Synthesis and Properties of 1 H-Pyrrolo[3',2':3,4]fluoreno[9,1- gh]quinolines and 7 H-Pyrrolo[2',3',4':4,10]anthra[1,9- fg]quinolines

Abstract

We report the synthesis of pyrrolo[3',2':3,4]fluoreno[9,1-gh]quinoline and pyrrolo[2',3',4':4,10]anthra[1,9-fg]quinoline derivatives. This novel class of N-doped polycyclic heteroaromatic compounds was synthesized by a site-selective cross-coupling reaction followed by acid-mediated cycloisomerization and Pd-catalyzed CH arylation as the final ring-closing reactions. Preliminary optical and aromatic properties were studied by means of steady-state absorption and fluorescence spectroscopy and DFT calculation. Special emphasis was placed on the impact of ring alternation and position of the N-doping within the scaffold.

Keywords: CH activation; PAHs; catalysis; cyclization; heterocycles; palladium; regioselectivity.

Scheme 1

Synthesis strategy for 1-methyl-1H-pyrrolo[3′,2′:3,4]fluoreno[9,1-gh]isoquinoline and 1-methyl-1H-pyrrolo[2′,3′,4′:4,10]anthra[9,1-gh]isoquinoline derivatives. Conditions: i (for 2a), 1a (1.0 equiv.), 1-Chloro-2-ethynylbenzene (1.2 equiv.), Pd(PPh₃)₄ (5 mol%), CuI (2 mol%), HNⁱPr₂, 40 °C, 1 h; (for 2b), 1b (1.0 equiv.), 1-Chloro-2-ethynylbenzene (1.2 equiv.), Pd(PPh₃)₄ (10 mol%), CuI (10 mol%), HNⁱPr₂, 40 °C, 3 h; ii, 2 (1.0 equiv.), 1-methyl-1H-indol-5-yl)boronic acid pinacol ester (1.2 equiv.), Pd(PPh₃)₄ (5 mol%), K₃PO₄ (2.0 equiv.), 1,4-dioxane/H₂O (6:1), 100 °C, 3 h; iii, 3 (1.0 equiv.), MsOH (30 equiv.), 120 °C, 18 h.

Scheme 2

Synthesis of 1-methyl-1H-pyrrolo[3′,2′:3,4]fluoreno[9,1-gh]isoquinoline (6) and 1-methyl-1H-pyrrolo[2′,3′,4′:4,10]anthra[9,1-gh]isoquinoline derivatives (7). Reaction conditions i: 4 or 5 (53–120 mg, 1.0 equiv.), PdCl₂ (0.2 equiv.), PCy₃ (0.4 equiv.), DBU (3.0 equiv.), NMP, 150 °C, 18 h.

Figure 1

X-ray crystal structure of compound 6a. (Left): single-crystal structure of 6a; (Middle): formed dimers within the lattice with short C⋯C-contacts; (Right): herringbone-type alignment of dimers.

Figure 2

Absorption (left) and emission (right) spectra of compounds 6a,b and 7a,b.

Figure 3

Visualization of HOMO and LUMO with associated energies.

Figure 4

NICS(1.7)_zz and NICS2BC(1.25) ring currents of 6a and 7a.