Si bond and the acetylene were 0.091 and 0.022 e, respectively. Therefore, at the initial stage of the [2 + 1] cycloaddition pathway, the acetylene interacted directly with the Si−Si σ bond. At s = −2.50 amu1/2 bohr, the CT progressed rapidly, and the bond paths still showed T shapes between the Si−Si σ bond and the acetylene. At s = −1.25 amu1/2 bohr, dramatic changes appeared: the charge on the acetylene moiety was −0.245e. The critical point of the T-shaped bond paths was split immediately, and the electron densities on the critical points of the Si−Si and Si−Cacetylene bonds were 0.073e and 0.047e, respectively. The bond path of Si−Cacetylene was greatly distorted toward the inside of the three-membered Si−Si− Cacetylene ring. At s = 0 amu1/2 bohr, i.e., the transition state, half of the charge was transferred. The charge on the acetylene moiety was −0.474e. The bond paths between the acetylene and the two silicon atoms clearly formed a three-membered ring. The electron densities on the critical points of the Si−Si and Si−Cacetylene bonds became approximately the same, 0.063e and 0.060e, respectively. At s = +1.25 amu1/2 bohr, the critical point of the Si−Si bond disappeared. At s = +2.50 amu1/2 bohr, the change in the charge on the acetylene moiety slowed and the structural change of the acetylenic group was nearly complete. Throughout the reaction, the acetylenic structural change occurred earlier than CT. At around s = +5.00 amu1/2 bohr, the molecular system had singlet biradical character. The reaction path maintained Cs symmetry from the reactant to TS2 through TS1. However, IM2 has C2 symmetry. Thus, a valley−ridge inflection point existed between TS1 and TS2. This problem is known as the instability of the IRC.55−57 We performed a projected frequency analysis along the IRC. The out-of-plane vibration mode for the Cs symmetry plane had an imaginary frequency around s = +8.0 amu1/2 bohr. Hence, this reaction path proceeds from TS1 to IM2 through a valley− ridge inflection point close to TS2. Proposal of an Experimental Verification of the [2 + 1] Cycloaddition Pathway. To provide direct proof for the [2 + 1] cycloaddition pathway, we propose an experiment to inspect the pathway. Generally, the trapping of a typical intermediate may be used to verify reaction mechanisms. For the [2 + 1] cycloaddition pathway, a good trapping candidate would be the five-membered-ring intermediate IM1. However, the instability of IM1 requires that we consider approaches that would stabilize the five-membered-ring structure. One possibility would use steric hindrance to increase the energy barrier from IM1 to IM2. Unfortunately, steric hindrance has only a small effect on increasing the reaction barrier height. Then, we focused on the cause of the instability in the five-memberedring structure, which was the conjugation between the Si− Cacetylene σ-bond orbital and the vacant p-type orbital on the carbene carbon. By NBO analysis, the interaction energy of the σ-bond orbital (donor) of the Si−Cacetylene with a vacant p-type orbital (acceptor) of the carbene carbon atom was 23.43 kcal mol−1 (see Figure S1 of the Supporting Information). Therefore, the C−C bond in the added acetylene group would be strengthened by electron acceptors, and the Si− Cacetylene σ bond would be weakened by electron donors. We assumed that further electron donation to the vacant p-type

Figure 3. (a) Si−Si σ-bond NBO for 6-TS1, (b) acetylene π* NBO for 6-TS1, (c) [2 + 1] orbital interaction between the Si−Si σ-bonding orbital (HOMO) of 6 and the π* antibonding orbital (LUMO) of acetylene, and (d) Si−Si σ-bond NBO for 6.

