Catalytic Access to Bridged Sila- N-heterocycles from Piperidines via Cascade sp3 and sp2 C-Si Bond Formation

Hydride Shuttle Catalysis: From Conventional to Inverse Mode

Hydride shuttle catalysis has emerged as a powerful synthetic platform, enabling the selective formation of C-C bonds to yield sp3-rich structures. By virtue of the compelling reactivity of sterically encumbered Lewis acids from the frustrated Lewis pair regime, hydride shuttle catalysis enables the regioselective functionalization of alkyl amines at either the α- or β-position. In contrast to classical Lewis acid reactivity, the increased steric hindrance prevents interaction with the Lewis basic amine itself, instead leading to reversible abstraction of a hydride from the amine α-carbon. The created positive charge facilitates the occurrence of transformations before hydride rebound or a similar capture event happen. In this Perspective, we outline a broad selection of transformations featuring hydride shuttle catalysis, as well as the recently developed approach of inverse hydride shuttle catalysis. Both strategies give rise to a wide array of functionalized amines and offer elegant approaches to otherwise elusive bond formations.

Electrophilic Hydrosilylation of Electron-Rich Alkenes Derived from Enamines

The low-electron count, air-stable, platinum complexes [Pt(ItBu')(ItBu)][BArF] (C1) (ItBu=1,3-di-tert-butylimidazol-2-ylidene), [Pt(SiPh)3(ItBuiPr)2][BArF] (C2) (ItBuiPr=1-tert-butyl-3-iso-propylimidazol-2-ylidene), [Pt(SiPh)3(ItBuMe)2][BArF] (C3), [Pt(GePh3)(ItBuiPr)2][BArF] (C4), [Pt(GePh)3(ItBuMe)2][BArF] (C5) and [Pt(GeEt)3(ItBuMe)2][BArF] (C6) (ItBuMe=1-tert-butyl-3-methylimidazol-2-ylidene) are efficient catalysts (particularly the germyl derivatives) in both the silylative dehydrocoupling and hydrosilylation of electron rich alkenes derived from enamines. The steric hindrance exerted by the NHC ligand plays an important role in the selectivity of the reaction. Thus, bulky ligands are selective towards the silylative dehydrocoupling process whereas less sterically hindered promote the selective hydrosilylation reaction. The latter is, in addition, regioselective towards the β-carbon atom of both internal and terminal enamines, leading to β-aminosilanes. Moreover, the syn stereochemistry of the amino and silyl groups implies an anti Si-H bond addition across the double bond. All these facts point to a mechanistic picture that, according to experimental and computational studies, involves a non-classical hydrosilylation process through an outer-sphere mechanism in which a formal nucleophilic addition of the enamine to the silicon atom of a platinum σ-SiH complex is the key step. This is in sharp contrast with the classical Chalk-Harrod mechanism prevalent in platinum chemistry.

Palladium-Catalyzed Allylation of Endocyclic 1-Azaallyl Anions

Endocyclic 1-azaallyl anions engage allyl acetates in a palladium-catalyzed allylation followed by reduction to give unprotected 2-(hetero)aryl-3-allylpiperidines and 2-allyl-3-arylmorpholines, products not easily accessible by other means. The allyl group is then readily transformed into a variety of functional groups. Preliminary studies on the asymmetric variant of the reaction using an enantiomerically pure BI-DIME-type ligand provide the product with moderate enantioselectivity. Computational studies suggest that energy barriers of inner-sphere reductive elimination and outer-sphere nucleophilic substitution are almost the same, which makes both of them possible reaction pathways. In addition, the inner-sphere mechanism displays an enantiodiscriminating C-C bond forming step, while the outer-sphere mechanism is much less selective, which combined to give the asymmetric variant of the reaction moderate enantioselectivity.

B(C6F5)3-Catalyzed Dehydrogenation of Pyrrolidines to Form Pyrroles

Pyrroles are important N-heterocycles found in medicines and materials. The formation of pyrroles from widely accessible pyrrolidines is a potentially attractive strategy but is an underdeveloped approach due to the sensitivity of pyrroles to the oxidative conditions required to achieve such a transformation. Herein, we report a catalytic approach that employs commercially available B(C6F5)3 in an operationally simple procedure that allows pyrrolidines to serve as direct synthons for pyrroles. Mechanistic studies have revealed insights into borane-catalyzed dehydrogenative processes.

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.

