Hydrogen-Atom-Transfer-Mediated Acceptorless Dehydrogenative Cross-Coupling Enabled by Multiple Catalytic Functions of Zwitterionic Triazolium Amidate

Catalytic Enantioselective α-Ethynylation of Oxindoles: Total Synthesis of (-)-Corynoxine, (-)-Isorhynchophylline, (-)-Aspidospermidine, and (-)-Limaspermidine

The all-carbon quaternary stereogenic center of oxindoles is a crucial structural element of a broad spectrum of indole alkaloids, imparting these molecules with rigid three-dimensional configurations essential for their biological activities. Here, we present a catalytic asymmetric α-ethynylation reaction of oxindoles taking advantage of the catalysis of a spiropyrrolidine amide (SPA) triazolium. This transformation enables the enantioselective construction of the C3 quaternary carbon stereocenter of oxindoles while introducing a versatile ethynyl functionality. Employment of this methodology has been demonstrated in the divergent total synthesis of indole alkaloids (-)-corynoxine, (-)-isorhynchophylline, (-)-aspidospermidine, and (-)-limaspermidine, featuring a protecting group-dependent 1,6-Michael addition or an aminolysis/1,6-Michael addition sequence to generate two distinct types of spiro-indoles, tailored for different late-stage synthetic purposes.

Visible-Light-Promoted α-C(sp3)-H Amidation of Cyclic Ethers under Redox-Neutral Conditions

Reported herein is a visible-light-promoted strategy for the α-C(sp3)-H amidation of cyclic ethers using N-acyloxyamide as an oxidative amidating reagent. This transformation provides a straightforward approach to various α-amidated cyclic ethers under metal- and additive-free conditions. The synthetic utility of the products was demonstrated through facile transformations, including reduction, allylation, acylation, sulfonamidation, and gram-scale reactions. Preliminary mechanistic studies suggest a radical/radical cross-coupling process, with C(sp3)-H bond cleavage identified as the rate-determining step.

Coordinating activation strategy enables 1,2-alkylamidation of alkynes

The radical 1,2-difunctionalization reaction of alkynes has been evolved into a versatile approach for expeditiously increasing the complexity of the common feedstock alkyne. However, intermolecular 1,2-carboamidation with general alkyl groups is an unsolved problem. Herein, we show that a coordinating activation strategy could act as an efficient tool for enabling radical 1,2-alkylamidation of alkynes. With the employment of diacyl peroxides as both alkylating reagents and internal oxidants, a large library of β-alkylated enamides is constructed in a three-component manner from readily accessible amides and alkynes. This protocol exhibits broad substrate scope with good functional group compatibility and is amenable for late-stage functionalization of natural molecules and biologically compounds.

Recent Advances in C-H Functionalisation through Indirect Hydrogen Atom Transfer

The functionalisation of C-H bonds has been an enormous achievement in synthetic methodology, enabling new retrosynthetic disconnections and affording simple synthetic equivalents for synthons. Hydrogen atom transfer (HAT) is a key method for forming alkyl radicals from C-H substrates. Classic reactions, including the Barton nitrite ester reaction and Hofmann-Löffler-Freytag reaction, among others, provided early examples of HAT. However, recent developments in photoredox catalysis and electrochemistry have made HAT a powerful synthetic tool capable of introducing a wide range of functional groups into C-H bonds. Moreover, greater mechanistic insights into HAT have stimulated the development of increasingly site-selective protocols. Site-selectivity can be achieved through the tuning of electron density at certain C-H bonds using additives, a judicious choice of HAT reagent, and a solvent system. Herein, we describe the latest methods for functionalizing C-H/Si-H/Ge-H bonds using indirect HAT between 2018-2023, as well as a critical discussion of new HAT reagents, mechanistic aspects, substrate scopes, and background contexts of the protocols.

Stepwise on-demand functionalization of multihydrosilanes enabled by a hydrogen-atom-transfer photocatalyst based on eosin Y

Organosilanes are of vital importance for modern human society, having found widespread applications in functional materials, organic synthesis, drug discovery and life sciences. However, their preparation remains far from trivial, and on-demand synthesis of heteroleptic substituted silicon reagents is a formidable challenge. The generation of silyl radicals from hydrosilanes via direct hydrogen-atom-transfer (HAT) photocatalysis represents the most atom-, step-, redox- and catalyst-economic pathway for the activation of hydrosilanes. Here, in view of the green characteristics of neutral eosin Y (such as its abundance, low cost, metal-free nature, absorption of visible light and excellent selectivity), we show that using it as a direct HAT photocatalyst enables the stepwise custom functionalization of multihydrosilanes, giving access to fully substituted silicon compounds. By exploiting this strategy, we realize preferable hydrogen abstraction of Si-H bonds in the presence of active C-H bonds, diverse functionalization of hydrosilanes (for example, alkylation, vinylation, allylation, arylation, deuteration, oxidation and halogenation), and remarkably selective monofunctionalization of di- and trihydrosilanes.

Zwitterionic Diphenylphosphinyl Amidate as a Powerful Photoinduced Hydrogen-Atom-Transfer Catalyst for C-H Alkylation of Simple Alkanes

The chemical and physical properties of amides change substantially when the electron-withdrawing groups attached to the nitrogen are varied. Herein, we report the superior performance of N-diphenylphosphinyl 1,2,3-triazolium amidate as a photoinduced hydrogen-atom transfer catalyst compared to its N-benzoyl analog. A binary catalyst system of the phosphinyl amidate and an Ir-based photocatalyst enables the alkylation of unbiased C-H bonds.

Ion-Pairing Catalysis in Stereoselective, Light-Induced Transformations

With the rapid development of photoredox catalysis, numerous concepts for asymmetric induction were successfully and broadly adapted from polar two-electron transformations to radical chemistry. While this applies to organocatalysis or transition metal chemistry, asymmetric ion-pairing catalysis remains a niche application within light-driven reactions today. This perspective gives an overview of recent examples, strategies, and their application in stereoselective transformations at the interface of ion-pairing and photo(redox) catalysis.

Photocatalytic C(sp3) radical generation via C-H, C-C, and C-X bond cleavage

C(sp³) radicals (R˙) are of broad research interest and synthetic utility. This review collects some of the most recent advancements in photocatalytic R˙ generation and highlights representative examples in this field. Based on the key bond cleavages that generate R˙, these contributions are divided into C–H, C–C, and C–X bond cleavages. A general mechanistic scenario and key R˙-forming steps are presented and discussed in each section.

C(sp³) radicals (R˙) are of broad research interest and synthetic utility.

Borane-Catalyzed Dehydrogenative C‒C Bond Formation of Indoles with N-Tosylhydrazones: An Experimental and Computational Study

A novel dehydrogenative C‒C bond formation of indoles and N-tosylhydrazones to di(indolyl)methanes (DIMs) has been demonstrated using tris(pentafluorophenyl)borane as catalyst. A wide range of functional groups can be tolerated under...

Metal-free polyporphyrin based photocatalysts for the functionalization of C(sp3)–H bonds in water

Insoluble polyporphyrin or water-dispersible nano-polyporphyrin was used to achieve visible light-induced functionalization of C(sp3)–H bonds in water under mild conditions.