Nucleophilic Attack on Nitrogen in Tetrazines by Silyl-Enol Ethers

The nucleophilic addition to nitrogen in 3-monosubstituted s-tetrazines under mild conditions is reported, by using silyl-enol ethers as the nucleophiles and mediated by BF3. The preference for this azaphilic addition over the usually observed inverse electron demand Diels-Alder reactions was determined experimentally and evaluated theoretically. In this regard, the influence of the effect of BF3-coordination to s-tetrazines was investigated thoroughly. The substrate dependency of this unusual reaction was rationalized by determination of the activation barriers and on the basis of the activation strain model by employing density functional theory. Lastly, the decomposition of the unstable adducts was examined and an interesting rearrangement to a triazine derived structure was observed.