Thel ast decades have seen at remendous expanse in the applicationo fC À Ha ctivation of many different substrate classes,including the invaluable indole scaffold. Following the exciting emergence of C À Ha ctivation as am ulti-faceted platform for functionalization, av ersatile toolb ox has been developed for the preparation of structurally diverse indoles. This review article discussesr ecent advances and strategies for transitionm etal-catalyzed C À Ha ctivationofi ndoles.

A mild and metal-free approach to C-N coupling is described that employs diaryliodonium salt electrophiles and secondary aliphatic amine nucleophiles. This reaction results in direct ipso-substitution of the iodonium moiety and unsymmetrical aryl(TMP)iodonium salts are primarily employed. Moreover, arene substituents and substitution patterns that currently pose a challenge to classical metal-free methods are accommodated and the alicyclic amine nucleophiles used here are unprecedented in other contemporary metal-free C-N coupling reactions.

A Sonogashira coupling reaction method to join terminal alkynes to the imidazole backbone was developed and investigated. The method exhibits good functional group tolerance and provides target 4‐alkynylated imidazoles in 70–93 % yield. The alkyne reagents were characterized by means of DFT calculations, from which electrostatic potential surfaces (EPS) were produced. A clear correlation between the EPS of the triple bond and the success of the coupling reaction was revealed. If the EPS is in range –94 to –105 kJ mol–1 the coupling is successful. An unsuccessful class of reagents (alkynols) was made compatible by means of an auxiliary group (tert‐butyldimethylsilyl). EPSs of these modified reagents then resembled those of the model and these auxiliary‐assisted reagents then coupled successfully in excellent yields.

The direct synthesis of aryl(2,4,6-trimethoxyphenyl)iodonium trifluoroacetate salts from aryl iodides is described. Stoichiometric quantities of trifluoroacetic acid and trimethoxybenzene are used as the counteranion and auxiliary precursors, respectively, under oxidizing conditions. The reaction occurs at mild temperature, is broad in scope, and does not require a separate anion exchange step to install the trifluoroacetate group. The intermediacy of two distinct dicarboxy aryl-λ-iodanes is hypothesized in the mechanism.

The synthesis of the Securinega alkaloid secu'amamine E (ent-virosine A) has been accomplished for the first time in 12 steps and 8.5% overall yield. In addition, bubbialine has been prepared and characterized. These two alkaloids and bubbialidine, all featuring an azabicyclo[2.2.2]octane core, were rearranged to their azabicyclo[3.2.1]octane congeners, a framework found in many Securinega alkaloids. These experiments suggest that azabicyclo[2.2.2]octane derivatives could serve as intermediates in the biosynthesis of the rearranged azabicyclo[3.2.1]octane products.

A three-way switchable Pd-catalyzed and microwave assisted process appropriate for selective arylation or hydrodehalogenation of the imidazole backbone was discovered and entirely optimized. The "arylation switch position" was adapted and optimized for the synthesis of 4,5-diaryl-1H-imidazoles, while the "hydrodehalogenation switch position" was used for the preparation of 4(5)-iodo-1H-imidazole. The hydrodehalogenation and the cross-coupling reactions were also successfully combined in "the third switch position" that performs an assisted tandem reaction sequence that produced 4(5)-aryl-1H-imidazole. All of the three pathways produced their corresponding products in excellent yield.

A general Cu-catalyzed, regioselective method for the N-3-arylation of hydantoins is described. The protocol utilizes aryl­(trimethoxyphenyl)­iodonium tosylate as the arylating agent in the presence of triethylamine and a catalytic amount of a simple Cu-salt. The method is compatible with structurally diverse hydantoins and operates well with neutral aryl groups or aryl groups bearing weakly donating/withdrawing elements. It is also applicable for the rapid diversification of pharmaceutically relevant hydantoins.

Computational chemistry has shown that backbone-alkylated imidazoles ought to be efficient ligands for transition metal catalysts with improved carbene-to-metal donation. In this work, such alkylated imidazoles were synthesized and complexed with silver(I) by means of an eight/nine-step synthetic pathway we devised to access a new class of biologically active silver complexes. The synthesis involves selective iodination of the imidazole backbone, followed by Sonogashira coupling to replace the backbone iodine. The installed alkyne moiety is then subjected to reductive hydrogenation with Pearlman’s catalyst. The imidazole N1 atom is arylated by the palladium-catalyzed Buchwald N-arylation method. The imidazole N3 position was then methylated with methyl iodine, whereupon the synthesis was terminated by complexation of the imidazolium salt with silver(I) oxide. The synthetic pathway provided an overall yield of ≈20 %. The resulting complexes were tested in vitro against HL60 and MOLM-13 leukemic cells, two human-derived cell lines that model acute myeloid leukemia. The most active compounds exhibiting low IC50 values of 14 and 27 μM, against HL60 and MOLM-13 cells, respectively. The imidazole side chain was found to be essential for high cytotoxicity, as the imidazole complex bearing a C7 side chain at the 4-position was four- to sixfold more potent than the corresponding imidazole elaborated with a methyl group.