Oxidative Coupling of 2-Naphthols

Polymers bearing a binaphthol unit in the main chain have also attracted interest as optically active and functional materials. In 2005, Habaue explored the asymmetric oxidative coupling polymerisation (AOCP) of 2,3-dihydroiynaphthalene (DHN) promoted by the catalyst prepared from 2,2 -methylenebis[(4R)-4-phenyl-2-oxazoline] (Phbox) and vanadyl sulfate  

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In recent years, axially chiral binaphthalene derivatives have emerged as important ligands and chiraUty inducers in organic synthesis. Oxidative coupling of 2-naphthols represents a well estabhshed method for the preparation of binaphthols. The couplings are usually carried out by treating naphthols with more than an equimolar amount of a metal such as Fe(III), Mn(III),... 

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. 

Table 10.3 Catalytic activities, selectivities and enantiomeric excess (e.e.) in homogeneous and heterogeneous vanadium (V) catalyses for the oxidative coupling of 2-naphthol .

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. 

ENANTIOSELECTIVE OXIDATIVE COUPLING OF 2-NAPHTHOLS CATALYZED BY A NOVEL CHIRAL VANADIUM COMPLEX... 

Table 8.1 Oxidative coupling of 2-naphthols using catalyst 1...

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

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.

Tab. 16. Oxidative coupling of 2-naphthol (6 a) and 2-naphthylamine (70a) by various iron (111) sources.

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-... 

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. ... 

Bi-2-naphthol was purchased from Aldrich Chemical Company, Inc. or prepared by oxidative coupling of 2-naphthol. ... 

TMSOTf may act as a Lewis acid promoter for the chiral oxova-nadium complex-catalyzed oxidative coupling of 2-naphthols. In the enantioselective version, chlorotrimethylsilane affords higher enantiomeric excesses (eq 124)4 ... 

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... 

Besides, it was disclosed that complex (S, Ra)-28 derived from Hg-BINOL is another highly efficient catalyst for the oxidative coupling of 2-naphthol derivatives. An attractive feature of this catalyst is that air (1 atm) can be used as the oxidant for a series of substrates with substitution at the C6 or C7 positions of the naphthalene ring, and the reaction time can also be shortened to 1.5 to 2 days (Table 3.9). 

Table 3.9 V-catalyzed oxidative coupling of 2-naphthols with air as the oxidant.

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