Catalytic Prenylation and Reverse Prenylation of Indoles with Isoprene: Regioselectivity Manipulation through Choice of Metal Hydride

Alkene Cross-Metathesis with 2,5-Dimethyl-2,4-Hexadiene Enables Isobutylenyl/Prenyl Functionalizations and Rubber Valorization

2,5-Dimethyl-2,4-hexadiene is a readily available and easily managable compound, whose symmetric and polymethylated dienic structure should be prone to engage in cross-metathesis reactions with other alkenes, but this has not been apparently exploited so far. Here we show that this reactant enables the easy synthesis of tri- and tetra-susbtituted alkenes (i. e. isobutylenyl and prenyl groups) from simple alkenes under mild reaction conditions, not only with the conventional 2nd generation Grubbs catalyst but also with other Grela-type catalyts such as StickyCat,TM AquaMetTM and GreenCatTM. The use of liquid and low volatile 2,5-dimethyl-2,4-hexadiene avoids the use of gaseous alkene reactants and, besides, showcases the reactivity of polyisoprene (rubber), thus allowing to optimize the reaction conditions for rubber upcycling, after metathesis reaction of the pristine or used polymer with simple alkenes. These results bring low volatile isoprene-type compounds as privileged poly-substituted reactants for alkene cross-metathesis reactions.

Synthesis of Branch-Type 3-Allylindoles from N-Alkyl-N-cinnamyl-2-ethynylaniline Derivatives Using π-Allylpalladium Chloride Complex as a Catalyst

The reaction of N-alkyl-N-cinnamyl-2-ethynylaniline derivatives 1 via annulation and aza-Claisen-type rearrangement easily afforded corresponding branch-type 3-allylindoles 2 with high regioselectivities in good yields using π-allylpalladium chloride complex as a catalyst.

Palladium-Catalyzed Intermolecular Dehydrogenative C-2 Pentenylation of Indoles with 2-Methyl-2-butene

Five-carbon (C5) units are the fundamental building blocks that constitute a multitude of natural products. Herein we report an unprecedented unusual C5 functionalization of indole regioselectively at the C-2-position enabled by a (2-pyridyl)sulfonyl-directing palladium-catalyzed dehydrogenative strategy with a bulk chemical 2-methyl-2-butene as a C5 source. Compared to typical C5 functionalization using pentenyl alcohols, carbonates, borates, or halides as the C5 source, the protocol not only has a low cost advantage but also is of atom and step economy.

HFIP-mediated C-3-alkylation of indoles and synthesis of indolo[2,3-b]quinolines & related natural products

An expeditious metal free C-3 alkylation of indoles and its NIS-mediated deviation to indolo[2,3-b]quinolines is reported. This protocol, executed in aqueous HFIP has broad substrate scope and is well inclined towards the ideas of sustainable chemistry. Applications of these strategies in accessing bioactive natural products like vibrindole, norcryptotakeine, neocryptolepine and indenoindolone scaffolds has also been demonstrated.

Construction of C2-indolyl-quaternary centers by branch-selective allylation: enabling concise total synthesis of the (±)-mersicarpine alkaloid

Herein we report a branch-selective allylation strategy for accessing C2-indolyl-all-carbon quaternary centers using allylboronic acids. This approach boasts broad functional group tolerance, scalability, and relies on easily accessible allyl alcohol precursors. Importantly, the C3-position of the indole remains free, offering a handle for further synthetic refinement. Mechanistic pathways, corroborated by density functional theory (DFT), suggest the involvement of an indolenine intermediate and a Zimmerman-Traxler-like transition state during allylboration. Demonstrating its efficacy, the method was applied to the total synthesis of the (±)-mersicarpine alkaloid and enabled formal synthesis of additional alkaloids, such as (±)-scholarisine G, (±)-melodinine E, and (±)-leuconoxine.

