Pressure Effect on the Mechanism of the Thermal (2+2) Cycloaddition Process
To explain the mechanisms of chemical reactions, it is very important to have information on the intermediates of these reactions. The thermal (2+2) cycloaddition in one step is forbidden by Woodward and Hoffmann’s rule . In other words, the orbital symmetry rule predicts that zwitterion or biradical intermediates may exist in thermal (2+2) cycloadditions. The zwitterionic intermediate or transition State was suggested first by Williams et al.  who found that the strong electron acceptor, tetracyanoethylene (TCNE) cycloadded to electron-rich olefins like methyl vinyl ether or p-methoxystyrene under mild conditions. This suggestion was based on the qualitative experimental results that these reactions were markedly accelerated by the polar solvents and the electron donating groups attached, directly or through the benzene ring, to the double bond of the nucleophilic olefins. The mechanism of the thermal cycloadditions by way of the zwitterionic intermediate was reviewed in detail by Gompper  and Bartlett . The stepwise mechanism was confirmed by the high but not complete stereospecificity , and by the trapping reaction with methanol . The large pressure effect on the cycloaddition of this type also was interpreted as strong evidence for the zwitterion formation .
KeywordsMaleic Anhydride Lower Wave Number Zwitterion Formation Enol Ether Stepwise Mechanism
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