Frustrated Radical Pairs: Insights from EPR Spectroscopy

Abstract

Progress in frustrated Lewis pair (FLP) chemistry has revealed the importance of the main group elements in catalysis, opening new avenues in synthetic chemistry. Recently, new reactivities of frustrated Lewis pairs have been uncovered that disclose that certain combinations of Lewis acids and bases undergo single-electron transfer (SET) processes. Here an electron can be transferred from the Lewis basic donor to a Lewis acidic acceptor to generate a reactive frustrated radical pair (FRP). This minireview aims to showcase the recent advancements in this emerging field covering the synthesis and reactivities of frustrated radical pairs, with extensive highlights of the results from Electron Paramagnetic Resonance (EPR) spectroscopy to explain the nature and stability of the different radical species observed.

Keywords: electron paramagnetic resonance; frustrated Lewis pair; main group chemistry; radicals; single-electron transfer.

Scheme 1

Generic representation of the reactivity of FLPs and FRPs in the activation of small molecules.

Figure 1

Isotropic and anisotropic EPR spectra of Mes₃P^.+ (1 left), and (Mes₂P)₂ ^.+ (15 right), simulated using data reported in Table 1.

Scheme 2

P/Al FLP‐mediated N₂O and C–H activation of toluene and bromobenzene.

Scheme 3

Contrasting reaction pathways between FLPs and FRPs when using tBu₃P/Mes₃P and B(C₆F₅)₃.

Scheme 4

P/B FLP‐mediated homolytic cleavage of peroxides.

Scheme 5

The divergent reactivities of FLPs towards diaryl esters.

Figure 2

Isotropic CW EPR spectra of 12, 10 and 7 (left), and 8‐TMS and 5 (right), simulated using data reported in Table 1.

Scheme 6

Single‐electron oxidation of an amine by B(C₆F₅)₃.

Scheme 7

Generation of radical species with or without light (top), and C−C bond formation reaction through single‐electron transfer (bottom).

Scheme 8

Synthesis of carbon‐based radical species bearing a N,N′‐dimesityldiamidocarbene group.

Scheme 9

Generation of radical pair species bearing Ge and B.

Scheme 10

Generation of carbon‐ and phosphorus‐based radical pairs.

Scheme 11

Formation of carbon‐based radicals.

Scheme 12

Germanium‐ and carbon‐/silicon‐based radical pairs.

Scheme 13

Reaction of silylium borate with Tipp₃P and Mes₃P (top), and the synthesis of solvent‐stabilized silylium borates and subsequent generation of phosphorus‐ and silicon‐based radicals (bottom).

Scheme 14

Synthesis and reactivity of intramolecular FLP‐NO radicals.

Scheme 15

Formation of persistent nitroxide radicals under frustrated Lewis pair conditions.

Scheme 16

Single‐electron oxidation of tBu₃P by a nitrosonium borate salt.