The quest for a stable silyne, RSi ≡ CR′. The effect of bulky substituents 
Rent the article at a discountRent now
* Final gross prices may vary according to local VAT.Get Access
The two major fundamental obstacles which so far have prevented theisolation of stable silynes, RSi≡CR′ (1), are: (a)the existence of more stable isomers, e.g., RR′C=Si: (2) and(b) their extremely facile (exothermic) dimerication. The steric andelectronic effects of various substituents R and R′ (R = alkoxy,alkyl, aryl and silyl; R′ = alkyl and aryl groups) on the stability ofRSi≡CR′ relative to the isomeric RR′C=Si:(ΔE(1-2)), and on the energy of dimerization tothe corresponding 1,3-disilacyclobutadienes (ΔE(D)), werestudied computationally using density functional theory (DFT) and theONIOM method. The goal was to find a combination of substituents thatwill make RSi≡CR′ more stable than RR′C=Si: and whichwill also prevent its dimerization. For R = R′ = H,ΔE(1-2)) = 40.7 kcal/mol (i.e., 2 islower in energy than 1), and ΔE(D) = −104.0kcal/mol. 1, R = OH, R′ = m-Tbt ≡2,6-bis[bis(silyl)methyl]phenyl, is by 11.1 kcal/mol morestable than the isomeric silylidene 2. However, thedimerization of 1, R = OH, R′ = m-Tbt remains highlyexothermic (by 101 kcal/mol). 1, R = R′ = m-Tbt and1, R = (t-Bu)3Si, R′ = m-Tbt, are by 5.8 and 2.0kcal/mol, respectively, less stable than the corresponding 2.However, the dimerization of 1, R = (t-Bu)3Si, = m-Tbt is exothermic by only 12 kcal/mol. For1, R = (t-Bu)3Si, and R′ = Tbt′ ≡2,6-bis[bis(trimethylsilyl)methyl]phenyl, the corresponding1,3-disilacyclobutadiene dimer 3, dissociates spontaneously.Thus, (t-Bu3Si)Si≡CTbt′ is predicted to be kineticallystable towards both, isomerization to (t-Bu3Si)Tbt′C=Si: anddimerization to 3, making it a viable synthetic target. Thereported energies were calculated atB3LYP/6-31G**//B3LYP/3-21G*; good agreement is found betweenthe DFT and the ONIOM results.
- Reported in part at the 12th International Symposium on Organosilicon Chemistry, May 23–28, 1999, Sendai, Japan; Book of Abstracts, p. 197 and at the 5th World Congress of Theoretically Oriented Chemists, WATOC'99, August, 1–6, 1999, London, U.K.; Book of Abstracts, p. 367.
- Brook, A.G., Abdesaken, F., Gutekunst, B., Gutekunst, R. & Kallury, K. 1981 J. Chem. Soc., Chem. Commun., 191.
- West, R., Fink, M.J. & Michl, J. 1981 Science 214, 1343.
- For comprehensive reviews, see: (a) The Chemistry of Organic Silicon Compounds, ed. S. Patai & Z. Rappoport. Wiley, New York, 1989. E.g., Apeloig, Y., p. 57; Raabe, G. & Michl, J., p. 1015; (b) The Chemistry of Organic Silicon Compouns,Vol. 2, ed. Z. Rappoport & Y. Apeloig. Wiley, New York, 1998. E.g., Sakurai, H., p. 827; Müller, T., Ziche, W. & Auner, N., p. 857; Tokitoh, N. & Okazaki, R., p. 1063; (c) Brook, A.G. & Brook, M.A. 1996 Adv. Organomet. Chem. 39, 71; (d) Driess, M. 1996 Adv. Organomet. Chem. 39, 193; (e) Okazaki, R. & West, R. 1996 Adv. Organomet. Chem. 39, 232.
- Maier, G. & Glatthaar, J. 1994 Angew. Chem., Int. Ed. Eng. 33, 473.
- The nature of the Si-N bond in RNSi compounds has still to be determined. (a) Bock, H. & Dammel, R. 1985 Angew. Chem., Int. Ed. Engl. 24, 111; (b) Elhanine, M., Farrenq, R. & Guelachvili 1991 J. Chem. Phys. 94, 2529; (c) Bogey, M., Demuynck, C., Destombes, J.L. & Walters, A. 1991 Astron. Astrophys. 244, 247; (d) Radziswski, J.G., Littmann, D., Bal-aji, V., Fabry, L., Gross, G. & Michl, J. 1993 Organometallics 12, 4816; (e) Goldberg, N., Iraqi, M., Hrusak, J. & Schwarz, H. 1993 Int. J. Mass Spectrum. Ion Processes 125, 267.
