Revised Structures of N-Substituted Dibrominated Pyrrole Derivatives and Their Polymeric Products. Termaleimide Models with Low Optical Band Gaps.
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This paper describes an unexpected rearrangement/oxidation of N-substituted 2,5-dibromopyrroles upon treatment with HNO(3). The bromides migrate from the 2,5-positions to the 3,4-positions with subsequent oxidation at the 2,5-positions to afford N-substituted 3,4-dibromomaleimides; the structure was confirmed by single-crystal X-ray analysis. The maleimides 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 nonoxidative acidic conditions (p-TsOH) to afford N-benzyl-3,4-dibromopyrrole; both the starting material and product structures were confirmed by single-crystal X-ray analysis. Several termaleimides were prepared from pyrrole, maleic anhydride, and citraconic anhydride. These trimeric compounds underwent enormous shifts in their optical absorbance maxima (ca. 200 nm) when bases or nucleophilic solvents were added. Therefore, the termaleimides served as excellent models for the polymeric systems that had undergone shifts of 350-400 nm upon treatment with the same additives. Ab initio Hartree-Fock and density functional theory were utilized to assess the minimum conformation of the trimeric system. Both terminal maleimides appear canted 37 degrees relative to the central maleimide unit. As the two end maleimide units were computationally forced into closer proximity, there was a dipolar stabilization that ensued between the two terminal maleimides with the formation of a 1,3-dioxetane. However, it is unlikely that there could be the formation of an isolable 1,3-dioxetane due to the large energy difference between the canted structure and the dioxetane. A significant decrease in the HOMO-LUMO energy of 13 kcal/mol was calculated upon formation of the 1,3-dioxetane, suggesting that nucleophiles likely move the canted structure more toward a planar form via addition to the alpha,beta-unsaturated carbonyl units.