Oxaziridinium Salts as Hydrophobic Epoxidation Reagents: Remarkable Hydrophobically-Directed Selectivity in Olefin Epoxidation

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Abstract
Selective epoxidation of cinnamates versus crotonate was used to detect hydrophobic binding of the cinnamates in the transition states with hydrophobic oxidizing agents in water solution. With peracids as oxidants, no such effect was seen, in accord with the calculated geometries of epoxidation in which the hydrophobic groups of substrate and oxidant could not stack. However, with oxaziridinium salts carrying fused benzene rings there was significantly high selectivity for the cinnamates in water solution, which could be suppressed with added 2-propanol. The hydrophobically induced selectivity changes were even larger, in free energy terms, than those reported previously for the atom-transfer reactions in hydride reductions. Furthermore, the oxaziridinium ions could be generated with oxone from catalytic amounts of the corresponding iminium salts. These substrate selectivities should also carry over to positional selectivities in polyenes.