Three-Component Coupling of Aldehydes, 2-Aminopyridines, and Diazo Esters via Rhodium(III)-Catalyzed Imidoyl C-H Activation: Synthesis of Pyrido[1,2- a]pyrimidin-4-ones

Solvent-Controlled Rh(III)-Catalyzed Mono- and Dual Functionalization of Quinolyl Aldoximes with Diazo Compounds

A solvent-controlled Rh(III)-catalyzed mono- and dual-C-H bond activation/carbene migratory insertion with diazo compounds as a single coupling partner was demonstrated. The reaction proceeded under mild conditions, yielding products in good to excellent yields. These results are significant for the development of the domino multiple functionalization of C(sp2)-H bonds via a carbenoid insertion approach.

Merging Cobalt-Catalyzed C-H Activation with the Mannich Reaction: A Modular Approach to α-Substituted N-Sulfonyl Amines

A three-component synthesis of α-substituted N-sulfonyl amines from aryl aldehydes, primary sulfonamides, and (hetero)arenes is described. This transformation enables a straightforward and modular synthesis of highly substituted sulfonamide scaffolds in good yields. The direct functionalization of C(sp2)-H bonds via cobalt-catalyzed C-H-activation offers an appealing and atom-economical alternative to classical methods for the synthesis of α-arylated amines such as the Petasis or Mannich-type reactions.

CuI-catalyzed synthesis of multisubstituted pyrido[1,2-a]pyrimidin-4-ones through tandem Ullmann-type C-N cross-coupling and intramolecular amidation reaction

Various multi-substituted pyrido[1,2-a]pyrimidin-4-ones were synthesized via a one-pot tandem CuI-catalyzed C-N bond formation/intramolecular amidation reaction at 130 °C in DMF. This protocol features simple operation, broad substrate scope, good functional group tolerance and gram scale preparation, thus allowing practical and modular synthesis of pyrido[1,2-a]pyrimidin-4-ones from readily available 2-halopyridine and (Z)-3-amino-3-arylacrylate ester in good to excellent yields.

Electro-Oxidative C3-Selenylation of Pyrido[1,2-a]pyrimidin-4-ones

In this work, we achieved a C3-selenylation of pyrido[1,2-a]pyrimidin-4-ones using an electrochemically driven external oxidant-free strategy. Various structurally diverse seleno-substituted N-heterocycles were obtained in moderate to excellent yields. Through radical trapping experiments, GC-MS analysis and cyclic voltammetry study, a plausible mechanism for this selenylation was proposed.

Imine Directed Cp*RhIII -Catalyzed N-H Functionalization and Annulation with Amino Amides, Aldehydes, and Diazo Compounds

A multicomponent annulation that proceeds by imine directed Cp*RhIII -catalyzed N-H functionalization is disclosed. The transformation affords piperazinones displaying a range of functionality and is the first example of transition metal-catalyzed multicomponent N-H functionalization. A broad range of readily available α-amino amides, including those derived from glycine, α-substituted, and α,α-disubstituted amino acids, were effective inputs and enabled the incorporation of a variety of amino acid side chains with minimal racemization. Branched and unbranched alkyl aldehydes and various stabilized diazo compounds were also efficient reactants. The piperazinone products were further modified through efficient transformations. Mechanistic studies, including X-ray crystallographic characterization of a catalytically competent five-membered rhodacycle with imine and amide nitrogen chelation, provide support for the proposed mechanism.

HFIP in Organic Synthesis

1,1,1,3,3,3-Hexafluoroisopropanol (HFIP) is a polar, strongly hydrogen bond-donating solvent that has found numerous uses in organic synthesis due to its ability to stabilize ionic species, transfer protons, and engage in a range of other intermolecular interactions. The use of this solvent has exponentially increased in the past decade and has become a solvent of choice in some areas, such as C-H functionalization chemistry. In this review, following a brief history of HFIP in organic synthesis and an overview of its physical properties, literature examples of organic reactions using HFIP as a solvent or an additive are presented, emphasizing the effect of solvent of each reaction.

Synthesis of Tetrahydrocarbazol-4-ones via Rh(III)-Catalyzed C-H Activation/Annulation of Arylhydrazines with Iodonium Ylides

The rhodium(III)-catalyzed C-H activation followed by intramolecular annulation reactions between arylhydrazines and iodonium ylides under suitable conditions has been described. Tetrahydrocarbazol-4-ones are readily achieved with moderate to excellent yields. The synthetic protocol features a wide range of substrates with high functional group tolerance. The gram-scale reaction and derivatization of the product demonstrate the synthetic practicality and utilization of this method.

Rh(III)-catalyzed C-H bond activation/annulation reactions of arylacyl ammonium salts with 4-diazoisochroman-3-imines and 4-diazoisoquinolin-3-ones

We report a C-H bond functionalization strategy for the construction of oxo- and aza-spirocyclic compounds from diazo compounds as coupling partners. Our method comprises ortho sp2 C-H bond activation of...

Rh(III)-Catalyzed Cascade Annulation to Produce N-acetyl Chain of Spiropyrroloisoquinoline Derivatives

A new rhodium(III)-catalyzed three-component multistep cascade spirocyclization approach was developed to synthesize nolvel N-acetyl chain of spiropyrroloisoquinoline derivatives using oxadiazoles as the directing group. This one-pot reaction also isolates aryloxadiazole...

Multicomponent Reactions Involving Diazo Reagents: A 5-Year Update

This review summarizes recent developments in multicomponent reactions of diazo compounds. The role of diazo reagent and the type of interaction between components was analyzed to structure the discussion. In contrast to previous reviews on related topics mostly focused on metal catalyzed transformations, a substantial amount of organocatalytic or catalyst-free methodologies is covered in this work.

