Aldol and (3+4)-Cycloaddition Reactions

The combination of CeCl₃ -SnCl₂ is widely used for the aldol reactions of α -bromoketones with aldehydes to afford β -hydroxy ketones in good to excellent yields (Scheme 21). The success of the reaction is attributed to activation of the carbonyl group by Lewis acidic CeCl₃ that promotes the reduction of bromine by SnCl₂ .

Scheme 21

The above described combined reagent also promotes the (3+4)-cycloddition of α,α -dibromo ketones with 1.3-dienes under mild conditions (Scheme 22).

Scheme 22

4.1.4 Cerium(III) Iodide (CeI₃)

CeI₃ is extremely hygroscopic and turns brown when exposed to air due to libration of iodine. The reagent is to be stored under nitrogen and handled in a glove bag.

 

1,2-Addition and Aldol Reactions

As CeCl₃ , CeI₃ reacts with organolithium compounds in THF to give orgnocerium compounds that can be used for the 1,2-addition of ketones (Scheme 23).

Scheme 23

Cross-aldol reaction of α -halo ketones with aldehydes can be accomplished using CeI₃ at room temperature (Scheme 24). Ketones do not react under these conditions.

Scheme 24

 

Selective Reduction

α,β -Unsaturated carbonyl compounds can be subjected to selective 1,2-reduction using CeI₃ -LiAlH4 in THF (Scheme 25).

Scheme 25