Construction of Bridged Benzazepines via Photo-Induced Dearomatization

Bridged benzazepine scaffolds, possessing unique structural and physicochemical activities, are widespread in various natural products and drugs. The construction of these skeletons often requires elaborate synthetic effort with low efficiency. Herein, we develop a simple and divergent approach for constructing various bridged benzazepines by a photocatalytic intermolecular dearomatization of naphthalene derivatives with readily available α-amino acids. The bridged motif is created via a cascade sequence involving photocatalytic 1,4-hydroaminoalkylation, alkene isomerization and cyclization. Interestingly, the diastereoselectivity can be regulated through different reaction modes in the cyclization step. Moreover, aminohydroxylation and its further bromination have also been demonstrated to access highly functionalized bridged benzazepines. Preliminary mechanistic studies have been performed to get insights into the mechanism. This method provides a divergent synthetic approach for construction of highly functionalized bridged benzazepines, which have been otherwise difficult to access.

Nucleophilic aromatization of monoterpenes from isoprene under nickel/iodine cascade catalysis

As a large number of organic compounds possessing two isoprene units, monoterpenes and monoterpenoids play important roles in pharmaceutical, cosmetic, agricultural, and food industries. In nature, monoterpenes are constructed from geranyl pyrophosphate (C10) via various transformations. Herein, the bulk C5 chemical-isoprene, is used for the creation of various monoterpenoids via a nucleophilic aromatization of monoterpenes under cascade catalysis of nickel and iodine. Drugs and oil mixtures from conifer and lemon can be convergently transformed to the desired monoterpenoid. Preliminary mechanistic studies are conducted to get insights about reaction pathway. Two types of cyclic monoterpenes can be respectively introduced onto two similar heterocycles via orthogonal C-H functionalization. And various hybrid terpenyl indoles are programmatically assembled from abundant C5 or C10 blocks. This work not only contributes a high chemo-, regio-, and redox-selective transformation of isoprene, but also provides a complementary approach for the creation of unnatural monoterpenoids.

Photo-induced imino functionalizations of alkenes via intermolecular charge transfer

A catalyst-free photosensitized strategy has been developed for regioselective imino functionalizations of alkenes via the formation of an EDA complex. This photo-induced protocol facilitates the construction of structurally diverse β-imino sulfones and vinyl sulfones in moderate to high yields. Mechanistic studies reveal that the reaction is initiated with an intermolecular charge transfer between oximes and sulfinates, followed by fragmentation to generate a persistent iminyl radical and transient sulfonyl radical. This catalyst-free protocol also features excellent regioselectivity, broad functional group tolerance and mild reaction conditions. The late stage functionalization of natural product derived compounds and total synthesis of some bioactive molecules have been demonstrated to highlight the utility of this protocol. Meanwhile, the compatibility of different donors has proved the generality of this strategy.

Photoredox-catalyzed diastereoselective dearomative prenylation and reverse-prenylation of electron-deficient indole derivatives

Prenylated and reverse-prenylated indolines are privileged scaffolds in numerous naturally occurring indole alkaloids with a broad spectrum of important biological properties. Development of straightforward and stereoselective methods to enable the synthesis of structurally diverse prenylated and reverse-prenylated indoline derivatives is highly desirable and challenging. In this context, the most direct approaches to achieve this goal generally rely on transition-metal-catalyzed dearomative allylic alkylation of electron-rich indoles. However, the electron-deficient indoles are much less explored, probably due to their diminished nucleophilicity. Herein, a photoredox-catalyzed tandem Giese radical addition/Ireland-Claisen rearrangement is disclosed. Diastereoselective dearomative prenylation and reverse-prenylation of electron-deficient indoles proceed smoothly under mild conditions. An array of tertiary α-silylamines as radical precursors is readily incorporated in 2,3-disubstituted indolines with high functional compatibility and excellent diastereoselectivity (>20:1 d.r.). The corresponding transformations of the secondary α-silylamines provide the biologically important lactam-fused indolines in one-pot synthesis. Subsequently, a plausible photoredox pathway is proposed based on control experiments. The preliminary bioactivity study reveals a potential anticancer property of these structurally appealing indolines.

