Abstract
Density functional M11 was used to study the mechanism and enantioselectivity of a binaphthophosphepine-catalyzed intramolecular [3 + 2] cycloaddition reaction. The computational results revealed that this reaction proceeds through nucleophilic addition of the phosphine catalyst to the allene, which yields a zwitterionic phosphonium intermediate. The subsequent stepwise [3 + 2] annulation process, which starts with the intramolecular nucleophilic addition of the allenoate moiety to the electron-deficient olefin group, determines the enantioselectivity of the reaction. This step is followed by a ring-closing reaction and water-assisted proton-transfer process to afford the final product with concomitant regeneration of the phosphine catalyst. Theoretical predictions of the enantioselectivity for various phosphine catalysts were consistent with experimental observations, and 2D contour maps played an important role in explaining the origin of the enantioselectivity. Moreover, on the basis of our theoretical study, new binaphthophosphepine catalysts were designed and that are expecting to afford higher enantioselectivity in this cycloaddition reaction.
Highlights
Triphenylphosphine is the most common catalyst for phosphine-catalyzed [3 + 2] cycloadditions because of its easy availability, low cost, and high catalytic activity[42,43,44,45]
The generally accepted mechanism of phosphine-catalyzed [3 + 2] cycloaddition starts with a zwitterionic phosphonium intermediate, which is formed by the nucleophilic addition of the phosphine catalyst to an allene species
The initiation step of this reaction is the nucleophilic addition of binaphthophosphepine catalyst I with the (R)-2 enantiomer to generate zwitterionic phosphonium intermediate II, which has a gauche conformation
Summary
Triphenylphosphine is the most common catalyst for phosphine-catalyzed [3 + 2] cycloadditions because of its easy availability, low cost, and high catalytic activity[42,43,44,45]. As a result of the broad application of this catalyst in the construction of five-membered carbocycles, the mechanism of www.nature.com/scientificreports/ This [3 + 2] cycloaddition has attracted considerable attention by both theoretical and experimental chemists[34, 51,52,53,54]. Fu and coworkers have reported the enantioselective intramolecular [3 + 2] cycloaddition of allene 2 to generate chiral [3.3.0] bicyclic products 3 using a series of binaphthophosphepines as catalysts. Fu and coworkers have reported the enantioselective intramolecular [3 + 2] cycloaddition of allene 2 to generate chiral [3.3.0] bicyclic products 3 using a series of binaphthophosphepines as catalysts76 This reaction occurs under mild conditions with good yields. The design of new binaphthophosphepines based on theoretical predictions is discussed
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