Enantioselective N-Alkylation of Indoles via an Intermolecular Aza-Wacker-Type Reaction

Desulfurative Functionalization of β-Acyl Allylic Sulfides with N-H Free Indoles Highly Regioselective at C3 and N1 Positions: Rapid Access to α-Branched Enones

A regiodivergent allylation of 1H-indoles highly selectively at the C3 and N1 positions with β-acyl allylic sulfides through desulfurative C-C/C-N bond-forming reactions has been developed under mild conditions. Notably, the remarkable site-selective switch can be achieved by a delicate choice of solvents and bases. This cost-efficient method displays a broad substrate scope, good functional compatibility, and excellent site-selectivity, thus offering a divergent synthesis of indole substituted α-branched enones, which possess diverse potential opportunities for further applications and derivatization.

Asymmetric N-Alkylation of 1H-Indoles via Carbene Insertion Reaction

An intermolecular enantioselective N-alkylation reaction of 1H-indoles has been developed by cooperative rhodium and chiral phosphoric acid catalyzed N-H bond insertion reaction. N-Alkyl indoles with newly formed stereocenter adjacent to the indole nitrogen atom are produced in good yields (up to 95 %) with excellent enantioselectivities (up to >99 % ee). Importantly, both α-aryl and α-alkyl diazoacetates are tolerated, which is extremely rare in asymmetric X-H (X=N, O, S et al.) and C-H insertion reactions. With this method, only 0.1 mol % of rhodium catalyst and 2.5 mol % of chiral phosphoric acid are required to complete the conversion as well as achieve the high enantioselectivity. Computational studies reveal the cooperative relay of rhodium and chiral phosphoric acid, and the origin of the chemo and stereoselectivity.

3-Nitroindoles Serving as N-Centered Nucleophiles for Aza-1,6-Michael Addition to para-Quinone Methides

An unprecedented N-alkylation of 3-nitroindoles with para-quinone methides was developed for the first time. Using potassium carbonate as the base, a wide range of structurally diverse N-diarylmethylindole derivatives were obtained with moderated to good yields via the protection group migration/aza-1,6-Michael addition sequences. The reaction process was also demonstrated by control experiments. Different from the previous advances where 3-nitrodoles served as electrophiles trapping by various nucleophiles, the reaction herein is featured that 3-nitrodoles is defined with latent N-centered nucleophiles to react with ortho-hydrophenyl p-QMs for construction of various N-diarylmethylindoles.

Rhodium-Catalyzed Asymmetric 1,4-Addition of α/β-(N-Indolyl) Acrylates

A rhodium-catalyzed asymmetric 1,4-addition of α/β-(N-indolyl) acrylates to access highly enantioenriched chiral N-alkylindoles promoted by chiral diene or sulfur-olefin ligands under mild reaction conditions has been developed, which provides an efficient and practical approach for constructing carbon stereocenters adjacent to the indole nitrogen. The reaction can be applied to various N-indolyl-substituted α,β-unstaturated esters and arylboron reagents, providing access to a wide range of α- and β-(N-indolyl) propionate derivatives in high yields with excellent enantioselectivities (≤99% ee).

Enantioselective synthesis of N-alkylindoles enabled by nickel-catalyzed C-C coupling

Enantioenriched N-alkylindole compounds, in which nitrogen is bound to a stereogenic sp³ carbon, are an important entity of target molecules in the fields of biological, medicinal, and organic chemistry. Despite considerable efforts aimed at inventing methods for stereoselective indole functionalization, straightforward access to a diverse range of chiral N-alkylindoles in an intermolecular catalytic fashion from readily available indole substrates remains an ongoing challenge. In sharp contrast to existing C-N bond-forming strategies, here, we describe a modular nickel-catalyzed C-C coupling protocol that couples a broad array of N-indolyl-substituted alkenes with aryl/alkenyl/alkynyl bromides to produce chiral N-alkylindole adducts in single regioisomeric form, in up to 91% yield and 97% ee. The process is amenable to proceed under mild conditions and exhibit broad scope and high functional group compatibility. Utility is highlighted through late-stage functionalization of natural products and drug molecules, preparation of chiral building blocks.

