Doubly fused fluorene embedded heterosapphyrins

Abstract

We report the synthesis of rare examples of nonaromatic doubly fused fluorene embedded heterosapphyrins by [3 + 2] condensation of 2,7-bis((1H-pyrrol-2-yl)(p-tolyl)methyl)-9H-fluorene (fluorene based tripyrrane) and [2,2′-bithiophene]-5,5′-diylbis(4-aryl)methanol (bithiophene diol) in CH₂Cl₂ using BF₃·OEt₂ as a mild acid and DDQ (2 equiv.) as an oxidant. The condensation resulted in the formation of unexpected doubly fused fluorene embedded heterosapphyrins due to the inversion of the bithiophene moiety followed by the fusion of β-thiophene carbons of the bithiophene unit with two adjacent pyrrole nitrogens. This led to the formation of two tripentacyclic moieties in the sapphyrin macrocycle. The molecular structures of the macrocycles were confirmed by HR-MS and 1D and 2D NMR spectroscopy. Since the formation of doubly-fused heterosapphyrins proceeded via the formation of mono-fused calix heterosapphyrins, three examples of mono-fused calix heterosapphyrins were synthesized under similar BF₃·OEt₂ catalysed reaction conditions using only one equivalent of DDQ as an oxidant. The mono-fused calix heterosapphyrins were not very stable and were characterized by ¹H, ¹³C, ¹H–¹H COSY, ¹H–¹H NOESY and ¹H–¹³C HSQC NMR studies. DFT studies on mono-fused and doubly-fused heterosapphyrins indicated that the fluorene moiety was in perpendicular orientation with the rest of the macrocycle. The macrocycles showed two broad featureless bands in the region of 350–800 nm and the electrochemical studies on doubly-fused heterosapphyrins indicated that the macrocycles are relatively electron rich. TD-DFT studies are in agreement with the experimental observations.