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Feb 18, 2010 - (Tsi)chlorosilylenoid [Tsi = C(SiMe3)3] reacted with acetaldehyde and benzaldehyde to give 2,4- dioxasilolane and 2,5-dioxasilolane, ...
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Organometallics 2010, 29, 1355–1361 DOI: 10.1021/om9008789

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New Synthetic Routes for Silaheterocycles: Reactions of a Chlorosilylenoid with Aldehydes Young Mook Lim,† Chang Hee Park,† Soo Jin Yoon,† Hyeon Mo Cho,† Myong Euy Lee,*,† and Kyoung Koo Baeck*,‡ †

Department of Chemistry & Medical Chemistry, College of Science and Technology, Yonsei University, Wonju, Gangwondo 220-710, Korea and ‡Department of Chemistry, College of Natural Science, Gangneung-Wonju National University, Gangneung, Gangwondo, 210-702, Korea Received October 9, 2009

(Tsi)chlorosilylenoid [Tsi = C(SiMe3)3] reacted with acetaldehyde and benzaldehyde to give 2,4dioxasilolane and 2,5-dioxasilolane, respectively. The direct addition of an aldehyde to a silaoxirane intermediate is proposed as a plausible mechanism based on a theoretical study. These results show a new synthetic application of the chlorosilylenoid and a new route for the synthesis of silaheterocycles.

Introduction Over the past few decades, a number of publications have appeared on novel silylenoids (R2SiMX), compounds in which an electropositive metal (M) and a leaving group (X) are bound to the same silicon atom.1-5 Recently, a few stable silylenoids have been reported, including TsiSiX2Li6 (Tsi =

C(SiMe3)3, X = Br, Cl), (Mes)2Si(SMes)Li,7 and (R3Si)2SiFLi (R3Si=t-Bu2MeSi).8 Such stable silylenoids bearing a halogen atom, which we reported, have been particularly important in the studies of their various reactivity, such as reduction, substitution, and transmetalation reactions.6 In this context, we now report some unprecedented examples of the reactivity of a halosilylenoid with aldehydes. These reactions of halosilylenoid can be used as new synthetic routes for functional silaheterocycles. In general, heterocycles have efficiently been used in biology, pharmacology, and electronic applications.9 Silicon isosteres of various heterocycles would be regarded as potential candidates for drug discovery and development of medicinal chemistry.10 The reactivities of silylenoids are often similar to those of silylenes. Reactions of silylenes with aldehyde compounds11 have been studied by a few research groups.12 Jutzi and coworkers reported the reaction of decamethylsilicocene with aldehydes such as benzaldehyde and cinnamaldehyde to give the respective dioxasilolane derivatives, in which a silaoxirane formed from [2þ1] cycloaddition of the silylene with CdO was suggested to be a key intermediate.11a The Komatsu group photochemically synthesized dioxasilolanes

*Corresponding authors. (M.E.L.) Tel: 82-33-760-2237. Fax: 82-33760-2182. E-mail: [email protected]. (For theoretical part, K.K.B.) Tel: 82-33-640-2307. Fax: 82-33-640-2264. E-mail: [email protected]. (1) For other experimental studies on silylenoids, see: (a) Oehme, H.; Weiss, H. J. Organomet. Chem. 1987, 319, C16. (b) Boudjouk, P.; Samaraweera, U. Angew. Chem., Int. Ed. Engl. 1988, 27, 1355. (c) Tamao, K.; Kawachi, A. Organometallics 1995, 14, 3108. (d) Kawachi, A.; Doi, N.; Tamao, K. J. Am. Chem. Soc. 1997, 119, 233. (e) Tamao, K.; Kawachi, A.; Asahara, M.; Toshimitsu, A. Pure Appl. Chem. 1999, 71, 393. (f) Wiberg, N.; Niedermayer, W. J. Organomet. Chem. 2001, 628, 57. (g) Driver, T. G.; Franz, A. K.; Woerpel, K. A. J. Am. Chem. Soc. 2002, 124, 6524. (h) Wiberg, N.; Niedermayer, W.; Fischer, G.; Noth, H.; Suter, M. Eur. J. Inorg. Chem. 2002, 1066. (i) Sekiguchi, A.; Lee, V. Y.; Nanjo, M. Coord. Chem. Rev. 2000, 210, 11. (2) For the theoretical studies on silylenoids, see: (a) Feng, S.; Feng, D. J. Mol. Struct. (THEOCHEM) 2001, 541, 171. (b) Feng, S.; Feng, D.; Li, J. Chem. Phys. Lett. 2000, 316, 146. (c) Clark, T.; Schleyer, P. v. R. J. Organomet. Chem. 1980, 191, 347. (d) Tanaka, Y.; Hada, M.; Kawachi, A.; Tamao, K.; Nakatsuji, H. Organometallics 1998, 17, 4573. (e) Feng, S.; Zhou, Y.; Feng, D. J. Phys. Chem. A 2003, 107, 4116. (f) Feng, D.; Xie, J.; Feng, S. Chem. Phys. Lett. 2004, 396, 245. (g) Flock, M.; Marschner, C. Chem.;Eur. J. 2005, 11, 4635. (3) Tamao, K.; Kawachi, A. Angew. Chem., Int. Ed. Engl. 1995, 34, 818. (4) (a) Tokitoh, N.; Hatano, K.; Sadahiro, T.; Okazaki, R. Chem. Lett. 1999, 931. (b) Hatano, K.; Tokitoh, N.; Takagi, N.; Nagase, S. J. Am. Chem. Soc. 2000, 122, 4829. (c) Tajima, T.; Hatano, K.; Sasaki, T.; Sasamori, T.; Takeda, N.; Tokitoh, N.; Takagi, N.; Nagase, S. J. Organomet. Chem. 2003, 686, 118. (5) (a) Tamao, K.; Asahara, M.; Saeki, T.; Toshimitsu, A. Angew. Chem., Int. Ed. 1999, 38, 3316. (b) Tamao, K.; Asahara, M.; Saeki, T.; Toshimitsu, A. J. Organomet. Chem. 2000, 600, 118. (6) (a) Lee, M. E.; Cho, H. M.; Lim, Y. M.; Choi, J. K.; Park, C. H.; Jeong, S. E.; Lee, U. Chem.;Eur. J. 2004, 10, 377–381. (b) Lim, Y. M.; Cho, H. M.; Lee, M. E.; Baeck, K. K. Organometallics 2006, 25, 4960. (c) Lee, M. E.; Lim, Y. M.; Son, J. Y.; Seo, W. G. Chem. Lett. 2008, 680. (7) Kawachi, A.; Oishi, Y.; Kataoka, T.; Tamao, K. Organometallics 2004, 23, 2949. (8) Molev, G.; Bravo-Zhivotovskii, D.; Karni, M.; Tumanskii, B.; Botoshansky, M.; Apeloig, Y. J. Am. Chem. Soc. 2006, 128, 2784.

