Dibenzoylbenzodipyrroles: Key Precursors for the Synthesis of Fused Meso-Aryl Sapphyrins.

A simple strategy has been developed for the synthesis of a,a' dibenzoylbenzodipyrroles, which are key synthons for the synthesis of fused porphyrinoids. alpha,alpha'-Dibenzoylbenzodipyrroles were characterized by high-resolution mass spectrometry (HRMS), NMR, and X-ray crystallography. To show the use of alpha,alpha'-dibenzoylbenzodipyrrole, we synthesized five different fused meso-aryl sapphyrins under acid-catalyzed reaction conditions. alpha,alpha'-Dibenzoylbenzodipyrrole was reduced to diol and condensed with five different tripyrranes such as aza, oxa, thia, selena, and telluratripyrranes under mild acid-catalyzed conditions to afford fused meso-aryl sapphyrins in 15-18% yields. One-dimensional (1D) and two-dimensional (2D) NMR studies revealed that in fused sapphyrins, the furan ring in oxabenzosapphyrin and the pyrrole ring in benzosapphyrin, which are present opposite to the benzodipyrrole moiety, attained ring inversion (inverted sapphyrins), whereas the selenophene ring in selenabenzosapphyrin and the tellurophene ring in tellurabenzosapphyrin did not show ring inversion (normal sapphyrins). However, thiabenzosapphyrin exhibits both normal and inverted conformations in different ratios. All fused sapphyrins showed typical aromatic absorption features; however, the absorption features of normal fused sapphyrins are different from the inverted fused sapphyrins. Redox studies indicate that normal fused sapphyrins are difficult to oxidize but easier to reduce compared to inverted fused sapphyrins. Density functional theory (DFT) studies support the experimental observations.