Substituent Effects in Thermal Reactions of a Silene with Silyl

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Substituent Effects in Thermal Reactions of a Silene with Silyl-Substituted Alkynes: A Theoretical Study Hiromasa Tanaka,† Yoshihito Shiota,† Kazunori Hori,† 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 Life Science, Kurashiki University of Science and the Arts, Kurashiki, Okayama 712-8505, Japan S Supporting Information *

ABSTRACT: Thermal reactions of a silene derivative (Me3Si)2SiC(OSiMe3)(t-Bu) (1) with silyl-substituted acetylenes, bis(trimethylsilyl)butadiyne, tert-butyldimethylsilylacetylene, and bis(trimethylsilyl)acetylene have been investigated with density functional theory calculations for the understanding of substituent effects in the reactivity of the silene. The first critical reaction step for determining the final product is the formation of a biradical intermediate from 1 and the acetylenes. A stepwise [2 + 2] cycloaddition via the biradical intermediate gives a silacyclobutene derivative, which is the precursor of the final product. The activation energy for this reaction step as well as thermodynamic stability of the biradical intermediate is governed by an interplay between geometric and electronic factors. The biradical intermediate is destabilized by steric hindrance between bulky substituents on 1 and acetylene, while it can be stabilized by delocalization of an unpaired electron in the acetylene moiety. Silacyclobutene derivatives undergo Si−C bond cleavage to give silabutadiene derivatives. The second critical reaction step is the attack of the OSiMe3 group on the silene Si atom in the silabutadienes. If the substituent on one of the acetylene C atoms does not easily migrate, the SiMe3 group of the OSiMe3 group rebounds to the silene C atom to form an oxasilacyclopentene derivative. If this substituent migrates easily, on the other hand, it migrates to the silene C atom with a simultaneous migration of the OSiMe3 group, resulting in the formation of an allene derivative.

1. INTRODUCTION Acylpolysilanes are useful precursors for the synthesis of silenes. In 1979, Brook and co-workers reported the photolysis and thermolysis of pivaloyltris(trimethylsilyl)silane to afford a silene with a moderate lifetime and confirmed the presence of the silene by trapping experiments (Scheme 1).1 Nowadays it is well known that photochemical and thermal reactions as well as Peterson-type reactions of the acylpolysilanes offer a convenient route for the preparation of silenes.1−6 Silenes having a highly reactive SiC bond are utilized as key reaction intermediates for the synthesis of various organosilicon compounds.2−7 On the other hand, the silenes have also been used as silicon-based reagents for organic synthesis.8 Ishikawa, Naka, and co-workers have been investigating the unique reactivity of silenes in the co-thermolysis of acylpolysilanes and silyl-substituted alkynes.3,4 Silenes derived from © XXXX American Chemical Society

Scheme 1

Received: April 17, 2012

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dx.doi.org/10.1021/om300310g | Organometallics XXXX, XXX, XXX−XXX

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Scheme 2

Scheme 3

Scheme 4

the acylpolysilanes generally react with alkynes to afford [2 + 2] cycloadducts, 1-silacyclobut-3-enes, which are isolable in many cases. For example, the [2 + 2] cycloadditions of silene 1 to bis(trimethylsilyl)butadiyne3g and tert-butyldimethylsilylacetylene3c at 120 °C gave the corresponding silacyclobutene derivatives 2a and 2b in high yields, respectively (Scheme 2). The generated silacyclobutene derivatives undergo a wide variety of thermal and photochemical reactions, strongly depending on the substituents on the sp2-hybridized carbon atoms in the four-membered ring. Thus, heating of 2a at 160 °C afforded 3-tert-butyl-1,1,3, 5-tetrakis(trimethylsilyl)-4-(trimethylsilyl)ethynyl-2-oxa-1-silacyclopent-4-ene (3a),3g while heating of 2b at 160 °C gave 1-tertbutyl-1-(tert-butyldimethylsilyl)-3-[(trimethylsiloxy)bis(trimethylsilyl)silyl]-1,2-propadiene (4b).3c Interestingly, the

