Tuning the reduction potentials of benzoquinone through the coordination to Lewis acids

Exploring frustrated radical pairs through the persistent radical effect: methods of generation and recent applications

Radicals have long fascinated chemists owing to their structure, reactivity, and other features. The recent discovery of frustrated radical pairs (FRPs) has added a new dimension to this field. These unique radicals, which do not conform to traditional radical behavior, have opened a world of intriguing possibilities. FRPs have been categorized into neutral and ionic frustrated radical pairs and both are addressed as FRPs in this review. These pairs consist of two different (transient and persistent) radicals or radical ion pairs that do not react with each other. Such orthogonal reactivities and the resultant "persistent radical effect" enable chemical transformations that are difficult to achieve using traditional radical chemistry. This highlight uses recent examples to explore the different ways of generating these radical pairs and their working principle, highlighting the novelty and potential of this emerging field.

Mechanistic studies on single-electron transfer in frustrated Lewis pairs and its application to main-group chemistry

Advances in the field of frustrated Lewis pair (FLP) chemistry have led to the discovery of radical pairs, obtained by a single-electron transfer (SET) from the Lewis base to the Lewis acid. Radical pairs are intriguing for their potential to enable cooperative activation of challenging substrates (e.g., CH4, N2) in a homolytic fashion, as well as the exploration of novel radical reactions. In this review, we will cover the two known mechanisms of SET in FLPs-thermal and photoinduced-along with methods (i.e., CV, DFT, UV-vis) to predict the mechanism and to characterise the involved electron donors and acceptors. Furthermore, the available techniques (i.e., EPR, UV-vis, transient absorption spectroscopy) for studying the corresponding radical pairs will be discussed. Initially, two model systems (PMes3/CPh3+ and PMes3/B(C6F5)3) will be reviewed to highlight the difference between a thermal and a photoinduced SET mechanism. Additionally, three cases are analysed to provide further tools and insights into characterizing electron donors and acceptors, and the associated radical pairs. Firstly, a thermal SET process between LiHMDS and [TEMPO][BF4] is discussed. Next, the influence of Lewis acid complexation on the electron acceptor will be highlighted to facilitate a SET between (pBrPh)3N and TCNQ. Finally, an analysis of sulfonium salts as electron acceptors will demonstrate how to manage systems with rapidly decomposing radical species. This framework equips the reader with an expanded array of tools for both predicting and characterizing SET events within FLP chemistry, thereby enabling its extension and application to the broader domain of main-group (photo)redox chemistry.

Homolytic C-H Bond Activation by Phosphine-Quinone-Based Radical Ion Pairs

Herein, we present the formation of transient radical ion pairs (RIPs) by single-electron transfer (SET) in phosphine-quinone systems and explore their potential for the activation of C-H bonds. PMes3 (Mes=2,4,6-Me3 C6 H2 ) reacts with DDQ (2,3-dichloro-5,6-dicyano-1,4-benzoquinone) with formation of the P-O bonded zwitterionic adduct Mes3 P-DDQ (1), while the reaction with the sterically more crowded PTip3 (Tip=2,4,6-iPr3 C6 H2 ) afforded C-H bond activation product Tip2 P(H)(2-[CMe2 (DDQ)]-4,6-iPr2 -C6 H2 ) (2). UV/Vis and EPR spectroscopic studies showed that the latter reaction proceeds via initial SET, forming RIP [PTip3 ]⋅+ [DDQ]⋅- , and subsequent homolytic C-H bond activation, which was supported by DFT calculations. The isolation of analogous products, Tip2 P(H)(2-[CMe2 {TCQ-B(C6 F5 )3 }]-4,6-iPr2 -C6 H2 ) (4, TCQ=tetrachloro-1,4-benzoquinone) and Tip2 P(H)(2-[CMe2 {oQtBu -B(C6 F5 )3 }]-4,6-iPr2 -C6 H2 ) (8, oQtBu =3,5-di-tert-butyl-1,2-benzoquinone), from reactions of PTip3 with Lewis-acid activated quinones, TCQ-B(C6 F5 )3 and oQtBu -B(C6 F5 )3 , respectively, further supports the proposed radical mechanism. As such, this study presents key mechanistic insights into the homolytic C-H bond activation by the synergistic action of radical ion pairs.

Insights into Single-Electron-Transfer Processes in Frustrated Lewis Pair Chemistry and Related Donor-Acceptor Systems in Main Group Chemistry

The activation and utilization of substrates mediated by Frustrated Lewis Pairs (FLPs) was initially believed to occur solely via a two-electron, cooperative mechanism. More recently, the occurrence of a single-electron transfer (SET) from the Lewis base to the Lewis acid was observed, indicating that mechanisms that proceed via one-electron-transfer processes are also feasible. As such, SET in FLP systems leads to the formation of radical ion pairs, which have recently been more frequently observed. In this review, we aim to discuss the seminal findings regarding the recently established insights into the SET processes in FLP chemistry as well as highlight examples of this radical formation process. In addition, applications of reported main group radicals will also be reviewed and discussed in the context of the understanding of SET processes in FLP systems.

Visible-Light Photoredox-Catalyzed Divergent 1,2-Diacylation and Hydroacylation of Alkenes with Carboxylic Acid Anhydride

A photoredox-catalyzed divergent 1,2-dicarbonylation and hydroacylation of alkenes with acid anhydride is presented. This approach offers a mild and efficient entry to 1,4-dicarbonyl compounds bearing all-carbon quaternary centers, exhibiting a broad substrate scope and high functional group compatibility. Hydrocarbonylaltion of alkenes can also be realized by simply introducing a proton source to the reaction system. Mechanism investigations support a radical addition/radical-polar crossover cascade.

Increasing the oxidation power of TCNQ by coordination of B(C6F5)3

The oxidation power of the cyanocarbon TCNQ (tetracyano-quinodimethane) can be significantly increased to approximately E = +0.9 V vs. Cp₂Fe by coordination of up to four equivalents of the strong fluorinated Lewis acid B(C₆F₅)₃, resulting in a highly reactive but easy-to-use oxidation system. Thianthrene and tris(4-bromophenyl)amine were oxidized to the corresponding radical cations. Dianionic [TCNQ·4 B(C₆F₅)₃]^2- was formed upon reduction with two equivalents of ferrocene or decamethylcobaltocene. [TCNQ·4 B(C₆F₅)₃]^- and [TCNQ·4 B(C₆F₅)₃]^2- are rare cases of redox-active weakly-coordinating anions.