Reaction of α-halogeno ketones with carbonyl compounds promoted by CeI3, CeCl3–NaI, or CeCl3–SnCl2

Reaction of α-halogeno ketones with aldehydes in the presence of CeI3 in tetrahydrofuran is found to give α,β-unsaturated ketones in excellent yields under mild conditions. In contrast, treatment of α-halogeno ketones and carbonyl compounds with CeCl3–NaI or CeCl3–SnCl2 affords β-hydroxy ketones in good yields. It is assumed that these reactions proceed via cerium enolates. The combined reagents, however, cannot be applied to a Reformatsky-type reaction. Regiospecific and aldehyde chemoselective aldol synthesis are also described.