IRC for the [2 + 1] Cycloaddition Pathway. Figure 4 illustrates the changes in the natural charge on the acetylene

Figure 4. IRC for the [2 + 1] cycloaddition pathway via 6-TS1 at the ωB97X-D/6-311G(d,p) level of theory: (top) relative energy profile and the electron charge on acetylene; (middle) snapshots of AIM analysis (red points are bond critical points, yellow points are ring critical points, and lines are bond paths); (bottom) selected geometrical parameters.

moiety, the relative energies, the AIM analysis, and the geometrical parameters along the IRC. CT progressed smoothly as the reaction proceeded. Early along the reaction coordinate (s = −5.00 amu1/2 bohr), CT had already begun. The charge on the acetylene moiety was −0.009e, and the Si− Cacetylene distance was 2.882 Å. Simultaneously, the structure 766

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orbital could stabilize IM1. We initially calculated the reaction of 6 with phenylacetylene (7), because the π system of the acetylenic phenyl substituent could conjugate with the vacant ptype orbital of the carbene carbon atom. Nevertheless, 7-IM1 was not stabilized, because the π system of the phenyl substituent was orthogonal to the vacant p-type orbital of the carbene carbon atom, as shown in Figure 5. Therefore, we

Figure 6. Energy profiles of the [2 + 1] cycloaddition pathway for 6 with diacetylene (top) and triacetylene (bottom) in kcal mol−1 at the ωB97X-D/6-311G(d,p) level of theory.

Biradical Pathway. Next, we investigated the biradical pathway for the reaction of disilacyclobutenes with acetylene. Figure 7 illustrates energy profiles for the biradical pathways for 5 and 6 with acetylene at the CCSD(T)//ωB97X-D level of theory. Figure 8 shows the optimized structures of the TS (6TS4) and intermediate (6-IM3) of the biradical pathway for 6 with acetylene at the ωB97X-D level of theory. At transition state TS4, acetylene initially attacked one Si atom of the Si−Si bond in the starting compounds from the back side. TS4 was similar to an SN2-type transition state. The Si−Si bond was broken and the Si−Cacetylene bond was formed simultaneously, with stereochemical inversion of the attacked Si atom. TS4 is the rate-determining step (48 kcal mol−1). IM3 was a biradical intermediate in which the spin density was mainly localized on the Si atom that was not attacked and the linked acetylene fragment. The relative energies for 5 and 6 were 18 and 30 kcal mol−1, respectively. TS5 and TS6 then underwent ring-closing reactions that led to IM2 and TS2, respectively. TS5 and TS6 corresponded to rotations around the C−Si bond with barriers of less than 5 kcal mol−1. In this reaction pathway, the activation energies of both 5 and 6 were 48 kcal mol−1 at the CCSD(T)//ωB97X-D level of theory. The activation energy of the biradical pathway was 15 kcal mol−1 higher than that of the [2 + 1] cycloaddition pathway. In addition, the stereochemistries of the starting compounds having chiral silicon atoms were inverted during the reaction with alkynes. Consequently, the biradical pathway was denied for the thermal reaction of disilacyclobutenes with alkynes. Reactivity Analogy between Si−Si and CC Bonds. In some cases, Si−Si σ bonds exhibit reactivities analogous to those of C−C π bonds.58−61 As a simple example, Si−Si σ-bond cleavage by halogens corresponds to the addition reactions of alkenes with halogens. Similarly, the recombination reaction of two Si−Si σ bonds in the presence of a catalyst corresponds to olefin metathesis (Scheme 3). Here, we briefly discuss analogies between the mechanism of the reaction of disilacyclobutenes

Figure 5. Optimized structures of the transition state (7-TS1) and five-membered-ring intermediates (8-IM1 and 9-IM1) of the [2 + 1] cycloaddition pathway for 6 with phenylacetylene, diacetylene, and triacetylene at the ωB97X-D/6-311G(d,p) level of theory. Selected bond lengths are given in Å. The blue arrows in 7-TS1 correspond to the reaction coordinate.