C-H Alkenylation of Indoles through a Dual 1,3-Sulfur Migration Process

Methods for the modification of indole derivatives are powerful techniques in organic, medicinal, and biomolecular chemistry. Here, we report a protocol for the C-H alkenylation of N-H indoles with β-chlorovinyl dithianes to furnish alkenyl indoles through dual 1,3-sulfur rearrangements. This alkenylation protocol could selectively prepare a variety of vinyl indoles in moderate to high yields with excellent functional group tolerance.

Palladium-Catalyzed Oxidative Amination of α-Olefins with Indoles

Herein we report the use of indoles, one of the most common nitrogen-containing heterocycles in FDA-approved drugs, as nucleophiles in the Pd-catalyzed aza-Wacker reaction. This N-functionalization of indoles is a Markovnikov selective olefin functionalization of simple alkenes using catalytic Pd(NPhth)₂(PhCN)₂ and O₂ as the terminal oxidant in the presence of catalytic Bu₄NBr. Various substituted indoles and alkenes are found to participate; 21 examples are presented with yields ranging from 41 to 97% isolated yield. Additionally, lactams and oxazolidinones are shown to participate under the reaction conditions. Mechanistic investigations suggest that the phthalimide ligand and Bu₄NBr additive slow undesired side reactions: indole decomposition and olefin isomerization, respectively.

Palladium-catalyzed oxidative Heck reaction of non-activated alkenes directed by fluorinated alcohol

A new reactivity pattern for the regio- and stereoselective oxidative arylation of non-activated alkenes by introducing a trifluoromethyl group in the substrate enol has been established.

Advances on the Merger of Electrochemistry and Transition Metal Catalysis for Organic Synthesis

Synthetic organic electrosynthesis has grown in the past few decades by achieving many valuable transformations for synthetic chemists. Although electrocatalysis has been popular for improving selectivity and efficiency in a wide variety of energy-related applications, in the last two decades, there has been much interest in electrocatalysis to develop conceptually novel transformations, selective functionalization, and sustainable reactions. This review discusses recent advances in the combination of electrochemistry and homogeneous transition-metal catalysis for organic synthesis. The enabling transformations, synthetic applications, and mechanistic studies are presented alongside advantages as well as future directions to address the challenges of metal-catalyzed electrosynthesis.

Sequential Metal Catalysis towards 7-Oxostaurosporine and Its Non-Natural Septanose Analogue

We report sequential metal catalysis towards indolocarbazole glycosides. The signature event is highlighted by i) Pd⁰ -catalyzed addition of indolocarbazole to alkoxyallene combined with ring-closing-metathesis; ii) Ru-catalyzed chemoselective olefin migration; iii) Pd^II -catalyzed oxidative cyclization to build the bicyclic core structure of the target compounds. This approach gave access to both natural pyranose- and non-natural septanose glycosides. A short formal synthesis of 7-oxostaurosporine was achieved via this strategy.

Nickel-Catalyzed 1,2-Carboamination of Alkenyl Alcohols

An alcohol-directed, nickel-catalyzed three-component umpolung carboamination of unactivated alkenes with aryl/alkenylboronic esters and electrophilic aminating reagents is reported. This transformation is enabled by specifically tailored O-(2,6-dimethoxybenzoyl)hydroxylamine electrophiles that suppress competitive processes, including undesired β-hydride elimination and transesterification between the alcohol substrate and electrophile. The reaction delivers the desired 1,2-carboaminated products with generally high regio- and syn-diastereoselectivity and exhibits a broad scope of coupling partners and alkenes, including complex natural products. Various mechanistic experiments and analysis of the stereochemical outcome with a cyclic alkene substrate, as confirmed by X-ray crystallographic analysis, support alcohol-directed syn-insertion of an organonickel(I) species.

A Metal-Free Reductive N-Alkylation of Indoles with Aldehydes

A simple metal-free method has been developed for the reductive N-alkylation of indoles employing aldehydes as the alkylating agent and inexpensive Et₃SiH as the reductant. A wide range of aromatic and aliphatic aldehydes are viable substrates along with a variety of substituted indoles. In addition, the method was applied to a one-pot sequential 1,3-alkylation of a substituted indole and successfully demonstrated on a 100 mmol scale.

