Synthesis of silicon heterocycles via gas-phase silylium ion reactions with cyclic acetals and ketals†

Abstract

Silylium cations react with a variety of cyclic acetals and ketals by initial O-silylation followed by ring-opening and then recyclization with elimination of an aldehyde or ketone, respectively. The reaction occurs with high efficiency and results in the replacement of C–O in the cyclic acetals and ketals by Si–O⁺. The cyclic nature of the product ions was established by multiple-stage mass spectrometry (MS² and MS³). For a series of silylium ions, (CH₃O)₃Si⁺, (CH₃O)₂Si⁺CH₃, CH₃OSi⁺(CH₃)₂, and Si⁺(CH₃)₃, increasing electron deficiency at silicon favors the formation of intact adducts; correspondingly, the highly oxygen-substituted silylium cations tend to undergo the recyclization reaction to form the replacement product ion. In the absence of oxygen in the reactant ion (e.g. Si⁺(CH₃)₃) an acyclic product is generated and characterized by its collision-induced dissociation (CID) spectrum, which shows silicon-to-carbon methyl migration. The reactivity of silylium ions was further explored using thiazolidine and 1,3-oxazinane and the products of replacement of C–N, C–S and C–O by Si–O⁺ were observed. The silylium ion reactions thus provide a general, structurally diagnostic, method for the gas-phase characterization of cyclic acetals and ketals.