Remote C-H Functionalization via Selective Hydrogen Atom Transfer

Remote Allylation of Unactivated C(sp3)-H Bonds Triggered by Photogenerated Amidyl Radicals

The allylation reaction is a highly versatile transformation in chemical synthesis. While many elegant direct C(sp²)-H allylation reactions have been developed, the direct allylation of unactivated C(sp³)-H bonds is underdeveloped. By applying photoredox catalysis and a [1,5]-HAT process, herein we report a direct allylation of unactivated C(sp³)‒H bonds. This photocatalyzed transformation is tolerant of several functional groups in the amide and allylic chloride substrates. Various allyl-substituted amide products were obtained with good yields and high δ-selectivity.

Chemistry With N-Centered Radicals Generated by Single-Electron Transfer-Oxidation Using Photoredox Catalysis

This review covers the recent literature on oxidative generation of N-centered radicals using photoredox catalysis. The concept of proton-coupled electron transfer is briefly discussed. Applications of such reactive N-centered radicals in cascade processes comprising arene amidation, alkene amidation, C—C bond cleavage reactions, and remote C—H functionalization are addressed. In addition, novel reagents allowing for clean oxidative N-radical generation are discussed.

Reaction of Nitrogen-Radicals with Organometallics Under Ni-Catalysis: N-Arylations and Amino-Functionalization Cascades

Herein, we report a strategy for the generation of nitrogen-radicals by ground-state single electron transfer with organyl-NiI species. Depending on the philicity of the N-radical, two types of processes have been developed. In the case of nucleophilic aminyl radicals direct N-arylation with aryl organozinc, organoboron, and organosilicon reagents was achieved. In the case of electrophilic amidyl radicals, cascade processes involving intramolecular cyclization, followed by reaction with both aryl and alkyl organometallics have been developed. The N-cyclization-alkylation cascade introduces a novel retrosynthetic disconnection for the assembly of substituted lactams and pyrrolidines with its potential demonstrated in the short total synthesis of four venom alkaloids.

Merging Photochemistry with Electrochemistry: Functional-Group Tolerant Electrochemical Amination of C(sp3 )-H Bonds

Direct amination of C(sp³ )-H bonds is of broad interest in the realm of C-H functionalization because of the prevalence of nitrogen heterocycles and amines in pharmaceuticals and natural products. Reported here is a combined electrochemical/photochemical method for dehydrogenative C(sp³ )-H/N-H coupling that exhibits good reactivity with both sp² and sp³ N-H bonds. The results show how use of iodide as an electrochemical mediator, in combination with light-induced cleavage of intermediate N-I bonds, enables the electrochemical process to proceed at low electrode potentials. This approach significantly improves the functional-group compatibility of electrochemical C-H amination, for example, tolerating electron-rich aromatic groups that undergo deleterious side reactions in the presence of high electrode potentials.

Copper-catalyzed remote C(sp3)-H azidation and oxidative trifluoromethylation of benzohydrazides

The Hofmann-Löffler-Freytag (HLF) reaction is a prototypical example of radical-based remote functionalization of unactivated C(sp3)-H bond. While 1,5-hydrogen atom transfer (1,5-HAT) of the amidyl radical is thermodynamically favorable and is well-established, the method for the subsequent functionalization of the translocated carbon radical is still limited. We report herein two catalytic remote C(sp3)-H functionalization protocols. Cu(MeCN)4PF6-catalyzed reaction of 2-alkyl benzohydrazides 3 with TMSN3 in the presence of MeCO2OtBu affords the γ-azido amides 4, while CuCl-catalyzed reaction of 3 with Togni's reagent provides 2-(β-trifluoromethylvinyl)benzamides 5 via an oxidative δ-trifluoromethylation of the alkyl group. Mechanistic studies suggest that the γ-azidation of benzohydrazides 3 goes through 1,5-HAT followed by a Cu-mediated azido transfer cascade, while the oxidative δ-trifluoromethylation of 3 proceeds via, after 1,5-HAT process, a radical-polar crossover mechanism.

