The role of organic electron donors in the initiation of BHAS base-induced coupling reactions between haloarenes and arenes

Photoinduced Single Electron Reduction of the 4-O-5 Linkage in Lignin Models for C-P Coupling Catalyzed by Bifunctional N-Heterocyclic Carbenes

Catalytic activation of Caryl-O bonds is considered as a powerful strategy for the production of aromatics from lignin. However, due to the high reduction potentials of diaryl ether 4-O-5 linkage models, their single electron reduction remains a daunting challenge. This study presents the blue light-induced bifunctional N-heterocyclic carbene (NHC)-catalyzed one-electron reduction of diaryl ether 4-O-5 linkage models for the synthesis of trivalent phosphines. The H-bond between the newly devised bifunctional NHC and diaryl ethers is responsible for the success of the single electron transfer. Furthermore, this approach demonstrates selective one-electron reduction of unsymmetric diaryl ethers, oligomeric phenylene oxide, and lignin model.

Direct C-H Hydroxylation of N-Heteroarenes and Benzenes via Base-Catalyzed Halogen Transfer

Hydroxylated (hetero)arenes are valued in many industries as both key constituents of end products and diversifiable synthetic building blocks. Accordingly, the development of reactions that complement and address the limitations of existing methods for the introduction of aromatic hydroxyl groups is an important goal. To this end, we apply base-catalyzed halogen transfer (X-transfer) to enable the direct C-H hydroxylation of mildly acidic N-heteroarenes and benzenes. This protocol employs an alkoxide base to catalyze X-transfer from sacrificial 2-halothiophene oxidants to aryl substrates, forming SNAr-active intermediates that undergo nucleophilic hydroxylation. Key to this process is the use of 2-phenylethanol as an inexpensive hydroxide surrogate that, after aromatic substitution and rapid elimination, provides the hydroxylated arene and styrene byproduct. Use of simple 2-halothiophenes allows for C-H hydroxylation of 6-membered N-heteroarenes and 1,3-azole derivatives, while a rationally designed 2-halobenzothiophene oxidant extends the scope to electron-deficient benzene substrates. Mechanistic studies indicate that aromatic X-transfer is reversible, suggesting that the deprotonation, halogenation, and substitution steps operate in synergy, manifesting in unique selectivity trends that are not necessarily dependent on the most acidic aryl position. The utility of this method is further demonstrated through streamlined target molecule syntheses, examples of regioselectivity that contrast alternative C-H hydroxylation methods, and the scalable recycling of the thiophene oxidants.

α-Diimine-mediated C-H functionalization of arenes for aryl-aryl cross-coupling reactions

Easily accessible methods for direct C-H arylation of arenes have been explored in the presence of transition metal catalysts to facilitate C-C bond formation; however, the absence of transition-metal impurities is a significant concern in the preparation of active pharmaceutical ingredients (APIs). Herein, we examine the use of bis(imino)acenaphthene (BIAN) as a potential single-electron transfer initiator in transition metal-free C-C bond-forming reactions. Using this approach, arenes are coupled to several aryl and heteroaryl halides. Based upon preliminary mechanistic evidence and crystallographic probation of an active initiator species, we tentatively propose a potassium-stabilized 'metal-free' radical pathway is in operation.

1H-1,2,3-Triazol-5-ylidenes as Catalytic Organic Single-Electron Reductants

Most of the known single-electron reductants are either metal based reagents, used in a stoichiometric amount, or a combination of an organic species and a photocatalyst. Here we report that 1H-1,2,3-triazol-5-ylidenes act not only as stoichiometric one-electron donors but also as catalytic organic reducing agents, without the need of a photocatalyst. As a proof of concept, we studied the reduction of quinones, which are well-known electron conveyors that are involved in various biological and industrial processes. This work also provides experimental evidence for the formation of a bis(triazolium)carbonate adduct, which acts as the resting state of the catalytic cycle and as the carbene reservoir.

Radical coupling of aryl halides to arenes facilitated by Ni(COD)(DQ) and other nickel sources

The air-stable complex Ni(COD)(DQ) (COD = 1,5-cyclooctadiene, DQ = duroquinone) promotes the coupling of aryl halides to arenes in the presence of KOtBu. This complex has recently been shown to perform coupling reactions based on organonickel intermediates, but in this case the coupling reactions proceed via aryl radicals as shown by our newly developed assay for aryl radicals. Coupling with this nickel source is more efficient than with Ni(COD)2, Ni(PPh3)4 and Ni(acac)2, all of which we also show to operate through aryl radical pathways.

