Palladium-catalyzed C–N bond formation via direct C–H bond functionalization. Recent developments in heterocyclic synthesis

Cu-Catalyzed Oxidative C(sp2)-H Cycloetherification of o-Alkenyl Arenols for the Preparation of Fused Furans

A practically efficient, copper-catalyzed approach for the synthesis of functionally embellished indeno-naphthofurans is developed from 1-(1H-inden-3-yl)naphthalen-2-ols. This intramolecular cycloetherification proceeds via C(sp2)-H oxygenation (C-H bond breaking and C-O bond forming), which enables the atom-economical synthesis of poly fused furans in high yields with large substrate diversity in the open air.

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.

Remote Construction of N-Heterocycles via 1,4-Palladium Shift-Mediated Double C-H Activation

In the past years, Pd0 -catalyzed C(sp3 )-H activation provided efficient and step-economical methods to synthesize carbo- and heterocycles via direct C(sp2 )-C(sp3 ) bond formation. We report herein that a 1,4-Pd shift allows access to N-heterocycles which are difficult to build via a direct reaction. It is shown that o-bromo-N-methylanilines undergo a 1,4-Pd shift at the N-methyl group, followed by intramolecular trapping by C(sp2 )-H or C(sp3 )-H activation at another nitrogen substituent and remote C-C bond formation to generate biologically relevant isoindolines and β-lactams. The product selectivity is influenced by the employed ligand, with NHCs favoring the product of remote C-C coupling against products arising from direct C-C coupling and N-demethylation.

Alkynoates as Versatile and Powerful Chemical Tools for the Rapid Assembly of Diverse Heterocycles under Transition-Metal Catalysis: Recent Developments and Challenges

Heterocycles, heteroaromatics and spirocyclic entities are ubiquitous components of a wide plethora of synthetic drugs, biologically active natural products, marketed pharmaceuticals and agrochemical targets. Recognizing their high proportion in drugs and rich pharmacological potential, these invaluable structural motifs have garnered significant interest, thus enabling the development of efficient catalytic methodologies providing access to architecturally complex and diverse molecules with high atom-economy and low cost. These chemical processes not only allow the formation of diverse heterocycles but also utilize a range of flexible and easily accessible building units in a single operation to discover diversity-oriented synthetic approaches. Alkynoates are significantly important, diverse and powerful building blocks in organic chemistry due to their unique and inherent properties such as the electronic bias on carbon-carbon triple bonds posed by electron-withdrawing groups or the metallic coordination site provided by carbonyl groups. The present review highlights the comprehensive picture of the utility of alkynoates (2007-2019) for the synthesis of various heterocycles (> 50 types) using transition-metal catalysts (Ru, Rh, Pd, Ir, Ag, Au, Pt, Cu, Mn, Fe) in various forms. The valuable function of versatile alkynoates (bearing multifunctional groups) as simple and useful starting materials is explored, thus cyclizing with an array of coupling partners to deliver a broad range of oxygen-, nitrogen-, sulfur-containing heterocycles alongside fused-, and spiro-heterocyclic compounds. In addition, these examples will also focus the scope and reaction limitations, as well as mechanistic investigations into the synthesis of these heterocycles. The biological significance will also be discussed, citing relevant examples of drug molecules highlighting each class of heterocycles. This review summarizes the recent developments in the synthetic methods for the synthesis of various heterocycles using alkynoates as readily available starting materials under transition-metal catalysis.

New frontiers in the transition-metal-free synthesis of heterocycles from alkynoates: an overview and current status

This review highlights the successful utilization of transition-metal-free approaches for the modular assembly of various heterocycles from alkynoates.

Asymmetric Aza-Wacker-Type Cyclization of N-Ts Hydrazine-Tethered Tetrasubstituted Olefins: Synthesis of Pyrazolines Bearing One Quaternary or Two Vicinal Stereocenters

We have developed an asymmetric aza-Wacker-type cyclization of N-Ts hydrazine-tethered tetrasubstituted olefins, affording optically active pyrazolines bearing chiral tetrasubstituted carbon stereocenters. This reaction is tolerant to a broad range of substrates under mild reaction conditions, giving the desired chiral products with high enantioselectivities. Generation of two vicinal stereocenters on the C═C double bonds was also achieved with high selectivities, a process which has been rarely studied for Wacker-type reactions. A mechanistic study revealed that this aza-Wacker-type cyclization undergoes a syn-aminopalladation process. It was also found that for substrates bearing two linear alkyl substituents on the outer carbon atom of the olefin, both of which are larger than a methyl group, the alkyl substituent that is cis to the intranucleophilic group participates more readily in β-hydride elimination. When one of the two alkyl substituents on the outer carbon atom of the olefin is a methyl group, β-hydride elimination proceeds selectively at the methylene side, thus both diastereomers can be prepared via switching the configuration of the olefin. Furthermore, the product can be converted to a pharmaceutical compound in high yields over three steps.

C-H functionalization by high-valent Cp*Co(iii) catalysis

Significant progress has been accomplished in directed C-H functionalization through the use of earth-abundant and inexpensive first-row transition metals. Among these base metals, Co is especially attractive in view of its versatile applications in C-H functionalization, in both low- and high-valent states. In this vein, catalytic Co(iii) species can be generated from the dissociation of a Cp*Co(iii) catalyst or through the oxidation of a low-valent cobalt catalyst in the presence of an oxidant. In this feature article, we will discuss the breakthroughs in Cp*Co(iii)-promoted C-H functionalization. In this field, C(sp²)-H functionalization has been extensively studied and developed. In contrast, few C(sp³)-H functionalization reactions have been reported.

Copper-catalyzed phosphorylation of sp(2) C-H bonds

The phosphorylation of the ortho C-H bonds in benzamides containing an 8-aminoquinoline moiety as a bidentate directing group with H-phosphonates using copper as a catalyst under mild temperature conditions is described. This method shows high functional group compatibility and selectively gives mono-substituted products.

Solvent-free, [Et3NH][HSO4] catalyzed facile synthesis of hydrazone derivatives

Eco-friendly and efficient synthesis of hydrazone derivatives.

[Et3NH][HSO4]-catalyzed eco-friendly and expeditious synthesis of thiazolidine and oxazolidine derivatives

Facile, sustainable and efficient synthesis of thiazolidine/oxazolidine derivatives.

One step C–N bond formation from alkylbenzene and ammonia over Cu-modified TS-1 zeolite catalyst

The formation of C–N bonds from toluene and ammonia, both on the ring and the side chain, was realized on a Cu-modified TS-1 catalyst, and the ring products had an ortho-orientation advantage.

Asymmetric palladium-catalysed intramolecular Wacker-type cyclisations of unsaturated alcohols and amino alcohols

The palladium (II)-catalysed reactions of alkenols and aminoalkenols such as oxycarbonylations or bicyclisations are powerful methods for the construction of oxygen and nitrogen-containing heterocyclic compounds. This review highlights recent progress in the development of the asymmetric palladium(II)-catalysed Wacker-type cyclisations of unsaturated polyols and aminoalcohols. The scope, limitations, and applications of these reactions are presented.

Palladium-catalyzed dual C-H or N-H functionalization of unfunctionalized indole derivatives with alkenes and arenes

This review highlights the development of palladium-catalyzed C-H and N-H functionalization reactions involving indole derivatives. These procedures require unactivated starting materials and are respectful of the basic principle of sustainable chemistry tied to atom economy.