Structural characterization of the decomposition product of hexamethylenetetramine (HMTA). A combined theoretical and spectroscopic study

Hexamethylenetetramine (HMTA) is a widely known nitrogen heterocycle characterized by a cage-like structure reminiscing the adamantane motif. This versatile compound has an ample array of scientific and technological applications. However, the reactivity of HMTA and the nature of its intermediates at different conditions remains unknown to date. To gain an understanding of its reactivity, we aim to elucidate the intermediates from this amine at the most common employed conditions (high temperature and acidic media). To be able to distinguish among all the possible compounds, we employed an array of analytical and computational techniques in a synergistic way. We combined the use of NMR experiments, chemometric tools, and DFT studies to unveil structural and mechanistic details about the stability and reactivity of the intermediates of HMTA. Applying these techniques, we found that the first stage entails a nitrogen atom-protonation, which is followed by a ring opening to afford an external imine. Then, at high temperature (90°C), a [1,5-H] sigmatropic rearrangement is observed, with the formation of the internal imine. This rearrangement involving two nitrogen atoms is not uncommon and can be carried out thermally or under Lewis acids assistance. We have found that theoretical studies give full support to the mechanism proposed for the formation of the internal imine. These findings add to the current knowledge of the stability of HMTA and may help to explain some of the challenges this substrate has in industrial applications.