Asymmetric Oxidative Coupling of Phenols and Hydroxycarbazoles

Abstract

The first examples of asymmetric oxidative coupling of simple phenols and 2-hydroxycarbazoles are outlined. Generation of a more vanadium catalyst by ligand design and by addition of an exogenous Brønsted or Lewis acid was found to be key to coupling the more oxidatively resistant phenols. The resultant vanadium complex is both more Lewis acidic and more strongly oxidizing. Good to excellent levels of enantioselectivity could be obtained, and simple trituration readily provided the products with ≥95% ee.

Synthetic Applications of Oxidative Aromatic Coupling—From Biphenols to Nanographenes

“…Although numerous chiral catalysts have been developed for the asymmetric synthesis of BINOLs by oxidative coupling, the enantioselective preparation of chiral biphenyl derivatives has remained challenging until very recently. Kozlowski and co‐workers addressed this issue and performed an extensive catalyst optimization study using dinuclear vanadium catalysts as a starting point . It turned out that nitro‐substituted mononuclear vanadium complex 109 actually provided the best results, especially in the presence of acetic acid or lithium chloride as additives (Scheme ).…”

“…Kozlowski and co-workers addressed this issue and performed an extensive catalyst optimization study using dinuclear vanadium catalysts as astarting point. [91,97] It turned out that nitro-substituted mononuclear vanadium complex 109 actually provided the best results,especially in the presence of acetic acid or lithium chloride as additives (Scheme 17). A series of biphenols of type 108 were obtained in good yields and moderate ee values.…”

Enantioselective Vanadium-Catalyzed Oxidative Coupling: Development and Mechanistic Insights

“…Furthermore, no highly enantioselective oxidative phenol homocoupling was known prior to our initial report. 12 In 1999, Uang and coworkers reported oxidative phenol dimerization by means of vanadium catalyst (Scheme 3B, I). 13 Recently, we (Cr 14 ) Pappo (Fe-catalyzed 15 ) and Guo (Co 16 ) disclosed metal catalysts for selective oxidative phenol cross-coupling to bisphenols that are not atropisomerically stable (Scheme 2B, III).…”

The Diaryliodonium(III) Salts Reaction With Free-Radicals Enables One-Pot Double Arylation of Naphthols

“…Finally, some mono-annular phenols assays did not display observable reaction, presumably due to their higher REDOX potential (2.1 eV) compared with naphthols (1.87 eV) (Brodwel and Cheng, 1991;Lee et al, 2014;Kang et al, 2017). In these cases, a stronger radical initiator different to TMP 2 O or higher temperatures must be employed.…”

“…The chemistry of radicals is a powerful tool in organic synthesis allowing chemical transformations with high activation energy profiles via HAT (Capaldo and Ravelli, 2017), SET (Kita et al, 1994(Kita et al, , 1996Rosen and Percec, 2009) or SOMO (Beesson et al, 2007). Both C-and O-centered radical formation on the naphthol moiety are known processes carried out by metals such as Cu (Nakajima et al, 1999;Li et al, 2001), Ru (Irie et al, 2000), Fe (Egami and Katsuki, 2009;Narute et al, 2016), Cr (Nieves-Quinones et al, 2019), or V (Brodwel and Cheng, 1991;Hon et al, 2001;Lee et al, 2014;Kang et al, 2017). These radicals can also be generated electrochemically (Elsler et al, 2014) or through the radical anion sulfate (SO •− 4 ) (More and Jeganmohan, 2015).…”