Radical Cation Diels‐Alder Reactions of Non‐Conjugated Alkenes as Dienophiles by Electrocatalysis

Abstract

of main observation and conclusion Radical cation Diels-Alder reactions provide a powerful method for the construction of six-membered ring systems between both electron-rich dienes and dienophiles. However, the most recent examples of this class have been limited to -methylstyrenes as dienophiles; the use of non-conjugated alkenes remains challenging. The present study describes the serendipitous development of novel radical cation Diels-Alder reactions by electrocatalysis that use non-conjugated alkenes as dienophiles. The key to successful transformation involves highly substituted cyclohexenyl radical cations that are stable enough to be reduced by intermolecular single electron transfer. www.cjc.wiley-vch.deOzaki et al.could be used as a dienophile; [15] however, the use of non-conjugated alkenes remains challenging. We questioned whether the redox tag strategy could be applied to overcome this limitation. Described herein is the serendipitous development of novel radical cation Diels-Alder reactions of non-conjugated alkenes as dienophiles by electrocatalysis.www.cjc.wiley-vch.de

Radical Cation [4+2] Cycloaddition of Non‐Conjugated Tetrasubstituted Alkenes by an FeCl3/AgSbF6 Co‐Initiator

“…Over the past few decades, a variety of strategies have been developed to generate the corresponding radical cation from electron‐rich dienophiles such as anetholes [4] ( A ) and heteroatom‐substituted alkenes ( B ) including vinyl ethers, [5] vinyl sulfides [6] and enamines [7] . Tetrasubstituted alkene ( C ) shows an oxidation potential comparable to those of the dienophiles, [8,9] and is likely to be oxidized to generate a radical cation, but little is known about its application to radical cation [4+2] cycloaddition [10] . Practical use of such readily accessible hydrocarbons for constructing valuable six‐membered rings is synthetically attractive, and therefore further study is desirable.…”

“…When 10 mol% of Fe(OTf) 3 was used, [4+2] cycloaddition of alkene 1 a with 2,3‐dimethyl‐1,3‐butadiene 2 proceeded, to afford the cycloadduct 3 a in 32% yield (entry 1). Encouraged by the works of Chiba, Kamiya, and Okada, [10,15] we synthesized alkenes with a non‐conjugated electron‐rich aromatic group which facilitates radical cation cycloadditions by intramolecular single electron transfer (SET). As intended, substituting a phenyl group for a 4‐methoxyphenyl group dramatically improved the yield (entry 2), while 1 c with a longer alkyl chain failed to work as well (entry 3).…”

“…[7] Tetrasubstituted alkene (C) shows an oxidation potential comparable to those of the dienophiles, [8,9] and is likely to be oxidized to generate a radical cation, but little is known about its application to radical cation [4 + 2] cycloaddition. [10] Practical use of such readily accessible hydrocarbons for constructing valuable six-membered rings is synthetically attractive, and therefore further study is desirable.…”

Construction and Application of Interfacial Inorganic Nanostructures

“…[14,15] Electrocatalysts, which are efficient and durable, play a crucial role in the green energy conversion technologies with lower energy consumption. [16][17][18][19][20] Currently, noble metal-based nanomaterials (e.g., Pt, Pd, Ru, and Ir) with high activity, selectivity, and stability, have attracted great attention. [21][22][23] Still, the high cost and low abundance significantly impede their large-scale industrialized applications.…”

Mechanistic Insights on Concentrated Lithium Salt/Nitroalkane Electrolyte Based on Analogy with Fluorinated Alcohols

“…A previous study proposed that the use of a fluorinated alcohol such as HFIP or 2,2,2-trifluoroethanol (TFE) is efficient both as a solvent and as an additive for electrochemical hole catalysis. [7] On the other hand, our group has also reported the use of lithium salt/nitroalkane as a promising electrolyte for hole catalysis, [9] such as in a Diels-Alder reaction, [10][11][12] [2+2] cycloaddition [13][14][15][16][17] and olefin metathesis. [18] Despite its great potential as a reaction medium in hole catalysis, the exact role of this electrolyte has been a long-standing mystery.…”