Three-Component Electrochemical Aminoselenation of 1,3-Dienes

Azoles and organoselenium compounds are pharmacologically important scaffolds in medicinal chemistry and natural products. We developed an efficient regioselective electrochemical aminoselenation reaction of 1,3-dienes, azoles, and diselenide derivatives to access selenium-containing allylazoles skeletons. This protocol is more economical and environmentally friendly and features a broad substrate scope; pyrazole, triazole, and tetrazolium were all tolerated under the standard conditions, which could be applied to the expedient synthesis of bioactive molecules and in the pharmaceutical industry.

Nickel-Catalyzed Unsymmetrical Bis-Allylation of Alkynes

The catalytic bis-allylation of alkynes is an important but challenging protocol to construct all-carbon tetra-substituted alkenes. Particularly, the catalytic unsymmetrical bis-allylation of alkynes remains as an underexplored task to date. We herein report an unprecedented unsymmetrical bis-allylation by simultaneously utilizing electrophilic trifluoromethyl alkene and nucleophilic allylboronate as the allylic reagents. With the aid of robust Ni⁰ /NHC catalysis, valuable skipped trienes can be obtained in high regio- and stereo-selectivities under mild conditions. Mechanistic studies indicate that the reaction may proceed through a β-fluorine elimination of a nickelacycle followed by a transmetalation step with allylboronate. The present method exhibits a good tolerance of various functional groups. Besides, the skipped triene products can undergo an array of elaborate transformations, which highlights the potential applications of this strategy.

Organocatalytic Regio- and Enantioselective Allylic Alkylation of Indolin-2-imines with MBH Carbonates toward 3-Allylindoles

An organocatalytic asymmetric C3-allylic alkylation of indolone-2-imines with MBH carbonates has been developed for the first time. As opposed to previous reports, an "interrupted" annulation was achieved, affording 3-allylindoles in generally high yields with excellent stereoselectivities. The representative scale-up reaction and transformation of 3-allylindoles were examined. A possible mechanism was also proposed.

Skeleton-Reorganizing Coupling Reactions of Cycloheptatriene and Cycloalkenones with Amines

Skeletal reorganization reactions have emerged as an intriguing tool for converting readily available compounds into complicated molecules inaccessible by traditional methods. Herein, we report a unique skeleton-reorganizing coupling reaction of cycloheptatriene and cycloalkenones with amines. In the presence of Rh/acid catalysis, cycloheptatriene can selectively couple with anilines to deliver fused 1,2-dihydroquinoline products. Mechanistic studies indicate that the retro-Mannich type ring-opening and subsequent intramolecular Povarov reaction account for the ring reorganization. Our mechanistic studies also revealed that skeleton-reorganizing amination between anilines and cycloalkenones can be achieved with acid. The synthetic utilization of this skeleton-reorganizing coupling reaction was showcased with a gram-scale reaction, synthetic derivatizations, and the late-stage modification of commercial drugs.

Iron-catalyzed hydroarylation of 1,3-dienes with indoles

An efficient iron-catalyzed hydroarylation of 1,3-dienes with indoles has been developed for the first time.

Ligand-controlled regiodivergence in cobalt-catalyzed hydrosilylation of isoprene

An atom-economical, regiodivergent hydrosilylation reaction of isoprene was developed using an Earth-abundant cobalt catalyst through variation of ligands.

Well-Defined NNS-Mn Complex Catalyzed Selective Synthesis of C-3 Alkylated Indoles and Bisindolylmethanes Using Alcohols

Herein, we demonstrated Mn-catalyzed selective C-3 functionalization of indoles with alcohols. The developed catalyst can also furnish bis(indolyl)methanes from the same set of substrates under slightly modified reaction conditions. Mechanistic studies reveal that the C-3 functionalization of indoles is going via a borrowing hydrogen pathway. To highlight the practical utility, a diverse range of substrates including nine structurally important drug molecules are synthesized. Furthermore, we also introduced a one-pot cascade strategy for synthesizing C-3 functionalized indoles directly from 2-aminophenyl ethanol and alcohol.

