(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, ac hiral 1,1'-bis-2-naphthol (BINOL) derivative, wasp repared via SuzukiC ÀCc oupling and immobilized onto diamine-functionalized multiwalled carbonn anotubes. The BINOL was first derivatized with ac arboxylic 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 ap robe 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 andf luoromethoxycarbonylphenyl BINOL moieties do not significantly influence the catalytic activity or selectivity of the process. However,w hen hybridm aterials were used, similar activities were obtained, but with lower enantioselectivities.T his result could be due to ac ompetitive alkylation reactiono ccurring on the carbon surfaceo rb yt he constraint caused by the ligand proximity on the support surface.

Modified Nanocarbons for Catalysis

“…Pereira et al . reported the multistep syntheses of two ( S )‐6‐(4‐(methoxycarbonyl)phenyl)‐1,1’‐bis‐2‐naphthol 77 a – b covalently linked to multi‐walled nanotubes (Figure ) . The hybrid materials have been employed as heterogeneous chiral ligands for Ti(O i Pr) 4 for the enantioselective alkylation of benzaldehyde with diethylzinc.…”

Regioselective Substitution of BINOL

“…For example, the Suzuki coupling of ( S )- or ( R )- 22 with polystyrene (PS) beads functionalized with boronic acid gave the PS-supported BINOL ( S )- or ( R )- 28 , which was used to catalyze the asymmetric oxidation of thioethers . ( S )-BINOL was also immobilized on carbon nanotubes to form hybrid materials such as ( S )- 29 for asymmetric catalysis . ( R )-BINOL was linked to silica gel to form ( R )- 30 as a chiral stationary phase for HPLC analysis …”

“…The monobromoBINOLs were used to prepare a variety of BINOL derivatives, − including many polymer- or solid-supported BINOLs. For example, the Suzuki coupling of ( S )- or ( R )- 22 with polystyrene (PS) beads functionalized with boronic acid gave the PS-supported BINOL ( S )- or ( R )- 28 , which was used to catalyze the asymmetric oxidation of thioethers .…”