in lewis acid catalysis of organic reactions, a metal-based lewis acid acts as an electron pair acceptor to increase the reactivity of a substrate. common lewis acid catalysts are based on main group metals such as aluminum, boron, silicon, and tin, as well as many early (titanium, zirconium) and late (iron, copper, zinc) d-block metals. the metal atom forms an adduct with a lone-pair bearing electronegative atom in the substrate, such as oxygen (both sp or sp), nitrogen, sulfur, and halogens. the complexation has partial charge-transfer character and makes the lone-pair donor effectively more electronegative, activating the substrate toward nucleophilic attack, heterolytic bond cleavage, or cycloaddition with 1,3-dienes and 1,3-dipoles.