Rhodium-Catalyzed Cycloadditions between 3-Diazoindolin-2-imines and 1,3-Dienes

Rhodium Catalyzed [4 + 1]-Annulation of o-Acylanilines with 3-Diazoindoline-2-imines

An efficient rhodium catalyzed [4 + 1]-annulation of o-acylanilines with 3-diazoindoline-2-imines has been successfully accomplished for the synthesis of spiroindolines in good to excellent yield. The reaction occurs through formation of N-ylide followed by cyclization and showed good tolerance to various functional groups. Gram-scale synthesis, diastereoselective construction of tetrasubstituted indoline, synthesis of spirooxindole, and isolation of potential intermediates have also been demonstrated.

Copper-Catalyzed [4 + 1] Annulation of Enaminothiones with Indoline-Based Diazo Compounds

A concise synthetic route to spiroindoline-fused S-heterocycles was developed through copper-catalyzed [4 + 1] annulation using enaminothiones as donor-acceptor synthons. Both 3-diazoindolin-2-imines and 3-diazooxindoles were amenable to work as effective C1 building blocks. The reaction proceeds via a copper-catalyzed cascade process involving the in situ generation of copper(I) carbene and C-S/C-C bond formation. This synthetic protocol features the use of readily available substrates, diverse substituent tolerance, and good to excellent yields.

Synthesis of dihydropyrroles from in situ-generated zwitterions via Rh2(adc)4/TBAI dual catalysis

A convenient rhodium/iodide dual catalysis strategy for the regioselective synthesis of dihydropyrroles was established.

Modern Synthetic Methods for the Stereoselective Construction of 1,3-Dienes

The 1,3-butadiene motif is widely found in many natural products and drug candidates with relevant biological activities. Moreover, dienes are important targets for synthetic chemists, due to their ability to give access to a wide range of functional group transformations, including a broad range of C-C bond-forming processes. Therefore, the stereoselective preparation of dienes have attracted much attention over the past decades, and the search for new synthetic protocols continues unabated. The aim of this review is to give an overview of the diverse methodologies that have emerged in the last decade, with a focus on the synthetic processes that meet the requirements of efficiency and sustainability of modern organic chemistry.

Recent advances in the synthesis of indole embedded heterocycles with 3-diazoindolin-2-imines

The preparations, reactivity and synthetic applications of 3-diazoindolin-2-imines, a valuable class of α-diazo amidines, are reviewed.

Syntheses of 2-Iminoindolin-3-ones and 2-Alknyl-2,3-dihydroquinazolin-4(1H)-ones from 3-Diazoindolin-2-imines

3-Diazoindolin-2-imines reacted with nitrones to furnish 2-iminoindolin-3-ones through a Au(I)-catalyzed cascade oxygen transfer/imine exchange process. The prepared 2-iminoindolin-3-ones could be further transformed into 2-alknyl-2,3-dihydroquinazolin-4(1H)-ones through a Ag(I)-catalyzed reaction with terminal alkynes. A MeOH-triggered ring expansion mechanism involving cyclic iminium formation and nucleophilic addition is proposed for this novel alkynylation reaction. This two-step procedure provides a general and convenient approach to 2-alknyl-2,3-dihydroquinazolin-4(1H)-ones, which are privileged structures in medicinal chemistry.

Recent Advances in Transition-Metal-Catalyzed (4+3)-Cycloadditions

A (4+3)-cycloaddition combines a four-atom synthon and three-atom synthon to form seven-membered rings. In the past decade, many improvements have been made to this class of cycloaddition, including excellent diastereo- and enantioselectivities, both intra- and intermolecularly. Through the strategic use of transition-metal catalysts, acids, bases, and organocatalysts, it is possible to perform the cycloaddition on a variety of substrates, generating novel seven-membered rings. With these advances, (4+3)-cycloaddition has also been applied to the synthesis of biologically relevant compounds and natural products. We exclude the cycloadditions of cyclic dienes such as furan, pyrrole, cyclohexadiene or cyclopentadiene as Chiu, Harmata, Mascareñas­ and others have recently published thorough reviews on that topic. We will however discuss the recent additions (2009–2020) to the literature for the (4+3)-cycloadditions involving other types of four-atom synthons.

1 Introduction

2 Rhodium

2.1 Cyclopropanation/Cope Rearrangement

2.2 C–H activation

3 Gold, Silver

4 Copper

5 Palladium, Platinum, Iridium

6 Dual-Activation

7 Conclusion

Palladium-catalyzed domino Heck cyclization/ring opening of sulfolenes/desulfitative coupling: regio- and stereoselective synthesis of alkylated conjugated dienes

An efficient cascade dienylation provides a range of alkylated conjugated diene compounds with the Z-configuration by introducing the C₄ unit directly.