Controllable Si-C Bond Formation from Trihydrosilanes En Route to Synthesis of 1,4-Azasilinanes with Diverse Silyl Functionalities

A B(C6F5)3-catalyzed controllable inter/intra-/intermolecular Si-C bond formation process has been developed from trihydrosilane and dienamide with alkenes, anilines, or aryl iodides. A variety of 1,4-azasilinanes have been generated with diverse exo-cyclic heteroleptic disubstitutions on silicon, thereby expanding the range of silaazacyclic rings available for the discovery of silicon-containing drugs.

Metal-Free Directed Site-Selective Csp3 -H Borylation of Saturated Cyclic Amines

The borylation of Csp3 -H bonds is a challenging transformation that is typically restricted to transition metal catalysis. Herein, we report the site-selective metal-free Csp3 -H borylation of saturated cyclic amines. It is possible to selectively borylate piperidine derivatives at the α or β positions according to the reaction conditions. The mechanism was supported by NMR spectroscopy, calorimetry experiments and density functional theory (DFT) computations. It suggests that the piperidine is dehydrogenated by complexation with BBr3 to produce an enamine intermediate, which is in turn borylated at either the α or β position according to the reaction conditions.

Palladium-Catalyzed Arylation of Endocyclic 1-Azaallyl Anions: Concise Synthesis of Unprotected Enantioenriched cis-2,3-Diarylpiperidines

Unprotected cis-2,3-diarylpiperidines are synthesized through an unprecedented palladium-catalyzed cross-coupling reaction between aryl halides and elusive endocyclic 1-azaallyl anions. These intermediates are generated in situ by the deprotonation of 2-aryl-1-piperideines, precursors that are readily prepared in two operations from simple piperidines. An asymmetric version of this reaction with (2R, 3R)-iPr-BI-DIME as the ligand provides products in moderate to good yields and enantioselectivities. This study significantly expands the synthetic utility of endocyclic 1-azaallyl anions.

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.

Synergistic effect of hydrogen bonds and π-π interactions of B(C6F5)3·H2O/amides complex: Application in photoredox catalysis

B(C₆F₅)₃·H₂O has been long recognized as a common Brønsted acid. The lack of X-ray crystal structure of B(C₆F₅)₃·H₂O with other substrates has greatly limited the development of a new catalytic mode. In this work, a complex of B(C₆F₅)₃·H₂O and amide 2-phenyl-3,4-dihydroisoquinolin-1(2H)-one with hydrogen bonds and π-π interactions is characterized by X-ray diffraction. Such noncovalent interactions in solution also exist, as verified by NMR, UV-Vis absorption, and fluorescence emission measurements. Moreover, the mixture of amide 2-phenyl-3,4-dihydroisoquinolin-1(2H)-one and B(C₆F₅)₃·H₂O, instead of other tested Brønsted acids, shows a tailing absorption band in the visible light region (400-450 nm). Based on the photoactive properties of the complex, a photoredox catalysis is developed to construct α-aminoamides under mild conditions.

Direct synthesis of sila-benzoazoles through hydrosilylation and rearrangement cascade reaction of benzoazoles and silanes

Sila-isosteres have attracted increasing attention due to their potential application in a variety of fields and their different properties compared to their carbon-containing analogs. However, the preparation of these silicon-containing compound remains challenging and thus the development of alternative synthetic methodologies is desirable. Here, we employ B(C6F5)3 as catalyst to enable the synthesis of highly functionalized sila-benzoazoles via hydrosilylation and rearrangement cascade reaction of benzoazoles and commercially available silanes. This strategy also exhibits remarkable features such as 100% atom-economy, good functional group tolerance, broad substrate scope, easy scale-up and good catalytic performance, demonstrating its potential application in sila-isostere synthesis.

Intramolecular Ring Expansion of 3-Silaazetidine with Alkynes Enabled by Pd-Catalyzed Si-C Bond Activation

An intramolecular ring expansion of in situ formed 3-silaazetidine with internal alkynes has been developed via Pd-catalyzed Si-C bond activation. The reaction gives rise to 6,5- and 6,6-fused bicyclic 1,3-azasilines, in which the silicon atom locates at the ring junction position.

Palladium-catalyzed synthesis of 4-sila-4H-benzo[d][1,3]oxazines by intramolecular Hiyama coupling

A palladium-catalyzed synthesis of 4-sila-4H-benzo[d][1,3]oxazines, silicon-switched analogs of biologically relevant 4H-benzo[d][1,3]oxazines, was developed by the intramolecular Hiyama coupling of 3-amido-2-(arylsilyl)aryl triflates.

