Nickel‑Copper Co‑Catalysed Atroposelective C‑Si Cross‑Coupling to Axially Chiral Heterobiaryl Silanes

Introduction

Creating axially chiral molecules is a frontier of modern organic synthesis. A recent breakthrough—nickel and copper co‑catalysed atroposelective C‑Si cross‑coupling—offers a direct route to heterobiaryl silanes with precise axial chirality. This method combines inexpensive base metals, mild conditions, and broad substrate scope, making it attractive for pharmaceuticals, material science, and asymmetric catalysis.

Why Axially Chiral Silanes Matter

Axial chirality provides a unique three‑dimensional element that can influence binding, reactivity, and physical properties. Silanes add further utility:

• Silicon can act as a protected alcohol, a directing group, or a handle for further functionalisation.
• Heterobiaryl frameworks are prevalent in bioactive compounds and organic materials.

The ability to install a silicon‑aryl bond with defined axial configuration opens new design possibilities.

Key Features of the Ni/Cu Co‑Catalysed System

• Dual‑metal synergy: Nickel activates the aryl halide, while copper mediates transmetalation of the silyl nucleophile.
• Atroposelectivity: Chiral phosphine ligands on copper enforce a single enantiomer of the biaryl axis.
• Mild conditions: Reactions run at 25–50 °C in THF with 1–2 % catalyst loading.
• Operational simplicity: No glove‑box required; air‑stable precursors are used.

Reaction Overview

 Ar–X  +  Ar'–SiR₃  —[Ni‑cat., Cu‑L*]→  Ar–Ar'–SiR₃ (axial chirality) 

Where Ar‑X is typically an aryl bromide or iodide bearing ortho‑substituents that bias the rotational barrier, and Ar’–SiR₃ is a silyl‑aryl nucleophile generated in situ from a silylboronate.

Mechanistic Insight

Step 1 – Oxidative Addition

Nickel(0)Lₙ inserts into the C–X bond, forming an aryl‑Ni(II)‑X intermediate. Electron‑rich ligands accelerate this step.

Step 2 – Transmetalation

Copper(I)‑L* (chiral phosphine) reacts with the silylboronate, yielding a Cu‑SiR₃ species. This species transfers the silyl group to the aryl‑Ni complex, generating a Ni‑Si bond.

Step 3 – Reductive Elimination

The key atroposelective step: the chiral copper ligand dictates the spatial arrangement of the two aryl groups during C–Si bond formation, delivering one axial enantiomer preferentially.

Substrate Scope

Functional groups such as –OMe, –F, –Cl, and heterocycles are tolerated, showcasing the method’s robustness.

Practical Tips for Beginners

1. Ligand selection: Use (R)-Segphos or (S)-BINAP on copper for high enantioselectivity.
2. Base choice: NaOtBu (1.5 equiv) efficiently generates the Cu‑Si species without over‑deprotonating the aryl halide.
3. Solvent: Anhydrous THF provides the best balance of solubility and reactivity.
4. Temperature control: Keep the reaction below 50 °C to prevent racemisation of the axle.
5. Work‑up: Quench with saturated NH₄Cl, extract with EtOAc, and purify by flash chromatography (hexane/EtOAc 9:1).

Applications

• Asymmetric catalysis: The chiral silane can be oxidised to a chiral alcohol or transformed into a silicon‑based Lewis acid for enantioselective reactions.
• Drug discovery: Axially chiral biaryl silanes serve as scaffolds for kinase inhibitors and CNS agents.
• Materials: Incorporating silicon improves photophysical stability of organic light‑emitting diodes (OLEDs).

Conclusion

The nickel‑copper co‑catalysed atroposelective C‑Si cross‑coupling represents a powerful, scalable solution for synthesising axially chiral heterobiaryl silanes. By leveraging inexpensive base metals and a well‑designed chiral ligand system, chemists can now access these valuable motifs with high enantioselectivity and functional‑group tolerance. Future work is expected to expand the method to other silicon nucleophiles and to integrate it into cascade syntheses of complex natural products.