Arenium-ion-catalysed halodealkylation of fully alkylated silanes.

'Organic silicon' is not found in nature but modern chemistry is hard to imagine without silicon bound to carbon. Although silicon-containing commodity chemicals such as those emerging from the 'direct process' look simple, it is not trivial to selectively prepare aryl-substituted and alkyl-substituted (functionalized) silicon compounds, known as silanes. Chlorosilanes such as MeSiCl (n = 1-3) as well as SiCl (n = 4) are common starting points for the synthesis of silicon-containing molecules. Yet these methods often suffer from challenging separation problems. Conversely, silanes with four alkyl groups are considered synthetic dead ends. Here we introduce an arenium-ion-catalysed halodealkylation that effectively converts MeSi and related quaternary silanes into a diverse range of functionalized derivatives. The reaction uses an alkyl halide and an arene (co)solvent: the alkyl halide is the halide source that eventually engages in a Friedel-Crafts alkylation with the arene to regenerate the catalyst, whereas the arenium ion acts as a strong Brønsted acid for the protodealkylation step. The advantage of the top-down halodealkylation methodology over reported bottom-up procedures is demonstrated, for example, in the synthesis of a silicon drug precursor. Moreover, chemoselective chlorodemethylation of the rather inert MeSi group attached to an alkyl chain followed by oxidative degradation is shown to be an entry into Tamao-Fleming-type alcohol formation.