INFLUENCE OF LEWIS-ACIDS ON THE CYCLOADDITION REACTIONS OF CYANO-SUBSTITUTED AND CARBOMETHOXY-SUBSTITUTED OLEFINS

The reactions of electrophilic olefins substituted with cyano and/or c arbomethoxy groups with donor olefins are investigated. The donor olef ins include, in order of decreasing donor character, phenyl vinyl sulf ide, isobutyl vinyl ether, tert-butyldimethylsilyl vinyl ether, p-meth oxy- and p-methylstyrene, and styrene. The reactions vary ii om [2 + 2 ] cycloadditions to Diels-Alder reactions to spontaneous free radical copolymerizations. The course of the reaction is determined by both th e electron disparity between the two reacting olefins and by the subst itution pattern on the electrophilic olefin. Lewis acids (ZnCl2 or eth ereal LiClO4) are added to form complexes with the acceptor olefins, r esulting in a larger electron disparity between the reactants. With th e highly electrophilic olefins with gem-dicyano groups, such as tetrac yanoethylene, dimethyl 1,1-dicyanoethylene-2,2-dicarboxylate, and meth yl beta,beta-dicyanoacrylate, [2 + 2] cycloadditions occur at room tem perature, except for the reaction of TCNE with styrene which has to be carried out in 5 M ethereal LiClO4. With olefins containing one cyano and one carbomethoxy group on the same carbon, dimethyl dicyanofumara te and dimethyl cyanofumarate, inverse electron demand Diels-Alder cyc loaddition involving the ester substituent dominates in reactions with the most nucleophilic olefins. The [2 + 2] cycloadducts can be obtain ed in the presence of Lewis acid. With olefins with two carbomethoxy-s ubstituents on the same carbon, dimethyl 2-cyanoethene-1,1-dicarboxyla te and trimethyl ethylenetricarboxylate, the reactions are much slower , and [2 + 4] cycloaddition dominates with the most reactive donor ole fins, while the [2 + 2] cycloadduct can be obtained in the presence of Lewis acid. With several acceptor olefins, free radical copolymerizat ions compete with the cycloadditions when reacted with the least react ive donor olefins. The reaction tendencies are discussed in light of t he electron disparity between the olefins. The influence of the Lewis acid on the conformational equilibria and on the reaction course is al so addressed.