Silicon−Carbon Unsaturated Compounds. 76. Photochemical and

The chemistry of silacyclobutenes generated from addi- ... years.1r3 In fact, these compounds show unique chemical ..... We thank Shin-Etsu Chemical C...
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Organometallics 2009, 28, 5641–5646 DOI: 10.1021/om900512f

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Silicon-Carbon Unsaturated Compounds. 76. Photochemical and Thermal Behavior of 1-Silacyclobut-3-enes Generated from the Reaction of Pivaloyltris(trimethylsilyl)silane with tert-Butylacetylene Akinobu Naka,*,† Norihito Senba,† Shingo Motoike,† Hiroki Fujimoto,† Toshiko Miura,‡ Hisayoshi Kobayashi,*,‡ Kazunari Yoshizawa,§ and Mitsuo Ishikawa*,† †

Department of Life Science, Kurashiki University of Science and the Arts, Nishinoura, Tsurajima, Kurashiki, Okayama 712-8505, Japan, ‡Department of Chemistry and Materials Technology, Kyoto Institute of Technology, Matsugasaki, Kyoto 606-8585, Japan, and §Institute for Material Chemistry and Engineering, Kyushu University, Fukuoka 812-8581, Japan Received June 16, 2009

Irradiation of trans-1-silacyclobut-3-ene 2, obtained by thermal isomerization of [2 þ 2] cycloadduct 1 arising from the co-thermoysis of pivaloyltris(trimethylsilyl)silane with tert-butylacetylene, with a low-pressure mercury lamp afforded an equilibrium mixture consisting of 2 and its isomer, cis1-silacyclobut-3-ene 3, in a ratio of 1:1. Similar irradiation of the cis isomer 3 gave the mixture involving equal amounts of 2 and 3. The photolysis of 2 in the presence of tert-butyl alcohol gave two isomers of the cyclopropane derivative, arising from the reaction of silabicyclobutane, A and A0 , with alcohol. The thermolysis of 3 at 250 °C produced 2 in quantitative yield. Similar thermolysis of 3 in the presence of tert-butyl alcohol, however, afforded no alcohol adducts, but 2 was obtained as the sole product. The DFT calculations with the use of cis-2,4-dimethyl-1-siloxy-1,2-bis(silyl)-1-silacyclobut-3-ene as a starting compound and trans-2,4-dimethyl-1-siloxy-1,2-bis(silyl)-1-silacyclobut-3ene as a product indicated that a pentacoordinate silicon intermediate plays an important role in the thermal isomerization of 3 to 2. Introduction The chemistry of silacyclobutenes generated from addition of silenes to alkynes has become of interest in recent years.1-3 In fact, these compounds show unique chemical behavior and undergo a wide variety of reactions, depending on the substituents on the silicon and carbon atoms in the ring2 (Scheme 1). Experimental and theoretical investigations have been extensively carried out to elucidate the mechanism for the addition reactions of the silenes to alkynes and isomerization of the resulting 1-silacyclobut3-enes. As a consequence, unambiguous evidence for the *Corresponding authors. E-mail: [email protected].; kobayashi@ chem.kit.ac.jp; [email protected]. (1) (a) Gusel’nikov, L. E.; Nametkin, N. S. Chem. Rev. 1979, 79, 529. (b) Wiberg, N. J. Organomet. Chem. 1984, 273, 141. (c) Raabe, G.; Michl, J. Chem. Rev. 1985, 85, 419. (d) Brook, A. G.; Baines, K. M. Adv. Organomet. Chem. 1986, 25, 1. (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. (2) (a) Ishikawa, M.; Matsui, S.; Naka, A.; Ohshita, J. Organometallics 1996, 15, 3836. (b) Naka, A.; Ishikawa, M.; Matsui, S.; Ohshita, J.; Kunai, A. Organometallics 1996, 15, 5759. (c) Naka, A.; Ishikawa, M. J. Organomet. Chem. 2000, 611, 248. (d) Naka, A.; Ikadai, J.; Motoike, S.; Yoshizawa, K.; Kondo, Y.; Kang, S.-Y.; Ishikawa, M. Organometallics 2002, 21, 2033. (e) Naka, A.; Ohnishi, H.; Miyahara, I.; Hirotsu, K.; Shiota, Y.; Yoshizawa, K.; Ishikawa, M. Organometallics 2004, 23, 4277. (f) Naka, A.; Ohnishi, H.; Ohshita, J; Ikadai, J.; Kunai, A.; Ishikawa, M. Organometallics 2005, 24, 5356. (g) Naka, A.; Fujioka, N.; Ohshita, J.; Ikadai, J.; Kunai, A.; Kobayashi, H.; Miura, T.; Ishikawa, M. Organometallics 2007, 26, 5535. (h) Naka, A.; Motoike, S.; Senba, N.; Ohshita, J.; Kunai, A.; Yoshizawa, K.; Ishikawa, M. Organometallics 2008, 27, 2750. (3) (a) Milnes, K. K.; Jennings, M. C.; Baines, K. M. J. Am. Chem. Soc. 2006, 128, 2491. (b) Milnes, K. K.; Baines, K, M. Can. J. Chem. 2009, 87, 307. r 2009 American Chemical Society