(9) Joule, J. A.; Mills, K. Heterocyclic Chemistry; Blackwell Science: Oxford, 2000. (10) Showell, D. A.; Mills, J. S. Drug Discovery Today 2003, 8, 551. (11) (a) Jutzi, P.; Eikenberg, D.; Bunte, E.; M€ ohrke, A.; Neumann, B.; Stammler, H. Organometallics 1996, 15, 1930. (b) Sakai, N.; Fukushima, T.; Minakata, S.; Ryu, I.; Komatsu, M. Chem. Commun. 1999, 1857. (c) Sakai, N.; Fukushima, T.; Okada, A.; Ohashi, S.; Minakata, S.; Komatsu, M. J. Organomet. Chem. 2003, 686, 368. (12) For the studies on silylenes with ketones, see: (a) Ando, W.; Ikeno, M.; Sekiguchi, A. J. Am. Chem. Soc. 1977, 99, 6447. (b) Ando, W.; Ikeno, M.; Sekiuchi, A. J. Am. Chem. Soc. 1978, 100, 3613. (c) Ando, W.; Ikeno, M. Chem. Lett. 1978, 609. (d) Ando, W.; Ikeno, M. J. Chem. Soc., Chem. Commun. 1979, 655. (e) Ishikawa, M.; Nishimura, K.; Sugisawa, H.; Kumada, M. J. Organomet. Chem. 1980, 194, 147. (f) Ando, W.; Hamada, Y.; Sekiguchi, A.; Ueno, K. Tetrahedron Lett. 1982, 23, 5323. (g) Ando, W.; Hamada, Y.; Sekiguchi, A. J. Chem. Soc., Chem. Commun. 1983, 952. (h) Belzner, J.; Ihmels, H.; Pauletto, L.; Noltemeyer, M. J. Org. Chem. 1996, 61, 3315. (i) Gehrhus, B.; Hitchcock, P. B.; Lappert, M. F. Organometallics 1997, 16, 4861. (j) Becerra, R.; Cannady, J. P.; Walsh, R. J. Phys. Chem. A 2002, 106, 11558.

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Scheme 1. Possible Intermediates: Silaoxirane (A) and Silacarbonyl Ylides (B and C)

by a trapping reaction of dimesitylsilylene with aldehydes, in which silacarbonyl ylide was a key intermediate.11b In the following we describe the reactions of a chlorosilylenoid with acetaldehyde and benzaldehyde and also theoretical computations to provide reasonable mechanisms considering silaoxirane (A) and silacarbonyl ylide (B and C) as intermediates (Scheme 1).

