Enantioselective Friedel-Crafts Alkylation Reactions of 3-Substituted Indoles with Electron-Deficient Alkenes

Cu-Catalyzed Asymmetric Arylation of Enone Diesters with Arylboronic Acids

The Cu-catalyzed asymmetric conjugate addition of arylboronic acids to enone diesters is reported. This operationally simple and scalable reaction proceeded under mild conditions. The practical utility of this approach was demonstrated by the synthesis of structurally important γ-lactones and a cyclopentene that bear an aryl group. Mechanistic studies revealed that two different copper catalytic species work synergistically in the reaction.

Synthesis of 2-(2-Nitroalkyl)indoles by Rhodium(III)-Catalyzed C-H Alkylation

The Rh(III)-catalyzed addition of the indole C2-H bond to nitroalkenes under an ambient atmosphere is disclosed, providing direct access to a wide range of 2-(2-nitroalkyl)indoles (33 examples) with excellent chemo- and regioselectivity. In addition, pyrrole derivatives also successfully participated in this Friedel-Crafts alkylation reaction. Representative nitroalkane products could be converted into structurally diverse and valuable indole derivatives. Furthermore, a series of control experiments were conducted, and a plausible mechanism was proposed.

Cu-Catalyzed Asymmetric Alkenylation of Enone Diesters with Trialkenylboroxines

The Cu-catalyzed asymmetric conjugate addition of trialkenylboroxines to enone diesters is reported. This operationally simple and scalable reaction proceeded at room temperature, and a wide range of enone diesters and boroxines were tolerated under the applied reaction conditions. The practical utility of this approach was demonstrated via the formal synthesis of (+)-methylenolactocin. Mechanistic studies revealed that two different catalytic species work synergistically in the reaction.

Three Component Cascade Reaction of Cyclohexanones, Aryl Amines, and Benzoylmethylene Malonates: Cooperative Enamine-Brønsted Acid Approach to Tetrahydroindoles

A three-component cascade reaction comprising cyclic ketones, arylamines, and benzoylmethylene malonates has been developed to access 4,5,6,7-tetrahydro-1H-indoles. The reaction was achieved through cooperative enamine-Brønsted catalysis in high yields with wide substrate scopes. Mechanistic studies identified the role of the Brønsted acid catalyst and revealed the formation of an imine intermediate, which was confirmed by X-ray crystallography.

Diastereoselective 1,6-Addition of α-Phosphonyloxy Enolates to para-Quinone Methides

The addition of α-ketoamide to p-quinone methide initiated by dialkylphosphite in the presence of organic base 1,8-diazabicyclo(5.4.0)undec-7-ene (DBU) is explored. Coupling of dialkylphosphites to α-ketoamides in the presence of a base follows [1,2]-phospha-Brook rearrangement, generating corresponding α-phosphonyloxy enolates that are subsequently seized by p-quinone methides (p-QMs). The two-step one-pot 1,6-conjugate addition provides effective access to a series of isatin-incorporated phosphate-bearing 1,6-adducts having two vicinal tertiary carbons with up to 90% yield and >20:1 dr.

2-Alkylation of 3-Alkyindoles With Unactivated Alkenes

An acid-catalyzed 2-alkylation of indole molecules is developed. Only catalytic amount of the commercially available, inexpensive and traceless HI is used as the sole reaction promoter. 2,3-Disubstituted indole molecules bearing congested tertiary carbon centers are afforded as the final products in moderate to excellent yields.

Ir-Catalyzed Enantioselective Formal C-H Conjugate Addition of Pyrrole and Indoles to α,β-Unsaturated Carbonyl Compounds

The chiral Ir(I)-catalyzed intermolecular reaction of N-carbamoylpyrrole and indole derivatives with α,β-unsaturated carbonyl compounds such as crotonates proceeded with high enantioselectivity. The obtained chirally functionalized pyrroles and indoles are formal C-H conjugate adducts. The reaction mechanism was studied by deuterium labeling experiments.

Further Developments and Applications of Oxazoline-Containing Ligands in Asymmetric Catalysis

The chiral oxazoline motif is present in many ligands that have been extensively applied in a series of important metal-catalyzed enantioselective reactions. This Review aims to provide a comprehensive overview of the most significant applications of oxazoline-containing ligands reported in the literature starting from 2009 until the end of 2018. The ligands are classified not by the reaction to which their metal complexes have been applied but by the nature of the denticity, chirality, and donor atoms involved. As a result, the continued development of ligand architectural design from mono(oxazolines), to bis(oxazolines), to tris(oxazolines) and tetra(oxazolines) and variations thereof can be more easily monitored by the reader. In addition, the key transition states of selected asymmetric transformations will be given to illustrate the features that give rise to high levels of asymmetric induction. As a further aid to the reader, we summarize the majority of schemes with representative examples that highlight the variation in % yields and % ees for carefully selected substrates. This Review should be of particular interest to the experts in the field but also serve as a useful starting point to new researchers in this area. It is hoped that this Review will stimulate both the development/design of new ligands and their applications in novel metal-catalyzed asymmetric transformations.

