, Volume 1, Issue 1, pp 59-65

The quest for a stable silyne, RSi ≡ CR′. The effect of bulky substituents [1]

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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.