Chemo- and Stereoselective Dearomative Coupling of Indoles and Bielectrophilic β-Imino Boronic Esters via Imine-Induced 1,2-Boronate Migration

Transition-metal-free iterative two-fold reductive coupling and 1,3-borotropic shift to form 1,4-skipped dienes

We report herein a transition-metal-free two-fold reductive coupling between prenal (allyl) tosylhydrazone and boronic acids/1,3-borotropic shift cascade to furnish 1,4-skipped dienes. In this work, a single batch operation produces (E,E)-1,4-skipped dienes by undergoing a second reductive coupling of the transient boronic acid, which developed in situ following the first reductive allylation and a cascade 1,3-boron migration. Remarkably, the protocol is compatible with various aryl- and alkyl-substituted boronic acids, is scalable and has demonstrated on 61 substrates with yields up to 98%.

Ag-Catalyzed Domino Decarboxylative Alkylation/Dearomative Annulation: Entry to Fused-Pyrido[4,3-b]Indolones

Here we report the development of unprecedented silver-catalyzed intramolecular annulations of N-acrolyl-2-(3-indolyl) benzimidazoles with alkyl carboxylic acids to construct complex fused-pentacyclic alkaloid scaffolds. Divergent reactivities are noticed with altered groups at C2-indole of the substrate. The reaction proceeds through decarboxylative alkylation, followed by dearomative annulation in a domino manner with excellent diastereoselectivity. Owing to the reactivity of the tert-OH group, these aza-enriched scaffolds can be further functionalized.

1,2-Metallate Rearrangement Using Indole Boronate Species to Access 2,3-Diarylindoles and Indolines

A transition metal-free multicomponent reaction using lithiated indole, boronic ester, pyridine, and ethyl chloroformate was developed to access C2,C3 bis-arylated indoles, which are present in several marketed drugs and bioactive compounds. One-pot access to unsymmetrical C2,C3-diaryl indole from the parent indole remains a huge synthetic challenge. Our group was able to achieve this goal through a transition metal-free 1,2-metalate rearrangement of the indole boronate complex. The reaction of indole boronate species with activated pyridine allows 1,2-migration to access pyridyl-indoleboronate species, which will convert to the corresponding indole upon oxidation and indoline after deborylation. The reaction tolerates substituted pyridines, quinolone, isoquinoline, and more. Both aryl and alkyl boronic esters were accommodated under optimized reaction conditions. Apart from mechanistic studies using ¹¹B-NMR, this methodology has been applied to the gram-scale synthesis of several bioactive compounds.

Annulative Coupling of Vinylboronic Esters: Aryne-Triggered 1,2-Metallate Rearrangement

A stereoselective annulative coupling of a vinylboronic ester ate-complex with arynes producing cyclic borinic esters has been developed. An annulation reaction that proceeded through the formation of two C–C bonds and a C–B bond was realized by exploiting a 1,2-metallate rearrangement of boronate triggered by the addition of a vinyl group to the strained triple bond of an aryne. The generated aryl anion would then cyclize to a boron atom to complete the annulation cascade. The annulated borinic ester could be converted to boronic acids and their derivatives by oxidation, halogenation, and cross-coupling. Particularly, halogenation and Suzuki–Miyaura coupling proceeded in a site-selective fashion and produced highly substituted alkylboronic acid derivatives.

A stereoselective annulative coupling of a vinylboronic ester ate-complex with arynes producing cyclic borinic esters has been developed.

A theoretical study based on DFT calculations on the different influence of functional groups on the C–H activation process via Pd-catalysed β-X elimination

We have performed a series of theoretical calculations for palladium-catalyzed β-X elimination reactions. The DFT calculation combined with energy decomposition analysis shows the determining factors of reactivity. Such as, the elemental composition, the structure of different functional groups and the stronger steric repulsions contribution.

The energy decomposition analysis (EDA) results show the quantitative contribution of nucleophile groups in intramolecular interactions.