General Approach To Construct Azepino[2,3-b:4,5-b']diindoles, Azocino[2,3-b:4,5-b']diindoles, and Azonino[2,3-b:4,5-b']diindoles via Rh(II)-Catalyzed Reactions of 3-Diazoindolin-2-imines with 3-(Bromoalkyl)indoles

Rh(II)-catalyzed reactions of 3-diazoindolin-2-imines with 3-(2-bromoethyl)indoles, 3-(3-bromopropyl)indoles, and 3-(4-bromobutyl)indoles, followed by treatment with 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU) in a one-pot operation furnished azepino[2,3-b:4,5-b']diindoles, azocino[2,3-b:4,5-b']diindoles, and azonino[2,3-b:4,5-b']diindoles, respectively. Structural uniqueness of the products, broad substrate scope, mild reaction conditions, and readily available starting materials are the merits of this approach.

Ring-opening formal hetero-[5+2] cycloaddition of 1-tosyl-2,3-dihydro-1H-pyrroles with terminal alkynes: entry to 1-tosyl-2,3-dihydro 2,3-dihydro-1H-azepines

Lewis acid-catalyzed formal hetero-[5+2] cycloaddition of 2,3-dihydro-1H-pyrroles with alkynes through C(sp²)–N bond cleavage toward 2,3-dihydro-1H-azepines is described.

Preparation of spiro[imidazolidine-4,3′-indolin]-2′-imines via copper(i)-catalyzed formal [2 + 2 + 1] cycloaddition of 3-diazoindolin-2-imines and triazines

We report a facile and efficient synthesis of spiro[imidazolidine-4,3′-indolin]-2′-imines via a copper(i)-catalyzed cascade reaction of 3-diazoindolin-2-imines with 1,3,5-triazines.

A Diverted Aerobic Heck Reaction Enables Selective 1,3-Diene and 1,3,5-Triene Synthesis through C-C Bond Scission

Substituted 1,3-dienes are valuable synthetic intermediates used in myriad catalytic transformations, yet modern catalytic methods for their preparation in a highly modular fashion using simple precursors are relatively few. We report here an aerobic boron Heck reaction with cyclobutene that forms exclusively linear 1-aryl-1,3-dienes using (hetero)arylboronic acids, or 1,3,5-trienes using alkenylboronic acids, rather than typical Heck products (i.e., substituted cyclobutenes). Experimental and computational mechanistic data support a pericyclic mechanism for C-C bond cleavage that enables the cycloalkene to circumvent established limitations associated with diene reagents in Heck-type reactions.

Metal-Free Synthesis of (E)-Monofluoroenamine from 1-Sulfonyl-1,2,3-triazole and Et2O·BF3 via Stereospecific Fluorination of α-Diazoimine

A general, stereospecific, and straightforward method for the rapid preparation of functionalized (E)-monofluoroenamines is reported. Rather than transition metals (Rh, Ni, Pd, Cu, Ag, etc.), Et₂O·BF₃ was employed to promote the formation of α-diazoimine through the Dimroth equilibrium of common 1-sulfonyl-1,2,3-triazole for the first time. An overall migration of fluoride from boron to the diazo-linked carbon of α-diazoimine was achieved. Derivations and late-stage modification of bioactive molecule were conducted. A plausible mechanism was also proposed.

Convenient synthesis of 2-amino-3-(arylthio)indoles via the Rh-catalyzed reaction of 3-diazoindol-2-imines with thioesters

2-Amino-3-(arylthio)indoles were conveniently synthesized via the Rh(ii)-catalyzed C–S/N–C coupling reaction between 3-diazoindol-2-imines and thioesters.

A copper-catalyzed reaction of 3-diazoindolin-2-imines with 2-(phenylamino)ethanols: convenient access to spiro[indoline-3,2′-oxazolidin]-2-imines

3-Diazoindolin-2-imines reacted with chiral 2-(phenylamino)ethanols under copper catalysis to furnish chiral spiro[indoline-3,2′-oxazolidin]-2-imines in good yields with excellent diastereoselectivity.

Metal-free synthesis of imidazole by BF3·Et2O promoted denitrogenative transannulation of N-sulfonyl-1,2,3-triazole

The first BF₃·Et₂O promoted metal-free denitrogenative transannulation of N-sulfonyl-1,2,3-triazole was reported, leading to the synthesis of functionalized imidazoles with a broad scope.

TfOH-Catalyzed Reaction between 3-Diazoindolin-2-imines and Electron-Rich Arenes: Access to 3-Aryl-2-aminoindoles

TfOH-catalyzed reactions between 3-diazoindolin-2-imines and electron-rich arenes were disclosed. These metal-free reactions furnished 3-aryl-2-aminoindoles in moderate to excellent yields with the tolerance of a broad range of functional groups.