Taming the Chlorine Radical: Enforcing Steric Control over Chlorine-Radical-Mediated C-H Activation

Area of Science:

• Organometallic Chemistry
• Photochemistry
• Organic Synthesis

Background:

• Chlorine radicals are highly reactive, limiting their application in selective C-H functionalization.
• Achieving regioselectivity in C-H bond activation remains a significant challenge in synthetic chemistry.

Purpose of the Study:

• To demonstrate that the secondary coordination sphere of a metal complex can confine photoeliminated chlorine radicals.
• To achieve steric control over chlorine radical reactivity for regioselective C-H functionalization.

Main Methods:

• Photochemical C(sp3)-H chlorination and bromination reactions using iron(III) chloride pyridinediimine complexes.
• Transient absorption spectroscopy to study radical confinement.
• Photocrystallography to elucidate the structural basis of selectivity.

Main Results:

• Iron complexes selectively chlorinated and brominated primary and secondary C-H bonds.
• Selectivity was achieved by confining chlorine radicals within the complex's secondary coordination sphere.
• Transient absorption spectroscopy confirmed Cl· confinement via a Cl·|arene complex.

Conclusions:

• The secondary coordination sphere of metal complexes can effectively control reactive radical intermediates.
• This strategy enables regioselective C-H functionalization, overriding thermodynamic preferences.
• The findings open new avenues for precise control in radical-mediated organic synthesis.