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.

Multi-component cascade reaction of 3-formylchromones: highly selective synthesis of 4,5-dihydro-[4,5'-bipyrimidin]-6(1H)-one derivatives

A novel protocol for the construction of highly functionalized bipyrimidine derivatives 4 and 5 from 3-formyl-chromones, ethyl 2-(pyridine-2-yl)acetate derivatives, and amidine hydrochlorides via an interesting and considerably complex multi-component cascade reaction was developed. The cascade reaction was manifested by refluxing a mixture of the three substrates in acetonitrile or DMF along with Cs2CO3. A series of 4,5-dihydro-[4,5'-bipyrimidin]-6(1H)-ones (DBPMOs) 4 was constructed regioselectively in suitable to excellent yields. Moreover, intermediates 4 then underwent a novel, metal- and oxidant-free cascade reaction to produce a series of [4,5'-bipyrimidin]-6(1H)-ones (BPMOs) 5. The formation of the bipyrimidine derivatives 4-5 was enabled by the formation of five bonds and the cleavage of one bond in one pot. This protocol can be used in the synthesis of functionalized bipyrimidine derivatives via a multi-component one-pot cascade reaction rather than multi-step reactions, which is suitable for both combinatorial and parallel syntheses of bipyrimidine derivatives.

Recent advances in Rh(iii)/Ir(iii)-catalyzed C–H functionalization/annulation via carbene migratory insertion

The review highlighted diverse annulations, including nitrogen, oxygen, sulfur heterocycles and carbocylizations via Rh(iii)/Ir(iii)-catalyzed C–H functionalization/annulation with various arene and carbene precursors.

Rh(iii)-Catalyzed one-pot three-component cyclization reaction: rapid selective synthesis of monohydroxy polycyclic BINOL derivatives

A Rh(iii)-catalyzed three-component C–H bond functionalization protocol has been successfully applied to access complex polycyclic BINOL derivatives in which the formation of intermediate amides occurred in situ from aldehydes and amines.

Rh(III)-Catalyzed Imidoyl C-H Carbamylation and Cyclization to Bicyclic [1,3,5]Triazinones

A Rh(III)-catalyzed synthesis of bicyclic [1,3,5]triazinones from a diverse array of imines coupled with ethyl (pivaloyloxy)carbamate is reported. The preparation of [5,6]- and [6,6]-bicyclic heterocycles substituted with aryl, alkyl, and alkoxy groups demonstrated a broad reaction scope. The efficiency of this approach was further enhanced with the development of a three-component variant featuring in situ imine formation. X-ray crystallographic characterization of a rhodacycle formed by imidoyl C-H activation provides support for the proposed mechanism.

Rh-Catalyzed C-H activation/intramolecular condensation for the construction of benzo[f]pyrazolo[1,5-a][1,3]diazepines

A novel and mild Rh(iii)-catalyzed C-H activation/intramolecular condensation of 1-aryl-1H-pyrazol-5-amines with cyclic 2-diazo-1,3-diketones was developed, giving access to various important benzo[f]pyrazolo[1,5-a][1,3]diazepine scaffolds through sequential C-C/C-N bond formation in a one-pot procedure under additive- and oxidant-free conditions. Furthermore, 3-([1,1'-biphenyl]-2-ylamino)-2-ethoxycyclohex-2-enones can be obtained in good yields by constructing C-O and C-N bonds through 1,1'-insertion, dehydration, and isomerization processes.

Annulation of Hydrazones and Alkynes via Rhodium(III)-Catalyzed Dual C-H Activation: Synthesis of Pyrrolopyridazines and Azolopyridazines

Hydrazones readily synthesized from N-aminopyrroles or N-aminoazoles and aldehydes undergo Rh(III)-catalyzed dual C-H activation and coupling with aryl- and alkyl-substituted alkynes to give pyrrolopyridazines or azolopyridazines, respectively. This transformation represents a rare example of hydrazoyl C-H activation and proceeds without heteroatom functionality to direct C-H activation. Hydrazones derived from aromatic, alkenyl, and aliphatic aldehydes were effective inputs, and tethering the alkyne to the hydrazone enabled annulations to more complex, tricyclic products.

Multicomponent Aromatic and Benzylic Mannich Reactions through C-H Bond Activation

Multicomponent Mannich reactions through C-H bond activation are described. These transformations allowed for the straightforward generation of densely substituted benzylic and homo-benzylic amines in good yields. The reaction involves a reaction between two transient species: an organometallic species, generated by transition-metal-catalyzed sp² or sp³ C-H bond activation and an in situ generated imine. The use of an acetal as an aldehyde surrogate was found essential for the reaction to proceed. The process could be successfully applied to Rh^III -catalyzed sp² C-H bond functionalization and extended to Cu^II -catalyzed sp³ C-H bond functionalization.

Spiro[indene-1,4'-oxa-zolidinones] Synthesis via Rh(III)-Catalyzed Coupling of 4-Phenyl-1,3-oxazol-2(3H)-ones with Alkynes: A Redox-Neutral Approach

Transition-metal-catalyzed C-H activation synthesis of heterocyclic spiro[4,4]nonanes has persistently witnessed the use of additional stoichiometric transition-metal oxidant when employing C═C bond as the spiro ring closure site. Herein, we have addressed the issue by reporting a redox-neutral strategy for spiro[indene-1,4'-oxa-zolidinones] synthesis via Rh(III)-catalyzed coupling of 4-phenyl-1,3-oxazol-2(3H)-ones with alkynes. The synthesis features a broad substrate scope and high regiospecificity.