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 carbomethoxy groups with donor olefins are investigated. The donor olefins include, in order of decreasing donor character, phenyl vinyl sulfide, isobutyl vinyl ether, tert-butyldimethylsilyl vinyl ether, p-methoxy- 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 the electron disparity between the two reacting olefins and by the substitution pattern on the electrophilic olefin. Lewis acids (ZnCl2 or ethereal LiClO4) are added to form complexes with the acceptor olefins, resulting in a larger electron disparity between the reactants. With the highly electrophilic olefins with gem-dicyano groups, such as tetracyanoethylene, dimethyl 1,1-dicyanoethylene-2,2-dicarboxylate, and methyl beta,beta-dicyanoacrylate, [2 + 2] cycloadditions occur at room temperature, 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 dicyanofumarate and dimethyl cyanofumarate, inverse electron demand Diels-Alder cycloaddition involving the ester substituent dominates in reactions with the most nucleophilic olefins. The [2 + 2] cycloadducts can be obtained in the presence of Lewis acid. With olefins with two carbomethoxy-substituents on the same carbon, dimethyl 2-cyanoethene-1,1-dicarboxylate and trimethyl ethylenetricarboxylate, the reactions are much slower, and [2 + 4] cycloaddition dominates with the most reactive donor olefins, while the [2 + 2] cycloadduct can be obtained in the presence of Lewis acid. With several acceptor olefins, free radical copolymerizations compete with the cycloadditions when reacted with the least reactive donor olefins. The reaction tendencies are discussed in light of the electron disparity between the olefins. The influence of the Lewis acid on the conformational equilibria and on the reaction course is also addressed.