Triple Regioselective Functionalization of Cationic [4]Helicenes via Iridium-Catalyzed Borylation and Suzuki Cross-Coupling Reactivity

Abstract

In essentially one-pot, using Ir- and Pd-catalysis, tris(arene)-functionalized cationic [4]helicenes are synthesized with full regioselectivity and enantiospecificity starting from a trivial precursor (17 examples). This poly-addition of aryl groups improves key optical properties, that is, fluorescence quantum yields and lifetimes. Electronic circular dichroism and circularly polarized luminescence signatures are observed up to the far-red domain, in particular with additional arenes prone to aggregation.

Keywords: Ir-catalyzed borylation •late stage functionalization; cationic helicenes; chiroptical spectroscopy; dyes and fluorophores.

Scheme 1

Late‐stage functionalization of polyaromatics. Selected examples. A: cationic [4]helicene 1 by S _E Ar mechanisms. B: Dioxa [6]helicene 2 by oxidative nucleophilic coupling. C: Ir‐catalyzed borylation of carbohelicenes, D: of cationic triaza triangulenes, E: of racemic and enantiopure DMQA 1 (M enantiomer shown) and subsequent cross‐coupling derivatizations. FG: functional group. Ar: aryl.

Figure 1

Scope of triple borylations and subsequent triple Suzuki−Miyaura cross couplings. Isolated yields for BF₄ ⁻ salts in racemic series and, in parenthesis, for (M)‐ and (P)‐enantiomers (average of both enantiospecific reactions). EWGs and EDGs in red and blue‐coded series, respectively. With aryl bromides in grey color, a lack of reactivity or purification issues were observed. [a] [Ir(cod)OMe]₂ (20 mol%), tmphen (40 mol%), B₂pin₂ (6 equiv). [b] Pd(OAc)₂ (60 mol%), dppf (60 mol%), ArBr (9 equiv), KOH (9 equiv). [c] SMCC reaction conditions: Pd(PPh₃)₄ (80 mol%), Cs₂CO₃ (10 equiv), 9‐Br‐anthracene (10 equiv), THF:1,4‐dioxane (2 : 1). For this single example, (M)‐6 l and (P)‐6 l products were obtained from the racemate via a chiral stationary phase HPLC resolution.

Figure 2

Chemical drawing of (P)‐6 e and (M)‐6 o and X‐ray structures of cationic helicenes; hydrogen atoms, BF₄ ⁻ counter‐ion and solvate molecules are omitted for clarity purpose.

Figure 3

Optical properties of selected derivatives, 1 (dotted black), 6 a (purple), 6 h (red) and 6 o (blue) in acetonitrile. a: Absorbance spectra. b: Emission spectra. c: Fluorescence decay.

Figure 4

ECD spectra of (M)‐1 (dotted black line), (P)‐1 (dotted grey line), (M)‐6 a (purple line) and (P)‐6 a (pink line) in acetonitrile solution. Inset: expansion between 500 and 700 nm.

Figure 5

a: Picture of 6 m with increasing amount of water (0‐90 %) in acetonitrile, taken under 365 nm irradiation. b: Evolution of absorption and ECD spectra of 6 m with increasing water content (0–90 %), (M)‐enantiomer only shown for clarity; c: Emission and CPL spectra of (M)‐6 m (full lines) and (P)‐6 m (dotted lines) with increasing water content (0–75 %).