REVISED STRUCTURES OF N-SUBSTITUTED DIBROMINATED PYRROLE DERIVATIVES AND THEIR POLYMERIC PRODUCTS - TERMALEIMIDE MODELS WITH LOW OPTICAL BAND-GAPS

This paper describes an unexpected rearrangement/oxidation of N-substi tuted 2,5-dibromopyrroles upon treatment with HNO3. The bromides migra te from the 2,5-positions to the 3,4-positions with subsequent oxidati on at the 2,5-positions to afford N-substituted 3,4-dibromomaleimides; the structure was confirmed by single-crystal X-ray analysis. The mal eimides were then polymerized to the poly(N-substituted-3,4-maleimide) s with copper bronze. This constitutes a revision of structure for the monomers and polymers. The propensity for the dibromide migration was further confirmed by treatment of N-benzyl-2,5-dibromopyrrole under n onoxidative acidic conditions (p-TsOH) to afford N-benzyl-3,4-dibromop yrrole; both the starting material and product structures were confirm ed by single-crystal X-ray analysis. Several termaleimides were prepar ed from pyrrole, maleic anhydride, and citraconic anhydride. These tri meric compounds underwent enormous shifts in their optical absorbance maxima (ca. 200 nm) when bases or nucleophilic solvents were added. Th erefore, the termaleimides served as excellent models for the polymeri c systems that had undergone shifts of 350-400 nm upon treatment with the same additives. Ab initio Hartree-Fock and density functional theo ry were utilized to assess the minimum conformation of the trimeric sy stem. Both terminal maleimides appear canted 37 degrees relative to th e central maleimide unit. As the two end maleimide units were computat ionally forced into closer proximity, there was a dipolar stabilizatio n that ensued between the two terminal maleimides with the formation o f a 1,3-dioxetane. However, it is unlikely that there could be the for mation of an isolable 1,3-dioxetane due to the large energy difference between the canted structure and the dioxetane. A significant decreas e in the HOMO-LUMO energy of 13 kcal/mol was calculated upon formation of the 1,3-dioxetane, suggesting that nucleophiles likely move the ca nted structure more toward a planar form via addition to the alpha,bet a-unsaturated carbonyl units.