Silyl cation intermediates

Overman15 discusses two conceivable mechanisms for the cyclization. One possibility assumes a direct cyclization of iminium ion 9 via p-silyl cation intermediate 24 to the indolizidinone 10. Cation 24 is stabilized by a p-effect of the silicon atom. Alternatively, iminium ion 9 might first undergo a charge-accelerated cationic aza-Co/fe rearrangement to allylsilaniminium ion 25, which would then cyclize to 10 with loss of a silyl cation. 

The low-temperature solution reactions of silylenes with arenes, which involve silyl cation intermediates, have been reviewed,80 and another pertinent review is available.81... 

A gold-catalyzed intramolecular alkenylsilylation reaction was reported and this unusual conversion was recently studied by DFT calculations. The electrophilic gold (I) catalyst is thought to generate carbocation and silyl cation intermediates in the conversion. The computational results suggest that the bistrifiimide anion modulates the reactivities of the cationic intermediates and controls rearrangement steps in the reaction. 

A computational study of the gold(I)-catalysed alkenylsilylation reaction of a silyl-tethered 1,6-enyne system has been shown to involve bistriflimide counterion-assisted rearrangements of carbocation and silyl cation intermediates (Scheme 132). ... 

When the reaction is performed in CH2CI2, without any protic additives, the transformation proceeds with poor catal)dic efficiency (31% yield). The catalyst turnover is assumed to be inhibited by loss of H2O from the catalytic cycle, presumably via formation of (TIPS)20. The use of protic additives might competitively scavenge the putative silyl cation intermediate. It is noteworthy that the selection of the appropriate cocatalyst and solvent allows access to the anti isomer with excellent selectivity (eq 9). 

In the second silylation step, the situation is opposite because the basicity of SENA is substantially higher than that of the starting AN. Hence, the second silylation should start with the electrophilic attack of nitronate (320) giving rise to the cationic intermediate B, whose deprotonation affords nitroso acetal... 

Under the action of a base, the first step (Ki) reversibly produces the a-nitro carbanion A, which then rapidly reacts with Si X (K2) to form SENA. The latter is reversibly silylated by the second equivalent of SiX (K3) to give the cationic intermediate B, whose deprotonation with the base (K4) affords the target nitroso acetal (or BENA). 

It can easily be seen that intermediates B and B" are generated, respectively, in silylation reactions of SENAs (Chart 3.20) or alkyl nitronates (Scheme 3.202). To estimate the efficiency of this approach, it is necessary to reveal whether cationic intermediates B and B" can exist as kinetically independent species. 

Steric hindrance in the silyl groups of cation (349) and nucleophile (352) has virtually no effect on the rate constant of the C,C-coupling reaction. Hence, it can be concluded that, at least for silyl-containing nucleophiles (352), elimination of the trialkylsilyl group from cationic intermediate A is not the rate-determining step of the reaction sequence (Scheme 3.207). 

SENAs derived from secondary AN are not involved in catalytic C,C-coupling reactions with silyl ketene acetals. This is possibly due to a decrease in both the effective concentration of the cationic intermediate (the steric effect) and its lower level of electrophilicity (see the lower entry in Table 3.23). 

Double silylation of the nitro fragment of substrate (372) would afford the [ > C=N(OSi)2]+ cation, which should be trapped. To hinder deprotonation of this cation, the substrate contains a bulky substituent R2 at the C-2 atom. A priori, based on the data on intermolecular C,C-coupling of the above-mentioned cations, it can be suggested that the introduction of substituents at the C-l atom should decrease the reactivity of the cation and can adversely affect the thermodynamics of such cations. Consequently, these substituents would hinder the generation as well as trapping of cationic intermediates. 

One approach is based on the reaction of BENAs with electrophiles E-X (path (a)) giving rise to cationic intermediates A, which react with the anionic intermediate [X-E-OSi]- without leaving the cell. As a result, the OSi fragment is transferred by the electrophile (LA) to the 3-carbon atom to give bis-silyl derivative (504). If the OSi anions leaves the cell, it can react with the starting BENA as the nucleophile through the pathway (b) (see below). 

The probable pathway giving rise to silylated cyclic nitrile (589), which is the most unusual reaction product, is shown on the left of Scheme 3.282. Apparently, this compound is generated through the cationic intermediate A. It undergoes cyclization at the terminal electron-rich C,C-double bond to form silylated oxime (587), which is transformed into nitrile (588). After silylation of the latter, nitrile (589) can be isolated. Desilylation of (589) according to standard procedures affords nitrile (588). 

The probable pathway resulting in the stereoselective formation of silylated ene nitrile (586) from enoxime (584) is presented on the right of Scheme 3.282. At higher temperature, the latter eliminates trimethylsilanol to give ene-nitrile (586) under the action of silyl Lewis acid (TfOSiMe3). Evidently, the reaction of compound (585) with TfOSiMe3 at room temperature involves initial silylation of the nitrogen atom to form the cationic intermediate B, which is deprotonated with triethylamine, followed by the thermodynamically favorable l,3-N,C-shift... 

Not only the ring size but also the number of stabilising silyl groups in the -position is essential for the stability of the vinyl cations. Thus, reaction of alkyne 16 with tityl cation gave both stereoisomers of aikenylsilane 18 as the only products in 80-85% isolated yield (Scheme 3). This result suggests, that the generated / -silyl-substituted vinyl cation intermediate 17 did not persist under the applied reaction conditions but underwent a second hydride transfer with the formation of compound 18. 

The reaction with silyl enol ethers 3f and 3g gave only the [3 + 2] cycloadducts in comparison with effective formation of acyclic adduct 15 in the reaction with ketene silyl acetals 3a and 3e at lower reaction temperature. This can be explained by the reactivity of cationic intermediates 19 the intermediates from 3f and 3g are more reactive owing to lower stabilization by the oxy group than those from 3a and 3e, and react with the internal allene more efficiently to give the cycloadduct(s). Cyclic product 17a could be obtained at higher temperature via the reaction of 3a (entry 2). 

Conjugated ketones and esters react with allenylsilanes to yield acylcyclopentenes (Eq. 9.60) [63]. These products are formed by initial 1,4-addition to the conjugated double bond to afford a silyl-stabilized vinyl cation intermediate. 1,2-Silyl migration gives rise to a second silyl-stabilized vinyl cation which cyclizes to the acyl cyclopen-tene (Scheme 9.14). 

On the basis of the above results, a possible mechanism for the allylsilylation of cyclohexene with la has been proposed as illustrated in Scheme 1. A silyl cation or a complex intermediate I is directly formed at the beginning stages of the reaction... 

The intermediate formation of the ferrocenyl-substituted silylium ion 16 by protonation of the ansa-ferrocenyl silane 17 can be regarded as a special case of electrophilic cleavage of an activated C-Si bond (see Scheme 7). The driving force for this reaction is the release of a strain by formation of the silyl cation. In a... 

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