Rhodium-Catalyzed Asymmetric C-H Functionalization Reactions

This review summarizes the advancements in rhodium-catalyzed asymmetric C-H functionalization reactions during the last two decades. Parallel to the rapidly developed palladium catalysis, rhodium catalysis has attracted extensive attention because of its unique reactivity and selectivity in asymmetric C-H functionalization reactions. In recent years, Rh-catalyzed asymmetric C-H functionalization reactions have been significantly developed in many respects, including catalyst design, reaction development, mechanistic investigation, and application in the synthesis of complex functional molecules. This review presents an explicit outline of catalysts and ligands, mechanism, the scope of coupling reagents, and applications.

Vinylation of N-Heteroarenes through Addition/Elimination Reactions of Vinyl Selenones

A new protocol for the synthesis of N-vinyl azoles using vinyl selenones and azoles in the presence of potassium hydroxide was developed. This reaction proceeded under mild and transition metal-free conditions through an addition/elimination cascade process. Both aromatic and aliphatic vinyl selenones and various mono-, bi- and tri-cyclic azoles can be tolerated and give terminal N-vinyl azoles in moderate to high yields. A plausible mechanism is also proposed.

Transition-metal-catalyzed C-H bond alkylation using olefins: recent advances and mechanistic aspects

Transition metal catalysis has contributed immensely to C-C bond formation reactions over the last few decades, and alkylation is no exception. The superiority of such methodologies over traditional alkylation is evident from minimal reaction steps, shorter reaction times, and atom economy while also allowing control over regio- and stereo-selectivity. In particular, hydrocarbonation of alkenes has grabbed increased attention due its fundamental ability to effectively and selectively synthesise a wide range of industrially and pharmaceutically relevant moieties. This review attempts to provide a scientific viewpoint and a systematic analysis of the recent developments in transition-metal-catalyzed alkylation of various C-H bonds using simple and activated olefins. The key features and mechanistic studies involved in these transformations are described briefly.

Diverse saturated heterocycles from a hydroacylation/conjugate addition cascade

Rhodium-catalyzed hydroacylation using alkynes substituted with pendant nucleophiles, delivers linear α,β-unsaturated enone intermediates with excellent regioselectivity. These adducts are used to construct a broad range of diversely substituted, saturated O-, N- and S-heterocycles in a one-pot process. Judicious choice of cyclisation conditions enabled isolation of O-heterocycles with high levels of diastereoselectivity. A variety of derivatisation reactions are also performed, generating functionalised hydroacylation products. This sequence serves as a general approach for the synthesis of fully saturated heterocycles.

Recent advances in the synthesis of pyrrolo[1,2-a]indoles and their derivatives

The pyrrolo[1,2-a]indole unit is a privileged heterocycle found in numerous natural products and has been shown to exhibit diverse pharmacological properties. Thus, recent years have witnessed immense interest from the synthesis community on the synthesis of this scaffold. In light of the ever-increasing demand for pyrrolo[1,2-a]indoles in drug discovery, this review provides an overview of recent synthesis methods for the preparation of pyrrolo[1,2-a]indoles and their derivatives. The mechanistic pathway and stereo-electronic factors affecting the yield and selectivity of the product are briefly explained. Furthermore, we have attempted to demonstrate the utility of the developed methods in the synthesis of bioactive molecules and natural products, wherever offered.

Catalytic Cycloisomerization onto a Carbonyl Oxygen

We have found that terminal N-vinylindoles bearing cycloalkanone substituents are excellent hydrogen atom acceptors, generating α-aminyl radicals with a variety of catalysts (Co(II)/H2 or Co(III)Cl precatalysts with silane reductants). These radicals can be converted to internal vinylindoles but eventually add to the oxygen of the cycloalkanone substituents. These cyclizations eventually furnish a densely functionalized dihydrofuran (a net cycloisomerization). The internal vinylindoles are slowly converted to the dihydrofurans, but the final cycloisomerization/isomerization ratio is affected by the size of the cycloalkanone ring (seven- and eight-membered rings give the highest ratio). These results demonstrate how HAT can isomerize substrates in nonintuitive ways, here leading to the first HAT-promoted formation of a C-O bond.

