Fluoroalkanesulfinate Salts as Dual Fluoroalkyl and SO2 Sources: Atom-Economical Fluoroalkyl-Sulfonylation of Alkenes and Alkynes by Photoredox Catalysis

TBHP-Promoted Trifluoromethyl-difluoromethylthiolation of Unactivated Alkenes with CF3SO2Na and PhSO2SCF2H

A TBHP-promoted trifluoromethyl-difluoromethylthiolation of alkenes was reported. Langlois' reagent was used as a stable and inexpensive trifluoromethyl source. In the presence of TBHP, the trifluoromethyl radical generated reacted with alkenes, achieving a new alkyl radical, which could be trapped by PhSO2SCF2H, forming C-C and C-S bonds in one step and incorporating trifluoromethyl and difluoromethylthio groups. The mild conditions and broad functional group tolerance endowed the reaction with great potential in the field of pharmaceuticals and agrochemicals.

Dawn of photoredox catalysis

Photoredox catalysis, which facilitates organic transformations under visible-light irradiation, including sunlight, has garnered considerable attention as a cornerstone of green chemistry. Since the early days of this field around 2010, the author's group has made substantial contributions to its advancement. This review article provides a concise overview of the history and fundamental principles of photoredox catalysis, along with highlights of the achievements by the author's group. Although colorless organic compounds cannot be directly activated by visible light, photo-excited colored catalysts, with their two half-occupied frontier orbitals, play dual roles via electron transfer processes with organic substrates. The hole in the lower-energy orbital functions as a single-electron oxidant, whereas the electron in the higher-energy orbital acts as a single-electron reductant, enabling the formation of reactive radical intermediates from diverse organic compounds, including colorless ones. The discussion will focus on the key transformations developed by the author's group, including bimetallic photocatalysis, fluoroalkylation, and catalysis in aqueous media.

Electrochemical trifluoromethylation of alkynes: the unique role of DMSO as a masking auxiliary

Recent advancements in eco-friendly radical fluoroalkylation have substituted traditional two-electron-based reactions. However, the radical trifluoromethylation of terminal alkynes remains a significant challenge, primarily due to the high reactivity of alkenyl radical intermediates, which predominantly engage in reactions other than the desired elimination. In this work, we have developed an electrochemical trifluoromethylation method for terminal alkynes, facilitating the efficient formation of CF3-alkynes. The success of this method centers on the use of DMSO as a "masking auxiliary", which effectively stabilizes the alkenyl radical intermediate, allowing the reaction to proceed smoothly under mild conditions. This approach is supported by extensive experimental and computational studies, which elucidate the unique mechanism and expand the potential applications of radical trifluoromethylation across chemical synthesis.

Catalyst-Free Radical Carbosulfonylation of Enamides with Indoles, Aryldiazonium Tetrafluoroborates, and DABCO·(SO2)2

Herein, we have developed a catalyst-free four-component carbosulfonylation of enamides involving indoles, DABCO·(SO2)2, and aryldiazonium tetrafluoroborates for the preparation of various β-amidosulfone products in moderate to excellent yields. This approach features mild reaction conditions, high step-efficiency, and broad substrate scope, which provides a green and efficient strategy for carbosulfonyl difunctionalization of enamides. Based on the results of mechanism studies, a radical tandem reaction process is proposed for the transformation.

Red-light-mediated copper-catalyzed photoredox catalysis promotes regioselectivity switch in the difunctionalization of alkenes

Controlling regioselectivity during difunctionalization of alkenes remains a significant challenge, particularly when the installation of both functional groups involves radical processes. In this aspect, methodologies to install trifluoromethane (-CF3) via difunctionalization have been explored, due to the importance of this moiety in the pharmaceutical sectors; however, these existing reports are limited, most of which affording only the corresponding β-trifluoromethylated products. The main reason for this limitation arises from the fact that -CF3 group served as an initiator in those reactions and predominantly preferred to be installed at the terminal (β) position of an alkene. On the contrary, functionalization of the -CF3 group at the internal (α) position of alkenes would provide valuable products, but a meticulous approach is necessary to win this regioselectivity switch. Intrigued by this challenge, we here develop an efficient and regioselective strategy where the -CF3 group is installed at the α-position of an alkene. Molecular complexity is achieved via the simultaneous insertion of a sulfonyl fragment (-SO2R) at the β-position. A precisely regulated sequence of radical generation using red light-mediated photocatalysis facilitates this regioselective switch from the terminal (β) position to the internal (α) position. Furthermore, this approach demonstrates broad substrate scope and industrial potential for the synthesis of pharmaceuticals under mild reaction conditions.

