Intramolecular Cyclizations of Vinyl-Substituted N,N-Dialkyl Arylamines Enabled by Borane-Assisted Hydride Transfer

Lewis Acid-Driven Inverse Hydride Shuttle Catalysis

Inverse hydride shuttle catalysis provides a multicomponent platform for the highly efficient synthesis of alkaloid frameworks with exquisite diastereoselectivity. However, a number of limitations hinder this method, primarily the strict requirement for highly electron-deficient acceptors. Herein, we present a general Lewis acid-driven approach to address this constraint, and have developed two broad strategies enabling the modular synthesis of complex azabicycles that were entirely unattainable using the previous method. The enhanced synthetic flexibility facilitates a streamlined asymmetric cyclization, leading to a concise total synthesis of the alkaloid (-)-tashiromine.

Diversity-oriented synthesis of indole-fused scaffolds and bis(indolyl)methane from tosyl-protected tryptamine

An efficient, diversity-oriented synthesis of indole-1,2-fused 1,4-benzodiazepines, tetrahydro-β-carbolines, and 2,2'-bis(indolyl)methanes was established starting from tosyl-protected tryptamine. These diverse privileged skeletons were controllably constructed by adjusting different hydride donors and Brønsted acids. A variety of indole-1,2-fused 1,4-benzodiazepines were facilely accessed using benzaldehydes bearing cyclic amines as hydride donors via a cascade N-alkylation/dehydration/[1,5]-hydride transfer/Friedel-Crafts alkylation sequence. The reaction site could be switched when benzaldehydes bearing an alkoxy moiety as hydride donors were used for the generation of tetrahydro-β-carbolines. On the other hand, the switchable synthesis of 2,2'-bis(indolyl)methanes could be achieved as well by applying p-TsOH·H2O as a catalyst. The reactions feature mild conditions, simple and practical operation, excellent efficiency and the use of EtOH as a green solvent. Using the concept of diversity-oriented, reagent-based synthesis, the inexpensive feedstock tryptamine was efficiently converted to three different types of privileged scaffolds, which facilitates rapid compound library synthesis for accelerating drug discovery.

β-C-H Allylation of Trialkylamines with Allenes Promoted by Synergistic Borane/Palladium Catalysis

Functionalization of the C(sp3 )-H bonds of trialkylamines is challenging, especially for reactions at positions other than the α position. Herein, we report a method for β-C(sp3 )-H allylation of trialkylamines. In these reactions, which involve synergistic borane/palladium catalysis, an enamine intermediate is first generated from the amine via α,β-dehydrogenation promoted by B(C6 F5 )3 and a base, and then the enamine undergoes palladium-catalyzed reaction with an allene to give the allylation product. Because the hydride and the proton resulting from the initial dehydrogenation are ultimately shuttled to the product by B(C6 F5 )3 and the palladium catalyst, respectively, these reactions show excellent atom economy. The establishment of this method paves the way for future studies of C-H functionalization of trialkylamines by means of synergistic borane/transition-metal catalysis.

B(C6F5)3-catalyzed β-C(sp3)-H alkylation of tertiary amines with 2-aryl-3H-indol-3-ones

The β-C-H functionalization of amines is one of the most powerful tools for the synthesis of saturated nitrogen-containing heterocycles in organic synthesis. However, the β-C-H functionalization of amines via redox-neutral addition with cyclic-ketimines is still unprecedented. Herein, the β-C-H functionalization of tertiary amines is described, providing the corresponding 1,3-diamines containing the indolin-3-one moiety in high yields via the B(C₆F₅)₃-catalyzed borrowing hydrogen strategy. According to the experimental results, a possible catalytic cycle has been proposed to rationalize the process of this reaction. Notably, the β-C-H alkylation of amines is external oxidant- and transition-metal-free, which makes a significant contribution to promoting economical chemical synthesis.

B(C6F5)3-Catalyzed α,β-Difunctionalization and C-N Bond Cleavage of Saturated Amines with Benzo[c]isoxazoles: Access to Quinoline Derivatives

Herein, we disclose a strategy to realize α,β-difunctionalization and C-N bond cleavage of saturated amines with benzo[c]isoxazoles via a B(C₆F₅)₃-catalyzed consecutive hydrogen-borrowing and [4 + 2] cycloaddition followed by a C-N bond cleavage process. In general, the reactions proceed efficiently in the absence of any oxidant and metal catalyst to afford a broad range of quinoline derivatives starting from easily accessible substrates in an atom-economical manner.

Borane-catalyzed cascade Friedel–Crafts alkylation/[1,5]-hydride transfer/Mannich cyclization to afford tetrahydroquinolines

We report a redox-neutral annulation reaction of tertiary amines with electron-deficient alkynes under metal-free and oxidant-free conditions.

