Substoichiometric TiCl₄-mediated vicinal difunctionalization of α,β-acetylenic ketones for the synthesis of β-halo Baylis-Hillman olefins

A substoichiometric amount of titanium tetrachloride was found to be effective to promote and participate in the tandem α-hydroxyalkylation/β-chlorination of α,β-acetylenic ketones in the presence of (n-Bu)₄NI. This method provides the concise synthesis of (E)-β-halo Baylis-Hillman adducts. No β-iodo products were detected when using this combination of halogen sources. The reaction process involves 1,4-addition of chloro anion released from TiCl₄ onto α,β-acetylenic ketones to give TiCl₃-allenolate intermediates followed by the titanium Lewis acid-promoted carbonyl addition. Modest to good yields (53-77%) and excellent E/Z stereoselectivity (>95%) have been obtained for 10 examples.