formation of the intermediates during these reactions has been provided.4,5 On the other hand, the metallocene-mediated intramolecular carbon-carbon bond formation of two alkynyl groups of bisand tetrakis(alkynyl)silanes can also be used for the synthesis of the silacyclobutenes. For example, it has been reported that treatment of bis- and tetrakis(phenylethynyl)silane with zirconocene complexes affords the silacyclobutene derivatives bearing π-conjugated systems.6 The chemical and optical properties of the resulting silacyclobutene derivatives have also been reported. Recently, we have demonstrated that the thermoysis of 2,3-di(tert-butyl)-2- (trimethylsiloxy)-1,1-bis(trimethylsilyl)1-silacyclobut-3-ene (1) obtained by the reaction of pivaloyltris(trimethylsilyl)silane with tert-butylacetylene proceeds stereospecifically to give trans-2,4-di(tert-butyl)-1-(trimethylsiloxy)-1,2-bis(trimethylsilyl)-1-silacyclobut-3-ene (2) as the sole product2d,h (Scheme 2). We have also elucidated that the thermal isomerization of 1 leading to 2 proceeds cleanly through the silylsubstituted cyclopropene derivative.2d,h It is of interest to us to investigate the photochemical properties of the silacyclobutenes, compared to their thermal ones. (4) Shiota, Y.; Yasunaga, M.; Naka, A.; Ishikawa, M.; Yoshizawa, K. Organometallics 2004, 23, 4744. (5) Tong, H.; Eklof, A. M.; Steel, P. G.; Ottosson, H. J. Mol. Struct. (THEOCHEM) 2007, 811, 153. (6) (a) Xi, Z.; Fischer, R.; Hara, R.; Sun, W.-H.; Obora, Y.; Suzuki, N.; Nakajima, K.; Takahashi, T. J. Am. Chem. Soc. 1997, 119, 12842. (b) Pellny, P.-M.; Peulecke, N.; Burlakov, V. V.; Baumann, W.; Spannenberg, A.; Rosenthal, U. Organometallics 2000, 19, 1198. (c) Jin, C. K.; Yamada, T.; Sano, S.; Shiro, M.; Nagao, Y. Tetrahedron Lett. 2007, 48, 3671. (d) Liu, J.; Zhang, S.; Zhang, W.-X.; Xi, Z. Organometallics 2009, 28, 413. Published on Web 09/18/2009

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Naka et al. Scheme 1

Scheme 2

In a preliminary communication,2d we have reported that the photolysis of trans-1-silacyclobut-3-ene 2 produces an equilibrium mixture, consisting of 2 and its stereoisomer, cis2,4-di(tert-butyl)-1-(trimethylsiloxy)-1,2-bis(trimethylsilyl)1-sila-cyclobut-3-ene (3), in a ratio of 1:1. We have also reported that the thermolysis of cis isomer 3 thus formed at 250 °C afforded trans isomer 2, quantitatively. To clarify the mechanism for the photochemical isomerization of 2 and 3, leading to the equilibrium mixture, and the thermal isomerization of 3 to 2, we have investigated their reactions in detail.