- Sekiguchi, A., Zigler, S.S., West, R. & Michl, J. 1986 J. Am. Chem. Soc. 108, 4241.
- Pietschnig, R., West, R. & Powell, D.R. 2000 Organometallics 19, 2724.
- Karni, M., Apeloig, Y., Schröder, D., Zummack, W., Rabezzana, R. & Schwarz, H. 1999 Angew. Chem., Int. Ed. 38, 332.
- Apeloig, Y. & Karni, M. 1997 Organometallics 16, 310.
- (a) Gordon, M.S. & Pople, J.A. 1981 J. Am. Chem. Soc. 103, 2945; (b) Gordon, M.S. 1982 J. Am. Chem. Soc. 104, 4352; (c) Hopkinson, A.C., Lien, M.H. & Csizmadia, I.G. 1983 Chem. Phys. Lett. 95, 232; (d) Hoffman, M.R., Yoshioka, Y. & Schaefer III, H.F. 1983 J. Am. Chem. Soc. 105, 1084; (e) Luke, B.T., Pople, J.A., Krogh-Jespersen, M.-B., Apeloig, Y., Karni, M., Chandrasekhar, J. & Schleyer, P.v.R. 1980 J. Am. Chem. Soc., 108, 270; (f) Colgrove, B.T. & Schaefer III, H.F. 1990 J. Phys. Chem. 94, 5593; (g) Grev, R. & Schaefer III, H.F. 1992 J. Chem. Phys. 97, 7990; (h) Jursic, B.S. 1999 J. Mol. Struct., 459, 221.
- (a) Nguyen, M.T., Sengupta, D. & Vanquickborne, L.G. 1995 Chem. Phys. Lett. 244, 83; (b) Stegmann, R. & Frenking, G. 1996 J. Comp. Chem. 17, 781.
- We have already pointed out this possibility in Ref. 10; Recent theoretical studies have shown that with very bulky substitu-ents, disilynes (RSi ≡SiR) are more stable than their isomeric silylidene (RR'si = Si:) [13a,b] and that they can also be stabilized towards dimerization [13c]. (a) Kobayashi, K. & Nagase, S. 1997 Organometallics 16, 2489; (b) Nagase, S., Kobayashi, K. & Takagi, N. 2000 J. Organomet. Chem. 611, 264; (c) Kobayashi, N., Takagi, N. & Nagase, S. 2001 Organo-metallics 20, 234; (d) Takagi, N. % Nagase, S. 2001 Chemistry Lett. 966.
- References(a) The calculations were performed using the Gaussian 98 package of programs: Gaussian 98, Revision A.7, Gausian, Inc., Pittsburgh PA, 1998; (b) Structures were optimized and frequencies and zero point vibrational energies were calculated using the hybrid-density functional method, with the B3LYP functional and the 3-21G* basis set (B3LYP/3-21G*). Single point energies at these geometries were calculated at B3LYP/6-31G**. B3LYP/6-31G**//B3LYP/3-21G* was the most elaborate level which we could use for the entire set of molecules. For small and medium size systems the reaction energies calculated at B3LYP/6-31G**//B3LYP/6-31G** are very similar to those calculated using B3LYP/3-21G* geometries, used for the larger systems. For some of the small systems, we have also used configuration interaction calculations at the QCISD(T)//6-31G**//QCISD/6-31G** level, and these energies served to calibrate the energies calculated for the larger systems. References to the computational methods and basis sets are given in Ref. 14a; (c) The isomerization and dimerization emergies of silynes substituted with very bulky substituents were calculated with the ONIOM method [14d]. The ONIOM method can be described as an extrapolation scheme where different levels of theory are applied to different parts of the molecule; e.g., in ONIOM(B3LYP/6-31G**:HF/STO-3G), the electronic active inner-layer (‘model’) is optimized at the B3LYP/6-31G** level, while the entire molecule (‘real’), i.e., inner-layer + outer-layer, is calculated at the lower HF/STO-3G level; (d) Daprich, S., Komáromi, I., Byun, K.S., Morokuma, K. & Frisch, M.J. 1999 J. Mol. Struct. 461–462,1.
- m-Tbt models the significantly larger Tbt group (Scheme 1) which was used successfully in the recent synthesis of sev-eral stable doubly-bonded silicon and germanium compounds. See: (a) Suzuki, H., Tokitoh, N., Okazaki, R., Nagase, S. & Goto, M. 1998 J. Am. Chem. Soc. 120, 11096; (b) Wakita, K., Tokitoh, N., Okazaki, R., Nagase, S., Schleyer, P.v.R. & Jiao, H. 1999 J. Am. Chem. Soc. 121, 11336; (c) Tokitoh, N., Kishikawa, K. & Okazaki, R. 1999 Phosphorous, Sulfur Silicon Relet. Elem. 150-151, 137; (d) Tokitoh, N. 1999 Pure Appl. Chem. 71, 495.
