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Reported in this Letter is a silver-catalyzed reaction between o -alkynylaryl aldehydes and trifluorodiazoethane that enables an expedient synthesis of trifluoromethylated benzo[ d ]oxepines. The reaction works through a silver-promoted 6- endo - dig cyclization of o -alkynylbenzaldehydes for the generation of an isochromenylium intermediate, which upon a ring-expansive addition of trifluorodiazoethane delivers a novel class of trifluoromethylated benzoxepine frameworks. This strategy was applied to the synthesis of phosphonylated benzo[ d ]oxepines using the Seyferth-Gilbert reagent.