Asymmetric epoxidation of allylic alcohols catalyzed by (≡SiO)_xTa(OEt)_3-x(dialkyl tartrate diolate): Influence of the reaction conditions

The influence of several experimental parameters (temperature, substrate concentration, presence of poisons, etc.) on the performance of the catalyst for the asymmetric epoxidation of two allylic alcohols, propenol and trans hex-2-en-lol, was studied. For propenol epoxidation, conversions ≤ 31% were obtained at a very high selectivity (> 95%). The maximum enantiomeric excess (ee) was obtained after a long impregnation of the solid with the chiral inducer (85 to 87% ee in (S)-glycidol with (+)-DIPT). The catalytic reaction was carried out in CH₂Cl, but it also gave good results in pentane and toluene. The use of cumyl hydroperoxide instead of tert-butyl hydroperoxide as epoxidizing agent did not improve the propanol epoxidation results. With hydrogen peroxide, a rather low ee (40% ee) was obtained at -20°C and the reaction was catalytic (TON > 30). Poisoning effects for this reaction were well understood and some of the poisons were identified. The formation of traces of di-tert-butyl peroxide and isobutene was shown. The temperature and the alcohol concentration had a great influence on the selectivity and enantioselectivity of the reaction. Indeed, if the propenol epoxidation was conducted at 10°C instead of 0°C, a kinetic resolution might occur, which leads to higher ee values and lower selectivities.