The use of the ring-closing enyne metathesis (RCEYM) as a methodology for the synthesis of the azonino[5,4-b]indole system, featuring the tricyclic substructure of the alkaloids cleavamine and quebrachamine, has been explored. Three series of enyne substrates were studied for their compatibility with the RCEYM reaction. In addition to the usual substrates bearing either a terminal or an internal alkyne, for the first time enynes with an alkynyl halide moiety were also considered. Although the metathesis cyclization allowed for assembly of the azoninoindole nucleus in all three series, an effective catalytic cycle was only noted for internal alkyne substrates. On the basis of the experimental results, the “yne-then-ene” pathway seems to be the mechanism at play in these reactions.

A palladium-catalyzed carbene insertion into C(sp³)−H bonds leading to pyrrolidines was developed. The coupling reaction can be catalyzed by both Pd⁰ and Pd^II, is regioselective, and shows a broad functional group tolerance. This reaction is the first example of palladium-catalyzed C(sp³)−C(sp³) bond assembly starting from diazocarbonyl compounds. DFT calculations revealed that this direct C(sp³)−H bond functionalization reaction involves an unprecedented concerted metalation–deprotonation step.

A palladium-catalyzed carbene insertion into C(sp³)−H bonds leading to pyrrolidines was developed. The coupling reaction can be catalyzed by both Pd⁰ and Pd^II, is regioselective, and shows a broad functional group tolerance. This reaction is the first example of palladium-catalyzed C(sp³)−C(sp³) bond assembly starting from diazocarbonyl compounds. DFT calculations revealed that this direct C(sp³)−H bond functionalization reaction involves an unprecedented concerted metalation–deprotonation step.

The influence of the heteroatom (nitrogen, oxygen, and sulfur) on the course of the palladium-catalysed intramolecular reactions of aryl iodides and aldehydes having heteroatom-containing tethers has been explored by an extensive experimental–computational (DFT) study. Two series of substrates were considered, namely aldehydes bearing either the α-(2-iodobenzylheteroatom) or β-(2-iodophenylheteroatom) moieties. While some experimental differences were observed when changing from nitrogen to oxygen or sulfur in the 2-iodobenzyl series, the aldehydes in which the heteroatom is directly bonded to the aromatic ring showed common chemical behaviour regardless of the nature of the heteroatom. The different reaction pathways leading to the experimentally observed reaction products were studied by computational means. Our calculations suggest that in all cases the initial nucleophilic addition involving a σ-aryl–Pd^II intermediate is preferred over the competing concerted metallation–deprotonation (CMD) process.

A new strategy for the synthesis of tetrahydroisoquinolines based on the Pd⁰-catalyzed intramolecular α-arylation of sulfones is reported. The combination of this Pd-catalyzed reaction with intermolecular Michael and aza-Michael reactions allows the development of two- and three-step domino processes to synthesize diversely functionalized scaffolds from readily available starting materials.

The factors that control the chemoselectivity of palladium-catalyzed cyclization reactions of (2-iodoanilino)carbonyl compounds have been explored by an extensive experimental computational (DFT) study. It was found that the selectivity of the process, that is, the formation of fused six- versus five-membered rings, can be controlled by the proper selection of the initial reactant, reaction conditions, and additives. Thus, esters or amides produce ketones by a nucleophilic addition process, whereas the addition of PhO⁻ ions leads to the formation of indolines by an α-arylation reaction. In contrast, the corresponding ketone reactants yield a mixture of both reaction products, the ratio of which depends on the base used, in the presence of phenol. The outcome of the processes can be explained by the formation of a common four-membered palladacycle intermediate from which the competitive nucleophilic addition and α-arylation reactions occur. The remarkable effect of phenol in the process, which makes the α-arylation reaction easier, favored the formation of enol complexes, which are stabilized by an intramolecular hydrogen bond between the hydroxy group of the enol moiety and the oxygen atom of the phenoxy ligand. Moreover, the chemoselectivy of the process can be also controlled by the addition of bidendate ligands that lead to the almost exclusive formation of indoles at expenses of the corresponding alcohols.