Organocatalytic C(sp3)-H Functionalization via Carbocation-Initiated Cascade [1,5]-Hydride Transfer/Cyclization: Synthesis of Dihydrodibenzo[b,e]azepines

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.

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.

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.

Construction of seven- and eight-membered carbocycles by Lewis acid catalyzed C(sp3)–H bond functionalization

Concise construction of seven- or eight-membered carbocycles was accomplished by Lewis acid catalyzed C(sp³)–H bond functionalization.

Redox-triggered cascade dearomative cyclizations enabled by hexafluoroisopropanol

An unprecedented cascade dearomative cyclization through hydrogen-bonding-assisted hydride transfer is realized. The aggregate effect of HFIP enables the rapid buildup of polycyclic amines directly from phenols and o-aminobenzaldehydes via a cascade dearomatization/rearomatization/dearomatization sequence. This unique transformation addressed the drawbacks of hydride transfer-involved redox-neutral reactions.

Highly diastereoselective synthesis of tricyclic fused-pyrans by sequential hydride shift mediated double C(sp3)-H bond functionalization

Described herein is the Brønsted acid-catalyzed double C(sp3)-H bond functionalization of alkyl phenethyl ether derivatives. In this process, a [1,5]-[1,5]-hydride shift occurred successively to afford tricyclic fused pyran derivatives in excellent chemical yields with excellent diastereoselectivities (up to >20 : 1). The key to achieve this reaction was the introduction of two methyl groups at the benzylic position, which was effective in both hydride shift processes: (1) the Thorpe-Ingold effect for the first hydride shift and (2) conformational control in the second hydride shift.

Organocatalytic Dearomative [4 + 2] Cycloadditions of Biomass-Derived 2,5-Dimethylfuran with ortho-Quinone Methides: Access to Multisubstituted Chromanes

The organocatalytic dearomative [4 + 2] cycloadditions of biomass-derived 2,5-dimethylfuran with ortho-quinone methides were developed, affording two diffferent types of multisubstituted chromanes in high yields and excellent diastereoselectivities. The controllable synthesis of these two types of multisubstituted chromanes could be achieved by succinctly varying the reaction conditions.

Substrate-controlled divergent synthesis of polycyclic indoloazepines and indolodiazepines via 1,5-hydride shift/7-cyclization cascades

Novel and practical acid-catalyzed cyclization of 2-indolyl aryl carbinols via tandem dehydration/1,5-hydride shift/7-cyclization sequences has been developed. By appropriate variation of the substrate, diverse polycyclic-fused indoles were synthesized in good yield, thus demonstrating the high efficiency, transition-metal-free nature, and high step/atom economy of the synthesis strategy.

Redox-Annulations of Cyclic Amines with 2-(2-Oxoethyl)malonates

Amines such as 1,2,3,4-tetrahydroisoquinoline undergo redox-neutral annulations with 2-(2-oxoethyl)malonates in the presence of catalytic amounts of benzoic acid. These reactions install a fully saturated five-membered ring and provide access to structures closely related to the natural products crispine A and harmicine.

Alkyl Ethers as Traceless Hydride Donors in Brønsted Acid Catalyzed Intramolecular Hydrogen Atom Transfer

A new protocol for the deoxygenation of alcohols and the hydrogenation of alkenes under Brønsted acid catalysis has been developed. The method is based on the use of either a benzyl or isopropyl ether as a traceless hydrogen-atom donor, and involves an intramolecular hydride transfer as a key step, which is achieved in a regio- and stereoselective manner.

A Stereoselective Reductive Hosomi-Sakurai Reaction

A novel reductive variant of the classical Hosomi-Sakurai reaction is reported. This transformation hinges on a redox-neutral, stereoselective internal reduction event under mild conditions. This operationally simple reaction relies on readily available starting materials and leads to useful products in diastereoselectivities of up to 7:1. The versatility of this new method is demonstrated through the stereoselective one-step synthesis of an AChE inhibitor.

Expeditious synthesis of multisubstituted indoles via multiple hydrogen transfers

Expeditious synthesis of multisubstituted indoles was achieved by multiple Lewis acid promoted hydrogen transfers.

Efficient construction of tetrahydroquinolines via fluorinated alcohol mediated cascade [1,5]-hydride transfer/cyclization

An environmentally benign cascade redox-neutral process was developed for the efficient construction of pharmaceutically important spirocyclic tetrahydroquinolines via a 3-step cascade Knoevenagel condensation/[1,5]-hydride transfer/cyclization.

Hydride transfer initiated ring expansion of pyrrolidines toward highly functionalized tetrahydro-1-benzazepines

The first example of ring expansion from a five-membered nitrogen-containing ring to a seven-membered heterocyclic ring has been developed.

Engaging 2-methyl indolenines in a tandem condensation/1,5-hydride transfer/cyclization process: construction of a novel indolenine–tetrahydroquinoline assembly

An efficient BF₃·Et₂O-catalyzed diastereoselective tandem condensation/1,5-hydride transfer/cyclization strategy for the synthesis of a novel indolenine–tetrahydroquinoline assembly was reported.

Divergent synthesis of CF3-substituted polycyclic skeletons based on control of activation site of acid catalysts

We report a divergent synthesis of CF₃-substituted fused skeletons based on precise control of the activation site through the selection of acid catalysts.