Rhenium-Catalyzed Intramolecular Carboalkoxylation and Carboamination of Alkynes for the Synthesis of C3-Substituted Benzofurans and Indoles

Transition metal catalysed cascade C-C and C-O bond forming events of alkynes

The past few decades have witnessed the emergence of domino reactions as a powerful tool for the multi-functionalization of alkynes for the rapid and smooth construction of complex molecular architectures. In this context, employing transition metal catalysis, vicinal/geminal cascade functionalization of alkynes involving C-C and C-O bond-formation reactions, has become a preferred strategy for the synthesis of oxygenated motifs. Despite this significant progress, reviews documenting such strategies are either metal/functional group-centric or target-oriented, thus hampering further developments. Therefore, in this review, different conceptual approaches based on C-C and C-O bond-forming events of alkynes such as carboxygenation (C-C and CO bond formation), carboalkoxylation (C-C and C-OR bond formation), and carboacetoxylation (C-C and C-OAc bond formations) are discussed, and examples from the literature from the last two decades are presented. Further, we have presented detailed insights into the mechanism of different transformations.

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.

Synthesis of 1-Aryltetralins via Re2O7/HReO4 Mediated Intramolecular Hydroarylations

Here, we describe highly efficient intramolecular hydroarylations mediated by Re2O7/HReO4. Styrene derivatives of different electronic properties have been activated to effect a challenging intramolecular hydroarylation for the facile access to various substituted 1-aryltetralin structures. This method is characterized by mild reaction conditions, broad substrate scope, high chemical yields, and 100% atom economy. The potential synthetic application of this methodology was exemplified by the efficient total synthesis of an isoCA-4 analogue.

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.

Re2O7/HReO4 Mediated Intramolecular Hydroacyloxylation of Unactivated Alkenes: A Dual Hydrogen-Bonding Effect

This publication describes the application of Re₂O₇ in hexafluoroisopropanol (HFIP) for the activation of inert as well as electronically deactivated olefins to facilitate a challenging intramolecular hydroacyloxylation reaction. Both HFIP and an internal carboxy group have been proven to be crucial for the successful implementation of this transformation; these are proposed to assist the formation and stabilization of the key cationic intermediate via hydrogen-bonding interactions with perrhenate anion (ReO₄^-).

Synthesis of Diversified Pyrazolo[3,4-b]pyridine Frameworks from 5-Aminopyrazoles and Alkynyl Aldehydes via Switchable C≡C Bond Activation Approaches

A cascade 6-endo-dig cyclization reaction was developed for the switchable synthesis of halogen and non-halogen-functionalized pyrazolo[3,4-b]pyridines from 5-aminopyrazoles and alkynyl aldehydes via C≡C bond activation with silver, iodine, or NBS. In addition to its wide substrate scope, the reaction showed good functional group tolerance as well as excellent regional selectivity. This new protocol manipulated three natural products, and the arylation, alkynylation, alkenylation, and selenization of iodine-functionalized products. These reactions demonstrated the potential applications of this new method.

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.

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.

Rhenium-catalysed reactions in chemical synthesis: selected case studies

This Perspective presents and discusses a selection of examples that reinforce the enabling and distinctive reactivity provided by homogeneous rhenium catalysis in chemical synthesis. Specifically, the ability for lower oxidation...

Sequential Sonogashira/intramolecular aminopalladation/cross-coupling of ortho-ethynyl-anilines catalyzed by a single palladium source: rapid access to 2,3-diarylindoles

We have developed a practical and efficient one-pot protocol for the synthesis of 2,3-diarylindoles via Pd-catalyzed bis-arylative cyclization of various o-ethynylanilines with aryl iodides. Mechanism studies showed that a Pd-catalyzed Sonogashira reaction took place firstly, giving an internal alkyne intermediate, which subsequently underwent intramolecular aminopalladation/cross-coupling to give access to 2,3-diarylindoles. The present methodology exhibits a broad substrate scope, producing various 2,3-diaryl indoles bearing two different aryl groups.

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

A Ni-catalyzed reaction was developed for the synthesis of multifunctionalized indoles. The reaction proceeded through oxidative cyclization of the Ni(0)/P^N complex with an enyne system, 2-alkynyl anilinoacrylate, to provide a nickelacycle intermediate. The trans-carboamination around the internal alkyne was achieved by syn/anti-rotation of the Ni-carbenoid intermediate formed by C-N bond cleavage of the nickelacycle, and 3-alkenylated indoles were formed by C-N bond-forming reductive elimination. Notably, the synthesized indoles could be successfully transformed to functionalized carbazoles.

Last Decade of Unconventional Methodologies for the Synthesis of Substituted Benzofurans

This review describes the progress of the last decade on the synthesis of substituted benzofurans, which are useful scaffolds for the synthesis of numerous natural products and pharmaceuticals. In particular, new intramolecular and intermolecular C-C and/or C-O bond-forming processes, with transition-metal catalysis or metal-free are summarized. (1) Introduction. (2) Ring generation via intramolecular cyclization. (2.1) C7a-O bond formation: (route a). (2.2) O-C2 bond formation: (route b). (2.3) C2-C3 bond formation: (route c). (2.4) C3-C3a bond formation: (route d). (3) Ring generation via intermolecular cyclization. (3.1) C7a-O and C3-C3a bond formation (route a + d). (3.2) O-C2 and C2-C3 bond formation: (route b + c). (3.3) O-C2 and C3-C3a bond formation: (route b + d). (4) Benzannulation. (5) Conclusion.

A competitive and highly selective 7-, 6- and 5-annulation with 1,3-migration through C-H and N-H - alkyne coupling

We demonstrated a highly competitive and selective C-C and N-C cross-coupled 7-, 6- and 5-annulation utilizing 2-ethynylanilides to afford functionalized 1H-benzo[b]azepin-2(5H)-ones, 2-quinolinones, and 3-acylindoles in high yield. ZnCl2 was found to be the smart catalyst for 7- and 5-annulation with 1,3-migration through C-H and N-H functionalization, respectively, whereas molecular iodine performed the C-H functionalized 6-annulation with a nonconventional 1,3 H-shift. The mechanism was investigated by intermediate trapping, control, and labeling experiments.