Radical anion chain process initiated by a dissociative electron transfer to a monocyclic endoperoxide.

Abstract

Electron transfer to 3,3,6,6-tetraphenyl-1,2-dioxane results in the cleavage of the oxygen-oxygen bond, generating a distonic radical anion intermediate whose fragmentation initiates an unprecedented radical anion chain process in competition with a second electron transfer.

A Radical‐Anion Chain Mechanism Initiated by Dissociative Electron Transfer to a Bicyclic Endoperoxide: Insight into the Fragmentation Chemistry of Neutral Biradicals and Distonic Radical Anions

“…[10][11][12] In these studies, it was discovered that the distonic radical anion can undergo a fragmentation in competition with the second heterogeneous ET [Eq. (3)].…”

“…The preference for a b-scission fragmentation was partially rationalized due to the increased flexibility in the monocyclic structure. [12] We now know of at least three competing processes available to the distonic radical anion generated electrochemically from the ET reduction of an endoperoxide: reduction, bscission fragmentation, and O-neophyl-type rearrangement. In the studies of the rigid bicyclic endoperoxides ASC, DASC, and ART with alkyl bridgehead substituents, only reduction of the distonic radical anion was observed to yield the corresponding cis-diol [Eq.…”

“…[6][7][8][9][10][11][12] These studies have not only provided previously unknown kinetic and thermochemical data, but also an insight into the reactivity of distonic radical anions, a reactive intermediate containing a spatially separated alkoxyl radical and alkoxyl anion. Initial studies focused on the di-A C H T U N G T R E N N U N G alkyl-substituted bicyclic endoperoxides, ascaridole (ASC) and dihydroascaridole (DASC), [6,7] and on the antimalarial drug, artemisinin (ART).…”

Efficient Homogeneous Radical‐Anion Chain Reactions Initiated by Dissociative Electron Transfer to 3,3,6,6‐Tetraaryl‐1,2‐dioxanes

“…Using various electrochemical techniques, we evaluated previously unknown kinetic and thermochemical data, delineated the reduction mechanism, and provided insight into the fragmentation chemistry of neutral biradicals and analogous distonic radical anions. [7,8] In addition to expanding on our original study on the electrochemical reduction of 1 a, [24] we extend our discussion to include three other derivatives: the methoxy-aryl-substituted TAD 1 b, a bicyclic analogue 1 c, and the nitro/methoxy-aryl-substituted TAD 1 d (Scheme 1). We report a thorough study of the ET-initiated reduction by employing cyclic voltammetry, constant potential electrolyses, DFT calculations, and digital simulation of the experimental CVs in order to obtain a better understanding of the ET mechanism and to evaluate kinetic and thermodynamic information, in particular, the rate constant for b-scission fragmentation of the distonic radical anions.…”

“…[23] The investigation of the ET reduction of 3,3,6,6-tetraphenyl-1,2-dioxane (TAD; 1 a) provided another reaction pathway for the distonic radical anion. [24] In our initial report, we proposed ET resulted in O À O bond cleavage generating a distonic radical anion that reacts by a b-scission fragmentation in competition with ET reduction of the oxygen-centered radical. We proposed the intermediates generated by fragmentation were responsible for initiating a homogeneous radical-anion chain reaction that accounted for the uncommon features observed in the cyclic voltammograms, notably an oxidative dip after the dissociative wave, and the pronounced potential dependence on the product ratios and low charge consumption values from the electrolysis studies.…”