A C-H Activation-Based Enantioselective Synthesis of Lower Carbo[n]helicenes (n = 4-6)

Abstract

The unique three-dimensional structure of carbohelicenes have fascinated generations of molecular chemists and has been exploited through a wide range of applications. In particular, their strong circularly polarized luminescence (CPL) has raised much attention in recent years due to promising applications in the design of new optical materials. Whereas a number of important precedents report enantioselective syntheses of fused carbo- and heterohelicenes, a direct catalytic enantioselective method allowing the synthesis of lower, nonfused carbo[n]helicenes (n = 4-6) is still lacking. We report 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 simple carbo[n]helicenes. Computational mechanistic studies indicate that both the C–H activation and reductive elimination steps contribute to the overall enantioselectivity. In addition, the observed enantio-induction seems to arise from a combination of noncovalent interactions and steric repulsion between the substrate and ligand during the two key reductive elimination steps. Moreover, the current method allows a comparative study of the CPL properties of lower carbo[n]helicenes, which led to the discovery that carbo[4]helicenes actually display CPL responses comparable to the higher carbo[6]helicene congeners.

Impact of helical elongation of symmetric oxa[n]helicenes on their structural, photophysical, and chiroptical characteristics

“…Therefore, the electric (μ) and magnetic (m) transition dipole moments, as well as the angle ( θ ) between μ and m, of ( M )‐ OH5 , ( M )‐ OH7 , and ( M )‐ OH9 for their S 1 → S 0 transitions were determined using TD‐DFT calculations (Figure 6B). 73,74 For organic materials, the |m| value is typically much lower than the|μ|value (Table 1). The above equation can thus be simplified as g lum = 4 cos θ |m|/|μ|.…”

“…S 0 transitions were determined using TD-DFT calculations (Figure 6B). 73,74 For organic materials, the jmj value is typically much lower than thejμjvalue (Table 1). The above equation can thus be simplified as g lum = 4 cos θjmj/jμj.…”

Rational Design of Chiral Single-Benzene-Based Fluorophores via Enantioselective Twofold C−H Activation: Solid-State and Circular Polarization Luminescence, and Cellular Imaging Application

“…Considering the apparent advantages, the development of new strategies to construct chiral uorophores via asymmetric catalysis is of great value. [34][35][36][37][38] Over the past decades, transition metal-catalyzed C − H activation has merged as a powerful and widely utilized approach for the rapid and modular generation of molecules, whose allure derives from its excellent e ciency, good functional group tolerance and atomeconomy. [39][40][41][42] Among these, the twofold C − H activation under single catalytic conditions has been shown as a class of eco-friendly and e cient approach since the overall process minimizes the solvent use, waste production and cost.…”

[5]-Helistatins: Tubulin-Binding Helicenes with Antimitotic Activity

“…Helicenes are polycyclic molecules consisting of ortho -fused aromatic or heteroaromatic rings that are angularly arranged to give rise to a helically shaped conformation . The synthetic challenge of assembling such helically chiral aromatics has generated much interest, with new synthesis strategies and methodologies regularly reported. − Furthermore, the coupling of a fully conjugated framework to a helical structure has led to a wide variety of applications for helicenes, including asymmetric catalysis, , molecular switches, − dyes, − circularly polarized light emitters, , spintronics, , and organic electronic devices, − to name a few. Despite the increasing number of applications for the use of helicenes in synthesis and materials science, these scaffolds are understudied in the area of chemical biology.…”