Precise Introduction of the -CHnX3-n (X = F, Cl, Br, I) Moiety to Target Molecules by a Radical Strategy: A Theoretical and Experimental Study

Haloalkane-driven dichotomous reactivity of aryl radicals as halogen and hydrogen atom transfer agents: photocatalytic olefin and alkyne functionalization cascades

A visible-light mediated protocol for the annulative trifluoroethylation, perfluoroalkylation and di/tri-chloromethylation of olefins and alkynes tethered to aromatic rings is described. Both PIDA and diazonium salts can independently promote this transformation by simultaneously acting as both an oxidant and halogen atom transfer reagent. The reaction is applicable to multiple precursor classes, leading to the synthesis of various N- and O-containing ring systems including dihydroisoquinolinones, coumarins, fused benzimidazoles, and quinazolinone. The activation of chloroform and dichloromethane was also achieved by hydrogen atom transfer, displaying a mechanistic dichotomy of aryl radicals.

C(sp3)-F Bond Activation by Lewis Base-Boryl Radicals via Concerted Electron-Fluoride Transfer

Selective C-F bond activation through a radical pathway in the presence of multiple C-H bonds remains a formidable challenge, owing to the extraordinarily strong bond strength of the C-F bond. By the aid of density functional theory calculations, we disclose an innovative concerted electron-fluoride transfer mechanism, harnessing the unique reactivity of Lewis base-boryl radicals to selectively activate the resilient C-F bonds in fluoroalkanes. This enables the direct abstraction of a fluorine atom and subsequent generation of an alkyl radical, thus expanding the boundaries of halogen atom transfer reactions.

Light-Induced Difunctionalization of Alkenes with Polyhaloalkanes and Quinoxalin-2(1H)-ones

Herein, we report a metal-free light-induced three-component reaction for the synthesis of polychloroalkyl-substituted quinoxalin-2(1H)-ones using commercially available alkenes, polyhalo alkanes, and quinoxalin-2(1H)-ones. Preliminary mechanistic studies suggested the generation of radical intermediates via an EDA-complex, single electron transfer, or halogen atom transfer pathway. Under mild reaction conditions, various alkenes and quinoxalin-2(1H)-ones containing different functional groups are compatible, providing the corresponding polychloroalkyl-substituted quinoxalin-2(1H)-ones in moderate to good yields.

Screening seven-electron boron-centered radicals for dinitrogen activation

The activation of dinitrogen is significant as nitrogen-containing compounds play an important role in industries. However, the inert NN triple bond caused by its large HOMO-LUMO gap (10.8 eV) and high bond dissociation energy (945 kJ mol-1 ) renders its activation under mild conditions particularly challenging. Recent progress shows that a few main group species can mimic transition metal complexes to activate dinitrogen. Here, we demonstrate that a series of seven-electron (7e) boron-centered radical can be used to activate N2 via density functional theory calculations. It is found that boron-centered radicals containing amine ligand perform best on the thermodynamics of dinitrogen activation. In addition, when electron-donating groups are introduced at the boron atom, these radicals can be used to activate N2 with low reaction barriers. Further analysis suggests that the electron transfer from the boron atom to the π* orbitals of dinitrogen is essential for its activation. Our findings suggest great potential of 7e boron radicals in the field of dinitrogen activation.

Cross-Electrophile C-PIII Coupling of Chlorophosphines with Organic Halides: Photoinduced PIII and Aminoalkyl Radical Generation Enabled by Pnictogen Bonding

Pnictogen bonding (PnB) has gained recognition as an appealing strategy for constructing novel architectures and unlocking new properties. Within the synthetic community, the development of a straightforward and much simpler protocol for cross-electrophile C-PIII coupling remains an ongoing challenge with organic halides. In this study, we present a simple strategy for photoinduced PnB-enabled cross-electrophile C-PIII couplings using readily available chlorophosphines and organic halides via merging single electron transfer (SET) and halogen atom transfer (XAT) processes. In this photomediated transformation, the PnB formed between chlorophosphines and alkyl amines facilitates the photogeneration of PIII radicals and α-aminoalkyl radicals through SET. Subsequently, the resulting α-aminoalkyl radicals activate C-X bonds via XAT, leading to the formation of carbon radicals. This methodology offers operational simplicity and compatibility with both aliphatic and aromatic chlorophosphines and organic halides.

Photocatalytic, Intermolecular Olefin Alkylcarbofunctionalization Triggered by Haloalkyl Radicals Generated via Halogen Atom Transfer

A visible-light-mediated, haloalkyl-radical-initiated, three-component olefin difunctionalization is reported. The application of haloalkyl radicals generated via halogen atom abstraction by α-aminoalkyl radicals has been demonstrated for accessing a new halogenated chemical space. Overall, the alkylcarbofunctionalization of styrenes was accomplished by employing them as (poly)haloalkyl radical acceptors and subsequent C-C bond formation with quinoxalinones.

