Rh(III)-Catalyzed C-H Activation/Annulation for the Construction of Quinolizinones and Indolizines

A catalytic-condition-controlled synthesis strategy was reported to build quinolizinone and indolizine derivatives from the easily available enamide and triazole substrates with high regioselectivity and good functional group tolerance. More especially, this transformation has successfully fulfilled a C-H bond activation of terminal olefin from enamides followed by a [3 + 3] and a [2 + 3] cyclization cascade under different catalytic conditions, respectively, to provide two kinds of potentially biologically active heterocyclic scaffolds with a ring-junction nitrogen atom. Mechanistically, the methoxyamine formyl group serves as either a traceless directing group (DG) or an oxidizing DG via the C-N and C-C cleavage in this protocol.

Construction of Functionalized 2H-Pyran-2-ones via N-Heterocyclic Carbene-Catalyzed [3 + 3] Annulation of Alkynyl Esters with Enolizable Ketones

A new synthesis of functionalized 2H-pyran-2-ones has been developed through N-heterocyclic carbene-catalyzed formal [3 + 3] annulation of alkynyl esters with enolizable ketones. The key to the success of this protocol relies on the use of an NHC instead of a tertiary amine as the catalyst. This protocol also features a broad substrate scope and mild metal-free conditions, offering simple and rapid access to the target molecules in a highly regioselective manner.

Recent Advances in Alkenyl sp2 C-H and C-F Bond Functionalizations: Scope, Mechanism, and Applications

Alkenes and their derivatives are featured widely in a variety of natural products, pharmaceuticals, and advanced materials. Significant efforts have been made toward the development of new and practical methods to access this important class of compounds by selectively activating the alkenyl C(sp²)-H bonds in recent years. In this comprehensive review, we describe the state-of-the-art strategies for the direct functionalization of alkenyl sp² C-H and C-F bonds until June 2022. Moreover, metal-free, photoredox, and electrochemical strategies are also covered. For clarity, this review has been divided into two parts; the first part focuses on currently available alkenyl sp² C-H functionalization methods using different alkene derivatives as the starting materials, and the second part describes the alkenyl sp² C-F bond functionalization using easily accessible gem-difluoroalkenes as the starting material. This review includes the scope, limitations, mechanistic studies, stereoselective control (using directing groups as well as metal-migration strategies), and their applications to complex molecule synthesis where appropriate. Overall, this comprehensive review aims to document the considerable advancements, current status, and emerging work by critically summarizing the contributions of researchers working in this fascinating area and is expected to stimulate novel, innovative, and broadly applicable strategies for alkenyl sp² C-H and C-F bond functionalizations in the coming years.

Synthesis of Furans via Rhodium(III)-Catalyzed Cyclization of Acrylic Acids with α-Diazocarbonyl Compounds

An efficient protocol for the synthesis of furans through Rh(III)-catalyzed vinyl C-H activation from acrylic acids and α-diazocarbonyl compounds has been developed. The reaction features broad functional group tolerance and affords a series of furans in moderate to good yields. Moreover, no additives such as copper or silver salts are required. Some control experiments are performed to give insight into the mechanism of this cascade transformation and the decarbonylation process is involved in the formation of the furan product.

Rh-Catalyzed Coupling of Acrylic/Benzoic Acids with α-Diazocarbonyl Compounds: An Alternative Route for α-Pyrones and Isocoumarins

A coupling of acrylic acids/benzoic acids with α-diazocarbonyl compounds has been realized by a combined catalytic system of rhodium catalyst and Zn(OAc)₂ additive. The presence of Zn(OAc)₂ obviously accelerates the C(sp²)-H activation and destructed the formation of carboxylic ester that is formed via a nucleophilic O-H insertion to metal carbenoid. The procedure featured mild reaction conditions and broad substrate scope, providing a straightforward approach to the synthesis of α-pyrones and isocoumarins without the transformation of carboxylic acids to the corresponding amides.

Transition-Metal-Catalyzed C-H Bond Functionalization of Arenes/Heteroarenes via Tandem C-H Activation and Subsequent Carbene Migratory Insertion Strategy

The past decade has witnessed tremendous developments in transition-metal-catalyzed C-H bond activation and subsequent carbene migratory insertion reactions, thus assisting in the construction of diverse arene/heteroarene scaffolds. Various transition-metal catalysts serve this purpose and provide efficient pathways for an easy access to substituted heterocycles. A brief introduction to metal-carbenes has been provided along with key mechanistic pathways underlying the coupling reactions. The purpose of this review is to provide a concise knowledge about diverse directing group-assisted coupling of varied arenes/heteroarenes and acceptor-acceptor/donor-acceptor diazo compounds. The review also highlights the synthesis of various carbocycles and fused heterocycles through diazo insertion pathways, via C-C, C-N and C-O bond forming reactions. The mechanism usually involves a C-H activation process, followed by diazo insertion leading to subsequent coupling.

Rhodium-Catalyzed Dehydrogenative Cycloisomerization of Dienylcyclopropane to Highly Substituted Toluene

A rhodium-catalyzed dehydrogenative cycloisomerization of dienylcyclopropane compounds is reported, which provides a straightforward approach to a variety of highly substituted toluene derivatives in 67-85% yields. The dienylcyclopropane-imides are produced by a single-step formal three-component olefination procedure. Preliminary mechanistic studies indicated that an electron-withdrawing group as R plays a critical role in completing this transformation.