Scheme 3

Results and Discussion As reported previously, the thermolysis of 2,3-di(tert-butyl)2-(trimethylsiloxy)-1,1-bis(trimethylsilyl)-1-silacyclobut-3-ene (1) produced trans-2,4-di(tert-butyl)-1-(trimethylsiloxy)-1,2bis(trimethylsilyl)-1-silacyclobut-3-ene (2) in almost quantitative yield.2h We thought that the photolysis of 1 might proceed cleanly to give the photoproduct, such as a ring-opened 1-silabuta-1,3-diene derivative, or isomerization product, 1-silacyclobut-3-ene 2. However, when 1 was irradiated in the presence of methanol with a low-pressure mercury lamp in a hexane solution, a complicated reaction mixture was produced. No volatile products were isolated from the resulting mixture. The photolysis of trans-1-silacyclobut-3-ene 2, obtained by the thermolysis of 1, however, proceeded cleanly to give the photoproduct. Thus, irradiation of 2 with a low-pressure mercury lamp bearing a Vycor filter in a hexane solution afforded an equilibrium mixture, consisting of 2 and its stereoisomer, cis-2,4-di(tert-butyl)-1-(trimethylsiloxy)-1,2bis(trimethylsilyl)-1-silacyclobut-3-ene (3), in a ratio of 1:1 in quantitative yield, as reported previously (Scheme 3).2d The cis isomer 3 was separated from the trans isomer 2 by recycling preparative HPLC, and its structure was verified by spectrometric analysis. That the reaction reaches an

equilibrium is confirmed by the observation that equal amounts of 2 and 3 are obtained quantitatively by the photolysis of the cis isomer 3. Surprisingly, when 3 was heated in a sealed glass tube at 250 °C for 7 h, the trans isomer 2 was obtained in quantitative yield. In order to get more information about the intermediate, which might be involved in the photochemical reaction of 2 or 3, leading to the equilibrium mixture, we carried out the photolysis of 2 in the presence of alcohol as a trapping agent. Thus, irradiation of 2 in the presence of a 10-fold excess of tert-butyl alcohol with a low-pressure mercury lamp in a hexane solution for 4 h afforded two stereoisomers of the alcohol adducts, c-2-[(tert-butoxy)(trimethylsiloxy)(trimethylsilyl)silyl]-1,t-3-di(tert-butyl)- and t-2-[(tert-butoxy)(trimethylsiloxy)(trimethylsilyl)silyl]-1,c-3-di(tert-butyl)-r1-(trimethylsilyl)cyclopropane (4a and 5a) in a ratio of 1:1, in 19% combined yield, after treatment of the reaction mixture with column chromatography. No adduct arising

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Organometallics, Vol. 28, No. 19, 2009 Scheme 4

from the reaction of a photochemical ring-opened product, 1-silabuta-1,3-diene derivative, with alcohol was detected in the reaction mixture. Similarly, the photolysis of 2 with tert-butyl alcohol-d1 under the same conditions again afforded two stereoisomers, c-2-[(tert-butoxy)(trimethylsiloxy)(trimethylsilyl)silyl]-1,t-3-di(tert-butyl)- and t-2-[(tertbutoxy)(trimethylsiloxy)(trimethylsilyl)silyl]-1,c-3-di(tertbutyl)-3-deuterio-r-1-(trimethylsilyl)cyclopropane (4b and 5b) in 22% combined yield. Again, no adduct originating from addition of tert-butyl alcohol-d1 to the 1-silabuta-1,3diene derivative was detected. The formation of 4a,b and 5a,b may be best understood by the reaction of the silabicyclobutane derivative (A or A0 ), generated photochemically directly from 2 or 3, with tertbutyl alcohol and tert-butyl alcohol-d1, respectively, as shown in Scheme 4.7 The silabicyclobutanes A and A0 thus formed presumably are labile under the photolysis conditions used, and some of them would undergo decomposition to give the unidentified products. In fact, small amounts of many unidentified products can be detected in the resulting photolysis mixture. Consequently, the low yields of the alcohol adducts may be ascribed to the lability of the silabicyclobutanes. The structures of 4a,b and 5a,b were verified by spectrometric analysis, and their cis and trans configurations were confirmed by NOE-FID difference experiments at 300 MHz. The long-range 1H-13C COSY NMR spectrum for 4a reveals that the proton at -0.01 ppm couples with the silylsubstituted ring carbon atom at 16.4 ppm, while the proton at 1.22 ppm couples with the ring carbon atom at 44.1 ppm. (7) Tortorelli, V. J.; Jones, M. J. Am. Chem. Soc. 1980, 102, 1425.

