Electro‐mediated PhotoRedox Catalysis for Selective C(sp³)–O Cleavages of Phosphinated Alcohols to Carbanions

Abstract

We report a novel example of electro‐mediated photoredox catalysis (e‐PRC) in the reductive cleavage of C(sp3)−O bonds of phosphinated alcohols to alkyl carbanions. As well as deoxygenations, olefinations are reported which are E‐selective and can be made Z‐selective in a tandem reduction/photosensitization process where both steps are photoelectrochemically promoted. Spectroscopy, computation, and catalyst structural variations reveal that our new naphthalene monoimide‐type catalyst allows for an intimate dispersive precomplexation of its radical anion form with the phosphinate substrate, facilitating a reactivity‐determining C(sp3)−O cleavage. Surprisingly and in contrast to previously reported photoexcited radical anion chemistries, our conditions tolerate aryl chlorides/bromides and do not give rise to Birch‐type reductions.

Electrochemical Activation of Diverse Conventional Photoredox Catalysts Induces Potent Photoreductant Activity**

“…[25] However,w hile these two discoveries validated the use of electrochemistry to generate potent photoreductants,both of these electrophotocatalysts remained structurally analogous to established conPET-based photocatalysts.W hile rapid progress has been made in net-oxidative electrophotocatalytic transformations, [40][41][42][43][44][45][46][47][48] electrophotocatalytic reductions remain comparatively underdeveloped. [49,50] Herein, we employ cathodic reduction to elicit new photocatalytic activity from numerous organic and inorganic structures.T hese studies reveal an ew electron-primed photoredox catalyst that enables cleavage of strong C(sp 2 ) À Na nd C(sp 2 ) À O bonds.…”

How Radical are ‘Radical’ Photocatalysts? A Closed-Shell Meisenheimer Complex is Identified as a Super-reducing Photoreagent

“…We note that the excitation spectrum in Figure 3C is the same as that recently assigned to an excitation spectrum of a purported quartet excited state of NMI -• . 27 As the excitation spectrum is not that of the NMI radical anion, our result establishes that emission originates from the excited state of the two-electron reduced NMI species and not from either a spin-allowed or a spin forbidden excited state of the radical anion.…”

Synthetic Molecular Photoelectrochemistry: New Frontiers in Synthetic Applications, Mechanistic Insights and Scalability

“…Adduct 33 or related species may be a candidate for the emitting state (ES 1 ) in Barham, König and co‐workers’ study, [19q] but comparisons are still questionable due to 1) UV‐vis absorptions ( λ max ) of NpMI . − agreeing within ±10 nm despite different solvents (DMAc [19r] vs. MeCN) in the two studies, [19q] yet ES 1 differing in peak shape and λ max (by ca. 40 nm); [19q] 2) notably lower cell potentials for electrolysis (U cell =−1.6 V) used both in spectroscopy and reactions where U cell =−3.0 V led to intractable complex mixtures [19q] .…”

“…− s UV‐vis/EPR signals upon irradiation in the presence of substrates [4a, 19q, s] and 4) absence of quenching of longer (ns or μs)‐lived emitting species derived from radical anion PRCat . ‐ s in other reports [4g, 19q] . Closed‐shell anionic species may be reservoirs/precursors to radical anion photocatalysts as proposed by Miyake and co‐workers [4g] …”