HFIP as Protonation Reagent and Solvent for Regioselective Alkylation of Indoles with All-Carbon Centers

The regio- and chemoselective construction of indole bearing an all-carbon center at the C3-position, a versatile bioactive building block, by C(sp²)-C(sp³) formation with olefins has been achieved through utilization of hexafluoroisopropanol (HFIP) as the protonation reagent and solvent. The catalytic reactions are operationally simple and green compared with previous reports utilizing elaborated olefins and catalysts. This protocol allows for alkylation of a variety of substituted indoles with diverse of styrene type alkenes in excellent yields and with high selectivity. Application of this protocol to the synthesis of drug was pursued and with an improved yield in contrast to previous art. Catalytic kinetics and deuterium-labeling experiments suggest that the rate-determining step involves the protonation of olefin by HFIP to generate carbocation, followed by electrophilic addition to indole derivative.

Palladium-Catalyzed Three-Component Regioselective Dehydrogenative Coupling of Indoles, 2-Methylbut-2-ene, and Carboxylic Acids

Five-carbon (C5) structural units are the fundamental building blocks of many natural products. An unprecedented palladium-catalyzed three-component dehydrogenative cascade coupling of indoles, 2-methylbut-2-ene, and carboxylic acids has been developed. The approach enables the straightforward introduction of a C3'-bonded five-carbon structural unit with a tertiary alcohol quaternary carbon center into indoles. The protocol employs 2-methylbut-2-ene as the C5 source and is featured by a broad substrate scope, atom and step economies, and high chemo- and regioselectivies.

Photo-induced catalytic halopyridylation of alkenes

The Mizoroki-Heck reaction and its reductive analogue are staples of organic synthesis, but the ensuing products often lack a chemical handle for further transformation. Here we report an atom-economical cross-coupling of halopyridines and unactivated alkenes under photoredox catalysis to afford a series of alkene halopyridylation products. This protocol with mild and redox neutral conditions contributes broad substrate scope. As a complement to conventional Heck-type reaction, this radical process avoids the involvement of β-H elimination and thus useful pyridyl and halide groups could be simultaneously and regioselectively incorporated onto alkenes. The success depends on TFA-promoted domino photocatalytic oxidative quenching activation and radical-polar crossover pathway. Plausible mechanism is proposed based on mechanistic investigations. Moreover, the reserved C - X bonds of these products are beneficial for performing further synthetic elaborations.

Redox-Divergent Construction of (Dihydro)thiophenes with DMSO

Thiophene-based rings are one of the most widely used building blocks for the synthesis of sulfur-containing molecules. Inspired by the redox diversity of these features in nature, we demonstrate herein a redox-divergent construction of dihydrothiophenes, thiophenes, and bromothiophenes from the respective readily available allylic alcohols, dimethyl sulfoxide (DMSO), and HBr. The redox-divergent selectivity could be manipulated mainly by controlling the dosage of DMSO and HBr. Mechanistic studies suggest that DMSO simultaneously acts as an oxidant and a sulfur donor. The synthetic potentials of the products as platform molecules were also demonstrated by various derivatizations, including the preparation of bioactive and functional molecules.

Pd-Catalyzed Tandem Isomerization/Cyclization for the Synthesis of Aromatic Oxazaheterocycles and Pyrido[3,4-b]indoles

An effient tandem process consisting of palladium-catalyzed double-bond isomerization of long-chain olefins and subsequent intramolecular cyclization promoted by B₂(OH)₂ for the synthesis of aromatic oxazaheterocycles is disclosed. This strategy can also provide rapid access to pyrido[3,4-b]indoles, trans-2-olefins, and eneamides bearing various functional groups with high regio- and stereoselectivity.

Orthogonal Regulation of Nucleophilic and Electrophilic Sites in Pd-Catalyzed Regiodivergent Couplings between Indazoles and Isoprene

Depending on the reactant property and reaction mechanism, one major regioisomer can be favored in a reaction that involves multiple active sites. Herein, an orthogonal regulation of nucleophilic and electrophilic sites in the regiodivergent hydroamination of isoprene with indazoles is demonstrated. Under Pd-hydride catalysis, the 1,2- or 4,3-insertion pathway with respect to the electrophilic sites on isoprene could be controlled by the choice of ligands. In terms of the nucleophilic sites on indazoles, the reaction occurs at either the N¹ - or N² -position of indazoles is governed by the acid co-catalysts. Preliminary experimental studies have been performed to rationalize the mechanism and regioselectivity. This study not only contributes a practical tool for selective functionalization of isoprene, but also provides a guide to manipulate the regioselectivity for the N-functionalization of indazoles.