Fe-Catalyzed Sequential C(sp3)-H/N-H Annulation of 2-Methylindoles with Ethyl Trifluoropyruvate at Room Temperature: Construction of Pyrrolo[1,2-α]indoles

An efficient and benign iron-catalyzed room-temperature method was developed for direct sequential C(sp³)-H/N-H annulation to construct pyrroloindole scaffolds. This strategy features cheap and readily available raw materials and mild room-temperature reaction conditions and provides a green and practical method for the one-pot rapid synthesis of a wide range of diversely functionalized pyrrolo[1,2-α]indoles.

Catalytic Asymmetric [3 + 2] Annulation via Indolyl Copper-Allenylidene Intermediates: Diastereo- and Enantioselective Assembly of Pyrrolo[1,2-a]indoles

A catalytic asymmetric decarboxylative [3 + 2] annulation via indolyl copper-allenylidene amphiphilic intermediates has been developed. This protocol offers a straightforward method for the synthesis of biologically important pyrrolo[1,2-a]indoles bearing contiguous quaternary and tertiary stereogenic centers with excellent diastereo- and enantioselectivities (up to >20:1 dr and >99% ee). In addition, the diversity-oriented synthesis of pyrrolo[1,2-a]indoles was achieved via versatile transformations of the alkyne-containing cycloadducts.

Recent advances in rhodium-catalyzed asymmetric synthesis of heterocycles

Heterocycles are crucial structural motifs that are ubiquitously present in biologically active natural products and pharmaceutically important compounds. Over the past few decades, great attention has been paid to develop efficient methodologies for the construction of diverse enantioenriched heterocyclic frameworks. This review focuses on the recent impressive progress and advances in the asymmetric synthesis of heterocycles under rhodium catalysis.

Catalytic, Enantioselective Synthesis of Polycyclic Nitrogen, Oxygen, and Sulfur Heterocycles via Rh-Catalyzed Alkene Hydroacylation

Enantioselective synthesis of polycyclic nitrogen, oxygen, and sulfur heterocycles by rhodium-catalyzed intramolecular alkene hydroacylation is reported. The intramolecular hydroacylation reactions generate 1,4-dihydrocyclopenta[b]indol-3(2H)-ones and 3,4-dihydrocyclopenta[b]indol-1(2H)-one in moderate-to-high yields (65-99%) with good-to-excellent enantioselectivities (84-99% ee). The catalyst system also promotes alkene hydroacylation of 3-vinylfuran-, 3-vinylbenzothiophene-, and 3-vinylthiophene-2-carboxaldehydes to generate the corresponding ketone products in moderate-to-high yields (71-91% yield) with excellent enantioselectivities (97-99% ee).

Diastereo- and enantioselective construction of biologically important pyrrolo[1,2-a]indole scaffolds via catalytic asymmetric [3 + 2] cyclodimerizations of 3-alkyl-2-vinylindoles

A catalytic asymmetric [3 + 2] cyclodimerization of 3-alkyl-2-vinylindoles has been established, which efficiently constructed a pyrrolo[1,2-a]indole scaffold in a diastereo- and enantioselective fashion.

Enantioselective Model Synthesis and Progress toward the Putative Structure of Yuremamine

An enantioselective model synthesis of the 2,3-dihydro-1H-pyrrolo[1,2-a]indole core of the putative structure of yuremamine is reported in 39% overall yield and 96% ee over five steps. The model synthesis leverages enantioselective, rhodium-catalyzed hydroacylation of an N-vinylindole-2-carboxaldehyde as the key step in the installation of the stereochemical triad. An enantioselective synthesis of a densely functionalized dihydropyrroloindolone that maps onto the putative structure of yuremamine is demonstrated in 26% yield and 97% ee over eight steps.