- The ‘real’ system is the entire molecule and it is calculated in the lower level of theory. For the two layer ONIOM calculations the inner layer, i.e., the ‘model’ was chosen as follows: for t -BuSi≡C (m-Tbt), the ‘model’ is H3CSi≡CPh; for t -Bu3SiSi≡(m-Tbt) it is H3SiSi≡CPh; for (m-Tbt) the ‘model’ is PhSi≡CPh. For the corresponding dimers, 3, the ‘model’ was chosen as the dimer of the ‘model’ system of the corresponding silynes, e.g., for 3,R = t -Bu3Si, R′ = m-Tbt, the model is 3,R = SiH3, R′ = Ph. For 3,R = t-Bu3Si, R′ =Tbt′ a three layer ONIOM was used: the most inner layer is 3, R = R′ = H calculated at B3LYP/6-31G**; the second, ‘intermediate model’ is 3,R = H3Si; R′ = Ph, Calculated at HF/3-21G *and the ‘real’ system, 3,R = t-Bu3Si, R′ =Tbt′, was calculated using the Dreiding force field implemented in G98 [14a].
- (a) For all alkyl and aryl substituents, ΔE(1-2) is smaller when the alkyl substituent is attached to the Si-end of the silyne and the aryl to the C-end of the silyne. (b) ΔE(1-2) for R = m-Tbt, R′ = t-Bu is higher, 19.8 kcal/mol [17a].
- This substituent was used successfully in the synthesis of a stable tetrasilatetrahedrane: Wiberg, N., Finger, C.M.M. & Polborn, K. 1993 Angew. Chem. Int. Ed. Engl. 32, 1054.
- The large effects of R = R′ = m-Tbt and R = t-Bu3Si, R′ = m-Tbt on ΔE (1-2) (Table 1), result from a cooperative effect: stabilization of the silyne by the aryl group and destabilization of the silylidene by the steric repulsions between R and R′ which amounts (according to isodesmic Equation 2) to ca.8.2 kcal/mol (11.7 kcal/mol, ONOIM) for R = R = m-Tbt and to 17.8 kcal/mol (ONIOM) for R = t-Bu3Si; R′ = m-Tbt,RR′C = Si:+ H2C = Si: –→ RHC = Si: + R′HC = Si: (2)
- The right reliability of the ONIOM(B3LYP/6-31G**:HF/STO-3G) calculations is clearly evident from the data in Tables 1 and 2, i.e., the differences between the ONIOM and B3LYP/6-31G** results for ΔE(1-2)and ΔE(D) are small. This applies also to the calculated geometries of the dimers. A detailed comparision between the ONIOM and B3LYP calculations will be presented elsewhere.
- Our study cannot however, exclude the possibility that other reactions such as insertions , or H-abstraction would com-plicate the isolation of the silyne. We are currently studying computationally these reactions.
- (a) Su, J., Li, X.-W., Critterdon, C. & Robinson, G.H. 1997 J. Am. Chem. Soc. 119, 5471; (b) Pu, L., Senge, M.O., Olmstead, M.M. & Power, P.P. 1998 J. Am. Chem. Soc. 120, 12682; (c) Pu, L., Twamley, B. & Power, P.P. 2000 J. Am. Chem. Soc. 122, 3524.
- White, D.P., Antony, J.C. & Oyefeso, A.O. 1999 J. Org. Chem. 64, 7707.
- A detailed VB analysis confirms that the bond in the trans-bent HSi ≡ CH is a genuine triple bond; see: Danovich, D., Ogliaro, F., Karni, M., Apeloig, Y., Cooper, D.L. & Shaik, S. 2001 Angew.Chem.Int.Ed. 113, 4146.
- (a) Wiberg, N., Niedermayer, W., Fisscher, G., Nöth, H. & Suter, M. 2002 Eur. J. Inorg. Chem. 1066; (b) Takagi, N. & Nagase, S. submitted, Eur. J. Inorg. Chem. (c) Karni, M. & Apeloig, Y., reported at the 6 th World Congress of Theoret-ically Oriented Chemists, WATOC'02, August, 4-9, 2002, Lugano, Switzerland (oral contribution No. 97).
- The quest for a stable silyne, RSi ≡ CR′. The effect of bulky substituents 
Volume 1, Issue 1 , pp 59-65
- Cover Date
- Print ISSN
- Online ISSN
- Kluwer Academic Publishers
- Additional Links
- 1,3-disilacyclobutadien isomerization
- MO calculations