Nitrogen heterocycles are some of the most important and sought-after structural motifs in synthetic chemistry. General methods that can be applied towards a structurally diverse range of different scaffolds are rare. This Focus Review discusses an emerging area with the field of π-acid catalysis based on the activation of carbon-carbon triple bonds in conjunction with nucleophilic nitrenoids. The resulting approach provides ready access into α-imino metal carbene reactivity patterns that can be employed in a number of quenching processes to realize a variety of powerful new transformations. The resulting methods are characterized by high efficiency, simple and straightforward reaction set ups, mild conditions, and excellent functional group tolerance. In this Focus Review the different nucleophilic nitrenoid types are explored showing how they can be used across a range of (poly)cyclization and formal cycloaddition processes to provide an alternative and direct disconnection pathway in the generation of N-heterocyclic motifs.

Gold carbene reactivity patterns were accessed by ynamide insertion into a C(sp³)H bond. A substantial increase in molecular complexity occurred through the cascade polycyclization of N-allyl ynamides to form fused nitrogen-heterocycle scaffolds. Exquisite selectivity was observed despite several competing pathways in an efficient gold-catalyzed synthesis of densely functionalized C(sp³)-rich polycycles and a copper-catalyzed synthesis of fused pyridine derivatives. The respective gold–keteniminium and ketenimine activation pathways have been explored through a structure–reactivity study, and isotopic labeling identified turnover-limiting CH bond-cleavage in both processes.

Gold carbene reactivity patterns were accessed by ynamide insertion into a C(sp³)H bond. A substantial increase in molecular complexity occurred through the cascade polycyclization of N-allyl ynamides to form fused nitrogen-heterocycle scaffolds. Exquisite selectivity was observed despite several competing pathways in an efficient gold-catalyzed synthesis of densely functionalized C(sp³)-rich polycycles and a copper-catalyzed synthesis of fused pyridine derivatives. The respective gold–keteniminium and ketenimine activation pathways have been explored through a structure–reactivity study, and isotopic labeling identified turnover-limiting CH bond-cleavage in both processes.

Unique α-hemiaminal ether gold carbene intermediates were accessed by a gold-catalysed 1,1-carboalkoxylation strategy and evolved through a highly selective 1,2-N-migration. This skeletal rearrangement gave functionalised indenes, and isotopic labelling confirmed the rare CN bond cleavage of the ynamide moiety. The effect of substituents on the migration has been explored, and a model is proposed to rationalise the observed selectivity.