Ligand-free iridium-catalyzed regioselective C-H borylation of indoles

We herein report a ligand-free Ir-catalyzed C-H borylation of N-acyl protected indoles. This simple protocol could tolerate a variety of functional groups, affording C3 borylated indoles in good yields with excellent regioselectivities. We also demonstrated that the current method is amenable to gram-scale borylation and the C-B bonds could be easily converted to C-C and C-heteroatom bonds.

Carbon dioxide-promoted palladium-catalyzed dehydration of primary allylic alcohols: access to substituted 1,3-dienes

A carbon dioxide promoted dehydration reaction of primary allylic alcohols gives synthetically important substituted 1,3-dienes in good yields under milder conditions compared with the reaction using a heterogeneous catalyst or carbon monoxide.

Isoprene: A Promising Coupling Partner in C–H Functionalizations

Five-carbon dimethylallyl units, such as prenyl and reverse-prenyl, are widely distributed in natural indole alkaloids and terpenoids. In conventional methodologies, these valuable motifs are often derived from substrates bearing leaving groups, but these processes are accompanied by the generation of stoichiometric amounts of by-products. From an economical and environmental point of view, the basic industrial feedstock isoprene is an ideal alternative precursor. However, given that electronically unbiased isoprene might undergo six possible addition modes in the coupling reactions, it is difficult to control the selectivity. This article summarizes the strategies we have developed to achieve regioselective C–H functionalizations of isoprene under transition-metal and acid catalysis.

1 Introduction

2 Catalytic Coupling of Indoles with Isoprene

3 Catalytic Coupling of Formaldehyde, Arenes and Isoprene

4 Catalytic Coupling of 4-Hydroxycoumarins with Isoprene

5 Catalytic Coupling of Cyclic 1,3-Diketones with Isoprene

6 Conclusion and Outlook

Nickel-catalyzed allyl–allyl coupling reactions between 1,3-dienes and allylboronates

A nickel-hydride catalysis has been developed to facilitate the allyl–allyl cross-coupling reactions between 1,3-dienes and allyl-B(pin) in excellent regioselectivity.

Palladium-catalyzed diastereo- and enantioselective allylic alkylation of oxazolones with 1,3-dienes under base-free conditions

Herein, we report a highly diastereo- and enantioselective allylic alkylation of oxazolones with 1,3-dienes by palladium-hydride catalyst under base-free conditions.

Rhodium-catalyzed regio- and enantioselective allylic alkylation of pyrazol-5-ones with alkynes

An atom-economical, regio- and enantioselective allylic alkylation of pyrazol-5-ones with alkynes was developed under rhodium catalysis.

Regiodivergent Intramolecular Nucleophilic Addition of Ketimines for the Diverse Synthesis of Azacycles

Azacycles such as indoles and tetrahydroquinolines are privileged structures in drug development. Reported here is an unprecedented regiodivergent intramolecular nucleophilic addition reaction of imines as a flexible approach to access N-functionalized indoles and tetrahydroquinolines, by the control of reaction at the N-terminus and C-terminus, respectively. Using ketimines derived from 2-(2-nitroethyl)anilines with isatins or α-ketoesters, the regioselective N-attack reaction gives N-functionalized indoles, while the catalytic enantioselective C-attack reaction affords chiral tetrahydroquinolines featuring an α-tetrasubstituted stereocenter. Mechanistic studies reveal that hydrogen-bonding interactions may greatly facilitate such unusual N-attack reactions of imines. The utility of this protocol is highlighted by the catalytic enantioselective formal synthesis of (-)-psychotrimine, and the construction of various fused aza-heterocycles.