Mass-spectrometric and quantum-chemical study of fragmentation of diallyl esters of dicarboxylic acids

Abstract

In a mechanistic study of thermal breakdown of diallyl polymers, six diallyl esters of dicarboxylic acids have been investigated by means of mass spectrometry. A characteristic feature of the fragmentation of these compounds is the low stability of the molecular ions and detachment of the allyl cation, the peak of which is usually the highest in the mass spectrum. In interpreting such an unusual path of fragmentation of esters, quantum-chemical calculations have been carried out in the MINDO/3 approximation; the preferred conformation of the diallyl maleate molecule and the distribution of electron density in this conformation have been determined, and the most stable cyclic structure of the intermediate cation [CH=CH(CO)2OH]+ has been established. The nature of the “bridge” between the carboxyl groups in diallyl esters of dicarboxylic acids has practically no influence on the direction of fragmentation. The decisive factor in the fragmentation is the electrostatic interaction of charges on the atoms of carbonyl carbon and ester oxygen of the second carboxyl group. A fragmentation scheme with the formation of intermediate cyclic cations is proposed.