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In the NOE-FID difference experiments at 300 MHz for 4a, saturation of a proton at -0.01 ppm on the silyl-substituted ring carbon atom led to enhancement of the signals at 0.15 ppm due to the trimethylsilyl protons and at 1.08 and 1.09 ppm attributed to the two nonequivalent tert-butyl protons. Similar irradiation of a signal at 1.22 ppm, due to the proton on the ring carbon atom, resulted in a strong enhancement of the signals at 0.15, 0.16, and 1.08 ppm, due to the two trimethylsilyl protons and tert-butyl protons, respectively. For 5a, irradiation of the signal at -0.17 ppm, attributed to the proton on the silyl-substituted ring carbon atom, showed enhancement of the signals at 0.14, 0.15, and 0.23 ppm due to the two trimethylsilyl protons and the trimethylsiloxy protons, as well as the signals at 1.00 ppm, attributable to the tert-butyl protons. Furthermore, saturation of the signal at 1.43 ppm, due to the proton on the ring carbon, led to enhancement of the signals at 0.15, 1.00, and 1.04 ppm, attributed to the trimethylsilyl protons and two nonequivalent tert-butyl protons. These results are wholly consistent with the structures proposed for 4a and 5a. According to the Woodward and Hoffmann rules,8 the photochemical isomerization of 2 or 3 can be expected to proceed stereospecifically to give the silabicyclobutane intermediate A or A0 . In a concerted process, the thermal ring expansion of the silabicyclobutane A or A0 thus formed should also proceed with stereospecificity to afford the silacyclobutene 3 or 2, as a single isomer. However, in the present system, a mixture consisting of two stereoisomers, 2 and 3, is produced. Presumably, ring expansion of the silabicyclobutane leading to the silacyclobutene proceeds with a stepwise process including a biradical species (Scheme 4). In this connection, theoretical treatments for the photochemical isomerization of butadiene, cyclobutene, and bicyclobutane9 and also isomerization of bicyclobutane to butadiene10 have been performed, and the formation of the biradical species has been suggested in these reactions. To clarify the mechanism for the thermal isomerization of 3 to 2, we carried out the thermolysis of 3 in the presence of tert-butyl alcohol. Thus, when 3 was heated in the presence of a 10-fold excess of tert-butyl alcohol in a degassed sealed tube at 250 °C, the product 2 was obtained as the sole product, indicating that no reactive species such as the silabicyclobutane A is involved in the thermolysis. As expected, similar thermolysis of 2 in the presence of tert-butyl alcohol afforded no alcohol adduct, but 2 was recovered unchanged. Next, we carried out the thermolysis of 2 and 3 using methanol, which seems to be a more reactive trapping agent. However, the thermolysis of 3 with methanol at 250 °C gave 2 in quantitative yield. Again, no alcohol adducts were detected in the reaction mixture by spectrometric analysis. Theoretical Calculations. We carried out DFT-based theoretical calculations to confirm the mechanism of the thermal isomerization from 3 to 2 using real and simplified models. The Becke three-parameter Lee-Yang-Parr hybrid-type density functional method11,12 implemented in the (8) Woodward, R. B.; Hoffmann, R. The Conservation of Orbital Symmetry; Verlag Chemie, Academic Press: New York, 1970. (9) Sakai, S. Chem. Phys. Lett. 2000, 319, 687. (10) Kinal, A.; Piecuch, P. J. Phys. Chem. A 2007, 111, 734. (11) Becke, A. D. J. Chem. Phys. 1993, 98 (1372), 5648. (12) Gill, P. M. W.; Johnson, B. G.; Pople, J. A. Int. J. Quantum Chem. Symp. 1992, 26, 319.

