Synthesis and Crystal Structure of a Silyl-Stabilized Allyl Cation Formed by Disruption of an Arene by a Protonation-Hydrosilylation Sequence

摘要

Allyl cations, which are well-known primarily from solution NMR spectroscopy in traditional superacidic media, have remained elusive in crystalline form. Indeed, these rare guests in the chemist's lab have normally arrived unannounced during attempts to prepare other reactive species. Recent investigations focused on the conversion of silane 1 into the silylium ion 2~+, which is stabilized by Si-π. interactions (Scheme 1). A side path of these studies led to the isolation of a crystalline bicyclic allylic cation 3~+ that is formed with concomitant disruption of an aromatic ring. Invited or not, the solid-state structure of 3~+ teaches us much about this important class of organic intermediates and suggests an intricate stereochemistry, which reveals a likely mechanism for this unexpected transformation.