An assay for aryl radicals using BHAS coupling

Aryl radicals are intermediates in many reactions, but determining their presence unambiguously is often challenging. As we recently reported, reaction of 2-iodo-1,3-dimethylbenzene (7) in benzene with KOtBu and a suitable organic additive, leads to a base-induced homolytic aromatic substitution (BHAS) coupling reaction giving 2,6-dimethylbiphenyl (12) and biphenyl (3) as coupled products, together with xylene (13). In this case, biphenyl arises from a radical translocation and is the major coupling product. This paper now quantitatively investigates that reaction, which shows a very similar ratio for 3 : 12 [ca. 4 : 1] when using different sources of radical initiation. Deuterium isotope studies provide detailed mechanistic support for the proposed mechanism; when carried out in C6D6vs. C6H6, the reaction is characterised by a strong isotope effect for formation of 3-d10vs. 3, but not for formation of 12-d5vs. 12. These distinctive properties mean that the transformation can act as an assay for aryl radicals. An advantage of such a BHAS process is its sensitivity, since it involves a chain reaction that can amplify radical activity.

Facile access to benzofuran derivatives through radical reactions with heteroatom-centered super-electron-donors

The development of suitable electron donors is critical to single-electron-transfer (SET) processes. The use of heteroatom-centered anions as super-electron-donors (SEDs) for direct SET reactions has rarely been studied. Here we show that heteroatom anions can be applied as SEDs to initiate radical reactions for facile synthesis of 3-substituted benzofurans. Phosphines, thiols and anilines bearing different substitution patterns work well in this inter-molecular radical coupling reaction and the 3-functionalized benzofuran products bearing heteroatomic functionalities are given in moderate to excellent yields. The reaction mechanism is elucidated via control experiments and computational methods. The afforded products show promising applications in both organic synthesis and pesticide development.

A Hierarchy of Ligands Controls Formation and Reaction of Aryl Radicals in Pd-Catalyzed Ground-State Base-Promoted Coupling Reactions

Palladium salts and complexes were tested separately and in the presence of added ligands as potential sources of aryl radicals in ground-state coupling reactions of aryl halide with arenes under basic conditions (KOtBu). Our recently developed assay for aryl radicals was employed to test for aryl radicals. In this assay, aryl radicals derived from the test substrate, 1-iodo-2,6-dimethylbenzene 7, undergo base-promoted homolytic aromatic substitution (BHAS) with benzene to produce 2,6-dimethylbiphenyl 8 and biphenyl 9 in an approximately 1:4 ratio as well as m-xylene 10. The biphenyl arises from a diagnostic radical transfer reaction with the solvent benzene. Using substrate 7 with a range of Pd sources as potential initiators led to formation of 8, 9, and 10 in varying amounts. However, when any one of a range of diphosphinoferrocenes (e.g., dppf or dippf) or BINAP or the monophosphine, diphenylphosphinoferrocene, was added as a ligand to Pd(OAc)2, the ratio of [2,6-dimethylbiphenyl 8: biphenyl 9] moved decisively to that expected from the BHAS (radical) pathway. Further studies were conducted with dppf. When dppf was added to each of the other Pd sources, the ratio of coupled products was also diverted to that expected for radical BHAS chemistry. Deuterium isotope studies and radical trap experiments provide strong additional support for the involvement of aryl radicals. Accordingly, under these ground-state conditions, palladium sources, in the presence of defined ligands, convert aryl iodides to aryl radicals. A rationale is proposed for these observations.

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.

Visible-Light-Induced N-Heterocyclic Carbene-Catalyzed Single Electron Reduction of Mono-Fluoroarenes

Fluoroarenes are abundant and readily available feedstocks. However, due to the high reduction potentials of mono-fluoroarenes, their photoreduction remains a continuing challenge, motivating the development of efficient activation modes to address this issue. This report presents the blue light-induced N-heterocyclic carbene (NHC)-catalyzed single electron reduction of mono-fluoroarenes for biaryl cross-couplings. We discovered that under blue light irradiation, NHC/tBuOK combination could construct powerful photoactive architectures to promote single electron transfer for C_aryl -F bond reduction via forming highly reducing NHC radical anion. Notably, the strategy was also successful to reduce C_aryl -O, C_aryl -N, and C_aryl -S bonds for biaryl cross-couplings.