Bifunctional Borane Catalysis of a Hydride Transfer/Enantioselective [2+2] Cycloaddition Cascade

Herein, we present a mild and efficient method for synthesizing enantioenriched tetrahydroquinoline-fused cyclobutenes through a cascade reaction between 1,2-dihydroquinolines and alkynones with catalysis by chiral spiro-bicyclic bisboranes. The bisboranes served two functions: first they catalyzed a hydride transfer to convert the 1,2-dihydroquinoline substrate to a 1,4-dihydroquinoline, and then they activated the alkynone substrate for an enantioselective [2+2] cycloaddition reaction with the 1,4-dihydroquinoline generated in situ.

Consecutive β,β'-Selective C(sp3 )-H Silylation of Tertiary Amines with Dihydrosilanes Catalyzed by B(C6 F5 )3

Tris(pentafluorophenyl)borane has been found to catalyze the two‐fold C(sp³)−H silylation of various trialkylamine derivatives with dihydrosilanes, furnishing the corresponding 4‐silapiperidines in decent yields. The multi‐step reaction cascade involves amine‐to‐enamine dehydrogenation at two alkyl residues and two electrophilic silylation reactions of those enamines, one inter‐ and one intramolecular.

Tris(pentafluorophenyl)borane catalyzed C-C and C-heteroatom bond formation

A series of boron based Lewis acids have been reported to date, but among them, tris(pentafluorophenyl)borane (BCF) has gained the most significant attention in the synthetic chemistry community. The viability of BCF as a potential Lewis acid catalyst has been vastly explored in organic and materials chemistry due to its thermal stability and commercial availability. Most explorations of BCF chemistry in organic synthesis has occurred in the last two decades and many new catalytic reactivities are currently under investigation. This review mainly focuses on recent reports from 2018 onwards and provides a concise knowledge to the readers about the role of BCF in metal-free catalysis. The review has mainly been categorized by different types of organic transformation mediated through BCF catalysis for the C-C and C-heteroatom bond formation.

Electron-deficient boron-based catalysts for C-H bond functionalisation

In contrast to transition metal-catalysed C-H functionalisation, highly efficient construction of C-C and C-X (X = N, O, S, B, Si, etc.) bonds through metal-free catalytic C-H functionalisation remains one of the most challenging tasks for synthetic chemists. In recent years, electron-deficient boron-based catalyst systems have exhibited great potential for C-H bond transformations. Such emerging systems may greatly enrich the chemistry of C-H functionalisation and main-group element catalysis, and will also provide enormous opportunities in synthetic chemistry, materials chemistry, and chemical biology. This article aims to give a timely comprehensive overview to recognise the current status of electron-deficient boron-based catalysis in C-H functionalisation and stimulate the development of more efficient catalytic systems.

Enantioselective Synthesis of N-Alkylamines through β-Amino C-H Functionalization Promoted by Cooperative Actions of B(C6F5)3 and a Chiral Lewis Acid Co-Catalyst

We disclose a catalytic method for β-C(sp³)-H functionalization of N-alkylamines for the synthesis of enantiomerically enriched β-substituted amines, entities prevalent in pharmaceutical compounds and used to generate different families of chiral catalysts. We demonstrate that a catalyst system comprising of seemingly competitive Lewis acids, B(C₆F₅)₃, and a chiral Mg- or Sc-based complex, promotes the highly enantioselective union of N-alkylamines and α,β-unsaturated compounds. An array of δ-amino carbonyl compounds was synthesized under redox-neutral conditions by enantioselective reaction of a N-alkylamine-derived enamine and an electrophile activated by the chiral Lewis acid co-catalyst. The utility of the approach is highlighted by late-stage β-C-H functionalization of bioactive amines. Investigations in regard to the mechanistic nuances of the catalytic processes are described.

o-Quinone methide with overcrowded olefin component as a dehydridation catalyst under aerobic photoirradiation conditions

An o-quinone methide (o-QM) featuring an overcrowded olefinic framework is introduced, which exhibits dehydridation activity owing to its enhanced zwitterionic character, particularly through photoexcitation. The characteristics of this o-QM enable the operation of dehydridative catalysis in the oxidation of benzylic secondary alcohols under aerobic photoirradiation conditions. An experimental analysis and density functional theory calculations provide mechanistic insights; the ground-state zwitterionic intermediate abstracts a hydride and proton simultaneously, and the active oxygen species facilitate catalyst regeneration.

