Antiaromatic interaction in the 9-methoxybicyclo[4.2.1]nona-2,4,7-trien-9-yl cation. Evidence of orbital symmetry control over 4p-electron interactions

Ionization reactions of anti-9-chloro-9-methoxybicyclo[4.2.l]nona-2,4,7-trie(n8e) proceed without skeletal rearrangements under conditions of short life. Rate constants were measured for the reaction of 8 and its more saturated analogues with pyridine. From the relatively low reaction rate of triene 8 it is concluded that the [4.2.1] cation is destabilized consequent to the homoantiaromatic interaction between the cationic center and the butadiene moiety. In addition to the kinetic results, the presence of this type of interaction is revealed by a NMR study of the 9-methoxybicyclo[4.2.1]nona-2,4-dien-9-cya1t ion (21) and the 1 I-methoxybicyclo[4.4. l]undeca-2,4,8-trien-ll-ylc ation (25) under conditions of long life, Le., superacid media. The IH and I3C NMR data point to an interaction of the cationic center with one of the double bonds of the butadiene moiety. Obviously the mode of homoconjugative interaction is controlled by orbital symmetry.