Silyloxyazadienes: one intermediate and two competitive pericyclic reactions

The two competing mechanisms in the reaction of 3-trialkylsilyloxy-2-aza-1,3 dienes to form b-lactams through a [2+2] electrocyclic ring closure or tetrahydrooxazin-2-ones via a [4+2] hetero Diels-Alder reaction were studied using Density Functional computations. Although the [2+2] and [4+2] mechanisms are typical of dienes, their competition, starting from the same diene intermediate, has not yet been observed and analyzed. This competition is governed by a delicate interplay between temperature and substituents at the diene and dienophile respectively. Clearly, entropy tends to favor the [4+2] hetero Diels-Alder at low temperatures and the [2+2] electrocyclic ring closure at high temperatures, but a simple substituent modification at the diene and dienophile, can make the [4+2] competitive at high temperatures and sometimes even trasform the [4+2] concerted mechanism into a two-step Mukaiyama-type process. Moreover a study of the global electrophilicity values showed that charge transfer in the hetero Diels-Alder transition states is driven by chemical hardness rather than by chemical potential