C–H Activation-Based Traceless Synthesis via Electrophilic Removal of a Directing Group. Rhodium(III)-Catalyzed Entry into Indoles from N-Nitroso and α-Diazo-β-keto Compounds

Radical-carbene coupling reaction: Mn-catalyzed synthesis of indoles from aromatic amines and diazo compounds

An unprecedented coupling reaction between aromatic amines and diazo compounds was well developed, which afforded a bridge connecting between radical chemistry and metal carbene chemistry. This Mn-catalyzed tandem reaction also provided a fundamentally different and practical approach to the indole skeleton under mild conditions.

An Efficient Metal‐Free Method for the Denitrosation of Aryl N‐Nitrosamines at Room Temperature

A simple and practical method for the denitrosation of aryl N‐nitrosamines to secondary amines is reported under metal‐free conditions using iodine and triethylsilane. Several reduction‐susceptible functional groups such as alkene, alkyne, nitrile, nitro, aldehyde, ketone and ester were found to be very stable during the denitrosation, which is remarkable. Broad substrate scope, room temperature reactions and excellent yields are the additional features of the current methodology.

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Oxidative ring-opening of 3-aminoindazoles for the synthesis of 2-aminobenzoates

A novel oxidative ring-opening of 3-aminoindazoles based on N–N bond cleavage was developed, in which various 2-aminobenzoates were obtained in good yields under mild conditions.

Rh(iii)-Catalyzed synthesis of pyrazolo[1,2-a]cinnolines from pyrazolidinones and diazo compounds

Pyrazolo[1,2-a]cinnolines were achieved via a rhodium(iii)-catalyzed redox-neutral annulation procedure, exhibiting good regioselectivity and high functional group tolerability.

Cp*Rh(iii) catalyzed ortho-halogenation of N-nitrosoanilines by solvent-controlled regioselective C–H functionalization

A highly efficient rhodium-catalyzed, solvent-controlled regioselective C–H halogenation of anilines by using N-nitroso as a suitable and removable directing group was achieved under mild reaction conditions.

Rh(iii)-Catalyzed α-fluoroalkenylation of N-nitrosoanilines with 2,2-difluorovinyl tosylates via C–H bond activation

Rh(iii)-Catalyzed α-fluoroalkenylation of N-nitrosoanilines with 2,2-difluorovinyl tosylates has been realized, leading to the synthesis of monofluoroalkenes with high Z-selectivity with respect to the olefin.

Divergent synthesis of 3,4-dihydrodibenzo[b,d]furan-1(2H)-ones and isocoumarins via additive-controlled chemoselective C–C or C–N bond cleavage

Rh(iii)-Catalyzed C–C/C–O bond formation between cyclic 2-diazo-1,3-diketones and salicylamides with additive-controlled chemoselectivity.

Rhodium-catalyzed triazole-directed C–H bond functionalization of arenes with diazo compounds

Rhodium(iii)-catalyzed alkylation reactions of arenes through triazole directed C–H activation that lead to a number of dialkylated and monoalkylated triazoles are described.

Rh(iii)-catalyzed regioselective intermolecular N-methylene Csp3-H bond carbenoid insertion

A Rh(iii)-catalyzed regioselective intermolecular carbenoid insertion into the N-methylene Csp³-H bond of acyclic aliphatic amides has been achieved, taking advantage of bidentate-chelation assistance. This methodology has been successfully applied to a broad range of linear and branched-chain N-alkylamides, thus providing a practical method for the assembly of diverse beta-amino esters. Mechanism studies and density functional theory (DFT) calculations revealed that a singlet Fischer type carbene insertion via an outer-sphere pathway was involved in this N-methylene Csp³-H bond carbenoid insertion.

Transition-Metal-Catalyzed Cross-Couplings through Carbene Migratory Insertion

Transition-metal-catalyzed cross-coupling reactions have been well-established as indispensable tools in modern organic synthesis. One of the major research goals in cross-coupling area is expanding the scope of the coupling partners. In the past decade, diazo compounds (or their precursors N-tosylhydrazones) have emerged as nucleophilic cross-coupling partners in C-C single bond or C═C double bond formations in transition-metal-catalyzed reactions. This type of coupling reaction involves the following general steps. First, the organometallic species is generated by various processes, including oxidative addition, transmetalation, cyclization, C-C bond cleavage, and C-H bond activation. Subsequently, the organometallic species reacts with the diazo substrate to generate metal carbene intermediate, which undergoes rapid migratory insertion to form a C-C bond. The new organometallic species generated from migratory insertion may undergo various transformations. This type of carbene-based coupling has proven to be general: various transition metals including Pd, Cu, Rh, Ni, Co, and Ir are effective catalysts; the scope of the reaction has also been extended to substrates other than diazo compounds; and various cascade processes have also been devised based on the carbene migratory insertion. This review will summarize the achievements made in this field since 2001.

