3.13 Oxidative Coupling of Phenols and Phenol Ethers

Chromium–Salen Complex / Nitroxyl Radical Cooperative Catalysis:A New Combination for Aerobic Intramolecular Dearomative Coupling of Phenols

“…Other metal-salen complexes or chromium salts did not give the desired compound. Following the precedents about intermolecular phenol coupling reaction in the literature, 4g, 4i, 4n, 4o, 4p, 4r, 8 we found that using 1,1,1,3,3, as a solvent is crucial for the reaction to proceed in a catalytic manner (entry 2). The presence of a nitroxyl radical is also essential for the reaction (entry 3).…”

“…There are limited examples of precedents that enable successful intramolecular dearomative phenol coupling, although many schemes for intermolecular phenol (arenol) coupling reaction 3,4 have been reported (Scheme 1). Most of the schemes require a stoichiometric or excess amount of oxidant such as vanadium oxychloride (VOCl3) or a hypervalent iodine reagent.…”

“…This novel catalytic system provides various dienone compounds from unprotected bisphenol substrates in good yield under mild reaction conditions. On the basis of our experience in the development of an aerobic alcohol oxidation system using copper(I)-nitroxyl radical catalysis, 9, 10 we envisioned that the combination of a transition metal that is known to activate phenols appropriately 3,4 and a radical mediator such as a nitroxyl radical would promote the desired coupling reaction. Our initial design is shown in Scheme 2, in which individual but precise simultaneous activation of one phenol by a transition metal and the other phenolic hydroxy group by a nitroxyl radical via the hydrogen atom transfer (HAT) pathway is crucial to escape from non-selective intermolecular coupling and to obtain thereby the intended coupled product efficiently.…”

Interplay of Hydrogen and Halogen Bonding in the Crystal Structures of 2,6‐Dihalogenated Phenols

“…Both classes of compounds represent key intermediates in organic synthesis. [6][7][8][9] Aryl halides are of particular importance as electrophiles in CÀ C coupling reactions, [10][11][12][13] which makes the development of new, efficient synthetic strategies a worthwhile task. [14][15][16][17][18][19] In recent years, efforts have been made to develop new synthetic routes particularly to halogenated phenols, [20][21][22][23][24] which have also been used as antibacterial agents.…”

Chiral Dinuclear Vanadium Complex-Mediated Oxidative Coupling of Resorcinols

“…However, the number of methods for the enantioselective synthesis of axially chiral biphenols is still limited due to high oxidation potentials requiring strong oxidants and harsh conditions, especially when compared to that for the catalytic asymmetric synthesis of 1,1′-bi-2-naphthol (BINOL) using chiral metal catalysts [mononuclear metal catalysts: Smrćina and Koćovský (Cu 1993), Nakajima (Cu 1999), Katsuki (Ru 2000, Fe 2009), , Kozlowski (Cu 2001), Uang (V 2001), Chen (V 2001), Iwasawa (V 2004), Habaue (V, Cu 2005), , and Pappo (Fe 2016); dinuclear metal complex: Gong (V 2002), Gao (Cu 2003), Sasai and Takizawa (V 2004)]. Accordingly, significant research efforts have been dedicated to the development of protocols for the oxidative coupling of phenol derivatives using peroxide initiators, hypervalent iodines, and electrochemical methods …”