Photoredox-catalysed chlorination of quinoxalin-2(1H)-ones enabled by using CHCl3 as a chlorine source

A photoredox-catalysed chlorination of quinoxalin-2(1H)-ones was developed by using CHCl₃ as a chlorine source, thus affording various 3-chloroquinoxalin-2(1H)-ones in moderate to high yields. This protocol is characterized by mild reaction conditions, excellent regioselectivity, and readily available chlorination agent. Considering the operational simplicity and low cost of this chlorination approach, this developed method offers an innovative pathway for rapid incorporation of chlorine functionality into heteroarenes, and will inspire broader exploitation of new chlorination strategies.

The polychloromethylation/acyloxylation of 1,6-enynes with chloroalkanes and diacyl peroxides through dual-role designs

The novel polychloromethylation/acyloxylation of 1,6-enynes with chloroalkanes and diacyl peroxides through dual-role designs has been developed to prepare 2-pyrrolidinone derivatives with polychloromethyl units with the use of an inexpensive copper salt under mild conditions. This strategy includes two dual-role designs, not only improving atomic utilization but also allowing a cleaner process. The wide substrate scope and simple reaction conditions demonstrate the practicability of this protocol.

Theoretical study for evaluating and discovering organic hydride compounds as novel trifluoromethylation reagents

Trifluoromethylation reaction is one of the significant and practical organic chemical reactions, and the design and discovery of novel trifluoromethylation reagents have been attracting more and more attention. Trifluoromethyl-substituted organic hydride compounds (XH) have the potential to be novel trifluoromethylation reagents in organic synthesis due to the favorable tendency of XH˙⁺ releasing ˙CF₃ to form stable aromatic structures in terms of thermodynamics. The key elementary step of the trifluoromethylation is the radical cation (XH˙⁺) generation by catalysis or single-electron activation releasing ˙CF₃ to form a stable aromatic structure, which also provides the thermodynamic driving force of the chemical process. In this work, 47 new trifluoromethylation reagent candidates of XHs were designed and calculated for the Gibbs free energy and activation free energy [ΔG^‡_RD(XH˙⁺)] of XH˙⁺ releasing ˙CF₃ using the density functional theory (DFT) method, in order to quantitatively measure the reactivity of XHs as trifluoromethylation reagents, and to establish the molecular library as well as reactivity database of novel trifluoromethylation reagents for synthetic chemists. According to the and ΔG^‡_RD(XH˙⁺) values, all the XHs can be reasonably divided into 3 classes, including class 1 (excellent trifluoromethylation reagents), class 2 (potential trifluoromethylation reagents) and class 3 (not trifluoromethylation reagents). To our delight, 15 XHs with a 1,4-dihydropyridine structure and 3 XHs with a 3,4-dihydropyrimidin-2-one structure are identified to be novel excellent and potential trifluoromethylation reagents, respectively, according to their reactivity data. The relationship between the structural features, including methylation, heteroatom, substituents, conjugated structure and so on, and the reactivity of XHs as trifluoromethylation reagents are also discussed in this work. The computation results indicate that trifluoromethyl-substituted 1,4-dihydropyridine compounds and 3,4-dihydropyrimidin-2-one analogues could be possible trifluoromethylation reagents in organic synthesis. This work may provide the theoretical basis and references for discovering organic hydride compounds as novel reagents for trifluoromethylation or other alkylation reactions.

Application of α-Aminoalkyl Radicals as Reaction Activators

α-Aminoalkyl radicals are easily accessible through multiple pathways from various precursors. Apart from their utilization as nitrogen-containing building blocks, they have recently been used as halogen atom abstraction reagents or single-electron reductants to transform organic halides or sulfonium salts into their corresponding highly reactive radical species. Benefiting from the richness of various halides and the diverse reactivity of radical intermediates, new transformations of halides and sulfonium salts have been developed. This short review summarizes this emerging chemistry that uses α-aminoalkyl radicals as the reaction activators.

1 Introduction

2 Activation of Halides as Halogen-Atom Transfer Agents

2.1 Addition to Unsaturated Bonds

2.1.1 Addition to C=C Bonds

2.1.2 Addition to C=O Bonds

2.2 Substitution Reactions

2.2.1 Deuteration

2.2.2 Olefination

2.2.3 Allylation

2.2.4 Aromatic Substitution

2.2.5 Amination

3 Activation of Sulfonium Salts as Single-Electron Reductants

4 Conclusion and Outlook