Addition of Benzyl Halides to Aldehydes and Imines Using Photoactivated TDAE: Access to 3,4-Dihydroisocoumarins, 1,2-Diarylethanols, and 1,2-Diarylcarbamates under Metal-Free Conditions

We describe herein the intermolecular addition reaction of benzyl halides to aldehydes and imines using photoactivated tetrakis(dimethylamino)ethylene (TDAE) as superphotoreductant. 3,4-Dihydroisocoumarins, 1,2-diarylethanols, and 1,2-diarylcarbamates were obtained with good functional group tolerance using simple, mild, and metal-free conditions.

Direct photo-induced reductive Heck cyclization of indoles for the efficient preparation of polycyclic indolinyl compounds

The photo-induced cleavage of C(sp2)-Cl bonds is an appealing synthetic tool in organic synthesis, but usually requires the use of high UV light, photocatalysts and/or photosensitizers. Herein is described a direct photo-induced chloroarene activation with UVA/blue LEDs that can be used in the reductive Heck cyclization of indoles and without the use of a photocatalyst or photosensitizer. The indole compounds examined display room-temperature phosphorescence. The photochemical reaction tolerates a panel of functional groups including esters, alcohols, amides, cyano and alkenes (27 examples, 50-88% yields), and can be used to prepare polycyclic compounds and perform the functionalization of natural product analogues in moderate to good yields. Mechanistic experiments, including time-resolved absorption spectroscopy, are supportive of photo-induced electron transfer between the indole substrate and DIPEA, with the formation of radical intermediates in the photo-induced dearomatization reaction.

Nucleophilic C-H Etherification of Heteroarenes Enabled by Base-Catalyzed Halogen Transfer

We report a general protocol for the direct C-H etherification of N-heteroarenes. Potassium tert-butoxide catalyzes halogen transfer from 2-halothiophenes to N-heteroarenes to form N-heteroaryl halide intermediates that undergo tandem base-promoted alcohol substitution. Thus, the simple inclusion of inexpensive 2-halothiophenes enables regioselective oxidative coupling of alcohols with 1,3-azoles, pyridines, diazines, and polyazines under basic reaction conditions.

Aryl radical-mediated N-heterocyclic carbene catalysis

There have been significant advancements in radical reactions using organocatalysts in modern organic synthesis. Recently, NHC-catalyzed radical reactions initiated by single electron transfer processes have been actively studied. However, the reported examples have been limited to catalysis mediated by alkyl radicals. In this article, the NHC organocatalysis mediated by aryl radicals has been achieved. The enolate form of the Breslow intermediate derived from an aldehyde and thiazolium-type NHC in the presence of a base undergoes single electron transfer to an aryl iodide, providing an aryl radical. The catalytically generated aryl radical could be exploited as an arylating reagent for radical relay-type arylacylation of styrenes and as a hydrogen atom abstraction reagent for α-amino C(sp³)–H acylation of secondary amides.

Use of aryl halides as coupling precursors typically occurs through transition metal catalysis and/or photoredox chemistry, which requires some combination of light, metals, and oxidants or reductants. Here, the authors show a method to generate aryl radicals from halides using only an NHC organocatalyst.

Photoinduced Cross-Coupling of Aryl Iodides with Alkenes

A protocol for photoinduced cross-coupling of aryl iodides having polar π-functional groups or elongated π-conjugation with alkenes has been developed. The radical cascade mechanism involving generation of aryl radicals via C-I bond homolysis of photoexcited aryl iodides and their subsequent addition to alkenes is proposed. The method enables iodide-selective cross-coupling over other halogen leaving groups with functional group compatibility on both arene and alkene motifs.

Small organic molecules with tailored structures: initiators in the transition-metal-free C–H arylation of unactivated arenes

A small organic molecule was tailored for the efficient synthesis of biphenyl and its derivatives from aryl iodides.

The base-induced regioselective radical arylation of 3-aminochromone with aryl hydrazine

A simple and efficient direct radical C-2 arylation of 3-aminochromone derivatives with aryl hydrazine under basic conditions is described.