A C–H activation-based enantioselective synthesis of lower carbo[n]helicenes

Abstract

The three-dimensional structure of carbohelicenes has fascinated generations of molecular chemists and has been exploited in a wide range of applications. Their strong circularly polarized luminescence has attracted considerable attention in recent years due to promising applications in new optical materials. Although the enantioselective synthesis of fused carbo- and heterohelicenes has been achieved, a direct catalytic enantioselective method allowing the synthesis of lower, non-fused carbo[n]helicenes (n = 4–6) is still lacking. We report here that Pd-catalysed enantioselective C–H arylation in the presence of a unique bifunctional phosphine-carboxylate ligand provides a simple and general access to these lower carbo[n]helicenes. Computational mechanistic studies indicate that both the C–H activation and reductive elimination steps contribute to the overall enantioselectivity. The observed enantio-induction seems to arise from a combination of non-covalent interactions and steric repulsion between the substrate and ligand during the two key reductive elimination steps. The photophysical and chiroptical properties of the synthesized scalemic [n]helicenes have been systematically studied.

CPL calculations of [7]helicenes with alleged exceptional emission dissymmetry values

“…Since g ∝ cos θ μm and cos (110°) ≈ −0.34, the orientation between μ and m found in fluorene-fused helicenes 1–3 poses an intrinsic limitation to reaching high g lum values. The situation is common among [6]helicenes or smaller analogues 14,37,38,44,45,48,50,51,77 and may be tackled by different structural designs, such as double-helicenes, S-shaped or 8-shaped helical conjugated systems, elongated or extended helicenes. 46,77–84 Interestingly enough, in several of the just mentioned reports, the molecular design was steered by a preliminary computational screening, which confirms the demand for accurate computational approaches for property predictions.…”

“…Our approach is based on time-dependent density functional theory (TD-DFT), 35,36 which in recent years has demonstrated good capability of reproducing CPL data of SOMs, including several helicenes and heterohelicenes. 14,37–51 Conversely, CPL calculations of intramolecular excimers or exciplexes are still relatively limited in number. 52–56 Moreover, with respect to the most common calculation approach, we treated the influence of the solvent by means of the vertical excitation method (VEM), a state-specific (SS) approach to the polarizable continuum model (PCM) for excited states, for which the excited state energy gradients are available, 57,58 going well beyond other SS or the standard linear response (LR) schemes in the PCM.…”

Der richtige Dreh: Helicene enantioselektiv synthetisieren

“…Quantenchemische Berechnungen zeigen, welche Wechselwirkungen die chirale Informationen weitergeben: Substrat und Ligand wechselwirken während der zweistufigen reduktiven Eliminierung nichtkovalent und stoßen sich sterisch ab. 8) Um nichthelikale C-H-Arylierungsprodukte zu vermeiden, wird ein difluorsubstituiertes Phenantren (8) genutzt (Abbildung 4). Dieses zersetzt sich außerdem nicht bei der Reaktionstemperatur von 140 °C.…”

“…Phenylgruppen in der 3-Position des Binaphthylkerns erhöhen die Ausbeuten; 3,5-Substitution des Arylrests am Phosphin macht die Reaktion enantioselektiver. 8) Alcarazo synthetisierte enantioselektiv aus einem polyaromatischen Dialkin goldkatalysiert 1,12-substituierte Carbo [4]helicene. You gelang dies durch rhodiumkatalysierte C-H Anellierung aus Isochinolinen mit Alkinen.…”

“…You gelang dies durch rhodiumkatalysierte C-H Anellierung aus Isochinolinen mit Alkinen. 8,9) Azahelicene über Fischer-Indol-Synthese Heterohelicene haben andere geometrische und elektronische Strukturen als Carbohelicene. 10) List nutzte im Jahr 2014 die enantiomerenreine Brönstedsäure (13) für eine asymmetrische Fischer-Indol-Synthese.…”