Nickel-Catalyzed trans-Carboamination across Internal Alkynes to Access Multifunctionalized Indoles

Mechanistic insights on C(acyl)-N functionalisation mediated by late transition metals

The carboxamide functional group has a privileged role in organic and biological chemistry due to its prevalence and utility across synthetic and natural products. Due to nN → π*CO delocalisation, amides and related functional groups are typically kinetically resistant to degradation. Nonetheless, over the past decade, transition metal catalysis has transformed our ability to utilise molecules featuring C(acyl)-N units as reactants. Alongside the burgeoning catalytic applications ranging from COx utilisation to small molecule synthesis, elucidation of the underlying mechanisms remains a critical ongoing effort. Herein, we aggregate and analyse current understanding of the mechanisms for C(acyl)-N functionalisation of amides and related functional groups with a focus on recent developments involving mechanisms unique to the late transition metals. Discussion is organized around three general mechanistic manifolds: redox-neutral mechanisms, 2e- redox-cycling mechanisms, and mechanisms involving 1e- redox steps. For each class, we focus on reactions that directly involve a transition metal mediator/catalyst in the C(acyl)-N cleavage step. We conclude with an outlook on the outstanding ambiguities and opportunities for innovation.

Metal Additive-Free Iodine-Promoted Reorganization of Ynamide-Ynes and Stereoselective 1,2-Diiodination

We report a metal additive-free iodine-promoted one-step structural reorganization of ynamide-ynes and simultaneous stereoselective 1,2-diiodination of the migrated alkyne to form stereospecific tetrasubstituted alkenyl diiodo-tethered indoles (E-isomer). Molecular iodine is cost effective, user friendly, less toxic, commercially available, and easy to handle. The key features of the reaction include metal-and additive-free environment, selectivity, structural reorganization, mild reaction conditions, simple workup, and gram-scale synthesis. This transformation generates multiple bonds [nitrogen (N)-carbon (C)sp2, Csp2-Csp2, 2 Csp2-iodine (I)] via N-Csp bond cleavage in ynamide-ynes.

Recent advances in electrochemically enabled construction of indoles from non-indole-based substrates

Indole motifs are important heterocycles found in natural products, pharmaceuticals, agricultural chemicals, and materials. Although there are well-established classical name reactions for indole synthesis, these transformations often require harsh reaction conditions, have a limited substrate scope, and exhibit poor regioselectivity. As a result, organic synthesis chemists have been exploring efficient and practical methods, leading to numerous strategies for synthesizing a variety of functionalized indoles. In recent years, electrochemistry has emerged as an environmentally friendly and sustainable synthetic tool, with widespread applications in organic synthesis. This technology allows for elegant synthetic routes to be developed for the construction of indoles under external oxidant-free conditions. This feature article specifically focuses on recent advancements in indole synthesis from non-indole-based substrates, as well as the mechanisms underlying these transformations.

Dual Photoredox and Nickel Catalysis in Regioselective Diacylation: Exploring the Versatility of Nickel Oxidation States in Allene Activation

We present a nickel-catalyzed regioselective radical diacylation of allenes with ketoacids to produce 1,4-dione products by dual photoredox and nickel catalysis. This integrated approach merges redox-active oxidative addition and reductive elimination steps with migratory insertion. The acyl radical generated in the photoredox cycle sequentially adds to Ni(I) and Ni(II) intermediates following a Ni(I)-Ni(II)-Ni(II)-Ni(III)-Ni(I) catalytic cycle. This methodology, supported by DFT calculations, demonstrates the potential of nickel catalysis in the creation of complex molecular architectures.

Update on novel synthetic approaches towards the construction of carbazole nuclei: a review

The carbazole scaffold is a significant entity in organic compounds due to its variety of biological and synthetic applications. Traditionally, carbazole skeletons have been synthesized either via the Grabe-Ullman method, Clemo-Perkin method or Tauber method. With the passage of time, these methods have been modified and explored to accomplish the synthesis of target compounds. These methods include hydroarylations, C-H activations, annulations and cyclization reactions mediated by a variety of catalysts to construct carbazole-based compounds. This brief review article intends to provide recent updates on important methodological developments reported for the synthesis of carbazole nuclei covering 2019-2023.

Base-Promoted Tandem Synthesis of 2-Substituted Indoles and N-Fused Polycyclic Indoles

Herein is developed a base-promoted approach for the synthesis of C2-substituted indoles and N-fused polycyclic indoles via 5-endo-dig cyclization of 2-alkynyl anilines, followed by a 1,3'-acyl migration or a dearomatizing Michael addition process. A range of N-H free indoles and 8,9-dihydropyrido[1,2-a]indol-6(7H)-one scaffolds were synthesized in good to excellent yields with broad scope.