reaction of pivaloyltris(trimethylsilyl)silane and bis(trimethylsilyl)acetylene at 160 °C afforded 1-[(tert-butyl)bis(trimethylsilyl)methyl]-1-(trimethylsiloxy)-2,3-bis(trimethylsilyl)-1-silacycloprop-2-ene (5c), and silacyclobutene intermediates such as 2c were not observed.3d It has been reported that some of the silacyclobutene derivatives show interesting optical properties, which may be used as functional materials, such as organic electroluminescent devices.3g,9 Experimental and theoretical studies have been reported for the elucidation of the mechanism of the formation of silacyclobutenes through [2 + 2] cycloaddition.4a,c,10,11 The thermal [2s + 2s] cycloaddition is symmetrically forbidden in carbon systems according to the Woodward−Hoffmann rules.12 The [2s + 2a] process is thermally allowed but energetically unfavorable because a distorted transition-state structure requires a high activation energy. Apeloig and co-workers theoretically investigated a stepwise mechanism of the [2 + 2] cycloaddition of acetylene and silene, in which a biradical •CC−Si−C• species is formed as an intermediate.11 Baines and co-workers provided experimental evidence for the involvement of a biradical intermediate in the addition of cyclopropyl alkynes to silene 1.10 Very recently we have theoretically proposed a plausible mechanism on the formation of silacyclopropene 5c from silene B


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Figure 1. Energy profile and optimized structures for the stepwise [2 + 2] cycloaddition of silene 1 and bis(trimethylsilyl)butadiyne calculated at the B3LYP/ 6-311+G**//B3LYP/6-31G* level of theory. Interatomic distances and Gibbs free energy changes (ΔG) at 120 °C are presented in Å and kcal/mol, respectively. energy surfaces using the B3LYP method14−16 combined with the 6-31G* basis set.17−19 Systematic vibrational analyses were carried out for all reaction species to characterize stationary-point structures. We considered the open-shell singlet state for the reaction pathways involving biradical species and the closed-shell singlet state for the other pathways. On application of density functional theory to the organosilicon compounds studied here, we showed that the activation energy for the formation of a biradical intermediate leading to 2c calculated with the unrestricted B3LYP method reasonably agrees with the energy calculated with the multireference CASSCF(4,4) method.4c An appropriate connection between a reactant and a product was confirmed by IRC (intrinsic reaction coordinate)20,21 and quasi-IRC calculations. In the quasi-IRC calculation, the geometry of a transition state was at first shifted by perturbing the geometries very slightly along the reaction coordinate and released for equilibrium optimization. This approach provides qualitatively identical results with IRC calculations at considerably lower computational cost. To evaluate energy profiles, we performed single-point calculations at the B3LYP/ 6-311+G** level of theory. The energy profiles are described by using Gibbs free energy changes (ΔG) at the reaction temperatures (120 °C for the formation of silacyclobutene intermediates, 160 °C for the formation of the final products).

1 and bis(trimethylsilyl)acetylene (Scheme 3).4c As predicted by the Woodward−Hoffmann rules, the concerted [2s + 2a] cycloaddition at 160 °C requires a very high activation energy (61.1 kcal/mol at the B3LYP/6-311+G(d,p) level of theory). By considering a stepwise mechanism involving the formation of the biradical intermediate 6c, the activation energy for the rate-determining step is reduced to 40.8 kcal/mol, which is still too high to be overcome even at 160 °C. The computational result is consistent with the experimental fact that no silacyclobutene intermediate was observed,3d and thus we concluded that the formation of 5c would not proceed through silacyclobutene 2c. In this system, instead, silene 1 should be transformed into a silylene intermediate. The silylene reacts with bis(trimethylsilyl)acetylene to yield 5c through a [2 + 1] cycloaddition (Scheme 3). However, it still remains unanswered why only the reaction of 1 and bis(trimethylsilyl)acetylene does not give any silacyclobutene derivative. In the present study, we have investigated the thermal reactions of 1 with bis(trimethylsilyl)butadiyne and tert-butyldimethylsilylacetylene by density functional theory (DFT) calculations for a systematic understanding of substituent effects on the reactivity of silene 1 with silyl-substituted acetylenes. Critical reaction steps for determining the final products (3a, 4b, and 5c) will be clarified by comparing energy profiles of possible reaction pathways.

