Mechanistic Studies of the Electrocatalytic Carbon–Bromine Cleavage and the Hydrogen Atom Incorporation from 1,1,1,3,3,3-Hexaflouroisopropanol

Abstract

Electrochemical dehalogenation of polyhalogenated compounds is an inefficient process as the working electrode is passivated by the deposition of short-chain polymers that form during the early stages of electrolysis. Herein, we report the use of 1, 1, 1, 3, 3, 3-hexaflouroisopropanol (HFIP) as an efficient reagent to control C–H formation over the radical association. Debromination of 1,6-dibromohexane was examined in the presence of Ni(II) salen and HFIP as the electrocatalyst and hydrogen atom source, respectively. Electrolysis of 10 mM 1,6-dibromohexane and 2 mM Ni(II) salen in the absence of HFIP yields 50% unreacted 1,6-dibromohexane and ∼40% unaccounted for starting material, whereas electrolysis with 50 mM HFIP affords 65% n-hexane. The mechanism of hydrogen atom incorporation was examined via deuterium incorporation coupled with high-resolution mass spectrometry, and density functional theory (DFT) calculations. Deuterium incorporation analysis revealed that the hydrogen atom originated from the secondary carbon of HFIP. DFT calculations showed that the deprotonation of hydroxyl moiety of HFIP, prior to the hydrogen atom transfer, is a key step for C–H formation. The scope of electrochemical dehalogenation was examined by electrolysis of 10 halogenated compounds. Our results indicate that through the use of HFIP, the formation of short-chain polymers is no longer observed, and monomer formation is the dominant product.