Oxidative aryl-coupling

P/A/)-5,6,7,8-Tetrahydrodibenzo ,c)cycloocta-l,3-dienes Oxidative Aryl Coupling with Thallium(lll) Oxidants General Procedures13 ... 

A broad range of biaryl structures, as is often encountered in various classes of naturally occurring compounds, such as alkaloids, lignans, and tannins, can be prepared by oxidative arylic coupling. Oxidative couplings have also been used to build non-natural skeletons, such as the binaphthol derivatives that play an important role in asymmetric synthesis. 

Further examples of oxidative arylic coupling within this family of lignan precursors include the preparation of (+)-schizandrin (169) and (+)-gomisin A (170) (Figure 5), as published by Tanaka and co-workers. In this instance, the key Ar-Ar coupling step with 194 was realized using either ruthenium tetrakis(trifluoroacetate) or iron perchlorate (Scheme 47) [124, 131-133]. As pointed out by the authors, this result was noteworthy as this simple... 

These examples illustrate one important point whereas oxidative arylic coupling is the natural (biosynthetic) pathway for linking two aryl units, it cannot be assumed that reproducing the reaction in vitro will furnish the same stereochemical outcome. This same issue of atrop-selectivity upon biaryl coupling was addressed by Evans and co-workers in an attempted biomimetic synthesis of 197, the M(5-7) unit of vancomycin [134, 135]. The more stable, natural isomer 197b is obtained only as a minor component in this kinetically controlled Ar-Ar coupling reaction (Scheme 48). However, subsequent thermal equilibration leads to a preponderance of the desired compound. 

The most intriguing work in the field of asymmetric oxidative aryl coupling has been directed towards finding catalytic enantioselective reactions. The main goal in these studies has been the synthesis of chiral binaphthyl units as an improvement over stoichiometric chiral reagent enantioselective syntheses. 

Photocatalytic enantioselective oxidative arylic coupling reactions have been investigated by two different groups. Both studies involved the use of ruthenium-based photocatalysts [142, 143]. In 1993, Hamada and co-workers introduced a photostable chiral ruthenium tris(bipyridine)-type complex (A-[Ru(menbpy)3]2+) 210 possessing high redox ability [143]. The catalytic cycle also employed Co(acac)3 211 to assist in the generation of the active (A-[Ru(menbpy)3]3+) species 212. The authors suggested that the enantioselection observed upon binaphthol formation was the result of a faster formation of the (R)-enantiomer from the intermediate 213 (second oxidation and/or proton loss), albeit only to a rather low extent (ee 16 %) (Scheme 54). 

Lessene, G. and Feldman, K.S. (2002) Oxidative aryl-coupling reactions in synthesis. In Astruc, D. (Ed.) Modern arene chemistry, Wiley-VCH, Weinheim, 479-538. 

The reaction could also be extended to an interesting two-step process, in which 2-allylaniline was first submitted to an oxidative aryl coupling with phenylboronic acid and copper(II) diacetate and myristic acid to give the N-arylated derivative, which was then submitted to catalytic carboamination reaction by the addition of manganese dioxide and base. 

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