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Gaussian 03 program13 was used with the 6-311G(d) basis set in this work. The structures of 2 and 3 shown in Scheme 1 were used as those of the real models. For the characterization of a local minimum and transition state (TS), the simplified models, where tert-butyl groups and trimethylsilyl groups of 2 and 3 were replaced by the methyl groups and silyl groups, respectively, were employed. In the course of the isomerization, TSs were searched first and then intrinsic reaction coordinate (IRC) analyses were carried out at each TS for both directions, i.e., reactant and product sides. Finally, we confirmed that (1) each TS connected with two local minima, corresponding to the reactant and product sides, and (2) any structures of local minima, except for LM1 (simplified 3) and LM-3 (simplified 2), were derived from the two TSs in the reactant side and the product side. (Hereafter each local minimum is denoted by the prefix “LM-” and its sequential number, such as LM-1 and LM-3.) Figure 1 shows optimized structures for local minima and TSs in the course of the thermal isomerization of 3 to 2. Some important internal coordinates and relative energies with respect to the total energy of LM-1 are indicated in the figure. The atomic charge is estimated from Mulliken population and indicated in square brackets for the important atoms for the bond formation and fission. For each TS, the number in parentheses represents contribution of that internal coordinate to the vibrational mode with imaginary frequency, and the larger numbers (in the absolute value) characterize the direction of the reaction coordinate. At TS-1, the Si atom on the C-2 carbon in the four-membered ring migrates onto the adjacent Si atom, and the five-coordinate intermediate (I-1) is formed. The structure of I-1 is characterized as a local minimum, where all the forces acting on nuclei are zero, due to a small hump on the reaction coordinate, to the product side. Both structures and energies for TS-1 and I-1 resemble each other. The dihedral angles O-Si-C-C for TS-1 and I-1 are 100.2° and 99.6°, respectively. We could not characterize a TS after I-1. However, when the optimization was restarted from a little larger dihedral angle, 102°, LM-2 was obtained straightforwardly. Thus we interpreted that the reaction coordinate is essentially continuous downhill until LM-2. The main difference in the structures between I-1 and LM-2 is in their dihedral angles of SiH3 and OSiH3 against the fourmembered ring. (As a practical parameter, the dihedral angles of O-Si-C-C are adopted.) In fact, compared to I-1, the O-Si-C-C angle of LM-2 considerably changes to a value of -170.4°, and its energy is stabilized by 36 kJ mol-1. There are two possibilities with respect to the migrating Si atom from LM-2, leading to LM-3, via TS-2. If the rear side (13) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Montgomery, J. A., Jr.; Vreven, T.; Kudin, K. N.; Burant, J. C.; Millam, J. M.; Iyengar, S. S.; Tomasi, J.; Barone, V.; Mennucci, B.; Cossi, M.; Scalmani, G.; Rega, N.; Petersson, G. A.; Nakatsuji, H.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa, J.; Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai, H.; Klene, M.; Li, X.; Knox, J. E.; Hratchian, H. P.; Cross, J. B.; Bakken, V.; Adamo, C.; Jaramillo, J.; Gomperts, R.; Stratmann, R. E.; Yazyev, O.; Austin, A. J.; Cammi, R.; Pomelli, C.; Ochterski, J. W.; Ayala, P. Y.; Morokuma, K.; Voth, G. A.; Salvador, P.; Dannenberg, J. J.; Zakrzewski, V. G.; Dapprich, S.; Daniels, A. D.; Strain, M. C.; Farkas, O.; Malick, D. K.; Rabuck, A. D.; Raghavachari, K.; Foresman, J. B.; Ortiz, J. V.; Cui, Q.; Baboul, A. G.; Clifford, S.; Cioslowski, J.; Stefanov, B. B.; Liu, G.; Liashenko, A.; Piskorz, P.; Komaromi, I.; Martin, R. L.; Fox, D. J.; Keith, T.; Al-Laham, M. A.; Peng, C. Y.; Nanayakkara, A.; Challacombe, M.; Gill, P. M. W.; Johnson, B.; Chen, W.; Wong, M. W.; Gonzalez, C.; Pople, J. A. Gaussian 03, Revision C.02; Gaussian, Inc.: Wallingford, CT, 2004.

