Lewis Donor and Acceptor Interactions of Silylenes:  A Theoretical Study

Abstract

Silylenes are known to show ambiphilic character. Ab initio study on (Lewis base)→H2Si coordination shows that the strength of the interaction depends mainly on two factors: (1) the nucleophilicity of the base and (2) the extent of π delocalization of the lone pair on silylene onto the π frame of the base. The stabilization energies due to the formation of H3N→SiH2, OC→SiH2, and HNC→SiH2 complexes at the G2 level are respectively 23.22, 20.84, and 29.59 kcal/mol. The base coordination triggers the nucleophilicity of silylenes, the strength of which is again dependent on the π interaction between the lone pair on Si and the Lewis base. The energy gains due to the formation of the LB→LA complexes (H3N)H2Si:→BH3, (OC)H2Si:→BH3, and (HNC)H2Si:→BH3 are 42.08, 24.65, and 29.16 kcal/mol, respectively, at the G2 level. The electrophilic nature of base-coordinated silylenes and the nucleophilicity of double-base-coordinated silylenes have been quantitatively estimated. Natural bond orbital (NBO) analysis and charge decomposition analysis (CDA) have been carried out to quantitatively estimate electron distribution.

Synthesis and Properties of the First Stable Silylene–Isocyanide Complexes

“…Further heating at 100 8C for 5 h led to the disappearance of the signals for 12, only 10 (60 %, isolated yield) being observed. This result suggests that silylene 2 undergoes both the [12] and [14] cycloaddition to 2,3-dimethyl-1,3-butadiene at the initial stage of this reaction.…”

“…Since the reaction of disilene 1 with 2,3-dimethyl-1,3-butadiene also gives 10 and 11, passing through silylene 2, [28] it is considered that these reactions also proceed by dissociation to silylene 2 and the corresponding free isocyanides 3 c ± e. The formation of 11 is probably interpreted in terms of hydrolysis of the [12] cycloadduct 12 during separation (Scheme 11), and this mechanism was supported by the reaction of 5 e with isoprene giving the corresponding vinylsilirane 13 (89 %), which was fully characterized by NMR spectroscopy [26] (Scheme 11). The reaction rates increased in the order of Scheme 10.…”

“…The 29 Si NMR spectrum showed three peaks (d À 76.3, À 72.9, and À 5.3 ppm with the peak height ratio of approximately 2:6:3) besides the peaks for trimethylsilyl groups. The peak at d À 5.3 was assigned to that for the [14] cycloadduct 10, and the peaks at d À 76.3, À 72.9 ppm were attributed to the geometric isomers of [12] cycloadducts 12. After heating the reaction mixture at 50 8C for 7 h, no change was observed in the 29 Si NMR spectrum.…”

“…[12] Experimentally, the reactions of silylenes, R 1 R 2 Si : , with carbon monoxide in low-temperature matrices have been [a] Prof. N. Tokitoh, Dr. N. Takeda, T. Kajiwarainvestigated by some groups.…”

“…Very recently, Bharatam et al carried out theoretical calculations on H 2 SiCNH indicating a strong contribution from structure C in the resonance hybrid. [12] Experimentally, some transient silaketenimines have been proposed as an intermediate in the reactions of di-tbutylsilylene with some isocyanides. [16,17] The spectroscopic data for silaketenimines had not been fully understood until we started studying the kinetically stabilized systems.…”