Immobilization of a BINOLate-Titanium Catalyst by Use of Aggregation Phenomenon

Abstract

Upon treatment with titanium tetraisopropoxide, tris-BINOL with a rigid aromatic tether forms an insoluble polymeric aggregate, which shows a catalytic activity in asymmetric addition of diethylzinc to aldehydes.

Key words

asymmetric catalysis - heterogeneous catalysis - Lewis acids - titanium - zinc

References and Notes

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10

Typical Experimental Procedure (Table 2, Entry 4).
To a solution of tris-BINOL 9 in CH₂Cl₂ (0.08 M) in a Schlenk flask at r.t. under argon atmosphere was added titanium tetraisopropoxide (3 equiv). The resulting dark orange suspension was stirred for 1 h. After addition of toluene, the mixture was concentrated by distillation of the solvents under atmospheric pressure. The residue was washed three times with toluene by centrifugation under argon and vacuum dried to give 9-[Ti(Oi-Pr)₂]₃, which was stored in an argon-filled glovebox prior to use. To a suspension of 9-[Ti(Oi-Pr)₂]₃ (27 mg, 0.015 mmol) in toluene (7.2 mL) and hexane (1.3 mL) at 0 °C under argon atmosphere was added titanium tetraisopropoxide (0.13 g, 0.46 mmol). The resulting suspension was sonicated for 15 min at 0 °C. To this at 0 °C was added diethylzinc (1.0 M in hexane) (1.4 mL, 1.4 mmol) and the mixture was stirred for 20 min. To the resulting mixture was added 1-naphthalde-hyde (72 mg, 0.46 mmol). After being stirred at 0 °C for 22 h, the reaction mixture was filtered in a glovebox under argon atmosphere. The filtrate was poured into aq 1 N HCl, extracted three times with EtOAc, and washed with aq 5% NaHCO₃. The organic layer was dried (MgSO₄) and concentrated in vacuo. The residue was purified by flash column chromatography (silica gel, 15% EtOAc in hexane) to give 81 mg (95% yield) of (R)-1-naphthyl-1-propanol (74% ee). Enantioselectivity was determined by HPLC analysis using a Chiralcel OD column (10% i-PrOH in hexane, 0.8 mL/min); t _R = 11.8 min (minor S enantiomer), 22.5 min (major R enantiomer). The absolute structure of the product was determined by comparing the retention time with that of an authentic sample prepared by asymmetric ethylation using (R)-BINOL as a ligand. [9]