Photoelectrochemical C−H Alkylation of Heteroarenes with Organotrifluoroborates

Abstract

Ap hotoelectrochemical method for the C À H alkylation of heteroarenes with organotrifluoroborates has been developed. The merger of electrocatalysis and photoredox catalysis provides ac hemical oxidant-free approach for the generation and functionalization of alkylr adicals from organotrifluoroborates.Avariety of heteroarenes were functionalized using primary,s econdary,a nd tertiary alkyltrifluoroborates with excellent regio-and chemoselectivity.

Electrophotocatalysis with a Trisaminocyclopropenium Radical Dication

“…With electrocatalysis,a na pplied potential effects the oxidation of as ubstrate through the intermediacyo fa ne lectrocatalyst (EC;F igure 1B), which obviates the need for external oxidants and can improve the kinetics of electron transfer, prevent electrode passivation, and provide catalyst control for selective transformations. Fore xample,X ud emonstrated the alkylation of arenes with organotrifluoroborates using photoredox catalysis coupled with anodic re-oxidation, [14] and Stahl developed aH ofmann-Lçffler-Freytagtype CÀHa mination reaction using electrochemical iodination paired with photochemical homolysis. Moutet and Reverdy demonstrated [11] the electrophotocatalytic oxidation of benzyl alcohol to benzal-dehyde,a lbeit with only three turnovers and no reported yield.…”

Neue Entwicklungen auf dem Gebiet der Minisci‐Reaktion

“…Die Autoren gehen davon aus, dass die direkte Oxidation des Tr ifluoroborats durch angeregtes Mes-Acr + den Reaktionszyklus initiiert und alle folgenden Oxidationen durch Persulfat oder Zersetzungsprodukte des Persulfats erreicht werden. [113] Ihrer Hypothese zufolge dient der Photoredoxzyklus der Radikalerzeugung während die elektrochemische Oxidation anstelle des üblicherweise bençtigten terminalen Oxidationsmittels stattfindet. Der Einsatz eines organischen Photokatalysators kann gegenüber anderen Methoden von Vorteil sein, die dieselben Produkte durch Einsatz von Edelmetallen liefern.…”

“…[112] Anfang 2019 berichteten Xu und Mitarbeiter von einer Minisci-Addition von Organotrifluoroboratena nH etero-arene,d ie auf der Arbeit von Molander aufbaut, statt eines terminalen Oxidationsmittels aber eine elektrochemische Oxidation nutzt. [113]…”

Electrophotocatalytic SNAr Reactions of Unactivated Aryl Fluorides at Ambient Temperature and Without Base

“…7). [15][16][17] In this Communication, we report that an electrophotocatalytic [18] strategy solves this problem and enables S N Ar reactions of unactivated aryl fluorides at room temperature and without strong base.…”

“…Am echanistic rationale for this chemistry is shown in Figure 2. Photoexcitation of DDQ (14)p roduces an excited state species 55 which is potent enough (E red = 3.18 Vv s. SCE) [22] to oxidize fluoroarene 11.N ucleophilic attack by pyrazole 9 to the resulting radical cation 57 then produces radical 58.W ep resume that 58 must undergo aone-electron reduction to furnish anion 59 in order to expel the fluoride leaving group to form product 21.H owever,b ecause DDQ radical anion 56 is apoor reductant, [15][16][17][18][19][20][21][22]27] the catalytic cycle likely cannot be closed by transfer of an electron from 56 to 58,b ut rather the reduction of 58 and the reoxidation of 56 must be accomplished by the electrotrodes.A dditional support for this mechanistic rationale is provided in the supporting information.…”