THE FIRST THREE AROMATIC TRIBROMIDES OF THE PARACYCLOPHANE SERIES

Article abstract of DOI:10.1007/BF00864341

From the reaction mixtures in the uncatalyzed polybromination of <2.2>paracyclophane by the action of excess Br2 in CCl4, there have been found along with the known products - 4,15- and 4,16-dibromo<2.2>paracyclophanes - two new aromatic tribromides of this series, which have been isolated in pure form; 4,12,15- and 4.15.16-tribromo<2.2>paracyclophanes.Special experiments demonstrated that the mixtures of these tribromides are formed as a result of competitive monobromination of 4,15-dibromo<2.2>paracyclophane; the 4,15,16-tribromo<2.2>paracyclophane, together with still another newly isolated isomer of this series - 4,8,12-tri bromo<2.2>paracyclophane - is formed as a result of competitive monobromination of 4,16-dibromo<2.2>paracyclophane.As an explanation of the features of the orienting effect of substituents in these competing reactions, a rule was proposed: On the conventional orientation (from the electronic point of view) of entry of the bromine atom into the substituted ring (para > ortho > meta), a steric limitation is imposed on its attack in the pseudo-gem-position, owing to the bulky bromine atom that is transannularly positioned above it in the neighboring aromatic ring.The structures of all of the tribromides were established on the basis of elemental analyses, mass spectrometry, and 1H NMR spectrometry (including PMR using the homonuclear Overhauser effect).The data obtained in this work indicate that the 4,12,15-tribromo<2.2>paracyclophane and 4,15,16-tribromo<2.2>paracyclophane are predecessors of the two tetrabromides previously obtained by Cram - 4,7,12,15- and 4,5,15,16-tetrabromo<2.2>paracyclophanes: and the 4,8,12-tribromo<2.2>paracyclophane is a possible predecessor of 4,8,12,16-tetrabromo<2.2>paracyclophane, which is unknown up to the present time . Keywords: <2.2>paracyclophane, dibromo<2.2>paracyclophane, tribromo<2.2>paracyclophane.