Cycloaddition with 1,2-silyl migration

The reaction of allenylsilanes with a,/8-unsaturated acylsilanes presents a new [3 + 3]-cycloaddition approach to a six-membered carbocycle [189]. Lewis acid-promoted ring expansion of the [3 + 2]-annulation product 260 is followed by a second cationic 1,2-silyl migration to produce the cyclohexenone 261 after desilylation. 

It has been reported that 1,3-silyl migration on acylpolysilane leads to the formation of silene. Silene thus formed undergoes [2+2] cycloaddition with acetylene in a two-step manner, giving rise to the formation of silacyclobutene <02OM3271>. 

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 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]. 

Another approach leading to pyrrolidine 210 via [3 + 2] cycloaddition is the reaction of A-protected a-amino aldehydes with allylsilanes in the presence of a catalytic amount of BF3 OEt2 (equation 172). No 1,2-silyl group migration occurs in these annulation processes313. 

As is the case with the [3 + 2] cycloaddition reactions with aldehydes, bulky silicon substituents are supposed to suppress the attack of a chloride and/or oxygen nucleophile to silicon (path a) and allow the silyl group to migrate to a positively charged /5-carbon possibly through 5-endo cyclization of siliranium ion intermediates to give five-membered products (Scheme 3-102). 

Those having bulky silicon substituents react with o y5-unsaturated ketones in the presence of a Lewis acid in a [3 + 2] cycloaddition pathway to give substituted cyclopentenes possibly via 1,2-migration of the silyl group in a manner similar to the reaction of allyl(triisopropyl)silane (Scheme 3-108). 

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