A Short‐Lived Radical Dication as a Key Intermediate in the Rearrangement of a Persistent Cation: The Oxidative Cyclization of 2,2‐Dimesityl‐1‐(4‐N,N‐dimethylaminophenyl)ethenol

Abstract

Almost completely unknown—the chemistry of radical dications. A reactive intermediate of this type (1•2+) plays a key role in the oxidative rearrangement of the persistent cation 1+.

Enol Radical Cations in Solution. 13. First Example of a Radical Dication Rearrangement. One-Electron Oxidation of Dihydrobenzofuranyl Cations Leads to Drastic Rate Enhancement in the Oxidative Benzofuran Formation from Enols

“…The second wave at E pa = 0.76 V Fc (wave Ib in Figure a) in the CV of E1 could be readily assigned to the oxidation of the protonated enol E1 − H + by control experiments in the presence of added base (4- N , N -(dimethylamino)pyridine) and acid (trifluoromethanesulfonic acid). The oxidation chemistry of E1 − H + has already been extensively described and shall not be repeated here . Wave II in the CV of enol E1 seems to correspond to the second waves in the CVs of the other enols as judged by its anodic shift vs E1 (Table ).…”

“…In the context of our ongoing investigations on the chemistry of enol radical cations in solution , we have discovered the rare event of a selective activation of a cationic intermediate via the corresponding radical dication, which shall be described here in detail . Since radical dications are usually formed through one-electron oxidation of electron-deficient cationic precursors, this class of mostly highly reactive intermediates is difficult to access.…”

“…The [1,2]-methyl shift constitutes the slowest step in the oxidative benzofuran formation and is slower the more electron-releasing the substituent R. The rate constant of the 1,2-methyl shift in the phenyl-substituted dihydrobenzofuranyl cation X + (R = Ph) was determined to be k = 1.8 × 10 -4 s -1 at −20 °C, whereas when stabilization is exerted by a p -C 6 H 4 NMe 2 group, the rate constant at room temperature is k = 2.5 × 10 -3 s -1 …”

Enol radical cations in solution. Part 12. Synthesis and electrochemical investigations of a stable enol linked to a ferrocene redox centre †

“…Of the two intermediates 14 ϩ and 15 ϩ the cyclohexadienyl cation 14 ϩ indeed is known to be long-lived when electron-releasing substituents are attached. 15 In contrast, intermediate 15 ϩ should be rather shortlived because (a) its deprotonation should proceed very rapidly even in neutral media; and (b) the oxidation wave at E 1/2 = 0.46 V Fc appeared under basic conditions. Hence, we may tentatively assign the reversible oxidation wave at E 1/2 = 0.46 V Fc to the oxidation of 14 ϩ to 14 ؒ2ϩ .…”

Electronic effects in polyarylvinyl propellers. Solid state structures and dynamic behaviour in solution of several crowded enol derivatives

“…These compounds were prepared by derivatisation of the appropriate enols, as described elsewhere. 6 The enols 12 and 13 7 were studied for comparison.…”