Sequential Ring Opening/In Situ SO2-Capture/Alkynylation of Cyclopropanols with Alkynyl Triflones Initiated by Energy Transfer

A visible-light-triggered ring opening/in situ SO2-capture/alkynylation sequence of cyclopropyl alcohols with alkynyl triflones using 4CzIPN as a triplet energy transfer photocatalyst is herein described. This metal-free protocol provides a straightforward and atom-economical approach to alkynyl-substituted γ-keto sulfones with a broad scope of substituents. In this transformation, alkynyl triflones could be used as both radical acceptors and SO2 donors. Preliminary experimental mechanistic studies and synthetic utility are also demonstrated.

Zinc 1,1,2,2-Tetrafluoroethanesulfinate: A Synthetically Useful Oxidative and Photoredox Source of the 1,1,2,2-Tetrafluoroethyl Radical

1,1,2,2-Tetrafluoroethyl-containing molecules are of potential importance in drug discovery, but the efficient synthesis of such compounds is still relatively unexplored due to the lack of readily available reagents for the incorporation of the HCF2CF2 group. Herein, we introduce a new reagent, zinc 1,1,2,2-tetrafluoroethanesulfinate, which can be useful for the oxidative tetrafluoroethylation of arylboronic acids and heteroarenes as well as for a novel photoredox, three component hydro-tetrafluoroethylation of two alkenes of complementary reactivity.

Iron-catalyzed fluoroalkylative alkylsulfonylation of alkenes via radical-anion relay

Transition metal-catalyzed reductive difunctionalization of alkenes with alkyl halides is a powerful method for upgrading commodity chemicals into densely functionalized molecules. However, super stoichiometric amounts of metal reductant and the requirement of installing a directing group into alkenes to suppress the inherent β-H elimination bring great limitations to this type of reaction. We demonstrate herein that the difunctionalization of alkenes with two different alkyl halides is accessible via a radical-anion relay with Na2S2O4 as both reductant and sulfone-source. The Na2S2O4 together with the electron-shuttle catalyst is crucial to divert the mechanistic pathway toward the formation of alkyl sulfone anion instead of the previously reported alkylmetal intermediates. Mechanistic studies allow the identification of carbon-centered alkyl radical and sulfur-centered alkyl sulfone radical, which are in equilibrium via capture or extrusion of SO2 and could be converted to alkyl sulfone anion accelerated by iron electron-shuttle catalysis, leading to the observed high chemoselectivity.

Recent advances in the application of Langlois' reagent in olefin difunctionalization

In this review, we summarise the recent applications of Langlois' reagent in the radical-mediated difunctionalization of alkenes. Among the various trifluoromethylation reagents, Langlois' reagent is an exceptional compound, and many important organic transformations have been realized by employing such reagents. Various organic transformations of Langlois' reagent, especially in radical chemistry, have been developed in recent years. This review describes several key activation methods for Langlois' reagent in the difunctionalization of alkenes by showcasing selected cornerstone research areas and related mechanisms to stimulate the interest of readers in promoting the wider development and application of Langlois' reagent.

Synthesis of Multifluoromethylated γ-Sultines by a Photoinduced Radical Addition-Polar Cyclization

Despite the significance of sultines in synthesis, medicine, and materials science, the chemistry of sultines has remained unexplored due to their inaccessibility. Herein, we demonstrate the development of a photoredox-catalyzed multifluoromethyl radical addition/SO₂ incorporation/polar cyclization cascade approach to multifluoromethylated γ-sultines. The reactions proceed by single electron transfer induced multifluoromethyl radical addition to an alkene followed by SO₂ incorporation, and single-electron reduction for polar 5-exo-tet cyclization. Key to the success of the protocol is the use of easily oxidizable multifluoroalkanesulfinates as bifunctional reagents. The reactions proceed with excellent functional-group tolerance to deliver γ-sultines in moderate to excellent yields.

Desulfinative Alkylation of Heteroarenes via an Electrostatic Electron Donor-Acceptor Complex

Functionalized pyridine and quinoline rings are important components of numerous bioactive molecules and natural products; however, diversification of these rings often requires de novo heterocycle ring synthesis or demanding reaction conditions. We report a method for desulfinative alkylation of pyridine and quinoline N-methoxide salts that operates under both photocatalytic and electrostatic electron donor-acceptor-mediated pathways. Unlike most EDA-mediated processes, this reaction operates in the absence of light and with the desulfination of the donor compound.

