Mechanism of Direct Electrophilic Aromatic Amination: an Electrophile is Found by Quantum-Chemical Study

Intramolecular amination via acid-catalyzed rearrangement of azides: a potent alternative to intermolecular direct electrophilic route

Although acid-catalyzed intramolecular rearrangement of organic azides is an attractive route to amines, its mechanism and synthetic prospective are still debated. Herein, through computational and experimental studies, we demonstrated that azide intramolecular rearrangement could serve as a potent synthetic route to a sought-after amine functionality including preparation of difficult to access and valuable heterocyclic amines. Using quantum chemical calculations at MP2/aug-cc-pVTZ and B3LYP/aug-cc-pVDZ levels, we discovered that this reaction proceeds via a concerted transition state with nitrogen elimination and alkyl/aryl migration occurring at the same time. Two conformers of protonated azides - syn- and anti- - were shown to precede corresponding transition states. It was shown that the reaction follows Curtin-Hammett scenario as the energy gap required for conformer interconversion was substantially lower than activation barrier of either transition state. Intramolecular amination via azide rearrangement was predicted to be a selective process with migratory aptitude increasing in a row alkyl<Ar-EWG<Ar-EDG (EWG - electron withdrawing group; EDG - electron donating group), which was supported by experimental results. We demonstrated experimentally that organic azides can be generated from stilbenes in situ and selectively undergo rearrangement to corresponding amines in a cascade fashion via amino-dealkenylation reaction.