Chiral polybinaphthyl

M. Johannsen, K. Jorgensen, X.-F. Zheng, Q.-S. Hu, and L. Pu, A highly enantioselective hetero-Diels-Alder reaction catalyzed by chiral polybinaphthyl-aluminum complexes, J. Org. Chem., 64 (1999) 299-301. 

In 1996, Enders and coworkers reported the asymmetric epoxidation of ( )-enones 91 in the presence of stoichiometric amounts of diethylzinc and (lR,2R)-A-methylpseudo-ephedrine (120) under an oxygen atmosphere to give fraw -epoxides 92 with excellent yields (94-99%), almost complete diastereoselectivity (>98% de) and with very good enantioselectivities (61-92%) (Scheme 54) . For the same reaction Pu and coworkers utilized achiral polybinaphthyl 121 as ligand (in excess) instead of the chiral aminoalcohol. For each substrate, only one diastereomer was formed, but in most cases yields were lower than observed with the Enders system. Enders catalyst shows high asymmetric induction for alkyl-substituted enones (ee 82-92%), but for substrates bearing only aromatic substituents only modest enantioselectivity was obtained (R = R = Ph ... 

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. 

The reaction of (/ )-14 with chloromethyl methyl ether in the presence of sodium hydride produces another chiral binaphthyl monomer (/ )-82. The specific optical rotation of (/ )-82 is [a]p = 23.1 (c = 1.0, THF). In the presence of a catalytic amount of nickel(II) chloride and excess zinc, (/ )-82 is polymerized to generate an optically active polybinaphthyl (/ )-83 (Scheme 35). GPC analysis of (/ )-83 shows = 6000 and M = 3800 (PDI = 1.6). Its specific optical rotation is [or]p = -301.9 (c = 1.0, THF). (/ )-83 is soluble in organic solvents and can easily be hydrolyzed under acidic conditions to generate the optically active polybinaphthol. 

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