A Neutral "Masked" Diborene and Its Reactivity Toward Metal-Free Pyridine Homocoupling.

Area of Science:

• Organometallic Chemistry
• Boron Chemistry
• Synthetic Organic Chemistry

Background:

• Diborenes are reactive boron compounds with limited synthetic utility.
• Stabilized diborenes often require external donors, limiting their reactivity.
• Elucidating the behavior of free diborenes is crucial for understanding boron chemistry.

Purpose of the Study:

• To synthesize and characterize a novel neutral, non-external-donor-stabilized diborene.
• To investigate the reactivity of this diborene in transition-metal-free reactions.
• To explore the potential of diborenes as electron donors in organic synthesis.

Main Methods:

• Synthesis of a masked diborene via intramolecular arene dearomatization.
• Photochemical generation of a free diborene intermediate.
• Experimental studies including chalcogenation reactions.
• Computational studies to determine electronic structure and ground state.
• Transition-metal-free pyridine activation and homocoupling reactions.
• Mechanistic investigations involving B-C bond cleavage and electron transfer.
• Oxidation studies for product release.

Main Results:

• An unprecedented neutral "masked" diborene stabilized by an N-heterocyclic imine (NHI) was synthesized.
• The free diborene intermediate exhibits an alkyne-like bonding pattern and a triplet ground state.
• The masked diborene efficiently activates pyridines for transition-metal-free C-C bond formation with high regioselectivity.
• A novel reactivity mode for diborenes was established, acting as a potent double single-electron donor.
• Free 2,2'-bipyridine derivatives were obtained after oxidation.

Conclusions:

• This study provides access to a previously elusive free diborene species.
• The findings establish a new reactivity paradigm for diborenes in organic synthesis.
• The masked diborene demonstrates potential to mimic transition-metal behavior, offering a powerful tool for molecule construction.