Iridium‐Catalyzed Direct C4‐ and C7‐Selective Alkynylation of Indoles Using Sulfur‐Directing Groups

Selective para-C-H Alkynylation of Aniline Derivatives via Pd/S,O-Ligand Catalysis

Herein, we report a nondirected para -selective C-H alkynylation of aniline derivatives by a Pd/S,O-ligand-based catalyst. The reaction proceeds under mild conditions and is compatible with a variety of substituted anilines. The scalability and further derivatizations of the alkynylated products have been also demonstrated.

Cascade Reaction to Selectively Synthesize Multifunctional Indole Derivatives by IrIII -Catalyzed C-H Activation

An effective and condition-controlled way to synthesize with high selectivity a variety of functionalized indoles with potent biological properties has been developed. Notably, 2,4-dialkynyl indole products were obtained by direct double C-H bond alkynylation, whereas alkynyl at the C4 position could convert to carbonyl to generate 2-alkynyl-3,4-diacetyl indoles fast and effectively. Additionally, a one-pot relay catalytic reaction led to 2,5-di-alkynyl-3,4-diacetyl indoles when using a carbonyl group as the directing group and by controlling the type and quantity of additives. A possible mechanism was proposed based on many studies including deuterium-exchange experiments, the necessary conditions of product conversion, and the effect of water on the reaction.

Palladium-Catalyzed Remote C-H Phosphonylation of Indoles at the C4 and C6 Positions by a Radical Approach

Palladium-catalyzed direct C-H activation of indole benzenoid moiety has been achieved in the past decade. However, palladium-catalyzed remote C-H activation of indoles is rare. Herein, we report a challenging palladium-catalyzed remote C4-H phosphonylation of indoles by a radical approach. The method provides access to a series of C4-phosphonylated indoles, including tryptophan and tryptophan-containing dipeptides, which are typically inaccessible by direct C4-H activation due to its heavy reliance on C3 directing groups. Notably, unexpected C6-phosphonylated indoles were obtained through blocking of the C4 position. The preliminary mechanistic studies indicated that the reactions may proceed via a C7-palladacycle/remote-activation process. Based on the strategy, examples of remote C4-H difluoromethylation with BrCF₂ COOEt are also presented, suggesting that the strategy may offer a general blueprint for other cross-couplings.

Palladium-Catalyzed Silacyclization of (Hetero)Arenes with a Tetrasilane Reagent through Twofold C-H Activation

The use of an operationally convenient and stable silicon reagent (octamethyl-1,4-dioxacyclohexasilane, ODCS) for the selective silacyclization of (hetero)arenes via twofold C-H activation is reported. This method is compatible with N-containing heteroarenes such as indoles and carbazoles of varying complexity. The ODCS reagent can also be utilized for silacyclization of other types of substrates, including tertiary phosphines and aryl halides. A series of mechanistic experiments and density functional theory (DFT) calculations were used to investigate the preferred pathway for this twofold C-H activation process.

Deconstructive Reorganization: De Novo Synthesis of Hydroxylated Benzofuran

An unprecedented deconstructive reorganization strategy for the de novo synthesis of hydroxylated benzofurans from kojic acid- or maltol-derived alkynes is reported. In this reaction, both the benzene and furan rings were simultaneously constructed, whereas the pyrone moiety of the kojic acid or maltol was deconstructed and then reorganized into the benzene ring as a six-carbon component. Through this strategy, at least one free hydroxyl group was introduced into the benzene ring in a substitution-pattern tunable fashion without protection-deprotection and redox adjustment. With this method, a large number of hydroxylated benzofuran derivatives with different substitution-patterns have been prepared efficiently. This methodology has also been shown as the key step in a collective total synthesis of hydroxylated benzofuran-containing natural products (11 examples).