Asymmetric oxidative coupling

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 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. 

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.

Asymmetric oxidative coupling polymerization of hydroxynaphthalene derivatives was investigated by Habaue and Okamoto et al. First, they studied the oxidative coupling polymerization of optically active 3,3/-hydroxy-2,2/-dimethoxy-1,1 -binaphthalene with copper catalysts bearing chiral ligands under an oxygen atmosphere (Scheme 41) [166]. The obtained polymers had molecular weights of 3100-5200. When the polymerization of (J )-monomer... 

Scheme 39. Strategies for the asymmetric oxidative coupling of aryl units.

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. ... 

Stereoselective Synthesis of Biaryl Compounds. The best known application of (1) to the asymmetric oxidative coupling of phenolic compounds is the copper(II)-catalyzed synthesis of l,r-binaphthyl-2,2 -diol in greater than 95% ee (eq 1). ... 

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... 

Very recently, Sasai and co-workers further realized the asymmetric oxidative coupling of polycyclic phenols such as 2-anthracenol, 9- or 3-phenan-throl, and 5-chrysenol by dinuclear vanadyl complexes (S,S,/ a)-31, (S,S,/ a)-32, or their mononuclear analog (S)-33 (Scheme 3.12). It should be noted that, apart from the corresponding axially chiral biaryl products obtained in up to 93% ee, no over-oxidation side product was observed. 

Scheme 3.12 V-catal3 ed asymmetric oxidative coupling of polycyclic phenols reported by Sasai.

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

Starting from an optically active dimer, tetramer, or octamer of 2,3-dihy-droxynaphthalene derivatives, Tsubaki and co-workers realized the synthesis of numerous optically active oligonaphthalene products by second-order asymmetric transformation under amine-copper conditions (Scheme 3.17). On the other hand, Okamoto and co-workers achieved asymmetric oxidative coupling polymerization (AOCP) reactions to synthesize poly(2,3-dihy-droxy-l,4-naphthylene) derivatives (Scheme 3.18). The starting materials of AOCP reactions can be either partially protected tetrahydroxybinaphthale-nes or 2,3-dihydroxynaphthalene. The chiral Cu complexes ligated by (-)-sparteine or bisoxazolines were identified as suitable catalysts for these reactions. However, the enantioselectivity attained in these AOCP reactions was estimated to be low. 

Based on the above achievements, the group of Habaue systematically studied Cu-catalyzed asymmetric oxidative coupling polymerization reactions. 

Scheme 3.19 Cu-catal) ed asymmetric oxidative coupling polymerization reactions reported by Kozlowski.

Three methods for the construction of chiral polynaphthalenes have been described. In the first case, 2,2 -dihydroxy-6,6 -binaphthalenes were subjected to the optimized asymmetric oxidative coupling conditions, yielding the corresponding chiral polymers (a). It was found that each biaryl coupling step during the polymerization process is independent of existing axial chirality... 

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 ... 

Oxidative Poiymerization of Naphthoi Derivatives. Oxidative polymerization of 2-naphtol (109) and 1,5-dihydroxynaphthalene (110) has been done using enzyme catalysts. Solid-state polycondensation of 2,6-dihydroxynaphthalene with FeCls catalyst (111) has been accomplished. Asymmetric oxidative coupling polymerization of 2,3-dihydroxynaphthalenes and their derivatives was achieved by chiral copper catalysts (112-115). 

CF3CSCCF2 to give selective asymmetric oxidative coupling of the cyclopalladated organo groups leading to the formation of the novel... 

Generation of the carbenium ion in an S il-type reaction with aldehydes can also be realized in different ways for example, the carbenium ion can be prepared directly from a CH bond by a formal CH oxidation. The CH bond can be oxidized by electrochemistry methodologies [40] or by the use of oxidants in stoichiometric amount. 2,3-Dichloro-5,6-dicyano-l,4-benzoquinone (DDQ) has been utQized in efficient oxidative coupling reactions involving CH activation. An organocatalytic asymmetric oxidative coupling of benzyUc compounds with aldehydes performed with the MacMillan catalysts 4 and DDQ was described [41]. Better enantiomeric... 

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