Visible Light-Induced Metal-Free Fluoroalkylations

Fluoroalkylation is a crucial synthetic process that enables the modification of molecules with fluoroalkyl groups, which can enhance the properties of compounds and have potential applications in medicine and materials science. The utilization of visible light-induced, metal-free methods is of particular importance as it provides an environmentally friendly alternative to traditional methods and eliminates the potential risks associated with metal-catalyst toxicity. This Account describes our studies on visible light-induced, metal-free fluoroalkylation processes, which include the use of organic photocatalysts or EDA complexes. We have utilized organophotocatalysts such as Nile red, tri(9-anthryl)borane, and an indole-based tetracyclic complex, as well as catalyst-free EDA chemistry through photoactive halogen bond formation or an unconventional transient ternary complex formation with nucleophilic fluoroalkyl source. A variety of π-systems including arenes/heteroarenes, alkenes, and alkynes have been successfully fluoroalkylated under the developed reaction conditions.

Enantioselective Amino- and Oxycyanation of Alkenes via Organic Photoredox and Copper Catalysis

The β-amino nitrile moiety and its derivatives frequently appear in natural product synthesis, in drug design, and as ligands in asymmetric catalysis. Herein, we describe a direct route to these complex motifs through the amino- and oxycyanation of olefins utilizing an acridinium photooxidant in conjunction with copper catalysis. The transformation can be rendered asymmetric by using a serine-derived bisoxazoline ligand. Mechanistic studies implicate olefin-first oxidation. The scope of amines for the aminocyanation reaction has been greatly expanded by undergoing a cation radical intermediate as opposed to previous N-centered radical-initiated aminocyanations. Furthermore, alkyl carboxylic acids were included as nucleophiles in this type of transformation for the first time without any decarboxylative side reactions.

Dodging the Conventional Reactivity of o-Alkynylanilines under Gold Catalysis for Distal 7-endo-dig Cyclization

A direct ring-closing strategy involving a less facile 7-endo-dig carbacyclization of o-alkynylaniline derivatives for the synthesis of benzo[b]azepines has been presented. The trivial well-documented 5-endo-dig cyclization in o-alkynylaniline derivatives due to high nucleophilicity of nitrogen has been overcome by using their vinylogous amides under gold catalysis to access a wide array of benzo[b]azepines in an atom economical way with excellent functional group compatibility. Deuterium scrambling experiments and DFT studies favor a mechanism involving stabilizing conformational change of the initially formed seven-membered vinyl gold intermediate through a key cyclopropyl gold carbene intermediate and its subsequent protodeauration mediated by the counter anion.

A Convergent Synthesis of Tetracyclic Indole Compounds by a ­Palladium-Catalyzed Cross-Coupling and Tandem Cyclization ­Reaction

A new strategy for the convergent synthesis of the ABCD ring system of indole terpene alkaloids has been developed based on a Sonogashira coupling of an o-iodoaniline (the A ring) with an alkyne bearing the D ring. After a tandem palladium-catalyzed cyclization, the tetracyclic ABCD ring structure found in the terpene indole alkaloids was obtained in good yield.

Transposition of an acrylate moiety in TMSOTf-mediated reaction of alkynyl vinylogous carbonates gives heterocyclic dienes

TMSOTf-mediated reaction of alkynyl vinylogous carbonates serendipitously gave 1,4-oxazepine and dihydropyran dienes via transposition of an ethyl acrylate moiety involving intramolecular cascade Prins-type cyclization/retro-oxa-Michael reaction/cycloisomerisation. The developed atom-economical protocol selectively provides an E double bond geometry. Dihydropyran dienes could be reduced diastereoselectively using Et₃SiH/TMSOTf or could be transformed into polycyclic heterocycles by Heck reaction.

Nickel-Catalyzed Reductive Borylation of Enaminones via C(sp2)-N Bond Cleavage

The cleavage and transformation of alkenyl C(sp²)-N bonds is a significant synthetic challenge. Herein we described an unprecedented nickel-catalyzed reductive borylation of enaminones to synthesize β-ketone boronic esters. Notably, B₂pin₂ played the dual role in this process, and water served as a hydrogen source, which was transferred to target products. The air-stable nickel catalyst was applied to the cleavage of alkenyl C(sp²)-N bonds, concomitant with the reductive process of the alkenyl boronic ester intermediates, on the basis of the mechanism study.

Anodic dearomatization of 2-alkynylanilines for the synthesis of multi-functionalized indoles

An anodic oxidative dearomatization reaction of 2-alkynylanilines was developed. The formed dearomatized compounds were used as versatile precursors in the synthesis of a variety of benzenoid ring multi-functionalized indoles through simple conversions.

