Copper nitroxyl radicals

The last decades have witnessed the emergence of new living Vcontrolled polymerizations based on radical chemistry [81, 82]. Two main approaches have been investigated the first involves mediation of the free radical process by stable nitroxyl radicals, such as TEMPO while the second relies upon a Kharash-type reaction mediated by metal complexes such as copper(I) bromide ligated with 2,2 -bipyridine. In the latter case, the polymerization is initiated by alkyl halides or arenesulfonyl halides. Nitroxide-based initiators are efficient for styrene and styrene derivatives, while the metal-mediated polymerization system, the so called ATRP (Atom Transfer Radical Polymerization) seems the most robust since it can be successfully applied to the living Vcontrolled polymerization of styrenes, acrylates, methacrylates, acrylonitrile, and isobutene. Significantly, both TEMPO and metal-mediated polymerization systems allow molec-... 

Early reports of Cu-catalyzed aerobic alcohol oxidation focused on simple homogeneous complexes [11] with nitrogenous ligands, such as 2,2 -bipyridine and 1,10-phenanthroline. However, copper-catalyzed methods that include nitroxyl radical or azodicarboxylate cocatalysts exhibit significant synthetic advantages and have been widely investigated in the literature (Figure 6.1). 

Figure 6.3 Common nitroxyl radicals used in copper-catalyzed aerobic alcohol oxidation.

Subsequent kinetic, spectroscopic, and computational studies, however, support a different mechanistic pathway [28, 29]. A simplified version of the mechanism features oxidation of Cu and TEMPOH by Oj to afford a Cu -OH species and TEMPO (Scheme 6.1). The copper hydroxide and nitroxyl radical... 

Usually the stable nitroxyl radicals alone cannot directly catalyze the oxidation of alcohols with dioxygen or peroxide, so they rely on the assistance of various cocatalysts that play an important role in activating the oxidation agent. The most used cocatalysts are first row transition-metal complexes where Cu compounds with various N-donor ligands account for the prime ones. In many instances this combination serves as some kind of model to compare catalytic properties of copper compounds. For example, the performances of two asymmetric tetranuclear (with the Cu4(p—0)2(p — 0)2 404 core) and dinuclear (with the Cu2(p-0)2N202 core) copper(II) complexes were compared in the catalytic TEMPO-mediated aerobic oxidation ofbenzylic alcohols. In spite of their similarity, the complexes perform differently the tetranuclear copper(II) (R) complex is highly active leading to yields up to 99% and TONs up to 770, while the (S,R)-2 dinuclear complex is not so efficient under the same conditions. However, no solid explanation of the activity differences was proposed. 

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