Synthesis and properties of allylic, benzylic, propargylic and allenylic oxonium ions

Despite numerous studies of trialkyloxonium ions in the literature, investigations into the chemistry of allylic, benzylic, propargylic and allenylic oxonium ions are rare. Existing reports on well characterized allylic and benzylic oxonium ions invariably construct these species based on constrained tricyclic oxatriquinane/oxatriquinacene scaffolds, and only one example of an unconstrained but thermally highly unstable benzylic oxonium ion has been reported in the literature without detailed characterization. Herein we report an investigation on a collection of allylic, benzylic and hitherto unknown propargylic and allenylic oxonium ions prepared on unconstrained scaffolds by a general, modular and unified strategy. Permutation of the substitution pattern of these oxonium ions allowed the extension of the strategy for the syntheses of various doubly-substituted oxonium ions. Most of these oxonium ions could be characterized at room temperature by NMR, and a series of unexpected reactions and chemical behavior pertinent to these species are briefly described.