Synthesis of Tetrahydro[1,3,4]triazepines via Redox-Neutral α-C(sp3)–H Amination of Cyclic Amines

Alkyl amines and ethers as traceless hydride donors in [1,5]-hydride transfer cascade reactions

The use of alkyl amines and ethers as traceless hydride donors in [1,5]-hydride transfer cascade reactions represents a promising strategy that greatly enriches redox-neutral hydride transfer chemistry. This review summarizes the remarkable progress made in this field, and focuses on (1) alkyl amines as traceless hydride donors in cascade [1,5]-hydride transfer/elimination reactions and (2) alkyl ethers as traceless hydride donors in [1,5]-hydride transfer cascade reactions. The reaction mechanisms, features, scope, limitations, and synthetic applications are included, where appropriate. Importantly, its powerful ability in allene synthesis and the combination with [Re]-vinylidene and carbocation chemistries render this strategy attractive enough to inspire chemists to develop colorful reactions for building molecular complexity.

peri-Interactions in 1,8-bis(dimethylamino)naphthalene ortho-ketimine cations facilitate [1,5]-hydride shift: selective synthesis of 1,2,3,4-tetrahydrobenzo[h]quinazolines

Selective heterocyclization leading to 1,2,3,4-tetrahydrobenzo[h]quinazolines from ortho-ketimines of 1,8-bis(dimethylamino)naphthalene (DmanIms) under acid catalysis has been revealed. In contrast to the rather unreactive N,N-dimethylaniline ortho-ketimine, DmanIms readily undergo this transformation without an additional catalyst. This distinction in the reactivity underscores the importance of the second peri-NMe₂ group in DmanIms, which facilitates a [1,5]-hydride shift and the subsequent cyclization. The cascade of peri-interactions emerging between 1-NMe₂ and 8-NMe₂ groups has been identified as a reason for the catalytic effect: (1) the hydrogen bond in the DmanIm dication constrains 1-NMe₂ in the desired position providing proximity of reaction centers, (2) the repulsion of the lone pairs of 8-NMe₂ group and unrelaxed 1-NMe₂ group arising right after deprotonation process reduces the Gibbs free energy of activation (ΔG^‡) for the straight hydride shift, and (3) the electrostatic interaction between 8-NMe₂ and the charged NCH₂⁺ group in the intermediate increases the ΔG^‡ for the reverse hydride shift.

Catalytic Asymmetric Construction of a 1,2,4-Benzotriazepine Skeleton via Diastereo- and Enantioselective Decarboxylative [4 + 3] Cyclization

We have developed a protocol for palladium-catalyzed decarboxylative [4 + 3] cycloaddition reactions between 4-vinyl benzoxazinanones and azomethine imines to generate moderate to good yields of structurally diverse 1,2,4-benzotriazepines bearing two stereogenic centers with good to excellent stereoselectivities. This protocol not only addresses the challenge of asymmetrically constructing compounds with a 1,2,4-benzotriazepine skeleton but also demonstrates the utility of decarboxylative cycloadditions for the synthesis of enantioenriched polycyclic compounds.

Facile syntheses of tetrahydroquinolines and 1,2-dihydroquinolines via vinylogous cascade hydride transfer/cyclization

The medicinally significant 3-monosubstituted tetrahydroquinolines and 1,2-dihydroquinolines were controllably constructed via redox-neutral vinylogous cascade condensation/[1,5]-hydride transfer/cyclization in EtOH.

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.

Catalyst-free construction of spiro [benzoquinolizidine-chromanones] via a tandem condensation/1,5-hydride transfer/cyclization process

A catalyst-free tandem 1,5-hydride shift/cyclization process to form spiro [benzoquinolizidine-chromanones] is developed, which features high atom- and step-economy, high levels of stereocontrol, mild conditions, and a simple workup process. A series of new polycyclic spiro [benzoquinolizidine-chromanones] were obtained in high yields with excellent diastereoselectivities (up to 91% yield, >20 : 1 dr).

Rapid access to 3-indolyl-1-trifluoromethyl-isobenzofurans by hybrid use of Lewis/Brønsted acid catalysts

We report herein a rapid access to 3-indolyl-1-trifluoromethyl-isobenzofurans via a [1,4]-hydride shift/cyclizatin/intermolecular nucleophilic addition reaction sequence. In this process, a Lewis acid promoted internal redox reaction ([1,4]-hydride shift/cyclization) followed by a Brønsted acid promoted intermolecular reaction (generation of cyclic oxonium cation/intermolecular Friedel-Crafts reaction) occurred to give various 3-indolyl-1-trifluoromethyl-isobenzofurans in good chemical yields.

Hydride Transfer Initiated Redox-Neutral Cascade Cyclizations of Aurones: Facile Access to [6,5] Spirocycles

Reported herein is the hydride transfer initiated redox-neutral cascade cyclizations of aurones, providing a variety of [6,5] spiro-heterocycles in satisfactory yields and good diastereoselectivities.

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.

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.

Palladium-catalyzed [4 + 3] dearomatizing cycloaddition reaction of N-iminoquinolinium ylides

An efficient approach to obtain saturated seven-membered ring containing three heteroatoms has been developed by using a palladium-catalyzed [4 + 3] dearomatizing cycloaddition. This approach features mild conditions and good functional group tolerance.

Synthesis of pyrazolone fused benzodiazepines via Rh(iii)-catalyzed [4 + 3] annulation of 1-phenylpyrazolidinones with propargyl alcohols

An efficient synthesis of pyrazolone fused benzodiazepines through an unprecedented regioselective [4 + 3] annulation of 1-phenylpyrazolidinones with propargyl alcohols via Rh(iii)-catalyzed redox-neutral C–H/N–H/C–O bond activation is presented.

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.