Sakurai Allylation
The Sakurai allylation (also known as the Hosomi–Sakurai reaction) allows the reaction between allylsilanes with a wide range of aldehydes and ketones in the presence of a Lewis acid to form the corresponding homoallylic alcohols.
General features:
1. Dichloromethane is a common solvent for this reaction, which takes place under nitrogen atmosphere at a temperature range between –78 °C and 25 °C. 2. Common Lewis acids: TiCl4, AlCl3, BF3·OEt2, SnCl4. 3. The reaction is highly regioselective: the electrophile attacking at the C3 terminus of the allylsilane. 4. α,β-unsaturated aldehydes react at the carbonyl group, while α,β-unsaturated ketones may undergo conjugate addition. 5. C1 substituted allylsilanes give the (E)-alkene product, while C3 monosubstituted allysilanes give the syn-diastereomer. 6. Protodesilylation is a common side reaction.
Sakurai Allylation
Reaction Mechanism
1. Activation of the carbonyl by the L.A. 2. C–C bond formation generates a silyl-stabilized carbocation. 3. C=C double bond formation after loss of the trimethylsilyl group. 4. Cleavage of the C–Ti bond.
Example
Experimental Procedure
To a solution of the aldehyde (2.90 mmol, 1.0 equiv) in DCM (29.0 mL) at –78 °C was slowly added TiCl4 (1.0 equiv), and the resultant mixture was stirred at –78 °C for 5 min. Next, allyltrimethylsilane (1.50 equiv) was added dropwise, and the reaction mixture was allowed to stir at – 78 °C for a further 30 min. Upon completion, the reaction contents were quenched by the addition of saturated aqueous NH4Cl, diluted with DCM, and poured into a separatory funnel. The two phases were separated, and the aqueous layer was extracted with DCM. The combined organic extracts were then dried (Na2SO4), filtered, and concentrated. Purification of the resultant residue by flash column chromatography afforded the desired allylic alcohol (89% yield).
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