(used as an easily recyclable ligand for asymmetric Lewis acid catalysis)
Solubility: soluble in chloroform, methylene chloride, THF, and toluene. Insoluble in alcohols, H2O, and hexane.
Form Supplied in: yellow solid.
Preparative Methods: 1,2 prepared from the Suzuki coupling of the phenylenediboronic acid1 with the optically pure binaphthyl monomer (R)- or (S)-2, made from optically pure 1,1´-bi-2-naphthol (BINOL), followed by acid hydrolysis. By using either Pd(OAc)2 or Pd(PPh3)4 as the catalyst, 3,3´-polyBINOL was obtained with a molecular weight in the range of MW = 6700 to 24 300 (polydispersity index = 1.5-2.5) as determined by gel permeation chromatography relative to polystyrene standard. It was purified by repeated precipitation with methanol from a methylene chloride solution.
Handling, Storage, and Precautions: for long-term storage, keep in closed bottle in the absence of light.
3,3´-PolyBINOL was used to catalyze the reaction of dialkylzincs with aldehydes to generate chiral secondary alcohols with good enantioselectivity. The reactions were normally carried out at 0 °C in toluene solution with 5 mol% (based on the repeating unit) of 3,3´-polyBINOL and 1-2 equiv of diethylzinc. The reactions of benzaldehyde and para-substituted benzaldehydes proceeded with up to 94% ee. The reactions of aliphatic aldehydes gave up to 83% ee. The polymer was recovered by precipitation with methanol and the recovered polymer showed the same catalytic properties as the original one. These reactions were also found to be independent of the molecular weight and molecular weight distribution of the polymer. The catalyst made of (R)-BINOL units gave (R)-alcohol products, and the (S)-BINOL derivative gave (S)-alcohols.
(R)-3,3´-PolyBINOL in combination with AlMe3 was used to catalyze the 1,3-dipolar cycloaddition of nitrones to vinyl ethers with excellent stereocontrol. The reactions were carried out at room temperature in methylene chloride in the presence of 20 mol% of (R)-3,3´-polyBINOL + AlMe3. After precipitation and centrifugation to remove the polymer catalyst following completion of the reactions, the isoxazolidine products were essentially both chemically and enantiomerically pure. The exo/endo product ratio was generally >98:2 and the ee was 93-99%. The recovered polymer showed very similar catalytic properties as the original polymer.
The zinc complex of (R)-3,3´-polyBINOL catalyzed the asymmetric epoxidation of a,b-unsaturated ketones. The reaction was typically conducted by first mixing 0.2 equiv of (R)-3,3´-polyBINOL with 0.36 equiv of diethylzinc in diethyl ether at room temperature. Then an a,b-unsaturated ketone and t-BuOOH (1.2 equiv) were added. The reaction mixture was then stirred at room temperature for a few hours, after which time it was quenched with 1N HCl and extracted with ethyl acetate. The polymer was recovered by precipitation with methanol from the ethyl acetate solution. The a,b-epoxyketone products were obtained with up to 97% yield and 81% ee. The diastereoselectivity was generally >99%.
(S)-3,3´-PolyBINOL in combination with diethylzinc catalyzed the asymmetric reduction of acetophenone with catecholborane. A 5.5 mol% of the polymer were mixed with 10 mol% of diethylzinc at room temperature, and the mixture was then combined with acetophenone. Subsequently, catecholborane (1.5 equiv) was added at -30 °C. The reaction mixture was stirred at this temperature for 48 h, and quenched with 1N HCl. The polymer was recovered by precipitation with methanol. The product (R)-1-phenylethanol was obtained with 62% ee and 90% yield.
University of Virginia, Charlottesville, VA, USA