Rhodium(iii)-catalyzed and MeOH-involved regioselective mono-alkenylation of N-arylureas with acrylates

A new and versatile Rh(iii)-catalyzed C–H activation/alkenylation/MeOH-involved ammonolysis/esterification cascade reaction has been disclosed.

One-pot synthesis of 2,3-difunctionalized indoles via Rh(iii)-catalyzed carbenoid insertion C–H activation/cyclization

A new and versatile Rh(iii)-catalyzed carbenoid insertion C–H activation/cyclization for one-pot synthesis of 2,3-difunctionalized indoles has been developed.

Rhodium(iii)-catalyzed indole-directed carbenoid aryl C–H insertion/cyclization: access to 1,2-benzocarbazoles

A rhodium(iii)-catalyzed indole-directed aryl C–H bond carbenoid insertion cascade of 2-arylindoles with diazo compounds has been developed.

RhIII-Catalyzed site-selective amidation with nitrone as a traceless directing group: an approach to functionalized arylaldehydes

A unique Rh^III-catalyzed C–H amidation of nitrones with 1,4,2-dioxazol-5-ones has been developed, providing a new approach to functionalized arylaldehydes.

An Ir(iii)-catalyzed aryl C–H bond carbenoid functionalization cascade: access to 1,3-dihydroindol-2-ones

An Ir(iii)-catalyzed relay aryl C–H bond carbenoid insertion cascade of N-aryl-2-pyridinamines with diazo Meldrum's acid has been developed.

Pd-Catalyzed oxidative annulation of enamides with diazabicyclic olefins: rapid access to cyclopentene fused 2-pyrrolines

A catalytic tandem transformation via C–H activation of enamides for the stereoselective synthesis of cyclopentene fused 2-pyrrolines has been realized.

Regio-selective synthesis of diversely substituted benzo[a]carbazoles through Rh(iii)-catalyzed annulation of 2-arylindoles with α-diazo carbonyl compounds

A novel synthetic approach toward benzo[a]carbazoles and 6-aminobenzo[a]carbazoles containing an unprotected NH unit through Rh(iii)-catalyzed cascade reactions of 2-arylindoles or 2-arylindole-3-carbonitriles with α-diazo carbonyl compounds is presented.

Rh(iii)-catalyzed sequential C–H activation and annulation: access to N-fused heterocycles from arylazoles and α-diazocarbonyl compounds

Valuable N-fused heterocycles were successfully obtained via Rh(iii)-catalyzed sequential C–H activation and annulation.

Synthesis of 2-Arylimino-6,7-dihydrobenzo[d][1,3]oxathiol-4(5H)-ones via Rh2(OAc)4-Catalyzed Reactions of Cyclic 2-Diazo-1,3-diketones with Aryl Isothiocyanates

A convenient and efficient synthesis of 2-arylimino-6,7-dihydrobenzo[d][1,3]oxathiol-4(5H)-ones was developed via a Rh₂(OAc)₄-catalyzed reaction of cyclic 2-diazo-1,3-diketones and aryl isothiocyanates in acetone at 60 °C. This reaction uses readily available stable cyclic 2-diazo-1,3-diketones as a starting material and generates the desired products in good to excellent yields (78-93%). The reaction proceeds under mild reaction conditions, produces only N₂ as the byproduct, and features a broad substrate scope. A plausible mechanism for this reaction is also discussed.

Bidentate Directing-Enabled, Traceless Heterocycle Synthesis: Cobalt-Catalyzed Access to Isoquinolines

Traceless heterocycle synthesis based on transition-metal-catalyzed C-H functionalization is synthetically appealing but has been realized only in monodentate directing systems. Bidentate directing systems allow for the achievement of high catalytic reactivity without the need for a high-cost privileged ligand. The first bidentate directing-enabled, traceless heterocycle synthesis is demonstrated in the cobalt-catalyzed synthesis of isoquinolines via 2-hydrazinylpyridine-directed C-H coupling/cyclization with alkynes. Convenient directing group installation through a ubiquitously present ketone group allows synthetic elaboration for complex molecules.

Synthesis of isoquinolines via Rh-catalyzed C–H activation/C–N cyclization with diazodiesters or diazoketoesters as a C2 source

Synthesis of isoquinolines based on efficient C–C and C–N bond formation through Rh(iii)-catalyzed C–H functionalization and subsequent intramolecular cyclization is reported. Diazodiesters serving as a C₂ source in the newly formed heterocycles are first demonstrated.

Rhodium-catalyzed C–H activation of 3-(indolin-1-yl)-3-oxopropanenitriles with diazo compounds and tandem cyclization leading to hydrogenated azepino[3,2,1-hi]indoles

Rh(iii)-Catalyzed C–H activation of 3-(indolin-1-yl)-3-oxopropanenitriles with diazo compounds and tandem cyclization leading to seven-membered ring scaffolds has been developed.

Advances in the development of catalytic tethering directing groups for C–H functionalization reactions

Inspired by the development of organocatalysis, transition-metal-catalyzed C–H functionalization reactions using tethering groups to accomplish site-selectivity catalytically have been demonstrated.