Rate Dependence on Inductive and Resonance Effects for the Organocatalyzed Enantioselective Conjugate Addition of Alkenyl and Alkynyl Boronic Acids to β-Indolyl Enones and β-Pyrrolyl Enones

Two key factors bear on reaction rates for the conjugate addition of alkenyl boronic acids to heteroaryl-appended enones: the proximity of inductively electron-withdrawing heteroatoms to the site of bond formation and the resonance contribution of available heteroatom lone pairs to stabilize the developing positive charge at the enone β-position. For the former, the closer the heteroatom is to the enone β-carbon, the faster the reaction. For the latter, greater resonance stabilization of the benzylic cationic charge accelerates the reaction. Thus, reaction rates are increased by the closer proximity of inductive electron-withdrawing elements, but if resonance effects are involved, then increased rates are observed with electron-donating ability. Evidence for these trends in isomeric substrates is presented, and the application of these insights has allowed for reaction conditions that provide improved reactivity with previously problematic substrates.

A three-component iodine-catalyzed oxidative coupling reaction: a heterodifunctionalization of 3-methylindoles

A metal-free method for the synthesis of heterodifunctional indole derivatives is developed through TBHP/KI-mediated oxidative coupling. The reaction constructs C-O and C-C bonds in succession with the help of tert-butyl peroxy radicals generated by the TBHP/KI catalytic system, enabling the direct realization of the heterodifunctionalization of indole in one pot. The product of this reaction is a novel heterodifunctional compound. This work might provide a new effective method for the synthesis of polycyclic indole compounds.

Chiral cis-iron(ii) complexes with metal- and ligand-centered chirality for highly regio- and enantioselective alkylation of N-heteroaromatics

Iron-catalyzed highly regio- and enantioselective organic transformations with generality and broad substrate scope have profound applications in modern synthetic chemistry; an example is herein described based on cis-Fe^II complexes having metal- and ligand-centered chirality. The cis-β Fe^II(N₄) complex [Fe^II(L)(OTf)₂] (L = N,N′-bis(2,3-dihydro-1H-cyclopenta-[b]quinoline-5-yl)-N,N′-dimethylcyclohexane-1,2-diamine) is an effective chiral catalyst for highly regio- and enantioselective alkylation of N-heteroaromatics with α,β-unsaturated 2-acyl imidazoles, including asymmetric N1, C2, C3 alkylations of a broad range of indoles (34 examples) and alkylation of pyrroles and anilines (14 examples), all with high product yields (up to 98%), high enantioselectivity (up to >99% ee) and high regioselectivity. DFT calculations revealed that the “chiral-at-metal” cis-β configuration of the iron complex and a secondary π–π interaction are responsible for the high enantioselectivity.

A cis-β Fe^II complex having metal- and ligand-centered chirality catalyzes highly regio- and enantioselective alkylation of indoles (at the N1, C2, or C3 position), pyrroles and anilines with α,β-unsaturated 2-acyl imidazoles (48 examples, up to 99% ee).

Organocatalytic enantioselective conjugate addition of pyrazolin-5-ones to arylomethylidene malonates

An efficient asymmetric organocatalytic conjugate addition reaction of pyrazolin-5-ones with arylomethylidene malonates has been successfully developed by employing bifunctional Cinchona derived thiourea organocatalysts. The protocol provides straightforward access to pyrazole substituted scaffolds in good yields with high stereocontrol.

A chiral bicyclic skeleton-tethered bipyridine-Zn(OTf)2 complex as a Lewis acid: enantioselective Friedel-Crafts alkylation of indoles with nitroalkenes

A conformationally rigid chiral bicyclic skeleton tethered bipyridine-Zn(OTf)2 complex facilitated the enantioselective Friedel-Crafts alkylation of indoles with trans-β-nitroarylalkenes in an enantioselective manner at elevated temperature. Indoles reacted smoothly with β-nitroarylalkenes to generate the corresponding 3-(2-nitroalkyl)indoles in good to excellent yields (up to 94%) with moderate to excellent enantioselectivities (up to 91%). The stereochemical outcome of the product from indole and trans-β-nitrostyrene in the presence of the CRCB tethered bipyridine-Zn(OTf)2 complex and the DFT calculation of the CRCB tethered bipyridine-Zn:trans-β-nitrostyrene complex support the si-face attack of indole on trans-β-nitrostyrene.

Enantioselective Friedel–Crafts C2-alkylation of 3-substituted indoles with trifluoropyruvates and cyclic N-sulfonyl α-ketiminoesters

Enantioselective Friedel–Crafts C2-alkylations of 3-substituted indoles with trifluoropyruvates and cyclic α-ketiminoesters were developed, delivering indole-containing α-hydroxyesters and α-aminoesters in excellent enantioselectivities.

Base-Controlled Reactions through an Aldol Intermediate Formed between 2-Oxoaldehydes and Malonate Half Esters

A practical, atom-economical, base-directed, and highly efficient method for the generation of different selective products through a common aldol intermediate of 2-oxoaldehydes and malonate half esters is successfully developed. The addition of a strong basic environment (potassium tert-butoxide) catalyzed the synthesis of stable decarboxylative aldol products (α-hydroxy ketones), while the Doebner modification procedure resulted in decarboxylative elimination to (E)-α,β-unsaturated esters in good yields. The application of this method in one pot and one pot/two steps with azoles helped to develop regioselective α- and β-azolated products in appreciable yields.

Recent progress in transition-metal-catalyzed enantioselective indole functionalizations

Recent progress on the transition-metal-catalyzed enantioselective functionalization reaction of indoles is reviewed, which is mainly focused on asymmetric indole alkylations, arylations, cycloaddition reactions, and other reactions.