Lewis acid catalysis for (2+2)-cycloadditions and alternating copolymerizations.

Abstract

(2+2) -Cycloaddition reactions between alkenes take place spontaneously when one alkene contains strong donors and the other strong acceptors. However, when the difference of electron-density between these nucleophilic and electrophilic partners is small, the (2+2) -cycloaddition does not take place. Lewis acids are known as catalysts for many purposes due to their capability to increase the accepting ability of an electrophile, and can be utilized as catalysts for these (2+2) -cycloadditions. In this study, it is shown that ethereal lithium perchlorate, which has recently been recognized as an astounding medium for many reactions, and zinc chloride can promote (2+2) -cycloadditions. This systematic study of the (2+2) -cycloadditions between various nucleophiles and electrophiles leads to an expansion of "The Organic Chemist's Periodic Table". When the electrophilic partner contains at least one ester at the olefinic end, the (2+2) -cycloaddition can occur in the presence of Lewis acid, instead of the reported inverse electron demand Diels-Alder reaction. The efficiency of Lewis acid catalysis to promote (2+2) -cycloadditions for olefin pairs where they normally do not occur is investigated. Weak Lewis acids are suitable for the (2+2) -cycloaddition in contrast to strong Lewis acids, which cause decomposition of the cyclobutanes. In addition, many cyclobutane derivatives containing an acidic proton at C₃ and a leaving group derived from sulfide at C₁ are synthesized. However, the aim of obtaining bicyclobutane by 1,3-elimination of these cyclobutane derivatives was not achieved. An exploratory study using zinc chloride as Lewis acid to promote the spontaneous alternating copolymerization between the weak acceptor olefin acrylonitrile and the donor olefins, p-methylstyrene, p-methoxystyrene and isobutyl vinyl ether was also performed.