2-Monosubstituted 1,3-dioxolanes and dithiolanes act as hydride-releasing fragments, transferring intramolecularly their acetalic H atom to the central carbon of ketenimine functions. The presumed products of these migrations, o-quinomethanimines, undergo in situ 6 pi-electrocyclization. A computational study supports this mechanism and the hydride-shift character of the first step. Carbodiimides were also suitable substrates, although less reactive.
Tandem 1,5-Hydride Shift/6π Electrocyclization of Ketenimines and Carbodiimides Substituted with Cyclic Acetal and Dithioacetal Functions: Experiments and Computations
beneficial effect of the acetal function, which gives hydride character to the migrating hydrogen atom. The capability to promote the H shift depends on the type of acetal function (acetal better than dithioacetal), its ring size (five-membered better than six-membered) and the heterocumulenic fragment (ketenimine better than carbodiimide). Changing the benzene ring connecting the acetal and ketenimine functions