Arylation of Unsaturated Compounds by Diazonium Salts (The M eerwein Arylation Reaction)

Abstract

The arylation of unsaturated compounds by diazonium salts with copper salt catalysis was first disclosed by Meerwein. Meerwein arylation proceeds best when the double bond is activated by an electron attracting groups such as carbonyl, cyano, aryl, vinyl, ethynyl or chloro. The net result is the union of the aryl group with the carbon atom beta to the activating group, either by substitution of a beta‐hydrogen atom of by addition of Ar and Cl to the double bond. The generally accepted mechanism of the reaction involves the aryl radical from the diazonium salt, though the manner of its formation and subsequent reaction is still controversial

A mild catalytic system for radical conjugate addition of nitrogen heterocycles

“…Here, we describe the development of a catalytic system for heteroaryl radical formation and direct coupling with electron-deficient alkenes, a reductive Meerwein arylation 9 process (illustrated in Fig. 1 ).…”

“…Aryl radicals are indispensable intermediates in organic synthesis, and redox processes of arenediazonium salts 8 , 9 , 16 or arylboronic acids 17 are reliable methods for their formation. However, these strategies are limited in the context of pyridine or diazine-based radical generation, due to the instability of the requisite heteroaryl-diazonium 18 a or -boronic acid reagents.…”

On the mechanism of photocatalytic reactions with eosin Y

“…They are readily available by diazotization of anilines, no toxic metals are required; the bond cleavage generates gaseous N 2 which escapes the reaction mixture. Photoredox reactions with arenediazonium salts are often more selective than traditional methods such as copper(II)-mediated Meerwein arylations [11] or protocols employing stoichiometric iron(II) or titanium(III) reductants in aqueous media [12–14]. This renaissance of arene diazonium chemistry has recently led to various applications of eosin Y to visible light-driven syntheses of biaryls [15], stilbenes [16], benzothiophenes [17], α-phenylketones [18], phenanthrenes [19], arylsulfides [20] and arylboron pinacolates [21] (Scheme 1).…”

Transition Metal‐Mediated Direct CH Arylation of Heteroarenes Involving Aryl Radicals

“…Then, CuCl reduces the diazonium salt into an aryl radical with concomitant loss of nitrogen. Whilst this mechanism could be acceptable in many cases, it cannot explain why in acetone‐free solvents, e.g., water, acetonitrile or NMP, the reaction still takes place nor why using directly CuCl tends to be less efficient than starting with CuCl 2 63. On the other hand, the formation of an arenediazonium tetrachlorocuprate(II) has been suggested when starting from CuCl 2 and arenediazonium chlorides, but this complex was not reduced by acetone 64…”