( S )-BINOL Immobilized onto Multiwalled Carbon Nanotubes through Covalent Linkage: A New Approach for Hybrid Nanomaterials Characterization

Abstract

(S)-6-(4-(Methoxycarbonyl)phenyl)-1,1'-bis-2-naphthol, a chiral 1,1'-bis-2-naphthol (BINOL) derivative, was prepared via Suzuki C¢C coupling and immobilized onto diamine- functionalized multiwalled carbon nanotubes. The BINOL was first derivatized with a carboxylic acid, capable of forming an amide linkage with the amine-carbon nanotube modified surface. The 4-methoxyphenylboronic acid was replaced by 3-fluoro-4-(methoxycarbonyl)phenylboronic acid and (S)-6-(3-fluoro-4-(methoxycarbonyl)phenyl)-1,1'-bis-2- naphthol was obtained and used as a probe to better quantify the organic material. It was found that in the hybrid material, 1/3 of amine groups have been transformed into the corresponding amide (S)-BINOL derivative. The chemical and electronic integrity of the chiral BINOL ligand is maintained after the ligand immobilization. Evaluation in diethyl zinc and Ti(OiPr)4-catalyzed alkylation of benzaldehyde showed that both methoxycarbonylphenyl and fluoromethoxycarbonylphenyl BINOL moieties do not significantly influence the catalytic activity or selectivity of the process. However, when hybrid materials were used, similar activities were obtained, but with lower enantioselectivities. This result could be due to a competitive alkylation reaction occurring on the carbon surface or by the constraint caused by the ligand proximity on the support surface.