Rhodium‐Catalyzed Cascade Reactions of Indoles with 4‐Hydroxy‐2‐Alkynoates for the Synthesis of Indole‐Fused Polyheterocycles

Ru-Catalyzed Redox-Neutral Coupling of N-Chlorobenzamides with Unsymmetrical Alkynes in Water

In water, Ru-catalyzed annulation of N-chlorobenzamides with unsymmetrical internal alkynes bearing aryl, hydroxy, ester, and sulfonyl functionalities has been accomplished to afford isoquinolone scaffolds under external oxidant-free conditions at room temperature. Use of water as reaction medium, redox-neutral conditions, regioselectivity, and substrate scope are important practical features.

Rh(III)-Catalyzed Chemoselective [4 + 2] Annulations for the Synthesis of [1,3]Oxazinoindolones: A Combined Experimental and Computational Study

The interaction of N-alkoxy-1H-indole-1-carboxamides with transition metals leads to indole-fused heterocyclic scaffolds through directing group leaving/migration, [3 + 2], N-C2 [4 + 1], and [4 + 2] annulations. However, the corresponding O-C2 [4 + 2] annulation reactions have never been reported. Herein, we report the chemoselective annulation of N-alkoxy-1H-indole-1-carboxamides catalyzed by Rh(III), affording [1,3]oxazinoindolones through a hitherto unknown reaction pathway. This unprecedented C2 oxygen cyclization, rather than the known C2 nitrogen cyclization to form oxazinoindolones via five key steps, has been explained using density functional theory.

Rhodium(III)-Catalyzed C-H/N-H Activation for Direct Synthesis of Pyrimidoindolones under Mild Conditions

Pyrimidoindolones are an important structural motif found in many natural products and are essential to the pharmaceutical and agrochemical industry. Direct synthesis of 3,4-unsubstituted pyrimidoindolones is not easily accessible. Here we report a rhodium(III)-catalyzed C-H/N-H activation and annulation approach for obtaining pyrimidoindolones from N-carbamoylindoles and vinylene carbonate. The reaction occurs at room temperature and does not require any external oxidants. A diverse spectrum of indoles were demonstrated to be viable substrates capable of producing the desired pyrimidoindolones in high yields. In addition, the reaction scope has been expanded to include pyrrole substrate. Furthermore, detailed mechanistic studies have been performed to delineate the working mode of the reaction.

Rh-catalyzed chemo-, stereo- and regioselective C-H cascade annulation of indolyloxopropanenitriles for pyranoindoles

Selective annulations of alkynes represent a powerful tool for constructing multicyclic scaffolds while installing desired substitution patterns with precision. Herein, we report a Rh-catalyzed unique annulation of indolyl oxopropanenitrile with hydroxy-alkynoates to access pyranoindole cyclic motifs, featuring enol oxygen as a rare chemoselective reactive terminal. The reaction proceeds via a five-membered oxy-rodacycle through C-H activation by a rhodium complex guided by enolic- and propargyloxy dual co-ordination to enable a regio- and stereoselective modular assembly.

Pd-catalyzed regioselective rollover dual C-H annulation cascade: facile approach to phenanthrene derivatives

Annulations of unsaturated systems through C-H activation represent a powerful tool for producing multicyclic scaffolds. Having coordinating centers in both annulation partners (a dual coordination strategy) would afford remarkable selectivities in the outcomes. Along this concept, we report herein a Pd-catalyzed regioselective rollover cascade dual C-H annulation of o-arylphenols with alkynols for constructing phenanthrene scaffolds. Control, KIE and deuteration studies were conducted to determine the reaction mechanism, and downstream transformations and scaled-up reactions were carried out to assess the robustness of the transformation.

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.

Rhodium(III)-Catalyzed Cascade Reactions of Imines/Imidates with 4-Hydroxy-2-alkynoates to Synthesize Regioselective Furanone-Fused Isoquinoline Scaffolds

A regioselective synthesis of furanone-fused isoquinoline heterocycles is developed in a single step using a Rh(III) catalyst. In this reaction, a cascade C-H activation, regioselective annulation, and lactonization occur in one pot. A wide range of alkynoates was examined, which showed good regioselectivity. The regioselectivity was guided by steric bulk at the C4 position of the 4-hydroxy-2-alkynoates. The synthetic utility was exemplified, and the model reaction was scaled up to a 1 g scale.

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.

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.

Redox-Neutral Rhodium(III)-Catalyzed Chemospecific and Regiospecific [4+1] Annulation between Indoles and Alkenes for the Synthesis of Functionalized Imidazo[1,5-a]indoles

Exploiting internal alkenes embedded with an oxidizing function/leaving group as a rare and unconventional one-carbon unit, a redox-neutral rhodium(III)-catalyzed chemo- and regiospecific [4+1] annulation between indoles and alkenes for the synthesis of functionalized imidazo[1,5-a]indoles has been achieved. Internal alkenes employed here can fulfill an unusual [4+1] annulation rather than normal [4+2] annulation/C-H alkenylation. This method is characterized by excellent chemo- and regioselectivity, broad substrate scope, good functional group tolerance, good to high yields, and redox-neutral conditions.

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

Herein, we report the divergent synthesis of alkynylated imidazo[1,5-a]indoles and α,α-difluoromethylene tetrasubstituted alkenes through Rh(III)-catalyzed [4 + 1] annulation/alkyne moiety migration and C-H alkenylation/DG migration, respectively. This protocol features tunable product selectivity, excellent chemo-, regio-, and stereoselectivity, broad substrate scope, moderate to high yields, good tolerance of functional groups, and mild redox-neutral conditions.

A Water-Soluble Rhenium(I) Catalyst for the Regio- and Stereoselective C(sp2)-H Alkenylation of N-Pyridyl-/N-Pyrimidylindole and the N-H Alkenylation of N-Pyrimidylaniline Derivatives with Ynamides

A water-soluble and low-valent rhenium(I) catalyst for the C2 alkenylation of N-pyridyl/N-pyrimidylindole derivatives with ynamides under mild conditions using water as the solvent has been described. The reaction of N-pyridyl/N-pyrimidyl indole with the ynamide afforded the C2-Z-selective alkenylation derivative as the sole product, and the reactions of N-pyrimidylanilines delivered the corresponding N-alkenylated product rather than the expected C-H alkenylation products in high yields under the same conditions.