Alkene Borylation-Hydrogenation Enables Highly Active, Site-Selective Cobalt-Catalyzed Borylation

Area of Science:

• Organometallic Chemistry
• Catalysis
• Organic Synthesis

Background:

• Cobalt-catalyzed C-H borylation is crucial for arene functionalization.
• Achieving high activity and site-selectivity remains a challenge, particularly for electron-rich arenes.
• Existing methods often struggle with substrate scope and reaction efficiency.

Purpose of the Study:

• To develop a highly active and site-selective cobalt-catalyzed C(sp2)-H arene borylation method.
• To investigate the role of tert-butyl ethylene (TBE) in enhancing catalytic activity and selectivity.
• To elucidate the reaction mechanism, including the origin of meta-selectivity.

Main Methods:

• Cobalt-catalyzed borylation reactions using various arenes.
• In situ monitoring of catalytic reactions via 1H NMR spectroscopy.
• Kinetic isotope effect studies and DFT calculations to probe the reaction mechanism.

Main Results:

• Addition of tert-butyl ethylene (TBE) significantly increased the activity of cobalt-catalyzed borylation for electron-rich arenes.
• High meta-selectivity was achieved for monosubstituted anisoles and anilines.
• A borylation-hydrogenation sequence of TBE was identified as key to enhanced activity, with borylation of the alkene preceding arene functionalization.
• Kinetic isotope effects supported irreversible and rate-determining C(sp2)-H bond oxidative addition to cobalt(I).

Conclusions:

• The developed method provides a highly active and meta-selective route for cobalt-catalyzed C(sp2)-H arene borylation.
• The use of TBE as an additive is crucial for enhancing activity and overcoming inhibitory effects.
• Irreversible oxidative addition of the meta-C(sp2)-H bond to cobalt(I) is the origin of the observed meta-selectivity.