Synthesis of Ring-Fused 1-Benzazepines via [1,5]-Hydride Shift/7-Endo Cyclization Sequences

Eco-friendly α,β-C(sp3)-H difunctionalization of tertiary amines via sequential [1,5]-hydride transfer and hetero-Diels-Alder cyclization

An unprecedented eco-friendly multi-component domino reaction for the synthesis of novel N,O-acetals is reported. The protocol involves sequential coupling, [1,5]-hydride transfer and hetero-Diels-Alder cyclization. This new strategy enables direct α,β-difunctionalization of cyclic amines utilizing enamines generated in situ. The methodology features high atom and step economy, excellent regioselectivity, a simple work-up procedure and molecular diversity.

Synthesis of Polysubstituted Naphthalenes by a Hydride Shift Mediated C-H Bond Functionalization/Aromatization Sequence

A synthetic strategy for forming multisubstituted naphthalenes based on hydride shift mediated C(sp3)-H bond functionalization was developed. This strategy consists of three successive transformations: (1) an intramolecular hydride shift mediated C(sp3)-H bond functionalization; (2) a decarboxylative fragmentation; and (3) an oxidation reaction. When benzylidene malonates having a 2-alkoxyethyl group at the ortho position were treated with a catalytic amount of Al(OTf)3, the hydride shift/cyclization reaction proceeded smoothly to afford tetralin derivatives in good chemical yields. The resulting tetralins were easily converted into naphthalenes by exposing them to modified Krapcho decarboxylation reaction conditions (LiCl, DMSO, and heating under an O2 atmosphere). The one-pot operation of these two reactions was also realized.

Silyl-Group Boosted Internal Redox Reaction: Hydride Shift from an Aliphatic Secondary Position for the Formation of Six- and Seven-Membered Carbocycles

We report a hydride shift/cyclization reaction at the aliphatic secondary position (methylene group). The key to accomplishing this reaction was the employment of benzylidene malonate having a silyl group β to the hydride donor carbon. When the corresponding malonates were treated with a catalytic amount of Al(OTf)3, the [1,5]-hydride shift from the simple aliphatic secondary position proceeded smoothly to afford silyl-group substituted tetralin derivatives in excellent chemical yields (up to 98%). This reaction system was applied to the formation of seven-membered carbocycles via the [1,6]-hydride shift mediated process.

Controllable Synthesis of N- and O-Containing Heterocycles via Formal [3 + 2] and [5 + 2] Cyclizations

The controllable synthesis of spirooxindole-dihydrofurans and spirooxindole-benzazepines was developed through formal [3 + 2] and [5 + 2] cyclization reactions from 2-(2-oxoindolin-3-yl)malononitriles and ortho-aminobenzaldehydes, respectively. A variety of spirooxindole-benzazepines were facilely constructed via a furan ring-open-involved hydride transfer/cyclization process. It is noteworthy that the application of the hydride-transfer-involved [5 + 2] cyclization strategy for construction of spirobenzazepines was unprecedented. In addition, the spiro N- and O-containing heterocycles were highly functionalized by amino, amide, and cyano groups, which were conducive to late-stage functionalization.

TFE-Facilitated Synthesis of Tetrahydropyridino[2,3-d]pyrimidine via Cascade [1,5]-Hydride Transfer/Cyclization

An efficient fluorinated alcohol-driven cascade [1,5]-hydride transfer/cyclization between o-amino pyridyl aldehydes and primary amines has been developed. This unique transformation enabled an array of tetrahydropyridino[2,3-d]pyrimidine construction. Furthermore, the encouraging antifungal activity of Thanatephorus cucumeris was demonstrated by this tetrahydropyridino[2,3-d]pyrimidine core structure.

I2-Mediated [6 + 1] Annulation of Alkynes with MsONH3OTf: Direct Synthesis of Benzo[b]azepines

The synthesis of benzo[b]azepines using protonated aminating reagent (MsONH₃OTf) and alkynes through I₂-mediated [6 + 1] annulation reaction has been developed. This protocol features excellent functional group tolerance and mild reaction conditions and affords the benzo[b]azepines in moderate to good yields under metal-free reaction conditions.

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.

Access to Polycyclic Indole-3,4-Fused Nine-Membered Ring via Cascade 1,6-Hydride Transfer/Cyclization

A cascade aldimine condensation/1,6-hydride transfer/Mannich-type cyclization of indole-derived phenylenediamine with aldehydes was developed for one-step construction of a polycyclic indole-3,4-fused skeleton. Aldehyde serves as a key to start the whole process, including 1,6-hydride transfer enabled δ-C(sp³)-H activation of the secondary amine. The challenges of construction of medium-sized rings are addressed via hydride transfer chemistry.