Selective oxidative β-C-H bond sulfenylation of tetrahydroisoquinolines with elemental sulfur

In this article, a convenient and efficient KIO₃-promoted oxidative sulfenylation at the β-position of tetrahydroisoquinolines and subsequent aromatization in the presence of elemental S₈ is presented. The reaction proceeds with moderate to good yields via a double C-S formation process. A wide range of structurally diverse 4-sulfenylisoquinolines/3-sulfenylpiperidine were synthesized with excellent functional group tolerance and high efficiency.

Facile construction of dibenzodioxo[3.3.1]nonanes bearing spirocyclohexadienones via domino [4 + 2] cycloaddition/C(sp3)-H oxidative dehydrogenation coupling reactions

A novel palladium catalyzed homodimerization of ortho-hydroxyphenyl substituted p-QMs has been developed via [4 + 2] cycloaddition/oxidative dehydrogenation coupling domino reactions. An interesting palladium catalyzed intramolecular benzyl C-H oxidation dehydrogenation to form a transannular C(sp³)-O bond was found. This protocol provided an efficient method to construct various dibenzodioxo[3.3.1]nonanes bearing spirocyclohexadienones.

Mechanistic insights into reductive deamination with hydrosilanes catalyzed by B(C6F5)3: A DFT study

Selective defunctionalization of synthetic intermediates is a valuable approach in organic synthesis. Here, we present a theoretical study on the recently developed B(C6F5)3/hydrosilane-mediated reductive deamination reaction of primary amines. Our computational results provide important insights into the reaction mechanism, including the active intermediate, the competing reactions of the active intermediate, the role of excess hydrosilane, and the origin of chemoselectivity. Moreover, the study on the substituent effect of hydrosilane indicated a potential way to improve the efficiency of the reductive deamination reaction.

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.

Oxoammonium Salt-Promoted Multifunctionalization of Saturated Cyclic Amines Based On β-Oxo Cyclic Iminium Ion Intermediates

Herein we describe a convenient method for multiple C(sp³)-H bond functionalization of saturated cyclic amines through oxoammonium salt-promoted oxidation to afford a β-oxo cyclic iminium ion as a key intermediate, followed by cascade addition with thiocyanate and diverse N-, O-, and S-containing nucleophiles in the green solvent and EtOH. Notably, chiral spiro azapolyheterocycles were prepared enantioselectively (>20:1 dr, up to 99% ee) when cysteine or serine esters were used as substrates. Moreover, the concise late-stage modification of several natural product derivatives was accomplished using this method.

(4 + 2) Annulation of Cl-NH3+CH2SiMe2CH2Cl and Propynones for the Synthesis of 1,3-Azasilinones

A useful 1,3-N,Si reagent (Cl^-NH₃⁺CH₂SiMe₂CH₂Cl) and its (4 + 2) annulation with propynones have been developed. The (4 + 2) annulation is promoted by NaHCO₃ via an intermolecular N-1,4-addition/intramolecular alkylation process, leading to 1,3-azasilinones in good yields. Diverse functionalization of the alkene, carbonyl, and nitrogen moieties on the 1,3-azasilinone has been demonstrated, showcasing the potential of the approach in the synthesis of bioactive molecules containing silaazacycles.

Visible-Light-Induced Dual C(sp3)-H Bond Functionalization of Tertiary Amine via Hydrogen Transfer to Carbene and Subsequent Cycloaddition

Herein, we describe the dual C(sp³)-H bond functionalization of a tertiary amine through hydride-transfer-induced dehydrogenation, followed by cycloaddition, using an easily preparable diazoester as a new type hydride-acceptor precursor under mild, redox-neutral conditions. With carbene as a hydrogen acceptor, this method was demonstrated by the preparation of a broad range of functionalized isoxazoldines in moderate to good yields.

Activation of Si-H and B-H bonds by Lewis acidic transition metals and p-block elements: same, but different

In this Perspective we discuss the ability of transition metal complexes to activate and cleave the Si-H and B-H bonds of hydrosilanes and hydroboranes (tri- and tetra-coordinated) in an electrophilic manner, avoiding the need for the metal centre to undergo two-electron processes (oxidative addition/reductive elimination). A formal polarization of E-H bonds (E = Si, B) upon their coordination to the metal centre to form σ-EH complexes (with coordination modes η1 or η2) favors this type of bond activation that can lead to reactivities involving the formation of transient silylium and borenium/boronium cations similar to those proposed in silylation and borylation processes catalysed by boron and aluminium Lewis acids. We compare the reactivity of transition metal complexes and boron/aluminium Lewis acids through a series of catalytic reactions in which pieces of evidence suggest mechanisms involving electrophilic reaction pathways.

