Mechanistic insights on the enantioselective (4+3) cycloaddition between oxyallylcations and furans catalyzed by BINOL-based phosphoramides

The mechanism of the enantioselective (4+3) cycloaddition between furan derivatives and oxyallylcations, which are generated from the corresponding oxiranes through in situ oxidation of allenamides, has been studied using DFT methods. The research has revealed that, under acid-catalysis by a chiral non-racemic phosphoramide, the epoxide ring-opening proceeds without any energy barrier, while the rate-limiting step is the electrophilic attack of the intermediate enaminium ion on the furan ring. The reaction exhibits low energy barriers when dealing with furan derivatives unsubstituted at C2 and C5. Calculations predict the formation of an achiral regioisomer for 2-substituted furans, a prediction that has been experimentally confirmed. Additionally, the calculations accurately predict the reaction with substituted allenamides.