Aliphatic Radical Relay Heck Reaction at Unactivated C(sp3 )-H Sites of Alcohols

The Mizoroki-Heck reaction is one of the most efficient methods for alkenylation of aryl, vinyl, and alkyl halides. Given its innate nature, this protocol requires the employment of compounds possessing a halogen atom at the site of functionalization. However, the accessibility of organic molecules possessing a halogen atom at a particular site in aliphatic systems is extremely limited. Thus, a protocol that allows a Heck reaction to occur at a specific nonfunctionalized C(sp3 )-H site is desirable. Reported here is a radical relay Heck reaction which allows selective remote alkenylation of aliphatic alcohols at unactivated β-, γ-, and δ-C(sp3 )-H sites. The use of an easily installed/removed Si-based auxiliary enables selective I-atom/radical translocation events at remote C-H sites followed by the Heck reaction. Notably, the reaction proceeds smoothly under mild visible-light-mediated conditions at room temperature, producing highly modifiable and valuable alkenol products from readily available alcohols feedstocks.

Photoredox-mediated remote C(sp3)-H heteroarylation of free alcohols

We report an efficient and economical method for remote δ C(sp3)-H heteroarylation of free aliphatic alcohols using a hypervalent iodine PFBI-OH oxidant under photoredox catalysis. The reaction sequence involves in situ alcoholysis of PFBI-OH with alcohol, generation of an alkoxy radical intermediate by SET reduction, 1,5-HAT, and Minisci-type C-C bond formation. This method uses a slight excess of alcohols, can facilitate reaction at δ methyl and methylene positions, and has been successfully applied to modification of complex drug molecules.

Catalytic β C-H amination via an imidate radical relay

An iodine-catalyzed strategy for β C–H amination of alcohols is enabled by a chemo-, regio-, and stereo-selective H-atom transfer mechanism.

The first catalytic strategy to harness imidate radicals for C–H functionalization has been developed. This iodine-catalyzed approach enables β C–H amination of alcohols by an imidate-mediated radical relay. In contrast to our first-generation, (super)stoichiometric protocol, this catalytic method enables faster and more efficient reactivity. Furthermore, lower oxidant concentration affords broader functional group tolerance, including alkenes, alkynes, alcohols, carbonyls, and heteroarenes. Mechanistic experiments interrogating the electronic nature of the key 1,5 H-atom transfer event are included, as well as probes for chemo-, regio-, and stereo-selectivity.

Catalytic cascade reactions by radical relay

Catalytic radical relays are an attractive tool for the rapid construction of complex molecular architectures.

δ C-H (hetero)arylation via Cu-catalyzed radical relay

A radical relay strategy has been developed to enable selective δ C–H arylation. The approach employs a chiral copper catalyst, which serves the dual roles of generating an N-centered radical to promote intramolecular H-atom transfer, and then intercepting a distal C-centered radical for C–C bond formation with (hetero)aryl boronic acids.

A Cu-catalyzed strategy has been developed that harnesses a radical relay mechanism to intercept a distal C-centered radical for C–C bond formation. This approach enables selective δ C–H (hetero)arylation of sulfonamides via intramolecular hydrogen atom transfer (HAT) by an N-centered radical. The radical relay is both initiated and terminated by a Cu catalyst, which enables incorporation of arenes and heteroarenes by cross-coupling with boronic acids. The broad scope and utility of this catalytic method for δ C–H arylation is shown, along with mechanistic probes for selectivity of the HAT mechanism. A catalytic, asymmetric variant is also presented, as well as a method for accessing 1,1-diaryl-pyrrolidines via iterative δ C–H functionalizations.

A General Approach to Site-Specific, Intramolecular C-H Functionalization Using Dithiocarbamates

Intramolecular hydrogen atom transfer is an established approach for the site-specific functionalization of unactivated, aliphatic C-H bonds. Transformations using this strategy typically require unstable intermediates formed using strong oxidants and have mainly targeted C-H halogenations or intramolecular aminations. Herein, we report a site-specific C-H functionalization that significantly increases the synthetic scope and convergency of reactions proceeding via intramolecular hydrogen atom transfer. Stable, isolable N-dithiocarbamates are used as precursors to amidyl radicals formed via either light or radical initiation to efficiently deliver highly versatile alkyl dithiocarbamates across a wide range of complex structures.

