Copper-Carbon Homolysis Competes with Reductive Elimination in Well-Defined Copper(III) Complexes

Area of Science:

• Organometallic Chemistry
• Catalysis
• Radical Chemistry

Background:

• Copper catalysis is crucial for cross-coupling reactions involving alkyl electrophiles.
• The role of high-valent alkyl-copper(III) intermediates is frequently proposed but poorly understood.
• Direct observation of alkyl-Cu(III) to alkyl radical/Cu(II) interconversion remains elusive.

Purpose of the Study:

• To synthesize and characterize alkyl-copper(III) complexes.
• To investigate the decomposition pathways and reactivity of these high-valent copper species.
• To provide mechanistic insights into copper-catalyzed cross-coupling and trifluoromethylation reactions.

Main Methods:

• Synthesis of well-defined four-coordinate alkyl-copper(III) complexes.
• Kinetic studies to determine Cu-C bond dissociation energies.
• Investigation of complex conversion to solvated alkyl-Cu(III)-(CF3)2 species and their subsequent reactions.

Main Results:

• Synthesized alkyl-copper(III) complexes exclusively undergo Cu-C homolysis, yielding alkyl radicals and Cu(II).
• Determined a Cu-C bond dissociation energy of 28.6 kcal/mol.
• Observed efficient C-CF3 bond-forming reductive elimination from solvated alkyl-Cu(III)-(CF3)2 at low temperatures.

Conclusions:

• The results provide strong evidence for the reversible recombination of alkyl radicals with Cu(II) to form alkyl-Cu(III) intermediates.
• Subtle changes in the coordination environment significantly influence Cu(III) reactivity.
• Highly reactive neutral alkyl-Cu(III)-(CF3)2 species are likely key intermediates in copper-catalyzed trifluoromethylation.