3. RESULTS Formation of Silacyclobutene Derivatives. In this section we describe the formation of silacyclobutene derivatives 2a and 2b from silene 1 and silyl-substituted acetylenes, which involves three reaction steps, shown in Scheme 4. First, an Si−C bond is formed between SiA and CA′ to give biradical intermediate 6. Isomeric biradical intermediates having a CB−CB′

2. METHOD OF CALCULATION All calculations were performed using the Gaussian 03 program package.13 Local minima and saddle points were located on potential C


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Figure 2. Energy profile and optimized structures for the stepwise [2 + 2] cycloaddition of silene 1 and tert-butyldimethylsilylacetylene calculated at the B3LYP/6-311+G**//B3LYP/6-31G* level of theory. Interatomic distances and Gibbs free energy changes (ΔG) at 120 °C are presented in Å and kcal/mol, respectively.

Scheme 5

ring closure, and then silacyclobutene 2 is generated by an intramolecular radical coupling. For the formation of biradical intermediates, Figure 1 shows an energy diagram for the addition

bond was ruled out because they are more than 20 kcal/mol less stable than 6 for all the systems. The CC bond in 6 is rotated along the SiA−CA′ bond to prepare intermediate 6′ for D


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Figure 3. Energy profile and optimized structures for the formation of oxasilacyclopentene 3a from silacyclobutene 2a calculated at the B3LYP/ 6-311+G**//B3LYP/6-31G* level of theory. Interatomic distances and Gibbs free energy changes (ΔG) at 160 °C are presented in Å and kcal/mol, respectively.

The addition of 1 and tert-butyldimethylsilylacetylene yielding 2b is exergonic by 16.2 kcal/mol. In summary, the formation of 2a and 2b proceeds in an exergonic way, and the highest activation energy in each reaction pathway is low enough to be overcome at 120 °C. These results are consistent with the experimental fact that both 2a and 2b are isolable.3c,g Transformation of Silacyclobutene Derivatives. The formation of an oxasilacyclopentene derivative 3a proceeds in two reaction steps, shown in Scheme 5. First a ring-opening reaction occurs to give the silabutadiene derivative 7a, and then the oxygen atom of the OSiMe3 group in 7a attacks SiA to form a five-membered ring structure. The formation of 3a is accomplished by a simultaneous silyl migration from the OSiMe3 group to CB. Figure 3 shows an energy profile and optimized intermediates for the formation of 3a. The SiA−CB bond cleavage of 2a via TS2a/7a requires an activation energy of 28.9 kcal/mol, the formation of 7a being endergonic by 9.5 kcal/mol. The SiC−CC framework in 7a is highly distorted (SiACA′CB′CB = −111.0°) because of the steric hindrance between the bulky substituents on SiA and CB. The ring-closing reaction of 7a leading to the final product 3a proceeds in an exergonic way (ΔG = −20.7 kcal/mol). To characterize the changes in geometry, we carried out a quasi-IRC calculation in the reaction path between TS7a/3a and 3a. Figure 4 describes an energy plot and selected

of silene 1 to bis(trimethylsilyl)butadiyne that proceeds in the open-shell singlet state. The generated silacyclobutene 2a is thermally converted into oxasilacyclopentene 3a. In the first step, the formation of biradical intermediate 6a from reactant complex RCa is calculated to be endergonic by 17.4 kcal/mol, its activation energy via TS6a being 25.9 kcal/mol. Mulliken spin densities calculated for 6a are localized on CB (−0.91) and CB′ (+0.57), indicating the biradical character of 6a. Although the rotation barrier was not evaluated, intermediates 6a and 6a′ lie close in energy. The ring-closing reaction of 6a′ via TS6a′/2a requires an activation energy of 4.1 kcal/mol. The addition of 1 and bis(trimethylsilyl)butadiyne yielding 2a is exergonic by 18.0 kcal/mol. Figure 2 shows an energy diagram for the addition of silene 1 and tert-butyldimethylsilylacetylene to give silacyclobutene 2b, which is the precursor of allene 4b. At first glance the energy profile for the formation of 2b is similar to that for the formation of 2a except for the absence of a conformer of biradical intermediate 6b, which is expected to connect 6b with TS6b/2b. The Gibbs free energy change of the formation of 6b is calculated to be 11.9 kcal/mol, the activation energy via TS6b being 21.5 kcal/mol. IRC calculations from TS6b/2b revealed a direct connection between 6b and 2b via TS6b/2b. The activation energy for the ring closure of 6b involving the rotation of the CA′CB′ bond along the SiA−CA′ bond is 11.6 kcal/mol. E


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Figure 4. Energy plot and selected snapshots along the minimum-energy path between TS7a/3a and 3a. Quasi-IRC calculations have been performed at the B3LYP/6-31G* level of theory.

