Allyl cation, Siloxy

Modes of cycloaddition of alkylideneallyl cation are also controlled by the reaction conditions. [4 + 3] Cycloaddition occurs in the reaction with furan. The [4 + 3] cycloaddition with furan was observed for the siloxy-substituted allyl cation 5S, but not for the methoxy-substituted allyl cation 5M. The lower electrophilicity of 5S may prefer the concerted pathway of [4 + 3] cycloaddition in competition with the stepwise pathway to yield a [3 + 2] cycloadduct and an electrophilic substitution product. 

Silver(I) compounds are known to promote different kinds of cycloaddition. Reactions of 2-alkoxyallyl halides with 1,3-dienes in the presence of silver(I) compounds provide a beneficial route to cycloheptanones [2,3]. When a mixture of 2-(trimethyl-siloxy)allyl chloride 1 and cyclopentadiene (2) is treated with 2 equiv. AgC104 in THF-ether (1 2) at 0 °C, bicyclo[3.2.1]oct-6-en-3-one 3 is produced in 91 % yield [3] (Sch. 1). The 2-(trimethylsiloxy)allyl cation 4 is believed to be involved as a reactive species in the reaction. 

The reaction of silica-supported bis(allyl)Rh complexes with PMe3 followed by hydrogen treatment leads to the synthesis of the surface SiO-RhH2(PMe3)4+ as the first example of a cationic organometallic complex attached to the silica surface by ion pairing. The counterion is presumed to be a siloxy group SiO- on the silica surface.276 277... 

Isomerization. Cationic iridium complex effects selective isomerization of unsymmetrical diallyl ethers and conjugated boronates containing an allylic ether group, including an access to 7-(siloxy)allylboronic esters. The conversion of allyl homoallyl ethers to -y,8-unsaturated carbonyl compounds is promoted by [(cod)IrCl]2. 

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