Regioselective and Enantioselective Addition of Sulfur Nucleophiles to Acyclic α,β,γ,δ-Unsaturated Dienones Catalyzed by an Iron(III)-Salen Complex.

The first regioselective, enantioselective conjugate addition of thiols to acyclic α,β,γ,δ-unsaturated dienones at the δ carbon is described. The reaction, catalyzed by a chiral iron(III)-salen complex derived from cis-2,5-diaminobicyclo[2.2.2]octane as the scaffold, provides δ-thia-α,β-unsaturated ketones in high yield and enantioselectivity. The bicyclooctane scaffold of (2R,3R,5R,6R) configuration affords a δ-thia-α,β-unsaturated ketone of (R) configuration, indicating that the sulfur nucleophile is introduced at the si face of the γ,δ-double bond. A model providing an explanation for this regio- and stereoselection is proposed.