Dimerization of phenols

Biaryls have also been prepared by coupling support-bound aryl halides with aryl-zinc compounds (Figure 5.20) or with aryl(fluoro)silanes [203]. As with Suzuki or Stille couplings, these reactions also require transition metal catalysis. An additional strategy for coupling arenes on solid phase is the oxidative dimerization of phenols (Figure 5.20). 

Not many catalyzed processes involving free radicals are known with these metals. Some vanadium-catalyzed pinacol coupling reactions were developed (reviews [129, 171], [172, 173] and cited ref, [174]). Niobium and tantalum complexes were applied in pinacol coupling reactions [130]. Vanadium(IV) [175-179] and vanadium(V) ([129], reviews [180-186]) complexes are known to catalyze asymmetric oxidative dimerizations of phenols and naphthols in moderate to excellent ees applying oxygen as the terminal oxidant. Biaryls are accessible by intramolecular coupling of sodium tetraarylborates, catalyzed by EtOVOCl2 in the presence of air [187]. 

Other radical-based transformations are ruthenium-catalyzed oxidative dimerizations of phenols [263] and reductive dimerizations [264], The isomerization of chiral c/s-epoxides to tram-epoxides catalyzed by 2-10 mol% TpRu(py)2Cl proceeds at 100 °C in 95-98% yields with inversion of configuration [265], A radical or SN2 mechanism was discussed for this process. 

There is less substantiation of the structure of the dimer of phenol, but the evidence favors cyclic dimers and trimers. Mecke s early work was based on this assumption (1375). More recently the caureful study by Rea (1698) hais led him to propose that at any concentration of phenol in CCI4 only a small fraction of the phenol is present as dimer, and that the more stable trimer is cyclic. 

Reports of Rh-catalyzed C-H/C-H arene arylations are rare and provide limited scope and mechanistic data. In a first report, the Barrett group describes the oxidative 0,0 -dimerization of phenols in the presence of a rhodium(III) catalyst... 

In nature, the oxidative dimerization of phenols is controlled by enzymes, as is demonstrated by the axial chirality of the 6,8 -coupled juglone derivative isodiospyrin. In synthesis, however, phenol oxidation only proceeds in high yields when the enzymatic reaction control is replaced by substituent control, that is, if all but one of the positions with high spin density in the radical (ortho-and para positions) are blocked. 

The oxidative dimerization of phenol 22.1 was simulated in vitro. Exposure of this compound to the complex Cu(NO 3) 2-pyridine gave rise to dimers 23.10 and 23.11 by a C-C coupling reaction. Compound 23.11 could be cyclodehydrogenated to 23.10 by reaction with DDQ. On the other hand, exposure of phenol 22.1 to K3Fe(CN)6 gave the product (22.3) of a C-0 coupling, which was then transformed into 23.12 by DDQ cyclodehydrogenation 140) (Scheme 9). 

Oxidative dimerization of phenols s. 20, 561 Furan ring synthesis s. 18, 802... 

Substituted biphenols can be prepared by oxidative dimerization of phenols under various conditions [20] (e.g., with air/oxygen [21] or by the effect of high temperatures) [22]. Because of the formation of several side products, the yields with phenols are generally only modest. Thus, in the reaction with aqueous NaOH, a 46% yield of the solid coupling product was isolated [21]. Only recently, improved results were reported by Jana and Tunge with KjFeJCNJg as a catalyst (Scheme 2.62) [23]. 

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