Binaphthyl couplings

Meyers has also reported the use of chiral oxazolines in asymmetric copper-catalyzed Ullmann coupling reactions. For example, treatment of bromooxazoline 50 with activated copper powder in refluxing DMF afforded binaphthyl oxazoline 51 as a 93 7 mixture of atropisomers diastereomerically pure material was obtained in 57% yield after a single recrystallization. Reductive cleavage of the oxazoline groups as described above afforded diol 52 in 88% yield. This methodology has also been applied to the synthesis of biaryl derivatives. 

Diheteroaryl-l,l -binaphthyls were prepared from 2,2 -diiodo-l,T-binaphthyl via microwave assisted cross-coupling by Putala and Kappe using several heteroarylzinc chlorides (2-thienyl)zinc chloride, (2-furyl)zinc chloride, and (3-pyridinyl)zinc chloride (Scheme 3) [22]. Importantly, no racem-ization occurred at the reaction temperature used, giving access to (R)-2,2 -diheter0aryl-1,T-binaphthyls starting from (i )-2,2 -diiodo-l,l -binaphthyl in excellent yields in 1 to 5 min of microwave irradiation. 

Aromatic and vinylic sulfides take part in cross-coupling reactions with Grignard reagents in the presence of Ni catalysts.336,337 This reaction has been applied to the enantioselective synthesis of binaphthyls using a standard chiral oxazoline ligand (Equation (25)) 338... 

Pu and co-workers incorporated atropisomeric binaphthols in polymer matrixes constituted of binaphthyl units, the macromolecular chiral ligands obtained being successfully used in numerous enantioselective metal-catalyzed reactions,97-99 such as asymmetric addition of dialkylzinc reagents to aldehydes.99 Recently, they also synthesized a stereoregular polymeric BINAP ligand by a Suzuki coupling of the (R)-BINAP oxide, followed by a reduction with trichlorosilane (Figure 10).100... 

Scheme 6.42 Synthesis of enantiopure 2,2 -diarylated 1,1 -binaphthyls utilizing stereoconservative Negishi cross-coupling reactions.

Some of the reactions (e.g., that of dimethylaluminum chloride in Table 2) involve redistribution of alkyl and halide groups between the metals. The boronic acids, ArB(OH>2, prepared by Sn/B transmetallation, have been used in Suzuki coupling reactions. It is remarkable that the bistributyltin derivative of 1,1 -binaphthyl undergoes... 

This biocatalytic method is therefore quite promising for the synthesis of complex molecules. Very recently, it was reported 1121] that HRP catalyzes the oxidation of 2-naphthols to l,l -binaphthyl-2,2 -diols with moderate enantiomeric excess (ee 38-64%) (Eq. 5). However, in view of the analytical techniques used, these data have to be questioned [167]. As shown recently, atrop-selective biaryl coupling can only be achieved by means of dirigent protein as chiral auxiliary [122]. 

By contrast, in 2000 Shibata reported the Ir-catalyzed enantioselective Pauson-Khand-type reaction of enynes [30aj. The chiral Ir catalyst was readily prepared in situ from [lrCl(cod)]2 and tolBINAP (2,2 -bis(di-p-tolylphosphino)-l,T-binaphthyl), both of which are commercially available and air-stable, and the reaction proceeded under an atmospheric pressure of carbon monoxide. The Ir-catalyzed carbonylative coupling had a wide generality in enynes with various tethers (Z), substituents on the alkyne terminus (R ) and the olefinic moiety (R ). In the case of less-reactive enynes, a lower partial pressure of carbon monoxide achieved a higher yield and ee-value (Table 11.1) [30b]. 

Tanaka et al. (1996,2000) studied the behavior of a series of naphthalene derivatives in AN solution containing NaN02 and CF3SO3H at 0°C in air. Naphthalene showed very low reactivity, and most of the starting material was recovered after the reaction. In case of 1-methylnaphthalene, a coupling reaction took place to produce 4,4 -dimethyl-l,T-binaphthyl in 91% yield alongside mononitro derivatives of the dimer in 1.5% yield. However, when the reaction is carried out on the same conditions but in inert (Nj) atmosphere, the yield of the dimer decreased from 97 to 15%, and no mononitro derivatives were formed. Therefore, the oxidation of NO with O2 to form NO2 (after the electron transfer to NO from 1-methylnaphthalene) is an obvious step of the reaction depicted in Scheme 4.42. 

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. 

In search of a convenient procedure for preparing diazo substrates for the cycloaddition to Cgg, Wudl introduced the base-induced decomposition of tosyl-hydrazones [116]. This procedure allows the in situ generation of the diazo compoimd without the requirement of its purification prior to addition to Cgg. Since they are rapidly trapped by the fullerene, even unstable diazo compounds can be successfully used in the 1,3-dipolar cycloaddition. In a one-pot reaction the tosyUiydrazone is converted into its anion with bases such as sodium methoxide or butylHfhium, which after decomposition readily adds to Cgg (at about 70 °C). This method was first proven to be successful with substrate 142. Some more reactions that indicate the versatility of this procedure are shown in Table 4.4. Reaction of 142 with CgQ under the previously described conditions and subsequent deprotection of the tert-butyl ester leads to [6,6]-phenyl-C5j-butyric acid (PCBA) that can easily be functionalized by esterification or amide-formation [116]. PCBA was used to obtain the already described binaphthyl-dimer (obtained from 149 by twofold addition) in a DCC-coupling reaction [122]. 

Pu reported the synthesis of axially chiral-conjugated polymer 82 bearing a chiral binaphthyl moiety in the main chain by the cross-coupling polymerization of chiral bifunctional boronic acid 80 with dibromide 81 (Equation (39)). The polymer is soluble in common organic solvents, such as THE, benzene, toluene, pyridine, chlorobenzene, dichloromethane, chloroform, and 1,2-dichloroethane. The polymer composed of racemic 80 was also synthesized, and the difference of characteristics was examined. Optically active polymer 82 was shown to enhance fluorescence quantum yield up to = 0.8 compared with the racemic 82 ( = 0.5). Morphologies of the optically active and racemic polymers were also compared with a systematic atomic-force microscopy (AEM). 

Unactivated aryl iodides undergo the conversion Arl — ArCHj when treated with tris(diethylamino)sulfonium difluorotrimethylsilicate and a palladium catalyst.131 A number of methods, all catalyzed by palladium complexes, have been used to prepare unsymmetrical biaryls (see also 3-16). In these methods, aryl bromides or iodides are coupled with aryl Grignard reagents,152 with arylboronic acids ArB(OH)2,153 with aryltin compounds Ar-SnR3,154 and with arylmercury compounds.155 Unsymmetrical binaphthyls were synthesized by photochemically stimulated reaction of naphthyl iodides with naphthoxide ions in an SrnI reaction.156 Grignard reagents also couple with aryl halides without a palladium catalyst, by the benzyne mechanism.157 OS VI, 916 65, 108 66, 67. 

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