Palladium-Catalysed Dehydrogenative Generation of Imines from Amines. A Nature-Inspired Route to Indoles via Cross-Couplings of Amines with Arylhydrazines

H-substituted imines are elusive compounds formed when aldehydes or ketones are mixed with ammonia. However, this class of molecules can be prepared through selective removal of a hydrogen molecule from a primary amine using a transition metal catalyst. This biomimetic transformation, previously employed for the N-alkylation of anilines, is applied here to a domino preparation of differently substituted indoles from aliphatic primary amines and arylhydrazines. Several different linear and branched aliphatic primary amines are oxidized with reusable palladium on charcoal to the corresponding primary imines entrapped as rylhydrazones that can be isolated as such. Arylhydrazones can be further converted into N-alkylindole derivatives via an acid-mediated indolisation reaction. The one-pot domino hydrogen transfer Fischer indole synthesis under heterogeneous catalysis is also possible, giving access to diverse substituted indoles, ncluding NH indoles obtained after deprotection of the corresponding benzyl compounds. The truly heterogeneous nature of the catalyst was demonstrated by recycling the catalyst in the same reaction, and in other palladium-catalysed transformations.