Scheme 6

snapshots along the minimum-energy path. The structural changes in TS7a/3a → 3a-i → 3a-ii clearly indicate that the ring closure simultaneously occurs with the silyl migration in a concerted manner. The activation energy for this concerted process is calculated to be 29.8 kcal/mol, which is the highest value in all the reaction steps. As shown in Scheme 6, the formation of allene 4b also starts with a ring-opening reaction of silacyclobutene 2b to give silabutadiene 7b. The formation of the SiA−O bond induces the migration of the SiMe2(t-Bu) group from CB′ to CB. This silyl migration is coupled with the CB−O bond cleavage and results in the formation of 4b. Oxasilacyclopentene 3b can be theoretically

formed from 2b if the SiMe3 group of the OSiMe3 group moves to CB before the migration of the SiMe2(t-Bu) group. Although such a product is not experimentally observed, we also searched the reaction pathway leading to 3b. Energy diagrams and optimized intermediates for the formations of 4b and 3b are presented in Figure 5. The activation energy for the ring-opening reaction of 2b yielding 7b is calculated to be 28.2 kcal/mol, which is comparable to that for 2a. In comparison with geometric parameters of TS7b/4b and TS7b/3b, the CB−O distance in TS7b/4b (1.582 Å) is longer than that in TS7b/3b (1.427 Å), while the SiA−O distance in TS7b/4b (1.949 Å) is much shorter than that in TS7b/3b (2.368 Å). These values F


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Figure 5. Energy profile and optimized structures for the formation of allene 4b and oxasilacyclopentene 3b from silacyclobutene 2b calculated at the B3LYP/6-311+G**//B3LYP/6-31G* level of theory. Interatomic distances and Gibbs free energy changes (ΔG) at 160 °C are presented in Å and kcal/mol, respectively.

pentene 3b (ΔG = −8.1 kcal/mol). These computational results are consistent with the experimental result that no oxasilacyclopentene derivative was observed in the thermal reaction of 2b.3c Quasi-IRC calculations starting from TS7b/4b and TS7b/3 give the minimum-energy paths leading to the corresponding final products. For the formation of 4b (Figure 6a), the migration of the OSiMe3 group (TS7b/4b → 4b-i) is followed

indicate that the OSiMe3 group is going to move from CB to SiA in TS7b/4b and only the SiA−O bond is being formed to give a five-membered ring structure in TS7b/3b. The activation energies for these reaction steps are 26.1 kcal/mol for the OSiMe3 migration and 36.0 kcal/mol for the ring closure. Moreover, the transformation of 2b into allene 4b (ΔG = −59.7 kcal/mol) is highly exergonic compared with the transformation into oxasilacycloG


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Figure 6. Energy plots and selected snapshots along the minimum-energy paths for (a) TS7b/4b → 4b and (b) TS7b/3b → 3b. Quasi-IRC calculations have been performed at the B3LYP/6-31G* level of theory.

(40.8 kcal/mol).4c In the reaction of 1 and bis(trimethylsilyl)acetylene, the high activation barrier leading to 6c inihibits the formation of silacyclobutene 2c, and instead silene 1 isomerizes into a silylene intermediate to react with bis(trimethylsilyl)acetylene by a [2 + 1] cycloaddition reaction (Scheme 3). The barrier height leading to 6 should strongly correlate with the stability of 6 because this reaction is endergonic. Actually 6a and 6b are less stable than RCa and RCb by 17.4 and 11.9 kcal/mol, respectively, while 6c is significantly higher in energy (32.1 kcal/mol)4c than the corresponding reactant complex. We here discuss the reactivity of 1 with acetylenes based on the stability of the biradical intermediates. Figure 7a compares geometric parameters and Mulliken spin densities of 6a, 6b, and 6c. It is noteworthy that the structures of the silene moiety in the three intermediates resemble each other. The SiA−CB bond distances are nearly 1.9 Å, and an unpaired electron is found at CB. Spin densities assigned to the acetylene moiety are localized at only CB′ in 6b and 6c, while they are delocalized over three carbon atoms, CB′ (0.57), CC′ (−0.30), and CD′ (0.64), in 6a. These results imply that 6a is electronically stabilized relative to 6b and 6c by electron delocalization in the acetylene moiety.

