REINVESTIGATION OF THE PHOTOCYCLIZATION OF 1,4-PHENYLENE BIS(PHENYLMALEIC ANHYDRIDE) - PREPARATION AND STRUCTURE OF [5]HELICENE 5,6-9,10-DIANHYDRIDE

The I-2-catalyzed photooxidative cyclization of 1,4-phenylene bis(phenylmaleic anhydride) [PPMA] yields two products in an overall yield approaching 80%. Contrary to a previous report, the major product has been identified by X-ray crystallography as the novel [5]helicene-5,6,9,10-tetracarboxylic 5,6:9,10-dianhydride [HLDA], while the minor product (<5% yield) is the isomeric dibenz[ah]anthracene-5,6,12,13-tetracarboxylic 5,6:12,13-dianhydride [DBAA]. Partial irradiation permits the isolation of the monocyclization intermediate PMPA, which under the photolysis conditions generates the observed dicyclization products. Unlike HLDA, DBAA resists prolonged methanolysis to the corresponding acid esters. This confirms molecular modeling predictions that the 1,6-interaction between the H-4, H-7, H-11, and H-14 aromatic hydrogens and the corresponding neighboring carbonyl oxygens in DBAA inhibit opening of the resulting O-CO anhydride bond. Noteworthy as well is that HLDA, unlike the parent [5]helicene, resists further photocyclization under the reaction conditions to the:corresponding benzo[ghi]perylene. AM1 semiempirical calculations indicate that, contrary to [5]helicene, there is no buildup of bonding interaction between the designated bonding carbons (C-1 and C-14) as one goes from the HOMO to the corresponding LUMO.