A palladium-catalyzed ring-expansion reaction of cyclobutanols with 2-haloanilines leading to benzazepines and quinolines

A general synthesis of 2-aryl benzazepines has been developed through palladium-catalyzed ring-expansion reactions of cyclobutanols with 2-haloanilines; the further oxidative rearrangement reaction of benzazepines provided an efficient synthesis of 2-acyl quinolines. These transformations feature the efficient construction of six- and seven-membered N-containing heterocycles from easily obtained materials with excellent functional group tolerance.

I2 -Promoted Intramolecular Oxidative Cyclization of Butenyl Anilines: A Facile Route to Benzo[b]azepines

A metal-free approach for the synthesis of seven-membered N-heterocycles has been developed by the I₂ -promoted intramolecular cross-coupling/annulation of butenyl anilines. This cyclization reaction involves C-H activation and C-C bond formation and exhibits good functional group tolerance. A series of benzo[b]azepine derivatives are obtained in moderate to good yields.

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.

tert-Amino Effect-Promoted Rearrangement of Aryl Isothiocyanate: A Versatile Approach to Benzimidazothiazepines and Benzimidazothioethers

A general and practical approach to benzimidazothiazepine and benzimidazothioether derivatives via an intramolecular nucleophilic addition/ring expansion rearrangement of aryl isothiocyanates promoted by the tert-amino effect has been developed. This reaction is catalyzed by low-cost camphorsulfonic acid and tolerates a broad substrate scope with complete atom economy. Structurally intriguing benzimidazothiazepine and benzimidazothioether products could be easily obtained by a simple operation in good to excellent yield (up to 98%).

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.

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.

Redox-Annulations of Cyclic Amines with ortho-Cyanomethylbenzaldehydes

Amines such as 1,2,3,4-tetrahydroisoquinoline undergo redox-neutral annulations with ortho-cyanomethylbenzaldehydes. These amine α-C-H bond functionalization reactions are promoted by acetic acid. The resulting β-aminonitriles can be converted to the corresponding β-aminoalcohols in diastereoselective fashion.

Expeditious Synthesis of Multisubstituted Quinolinone Derivatives Based on Ring Recombination Strategy

We achieved a concise construction of 3-substituted quinolinone derivatives based on a ring recombination strategy. In this process, seven transformations involving two types of cyclization proceeded in one pot to afford various quinolinone derivatives in good to excellent chemical yields (up to 98%).

Hydride transfer enabled switchable dearomatization of indoles in the carbocyclic ring and the pyrrole ring

Hydride transfer enabled the first success of the regioselective dearomatization of indoles in the carbocyclic ring and the pyrrole ring, which was induced by ortho-quinone methides and vinylogous iminium intermediates, respectively.

Electrochemical trifluoromethylation/semipinacol rearrangement sequences of alkenyl alcohols: synthesis of β-CF3-substituted ketones

Electrochemical oxidative radical trifluoromethylation/semipinacol rearrangement sequences of alkenyl alcohols were developed in this study. This approach is environmentally benign and uses the shelf-stable Langlois reagent as a trifluoromethyl radical precursor and electrons as the oxidizing reagents. The present protocol offers a facile route to prepare β-trifluoromethylated ketone derivatives.

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.

Mild, Metal-Free Oxidative Ring-Expansion Approach for the Synthesis of Benzo[ b]azepines

Benzo[ b]azepines are important structural motifs for the pharmaceutical industry. However, their syntheses are usually lengthy, involving several steps, transition-metal catalysts, and/or harsh conditions. A novel, general, mild, and metal-free oxidative ring expansion tandem reaction of hydroquinolines with TMSCHN₂ as a versatile soft nucleophile to gain access to these valuable compounds in a simple and straightforward manner is presented.

α-Functionalization of Cyclic Secondary Amines: Lewis Acid Promoted Addition of Organometallics to Transient Imines

Cyclic imines, generated in situ from their corresponding N-lithiated amines and a ketone hydride acceptor, undergo reactions with a range of organometallic nucleophiles to generate α-functionalized amines in a single operation. Activation of the transient imines by Lewis acids that are compatible with the presence of lithium alkoxides was found to be crucial to accommodate a broad range of nucleophiles including lithium acetylides, Grignard reagents, and aryllithiums with attenuated reactivities.

Diastereoselective Synthesis of CF3-Substituted Spiroisochromans by [1,5]-Hydride Shift/Cyclization/Intramolecular Friedel-Crafts Reaction Sequence

Developed herein is a diastereoselective synthesis of CF₃-substituted spiroisochromans via C(sp³)-H bond functionalization involving sequential transformations ([1,5]-hydride shift/cyclization/elimination of MeOH/intramolecular Friedel-Crafts reaction).

Redox-Annulations of Cyclic Amines with Electron-Deficient o-Tolualdehydes

Amines such as 1,2,3,4-tetrahydroisoquinoline undergo redox-neutral annulations with 2-methyl-3,5-dinitrobenzaldehyde and closely related substrates. Acetic acid serves as the solvent and sole promoter of these transformations which involve dual C-H functionalization.