Naka et al.

of the Si atom (indicated by a hatched wedged bond in the Lewis formula) migrates to the adjacent carbon atom to produce the isomerization product, LM-3 (simplified 2) is formed through TS-2, as shown in Figure 1. On the other hand, in the case of the migration of the front side of the Si atom (indicated by a filled wedged bond), the resulting product is an enantiomer of LM-3 (not shown in the figure). Both reaction paths were examined, and the identical energies were computed as expected for the corresponding enantiomers. Undoubtedly both routes led to the formation of the trans isomer, consistent with the experimental results. The energy change along the reaction coordinate is shown in Figure 2. At first glance, the reaction proceeds across a plateau with an energy of 264-271 kJ mol-1, above simplified 3 (activation energy of 271 kJ mol-1). There are two TSs and one local minimum on the plateau, and their energies are in the range within 43 kJ mol-1. Unfortunately, the energy difference between simplified 2 and 3 is underestimated compared to that expected by the experiment. Table 1 shows the results including additional calculations. Simplified 2 is more stable by 5.3 kJ mol-1 than simplified 3. The energies of compounds 2 and 3 are evaluated with the real models, and the energy difference is somewhat increased to 7.3 kJ mol-1. Using the structure optimized by the B3LYP method, singlepoint energies are also evaluated at the MP2 level of theory. The energy difference is still increased to 10.4 kJ mol-1. The energy difference between the reactant 3 and the product 2 is reproduced only qualitatively, and it is too small to explain the irreversible conversion observed experimentally. However, the reaction mechanism is rationalized by the characterization of the local minima and TSs in the course of the thermal isomerization. Table 1. Total Energy Difference between Reactant and Producta model/method

reactant 3

product 2

simplified/B3LYP real/B3LYP real/MP2b

-1433.5602 -2023.4894 -2018.4428

-1433.5622 -2023.4922 -2018.4468

difference -5.3 -7.3 -10.4

a Total energy is in hartrees, and energy difference is in kJ mol-1. Second-order Moeller-Plesset perturbation theory. The structures optimized by the B3LYP method are used. b

In conclusion, the photolysis of the trans isomer 2 with a low-pressure mercury lamp in hexane gave an equilibrium mixture consisting of 2 and its cis isomer 3, via silabicyclobutane A and A0 . Irradiation of the cis isomer 3 afforded the same mixture as that of 2. When 3 was heated in a sealed glass tube at 250 °C, 2 was obtained quantitatively. The photolysis of 2 in the presence of a large excess of tert-butyl alcohol gave two stereoisomers of the cyclopropane derivative, indicting that the silabicyclobutanes A and A0 were produced as the key intermediates. The theoretical calculations indicated that the pentacoordinate silicon intermediate plays an important role for the thermal isomerization of 3 to 2.

Experimental Section General Procedures. The photochemical reactions were carried out under an atmosphere of dry argon. Yields of the products were calculated on the basis of the isolated products. NMR spectra were recorded on a JNM-LA 300 spectrometer and JNM-500 spectrometer. Infrared spectra were recorded on a JEOL model JIR-DIAMOND 20 infrared spectrophotometer. Mass spectra were measured on a JEOL model JMS-700 instrument. Column chromatography was performed by using

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Figure 1. Optimized structures for LM-1 (3), TS-1, I-1, LM-2, TS-2, and LM-3 (2) with the simplified models. Some important bond lengths and angles are indicated together with the relative energy with respect to the total energy of LM-1. For TSs, the leading internal parameters characterizing the reaction coordinate are shown in parentheses. The atomic charges estimated from Mulliken population are indicated in square brackets for important atoms of the reaction. For LM-1 (3), they are indicated for all the atoms except for hydrogen to show their reference values. Wakogel C-300 (Wako). Gel permeation chromatographic separation was carried out with the use of a model LC-908 recycling preparative HPLC (Japan Analytical Industry Co., Ltd.).