Results and Discussion Syntheses and Characterizations. Trichloro[tris(trimethylsilyl)methyl]silane (1), containing the bulky Tsi group, was prepared in high yield as described previously.6 LiNp (lithium naphthalenide) in THF was added slowly to compound 1 in THF at -78 °C. The solution was stirred for 12 h at the same temperature; all the starting reactants were consumed to give the chlorosilylenoid 2, which was monitored by GC/MS. To the resulting dark brown solution was added an excess of acetaldehyde at -78 °C, whereupon the solution rapidly became light yellow, indicating that the reaction was complete. Treatment of the reaction mixture with an excess of MeOH gave the trapping product 2,4-dioxasilolane (3) in 83% GC yield as a mixture of four isomers (Scheme 2).13 From the crude material one of the isomers, 3, was isolated by silica gel chromatography (n-hexane) as a colorless oil in 28% yield and was characterized by NMR, GC/MS, EA, and HRMS. Due to the nonsymmetrical structure, 3 showed different resonances for the nonequivalent CH3 (1H NMR; 1.36, 1.40 ppm; 13C NMR 16.4, 22.9 ppm) and CH (1H NMR 3.41, 5.17 ppm; 13C NMR 67.6, 96.3 ppm) in the NMR spectra. The reaction of 2 with benzaldehyde was carried out using similar procedures to those described above. Treatment of the reaction mixture with excess MeOH gave the product 2,5-dioxasilolane (4) in 80% GC yield. After separation using column chromatography, 4 was isolated as a colorless oil in 55% yield and characterized by GC/MS, NMR, EA, and HRMS. Although four isomers of 4 were expected to be formed considering product 3, the NMR spectra of 4 indicated that only one isomer was produced. This may result from the (13) The four isomers appeared in the GC/MS due to the different up and down position of methyl and methoxy substituents in the ring (TsiMe-Me: cis-cis, cis-trans, trans-trans, trans-cis), but unfortunately each isomer could not be isolated individually.

Lim et al. Scheme 2. Reactions of a Chlorosilylenoid with Aldehydes

greater hindrance of the two phenyl groups, leading to formation of a single isomer in the cycloaddition reaction. Interestingly the Si-Cl bond in 4 did not react with MeOH, suggesting severe steric hindrance of this compound. As shown in Scheme 1, the silaoxirane and silacarbonyl ylide generated from the cleavage of the Si-C bond of the silaoxirane are thought to be candidates for key intermediates in these reactions based on the previous works.11,12 However, little theoretical work has been conducted so far to directly investigate the mechanisms involving the silaoxirane and silacarbonyl ylide intermediates. We carried out a theoretical investigation to provide a reasonable mechanism and further insights on new experimental findings. Theoretical Computations and Discussion. The molecular geometry, harmonic vibration frequency, and energy of the transitions-state (TS) structures as well as reactants and products are calculated by using two density-functionaltheory (DFT) methods. The B3LYP14 and the M05-2X15 DFT functional forms implemented in the Gaussian03 suite of programs16 are selected in the present work, because B3LYP is one of the most widely used functional form, whereas M05-2X has recently been claimed to be one of the best functional forms for activation energies.15 Though the standard 6-311þG(2d) basis sets are employed in some test calculations to check the dependence of relative energies on the size of basis sets, the main discussions will be based on the results with the standard 6-31þG(d) basis sets. As will be shown below, the results with the 6-311þG(2d) basis sets did not bring any significant change in both qualitative and quantitative aspects. The tight criteria options of the Gaussian03 are applied to both the SCF iterations and the geometry optimization procedures. Six Cartesian d-type functions are employed in the 6-31þG(d) sets, whereas five spherical d-type functions are used in the 6-311þG(2d) sets. The atomic partial charges are determined by the natural bond orbital analysis (NBO version 3)17 with the electron (14) (a) Becke, A. D. J. Chem. Phys. 1993, 98, 5648. (b) Hermanns, J.; Schmit, B. J. Chem. Soc., Perkin Trans. 1998, 2209. (c) Hermanns, J.; Schmit, B. J. Chem. Soc., Perkin Trans. 1999, 81. (d) Chinchila, R.; Najera, C.; Yus, M. Chem. Rev. 2004, 104, 2667. (e) Lee, C.; Yang, W.; Parr, R. G. Phys. Rev. B 1988, 37, 785. (15) Zhao, Y.; Truhlar, D. G. Acc. Chem. Rev. 2008, 41, 157. (16) Pople, J. A.; et al. et al. Gaussian03, Revision E.01; Gaussian, Inc.: Wallingford, CT, 2004 (see Supporting Information for program's authors ). (17) (a) Carpenter, J. E.; Weinhold, F. J. Mol. Struct. (THEOCHEM) 1998, 169, 41. (b) Reed, A. E.; Curtiss, L. A.; Weinhold, F. Chem. Rev. 1988, 88, 889.

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Figure 1. The model compounds used in theoretical computations. Scheme 3. Proposed Direct Mechanism

Figure 2. Relative energy profile for the reaction with acetaldehyde.

