Conformation et réactivité de dérivés [4.n.0] bicycliques à jonction trans—XXVI: Méthanolyse du tosylate de phényl-4a bicyclo(4.2.0]-octyle-3a trans et du tosylate de phényl-4e bicyclo[4.2.0]octyle-3e trans en l'absence et en présence de MeONa. mise en évidence d'un intérmediaire ion phénonium

The structure of the reaction products and the rate constants for methanolysis of 4a-phenyl 3a-tosyloxy bicyclo4.2.0]octane trans 1 and 4e-phenyl 3e-tosyloxy bicyclo(4.2.0]octane trans 2 in different MeO?/MeOH systems, suggest that these reactions do not involve a phenyl-assisted, but an ion-pairing process. The secondary cation formed in a first slow step leads to the phenyl-bridged ion by a phenyl shift. Owing to steric hindrance, solvent attack is not observed on the covalent compound 1, and a solvent change causes the mechanism to change: phenyl participation which has been previously established in CF₃CH₂OH, a convenient solvent for this assisted process, is not observed in MeOH. So in a more nucleophilic medium when solvent assistance is inhibited, unlike β phenylalkyl primary derivatives, a secondary β phenyl substrate may not react by phenyl participation but may present a nearly limiting behavior.