Boron Lewis Acid Catalyzed Intermolecular trans-Hydroarylation of Ynamides with Hydroxyarenes

An atom-economic protocol for the efficient and highly chemo- and stereoselective trans-hydroarylation of ynamides with hydroxyarenes catalyzed by B(C₆F₅)₃ has been developed. Use of readily available starting materials, low catalyst loading, mild reaction conditions, a broad substrate scope, ease of scale-up, and versatile functionalizations of the enamide products make this approach very practical and attractive.

Rhodium hydride enabled enantioselective intermolecular C–H silylation to access acyclic stereogenic Si–H

The tremendous success of stereogenic carbon compounds has never ceased to inspire researchers to explore the potentials of stereogenic silicon compounds. Intermolecular C–H silylation thus represents the most versatile and straightforward strategy to construct C–Si bonds, however, its enantioselective variant has been scarcely reported to date. Herein we report a protocol that allows for the enantioselective intermolecular C–H bond silylation, leading to the construction of a wide array of acyclic stereogenic Si–H compounds under simple and mild reaction conditions. Key to the success is (1) a substrate design that prevents the self-reaction of prochiral silane and (2) the employment of a more reactive rhodium hydride ([Rh]-H) catalyst as opposed to the commonly used rhodium chloride ([Rh]-Cl) catalyst. This work unveils opportunities in converting simple arenes into value-added stereogenic silicon compounds.

Construction of chiral organosilicon compounds could have implications in photophysical, biological, and chemical fields, as silicon is isoelectronic with carbon, and can mimic carbon atoms while providing slightly different properties. Here the authors present an intermolecular, enantioselective C–H silylation of heterocycles via rhodium catalysis.

Synthetic Approaches for the Construction of Five- and Six-Membered Silaazacycles

Silaazacycles (or azasilacycles), containing both nitrogen and silicon atoms, are appealing ring structures in the development of silicon-containing functional molecules. The development of general and efficient methods towards these motifs has therefore attracted considerable attention from synthetic chemists. This short review intends to highlight representative advances in the synthesis of five- and six-membered silaazacycles.

1 Introduction

2 Five-Membered Silaazacycles

2.1 Five-Membered Silaazacycles bearing a 1,2-N/Si Moiety

2.2 Five-Membered Silaazacycles bearing a 1,3-N/Si Moiety

3 Six-Membered Silaazacycles

3.1 Six-Membered Silaazacycles bearing a 1,2-N/Si Moiety

3.2 Six-Membered Silaazacycles bearing a 1,3-N/Si Moiety

3.3 Six-Membered Silaazacycles bearing a 1,4-N/Si Moiety

4 Conclusion

(3 + 2)-Annulation of 1,3-N,Si-tetraorganosilane reagents TsHNCH2SiBnR1R2 with arynes for efficient synthesis of 3-silaindolines

1,3-N,Si-Tetraorganosilane reagents TsHNCH2SiBnR1R2 were developed as robust synthons to prepare 3-silaindolines via a Cs2CO3-promoted (3 + 2)-annulation reaction with arynes.

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.

Site-Selective Molecular Transformation: Acylation of Hydroxy Groups and C-H Amination

Control of site selectivity is an exciting direction for synthetic organic chemistry owing to the possibility of selective modification of multifunctionalized molecules, ultimately including biomacromolecules. In this review, our recent research related to site selectivity in two types of transformation, namely, the acylation of hydroxy groups and C-H amination, is summarized. Regarding the acylation of hydroxy groups, catalyst-controlled site selectivity enables unconventional retrosynthetic analysis, leading to efficient syntheses of sugar-related natural and unnatural products. Regarding C-H amination, the discovery of unprecedented reaction sites in intermolecular amination mediated by dirhodium nitrenes is described. The findings of this research demonstrate the power of site-selective transformation in the synthesis of a particular class of compounds.

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.

3-Silaazetidine: An Unexplored yet Versatile Organosilane Species for Ring Expansion toward Silaazacycles

Small-ring silacycles are important organosilane species in main-group chemistry and have found numerous applications in organic synthesis. 3-Silaazetidine, a unique small silacycle bearing silicon and nitrogen atoms, has not been adequately explored due to the lack of a general synthetic scheme and its sensitivity to air. Here, we describe that 3-silaazetidine can be easily prepared in situ from diverse air-stable precursors (RSO₂NHCH₂SiR¹₂CH₂Cl). 3-Silaazetidine shows excellent functional group tolerance in a palladium-catalyzed ring expansion reaction with terminal alkynes, giving 3-silatetrahydropyridines and diverse silaazacycle derivatives, which are promising ring frameworks for the discovery of Si-containing functional molecules.