density calculated by the M05-2X/6-311þþG(3df,2pd) method at the geometry optimized by the M05-2X/6-31þG(d) method. The use of the NBO method as well as the very large 6-311þþG(3df,2pd) basis sets provides less variation of the atomic partial charges depending on methodologies. The zero-point energy (ZPE), included in all of the relative energies in the present work, is obtained by the calculations of harmonic frequencies at stationary structures by exactly the same method used in the geometry optimization, and no scaling factor was used. In order to reduce computational resources, the Tsi group of actual experiments is replaced by a C(SiH3)3 group in all of the present computations, and the model molecules with the C(SiH3)3 group will be designated by 5Me, 5Ph, 4Me, and 4Ph, as depicted in Figure 1. The first thing uncovered by the present computation is the fact that the two sila-carbonyl ylide structures, B and C in Scheme 1, are not distinguishable, at least within the level of DFT methods used in the present work. A more important fact is that the silaoxirane structure (A in Scheme 1) is more stable than the sila-carbonyl ylide structure (B and C in Scheme 1); according to the calculations with the model (18) The total energy (including ZPE) of silaoxirane, (H3Si)3C-ClSi-OCHMe with trans-Tsi-Me configurations, is -1815.2829 (-1815.4540) au by the M05-2X/6-31þG(d) (B3LYP/6-31þG(d)) method, and the corresponding silacarbonyl ylide has 23.7 (19.8) kcal/mol higher energy. The energy of silaoxirane, (H3Si)3C-ClSi-O-CHPh with trans-Tsi-Ph configuration, is -2006.9620 (-2007.1455) au by the M05-2X/6-31þG(d) (B3LYP/6-31þG(d)) method, and the corresponding sila-carbonyl ylide has 23.2 (16.4) kcal/mol higher energy. The energy difference between the two isomers (cis- and trans-Tsi-R) of silaoxirane is less than 2 kcal/mol for both R = Me and R = Ph cases, whereas the difference between the two isomers of sila-carbonyl ylide is about 3 and 6 kcal/mol for R = Me and R = Ph, respectively. Detailed structures, energies, and atomic coordinates of silaoxiranes and sila-carbonyl ylides (trans-Tsi-R isomer) are given in the Supporting Information.

compounds with a C(SiH3)3 group instead of a Tsi group, sila-carbonyl ylide is about 20 kcal/mol higher energy than silaoxirane (more details are given elsewhere).18 Considering the low temperature (-78 °C) of the present experiments, a mechanism based on the silaoxirane seems more reasonable than that based on sila-carbonyl ylide species. Moreover, all of our efforts to generate computational results supporting any plausible mechanism starting from sila-carbonyl ylide were in vain. Therefore, we propose the following simpler mechanism: the direct addition of aldehyde to silaoxirane, as shown in Scheme 3. In order to study the possibility and plausibility of the direct addition mechanism, the molecular structures and energies of reactants (silaoxirane and aldehyde), possible products (2,5-dioxasilolane or 2,4-dioxasilolane), and the corresponding TS structures were calculated, and the energetic relationship is depicted in Figures 2 and 5 for the reactions of 2 with acetaldehyde and benzaldehyde, respectively. The characteristics of the TSs are confirmed not only by observing the one imaginary vibration frequency but also by obtaining the structures of the initial encounters of reactants as well as the final products, as will be discussed in detail below. As shown in Figure 2, the reaction energy from reactants to 5Me is calculated to be -49.6 (-35.4) kcal/mol by the M052X/6-31þG(d) (B3LYP/6-31þG(d)) method, and the magnitude is reduced a little by using a larger basis set: -48.5 (-33.5) kcal/mol by the M05-2X/6-311þG(2d) (B3LYP/6311þG(2d)) method. Meanwhile, the exothermic energy to form 4Me is calculated to be -81.3 (-65.0) kcal/mol by the M05-2X/6-31þG(d) (B3LYP/6-31þG(d)) method, and the magnitude is almost not changed even with a larger basis set: -81.4 (-65.0) kcal/mol by the M05-2X/6-311þG(2d) (B3LYP/6-311þG(2d)) method. The exothermic energy of 4Me is much larger than (almost twice of) that of 5Me, which implies that the cleavage of the Si-C bond of silaoxirane is more favored thermodynamically, as expected.11,12 The experimental results, however, contradict the above; the 2,5-dioxasilorane corresponding to 4Me was not produced, but 3 is actually obtained after the reaction of MeOH with the compound corresponding to 5Me. Because the

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Figure 3. Interatomic distances (in pm) and atomic charges (in e) of TS-5Me (left) and its initial associate of reactants (right). The normal mode displacement vectors of the imaginary frequency of TS-5Me are shown by black arrows.

Figure 4. Interatomic distances (in pm) and atomic charges (in e) of TS-4Me (left) and its initial associate of reactants (right).

