Titanium(IV) Chloride and Quaternary Ammonium Salt Promoted Baylis−Hillman Reaction

The Baylis−Hillman reaction of aldehydes with α,β-unsaturated ketones can be drastically affected by the reaction temperature and Lewis bases. When the reaction was carried out at −78 °C using catalytic amounts of quaternary ammonium salts (R₄N⁺X^-, X = Cl, Br, I) as Lewis bases, in the presence of titanium(IV) chloride, the chlorinated aldol adduct 1 was obtained as the major product. Quaternary ammonium bromides and iodides (R₄N⁺X^-, X = Br, I) have higher catalytic activity than corresponding chlorides (R₄N⁺Cl^-). Quaternary ammonium fluorides (R₄N⁺F^-) do not have activity at all. The amounts of Lewis acid and quaternary ammonium salts used affect the reaction rate and product. A plausible reaction mechanism is proposed. If the reaction was carried out at room temperature (about 20 °C) in the presence of titanium(IV) chloride and quaternary ammonium salts (R₄N⁺X^-, X = Cl, Br, I), the elimination product 3, derived from 1, was formed as the major product.