Hypervalent-Iodine-Mediated Formation of Epoxides from Carbon(sp2)-Carbon(sp3) Single Bonds.

We have developed an efficient method for direct formation of epoxide groups from carbon(sp2)-carbon(sp3) single bonds of ß-keto esters; the reaction is mediated by the water-soluble hypervalent iodine(V) reagent AIBX (5-trimethylammonio-1,3-dioxo-1,3-dihydro-1λ5-benzo[d][1,2]iodoxol-1-ol anion). On the basis of the results of density functional theory calculations and experimental studies, we propose that the reaction proceeds by a two-stage mechanism involving dehydrogenation of the ß-keto ester substrates and epoxidation of the resulting enone intermediates. The rate-limiting step is abstraction of the ß'-C-H (calculated free energy of activation, 24.5 kcal/mol).

Mechanism and Origin of the Unexpected Chemoselectivity in Fluorocyclization of o-Styryl Benzamides with a Hypervalent Fluoroiodane Reagent.

The mechanism and origin of the unexpected chemoselectivity in fluorocyclization of o-styryl benzamide with a cyclic hypervalent fluoroiodane reagent were explored with DFT calculations. The calculations suggested an alternative mechanism that is broadly similar to, but also critically different from, the previously proposed mechanism for the formation of an unexpected structurally novel seven-membered 4-fluoro-1,3-benzoxazepine. The amide group of o-styryl benzamide was revealed to be crucial for activating the fluoroiodane reagent and facilitating C-F bond formation. In contrast to the popular electrophilic N-F reagent Selectfluor, the F atom in the fluoroiodane reagent is nucleophilic, and the I(III) atom is the most electrophilic site, thus inducing a completely different reactivity pattern. The insights reported here will be valuable for the further development of new reactions based on the hypervalent fluoroiodane reagent.

Mechanism of Silver-Mediated Geminal Difluorination of Styrenes with a Fluoroiodane Reagent: Insights into Lewis-Acid-Activation Model.

Fluorination mediated by the cyclic hypervalent fluoroiodane reagent (1) often requires an exogenous Lewis acid. The widely accepted Lewis-acid-activation model is that a given Lewis acid binds to the oxygen atom of 1 (O-coordination) to polarize the I-O bond. Computational studies of silver-mediated geminal difluorination of styrenes with 1 reveal a new "F-coordination" model that is energetically much preferred over the commonly accepted "O-coordination" model. The calculations rationalize the regioselective formation of the geminal difluorination product.