2-Naphthol oxidative dimerization

Organometallic compounds asymmetric catalysis, 11, 255 chiral auxiliaries, 266 enantioselectivity, 255 see also specific compounds Organozinc chemistry, 260 amino alcohols, 261, 355 chirality amplification, 273 efficiency origins, 273 ligand acceleration, 260 molecular structures, 276 reaction mechanism, 269 transition state models, 264 turnover-limiting step, 271 Orthohydroxylation, naphthol, 230 Osmium, olefin dihydroxylation, 150 Oxametallacycle intermediates, 150, 152 Oxazaborolidines, 134 Oxazoline, 356 Oxidation amines, 155 olefins, 137, 150 reduction, 5 sulfides, 155 Oxidative addition, 5 amine isomerization, 111 hydrogen molecule, 16 Oxidative dimerization, chiral phenols, 287 Oximes, borane reduction, 135 Oxindole alkylation, 338 Oxiranes, enantioselective synthesis, 137, 289, 326, 333, 349, 361 Oxonium polymerization, 332 Oxo process, 162 Oxovanadium complexes, 220 Oxygenation, C—H bonds, 149... 

Numerous cationic azo dyes are prepared by the action of /V-hydroxymcthyl-chloroacetamide on azo dyes in sulfuric acid medium, followed by displacement of the reactive chloro substituent by pyridine or trialkylamine. Of special significance for dyeing paper are dyes that are prepared by coupling of diazotized 2-(4 -aminophenyl)-5-methylbenzothiazole to acetoacetaiylides, pyrazolones, naphthols [143], or barbituric acid derivatives [144], followed by reaction with N-hydroxymethylchloro-acetamide and pyridine. The azo dye obtained by oxidative dimerization of 2-(4 -aminophenyl)-5-methylbenzothiazole may also be subjected to this conversion [145], Dye 51 colors paper yellow. 

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]. 

Scheme 16. Large-scale iron(lll)-mediated oxidative dimerization of 2-naphthol (68a).

Tab. 15. Survey of iron(lll) sources used in the two-phase oxidative dimerization of 2-naphthol (6 a).

Tab. 17. Microwave-assisted oxidative dimerization of 2-naphthol derivatives by iron(lll).

Tab. 19. Vanadium-mediated oxidative dimerization of naphthol derivatives.

Tab. 25. Copper(ll)-catalyzed oxidative dimerizations of substituted 2-naphthol derivatives.

Wulff and co-workers observed that the oxidative dimerization of 147 can be accomplished extremely efficiently when it is melted in a sealed tube in the presence of air, to furnish the biaryl product 148, a precursor to the natural compound gossypol (149) (Scheme 35) [99]. The same reaction with iron(III) chloride gives low yields and a less clean product distribution. It is interesting to note that in the case of 3-phenyl-l-naphthol (150), the regioselectivity of the iron(III) chloride-mediated oxidation is completely different from that observed with 02 as the oxidant, with the para-para-coupled product 152 being favored. This air oxidation procedure is also applicable to phenanthrol units (e.g. 153 —> 154), giving similarly high yields. 

Tab. 34. Sparteine-mediated electrocatalytic asymmetric oxidative dimerization of 2-naphthol and...

Nakajima and co-workers have carried out extensive investigations into the influence of different chiral diamine-copper complexes on the oxidative dimerization of naphthols [146-148]. As emerged from Smrcina s work, the inclusion of an ester moiety on the naphthol precursor is an important factor for optimizing the enantioselectivity. After establishing a catalytic cycle with TMDA as the base and showing that sparteine gave promising results (Scheme 57), they focused their work on other chiral diamines (Table 39) [147]. 

Scheme 57. Enantioselective oxidative dimerization of naphthol derivative 68f in the presence of a copper(l) pre-catalyst and sparteine.

Oxidations. Clean products are generally obtained by using alumina-supported reagents. For the oxidative dimerization of thiols by OMSO or NalOj, the conversion of hydroquinones to quinones and 2-naphthols to BINOLs in the presence of CuSO, the formation of lactones from a,co-diols under the influence of NaBrOj, and the hydrolytic defluorination of certain perfluoroalkylarenes, alumina has unique or positive effects. 

Manganese chelates, such as manganese(III) acetonylacetonate were also tried as oxidants in the coupling of phenols and phenol ethers [125]. In one of the first examples binaphthol was prepared by the oxidative dimerization of 2-naphthol [126]. Although with this reagent overoxidation can be mostly avoided, low conversions make this method inferior to others. 

SCHEME 5 Asymmetric oxidative dimerization of naphthols mediated by a chiral copper complex. 

SCHEME 6 Gong s catalytic, asymmetric oxidative dimerization of naphthols using vanadium complex 35. Reboot of viriditoxin/pigmentosin A S5mthetic plan. 

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