Site-Selective Pyridylic C-H Functionalization by Photocatalytic Radical Cascades

An efficient pyridylic C(sp³ )-H functionalization has been developed through photocatalytic radical-mediated fluoroalkylation or cascade reactions. This method is enabled by the reversible formation of alkylidene dihydropyridine intermediates via the facile enolate formation of C4-alkyl N-amidopyridinium salts in the absence of an external base, thereby establishing the conditions necessary for subsequent intermolecular radical trapping. Rapid structural diversification of the pyridylic site can be achieved through photocatalytic multicomponent cascade reactions involving alkene trifluoromethylation, SO₂ -reincorporation, and sulfonyl radical addition. This operationally simple method features a broad substrate scope and high chemoselectivity and offers a unique approach for the rational modification of the heterobenzylic C-H bonds of pyridines and quinolines with uniform site-selective control. Furthermore, experimental and theoretical studies were performed to elucidate the reaction mechanism.

Recent advances in visible-light mediated functionalization of olefins and alkynes using copper catalysts

Over the past decade, visible-light photoredox catalysis has blossomed as a powerful strategy and offers a discrete activation mode complementary to thermal controlled reactions. Visible-light-mediated photoredox catalysis also offers exciting...

Graphene oxide-catalyzed trifluoromethylation of alkynes with quinoxalinones and Langlois' reagent

The direct C–H trifluoromethylation of alkynes and quinoxalinones has been achieved using a graphene oxide/Langlois' reagent system. This multi-component tandem reaction using graphene oxide as the catalyst and Langlois' reagent as the robust CF₃ radical source results in the formation of olefinic C–CF₃ to access a series of 3-trifluoroalkylated quinoxalin-2(1H)-ones.

The direct C–H trifluoromethylation of alkynes and quinoxalinones using a graphene oxide/Langlois' reagent system.

Photocatalytic functionalizations of alkynes

Visible light mediated functionalizations have significantly expanded the scope of alkynes by unraveling new mechanistic pathways and enabling their transformation to diverse structural entities. The photoredox reactions on alkynes rely on their innate capability to generate myriad carbon-centred radicals via single electron transfer (SET), thereby, allowing the introduction of new radical precursors. Moreover, an array of methods have been developed facilitating transformations such as vicinal or gem-difunctionalization, annulation, cycloaddition and oxidative reactions to construct numerous key building blocks of natural and pharmaceutically important molecules. In addition, the introduction of photoredox chemistry has successfully been used to deal with the challenges associated with alkyne functionalization such as stereoselective and regioselective control. This article accounts for several visible light mediated functionalization reactions of alkynes, wherein they have been transformed into α-oxo compounds, β-keto sulfoxides, substituted olefins, N-heterocycles, internal alkynes and sulfur containing compounds. The article has been primarily categorized into various sections based on the reaction type with particular attention being paid to mechanistic details, advancement and future applications.

Radical-mediated vicinal addition of alkoxysulfonyl/fluorosulfonyl and trifluoromethyl groups to aryl alkyl alkynes

The addition of sulfonyl radicals to alkenes and alkynes is a valuable method for constructing useful highly functionalized sulfonyl compounds. The underexplored alkoxy- and fluorosulfonyl radicals are easily accessed by CF₃ radical addition to readily available allylsulfonic acid derivatives and then β-fragmentation. These substituted sulfonyl radicals add to aryl alkyl alkynes to give vinyl radicals that are trapped by trifluoromethyl transfer to provide tetra-substituted alkenes bearing the privileged alkoxy- or fluorosulfonyl group on one carbon and a trifluoromethyl group on the other. This process exhibits broad functional group compatibility and allows for the late-stage functionalization of drug molecules, demonstrating its potential in drug discovery and chemical biology.

An unprecedented method for vicinal addition of alkoxysulfonyl/fluorosulfonyl and trifluoromethyl groups to aryl alkyl alkynes has been developed to afford useful alkenylsulfonate esters and alkenylsulfonyl fluorides.

Bifunctional reagents in organic synthesis

Developments in synthetic chemistry are increasingly driven by improvements in the selectivity and sustainability of transformations. Bifunctional reagents, either as dual coupling partners or as a coupling partner in combination with an activating species, offer an atom-economic approach to chemical complexity, while suppressing the formation of waste. These reagents are employed in organic synthesis thanks to their ability to form complex organic architectures and empower novel reaction pathways. This Review describes several key bifunctional reagents by showcasing selected cornerstone research areas and examples, including radical reactions, C-H functionalization, cross-coupling, organocatalysis and cyclization reactions.

Visible-light photoredox-catalyzed aryl radical in situ SO2-capture reactions

An aryl radical in situ SO₂-capture reaction is developed for the synthesis of various β-keto, allyl and alkynyl arylsulfone derivatives.