Hyperconjugative stabilization of silicenium ions: kinetics of hydride abstractions from .beta.-element-substituted silanes

Abstract

Abstract:Rates of hydride abstractions from j3-silyl-, j3-germyl-, and B-stannyl-substituted silanes [HSiMeZ(CHzMMe3), M = Si, Ge, Sn] and related compounds by diarylcarbenium ions have been measured. The intermediate silicenium ions, produced in the rate-determining step, are stabilized by hyperconjugation, but the effects are much smaller than in comparable carbenium ions.

Hyperconjugative Effects in Carbenium and Silicenium Ions

“…We now turn to a discussion of the trends in the energy and geometry results tabulated in Tables −4, and their implications to the subject of hyperconjugative interactions in the R−M bond in these cation systems. ,,,,,,, Comparing Δ E BDE values for R 3 M + −L from CH 3 + to SiH 3 + (Table ) for a given L, there is a clear and substantial decrease in value for all the ligands. Thus, methyl cation affinity is always greater than silyl cation affinity. , On the other hand, Me 3 Si + affinity is consistently greater than for Me 3 C + .…”

“…These model systems are VB analyzed for the various bonding component interactions that determine the total electronic and geometric structure description. Besides the usual electron pair σ bond between M + and A, with its covalent and ionic components, , the possibility of an additional, π type bond between the saturated AH n group and the electron deficient MH 2 + fragment is the key to the presence and magnitude of the hyperconjugative effect . In this context the hyperconjugative interaction can also be termed a resonance effect between σ bond and σ + π bond VB configurations, or a delocalization from an electron rich AH n to an electron deficient MH 2 + .…”

Silicon‐Modified Carbohydrate Surfactants V: The Wetting Behaviour of Low‐Molecular‐Weight Siloxane, Carbosilane, Silane and Polysilane Precursors on Low‐Energy Surfaces

“…To complete the reaction this temperature was maintained for 1 h. Afterwards (CH 3 ) 2 SiCl 2 (16.4 g, 1.3ϫ 10 Ϫ 1 mol) was dropped into the refluxing solution during 1 h and this temperature was maintained for 9 h. The precipitated salt was filtered off from the THF solution and the salt extracted with npentane. The THF solution and the extract were united and after distillation 8.4 g (CH 3 ) 3 In complete analogy to the Grignard-type procedure described above, the neopentyl-substituted silanes 25 and 26 27,28 were synthesized from (CH 3 ) 3 3 [3] | H The di-, tri-and tetra-silanes 18-22 and 24 were synthesized from chlorosilanes by a sequence including dehalogenating reductions, halogenations and hydrogenations [30][31][32][33] (Scheme 2). Thus, in a 200 ml three-necked bottle equipped with stirrer, refluxing condenser and dropping funnel, 700 ml dry THF was placed and 34 g (4.85 mol) lithium powder was added under argon.…”

Steric effects in the ionic hydrogenation of aryldi(1-adamantyl)methanols to the corresponding methanes by trifluoroacetic acid and hydrosilanes or sodium borohydride

“…Crystallographic studies on silylium ion-like species obtained from trialkylsilanes and a large, weakly coordinating halogenated carborane ligand suggest that the triisopropyl derivative most nearly approaches the ideal planar structure [more than tri(tert-butyl)], this geometry favouring C᎐H hyperconjugative stabilization, 4 though this factor is less important in silylium ions than in carbocations. 8, 22 The very low reactivity of TIPS with diadamantylbenzyl cations would appear to indicate that in this highly congested system steric repulsion outweighs inductive and hyperconjugative stabilization of the incipent triisopropylsilylium ion. TIPS is reported to react about 100 times more slowly with Ag ϩ than does TES.…”