Silyl cations structure

There is no general theoretical study for trialkyl-substituted cations R3E, which investigates the relationship of the classical planar trigonal structure to isomeric complexes RE /R2 and its relative energy compared to the dissociation products, the singly coordinated four-valence-electron species R E and the hydrocarbon R2. The only exceptions are 7-norbornadienyl cations 37 for which the germyl and silyl cation has been intensively studied theoretically by Radom and Nicolaides. ... 

Silyl cations like 3 and 7 in which the positively charged silicon is part of a n-conjugated system attracted particular interest. The marginally stable silatropylium ion 7, is characterized by a Si NMR resonance at 8 Si = 149 in CD2CI2 at —50 °C, downfield-shifted by 192 ppm compared to the precursor silane.This experimental value is in fair agreement with the calculated silicon NMR chemical shift for the optimized gas phase structure of 7 (8 Si = 159.9, at GIAO/HF/6-311 + G(2df,p)(Si), 6-31G(d) (C,H)). This indicates only small interactions between the cation and dichloromethane, the solvent used for the NMR investigations. 

The reaction of cyclotrisilene 15 with the silylated benzenium ion [Et3Si/C6H6] results in the formation of the cyclotetrasilenylium ion 3 (see Scheme 6). The solid state structure of silyl cation 3 indicates its homoconjugative nature (see Fig. 7 and Table XIII). ... 

In a series of papers Reed and colleagues present the solid state structures of a series of simple trialkyl-substituted silyl cations with halogenated carborane anions... 

Quantum mechanical calculations show that the silyl cation (19) has a twisted structure, and that the a-C02 group provides substantial electrostatic stabilization.58 Isotope effects for its formation reaction are also reported.58 Evidence is provided for the stabilization of incipient oxocarbenium ions by axial electronegative substituents, as in (20) the presence of the most electronegative substituent results in the fastest reaction.59 Lewis acid-promoted cleavage of spirocyclic dioxanes such as (21) involves oxonium ions, and high axial vs equatorial product selectivities are possible with the correct choice of Lewis acid and nucleophile.60 Reactions which lead to 1,3-dioxenium salts have been reviewed.61... 

The internally stabilized silyl cation (36), formed by hydride transfer as shown, has a 2-silanorbomyl cation structure, and is not coordinated to die solvent or the counterion.78 NMR chemical shifts calculated on the basis of die bridged structure shown are in agreement with the experimental values. The autiiors describe it as free but internally re-stabilized .78 The silicon-stabilized oxonium ion (37) shows considerable stereoselectivity in its reactions (38) is the preferred product isomer by a 92 8 ratio, and (39) by 98 2.79... 

Further support comes from calculation of 1 Jch spin-spin coupling constants at IGLO-DFPT/BIII level. The data were interpreted as giving strong evidence for intramolecularly TT-stabilized silanorbomyl cation structure lacking coordination to solvent or counterion. The species can also be regarded as an almost symmetrically bridged /J-silyl carbocation with siliconium ion character. 

Several major advances have been made in the field of R3M+ cations (M = Ge, Sn, Pb) in recent years, including the preparation of free cyclotrigermenium cations and possibly close to free trimesitylgermyl and trimesitylstannyl cations. However, many challenges remain, for example, a structural characterization of free R3M+ cations (M = Ge, Sn, Pb). In many respects these cations appear to be even more challenging than the silyl cations, due to their weaker and longer bonds to carbon. Presently inadequate tools which need to be improved are computational methods for the NMR chemical shifts of the Sn and Pb nuclei. 

Calculations revealed that the parent structure of the silylenium ion H3Si+ is more stable than that of H3C+ by 41-73.5 kcal/mol. However AE tends to decrease strongly on replacement of the hydrogen atom, indicating that substituents, in particular it donors, are decidedly more effective in stabilizing carbenium ions than in stabilizing silylenium ions. Thus, the methyl cation is stabilized by an amino substituent by 93.8 kcal/mol but the silyl cation by only 38.3 kcal/mol (27). +... 

These epoxides react stereospecifically with nucleophiles to give single diastereoisomers of adducts. If a carbon nucleophile is used (cuprates are best), it is obvious from the structure of the products that nucleophilic attack has occurred at the end of the epoxide next to silicon. This is obviously an S>j2 reaction because it is stereospecific in any case an S>jl reaction would have occurred at the other end of the epoxide through the p-silyl cation. 

Until the year 2002 no experimental data existed on the structures of unperturbed R3E+ cations, the exact analogues of the carbenium ions. Computational data combined with NMR chemical shift calculations, which could be compared to experiment, were the only source of reliable structural information for silylium ions6,7,13,77,121 while for germylium, stannylium and plumbylium ions this combined approach was not attractive due to either the non-existence of the experimental data (Ge) or the complexity of the computational problem (Sn, Pb). On the other hand, a series of excellent experimental studies demonstrated, for example, the high coordination tendency of small trialkylsilylium ions either toward the counteranion38,114,127,138 or toward the solvent.36,37,67,116,127 The solid state structures of these silyl cation salts showed clear indications either of cation/anion or cation/ solvent coordination. Thus, the nature of the observed cation, i.e. the degree of silylium ion character remained disputable.10,11,13... 

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