The cationic oligomerization of p-aminostyrene 2. Dimerization in the intial stage

Summary It was found that p-aminostyrene (PAS) could be oligomerized by using several aminium perchlorates as catalysts in acetonitrile to give a dimer D in the initial stage of the reaction and finally the isomerization oligomer 1 at the end, in quite similar manner to the preceding paper (1), where dimerization (hydroamination) is the main reaction in the initial stage. The pK_a values for the amines were evaluated by the indicator method, and the H⁺]₀ and MH⁺]₀ values were calculated from equilibrium equations. As a result, the rate increased with decreasing pK_a of the catalytic amine used and reached an overall constant value. From the relationships between the rate of the reaction and the H⁺]₀ or MH⁺]₀ values calculated numerically, however, it was determined that the rate was more affected by the H⁺]₀ or M⁺]₀ values rather than the pK_a of the catalyst. It was assumed that the reaction would be initiated by the electrophilic attack of MH⁺ on the double bond of the monomer. Three successive reactions were proposed for the dimerization step: slow protonation followed by subsequent fast reactions; amine addition to the resultant styryl cation and proton elimination. It was assumed that oligomerization might proceed via the same three successive reactions as proposed for the dimerization step. Received: 24 July 2000/Revised version: 8 September 2000/Accepted: 18 September 2000