Substituent effect and ligand exchange control the reactivity in ruthenium(II)-catalyzed hydroacylation of isoprenes and aldehydes ‖ A DFT study

Density functional theory (DFT) was used to investigate the reaction mechanisms of ruthenium(II)-catalyzed hydroacylation of isoprene with benzaldehyde, and o-methoxyl, m-methoxyl and p-methoxyl benzaldehyde. All intermediates and transition states were entirely optimized at the B3LYP/6-31G(d,p) level (LANL2DZ(f) for Ru). The results demonstrated that the hydroacylation had two different catalytic cycles (path I and II), path II was more favorable than path I. Ru(II)-catalyzed hydroacylation began from the first catalytic cycle, and the nucleophilic reaction was the rate-determining step. The activation barriers of hydrogen migration were the highest in two catalytic cycles, so the hydrogen migration was the rate-determining step. The activation barrier of hydrogen migration could be broken down to two parts: the free energy of exchange ([Formula: see text]) and the relative free energy of transition state ([Formula: see text]). The ligand exchange energy ([Formula: see text]) had more contribution to the activation barrier than the relative free energy of transition state ([Formula: see text]), so the ligand exchange would control these hydroacylation. Furthermore, our calculations also described the substituent effect, and the results indicated that four aldehydes showed different chemical reactivity, and benzaldehyde and m-methoxyl benzaldehyde were predicted to have the best reactivity in ruthenium hydride-catalyzed hydroacylation.

Diastereoselective Radical Hydroacylation of Alkylidenemalonates with Aliphatic Aldehydes Initiated by Photolysis of Hypervalent Iodine(III) Reagents

Diastereoselective radical hydroacylation of chiral alkylidenemalonates with aliphatic aldehydes is realized by the combination of a hypervalent iodine(III) reagent and UV-light irradiation. The reaction is initiated by the photolysis of hypervalent iodine(III) reagents under mild, metal-free conditions, and is the first example of diastereoselective addition of acyl radicals to olefins to afford chiral ketones in a highly stereoselective fashion. The obtained optically active ketones are useful chiral synthons, as exemplified by the short formal synthesis of (-)-methyleneolactocin.

Recent advances in transition metal-catalysed hydroacylation of alkenes and alkynes

This highlight presents advances in transition metal-catalysed alkene and alkyne hydroacylation over the past three years.

Synthetic studies towards putative yuremamine using an iterative C(sp3)–H arylation strategy

An overview of synthetic efforts toward the initially proposed structure of yuremamine using iterative C(sp³)–H arylations is described.

Homogeneous rhodium(i)-catalysis in de novo heterocycle syntheses

Rh(i)-catalysed reactions often employ mild reaction conditions and offer excellent functional group tolerance, making them ideal transformations for the preparation of complex molecules. This review surveys examples of these synthetically useful transformations as applied to the synthesis of various heterocycles.

N-Heterocyclic carbene-catalysed intramolecular hydroacylation to form basic nitrogen-containing heterocycles

N-Heterocyclic carbene-catalysed intramolecular hydroacylation of N-allylimidazole-2-carboxaldehydes and N-allylbenzimidazole-2-carboxaldehydes is reported. These exo-selective hydroacylations of unactivated alkenes enable the synthesis of a variety of basic, polycyclic nitrogen heterocycles in good-to-excellent yields.

Bioinspired total synthesis and structural revision of yuremamine, an alkaloid from the entheogenic plant Mimosa tenuiflora

Guided by a biosynthetic hypothesis, a serendipitous total synthesis of yuremamine has resulted in its structural revision from the putative pyrroloindole (1) to the flavonoidal indole (2), which was initially proposed as a biosynthetic intermediate.

Rh-Catalyzed Desymmetrization of α-Quaternary Centers by Isomerization-Hydroacylation

We describe a Rh-catalyzed desymmetrization of all-carbon quaternary centers from α,α-bis(allyl)aldehydes by a cascade featuring isomerization and hydroacylation. This desymmetrization competes with two other novel olefin functionalizations that are triggered by C-H bond activation, including carboacylation and bisacylation. A BIPHEP ligand promotes enantioselective formation of α-vinylcyclopentanones. Mechanistic studies support irreversible and enantioselective olefin-isomerization followed by olefin-hydroacylation.