Straightforward Synthesis of Indenes by Gold-Catalyzed Intramolecular Hydroalkylation of Ynamides

Base-Assisted and Silica Gel-Promoted Indole-Substituted Indene Synthesis

Herein, we report for the first time a transition-metal-free and mild protocol that requires inexpensive K2CO3 and silica gel for direct access to polysubstituted indenes from readily obtainable starting precursors. Notably, sequential Michael addition, intramolecular cyclization, and silica gel-promoted nucleophilic substitution reactions afford the desired products. A broad range of indoles and other aromatic nucleophiles are well-tolerated, affording indenes in moderate to good yields. Gratifyingly, indene could be easily converted into synthetically useful 3-indole-substituted indanone and indanol. Nonetheless, the successful isolation of a reaction intermediate highlights the crucial role of methanol in this reaction.

Mechanistic Studies on the Gold-Catalyzed Intramolecular Hydroalkylation of Ynamides to Indenes

An in-depth experimental and computational study to rationalize the mechanism underlying the gold-catalyzed intramolecular hydroalkylation of ynamides to indenes is reported. Evaluating the reactivity of a set of deuterated ynamides and gold complexes allowed to get valuable insights into the mechanism of this reaction, while DFT calculations allowed to determine a plausible reaction pathway for this unprecedented transformation. This pathway involves the activation of the ynamide followed by a [1,5]-hydride shift from the highly reactive, in situ generated keteniminium ion, and a subsequent cyclization before deprotonation followed by a final protodeauration. According to DFT calculations, the initial [1,5]-hydride shift was identified as the rate-determining step of the reaction mechanism. Additionally, computational studies allowed to rationalize the differences in reactivity of various ynamides and the pivotal role of gold complexes in the catalysis of this reaction.

TfOH-Catalyzed Reactions of Aryl Methyl Ketones with Ynamides: Synthesis of 1-Amino-1H-indenes and 2,4-Dienamides

The efficient synthesis of 1-amino-1H-indenes and 2,4-dienamides was realized via TfOH-catalyzed reactions of aryl methyl ketones with terminal ynamides in two distinct pathways. Aromatic ketones with high electrophilicity underwent [3 + 2] annulation with ynamides to produce 1-amino-1H-indenes, while aromatic ketones with low electrophilicity proceeded under the same conditions to afford 2,4-dienamides. Furthermore, the obtained 1-amino-1H-indenes could be converted into the corresponding 1H-indenes and dihydro-1H-indenes in excellent yields.

Steric, Electronic and Conformational Synergistic Effects in the Gold(I)-catalyzed α-C-H Bond Functionalization of Tertiary Amines

Direct C-H bond functionalization is a useful strategy for the straightforward formation of C-C and C-Heteroatom bonds. In the present work, a unique approach for the challenging electrophilic Au-catalyzed α-C-H bond functionalization of tertiary amines is presented. Electronic, steric and conformational synergistic effects exerted by the use of a malonate unit in the substrate were key to the success of this transformation. This new reactivity was applied to the synthesis of tetrahydro-γ-carboline products which, under oxidative conditions, could be converted into valuable structural motifs found in bioactive alkaloid natural products.

Recent Advances in Gold(I)-Catalyzed Approaches to Three-Type Small-Molecule Scaffolds via Arylalkyne Activation

Gold catalysts possess the advantages of water and oxygen resistance, with the possibility of catalyzing many novel chemical transformations, especially in the syntheses of small-molecule skeletons, in addition to achieving the rapid construction of multiple chemical bonds and ring systems in one step. In this feature paper, we summarize recent advances in the construction of small-molecule scaffolds, such as benzene, cyclopentene, furan, and pyran, based on gold-catalyzed cyclization of arylalkyne derivatives within the last decade. We hope that this review will serve as a useful reference for chemists to apply gold-catalyzed strategies to the syntheses of related natural products and active molecules, hopefully providing useful guidance for the exploration of additional novel gold-catalyzed approaches.