Oxidative Naphthol Coupling Reactions

Oxidation of phenols and aromatic amines using HRP is generally of little synthetic value, as oligomers and polymers are the main products (5, 260). Under certain conditions oxidative coupling of phenols or naphthols to give biaryls can be achieved, but with low selectivity (262). In contrast, HRP can catalyze a number of useful oxidative N-and 0-deaIkyIation reactions that are relatively difficult to carry out synthetically. This area has been described in detail by Meunier (263). A method for the preparation of optically active hydroperoxides using HRP C has been developed (264). Optically pure (S)-hydroperoxides... 

We have found that the novel chiral self-dimerizahon of V complexes produces an active unsaturated structure with a new chiral conformation on a Si02 surface, and provides highly enanhoselective achve sites for the asymmetric oxidative coupling of 2-naphthol [35, 36]. Table 10.3 summarizes catalyhc activities, selectivities and enantiomeric excess (e.e.) of homogeneous and heterogeneous V catalysts for the oxidative coupling of 2-naphthol. The homogeneous reactions on a representative L-leucine based V monomer did not exhibit the catalyhc activity. Only in the presence of chlorotrimethylsilane as an acidic promoter did the reachon proceed, but conversion was as low as 15% at 293 K, selectivity to BINOL and e.e. to (P)-BINOL were quite low at 73% and 8%, respectively (Table 10.3). Furthermore, loss of activity occurred and no reachon proceeded after 9 days. 

The enantioselective oxidative coupling of 2-naphthol itself was achieved by the aerobic oxidative reaction catalyzed by the photoactivated chiral ruthenium(II)-salen complex 73. 2 it reported that the (/ ,/ )-chloronitrosyl(salen)ruthenium complex [(/ ,/ )-(NO)Ru(II)salen complex] effectively catalyzed the aerobic oxidation of racemic secondary alcohols in a kinetic resolution manner under visible-light irradiation. The reaction mechanism is not fully understood although the electron transfer process should be involved. The solution of 2-naphthol was stirred in air under irradiation by a halogen lamp at 25°C for 24 h to afford BINOL 66 as the sole product. The screening of various chiral diamines and binaphthyl chirality revealed that the binaphthyl unit influences the enantioselection in this coupling reaction. The combination of (/f,f )-cyclohexanediamine and the (R)-binaphthyl unit was found to construct the most matched hgand to obtain the optically active BINOL 66 in 65% ee. 

There are no reports of chiral complex catalysts that could catalyze the aerobic oxidative coupling of 2-naphthol at a reasonable reaction rate with high enantioselectivity. The reactions mentioned above are accomplished in 24 h or in 7-10 days. These drawbacks should be overcome in the near future. 

CATALYTIC OXIDATIVE CROSS-COUPLING REACTION OF 2-NAPHTHOL DERIVATIVES... 

Table 8.2 shows the results of the cross-coupling reaction between two differently substituted 2-naphthol derivatives using the CuCl-(5)Phbox catalyst. In conclusion, the first catalytic asymmetric oxidative coupling with a high cross-coupling selectivity was accomplished under mild conditions. 

Catalysis in reaction systems with undissolved substrates and products is not restricted to biocatalysis. High yields in sobd-state synthesis, sohd-to-sohd reactions, and solvent-free systems have also been reported for aldol condensation, Baeyer-Villiger oxidation, oxidative coupling of naphthols, and condensation of amines and aldehydes [1, 2]. 

Systems based on the reaction of iron salts with heterocyclic hydrazines and on the oxidative coupling of 2-aminothiazoles with naphthols have also been used (64USP3129109). In a similar process a heterocyclic hydrazone, such as the benzothiazole (174), is cleaved in the presence of an oxidizing agent, and the resulting oxidation products react immediately to give an azo dye (175) (62USP3076721). 

Dehydrogenation reactions of ketones have been applied to the aromatization and oxidative coupling of polycycles. High temperature dehydrogenation of 7-fluoro-3,4-dihy-dronaphthalen-l(2/7)-onc with sulfur gives 7-fluoro-1-naphthol in 90% yield.196 During the oxidation of fluorinated anthraccn-9(10//)-one 9 with iron(III) chloride, a radical is formed that collapses by a termination reaction to give the bianthryl derivative 10.197... 

BINOL and its derivatives have been utilized as versatile chiral sources for asymmetric catalysis, and efficient catalysts for their syntheses are, ultimately, required in many chemical fields [39-42]. The oxidative coupling of 2-naphthols is a direct synthesis of BINOL derivatives [43, 44], and some transition metals such as copper [45, 46], iron [46, 47] and manganese [48] are known as active metals for the reaction. However, few studies on homogeneous metal complexes have been reported for the asymmetric coupling of 2-naphthols [49-56]. The chiral self-dimerized V dimers on Si02 is the first heterogeneous catalyst for the asymmetric oxidative coupling of 2-naphthol. 

Other Reagents for the Oxidative Coupling Reaction Tab. 23. Copper(ll)/amine-mediated cross-coupling of variously substituted 2-naphthol derivatives. 

A catalytic version of the copper(II)-mediated stoichiometric chiral amine naphthol oxidative couplings has been developed as an extension of the successful achiral version. Smrcina and co-workers first carried out such a reaction in order to obtain information on the mechanism of their stoichiometric amine reactions. They obtained the biphenyl product (S)-207 in 41 % yield with a modest enantiomeric excess (32 % ee) (Scheme 56) [88]. 

The effect of polyoxyethylene(9-5) diisobutylphenol, Triton X-100, on the rate of oxidative coupling between disubstituted p-phenylenedi-amines and phenols or naphthols (see, for example, equation 46) has been investigated by Tong and Glesmann (1957b). The observed rate constants were resolved into those for the reactions occurring in the micellar phase and in the bulk aqueous phase. The rate of oxidative coupling... 

Preparative Methods racemic l,l -bi-2,2 -naphthol (BINOL) is most conveniently prepared by the oxidative coupling reaction of 2-naphthol in the presence of transition metal complexes (eq 1). The resolution of racemic BINOL with cinchonine may be performed via the cyclic phosphate (eq 2). An alternative procedure to provide directly optically active BINOL is the oxidative coupling of 2-naphthol catalyzed by Cu salt in the presence of chiral amines (eq 3). The best procedure uses (+)-amphetamine as the chiral ligand and provides BINOL in 98% yield and 96% ee. Above 25 °C the Cu /(+)-amphetamine/(5)-BINOL complex precipitates, while the more soluble Cu /(+)-amphetamine/(I )-BINOL complex is slowly transformed into the former complex. 9,9 -Biphenanthrene-10,10 -diol has also been prepared in 86% yield and with 98% ee by a similar asymmetric oxidative coupling of 9-phenanthrol in the presence of (I )- 1,2-diphenylethylamine. ... 

Li and co-workers also showed that an electron-rich aromatic C-H bond could undergo oxidative coupling reactions with tertiary amines. For example, 2-naphthol derivatives 20 were to be good substrates for the oxidative coupling reaction. While... 

Intermolecular dehydrogenative oxidative homocouplings of (hetero)arenes turned out to be among the most important methods for the synthesis of symmetrically substituted biaryls [122]. A recent illustrative example is oxidative coupling reactions of 2-naphthols, which were accomplished in an asymmetric fashion employing an inexpensive iron catalyst (Scheme 9.47) [123]. 

A very efficient asymmetric variant of the oxidative coupling reaction of /(-naphthol using a copper(II) complex with (S)-amphetamine as a chiral reagent has been developed 14, giving the binaphthol with 95% ee. 

---