Enantioselective synthesis of polycyclic nitrogen heterocycles by Rh-catalyzed alkene hydroacylation: constructing six-membered rings in the absence of chelation assistance

Amide-Directed, Rhodium-Catalyzed Regio- and Enantioselective Hydroacylation of Internal Alkenes with Unfunctionalized Aldehydes

Despite the current progress achieved in asymmetric hydroacylations, highly enantioselective catalytic addition of unfunctionalized aldehydes to internal alkenes remains an unsolved challenge. Here, using a coordination-assisted strategy, we developed a rhodium-catalyzed regio- and enantioselective addition of unfunctionalized aldehydes to internal alkenes such as enamides and β,γ-unsaturated amides. Valuable α-amino ketones and 1,4-dicarbonyl compounds were directly obtained with high enantioselectivity from readily available materials.

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.

Regio- and Enantioselective Synthesis of Dihydropyrido[1,2-a]indoles via Catalytic Asymmetric Annulative Allylic Alkylation

A palladium-catalyzed asymmetric annulative allylic alkylation reaction of 2-[(1H-indol-2-yl)methyl]malonates with (E)-but-2-ene-1,4-diyl dicarbonates is described, leading to the regio- and enantioselective synthesis of dihydropyrido[1,2-a]indoles with a chiral cyclic allyl stereocenter adjacent to the ring-junction nitrogen atom in moderate to good yields. The salient features of this protocol include mild conditions, a broad substrate scope, and good compatibility with substituents as well as high regio- and stereoselectivities, providing a catalytic asymmetric entry for fabricating chiral pyridoindole scaffolds.

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.

Total synthesis of (±)-vinoxine: construction of the bridged pyrido[1,2-a]indole skeleton via Tf2O-mediated Bischler-Napieralski reaction and stereoselective radical cyclization

The total synthesis of (±)-vinoxine was achieved featuring the assembly of a multi-substituted tetrahydropyrido[1,2-a]indole skeleton through the Tf₂O-mediated Bischler-Napieralski reaction. The characteristic diazabicyclo[3.3.1]nonane skeleton was stereoselectively constructed via radical cyclization based on the one stereochemistry of the C3 position. The established methodology provides new options for the synthesis of natural products and pharmaceuticals containing the multi-substituted pyrido[1,2-a]indole skeleton.

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.

Iridium-catalysed branched-selective hydroacylation of 1,3-dienes with salicylaldehydes

Herein, we report an iridium-catalyzed branched-selective hydroacylation of 1-aryl 1,3-dienes with salicylaldehydes under mild conditions with no need of phosphine ligands. With this protocol, a series of α-branched β,γ-unsaturated o-hydroxyacetophenones with biological potentials were synthesized in high efficiency with excellent regioselectivities. When simple 1,3-butadiene or isoprene instead of 1-aryl 1,3-dienes were used, exclusive linear-selective hydroacylation products were obtained.

Machine Learning Classification of One-Chiral-Center Organic Molecules According to Optical Rotation

In this study, machine learning algorithms were investigated for the classification of organic molecules with one carbon chiral center according to the sign of optical rotation. Diverse heterogeneous data sets comprising up to 13,080 compounds and their corresponding optical rotation were retrieved from Reaxys and processed independently for three solvents: dichloromethane, chloroform, and methanol. The molecular structures were represented by chiral descriptors based on the physicochemical and topological properties of ligands attached to the chiral center. The sign of optical rotation was predicted by random forests (RF) and artificial neural networks for independent test sets with an accuracy of up to 75% for dichloromethane, 82% for chloroform, and 82% for methanol. RF probabilities and the availability of structures in the training set with the same spheres of atom types around the chiral center defined applicability domains in which the accuracy is higher.

Origin of the Diastereoselectivity of the Heterogeneous Hydrogenation of a Substituted Indolizine

In this work, the stereoselective heterogeneous hydrogenation of a tetrasubstituted indolizine was studied. Partial hydrogenation products were obtained in three steps from a substituted pyridine-2-carboxaldehyde prepared from commercial pyridoxine hydrochloride. The hydrogenation of the indolizine ring was shown to be diastereoselective, forming trans-6b and cis-9. Theoretical calculations (ab initio and DFT) were used to rationalize the unusual trans stereoselectivity for 6b, and a keto–enol tautomerism under kinetic control has been proposed as the source of diastereoselectivity.

Tandem sequential catalytic enantioselective synthesis of highly-functionalised tetrahydroindolizine derivatives

An isothiourea-catalysed enantioselective synthesis of novel tetrahydroindolizine derivatives is reported through a one-pot tandem sequential process. The application of 2-(pyrrol-1-yl)acetic acid in combination with either a trifluoromethyl enone or an α-keto-β,γ-unsaturated ester in an enantioselective Michael addition–lactonisation process, followed by in situ ring-opening and cyclisation, led to a range of 24 tetrahydroindolizine derivatives containing three stereocentres in up to >95 : 5 dr and >99 : 1 er.

The isothiourea-catalysed enantioselective synthesis of tetrahydroindolizine derivatives containing three stereocentres is reported through a one-pot tandem sequential process.

Nickel-catalyzed exo-selective hydroacylation/Suzuki cross-coupling reaction

The first nickel-catalyzed intramolecular hydroacylation/Suzuki cross coupling cascade of o-allylbenzaldehydes with a broad range of phenylboronic acid neopentyl glycol esters has been developed.

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.

Enantioselective CuH-Catalyzed Hydroacylation Employing Unsaturated Carboxylic Acids as Aldehyde Surrogates

The direct asymmetric copper hydride (CuH)-catalyzed coupling of α,β-unsaturated carboxylic acids to aryl alkenes to access chiral α-aryl dialkyl ketones is reported. A variety of substrate substitution patterns, sensitive functional groups, and heterocycles are tolerated in this reaction, which significantly expands the range of accessible products compared with existing hydroacylation methodology. Although mechanistic studies are ongoing, we propose that CuH-catalyzed silylation of unsaturated acids occurs to access a uniquely effective acyl electrophilic coupling partner.

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.

Rhodium-catalyzed pyridannulation of indoles with diazoenals: a direct approach to pyrido[1,2-a]indoles

A new rhodium-catalyzed [4 + 2]-pyridannulation of 3-substituted indoles with diazoenals resulted in the biologically important pyrido[1,2-a]indoles.

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.

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.

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.