Biocatalytic hydrogen atom transfer: an invigorating approach to free-radical reactions

Initiating and terminating free-radical reactionis via hydrogen atom transfer (HAT) is an attractive means of avoiding substrate prefunctionalization. Small molecule catalysts and reagents, however, struggle to execute this fundamental step with useful levels of diastereoselectivity and enantioselectivity. In contrast, nature often carries out HAT with exquisite levels of selectivity for even electronically unactivated carbon-hydrogen bonds. By understanding how enzymes exploit and control this fundamental step, new strategies can be developed to address several long-standing challenges in free-radical reactions. This review will cover recent discoveries in biocatalysis that exploit a HAT mechanism to either initiate or terminate novel one-electron reactions.

Photoinduced Remote Functionalization of Amides and Amines Using Electrophilic Nitrogen Radicals

The selective functionalization of C(sp3 )-H bonds at distal positions to functional groups is a challenging task in synthetic chemistry. Reported here is a photoinduced radical cascade strategy for the divergent functionalization of amides and protected amines. The process is based on the oxidative generation of electrophilic amidyl radicals and their subsequent transposition by 1,5-H-atom transfer, resulting in remote fluorination, chlorination and, for the first time, thioetherification, cyanation, and alkynylation. The process is tolerant of most common functional groups and delivers useful building blocks that can be further elaborated. The utility of this strategy is demonstrated through the late-stage functionalization of amino acids and a dipeptide.

Site-Selective Remote Radical C-H Functionalization of Unactivated C-H Bonds in Amides Using Sulfone Reagents

A general and practical strategy for remote site-selective functionalization of unactivated aliphatic C-H bonds in various amides by radical chemistry is introduced. C-H bond functionalization is achieved by using the readily installed N-allylsulfonyl moiety as an N-radical precursor. The in situ generated N-radical engages in intramolecular 1,5-hydrogen atom transfer to generate a translocated C radical which is subsequently trapped with various sulfone reagents to afford the corresponding C-H functionalized amides. The generality of the approach is documented by the successful remote C-N₃ , C-Cl, C-Br, C-SCF₃ , C-SPh, and C-C bond formation. Unactivated tertiary and secondary C-H bonds, as well as activated primary C-H bonds, can be readily functionalized by this method.

Diverse secondary C(sp3)-H bond functionalization via site-selective trifluoroacetoxylation of aliphatic amines

We describe a coinage-metal-catalyzed site-selective oxidation of secondary C(sp3)-H bonds for aliphatic amine substrates. Broad amine scope, good functional compatibility and late-stage diversification are demonstrated with this method. The steric demand of the β-substituents controlled diastereoselectivities under this catalytic system. The site selectivity favors secondary C(sp3)-H bonds over tertiary ones underscoring the unique synthetic potential of this method.

Remote C-H functionalization using radical translocating arylating groups

Site selective chemical functionalization at unactivated C(sp³)−H bonds is highly challenging and recent successful studies mostly focus on the use of transition metal catalysis in combination with directing groups. Radical chemistry offers a complementary approach with the Barton and the Hofmann-Löffler-Freytag reactions being landmark contributions in the area of remote C−H functionalization at unactivated aliphatic sites. Herein we introduce the concept of radical translocation arylation at unactivated secondary and tertiary C(sp³)−H bonds in various alcohols. The straightforward two-step sequence comprises an ionic alcohol sulfonylation with especially designed ortho-iodoaryl sulfonyl chlorides followed by a radical cascade reaction including aryl radical generation, remote radical translocation, aryl migration, reduction, and SO₂ extrusion to give the corresponding γ-arylated alcohols. Moderate to good yields are obtained, remote C−H arylation occurs with excellent regioselectivity and for secondary C(sp³)−H bonds good to excellent stereoselectivity is achieved.

Selective remote functionalization of aliphatic C(sp³)−H bonds is highly challenging and often requires transition metals and/or directing groups. Here, the authors show the γ-arylation of aliphatic alcohols via a two-step radical translocation and subsequent radical aryl migration.

β C-H di-halogenation via iterative hydrogen atom transfer

A radical relay strategy for mono- and di-halogenation (iodination, bromination, and chlorination) of sp3 C-H bonds has been developed. This first example of β C-H di-halogenation is achieved through sequential C-H abstraction by iterative, hydrogen atom transfer (HAT). A double C-H functionalization is enabled by in situ generated imidate radicals, which facilitate selective N˙ to C˙ radical translocation and tunable C-X termination. The versatile, geminal di-iodide products are further elaborated to β ketones and vinyl iodides. Mechanistic experiments explain the unique di-functionalization selectivity of this iterative HAT pathway, wherein the second C-H iodination is twice as fast as the first.