Understanding the origin of the asynchronicity in bond-formation in polar cycloaddition reactions. A DFT study of the 1,3-dipolar cycloaddition reaction of carbonyl ylides with 1,2-benzoquinones

The origin of the asynchronicity in bond-formation in polar cycloadditions has been studied by an ELF analysis of the electron reorganisation along the 1,3-dipolar cycloaddition of Padwa's carbonyl ylide 4 with the 1,2-benzoquinone 8. This reaction presents an unexpected asynchronous bond-formation, which is initialised through the nucleophilic attack of Padwa's carbonyl ylide on the carbonyl oxygen atom of the strongly electrophilically activated 1,2-benzoquinone. The present study allows for the establishment that along an asynchronous bond-formation, the more favourable two-center interaction begins at the most electrophilic center, which is the center with the highest spin density achieved through the charge transfer process, and not, as expected, at the center that presents the larger positive charge.