Pincerlike Cyclic Systems for Unraveling Fundamental Coinage Metal Redox Processes

Abstract

Pincerlike cyclic ligands have overcome the high instability of transition metals in their higher oxidation states and have permitted the isolation of such species and the exhaustive study of their properties and reactivity. The formation and isolation of organometallic Cu II and M III (M¼Cu, Ag, Au) complexes stabilized by NCPs, carbaporphyrins, carbaporphyrinoids, heterocalixarenes, and triaza macrocyclic ligands will be discussed in this chapter. The study of these complexes have led to the discovery of unprecedented reactivity and proved the plausibility of often invoked pathways in copper-catalyzed cross-coupling reactions. Aryl-M III (M¼Cu, Ag) stable species have been implicated as the key intermediate species that operate in coupling catalysis through two-electron redox cycles involving oxidative addition and reductive elimination fundamental steps.

Mechanistic Study on Cu(II)-Catalyzed Oxidative Cross-Coupling Reaction between Arenes and Boronic Acids under Aerobic Conditions

“…The clear-cut distinction between the reactivity of boronic acid with organocopper­(II) and organocopper­(III) compounds was not expected. Our previous studies have shown that azacalix­[1]­arene­[3]­pyridine-derived arylcopper­(III) complexes exhibit remarkable reactivity toward diverse nucleophiles ranging from halides, acetates, aryloxy and alkoxy, nitrate and amides, and thiocynate and cyanide to alkyl and alkynyl lithium reagents. ,− An analogous cross-coupling reaction property has also been observed by Stahl and Ribas ,,− for a similar macrocyclic arylcopper­(III) species which is derived from a m -phenylene-embedded azacrown ether . It should be addressed that the outcomes of the stoichiometric reactions of high valent arylcopper compounds with arylboronic acid were very important in comprehending the mechanism of catalysis.…”

“…Since the Cu­(II)/Cu(0) catalytic cycle in Figure was excluded, the aryl–aryl coupling is most likely to occur via reductive elimination from the diaryl copper­(III) intermediate B . Concurring with the documented results of the reactions of high-valent organocopper­(III) complexes, ,,− ,− , the reductive elimination from diarylcopper­(III) complex B was calculated to be almost barrierless (Figure S15 in the Supporting Information). Therefore, the central question was focused on the intriguing step of transmetalation of a boronic acid to organocopper species.…”

“…In this regard, a number of hypotheses comprising various oxidation states of copper and organocopper species have been proposed. − Reactions proceeding through plausible Cu­(III)/Cu­(I) ,, and Cu­(II)/Cu(0) − catalytic cycles, and the Cu­(II)/Cu­(I) , catalytic cycle through a single-electron transfer (SET) pathway, have been invoked to rationalize the reported Cu­(II)-catalyzed functionalizations of arene C–H bonds. By means of the systematic synthesis and reactivity studies of organocopper­(III) compounds using macrocyclic substrates, we, − Stahl, , and Ribas have provided sufficient experimental evidence that a Cu­(I)/Cu­(III) catalytic cycle operates in the C–H bond functionalization reactions with a number of “anionic” carbon and heteroatom nucleophiles. The aryl-Cu­(III) intermediates are formed through two distinct pathways: either an electrophilic metalation of arene followed by the oxidation of the resulting aryl-Cu­(II) species with a free copper­(II) ion or a proton-coupled electron transfer (PCET) reaction .…”

Silver(I)-Catalyzed C–X, C–C, C–N, and C–O Cross-Couplings Using Aminoquinoline Directing Group via Elusive Aryl-Ag(III) Species

“…[1][2][3][4][5] However, there is continuous need to discover new synthetic tools in order to have in hand versatile solutions for a given cross-coupling transformation. Silver has been completely forgotten in its use as catalyst for cross-coupling, [6][7] which typically undergoes 2-electron redox processes, since it is generally believed that Ag only shows 1-electron redox chemistry. [8][9] Nevertheless, albeit scarce, there are reports showing that silver catalyzes Sonogashira-type couplings 10 and Ullmann-type C-N and C-O couplings, 11 although with complete lack of mechanistic understanding.…”

The First Organosilver(III) Fluoride, [PPh4][(CF3)3AgF]

“…[13] Interestingly,t he importanceo fA g(III) intermediates in silver-catalyzed CÀX, CÀC, CÀN, and CÀOc rosscouplings was recentlyd emonstrated. [14] The handful of organosilver(III) compounds currently known are only stabilizedb y eitherm acrocyclic ligands [15] or by fluorinated methyl groups. [16] In fact, the non-fluorinated complexes [(CH 3 ) 3 AgI] À and [(CH 3 ) 4 Ag] À were found to be unstable.…”