Rh(III)-Catalyzed Divergent Synthesis of Alkynylated Imidazo[1,5-a]indoles and α,α-Difluoromethylene Tetrasubstituted Alkenes

Rhodium(III)-Catalyzed Annulation Synthesis of Difluorinated Quinazolinone Derivatives Using an Amide Carbonyl as the Directing Group

The use of amide carbonyl groups of substrates as weakly coordinating directing groups has received a significant amount of attention. Recently, difluoromethylene alkynes have been successfully used in fluorination reactions, resulting in the preparation of various fluorine-containing compounds. This work describes a [4+2] annulation method for creating a range of fluorinated quinolino[2,1-b]quinazolinone derivatives. The derivatives are formed through Rh(III)-catalyzed cascade cyclization of 3-phenylquinazolinones and gem-difluoromethylene alkynes.

Synthesis of CF3-Substituted N-Heterocyclic Compounds Based on C-H Activation-Initiated Formal [2 + 3] Annulation Featuring with a Latent Nucleophilic Site

Presented herein is a novel synthesis of CF3-substituted pyrrolo[1,2-a]indole derivatives based on the cascade reactions of N-alkoxycarbamoyl indoles with CF3-ynones. Mechanistically, the formation of a product involves a tandem process initiated by Rh(III)-catalyzed and N-alkoxycarbamoyl group-directed regioselective C2-H alkenylation of the indole scaffold followed by in situ removal of the directing group and intramolecular N-nucleophilic addition/annulation under one set of reaction conditions. To our knowledge, this is the first example in which a N-alkoxycarbamoyl unit initially acts as a directing group for C2-H functionalization of the indole scaffold and is then removed to provide the required reactive NH-moiety for subsequent intramolecular condensation. Moreover, the products thus obtained could be conveniently transformed into structurally and biologically attractive cycloheptenone fused indole derivatives through an acid-promoted cascade transformation. In addition, studies on the activity of selected products against human cancer cell lines demonstrated their potential as lead compounds for the development of novel anticancer drugs.

Redox-neutral rhodium(III)-catalyzed divergent synthesis of tetrasubstituted 1,3-enynes and alkynylated benzofurans

With the assistance of the acetamido directing group (DG), a rhodium-catalyzed C-H alkenylation/DG migration cascade for the synthesis of tetrasubstituted 1,3-enynes from N-phenoxyacetamides and 1,3-diynes has been achieved in this work. Alternatively, a rhodium-catalyzed [3 + 2] annulation for the synthesis of alkynylated benzofurans from the same set of substrates has also been achieved by simply changing the reaction conditions. This work highlights the tunable divergent synthesis of valuable compounds triggered by C-H activation.

Formation of Fluorovinyl Spiro-[imidazole-indene] and α-Amino-β-naphthalenones via Rh(III)-Catalyzed Cascade C-H Functionalization

An efficacious method for building fluorovinyl spiro-[imidazole-indene] and α-amino-β-naphthalenone skeletons synchronously has been shown to consist of Rh(III)-catalyzed C-H functionalization between 2H-imidazoles and difluoromethylene alkynes. This protocol demonstrates a practical and straightforward route for installing fluorine elements in the envisioned position of heterocyclic compounds.

Rhodium(III)-Catalyzed Sequential Cyclization of N-Boc Hydrazones with Propargylic Monofluoroalkynes via C-H Activation/C-F Cleavage for the Synthesis of Spiro[cyclobutane-1,9'-indeno[1,2-a]indenes]

An effective rhodium(III) catalysis for the construction of valuable tetracyclic compounds is described herein. This domino process involving the C-H activation/[3 + 2] annulation/intramolecular Friedel-Crafts reaction sequences of simple and readily available N-Boc hydrazones and propargylic monofluoroalkynes afforded fused tetracyclic spiro[cyclobutane-1,9'-indeno[1,2-a]indenes] in moderate to good yields, featuring three C-C bond formation. Moreover, control experiments indicated that the C-H activation might be involved in the rate-determining step.

Multiple-fold C–F bond functionalization for the synthesis of (hetero)cyclic compounds: fluorine as a detachable chemical handle

We highlighted the recent advances in the field of multiple-fold C–F bond functionalization for the synthesis of (hetero)cyclic compounds.

Redox-neutral rhodium(III)-catalyzed chemo- and regiospecific [4 + 1] annulation between benzamides and alkenes for the synthesis of functionalized isoindolinones

Herein, using electron-deficient alkenes embedded with an oxidizing function/leaving group as a rare and nontraditional C₁ synthon, we have achieved the redox-neutral Rh(III)-catalyzed chemo- and regioselective [4 + 1] annulation of benzamides for the synthesis of functionalized isoindolinones. This method features broad substrate scope, good to excellent yields, excellent chemo- and regioselectivity, good tolerance of functional groups and mild external-oxidant-free conditions.

TBN-triggered, manipulable annulations of o-hydroxyarylenaminones for divergent syntheses of oximinochromanones and oximinocoumaranones

Divergent synthesis provides an indispensable route to rapid acquisition of structurally diverse chemical scaffolds from identical starting materials. Herein, we describe unprecedented divergent annulations of o-hydroxyarylenaminones promoted by tert-butyl nitrite (TBN) under mild conditions. Two different types of benzo-oxa-heterocycle, including oximinochromanones and oximinocoumaranones, were smoothly assembled with a broad substrate scope and good functional group compatibility.

Synthesis of Indole-Fused Oxepines via C-H Activation Initiated Diastereoselective [5 + 2] Annulation of Indoles with 1,6-Enynes

A rhodium-catalyzed diastereoselective formal [5 + 2] annulation of indoles with cyclohexadienone-containing 1,6-enynes has been established via indole 2,3-difunctionalization. The reaction, probably proceeding through tandem indole C2-H alkenylation and intramolecular Friedel-Crafts alkylation relay, provides rapid construction of indole-fused oxepines in good to excellent yields with a broad substrate scope. This method also features concomitant construction of cis-hydrobenzo[b] oxepine scaffolds, a core unit found in numerous natural products of important biological activities.

The C-H functionalization of N-alkoxycarbamoyl indoles by transition metal catalysis

Indole and its congeners are ubiquitous nitrogen-containing organic scaffolds present in a plethora of natural products, marketed drugs, and other organic functional molecules. Recent years have witnessed tremendous advances in the diversification of this motif and its biological applications via transition-metal-catalyzed auxiliary assisted site-selective inert C-H functionalization. In this burgeoning field, N-methoxy/ethoxy/pivaloxy amide functionality has emerged as a most potent auxiliary/DG (directing group) for a wide range of C-C and C-heteroatom bond formations, providing a new advance for forging structurally fabricated polycyclic indole frameworks. This review aims to highlight evolved transformations, like arylation, alkylation, alkenylation, allylation, amidation, difluorovinylation, deuteration, hydroarylation, etc., and the applications of N-alkoxycarbamoyl indole derivatives made within the period of 2014-August 2021. Additionally, explicit mechanistic underpinnings have also been provided in the appropriate places.