Abstract

2,5-disubstituted tetrazoles decompose thermally to give nitrilimines (R1–CNN–R2), whose electronic structure widely varies with the substituents R1 and R2. In particular, for R1, R2 = H, the carbenic contribution to the resonance structure is small, while for NH2-disubstitution Natural Resonance Theory predicts 70% carbene character. This has large implications for the reactivities of substituted nitrilimines. The goal of this work is to identify nitrilimines with large carbenic contributions to their resonance structure that can undergo [1+2] cycloaddition reactions with unsaturated systems, in contrast to the common [3+2] cycloadditions of nitrilimines with only a small carbenic character.
In this study, twelve tetrazoles known in the literature have been analyzed with respect to changes in geometry and orbital interactions. A Natural Bond Orbital analysis of the corresponding nitrilimines indicates that several possess an electron lone pair on carbon, which could be indicative of a sizeable carbenic contribution to the resonance structure. Based on this finding, the decomposition of the respective tetrazoles has been investigated. We have shown that, depending on the substituents, decomposition is either stepwise or concerted: C-substituted tetrazoles with electron donating groups follow a concerted pathway while electron withdrawing groups promote a stepwise path.
Nitrilimines have been widely used in 1,3-dipolar ([3+2]) cycloadditions, in which they add to a dipolarophile, typically an alkene or alkyne, to form five-membered heterocyclic rings. In fact, all nitrilimines studied experimentally to date have been shown to undergo [3+2] cycloaddition reactions and their mechanism has been extensively studied in the past. On the other hand, nitrilimines can be described through a carbenic valence-bond structure, and while intermolecular carbenic reactions from nitrilimines are unknown, intramolecular reaction products from ortho-vinyl MeCOO–CNN–Ph and Ph–CNN–Ph that seem to have followed two typical carbene reaction mechanisms, [1+2] cycloaddition and C–H insertion, have been reported. In an attempt to promote the intermolecular [1+2] cycloaddition, reactions of F–CNN–F and NH2–CNN–NH2, both possessing substantial carbenic character, with ethene and electron-poor alkenes were compared to those for the unsubstituted H–CNN–H with smaller carbene character. The intermolecular [1+2] reaction is observed for H–CNN–H and F–CNN–F with tetrafluoroethene. NH2–CNN–NH2, as a nucleophilic species and possibly a stable carbene, does not tend to react with alkenes. To render these findings feasible for experiments, the carbenic character of readily accessible nitrilimines was determined from natural resonance theory.
Nitrilimines with even a small carbenic character can undergo [1+2] cycloaddition with tetrafluoroethene. For example, COOH–CNN–OH and CHO–CNN–OH, with as little as 13% carbenic character, undergo [1+2] cycloaddition preferentially (B3LYP/6-31+G(d)).