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Studies toward the chemistry of N-confused oligopyrroles Liu, Runchang


The objectives of this work were to explore the synthesis of linear N-confused (α-meso-β) oligopyrrolic pigments (variants of the corresponding α,α’-linear oligopyrrolic pigments), as well as to study the stability, optical properties and cyclization reactivity of these unprecedented N-confused oligopyrrole structures. MSA (methanesulfonic acid)-catalyzed condensation of aryl aldehydes with a variety of electron-withdrawing group (EWG)-substituted pyrroles to afford the corresponding meso-aryl N-confused diacyl dipyrromethanes was established. The directing effect of these EWGs for α-meso-β-linkage formation in N-confused dipyrromethanes was evaluated. The key for the successful transformation appears to depend upon the reactivities of the aldehyde and the electron-poor pyrrole substrates used. This novel one-pot synthetic approach for construction of α-meso-β-linkages between pyrroles is a substantial improvement over the classic multi-step synthesis (Chapter 2). Oxidation of an N-confused dipyrromethane to the corresponding conjugated dipyrrin system was explored by applying different oxidants. An efficient DDQ-mediated benzylic C-H activation for hydroxylation, alkoxylation and heteroarylation of meso-aryl DPMs (three different forms) to the corresponding meso-modified dipyrromethanes (DPMs) was established. Mechanistic study shows the reaction proceeds through a highly-unstable protonated dipyrrin intermediate (Chapter 3). Exploration of the oxidation process of the dipyrromethane allows discussion of the meso-steric effect observed during the oxidation, where substituents on the 2- and 6- positions of the meso-aryl group play important roles in producing and stabilizing the conjugated pyrrolemethene. N-confused (α-meso-β) dipyrrins were studied with regards to their synthesis, stability, reactivity, and configurational interconversion. The N-confused dipyrrinodiol can be imbedded as a building block into oligopyrrole frameworks efficiently with retention of the Z-syn N-confused moiety, which provides a concise way to create higher N-confused oligopyrrolic pigments (Chapter 4). Subsequently, in chapter 5, we report our synthesis of the conjugated N-confused oligopyrrolic macrocycle [12] tripyrrin (0.1.1) via an oxidative amination cyclization. As a macrocycle constructed purely of meso-aryl groups and pyrrole, N-confused [12] tripyrrin (0.1.1) was synthesized using an unprecedented head-to-tail cyclization. Also, linear N-confused tri- and tetrapyrrolemethenes were synthesized and fully characterized by X-ray analysis. Conversion of α,α’-linear oligopyrroles to their corresponding N-confused counterparts was achieved despite their presumed instability.

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