thermodynamic viewpoint is in contrast with actual experimental results, the kinetic aspect, i.e., the reaction barrier, has to be investigated. The activation energy (ΔE0q) from the reactants (silaoxirane and acetaldehyde) to the transition state (TS-5Me) leading to 2,4-dioxasilolane 5Me is calculated to be 8.6 (18.6) kcal/mol by the M05-2X/6-31þG(d) (B3LYP/ 6-31þG(d)) method. The value is not much changed by increasing the size of the basis set to a larger triple-ζ quality set; ΔE0q(TS-5Me) is 9.1 (19.6) kcal/mol by the M05-2X/ 6-311þG(2d) (B3LYP/6-311þG(2d)) method. The other activation energy to TS-4Me leading to 2,5-dioxasilolane 4Me is 10.9 (21.6) kcal/mol by the M05-2X/6-31þG(d) (B3LYP/6-31þG(d)) method, and the value is almost not (19) Total energies (including ZPE) of acetaldehyde, (H3Si)3C-ClSiO-HMe (silaoxirane with trans-(H3Si)3C-Me), TS-5Me, 5Me, TS4-Me, and 4Me are -153.7476 (-153.7841), -1815.2829 (-1815.4540), -1969.0167 (-1969.2085), -1969.1096 (-1969.2944), -1969.0132 (-1969.2037), and -1969.1600 (-1969.3417), respectively, by the M052X/6-31þG(d) (B3LYP/6-31þG(d)) method. The same values by the M05-2X/6-311þG(2d) (B3LYP/6-311þG(2d)) method are -153.7894 (-153.8241), -1815.4758 (-1815.6313), -1969.2505 (-1969.4241), -1969.3419 (-1969.5087), -1969.2482 (-1969.4213), and -1969.3949 (-1969.6043), respectively. Detailed structures are given in the Supporting Information.

changed by the increase of the basis set size; ΔE0q(TS-4Me) is 10.8 (21.4) kcal/mol by the M05-2X/6-311þG(2d) (B3LYP/ 6-311þG(2d)) method. The main features of the TS-5Me and the TS-4Me, without H atoms for clarity, are shown in the left part of Figures 3 and 4; more details of the structures and energies of the TSs and products are given elsewhere.19 According to Figure 2, ΔE0q(TS-5Me) is 2-3 kcal/mol lower than ΔE0q(TS-4Me). The energetic relationship shown in Figure 2, however, is just one of several possible cases because there are four isomers of 4Me. If the other stereoisomers of the TS-5Me and the TS-4Me shown in Figures 3 and 4 are used in the calculations, the magnitudes of these ΔE0q are slightly different, and the ΔE0q(TS-5Me) is 3-4 kcal/mol lower than the ΔE0q(TS-4Me). The results summarized in Figure 2 correspond to the stereoisomer giving the smallest difference between ΔE0q(TS-5Me) and ΔE0q(TS-4Me). The simple sum of the energies of the two separated reactants is taken as the reference in the calculations of the activation energies, and the difference between ΔE0q of the same TS by the M05-2X and the B3LYP is 10.0 and 10.7 kcal/mol for Ts-5Me and TS-4Me, respectively. If the energy of the initial associates of reactants is taken as the reference of the activation barriers, then the above dependence

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of ΔE0q on the DFT functional form is reduced to about 5 kcal/mol, but the difference between ΔE0q(TS-5Me) and ΔE0q(TS-4Me) does not depend on the choice of the reference. However, the configuration of the initial associate for TS-5Me (the right part of Figure 3) differs from that for TS-4Me (the right part of Figure 4). The interaction between the O atom of aldehyde and the Si atom of the silaoxirane ring seems to be the main interaction forming both of the initial associates of TS-5Me and TS-4Me. The initial associate of TS-5Me has an additional interaction between the H atom of the aldehyde and the O atom of the silaoxirane, whereas that of TS-4Me has the additional interaction between the H atom of the aldehyde and the Cl atom of the silaoxirane. The initial associate of TS-5Me was calculated to be 2.4 (0.8) kcal/mol lower than the initial associate of TS-4Me by the M05-2X/6-31þG(d) (B3LYP/6-31þG(d)) method, and the magnitude slightly reduced to 2.1 (0.7) kcal/mol by using the larger 6-311þG(2d) basis sets. The lower energy of the initial associate of TS-5Me could be an additional advantage of the reaction path via TS-5Me, but several delicate considerations have to be noted about the degree of the advantage, and such complicated and too detailed considerations are refrained from here, partially because all such details do not change any qualitative aspect given below. In spite of some variations of the activation energies depending on the functional form of DFT, the size of the basis sets, and the choice of the reference of the relative energies, the important point is the fact that ΔE0q(TS-5Me) is at least 2 kcal/mol lower than ΔE0q(TS-4Me) regardless of the methodology. This means that 5Me is kinetically more favorable than 4Me, while 4Me is thermodynamically more stable than 5Me. Though the magnitude of the difference (2 kcal/mol) is not large enough to be conclusive evidence for the domination of the kinetic factor over the thermodynamic factor, it can be one possible explanation for the actual experimental results because of the very low reaction temperature, -78 °C. Considering the significant yields (∼80%) of rapidly completed reactions at such a low reaction temperature, the activation barrier computed by the B3LYP functional (18.6 kcal/mol) is rather too high, whereas the magnitude by the M05-2X functional (8.6 kcal/mol) seems more reasonable. As an important characteristic of TS structures, only one imaginary frequency is observed in the TS structures; the values of the imaginary frequencies of TS-5Me and TS-4Me by the M05-2X/6-31þG(d) (B3LYP/6-31þG(d)) method are 390i (335i) and 368i (365i) cm-1, respectively. The magnitudes of the imaginary frequency are not much changed even by calculations with a larger size of basis sets: 385i (332i) and 375i (353i) cm-1 by the M05-2X/6-311þG(2d) (B3LYP/ 6-311þG(2d)) method for TS-5Me and TS-4Me, respectively. The normal mode displacement vectors of the imaginary frequency, represented by arrows in Figures 3 and 4 for TS-5Me and TS-4Me, respectively, show that the nuclear motion along the imaginary frequency would lead to the product structure 5Me and 4Me, respectively. We actually confirmed that not only the corresponding product 5Me or 4Me but also the initial associate structures of reactants are obtained by the geometry optimization after a few steps of forward and backward nuclear movements along the intrinsic reaction coordinate (IRC) corresponding to the imaginary frequency. It is now clear that TS-5Me and TS-4Me are the real transition-state structures leading to 5Me and 4Me,

