Understanding the selectivity in the formation of delta-lactams vs. beta-lactams in the Staudinger reactions of chloro-cyan-ketene with unsaturated imines. A DFT study

作者：Domingo Luis R^*; Saez Jose A

来源：RSC Advances, 2014, 4(102): 58559-58566.

DOI：10.1039/c4ra10291f

The reactions of chloro-cyan-ketene with two phenyl substituted unsaturated imines yielding beta- or delta-lactams have been investigated using DFT methods at the MPWB1K/6-311G(d,p) level in diethyl ether. The reactions are initialised by the nucleophilic attack of the unsaturated imines on the ketene with formation of zwitterionic intermediates. The subsequent C-C single bond formation at the imine carbon or at the beta-conjugated position enables the formation of beta- or delta-lactams. Analysis of the energies involved in the two competitive channels explains the selectivity experimentally observed; in the absence of any steric hindrance, formation of delta-lactams is favoured over the formation of beta-lactams. ELF topological analysis allows explaining the bonding changes along the two competitive channels. While formation of the N-C bond takes place by participation of the nitrogen lone pair, formation of the C-C bonds takes place through a retrodonation process involving the C-C double bond of the ketene and the C-N or C-C double bonds of the unsaturated imine. ELF topological analysis makes it possible to rule out an electrocyclic mechanism for the cyclisation step.