Oxidative Homocoupling of Aryl, Alkenyl, and Alkynyl Grignard Reagents with TEMPO and Dioxygen

Abstract

Without a transition metal, unsaturated organomagnesium compounds undergo homocoupling in the presence of a substoichiometric amount of the oxidant 2,2,6,6‐tertramethylpiperidine‐N‐oxyl radical (TEMPO) and oxygen. Alkynyl Grignard reagents can even be coupled to give the corresponding diynes with dioxygen in the absence of a catalyst (see scheme).

TEMPO Derivatives as Alternative Mild Oxidants in Carbon-Carbon Coupling Reactions

“…[7][8][9] One equivalent of the oxidant was required to achieve good to excellent yields of the corresponding biaryl and dialkenyl products 5 under heating conditions (66°C) in THF. Comparable good yields were obtained with alkynylmagnesium reagents; however, significantly longer reaction times were normally required (4-6 h vs 5-30 min).…”

Nitroxide Radicals: Properties, Synthesis and Applications

“…238 Recently, "transition metal free" oxidative homocoupling reactions of aryl, alkenyl, and alkynyl based Grignard reagents have been reported using TEMPO as the stoichiometric oxidant. 239 The method appears to be versatile and very efficient. Reaction of TEMPO with Grignard reagents usually affords the corresponding alkoxyamines, as illustrated in Scheme 5.53.…”

Transition-Metal-Free Oxidative Homocoupling of Aryl, Alkenyl, and Alkynyl Grignard Reagents with TEMPO

“…In fact, we recently published that alkenyl, aryl, and alkynyl Grignard reagents 1 undergo highly efficient homocoupling upon oxidation with TEMPO to give 2. 3 This is surprising since transition metals are generally necessary to conduct such oxidative coupling reactions. 4 There is only one precedence in the literature on a high yielding homocoupling of Grignard reagents by using an organic oxidant as a coupling reagent.…”