by the migration of the SiMe2(t-Bu) group from CB′ to CB (4b-i → 4b-ii → 4b-iii), resulting in the formation of 4b. Because 4b-i, 4b-ii, and 4b-iii are not stationary points on the potential energy surface, the two migration processes should occur in a concerted manner. On the other hand, structural changes along the minimum-energy path for TS7b/3b → 3b are very similar to those for TS7a/3a → 3a. The transformation of 7b into 3b is achieved by the ring closure (TS7b/3b → 3b-i) and the following SiMe3 transfer (3b-ii → 3b-iii → 3b). These two processes also occur in a concerted manner.

4. DISCUSSION In this section, we will first discuss substituent effects in the formation of biradical intermediates from silene 1 and silylsubstituted acetylenes. One of the major differences in the three thermal reactions studied here should be the presence of silacyclobutene derivatives (2a, 2b, and 2c). As mentioned above, 2a and 2b were isolated, while 2c was not observed.3c,d,g We have shown that the experimental findings can be associated with the activation energies for the formation of biradical intermediates 6a (25.9 kcal/mol), 6b (21.5 kcal/mol), and 6c H


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Figure 7. (a) Optimized structures of biradical intermediates 6a, 6b, and 6c. Mulliken spin densities and bond distances (Å) are indicated in roman and italic types, respectively. (b) Space-filling models of 6a, 6b, and 6c. Top views (right column, rotated by 90°) are also shown to clarify steric hindrance between the substituents. Gray, orange, purple, and white spheres represent carbon, silicon, oxygen, and hydrogen atoms, respectively.

Structural contributions must also be considered for discussing the stability of the biradical intermediates. The SiA−CA′ bond distances are calculated to be 1.937, 1.920, and 1.947 Å for 6a, 6b, and 6c, respectively. The shortest SiA−CA′ distance in 6b can be rationalized by the smallest substituent (H) on CA′ in the three biradical intermediates (Figure 7b). The bulky SiMe3 group on CA′ in 6a and 6c would hamper the formation of a strong Si−C bond. However, the degree of steric hindrance between the silene and acetylene moieties seems smaller in 6a than in 6c. As seen in Figure 7b, the silene moiety of 6a has a longer “neck” (CB′−CC′CD′−SiMe3) than that of 6c (CB′− SiMe3), and thereby the total steric hindrance would be reduced in 6a. In summary, an interplay between geometric and electronic factors controls the reactivity of silene 1 with the acetylenes. Steric hindrance between bulky substituents (particularly on CA′) destabilizes a biradical intermediate, and electron delocalization stabilizes the intermediate. Owing to the latter contribution, the activation energy for the formation of 6a is reduced to 21.5 kcal/mol, which is comparable to the value of 6b (25.9 kcal/mol). On the other hand, the system consisting of 1 and bis(trimethylsilyl)acetylene has neither geometric nor

electronic advantages in the biradical intermediate, and thus the activation energy rises to 40.8 kcal/mol, which is too high to be overcome even at 160 °C. Finally, we propose that the reactivity of silacyclobutenes 2a and 2b is controlled by two major factors: mobility of the substituent on CB′ and the steric hindrance between the substituent on CB′ and the SiMe3 group derived from the OSiMe3 group. As presented in Figure 5, after the ring-opening reaction of silacyclobutene yielding silabutadiene, there are two possible reaction pathways that lead to the two final products. If the substituent on CB′ can easily migrate such as a silyl group, the SiA−O bond formation should cause the migration of the substituent on CB′ to CB. The generated allene framework is capable of separating bulky silyl substituents to avoid steric hindrance. If the substituent on CB′ cannot easily migrate, the SiMe3 group of the OSiMe3 group rebounds to CB to generate an oxasilacyclopentene framework. The stability of the oxasilacyclopentene derivative depends on the degree of steric hindrance between the substituents on CB′ and the SiMe3 group moved to CB. In the case of the reaction of silabutadiene 7a, low mobility of the CCSiMe3 group on CB′ results in the formation of an I


