Binaphthol-aluminum complexes

Optically active l,l -binaphthols are among the most important chiral ligands of a variety of metal species. Binaphthol-aluminum complexes have been used as chiral Lewis acid catalysts. The l,T-binaphthyl-based chiral ligands owe their success in a variety of asymmetric reactions to the chiral cavity they create around the metal center [107,108]. In contrast with the wide use of these binaphthyls, the polymer-supported variety has been less popular. The optically active and sterically regular poly(l,l -bi-naphthyls) 96 have been prepared by nickel-catalyzed dehalogenating polycondensation of dibromide monomer 95 (Sch. 7) [109] and used to prepare the polybinaphthyl aluminum(III) catalyst 97 this had much greater catalytic activity than the corresponding monomeric catalyst when used in the Mukaiyama aldol reaction (Eq. 29). Unfortunately no enantioselectivity was observed in the aldol reaction. 

Asymmetric ene Reaction In 1988 Yamamoto and coworkers provided the first indication that asymmetry in ene-reactions could be induced by catalytic amounts of chiral Lewis acids in the presence of 4-A molecular sieves (Scheme 6.64) [88]. They described the first example of asymmetric ene-reaction between prochiral, halogenated aldehydes and alkenes catalyzed by chiral binaphthol-derived aluminum complexes. The hindered 3,3-silyl substituents in the chiral catalyst are essential to achieve good enantioselectivity and high yield. In fact, the use of a catalyst derived from MesAl and 3,3 -biphenylbinaphthol led to the racemic product in a low yield. 

Yamamoto et al. were probably the first to report that chiral aluminum(III) catalysts are effective in the cycloaddition reactions of aldehydes [11]. The use of chiral BINOL-AlMe complexes (R)-S was found to be highly effective in the cycloaddition reaction of a variety of aldehydes with activated Danishefsky-type dienes. The reaction of benzaldehyde la with Danishefsky s diene 2a and traws-l-methoxy-2-methyl-3-(trimethylsilyloxy)-l,3-pentadiene 2b affords cis dihydropyrones, cis-3, as the major product in high yield with up to 97% ee (Scheme 4.6). The choice of the bulky triarylsilyl moiety in catalyst (J )-8b is crucial for high yield and the en-antioselectivity of the reaction in contrast with this the catalysts derived from AlMe3 and (J )-3,3 -disubstituted binaphthol (substituent = H, Me, Ph) were effective in stoichiometric amounts only and were less satisfactory with regard to reactivity and enantioselectivity. 

With Chiral Al Complexes Chiral bis(silyl)binaphthol-modified aluminum catalyst, which is originally developed for asymmetric hetero-Diels-Alder reaction [50], is successfully applied to asymmetric Diels-Alder reaction of cyclopentadiene with methyl acrylate or methyl propio-late [51 ] (Eq. 8A.28). The latter is a rather rare example in the literatures. 

Rebiere F, Riant O, Kagan HB (1990) Tetrahedron Asymmetry 1 199 An aluminum-binaphthol complex catalyzes the Diels-Alder reaction of cyclopentadiene and methyl acrylate in 82% yield and 67% ee Maruoka K, Concepcion AB, Yamamoto H (1992) Bull Chem Soc Jpn 65 3501... 

The first catalytic asymmetric radical-mediated allylation was reported in late 1997 by Hoshino and coworkers, who studied the allylation of an a-iodolac-tone substrate, Eq. (19) using trimethylaluminum as Lewis acid and a silylated binaphthol as the chiral catalyst, with triethylborane as radical initiator [62]. Use of one equiv. of diethyl ether was crucial for high enantioselectivity, providing an ee up to 91% in the presence of one equiv. of catalyst, with only a 27% ee in the absence of ether, and poorer ee s when other ethers were employed. In the catalytic version, the ee s dropped off vs. the stoichiometric reaction, with an ee of 81% with 0.5 equiv., and 80% with 0.2 equiv., and 72% with 0.1% catalyst. As in the above example, the presumed chiral intermediate involves complexation of the lactone radical with the Lewis acid-binaphthol complex, with the diethyl ether perhaps as a ligand on the aluminum. 

As for the chiral ytterbium and scandium catalysts, the following structures were postulated. The unique structure shown in scheme 13 was indicated by 13C NMR and IR spectra. The most characteristic point of the catalysts was the existence of hydrogen bonds between the phenolic hydrogens of (R)-binaphthol and the nitrogens of the tertiary amines. The 13 C NMR spectra indicated these interactions, and the existence of the hydrogen bonds was confirmed by the IR spectra (Fritsch and Zundel 1981). The coordination form of these catalysts may be similar to that of the lanthanide(III)-water or -alcohol complex (for a review see Hart 1987). It is noted that the structure is quite different from those of conventional chiral Lewis acids based on aluminum (Maruoka and Yamamoto 1989, Bao et al. 1993), boron (Hattori and Yamamoto 1992), or titanium... 

Apart from the aluminum-containing catalyst, the same group also reported a gallium-sodium-binaphthol complex ((/J)-GaSB) and applied them in the Michael reaction of dibenzyl malonate with cyclohexenone (Table 9.3). In the presence of (/ )-GaSB, up to 98% ee was obtained, whereas the yield dropped to 45% even after a prolonged reaction time. Interestingly, when sodium malonate was added to the reaction system as the additive, 2-cyclohexen-l-one could be transformed completely into 1,4-adduct with 96% ee. 

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