An o-quinone methide (o-QM) featuring an overcrowded olefinic framework is introduced, which exhibits dehydridation activity owing to its enhanced zwitterionic character, particularly through photoexcitation.

Recent advances in hydride transfer-involved C(sp3)–H activation reactions

This review summarizes the recent progresses (2016–2020) in the hydride transfer-enabled C(sp³)–H activation according to the reaction types, categorized into the intramolecular/intermolecular C(sp³)–H functionalization, and hydride reduction.

Electron deficient borane-mediated hydride abstraction in amines: stoichiometric and catalytic processes

Borane mediated hydride abstraction of amines efficiently generates useful iminium salts. This review explores this fascinating reactivity and discusses how the iminium intermediates are utilised in a variety of stoichiometric and catalytic processes.

Direct Conversion of N-Alkylamines to N-Propargylamines through C-H Activation Promoted by Lewis Acid/Organocopper Catalysis: Application to Late-Stage Functionalization of Bioactive Molecules

An efficient catalytic method to convert an α-C-H bond of N-alkylamines into an α-C-alkynyl bond was developed. In the past, such transformations were carried out under oxidative conditions, and the enantioselective variants were confined to tetrahydroisoquinoline derivatives. Here, we disclose a method for the union of N-alkylamines and trimethylsilyl alkynes, without the presence of an external oxidant and promoted through cooperative actions of two Lewis acids, B(C6F5)3 and a Cu-based complex. A variety of propargylamines can be synthesized in high diastereo- and enantioselectivity. The utility of the approach is demonstrated by the late-stage site-selective modification of bioactive amines. Kinetic investigations that shed light on various mechanistic nuances of the catalytic process are presented.

cine-Silylative Ring-Opening of α-Methyl Azacycles Enabled by the Silylium-Induced C-N Bond Cleavage

Described herein is the development of a borane-catalyzed cine-silylative ring-opening of α-methyl azacycles. This transformation involves four-step cascade processes: (i) exo-dehydrogenation of alicyclic amine, (ii) hydrosilylation of the resultant enamine, (iii) silylium-induced cis-β-amino elimination to open the ring skeleton, and (iv) hydrosilylation of the terminal olefin. The present borane catalysis also works efficiently for the C-N bond cleavage of acyclic tertiary amines. On the basis of experimental and computational studies, the silicon atom was elucidated to play a pivotal role in the β-amino elimination step.

Hydrogen-bonding-assisted redox-neutral construction of tetrahydroquinolines via hydride transfer

The hydrogen-bonding-assisted construction of tetrahydroquinolines decorated with structurally diverse 3,3'-difunctional groups has been realized via a hydride transfer-involved three-step cascade reaction in the presence of morpholine. This protocol solves the limitation of acyclic 1,3-dicarbonyl compounds by one-pot synthesis of tetrahydroquinolines, featuring operational simplicity, broadly applicable substrates, and metal- and acid-free conditions with EtOH as a hydrogen-bonding donor.

Desaturation via Redox-Neutral Hydrogen Transfer Process: Synthesis of 2-Allyl Anilines, Mechanism and Applications

An unprecedented desaturation method via redox-neutral hydrogen transfer process has been disclosed under mild conditions for the selective formation of terminal alkene with alkyl diazo compounds and aza-o-QMs. The control experiments and DFT calculations suggest that the visible light was introduced as a key parameter to enhance the reactivity via a radical process in the formation of closed-shell cyclopropane intermediate, followed by a ring opening and redox-neutral hydrogen transfer process to give the desaturated product. The high regioselectivity in this transformation is enabled by the internal amino species as an ancillary group (AG) in the final olefin formation step. This method provides a missing link in the expeditious preparation of synthetically useful 2-allyl anilines with broad substrate generality. Further applications of these generated products in N-heterocycle construction, including 5- and 6-membered rings with structural diversity, have been tactfully explored, which highlight the potential in methodology development and drug discovery.

•

Highly site and regioselective synthesis enabled by ancillary group

•

Desaturation via redox-neutral inert hydrogen transfer process

•

Missing link in the synthesis of 2-allyl anilines with board substrate scope

•

Methodology development and diversity synthesis based on 2-allyl anilines

B(C6F5)3-Catalyzed Direct C3 Alkylation of Indoles and Oxindoles

The direct C3 alkylation of indoles and oxindoles is a challenging transformation, and only a few direct methods exist. Utilizing the underexplored ability of triaryl boranes to mediate the heterolytic cleavage of α-nitrogen C–H bonds in amines, we have developed a catalytic approach for the direct C3 alkylation of a wide range of indoles and oxindoles using amine-based alkylating agents. We also employed this borane-catalyzed strategy in an alkylation-ring opening cascade.

