THE 1ST 3 AROMATIC TRIBROMIDES OF THE [2.2]PARACYCLOPHANE SERIES

From the reaction mixtures in the uncatalyzed polybromination of [2.2] paracyclophane by the action of excess Br2 in CCl4, there have been fo und along with the known products - 4,15- and 4,16-dibromo[2.2]paracyc lophanes - two new aromatic tribromides of this series, which have bee n isolated in pure form: 4,12,15- and 4,15,16-tribromo[2.2]paracycloph anes. Special experiments demonstrated that the mixtures of these trib romides are formed as a result of competitive monobromination of 4,15- dibromo[2.2]paracyclophane; the 4,15,16-tribromo[2.2]paracyclophane, t ogether with still another newly isolated isomer of this series - 4,8, 12-tribromo[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 compe ting reactions, a rule was proposed: On the conventional orientation ( from the electronic point of view) of entry of the bromine atom into t he substituted ring (para > ortho > meta), a steric limitation is impo sed on its attack in the pseudo-gem-position, owing to the bulky bromi ne atom that is transannularly positioned above it in the neighboring aromatic ring. The structures of all of the tribromides were establish ed on the basis of elemental analyses, mass spectrometry, and H-1 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 predecessor s of the two tetrabromides previously obtained by Cram - 4,7,12,15- an d 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.