Asymmetric coupling of 2-naphthol

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. 

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. 

Chiral Self-Dimerization of Vanadium Schiff-Base Complexes on Si02 and Their Catalytic Performances lor Asymmetric Oxidative Coupling of 2-Naphthol... 

Table 2.1 Catalytic performances of homogeneous and heterogeneous V catalysts for asymmetric oxidative coupling of 2-naphthol. The Vdimer/2-naphthol ratio was 1 36 and 100 mg of supported catalysts were used in 5 ml of toluene.

The use of a chiral reagent for the oxidative coupling of naphthols has received much attention as the product chiral binaphthols are widely used in asymmetric synthesis [138]. The well-known oxidative coupling of 2-naphthol by dioxygen in the presence of a copper com-... 

Fig. 6. Rationale for the observed sense of asymmetric induction upon cross-coupling of 2-naphthol (68a), as mediated bycopper(ll) and sparteine.

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. ... 

Based on their mechanistic investigation on the Cu -mediated oxidative coupling of 2-naphthol derivatives, Smrcina, Kocovsky, and co-workers were able to develop the first catalytic asymmetric oxidative biaryl coupling reaction (Scheme 3.3). AgCl was used as the stoichiometric oxidant to regenerate the Cu species. In order to avoid the formation of detrimental HCl by-product, the corresponding sodium 2-naphthol salts were employed as the coupling partners. The desired product 3 was obtained in 41% yield (8 turnovers) with 32% ee. 

The group of Palmisano and Sisti reported the synthesis of (+)-camphor-based C2-symmetrical diamine ligand (-)-14 and the application of its Cu complex to the asymmetric oxidative coupling of 2-naphthol derivatives (Scheme 3.6). As observed by Kocovsky, Nakajima, and Kozlowski, an ester group installed at the C3 position of the naphthalene is crucial for achieving... 

Scheme 3.15 Proposed catalytic cycle for Fe-catalyzed asymmetric oxidative coupling of 2-naphthols.

Enantioselective vanadium and niobium catalysts provide chemists with new and powerful tools for the efficient preparation of optically active molecules. Over the past few decades, the use of vanadium and niobium catalysts has been extended to a variety of different and complementaiy asymmetric reactions. These reactions include cyanide additions, oxidative coupling of 2-naphthols, Friedel-Crafts-type reactions, pinacol couplings, Diels-Alder reactions, Mannich-type reactions, desymmetrisation of epoxides and aziridines, hydroaminations, hydroaminoalkylations, sulfoxida-tions, epoxidations, and oxidation of a-hydroxy carbo) lates Thus, their major applications are in Lewis acid-based chemistiy and redox chemistry. In particular, vanadium is attractive as a metal catalyst in organic synthesis because of its natural abundance as well as its relatively low toxicity and moisture sensitivity compared with other metals. The fact that vanadium is present in nature in equal abundance to zinc (albeit in a more widely distributed form and more difficult to access) is not widely appreciated. Inspired by the activation of substrates in nature [e.g. bromoperoxidase. 

Asymmetric Catalysis for Oxidative Coupling of 2-Naphthol to BINOL... 

Scheme 7.22 Cu-Catalyzed asymmetric oxidative cross-coupling of 2-naphthols. DCE = 1,2-dichloroethane.

With the bisoxazoline hgand (S)-Phbox and CuCl, the asymmetric oxidative couphng of 2-naphthol and hydroxy-2-naphthoates resulted in an asymmetrically substituted 2,2 -binaphthol with ee s of up to 65% [260]. On the basis of the previous results obtained with this catalyst system, the asymmetric oxidative cross-coupling polymerization of 2,3-dihydroxynaphthalene [261] and methyl 6,6 -dihydroxy-2,2 -binaphthalene-7,7 -dicarboxylate [262] as well as the copolymerization of 6,6 -dihydroxy-2,2 -binaphthalene and dihexyl 6,6 -dihydroxy-2,2 -binaphthalene-7,7 -dicarboxylate with Cu diamine catalysts were carried out imder aerobic conditions, using O2 as the oxidant, and a cross-coupling selectivity of 99% was achieved [263]. 

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]. 

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