Ground State Cross-Coupling of Haloarenes with Arenes Initiated by Organic Electron Donors, Formed in situ: An Overview

Abstract

Many reactions have been discovered that lead to coupling of haloarenes to arenes using potassium tert-butoxide as the base, and one of a variety of organic compounds as an additive. The organic additive reacts with the base to form a strong organic electron donor in situ that initiates a base-induced homolytic aromatic substitution (BHAS) coupling reaction, by converting the haloarene into an aryl radical. This brief report presents an overview of the wide range of organic additives that can be used, and the organic electron donors that they form.

Radical–anion coupling through reagent design: hydroxylation of aryl halides

“…Thus, considering these results and our previous observations, it is reasonable to assume that many of the substitution reactions described herein likely proceed via an open-shell mechanism. We therefore propose that an electron-catalysed 7 c S RN 1 chain is initiated by either the formation and activation of a CTC, or a slow thermal (concerted) dissociative ET 20 from an anionic electron donor 21 ( e.g. the oxime anion 1 ) to the aryl halide 3 ( Scheme 4b ).…”

Radical condensation between benzylic alcohols and acetamides to form 3-arylpropanamides

“…Replacing the base with sodium tert-butoxide again gave exclusive formation of amide 2 although in a slightly lower yield due to some unreacted alcohol remaining (entry 14). A number of other bases were also included in the study, but very poor results were obtained in all cases (entries [15][16][17][18][19][20][21]. Consequently, the optimum conditions for the condensation uses 2 equiv.…”

“…A number of organic compounds have been shown to undergo deprotonation or double deprotonation by potassium tert-butoxide to generate anionic intermediates, which can serve as strong single electron donors. 21 A relevant example is the double deprotonation of 2-pyridinemethanol to yield the dianion capable of initiating haloarenearene couplings. 22 Thus, anion A is most likely deprotonated by potassium tert-butoxide at the elevated temperature to form PhCHO 2À , which will convert into radical anion B by single electron transfer to the acetamide (during the reaction) or to the aromatic moiety of the solvent or the alcohol (in the absence of the amide).…”

[2,3‐Bis‐(2‐pyridyl) pyrazine] as an Efficient Organocatalyst for the Direct C(sp2)‐H Arylation of Unactivated Arenes/Heteroarenes via C−H Bond Activation

“…In the last two decades, the concept of organocatalyzed C (sp 2 ) −H bond activation for the direct arylation of arenes/heteroarenes has been recognized as a straightforward methodology over traditional transition metal‐catalyzed cross‐coupling reactions . Being a novel and powerful substitute, the organocatalyzed C−H bond activation reactions have demonstrated greater forbearance for the diversity of functional groups and efficiently overcome several limitations associated with the transition metal‐catalyzed cross‐coupling reactions such as pre‐functionalization of substrate/reagents and sensitivity towards air and moisture .…”