Hexafluoroisopropanol-Mediated Redox-Neutral α-C(sp3)-H Functionalization of Cyclic Amines via Hydride Transfer

Hexafluoroisopropanol has been demonstrated as the versatile promoter for redox-neutral α-C(sp³)-H functionalization of cyclic amines via the cascade [1,5]-hydride transfer/cyclization strategy. A wide range of cyclic amines are functionalized into bioactive tetrahydroquinolines, quinazolines, benzoxazines, and benzotriazepines in moderate to excellent yields. This protocol features additive-free conditions, operational simplicity, and wide substrate scope.

Diiodomethane-Mediated Generation of N-Aryliminium Ions and Subsequent [4 + 2] Cycloadditions with Olefins

Herein, we report a method for in situ generation of N-aryliminium ions via reactions of N,N-dimethylanilines with diiodomethane. We used the method to prepare tetrahydroquinolines via one-pot three-component reactions between N,N-dimethylanilines, diiodomethane, and olefins. This transformation involves initial reaction of the aniline with diiodomethane to form an iodomethylammonium salt, which undergoes fragmentation accompanied by elimination of methyl iodide to give an N-aryliminium ion, which is trapped by the olefin via [4 + 2] cycloaddition to give the final product. This method for generating N-aryliminium ions requires neither a catalyst nor a strong oxidant, suggesting that it can be expected to find broad utility, especially for substrates that are sensitive to Lewis acids, transition metals, or strong oxidants.

Hydrosilylation-Promoted Furan Diels-Alder Cycloadditions with Stereoselectivity Controlled by the Silyl Group

Herein we describe an unprecedented B(C₆F₅)₃-catalyzed cascade reaction of N-allyl-N-furfurylamides involving an initial intramolecular furan Diels-Alder reaction and subsequent ether cleavage. The reaction has a broad substrate scope, even tolerating a trialkyl-substituted olefin as the dienophile, which has not previously been observed with conventional furan Diels-Alder reactions. In addition, the relative configuration of the product can be controlled by the choice of the silyl group: reactions involving Et₃SiH and ⁱPr₃SiH gave different diastereomers. Control experiments and the computational studies revealed that the steric bulk of the silyl group plays a key role in determining the reaction pathway and thus the relative configuration of the product.

Redox-Neutral β-C(sp3)-H Functionalization of Cyclic Amines via Intermolecular Hydride Transfer

Herein, we report the first redox-neutral and transition-metal-free β-C(sp³)-H functionalization of cyclic amines via a consecutive intermolecular hydride transfer process. A series of N-aryl pyrrolidines and N-aryl 1,2,3,4-tetrahydropyridines decorated with CF₃ and carboxylic ester functionalities are directly accessed in good yields from pyrrolidines and piperidines. This work pushes forward the application of the intermolecular hydride transfer strategy in one-step assembly of molecular complexity.

Catalytic Deuterium Incorporation within Metabolically Stable β-Amino C-H Bonds of Drug Molecules

An efficient deuteration process of β-amino C-H bonds in various N-alkylamine-based pharmaceutical compounds has been developed. Catalytic reactions begin with the action of Lewis acidic B(C₆F₅)₃ and Brønsted basic N-alkylamine, converting a drug molecule into the corresponding enamine. The acid/base catalysts also promote the dedeuteration of acetone-d₆ to afford a deuterated ammonium ion. Ensuing deuteration of the enamine then leads to the formation of β-deuterated bioactive amines with up to 99% deuterium incorporation.

Cascade [1,5]-Hydride Transfer/Cyclization for Synthesis of [3,4]-Fused Oxindoles

The scandium-catalyzed redox-neutral cascade [1,5]-hydride transfer/cyclization between C4-amine-substituted isatins and 1,3-dicarbonyl compounds has been developed. This protocol enabled the synthesis of tricyclic [3,4]-fused oxindoles in good to high yields and excellent diastereoselectivities, featuring high atom- and step economy as well as good functional group tolerance.

B(C6F5)3-Catalyzed formal (4+1)-annulation of ortho-quinone methides with diazoacetates: access to 2,3-dihydrobenzofurans

A novel B(C₆F₅)₃-catalyzed formal (4+1)-cycloaddition reaction of alkyne-tethered ortho-quinone methides with diazoacetates has been well established.

B(C6F5)3-Catalyzed redox-neutral β-alkylation of tertiary amines using p-quinone methides via borrowing hydrogen

A transition metal-free intermolecular redox-neutral β-alkylation of acyclic tertiary amines via borrowing hydrogen catalyzed by commercially available B(C₆F₅)₃ was achieved.