On the Unique Reactivity of Pd(OAc)2 with Organic Azides: Expedient Synthesis of Nitriles and Imines

Organic azides are well-established as versatile compounds that can act as precursors of different heterocycles (triazoles, triazolines, tetrazoles, etc.) or other nitrogen-containing compounds, such as amines (Staudinger reduction, Curtius rearrangement) or imines (Schmidt rearrangement, aza-Wittig reaction). Besides the ubiquitous copper-catalysed azide–alkyne cycloaddition reaction, two applications of organic azides have recently attracted the interest of the synthetic community: 1) the preparation of aziridines through the generation of nitrenes and 2) the synthesis of nitriles. We were particularly interested in the latter application, owing to the importance of the cyano group in industry, as well as its utility as an organic synthon. Traditional cyanation methods suffer from serious drawbacks, such as the use of highly toxic reagents, the generation of stoichiometric quantities of metal waste, the need for harsh conditions, and poor functional-group tolerance. Azides allow the cyanide-free preparation of nitriles with no elongation of the skeletal carbon chain under several conditions: strong stoichiometric oxidants (such as BF3, [5a] 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ), or KI/tert-butyl hydroperoxide), CuSO4/phenyliodonium diacetate, [6] CuI/tertbutyl hydroperoxide, or a supported Ru OH catalyst. An earlier report, which employed palladium on charcoal with an alkyne as a hydrogen acceptor, caught our attention for its neutral conditions and its scope, which was not restricted to the formation of benzonitrile derivatives [Eq. (1)] . In contrast to the other methods, this Pd system required the rigorous exclusion of moisture and oxygen for any reaction to take place at reflux in benzene or diethylamine. However, despite our efforts, we could not reproduce the reported results and only conversions of <20% were obtained. Considering that different Pd/C catalysts can lead to adventitious results, we optimised the reaction conditions (Table 1). We