Silylium Ions: From Elusive Reactive Intermediates to Potent Catalysts

The history of silyl cations has all the makings of a drama but with a happy ending. Being considered reactive intermediates impossible to isolate in the condensed phase for decades, their actual characterization in solution and later in solid state did only fuel the discussion about their existence and initially created a lot of controversy. This perception has completely changed today, and silyl cations and their donor-stabilized congeners are now widely accepted compounds with promising use in synthetic chemistry. This review provides a comprehensive summary of the fundamental facts and principles of the chemistry of silyl cations, including reliable ways of their preparation as well as their physical and chemical properties. The striking features of silyl cations are their enormous electrophilicity and as such reactivity as super Lewis acids as well as fluorophilicity. Known applications rely on silyl cations as reactants, stoichiometric reagents, and promoters where the reaction success is based on their steady regeneration over the course of the reaction. Silyl cations can even be discrete catalysts, thereby opening the next chapter of their way into the toolbox of synthetic methodology.

Non-Directed β- or γ-C(sp3)–H Functionalization of Saturated Nitrogen-Containing Heterocycles

Reactions that take place via C–H functionalization are valuable tools in organic synthesis because they can be used for the synthesis of target compounds and for the late-stage functionalization of bioactive compounds. Among these, non-directed C(sp3)–H functionalization reactions of saturated nitrogen-containing heterocycles have been developed in recent years. However, most of these lead to functionalization at the α-position relative to the heteroatom, and reactions at the β- or γ-positions are limited since these bonds are stronger and less electron-rich. Hence, in this review, we will discuss non-directed β- or γ-C(sp3)–H functionalization reactions of saturated nitrogen-containing heterocycles, which are of recent interest to medicinal chemists. These methods are attractive in order to avoid the pre-functionalization of substrates, and to reduce the number of synthetic steps and the formation of byproducts. Such non-directed β- and γ-C(sp3)–H functionalization reactions can be divided into enamine-intermediate-mediated processes and other reaction types described in this review.

1 Introduction

2 Non-Directed β-C(sp3)–H Functionalization of Saturated Nitrogen­-Containing Heterocycles via an Enamine Intermediate

2.1 Non-Directed β-C(sp3)–H Functionalization of Saturated Nitrogen­-Containing Heterocycles under Acidic, Basic or Thermal Conditions

2.2 Non-Directed β-C(sp3)–H Functionalization of Saturated Nitrogen­-Containing Heterocycles under Oxidative Conditions

2.3 Non-Directed β-C(sp3)–H Functionalization of Saturated Nitrogen­-Containing Heterocycles under Redox-Neutral Conditions

3 Strategies for Non-Directed β- or γ-C(sp3)–H Functionalization of Saturated Heterocycles Excluding Examples Proceeding via an Enamine Intermediate

4 Summary

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.

Metal-free C(sp3)-H functionalization of sulfonamides via strain-release rearrangement

With the increasing awareness of sustainable chemistry principles, the development of an efficient and mild strategy for C(sp3)-H bond activation of nitrogen-containing compounds without the utilization of any oxidant and metal is still highly desired and challenging. Herein, we present a metal-free reaction system that enables C-H bond functionalization of aliphatic sulfonamides using DABCO as a promoter under mild conditions, affording a series of α,β-unsaturated imines in good yields with high selectivities. This protocol tolerates a broad range of functionalities and can serve as a powerful synthetic tool for the late-stage modification of complex compounds. More importantly, control experiments and detailed DFT calculations suggest that this process involves [2 + 2] cyclization/ring-cleavage reorganization, which opens up a new platform for the establishment of other related reorganization reactions.

Diversification of Unprotected Alicyclic Amines by C-H Bond Functionalization: Decarboxylative Alkylation of Transient Imines

Despite extensive efforts by many practitioners in the field, methods for the direct α-C-H bond functionalization of unprotected alicyclic amines remain rare. A new advance in this area utilizes N-lithiated alicyclic amines. These readily accessible intermediates are converted to transient imines through the action of a simple ketone oxidant, followed by alkylation with a β-ketoacid under mild conditions to provide valuable β-amino ketones with unprecedented ease. Regioselective α'-alkylation is achieved for substrates with existing α-substituents. The method is further applicable to the convenient one-pot synthesis of polycyclic dihydroquinolones through the incorporation of a SN Ar step.