Effect of Water on Keck’s Catalytic Asymmetric Allylations of Aldehydes

Abstract

The complex generated from BINOL, Ti(i-PrO)₄, and unactivated or a large amount of activated 4 Å MS in toluene is very effective in catalytic allylations of aldehydes using allyltributyltin. Allylations with 2.5 mol% of the catalyst provide homoallylic alcohols with greater than 95% ee. Very high syn-selective allylations of protected b-hydroxyaldehydes are achieved with 5 mol% of the BINOL/Ti(i-PrO)₄ complex.

Key words

catalytic asymmetric allylation - 4 Å molecular sieves - BINOL - Ti(i-PrO)₄ - allyltributyltin

References

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11

Powdered 4 Å MS (available from Aldrich) were used for this study.

12

In our hands the same reaction under the reported conditions by Keck [6b] afforded (R)-1-phenylhex-5-en-3-ol in 50% yield with 75.0% ee. For an analogous observation see reference 9.

14

¹H NMR spectra (300 MHz, CD₂Cl₂) of (S)-BINOL and the active BINOL/Ti complex.

16

Powdered 4 Å MS (available from Aldrich) were activated at 200 °C under high vacuum for 12 h, it is believed that water still remained inside activated 4 Å MS. Activated 3 Å and 5 Å MS were less effective in the formation of an active catalyst than 4 Å MS.

17

The allylstannation of hydrocinnamaldehyde using the complex generated in the presence of 50-100 mg of activated 4 Å MS [against 200 mg of (S)-BINOL] afforded (R)-1-phenylhex-5-en-3-ol in 70-80% yields with 60-70% ee. The same scale experiment using the complex generated with 150 mg of activated 4 Å MS afforded the product in 82% yield with 85% ee.

22

(S)-BINOL was purchased from Kankyo Kagaku Center Co. Ltd.