Asymmetric reduction of ketones catalyzed by α,α-diphenyl-(L)-prolinol modified with imidazolium ionic liquid and BH3·SMe2 as a recoverable catalyst

• Easily recoverable ionic liquid for asymmetric reduction of ketones was developed.
• Ionic liquid with ether linkage is the stable compared to ester linkage.
• Recovered and reused up to 4 cycles.
• Yield up to 99% and ee up to 88%.

The synthesis of α,α-diphenyl-4-trans-hydroxy-(L)-prolinol modified with imidazolium based ionic liquids was carried out with trans-α,α-diphenyl-4-hydroxy-(L)-prolinol, 5-bromovaleric acid or 1,5-dibromopentane and imidazole. α,α-Diphenyl-4-hydroxy-(L)-prolinol modified with imidazolium ionic liquid was treated with BH3·SMe2 which generate 1,3,2-oxazaborolidine, that acts as a catalyst for asymmetric reduction of prochiral ketones. α,α-Diphenyl-4-hydroxy-(L)-prolinol modified with imidazolium ionic liquids (PF6 anion) with BH3·SMe2 found to be an efficient catalyst (10 mol%) for the reduction of the acetophenone, gave 99% yield and 87–84% ee. The catalytic method has wide applicability for a variety of substrates. 1,3,2-oxazaborolidine containing ether linkage ionic liquid was recovered and reused up to 4 cycles with 99–91% yields and 87–81% ee’s.

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