On the oxidation of dibenzo[fg,ij]pentaphene and dinaphtho[2,1,8,7-defg:2',1',8',7'-ijkl]pentaphene dianions

The dianion of dibenzoSfg,ijCpentaphene has recently been synthesized and shown to react with oxygen, forming the 15,16-dioxo derivative in nearly quantitative yield. Oxidation of the dianion of dinaphtho[2,1,8,7-defg:2',1',8',7'-ijkl]pentaphene under similar conditions does not result in any dioxo product. It is shown that these findings are a consequence of the different modes of cyclic conjugation in the two dianions. Cyclic conjugation in the mentioned two dianions, as well as in a number of other structurally related benzenoid dianions is assessed by means of a quantum-chemical method, developed earlier by the author.