Copper-Mediated Radical-Polar Crossover Enables Photocatalytic Oxidative Functionalization of Sterically Bulky Alkenes

Oxidative heterofunctionalization reactions are among the most attractive methods for the conversion of alkenes and heteroatomic nucleophiles into complex saturated heterocycles. However, the state-of-the-art transition-metal-catalyzed methods to effect oxidative heterofunctionalizations are typically limited to unhindered olefins, and different nucleophilic partners generally require quite different reaction conditions. Herein, we show that Cu(II)-mediated radical-polar crossover allows for highly efficient and exceptionally mild photocatalytic oxidative heterofunctionalization reactions between bulky tri- and tetrasubstituted alkenes and a wide variety of nucleophilic partners. Moreover, we demonstrate that the broad scope of this transformation arises from photocatalytic alkene activation and thus complements existing transition-metal-catalyzed methods for oxidative heterofunctionalization. More broadly, these results further demonstrate that Cu(II) salts are ideal terminal oxidants for photoredox applications and that the combination of photocatalytic substrate activation and Cu(II)-mediated radical oxidation can address long-standing challenges in catalytic oxidation chemistry.

Rhodium-Catalyzed Chemodivergent Regio- and Enantioselective Allylic Alkylation of Indoles

The control of C3/N1 chemoselectivity in indole alkylation with the same electrophiles is still challenging. An Rh/bisoxazolinephosphane-catalyzed chemodivergent regio- and enantioselective allylic alkylation of indoles was developed. Chiral C3- and N1-allylindoles can be selectively obtained with high branched/linear ratio and up to 99 % ee by changing the counteranion of Rh, the allylic carbonate, the reaction temperature, and the ligand.

Enantioselective Synthesis of Alkyl Allyl Ethers via Palladium-Catalyzed Redox-Relay Heck Alkenylation of O-Alkyl Enol Ethers

Herein we report a transformation that generates an array of enantiomerically enriched, alkyl allyl ethers. Cyclic, acyclic, and heteroatom-bearing alkenyl triflates undergo an enantioselective, palladium-catalyzed C-C bond formation with diverse acyclic O-alkyl enol ethers in good yields and excellent enantioselectivities.

Stereoselective Remote Functionalization via Palladium-Catalyzed Redox-Relay Heck Methodologies

Exploration of novel, three-dimensional chemical space is of growing interest in the drug discovery community and with this comes the challenge for synthetic chemists to devise new stereoselective methods to introduce chirality in a rapid and efficient manner. This Minireview provides a timely summary of the development of palladium-catalyzed asymmetric redox-relay Heck-type processes. These reactions represent an important class of transformation for the selective introduction of remote stereocenters, and have risen to prominence over the past decade. Within this Minireview, the vast scope of these transformations will be showcased, alongside applications to pharmaceutically relevant chiral building blocks and drug substances. To complement this overview, a mechanistic summary and discussion of the current limitations of the transformation are presented, followed by an outlook on future areas of investigation.

Nickel-Catalyzed Highly Selective Hydroalkenylation of Alkenyl Boronic Esters to Access Allyl Boron

Allyl boron derivatives are valuable building blocks in the synthesis of natural products and bioactive molecules. Herein, a practical strategy of nickel-catalyzed highly selective hydroalkenylation of alkenyl boronic esters was developed. Under the mild reaction conditions, a variety of allyl boronic esters were accessed with excellent chemo- and regioselectivity. The mechanism of this transformation was illustrated by control experiments and kinetic studies.

Enantioselective Catalytic Synthesis of N-alkylated Indoles

During the past two decades, the interest in new methodologies for the synthesis of chiral N-functionalized indoles has grown rapidly. The review illustrates efficient applications of organocatalytic and organometallic strategies for the construction of chiral α-N-branched indoles. Both the direct functionalization of the indole core and indirect methods based on asymmetric N-alkylation of indolines, isatins and 4,7-dihydroindoles are discussed.

Development and Mechanistic Interrogation of Interrupted Chain-Walking in the Enantioselective Relay Heck Reaction

The formation of alkyl-palladium complexes via the nucleopalladation of alkenes is the entry point for a wide range of diverse reactions. One possibility is that the intermediate alkyl-Pd complexes can undergo a "chain-walking" event, to allow for remote functionalization through various termination processes. However, there are few methods to selectively interrupt the chain-walking process at a prescribed location. Herein, we demonstrate that a variety of homoallylic protected amines undergo an interrupted enantioselective relay Heck reaction to give enantioenriched allylic amine products. The selectivity of this process can be diverted to exclusively yield the ene-amide products by virtue of changing the nature of the amine protecting group. To rationalize this observation, we combine experiment and computation to investigate the mechanism of the chain-walking process and termination events. Isotopic labeling experiments and the computed reaction pathways suggest that the system is likely under thermodynamic control, with the selectivity being driven by the relative stability of intermediates encountered during chain-walking. These results illustrate that the chain-walking of alkyl-palladium complexes can be controlled through the alteration of thermodynamic processes and provides a roadmap for exploiting these processes in future reaction development.

