Benzannelation mode

Arenium ion energies (AAH ion - neutral) and changes in carbon charges [Aq = (ion) - q (neutral)] for protonation of 90 and 91 were probed by the AMI method. The singlet oxidation dication of 90 was also calculated. The charge delocalization modes in the PAH arenium ions were discussed and compared. The AMI studies indicated that benzo[<3]coronene cations were less delocalized than benzo[g/z/]perylene ions. Benzannelation (91 92) severely limited the conjugation path in the carboca-tions despite the fact that coronene 92 was still planar. Further benzannelation (92 90) had a minimal effect on the charge delocalization mode. 

Betaines analogous to the 3-oxidopyridinium species have been prepared, and found to dimerize rapidly on liberation from the perchloric acid salts. Two dimers are formed, (121) and (122), in the ratio ca. 12 1 when R = methyl, though only (121) is formed when R = H. When the betaine is benzannelated, however, the mode of dimerization is different, and affords (123) and (124) in the ratio 7 1 (75JCS(P1)1366,75JCS(Pl)2099>. 

The most exciting and perhaps the most versatile application of the electrocyclic ring-closure elimination method may be the benzannelation reaction. The ready availability of the starting compounds and in general the high yields make this new mode of formation of long- and well-known aromatics the synthesis of choice, especially if an aimed substitution is desired in the compound. 

Of di-7c-methane and oxa-di-7t-methane rearrangements reported, that of greatest preparative value involves the photolysis of bicyclo[2,2,2]octadienones. The triplet-state reactions of these latter compounds have been examined for the first time in the absence of benzannelation. Bicyclo-octadienones (42) have available to them four di-7t-methane and two oxa-di-ii-methane modes of rearrangement, and, remarkably, only one di-7c-methane route is followed, to give (43) regiospecifically and in good (40—70 %) yields. 

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