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Figure 5. The relative energy profile for the reaction with benzaldehyde.

Figure 6. Structure, atomic charges, and normal mode displacements of TS-5Ph (left) and its initial associate of reactants (right).

Figure 7. Structure, atomic charges, and normal mode displacements of TS-4Ph (left) and its initial associate of reactants (right).

respectively, by a single-step addition of acetaldehyde to silaoxirane. The energetic relationship shown in Figure 2 also provides an explanation why not 5 but 3 is the final product in the actual experiment; 5Me is thermodynamically not stable enough compared with 4Me, and therefore 5Me converts to 3 by the reaction with MeOH. The magnitude of the further stabilization energy due to the reaction of 5Me with MeOH is not actually computed here due to the difficulties and uncertainties in the calculation of the solvated states of Hþ and Cl- ions expected to be generated in actual reaction media. The thermodynamic and kinetic aspects of the direct addition mechanism of benzaldehyde to silaoxirane are also

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Figure 8. Molecular structure (without H atoms) and atomic charges of two silaoxiranes.

calculated by the same methods as used in the above acetaldehyde case, but only the 6-31þG(d) basis sets are used here, partially because the 6-311þG(2d) sets are too large for compounds containing two phenyl groups and partially because the changes caused by using the larger basis sets are not expected to be significant, as seen in the above case of acetaldehyde. The energetic relationship shown in Figure 5 uncovers different features compared with those in Figure 2. In the case of benzaldehyde, the ΔE0q of TS-4Ph is about 2-3 kcal/mol lower than the ΔE0q of TS-5Ph, and 4Ph is also more stable than 5Ph. In other words, the reaction path to 4Ph is both kinetically and thermodynamically more favorable than the path to 5Ph. Both the thermodynamic and the kinetic aspects are in accordance with the fact that 4 is the major product in actual experiments. The large magnitude of the exothermic reaction energy (-77.6 kcal/mol by the M05-2X/6-31þG(d) method) in the formation of 4Ph also provides an explanation why 4 does not undergo any further reaction with MeOH; 4 is stable enough and does not need a further stabilization, whereas 5 is not stable enough and needs a conversion to 3 by the reaction with MeOH in actual experiments. The main features of the TS-5Ph and the TS-4Ph structures are shown in Figures 6 and 7. The only imaginary frequencies of TS-5Ph and TS-4Ph by the M05-2X (B3LYP) method are 236i (207i) and 405i (369i) cm-1, respectively. The normal mode displacement vectors of the imaginary frequency of TS-5Ph and TS-4Ph also show proper nuclear motions, leading to the product structure 5Ph and 4Ph, respectively. The geometries of 5Ph, 4Ph, and the initial associates of reactants are also obtained by the direct geometry optimization after a few steps of forward and backward nuclear movements along the intrinsic reaction coordinate of the imaginary frequency of the corresponding TS structure. The molecular structures of the initial associates in Figures 6 and 7 are basically the same as the corresponding ones in Figures 3 and 4. More details of the energies and structures are given elsewhere.20 It is clear that the TS-5Ph and TS-4Ph are the real TS structures leading to 5Ph and 4Ph, respectively, by the mechanism of a single step addition. One of the most noticeable change from Figure 2 to Figure 5 is the big stabilization of TS-4Ph; ΔE0q(TS-5Ph) (20) Total energies (including ZPE) of benzaldehyde, (H3Si)3C-ClSiO-HPh (silaoxirane with trans-(H3Si)3C-Ph), TS-5Ph, 5Ph, TS-4Ph, and 4Ph are -345.4280 (-345.4788), -2006.9620 (-2007.1455), -2352. 3785 (-2352.5971), -2352.4610 (-2352.6681), -2352.3807 (-2352. 6020), and -2352.5136 (-2351.7181), respectively by the M05-2X/631þG(d) (B3LYP/6-31þG(d)) method. Detailed structures are given in the Supporting Information.