Photoinduced ynamide structural reshuffling and functionalization

The radical chemistry of ynamides has recently drawn the attention of synthetic organic chemists to the construction of various N-heterocyclic compounds. Nevertheless, the ynamide-radical chemistry remains a long-standing challenge for chemists due to its high reactivity, undesirable byproducts, severe inherent regio- and chemoselective problems. Importantly, the ynamide C(sp)-N bond fission remains an unsolved challenge. In this paper, we observe Photoinduced radical trigger regio- and chemoselective ynamide bond fission, structural reshuffling and functionalization of 2-alkynyl-ynamides to prepare synthetically inaccessible/challenging chalcogen-substituted indole derivatives with excellent step/atom economy. The key breakthroughs of this work includes, ynamide bond cleavage, divergent radical precursors, broad scope, easy to handle, larger-scale reactions, generation of multiple bonds (N-C(sp²), C(sp²)-C(sp²), C(sp²)-SO₂R/C-SR, and C-I/C-Se/C-H) in a few minutes without photocatalysts, metals, oxidants, additives. Control experiments and ¹³C-labeling experiments supporting the conclusion that sulfone radicals contribute to ynamide structural reshuffling processes via a radical pathway.

Although ynamides have emerged as a versatile class of compounds for organic synthesis, the radical chemistry of ynamides usually proceeds with the expected connectivity largely intact. Here the authors show a methodology by which the C(sp)–N bond undergoes scission, alkyne migration and functionalization under blue LED light in the absence of metals or additives.

Synthesis of C7-Functionalized Indoles through an Aromaticity Destruction-Reconstruction Process

A process for the synthesis of C7-functionalized indoles using para-substituted 2-alkynylanilines as starting materials was reported. The process involves a dearomatization, an 1,2-addition by organic lithium or Grignard reagents, an aromatization-driven allylic rearrangement, and a cyclization. A variety of groups including alkyl, aryl, alkenyl, or alkynyl groups were selectively installed at the C7 site of indoles leading to the formation of 2,5,7-trisubstituted indoles.

TfOH-promoted synthesis of indoles and benzofurans involving cyclative transposition of vinyl ketone

A metal-free approach to construct indole ring from vinylogous amides derived from o-alkynylanilines involving cyclization, retro-aza-Michael and amine trapping cascade is reported here. This atom-economical transformation has been extended to...

1,2-Difunctionalizations of alkynes entailing concomitant C–C and C–N bond-forming carboamination reactions

Vicinal carboamination of alkynes is a highly reliable and efficient practical strategy for the quick preparation of valuable and diverse amine derivatives starting from simple synthons. The last decade has witnessed numerous practical methods employing transition-metal-based/metal-free carboamination approaches using alkynes for the synthesis of these N-bearing entities. Driven by the renaissance of transition metal catalysis, intermolecular and intramolecular carboamination of alkynes comprising concomitant C–N and C–C bond formation has been studied extensively. In contrast to metal catalysis, though analogous metal-free approaches have been relatively less explored in the literature, they serve as alternatives to these expensive approaches. Despite this significant progress, reviews documenting such examples are sporadic; as a result, most reports of this type remained scattered throughout the literature, thereby hampering further developments in this escalating field. In this review, different conceptual approaches will be discussed and examples from the literature will be presented. Further, the reader will get insight into the mechanisms of different transformations.

The 1,2-difunctionalization of alkynes happening through concomitant C–C and C–N bond formation strategies have provide an unified access to diversely functionalized N-bearing heterocycles.

Diastereoselective [2+2+1] Hydrative Annulation of Phenol-Linked 1,6-Enynes with Acetylenes Through Rhodium Catalysis: A Rapid Access to Cyclopenta[b]benzofuranols

An unprecedented [2+2+1] hydrative annulation of 1,6-enynes with terminal alkynes is achieved using catalytic cationic Rh(I). Thus, a modular assembly of cyclopenta[b]benzofuranols with two consecutive quarternary stereocenters is achieved from readily available alkynes. The reaction is proposed to go through a sequence of 5-membered rhoda-cycle formation, regioselective acetylene insertion, 1,5 H-shift, substrate controlled stereoselective addition of water molecule followed by 1,2-rhodium migration gave contracted rhoda-cycle D and reductive elimination. Necessary control/labelling experiments were conducted to gain insight in to the mechanism.

Synthesis of 4-Alkylindoles from 2-Alkynylanilines via Dearomatization- and Aromatization-Triggered Alkyl Migration

A simple method for rapid synthesis of 4-alkylindoles from 2-alkynylanilines was reported. The protocol involves an oxidative dearomatization and an aromatization triggered regioselective alkyl migration. A range of alkyl groups including linear, branched, or cycloalkyl groups can be introduced into the C4 position of indole.

Facile synthesis of 4-acetoxyindoles via PhI(OAc)2-mediated dearomatization of 2-alkynylanilines

A simple process for rapid synthesis of 4-aetoxyindoles from readily available 2-alkynylanilines via dearomatization was reported.