High-Pressure Enantioselective Allylation of Aldehydes Catalyzed by (Salen)Chromium(III) Complexes

Abstract

The enantioselective addition of allyltributyltin to simple aldehydes (2a-l), catalyzed by chiral (salen)Cr(III) complexes 1, has been studied. The reaction proceeds smoothly with low loading (1-2 mol%) of (salen)Cr(III)BF₄ (1a) and allyltributyltin under high-pressure conditions (10 kbar) in good yield and ee values in the range of 35-79%.

Key words

aldehydes - allylstannane - asymmetric catalysis - high-pressure technique - homoallylic alcohols - salen-chromium complexes

References

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20

General Procedure for the High-Pressure Allylation:
In a 2 mL Teflon ampoule were placed (salen)CrBF₄ (2a, usually 13.7mg, 2 mol%), ca. 1 mL of solvent (usually CH₂Cl₂), followed by aldehyde (usually 1 mmol) and allyltributyltin (1.1 equiv). Finally, the ampoule was made up with solvent, closed and placed in a high-pressure vessel, and pressure was slowly increased to 10 kbar at 20 °C. After stabilization of pressure, the reaction mixture was kept under these conditions for 24 h. After decompression, the reaction mixture was diluted with wet Et₂O, and dried over MgSO₄. After evaporation of solvents, the residue was chromatographed on a silica gel column using hexane-EtOAc. All liquid aldehydes were distilled prior to use.

21

Enantioselectivity of homoallylic alcohols 3a-l was determined by GC employing a capillary chiral β-dex 120 column. Alcohols 3c,k were analyzed directly, 3a,b,d-h,j as their O-trimethylsilyl derivatives, 3i as a trifluoroacetate and 3l as a isopropylidene derivative of pent-4-ene-1,2-diol.