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(phen ylmaleic anhydride) [PPMA] yields two products in an overall yield app roaching 80%. Contrary to a previous report, the major product has bee n identified by X-ray crystallography as the novel [5]helicene-5,6,9,1 0-tetracarboxylic 5,6:9,10-dianhydride [HLDA], while the minor product (<5% yield) is the isomeric dibenz[ah]anthracene-5,6,12,13-tetracarbo xylic 5,6:12,13-dianhydride [DBAA]. Partial irradiation permits the is olation of the monocyclization intermediate PMPA, which under the phot olysis conditions generates the observed dicyclization products. Unlik e HLDA, DBAA resists prolonged methanolysis to the corresponding acid esters. This confirms molecular modeling predictions that the 1,6-inte raction between the H-4, H-7, H-11, and H-14 aromatic hydrogens and th e corresponding neighboring carbonyl oxygens in DBAA inhibit opening o f 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 s emiempirical calculations indicate that, contrary to [5]helicene, ther e 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.