Photolysis of 2. A solution of 0.336 g (0.81 mmol) of 2 in 60 mL of dry hexane was placed in a reaction vessel fit internally with a low-pressure mercury lamp bearing a Vicor

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Organometallics, Vol. 28, No. 19, 2009

Figure 2. Energy change along the isomerization reaction from LM-1 (3) to LM-3 (2) with the simplified models. filter.14 The solution was irradiated at room temperature for 30 min. After evaporation of the solvent, the residue was analyzed by 1H NMR spectrometry as being a mixture consisting of 2 and 3 in the ratio of 1:1 in quantitative yield. Compound 3 was separated from 2 by preparative HPLC. Anal. Calcd for C20H46OSi4: C, 57.89; H, 11.17. Found: C, 57.80; H, 11.22. MS: m/z 414 (Mþ). 1H NMR δ (CDCl3): 0.07 (s, 9H, Me3Si), 0.15 (s, 9H, Me3Si), 0.20 (s, 9H, Me3Si), 1.04 (s, 9H, t-Bu), 1.06 (s, 9H, t-Bu), 6.71 (s, 1H, HCdC). 13C NMR δ (CDCl3): 0.5, 3.1, 3.5 (Me3Si), 30.5, 31.8 (Me3C), 33.6, 35.4 (CMe3), 145.4, 164.7 (olefinic carbons). 29Si NMR δ (CDCl3): -21.7, -5.6, -1.7, 6.5. Photolysis of 3. A solution of 7.5 mg (0.018 mmol) of 3 in 0.8 mL of dry cyclohexane-d12 was placed in a quartz NMR tube. The tube was thoroughly degassed by freeze-pump-thaw cycles and then irradiated externally with a low-pressure mercury lamp. After 20 min of irradiation, a mixture consisting of 2 and 3 in the ratio of 1:1 by 1H NMR spectrometric analysis was obtained quantitatively. Photolysis of 2 in the Presence of 10 equiv of tert-Butyl Alcohol. A solution of 0.301 g (0.73 mmol) of 2 and 0.5423 g (7.33 mmol) of tert-butyl alcohol in 65 mL of dry hexane was placed in a reaction vessel fit internally with a low-pressure mercury lamp bearing a Vicor filter. After the solution was irradiated for 4 h, the solvent hexane was evaporated, and the residue was chromatographed on a silica gel column, with hexane as eluent, to give 0.067 g (19% yield) of the mixture consisting of 4a and 5a in a ratio of 1:1. Pure 4a and 5a were isolated by preparative HPLC. Data for 4a: Exact mass calcd for C24H56O2Si4 ([Mþ]) 448.3357, found 448.3348. MS: m/z 448 (Mþ). 1H NMR δ (CDCl3): -0.01 (d, 1H, CH, J = 10.2 Hz), 0.15 (s, 9H, Me3Si), 0.16 (s, 9H, Me3Si), 0.18 (s, 9H, Me3Si), 1.08 (s, 9H, t-Bu), 1.09 (s, 9H, t-Bu), 1.22 (d, 1H, CH, J = 10.2 Hz), 1.28 (s, 9H, t-Bu). 13 C NMR δ (CDCl3): 0.0, 2.7, 3.9 (Me3Si), 16.4 (CH), 32.3, 32.4, 32.4 (Me3C), 32.5 (ring quaternary-C), 34.0, 35.5 (CMe3), 44.1 (CH), 72.4 (OC). 29Si NMR δ (CDCl3): -25.0, -24.2, 3.8, 4.3. (14) Ishikawa, M.; Kumada, M. Adv. Organomet. Chem. 1981, 19, 51.