calculated the reaction of 6 with diacetylene (8) and triacetylene (9) having sp rather than sp2 carbons. As a result, the five-membered-ring intermediates (8-IM1 and 9−IM1) were stabilized successfully. Similarly, stabilization of IM1 may also be possible by halogen or amine substitution, but we discuss only di- and triacetylenes in this study. Figure 5 illustrates the optimized structures of the five-membered-ring intermediates (8-IM1 and 9-IM1) along the [2 + 1] cycloaddition pathway for 6 with 8 and 9. Figure 6 displays the energy profiles of the pathways. The relative energies of 8IM1 and 9-IM1 were −14.71 and −26.39 kcal mol−1, respectively. The energy barrier between 8-IM1 and 8-IM2 was 3.32 kcal mol−1, and that between 9-IM1 and 9-IM2 was 11.24 kcal mol−1. Additionally, the activation energies for 8TS1 and 9-TS1 were 23.72 and 19.95 kcal mol−1, respectively, which were lower than that of 6-IM1 (see Figure 1). In the case of the reaction of 6 with triacetylene 9, 9-TS1 is still the ratedetermining step. However, 9-IM1 may exist with a short lifetime at low temperature. Thus, trapping the five-memberedring IM1 derivatives may be possible by further reaction of the carbene carbon atom with some appropriate reactant. Therefore, the use of triacetylene may potentially provide experimental verification of the [2 + 1] cycloaddition pathway. 767

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Scheme 3

Scheme 4

Figure 7. Energy profiles for the biradical pathway for the reactions of 5 (top) and 6 (bottom) with acetylene in kcal mol−1 at the CCSD(T)/ 6-311G(d,p)//ωB97X-D/6-311G(d,p) level of theory.

Scheme 5

alkyne reaction and that for strained alkynes with alkenes seem analogous in terms of the formation of the three-memberedring TSs and the carbene intermediates. Additionally, CT also occurs for the [2 + 1] cycloaddition pathway in the reactions of cyclopentyne and benzyne with ethylene.45 Therefore, there also exists an analogy between the reactivity of the Si−Si σ bond and that of the C−C π bond. However, it should be noted that the Si−Si bond in the disilacyclobutene is strained, whereas the CC bond is strained in the cyclic alkynes. Furthermore, their reactivities are different. For the reactions of strained alkynes with alkenes, the calculated activation energy of the [2 + 1] cycloaddition pathway is almost the same as that of the biradical pathway, as reported by Kinal and Piecuch.64 Therefore, the question of these reaction mechanisms is still open. In contrast, for the reaction of disilacyclobutenes with terminal alkynes, the biradical pathway is clearly denied, because it cannot explain the retention of stereochemistry. Moreover, the activation energy is higher than that of the [2 + 1] cycloaddition pathway.

Figure 8. Optimized structures of the transition state (6-TS4) and intermediate (6-IM3) of the biradical pathway for the reaction of 6 with acetylene at the ωB97X-D/6-311G(d,p) level of theory. Selected bond lengths are given in Å. The blue arrows in 6-TS4 correspond to the reaction coordinate.

with alkynes and that of alkenes with alkynes. Although alkenes do not usually react thermally with alkynes, the thermal [2 + 2] cycloaddition reactions of strained alkynes such as cyclopentyne and benzyne with alkenes are known.62,63 The reactions of strained alkynes with alkenes are proposed to proceed via a [2 + 1] cycloaddition pathway and/or a biradical intermediate pathway (Scheme 4).64 Scheme 5 displays the [2 + 1] cycloaddition and biradical pathways for the reactions of disilacyclobutenes with terminal alkynes, for comparison with the reactions of alkenes with strained alkynes. There are several points of similarity for the two processes. The [2 + 1] cycloaddition pathways for the disilacyclobutene and terminal 768

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IM2 underwent trans−cis isomerization (TS3) to give the final product (P1). TS1 is the rate-determining step. The five-membered-ring structure (IM1) was not stable. However, for the reaction using triacetylene instead of acetylene, IM1 became an equilibrium point from the TS, and the energy barrier height from IM1 to IM2 was estimated to be 11.24 kcal mol−1. This system may provide a possible opportunity to trap the five-membered-ring derivatives experimentally. We also investigated the biradical pathway. This pathway was initiated with an SN2-like reaction in which the acetylene directly added to a silicon atom of the ring-closed structure. Then, a biradical intermediate was generated, giving IM2 by a ring-closure reaction. The activation energy of the biradical pathway was higher than that of the [2 + 1] cycloaddition path; therefore, the reaction with acetylene would not proceed via the biradical pathway but, rather, the [2 + 1] cycloaddition pathway.

