A-Silylcarbenium ions

Palladium-mediated substitutions of silylated allylic compounds are not subject to steric hindrances. The silicon atom exerts control over the reaction site snch thaty-substitution results [64]. While the electrophiles are unavoidably disjoint with respect to the acceptor silicon, one of the two limiting forms, i.e., the a-silylcarbenium ion, is much more unfavorable (a-a arrangement) and therefore its population is expectedly low. 

In the presence of a Lewis acid, 1-seleno- and l-thio-2-silylethenes react with 2-phosphonoacrylates to give cyclopropane products (Scheme 2).31,31a The [2 + l]-cycloaddition would involve 1,2-silyl migration of an cr-seleno-or a-thio-/3-silylcarbenium ion intermediate and subsequent ring closure. 

The BF3-OEt2-promoted [3 + 2]-cycloaddition of l-morpholino-2-trimethylsilylethyne to homochiral epoxides is very valuable for direct asymmetric synthesis of 7-butanolides (Equation (4)).36 The initial product, a 4-sily 1-2,3-dihydrofuran, may be formed by ring closure of the /3-silylcarbenium ion generated from a BF3-activated epoxide and... 

Intramolecular /ra r-carbosilylation of terminal alkynes with alkenyl- and arylsilanes proceeds efficiently under catalysis by a Lewis acid (Equation (8)).41,41a Alkenyl- and arylsilanes bearing an alkynyl group at the (1- or ortho-position undergo -cyclization, while a-alkynyl-substituted alkenylsilanes are converted into tv/r/a-cyclization products. These cyclizations have been proposed to proceed also via a /3-silylcarbenium ion intermediate. However, the cationic center does not participate in bond formation. The intermolecular alkenylsilylation of terminal alkynes is rather limited in applicability.42... 

In the presence of Mn(OAc)3 or a Ce(iv) salt, 1,3-dicarbonyl compounds and /3-carbonyl imines react with allylsilanes to give silylmethylated dihydrofurans and dihydropyrroles, respectively.199,200,20011 A proposed mechanism involves the formation of a /3-silylcarbenium ion intermediate via two-electron oxidation and subsequent intramolecular nucleophilic attack (Equation (51)). a... 

The Lewis acid-catalyzed aUylsilylation of alkynes and alkenes with allylsilanes was introduced by Jung [511] and Yamamoto [512]. Jung and co-workers found fhat AICI3 catalyzes the aUylsilylation of simple alkenes and phenyl-substituted alkynes wifh allylsilanes (Scheme 10.191) [513]. The allylation occurs regiospecifically at the y-position of the allylsilanes. When cycloalkenes are used as substrates, trans adducts are obtained exclusively. Jung et al. initially reported [513 a] that the aUylsilylation of alkynes proceeded in a cis-addition mode they later noted, however, fhat fhe stereochemical assignment of fhe aUylsilylation products was incorrect and the products were not cis but trans adducts [511]. It has been proposed that the mechanism of fhe AlCls-catalyzed aUylsilylation involves addition of a silyl cation or its equivalent to fhe carbon-carbon multiple bond and subsequent aUylation of the resulting -silylcarbenium ion [511]. 

The Lewis acid-promoted reactions of l-seleno-2-silylethene with a-enones and 1-phosphonoacrylates give cyclopropane products (Scheme 10.203) [534]. The [24-1] cycloaddition would proceed via 1,2-silyl migration of a-seleno-/ -silylcarbenium ion intermediate 146 and subsequent ring-closure. Interestingly, dimethyl 1,1-di-cyanoethene-2,2-dicarboxylate, a highly electron-deficient alkene, undergoes [24-2] cycloaddition under similar conditions [535]. 

In the present paper we pursue the question of whether tris(trimethylsilyl)silylcarbenium ions with a delocalized positive charge similarly undergo the above-described isomerization. The delocalization is expected to decrease the energy difference between the carbenium ion and the silylium ion and the rearrangement may fail. 

The presence of cyclopentyl ketone results from a long life-time of the ]8-silylcarbenium ion The ratio divinylcyclopentanol 24/cvclopentyl ketone 25 is slightly dependant on the structure of R. 

---