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Notes

oxasilacyclopentene framework. The steric hindrance between the CCSiMe3 group and the substituents on CB is not expected to be very serious because an “extra” acetylene unit keeps the connected SiMe3 group away from the substituents on CB. In the case of 7b, high mobility of the SiMe2(t-Bu) group on CB′ induces the formation of an allene framework, in which bulky substituents are separated well on the terminal CA′ and CB atoms of the framework. In the reaction pathway to oxasilacyclopentene from 7b, the bulkier SiMe2(t-Bu) group directly bound to CB′ effectively inihibits the migration of the SiMe3 group to CB.

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by Grants-in-Aid (Nos. 21750063 and 22245028) for Scientific Research 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.

5. CONCLUSIONS We have theoretically investigated the three thermal reactions of silene derivative (Me3Si)2SiC(OSiMe3)(t-Bu) (1) with silyl-substituted acetylenes, bis(trimethylsilyl)butadiyne, tertbutyldimethylsilylacetylene, and bis(trimethylsilyl)acetylene, for a systematic understanding of substituent effects in the reactivity of the silene. The first critical reaction step for determining the final product is the formation of a biradical intermediate (•CC−Si−C•) from 1 and the acetylenes. A stepwise [2 + 2] cycloaddition via the biradical intermediate gives a silacyclobutene derivative, which is the precursor of the final product. The activation energy for this reaction step as well as thermodynamic stability of the biradical intermediate is governed by an interplay between geometric and electronic contributions. The biradical intermediate is destabilized by the steric hindrance between the bulky substituents on 1 and acetylene, while it can be stabilized by delocalization of an unpaired electron in the acetylene moiety. The thermal reaction of 1 and bis(trimethylsilyl)acetylene does not proceed via a silacyclobutene derivative because of significant steric hindrance between the neighboring SiMe3 groups as well as no electron delocalization to stabilize the biradical intermediate. In this case, 1 is first transformed into a silylene intermediate, and then the silylene reacts with acetylene to form a silacyclopropene derivative via a [2 + 1] cycloaddition. For the other acetylenes, silacyclobutene derivatives undergo Si−C bond cleavage to give silabutadiene derivatives. The second critical reaction step for determining the final product is the attack of the OSiMe3 group at the silene Si atom in the silabutadienes. If the substituent on one of the acetylene C atoms cannot easily migrate, the SiMe3 group of the OSiMe3 group rebounds to the silene C atom to form an oxasilacyclopentene derivative. If this substituent can easily migrate, it migrates to the silene C atom with a simultaneous migration of the OSiMe3 group, resulting in the formation of an allene derivative. The allene derivative is energetically more favorable than the oxasilacyclopentene derivative because the allene framework is capable of separating bulky substituents on its terminal carbon atoms. All the calculated results reasonably agree with the experimental findings reported by Naka and Ishikawa.3c,d,g