Hence, by analogy with C−C π-bond chemistry, we explored the reaction in which acetylene was added to the CC bond of disilacyclobutenes (5), as shown in Figure 9. For this reaction,

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ASSOCIATED CONTENT

S Supporting Information *

A table giving relative energies for all optimized structures of the [2 + 1] cycloaddition pathway at the CCSD(T)//ωB97XD, ωB97X-D, B3LYP, and CAM-B3LYP level of theory, a figure detailing the orbital interaction of the σ-bond orbital (donor) of the Si−Cacetylene bond with a vacant p-type orbital (acceptor) of the carbene carbon atom for 6-TS2, tables giving Cartesian coordinates for all optimized structures at the ωB97X-D level of theory, and XYZ files for the calculated structures. This material is available free of charge via the Internet at http://pubs.acs.org.

Figure 9. Reaction scheme for acetylene addition to the CC bond of disilacyclobutene (5). Relative energies are given in kcal mol−1 at the CCSD(T)/6-311G(d,p)//ωB97X-D/6-311G(d,p) level of theory.

we also found two reaction courses: the [2 + 1] cycloaddition and biradical pathways. The [2 + 1] cycloaddition pathway gives the final product (P2) via the [2 + 1] cycloaddition TS (TS7), the three-membered-ring intermediate (IM4), and the four-membered-ring TS (TS8). The biradical pathway gives the final product (P2) via the radical addition TS (TS9), the biradical intermediate (IM5), and the ring-closure TS (TS8). The activation energies for the acetylene addition TSs (TS7 and TS9) were 46.24 and 40.59 kcal mol−1, respectively, which were lower than or the same as that for the ring-opening reaction of 5, as reported by Yoshizawa et al. (46.7 kcal mol−1).2 The activation energy for the ring closure and fourmembered-ring TS (TS8) was 54.88 kcal mol−1, which was relatively high because the acetylene moiety was greatly strained. TS8 is the rate-determining step for both pathways, and the activation energy of TS8 was higher than that of the [2 + 1] cycloaddition pathway for acetylene addition to the Si−Si bond (32.95 kcal mol−1, see Figure 2). In addition, the final product (P2) was not detected experimentally. Therefore, acetylene addition to the CC bond of 5 could be neglected.

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AUTHOR INFORMATION

Corresponding Author

*E-mail for S.K.: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The numerical calculations were carried out on the TSUBAME2.5 supercomputer at the Tokyo Institute of Technology, Tokyo, Japan, and on the supercomputer at the Research Center for Computational Science, Okazaki, Japan.

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REFERENCES

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CONCLUSION In this study, we performed extensive theoretical calculations to explore the mechanistic pathways for the thermal reactions of disilacyclobutenes and acetylene. We found two new reaction pathways: the [2 + 1] cycloaddition pathway and the biradical pathway. Among these, we focused on the novel [2 + 1] cycloaddition, because this was the first and only pathway that could explain the experimental results without contradiction. The [2 + 1] cycloaddition pathway was attributed to CT, in which an electron was transferred mainly from the Si−Si σbond orbital to the acetylene π* orbital. In this CT step (TS1), the acetylene initially interacted directly with the Si−Si σ bond to form a three-membered ring. Next, the Si−Si bond was cleaved to form a five-membered ring (TS2 or near the valley− ridge inflection point). Then, the structure smoothly changed toward the six-membered-ring intermediate (IM2). Finally, 769

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