Substituent-Dependent Nitration of 9-Substituted 9,10-Dihydro-9,10-ethanoanthracenes

Abstract

Mononitration of 9-substituted ethanoanthracenes, bearing Me, Bu^t, F, Br, I, OMe , NO₂, CN, CHO or CO₂Me substituents at the bridgehead carbon, was found to occur exclusively at the β-positions of the aromatic ring. The mononitro products were isolated, identified by ¹H n.m.r. spectroscopy, and their relative proportions were estimated by quantitative g.l.c . and/or by ¹H n.m.r. spectroscopy. For all the above substrates the proportion of nitration at the β-position meta to the bridgehead carbon bearing the substituent [to give compounds of the general form (4)] was greater than the proportion of nitration at the corresponding β-position para to the bridgehead substituent [to give compounds of the general form (3)]. Whilst the preferential nitration at the β-positions of the aromatic rings is consistent with the previously reported nitration of 9,10-dihydro-9,10-ethanoanthracene (2a) itself, no observations of this preferential meta attack have been made previously. No correlation could be made of this behaviour with available substituent parameters for the widely sterically and electronically disparate set of substituents used in this study, and the origin of this preferential attack remains unclear. Dinitration in this system was studied only superficially. The influence of the bridgehead substituent together with that of the nitro group already present on one aromatic ring appear to combine with quite unpredictable results in orienting the position of attack of the incoming nitro group onto the other (non-nitrated) aromatic ring.