A DFT study of the [4+2] cycloadditions of conjugated ketenes (vinylketene, imidoylketene and formylketene) with formaldimine. The pericyclic or pseudopericyclic character from magnetic properties

A comprehensive B3LYP/6-31+G∗ study of the nature of the [4+2] cycloadditions of conjugated ketenes, vinylketene, imidoylketene and formylketene, with formaldimine was conducted. For each reaction, the complete pathway was determined and changes in different magnetic properties (magnetic susceptibility, χ, magnetic susceptibility anisotropy, χanis, and the nucleus-independent chemical shift, NICS) were monitored along the reaction profile with a view to estimate the aromatization associated to the process. We have also applied the ACID (anisotropy of the current-induced density) method with the same purpose. The deep analysis of the results indicates the existence of both disrotatory and conrotatory pericyclic paths for the cyclization step of the cycloaddition of vinylketene with formaldimine and the pseudopericyclic character of reactions with imidoylketene and formylketene.

The mechanism of the [4+2] cycloadditions of vinylketene, imidoylketene and formylketene with formaldimine was theoretically studied, analyzing the pericyclic or pseudopericyclic character of the cyclization step.