Concerning the Reactivity of Dioxiranes. Observations from Experiments and Theory

The challenging hypothesis of a "biphilic" (i.e., electrophilic vs nucleophilic) character for dioxirane reactivity, which envisages that electron-poor alkenes are attacked by dioxiranes in a nucleophilic fashion, could not be sustained exptl. Rate data, which est. Hammett ? values for the epoxidn. of 3- or 4-substituted cinnamonitriles X·Ph-CH:CH-CN, unequivocally allow one to establish that dioxiranes epoxidize electrophilically even alkenes carrying electron-withdrawing groups. The greater propensity of methyl(trifluoromethyl)dioxirane TFDO (1b) to act as an electrophilic oxidant with respect to dimethyldioxirane DDO (1a) parallels the cathode redn. potentials for the two dioxiranes, as measured by cyclic voltammetry. A simple FMO approach for alkene epoxidn. is helpful to conceive a likely rationale for the greater oxidizing power of TFDO as compared to DDO.