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(1) Brook, A. G.; Harris, J. W.; Lennon, J.; Sheikh, M. E. J. Am. Chem. Soc. 1979, 101, 83. (2) (a) Brook, A. G. J. Organomet. Chem. 1986, 300, 21. (b) Brook, A. G. In The Chemistry of Organic Silicon Compounds; Patti, S., Rappoport, Z., Eds.; Wiley: New York, 1989; Chapter 15. (c) Brook, A. G. Adv. Organomet. Chem. 1996, 39, 71. (d) Brook, A. G. In The Chemistry of Organic Silicon Compounds; Rappoport, Z., Apeloig, Y., Eds.; Wiley: New York, 1998; Vol. 2, Chapter 21. (e) Brook, A. G.; Abdesaken, F.; Gutekunst, B.; Gutekunst, G.; Kallury, R. K. Chem. Commun. 1981, 191. (f) Brook, A. G.; Nyburg, S. C.; Abdesaken, F.; Gutekunst, B.; Gutekunst, G.; Kallury, R. K. M.; Poon, Y. C.; Chang, Y.-M.; Wong-Ng, W. J. Am. Chem. Soc. 1982, 104, 5667. (g) Lassacher, P.; Brook, A. G.; Lough, A. J. Organometallics 1995, 14, 4359. (h) Brook, A. G.; Ionkin, A.; Lough, A. J. Organometallics 1996, 15, 1275. (3) (a) Ishikawa, M.; Matsui, S.; Naka, A.; Ohshita, J. Organometallics 1996, 15, 3836. (b) Naka, A.; Ishikawa, M. Organometallics 1996, 15, 5759. (c) Naka, A.; Ishikawa, M. J. Organomet. Chem. 2000, 611, 248. (d) Naka, A.; Ishikawa, M. Organometallics 2000, 19, 4921. (e) Naka, A.; Ikadai, J.; Motoike, S.; Yoshizawa, K.; Kondo, Y.; Kang, S.-Y.; Ishikawa, M. Organometallics 2002, 21, 2033. (f) Naka, A.; Ishikawa, M. Organometallics 2002, 364. (g) Naka, A.; Ishikawa, M. Chem. Lett. 2002, 364. (h) Naka, A.; Motoike, S.; Senba, N.; Ohshita, J.; Kunai, A.; Yoshizawa, K.; Ishikawa, M. Organometallics 2008, 27, 2750. (i) Naka, A.; Ueda, S.; Ohshita, J.; Kunai, A.; Miura, T.; Kobayashi, H.; Ishikawa, M. Organometallics 2008, 27, 2922. (j) Naka, A.; Senba, N.; Motoike, S.; Fujimoto, H.; Miura, T.; Kobayashi, H.; Yoshizawa, K.; Ishikawa, M. Organometallics 2009, 28, 5641. (4) (a) Yoshizawa, K.; Kondo, Y.; Kang, S.-Y.; Naka, A.; Ishikawa, M. Organometallics 2002, 21, 3271. (b) Shiota, Y.; Yasunaga, M.; Naka, A.; Ishikawa, M.; Yoshizawa, K. Organometallics 2004, 23, 4744. (c) Tanaka, H.; Kondo., Y.; Shiota, Y.; Naka, A.; Ishikawa, M.; Yoshizawa, K. Organometallics 2011, 30, 3160. (5) (a) Brook, A. G.; Chiu, P.; McClenaghnan, J.; Lough, A. J. Organometallics 1991, 10, 3292. (b) Ohshita, J.; Masaoka, Y.; Ishikawa, M. Organometallics 1991, 10, 3775; 1993, 12, 876. (c) BravoZhivotovskii, D.; Brende, V.; Stanger, A.; Kapon, M.; Apeloig, Y. Organometallics 1992, 11, 2326. (d) Apeloig, Y.; Bendikov, M.; Yuzefovich, M.; Nakash, M.; Bravo-Zhivotovskii, D.; Blaser, D.; Boese, R. J. Am. Chem. Soc. 1996, 118, 12228. (e) Krempner, C.; Oehme, H. J. Organomet. Chem. 1994, 464, C7. (f) Krempner, C.; Reinke, H.; Oehme, H. Angew. Chem., Int. Ed. Engl. 1994, 33, 1615. (6) (a) Gusel’nikov, L. E.; Flowers, M. C. J. Chem. Soc., Chem. Commun. 1967, 864. (b) Gusel’nikov, L. E.; Nametkin, N. S. Chem. Rev. 1979, 79, 529. (c) Gusel’nikov, L. E. Coord. Chem. Rev. 2003, 244, 149. (7) (a) Coleman, B.; Jones, M., Jr. Rev. Chem. Intermed. 1981, 4, 297. (b) Wiberg, N. J. Organomet. Chem. 1984, 273, 141. (c) Raabe, G.; Michl, J. Chem. Rev. 1985, 85, 419. (d) Müller, T.; Ziche, W.; Auner, N. In The Chemistry of Organic Silicon Compounds; Rappoport, Z., Apeloig, Y., Eds.; Wiley: New York, 1998; Vol. 2, p 857. (e) Morkin, T. L.; Leigh, W. J. Acc. Chem. Res. 2001, 34, 129. (f) Mohseni-Ala, J.; Auner, N. Inorg. Chim. Acta 2006, 359, 4677.

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Cartesian coordinates for all optimized intermediates and transition structures and the complete author list of ref 13. This material is available free of charge via the Internet at http:// pubs.acs.org.

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REFERENCES

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. J


Organometallics

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Substituent Effects in Thermal Reactions of a Silene with Silyl

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