Naka et al. Data for 5a: Exact mass calcd for C24H56O2Si4 ([Mþ]) 448.3357, found 448.3348. MS: m/z 448 (Mþ). 1H NMR δ (CDCl3): -0.17 (d, 1H, CH, J = 9.9 Hz), 0.14 (s, 9H, Me3Si), 0.15 (s, 9H, Me3Si), 0.23 (s, 9H, Me3Si), 1.00 (s, 9H, t-Bu), 1.04 (s, 9H, t-Bu), 1.28 (s, 9H, t-Bu), 1.43 (d, 1H, CH, J = 9.9 Hz). 13C NMR δ (CDCl3): -0.2, 2.6, 5.9 (Me3Si), 18.9 (CH), 31.4, 31.7, 32.1 (Me3C), 32.2 (ring quaternary-C), 31.5, 35.9 (CMe3), 41.9 (CH), 72.4 (OC). 29 Si NMR δ (CDCl3): -24.4, -24.1, 1.9, 4.2. Photolysis of 2 in the Presence of 10 equiv of tert-Butyl Alcohold1. A solution of 0.302 g (0.73 mmol) of 2 and 0.5430 g (7.24 mmol) of tert-butyl alcohol-d1 in 65 mL of dry hexane was placed in a reaction vessel fitted internally with a low-pressure mercury lamp bearing a Vicor filter. After the solution was irradiated for 4 h, the solvent was evaporated, and the residue was chromatographed on a silica gel column, with hexane as eluent, to give 0.078 g (22% yield) of the mixture of 4b and 5b in a ratio of 1:1. Pure 4b and 5b were isolated by preparative HPLC. Data for 4b: 1H NMR δ (CDCl3): -0.03 (s, 1H, CH), 0.13 (s, 9H, Me3Si), 0.14 (s, 9H, Me3Si), 0.16 (s, 9H, Me3Si), 1.06 (s, 9H, tBu), 1.07 (s, 9H, t-Bu), 1.26 (s, 9H, t-Bu). 13C NMR δ (CDCl3): -0.0, 2.7, 3.9 (Me3Si), 16.3 (CH), 32.3, 32.4, 32.4 (Me3C), 32.6 (ring quaternary-C), 34.0, 35.4 (CMe3), 43.1 (t, CD, J = 20 Hz), 72.4 (OC). 2H NMR δ (CDCl3): 1.23. Data for 5b: 1H NMR δ (CDCl3): -0.19 (s, 1H, CH), 0.13 (s, 9H, Me3Si), 0.14 (s, 9H, Me3Si), 0.22 (s, 9H, Me3Si), 0.98 (s, 9H, t-Bu), 1.02 (s, 9H, t-Bu), 1.26 (s, 9H, t-Bu). 13C NMR δ (CDCl3): -0.2, 2.6, 5.9 (Me3Si), 18.9 (CH), 31.4, 31.7, 32.1 (Me3C), 32.2 (ring quaternary-C), 31.45, 35.9 (CMe3), 40.9 (t, CD, J = 20 Hz), 72.4 (OC). 2H NMR δ (CDCl3): 1.45. Thermolysis of 3. Compound 3 (7.1 mg, 0.017 mmol) in a sealed degassed tube was heated at 250 °C for 7 h. The resulting product was analyzed by 1H NMR spectrometry as being 2 quantitatively. Thermolysis of 2 in the Presence of tert-Butyl Alcohol. A mixture of 2 (0.1474 g, 0.36 mmol) and tert-butyl alcohol (0.1154 g, 1.56 mmol) was heated in a sealed tube at 250 °C for 7 h. Compound 2 was recovered quantitatively. Thermolysis of 3 in the Presence of tert-Butyl Alcohol. A mixture of 3 (0.037 g, 0.088 mmol) and tert-butyl alcohol (0.0227 g, 0.31 mmol) was heated in a sealed tube at 250 °C for 7 h. The resulting product was analyzed by 1H NMR spectrometry as being 2 quantitatively. Thermolysis of 2 in the Presence of Methanol. A mixture of 2 (0.0595 g, 0.14 mmol) and methanol (0.0186 g, 0.58 mmol) was heated in a sealed tube at 250 °C for 7 h. Compound 2 was recovered quantitatively. Thermolysis of 3 in the Presence of Methanol. A mixture of 3 (0.0491 g, 0.12 mmol) and methanol (0.0145 g, 0.45 mmol) was heated in a sealed tube at 250 °C for 7 h. The resulting product was analyzed by 1H NMR spectrometry as being 2 quantitatively.

Acknowledgment. We thank Shin-Etsu Chemical Co. Ltd. for a gift of organochlorosilanes. Supporting Information Available: Cartesian coordinates and total energies for TSs and LMs. This material is available free of charge via the Internet at http://pubs.acs.org.