is 1.4 kcal/mol lower than ΔE0q(TS-5Me), whereas ΔE0q(TS4Ph) is 6.1 kcal/mol lower than ΔE0q(TS-4Me). The big change causes the totally different experimental results for acetaldehyde and benzaldehyde. The partial atomic charges in Figures 3, 4, 6, and 7 provide a clue where the difference stems from. The partial charges of TS-5Me are not much different from those of TS-5Ph; the magnitudes of atomic charge (in units of electron) of O (of aldehyde), C (carbonyl carbon of aldehyde), O (of silaoxirane), and C (of silaoxirane ring) in TS-5Me are -0.80, þ0.60, -0.94, and -0.35, respectively, whereas the values in TS-5Ph are -0.80, þ0.59, -0.91, and -0.37, respectively. The changes from TS-5Me to TS-5Ph are almost negligible in spite of the big difference between methyl and phenyl groups. Due to this small effect, the magnitude of ΔE0q(TS-5) is not much changed, just 1.4 kcal/mol decreased by substituting Me with Ph. On the other hand, the partial charges of TS-4Me show a noticeable difference from those of TS-4Ph; the values are -0.74, þ0.46, -0.31, and -0.93 in TS-4Me and -0.80, þ0.63, -0.34, and -0.92 in TS-4Ph. By substituting the methyl with phenyl, the positive charge on the carbonyl C of aldehyde increased from þ0.46 to þ0.63, while the negative charge on the C of the silaoxirane ring is also slightly intensified from -0.31 to -0.34. The phenyl group seems to stabilize both the positive and negative charge by the resonance effect of the phenyl ring, and the activation energy is decreased from 10.9 to 4.8 kcal/mol by the effect. The partial charges of the oxygen and the carbonyl carbon of an isolated acetaldehyde, calculated by the NBO-M05-2X/6-311þþG(3df,2pd)//M05-2X/ 6-31þG(d) method, are -0.53 and þ0.45, respectively, and the values remain almost the same even when the methyl is replaced by phenyl, i.e., -0.54 and þ0.43, respectively, in benzaldehyde. Figure 8 also shows that the bond lengths and partial charges on the O and C atoms of the silaoxirane ring remain almost the same even when the methyl is replaced by phenyl. Therefore, the difference between the partial charge in TS-4Me and the corresponding value in TS-4Ph can be regarded as the result of different effects of the methyl and the phenyl group on the developed partial charges during the reaction processes. According to the above analyses on atomic charges, the methyl group cannot stabilize the developing partial atomic charges, whereas the phenyl group can in the reaction path to 2,5-dioxasilolane (4Me/4Ph). In the reaction path to 2,4-dioxasilolane (5Me/5Ph), on the other hand, the location and the big negative charge of the O atom of the silaoxirane (see Figures 3 and 6) seem to play a dominating role in TS-4Me and TS-4Ph, but the methyl (phenyl) group of the silaoxirane ring is not directly connected to the reacting O atom. As a result, the effect of the phenyl group is rather small in this case, and ΔE0q(TS-5) is reduced by just 1.4 kcal/mol by

Article

Organometallics, Vol. 29, No. 6, 2010

replacing methyl with phenyl. This implies that the effect of any change of alkyl (aryl) group of aldehyde on ΔE0q(TS-5) would be small, whereas the effect on ΔE0q(TS-4) would be large, which will be studied in future experimental and theoretical works. Although the effects of reaction media are not included in the present calculations, the effects are expected to be very small because the dielectric constant of THF, the main reaction medium of the actual experiments, is very small. The relative energies of actual compounds with a Tsi group may be a little different from the calculated results with the model compounds of the present calculations. In spite of such approximations, our theoretical results depicted in Figures 2 and 5 strongly suggest that an addition mechanism of aldehydes to silaoxirane is a very plausible one.

Summary and Conclusions The reaction of (Tsi)chlorosilylenoid 2 with acetaldehyde and benzaldehyde gave the corresponding products, 2,4-dioxasilolane (3) and 2,5-dioxasilolane (4), respectively. This first example of the reaction of the silylenoid with aldehydes showed a synthetic application of the chlorosilylenoid as a new route for the synthesis of functional silaheterocycles. To investigate the possible pathway for the formation of the silaheterocycles, theoretical computations were carried out by using the B3LYP and the M05-2X DFT functional forms. From the theoretical results, the direct addition of aldehyde to the silaoxirane intermediate, shown in Scheme 3, is proposed as a plausible mechanism to explain the experimental facts, and the theoretical results supporting the mechanism are depicted in Figures 2 and 5. The production of 2,5-dioxasilolane by the cleavage of the Si-C bond of silaoxirane seems to be more natural result because the reaction is both thermodynamically and kinetically favored over the formation of 2,4-dioxasilolane by the cleavage of the Si-O bond. In this context, the production of 5Me is a rather exceptional result in a sense, but we think that the main reason for the exception seems to be because a single methyl group of acetaldehyde is not enough to stabilize the partial charges developed at the reacting atoms in the reaction path toward 2,5-dioxasilolane. Further experimental and theoretical studies to support this mechanism or to suggest another one surely deserve to be conducted in the near future, but our mechanistic studies provide experimentally and theoretically consistent, reasonable, and insightful understandings.

