Vinyl cation 3-silylated

An important contribution to silylium ion chemistry has been made by the group of Muller, who very recently published a series of papers describing the synthesis of intramolecularly stabilized silylium ions as well as silyl-substituted vinyl cations and arenium ions by the classical hydride transfer reactions with PhjC TPEPB in benzene. Thus, the transient 7-silanorbornadien-7-ylium ion 8 was stabilized and isolated in the form of its nitrile complex [8(N=C-CD3)]+ TPFPB (Scheme 2.15), whereas the free 8 was unstable and possibly rearranged at room temperature into the highly reactive [PhSi /tetraphenylnaphthalene] complex. ... 

A jS-silyl stabilized vinyl cation, the l-bis(trimethylsilyl)methyl-2-bis(trimethylsilyl) ethenyl cation (32) was investigated by dynamic 13C NMR spectroscopy (Fig. 6).55... 

The generation of a-ferrocenyl-P-silyl substituted vinyl cations of type 28 does not require superacidic conditions, they can be generated by protonation of l-ferrocenyl-2-trialkylsilyl alkynes with trifluoroacetic acid at room temperature. The SiR3-groups with larger alkyl substituents increase the lifetime of this type of carbocations. 

Vinyl cations of type 28 with a-aryl or a-alkyl substituents and two P-silyl groups and with an anion of very low nucleophilicity can be generated at room temperature in non-coordinating solvents from 30 by a Si-H to C-H hydride transfer reaction. For 29 (R = t-butyl), an X-ray structure determination has been reported (43, 52, 53). 

The l3C NMR spectrum of 31 (R = H) is shown in Figure 8. This is likely to be the the smallest member of P-silyl substituted vinyl cations which can be generated and observed as persistent species in solution. 

Recent results on the chemistry of persistent vinyl cations are summarized. / , / -Disilyl-substituted vinyl cations were synthesized by intramolecular addition of transient silylium ions to alkynes. The vinyl cations are stable at ambient temperature and were isolated in the form of their tetrakispentafluorophenylborate and hexabromocarboranate salts. The vinyl cations were characterized by IR and NMR spectroscopy and by X-ray crystallography. The experimental results for the a-alkyl- and a-aryl-substituted vinyl cations confirm their Y-shape structures, consisting of a linear dicoordinated, formally positively charged a-carbon atom and a trigonal planar coordinated /f-carbon atom. In addition, the spectroscopic data clearly indicate the consequences of, / -silyl hyperconjugation in these vinyl cations. Scope and limitations of the synthetic approach to vinyl cations via addition of silylium ions to C=C triple bonds are discussed. 

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. 

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). 

The reaction of acylsilanes with acid chlorides in the presence of A1C13 leads to furans (Table 9.41) [45]. In these reactions an acyl cation initiates the addition with ensuing silyl migration yielding an intermediate vinyl cation. Attack of the carbonyl oxygen followed by proton loss affords the observed products (Scheme 9.16). An analogous reaction with nitrosyl fluoroborate provides a route to oxazoles (Table 9.42) [65]. The nitrosyl cation serves as the electrophile in this application. 

A regioselective [3 + 2]-cycloaddition approach to substituted 5-membered carbo-cycles was made available by the use of allenylsilanes [188]. The reaction involves regioselective attack of an unsaturated ketone by (trimethylsilyl)allene at the 3-position. The resulting vinyl cation undergoes a 1,2-silyl migration. The isomeric vinyl cation is intercepted intramolecularly by the titanium enolate to produce a highly substituted (trimethylsilyl)cyclopentene derivative. 

The ethoxycarbocation intermediate (363) produced by the action of acid on the cyclobutenedione monoacetal (362) has been found to react with bis(trimethylsilyl)-acetylene to afford a 2-methylenecyclopent-4-ene-l,3-dione derivative (365). The authors426 proposed that the rearrangement results from an unprecedented cationic 1,2-silyl migration on the alkynylsilane, subsequent ring expansion via a vinyl cation intermediate (364), and re-closure by intramolecular addition of an acyl cation to a silylallene in a 5-exo-trig mode (see Scheme 90). 

A variety of other methods have been used to measure the /3-silicon stabilization of car-bocations. From gas-phase studies, Hajdasz and Squires50 derived a value of 39 kcal mol-1 for the stabilization of the cation Me3SiCH2CH2+ relative to the ethyl cation. This is in agreement with calculations by Ibrahim and Jorgenson39. Siehl and Kaufmann51 have used carbon-13 NMR spectroscopic data to give an indication of the /1-silyl stabilizing effect in some aryl vinyl cations. 

Alkynylsilanes have been studied less than vinylsilanes. Electrophilic addition to an alkynylsilane can give a a- or /3-silyl-substituted vinyl cation, in the first step (equation 59). 

In contrast, /1-silyl-substitution is predicted by the calculations to be far more stabilizing than /1-alkyl substitution. Thus, the isodesmic equation 3 predicts for 9 a stabilization by the /1-silyl group of 38 kcalmol-1, while the /1-methyl substitution in 10 gives only a stabilization of 28 kcalmol-1 [MP3/6-31G(d)//3-21G(d)]5. The stabilization by the silyl substituent is markedly orientation-dependent. Thus, the perpendicular conformation of the /1-silylethyl cation 9p is higher in energy by 29.6 kcalmol-1 compared with the bisected conformation 9 [MP3/6-31G(d)//3-21G(d)]5. The open /1-silyl-substituted vinyl cation 11 is lower in energy by 28.6 kcalmol-1 and 20.5 kcalmol-1 [MP3/6-31G(d)//3-21G(d)] compared with the vinyl cation (equation 4, R = H) and the 1-propenyl cation (equation 4, R = Me), respectively5. 

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