Palladium-Catalyzed Regioselective C-H Functionalization/Annulation Reaction of Amides and Allylbenzenes for the Synthesis of Isoquinolinones and Pyridinones

A regioselective C-H functionalization/annulation reaction of N-sulfonyl amides and allylbenzenes through a palladium-catalyzed C(sp²)-H allylation/aminopalladation/β-H elimination/isomerization sequence has been reported. Various aryl and alkenyl carboxamides are found to be efficient substrates to construct isoquinolinones and pyridinones in up to 96% yield. Using ambient air as the terminal oxidant is another advantage regarding environmental friendliness and operational simplicity.

Rhodium(iii)-catalyzed C–H activation/annulation of N-iminopyridinium ylides with alkynes and diazo compounds

Rh(iii)-Catalyzed C–H activation/annulation of N-iminopyridinium ylides with alkynes and diazo compounds has been realized for the synthesis of isoquinolones and isocoumarins.

Rh(III)-Catalyzed Annulation of Boc-Protected Benzamides with Diazo Compounds: Approach to Isocoumarins

A mild rhodium-catalyzed annulation of Boc-protected benzamides with diazo compounds via C−C/C−O bond formation has been explored. In the presence of [Cp*RhCl₂]₂, AgSbF₆ and Cs₂CO₃, Boc-protected benzamides can be effectively annulated to yield isocoumarins in 0.5–2 h.

[RhIII(Cp*)]-catalyzed arylfluorination of α-diazoketoesters for facile synthesis of α-aryl-α-fluoroketoesters

Here we describe the Cp*Rh(iii)-catalyzed cascade arylfluorination reactions of α-diazoketoesters with arylboronic acids and N-fluorobenzenesulfonimide for one-pot C(sp3)-C(aryl) and C(sp3)-F bond formation. The arylfluorination reaction can be accomplished with remarkable chemo- and regioselectivity. Our mechanistic investigation showed that the Rh-catalyzed arylfluorination of diazoacetates occurred by (1) transmetalation of arylboronic acids to form an arylrhodium(iii) complex, (2) coupling of diazomalonates with the arylrhodium(iii) complex to form carbene-rhodium, (3) migratory carbene insertion to form a diketonato-rhodium(iii) complex - probably via rearrangement of the putative σ-alkylrhodium(iii) complex, and (4) electrophilic fluorination of the diketonato-rhodium to form the α-aryl-α-fluoromalonates.

Controllable construction of isoquinolinedione and isocoumarin scaffolds via RhIII-catalyzed C–H annulation of N-tosylbenzamides with diazo compounds

An efficient protocol for the synthesis of isoquinolinediones by Rh^III-catalyzed C–H activation/annulation/decarboxylation of N-tosylbenzamides with diazo compounds is reported.

One-Pot Synthesis of Deuterated Aldehydes from Arylmethyl Halides

A facile, one-pot approach for synthesizing deuterated aldehydes from arylmethyl halides was developed using D₂O as the deuterium source. The efficient process is realized by a sequence of formation, H/D exchange, and oxidation of pyridinium salt intermediates. The mild and air-compatible reaction conditions enable efficient synthesis of diverse deuterated aldehydes with high deuterium incorporation.

Rhodium-Catalyzed Cyclization of O,ω-Unsaturated Alkoxyamines: Formation of Oxygen-Containing Heterocycles

O,ω-Unsaturated N-tosyl alkoxyamines undergo unexpected Rh^III -catalyzed intramolecular cyclization by oxyamination to produce oxygen-containing heterocycles. Mechanistic studies show that an aziridine intermediate seems to be responsible for the formation of the heterocycles, possibly via a Rh^V species.

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.

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.

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.

The synthesis of cycloalka[b]furans via an Au(i)-catalyzed tandem reaction of 3-yne-1,2-diols

A rapid and convenient approach toward substituted cycloalka[b]furan compounds has been developed via an Au(i)-catalyzed cyclization/1,2-rearrangement/aromatization cascade.

Silver-mediated direct trifluoromethoxylation of α-diazo esters via the (-)OCF3 anion

Silver-mediated direct trifluoromethoxylation of α-diazo esters and ketosteroid was disclosed. The reactions of alkyl α-diazo arylacetates with AgOCF3 or CF3SO2OCF3/AgF at -30 to 10 °C under a N2 atmosphere provided α-trifluoromethoxyl arylacetates in up to 90% yield, while alkyl α-diazo vinylacetates reacting with CF3SO2OCF3/AgF or AgOCF3 afforded γ-trifluoromethoxyl α,β-unsaturated esters in up to 94% yield. The α-diazo ketosteroid was also trifluoromethoxylated under the standard reaction conditions. This protocol allows for an effective and convenient access to a large number of synthetic building blocks, which are promising in the development of new functional OCF3-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.

Copper-catalyzed three-component cascade reaction of alkynes, sulfonyl azides and simple aldehydes/ketones

A copper-catalyzed dimethylzinc-promoted three-component cascade reaction of alkynes, sulfonyl azides, and simple aldehydes or ketones is described, which delivers polysubstituted olefins expeditiously in one pot under mild conditions.

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.