Experimental Section General Considerations. In all reactions in which air-sensitive chemicals were used, the reagents and solvents were dried prior to use. THF was distilled from Na/Ph2CO. Other starting materials were purchased as reagent grade and used without further purification. Glassware was flame-dried under nitrogen or argon flushing prior to use. All manipulations were performed using standard Schlenk techniques under a nitrogen or argon atmosphere. 1H NMR and 13C NMR spectra were recorded on a Bruker Avance IIþ BBO 400 MHz S1 spectrometer in CDCl3. Analyses of product mixtures were accomplished using an HP 5890 II Plus instrument with FID (HP-5, 30 m column) with dried n-decane as an internal standard. Mass spectra were recorded on a low-resolution (Agilent Technologies GC/MS: 6890N, 5973N mass selective detector) EI mass

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spectrometer and a high-resolution (JEOL JMS-600W-Agilent 6890 Series) instrument. The preparation of trichloro[tris(trimethylsilyl)methyl]silane, 1, and chlorosilylenoid 2 was reported in previous papers.7 Synthesis of 3. Naphthalene (1.28 g, 10 mmol) dissolved in THF (40 mL) was added to Li (0.060 g, 8.7 mmol) at room temperature. After stirring for 3 h at room temperature, LiNp was obtained as a dark green solution. LiNp in THF was added slowly to TsiSiCl3 (1.46 g, 4.0 mmol) in THF (40 mL) at -78 °C using a cannula technique within 10 min. The solution was stirred for 12 h at the same temperature; all the starting reactants were consumed to give the chlorosilylenoid 2.7 To the dark brown reaction mixture was added an excess of acetaldehyde (2.2 mL, 39 mmol) to the reaction products at -78 °C, whereupon the solution rapidly became light yellow. Then treatment of the reaction mixture with an excess of MeOH gave the trapping product 3 (in 83% GC yields). The reaction mixture was slowly warmed to room temperature, and then the solvent was evaporated under reduced pressure. After the sublimation of naphthalene, to the resulting viscous light yellow oil was added n-hexane (100 mL), and then precipitated species were removed by filtration. The crude material was purified by silica gel chromatography (n-hexane) to afford 3 as a colorless oil in 28% yield (0.42 g, 1.1 mmol). Compound 3 was characterized by NMR, GC/MS, EA, and HRMS. 1H NMR (400 MHz, chloroform-d): δ 0.25 (s, 27H), 1.36 (d, J = 4.8, 3H), 1.40 (d, J = 7.2, 3H), 3.41 (q, J = 7.2, 1H), 3.68 (s, 3H), 5.17 (q, J = 4.8, 1H). 13C NMR (100 MHz, chloroform-d): δ 1.0 (C(Si(CH3)3)3), 4.5 (SiCH3), 16.4 (CH3), 22.9 (CH3), 51.1 (SiOCH3), 67.6 (CHCH3), 96.3 (CHCH3). GC/MS: m/z (%) 363 (Mþ - 15, 1.9), 305 (7.6), 275 (100), 261 (37.6), 189 (23.6), 129 (14.3), 73 (35.8). HRMS: C14H35O3Si4 363.1663 (calcd), 363.1662 (found). Anal. Calcd for C15H38O3Si4: C, 47.56; H, 10.11; O, 12.67. Found: C, 47.46; H, 10.01; O, 12.74. Synthesis of 4. The product mixture of chlorosilylenoid was trapped by benzaldehyde (4.0 mL, 39 mmol) in a similar manner to those described above to give compound 4 (80%, GC yield). We carried out the workup procedures in a similar manner to those used in the trapping experiment described above. The crude material was purified by silica gel chromatography (n-hexane) to afford 4 as a colorless oil in 55% yield (1.2 g, 2.2 mmol). Compound 4 was characterized by NMR, GC/MS, EA, and HRMS. 1H NMR (400 MHz, chloroform-d): δ 0.43 (s, 27H), 4.86 (d, J = 9.3, 1H), 5.07 (d, J = 9.3, 1H), 7.15-7.35 (m, 10H). 13C NMR (100 MHz, chloroform-d): δ 4.5 (SiCH3), 7.0 (C(Si(CH3)3)3), 82.6, 84.3 (CH), 126.6, 127.5, 127.9, 128.2, 128.2 (ph-CH), 137.9, 139.2 (ph-ipso-C). GC/MS: m/z (%) 506 (Mþ, 3.1), 491 (21.2), 295 (100), 187 (37.8), 179(53.0), 90 (29.8), 73 (71.5). HRMS: C24H39ClO2Si4 506.1716 (calcd), 506.1715 (found). Anal. Calcd for C24H39ClO2Si4: C, 56.82; H, 7.75; O, 6.31. Found: C, 56.75; H, 7.56; O, 6.35.

Acknowledgment. This work was supported by the Korea Research Foundation Grant funded by the Korean Government (No. KRF-2007-521-C00149). The authors thank Professor Robert West, distinguished professor in the WCU (World Class University) Program through the National Research Foundation of Korea funded by the Ministry of Education, Science and Technology for his valuable discussions. Supporting Information Available: Molecular structures including H atoms, total energies including zero-point energy (ZPE), and atomic xyz-coordinates produced by computations. These materials are available free of charge via the Internet at http://pubs.acs.org.