Structure-Reactivity Relationships of Zwitterionic 1,3-diaza-Claisen Rearrangements

Bridged bicyclic tertiary allylic amines aza-norbornene 1 and isoquinuclidene 2 add to isocyanates, isothiocyanates, and in situ-generated carbodiimides to form zwitterionic intermediates that undergo 1,3-diaza-Claisen rearrangements to afford highly substituted ureas, thioureas, and guanidines, respectively. Aza-norbornene 1 is significantly more reactive toward 1,3-diaza-Claisen rearrangements than isoquinuclidene 2. This reactivity difference is most likely due to the inherent ring strain in the aza-bicyclo[2.2.1]heptene ring system of aza-norbornene 1. The most apparent reactivity trend of the heterocumulenes is that the most electron-deficient heterocumulenes are more reactive toward 1,3-diaza-Claisen rearrangements. The introduction of a new stereocenter α- to the nucleophilic nitrogen in aza-norbornene 1 and isoquinuclidine 2 decreases the reactivity toward 1,3-diaza-Claisen rearrangements, while the exodiastereomers 3b and 4b are less reactive than the corresponding endodiastereomers 3a and 4a. Is...