Palladium-Catalyzed Enantioselective Decarboxylative Allylic Alkylation of Acyclic α-N-Pyrrolyl/Indolyl Ketones

The synthesis of fully substituted α-N-pyrrolyl and indolyl ketones via enantioselective palladium-catalyzed allylic alkylation is described. The acyclic ketones are alkylated in high yields with high enantioselectivities through the use of an electron-deficient phosphinooxazoline ligand, furnishing a highly congested and synthetically challenging stereocenter. The obtained alkylation products contain multiple reactive sites poised for additional functionalizations and diversification.

Iridium-Catalyzed Enantioselective Intermolecular Indole C2-Allylation

The enantioselective intermolecular C2-allylation of 3-substituted indoles is reported for the first time. This directing group-free approach relies on a chiral Ir-(P, olefin) complex and Mg(ClO4 )2 Lewis acid catalyst system to promote allylic substitution, providing the C2-allylated products in typically high yields (40-99 %) and enantioselectivities (83-99 % ee) with excellent regiocontrol. Experimental studies and DFT calculations suggest that the reaction proceeds via direct C2-allylation, rather than C3-allylation followed by in situ migration. Steric congestion at the indole-C3 position and improved π-π stacking interactions have been identified as major contributors to the C2-selectivity.

Direct asymmetric N-propargylation of indoles and carbazoles catalyzed by lithium SPINOL phosphate

Catalytic asymmetric functionalization of the N–H groups of indoles and carbazoles constitutes an important but less developed class of reactions. Herein, we describe a propargylation protocol involving the use of a lithium SPINOL phosphate as the chiral catalyst and our recently developed C-alkynyl N,O-acetals as propargylating reagents. The direct asymmetric N-propargylation of indoles and carbazoles provides hitherto inaccessible N-functionalized products. Notably, the efficiency of the system allows reactions to be run at a very low catalyst loading (as low as 0.1 mol%). Mechanistic information about the titled reaction is also disclosed. This study represents an advance in the direct asymmetric functionalization of the N–H bonds of indoles and carbazoles, and additionally expands on the application of chiral alkali metal salts of chiral phosphoric acids in asymmetric catalysis.

Asymmetric functionalization of N–H bonds constitutes an important but less developed class of reactions. Here, the authors report the asymmetric direct N-propargylation of indoles and carbazoles with a lithium SPINOL phosphate as the chiral catalyst and shed light on the reaction mechanism.

Recent advances in Wacker oxidation: from conventional to modern variants and applications

Recent developments in the well-known Wacker oxidation process from conventional to modern variants and applications to natural products' synthesis are compiled in this review.

Asymmetric N-aminoalkylation of 3-substituted indoles by N-protected N,O-acetals: an access to chiral propargyl aminals

A direct enantioselective N1 aminoalkylation of 3-substituted indoles is efficiently catalyzed by a phosphoric acid catalyst under mild conditions, which could be applied to the modification of tryptophan containing oligopeptides.

Photochemical Asymmetric Nickel-Catalyzed Acyl Cross-Coupling

Photochemical enantioselective nickel-catalyzed cross-coupling reactions are difficult to implement. We report a visible-light-mediated strategy that successfully couples symmetrical anhydrides and 4-alkyl dihydropyridines (DHPs) to afford enantioenriched α-substituted ketones under mild conditions. The chemistry does not require exogenous photocatalysts. It is triggered by the direct excitation of DHPs, which act as a radical source and as a reductant, facilitating the turnover of the chiral catalytic nickel complex.

Enantioselective Synthesis of γ-Functionalized Cyclopentenones and δ-Functionalized Cycloheptenones Utilizing a Redox-Relay Heck Strategy

In this report, the desymmetrization of cyclic enones under relay Heck conditions with an array of aryl boronic acids, alkenyl triflates and indole derivatives is described. This method grants facile access to diverse γ-functionalized cyclopentenones and δ-functionalized cycloheptenones. Using this approach, a formal synthesis of (S)-baclofen was completed in high yield and excellent enantioselectivity.