IS THROUGH-BOND DIHYDROAROMATICITY ATTAINABLE - PREPARATION OF [4,5]DIHOMOTROPONE, INVESTIGATION OF ITS GROUND-STATE PROPERTIES, AND AN ATTEMPT TO GENERATE THE DIHOMOTROPYLIUM CATION

Construction of the first dihomotropone (6) has been accomplished. The key elements of the synthesis were a regiocontrolled expansion of a [ 4.1.1] bicyclic cr-bromo ketone and introduction of the dienone double bonds by means of the Garbisch method. The spectral properties of 6 u nder neutral and strongly acidic conditions are described and interpre ted in terms of a lack of dihomoaromatic character. The parent bicyclo [5.1.1]nonadienyl cation is unstable and experiences ready Wagner-Meer wein rearrangement to the bicyclo[4.2.1]nonadien-7-yl cation. These fe atures appear well suited to the ultimate evaluation of through-bond i nteraction in tricyclic homologues of 6 that carry perpendicular pi ar rays in the third ring.