Theoretical studies on selectivities in the Staudinger reaction of vicinal diimines and ketenes

The selectivities, including peri-, regio-, and diastereoselectivities, in the Staudinger reaction involving vicinal diimines and ketenes were investigated theoretically via the density functional theory (DFT) calculation. The results indicate that vicinal diimines prefer stepwise 2+2] cycloaddition rather than 2+4] cycloaddition to generate cis-4-imino-β-lactams. The diimines attack the less sterically hindered exo-side of ketenes to generate zwitterionic intermediates, which directly undergo a conrotatory ring closure to produce cis-4-imino-β-lactams whatever diimines with less or more bulky N-substituents. For unsymmetric vicinal ketoaldehyde-derived diimines, their ketimines attack the exo-side of ketenes and undergo a conrotatory ring closure to produce cis-4-aldimino-β-lactams due to less steric effect. The current theoretical studies provide very important information for in-depth understanding of the selective formation of mono-cis-β-lactams from vicinal diimines and ketenes.