Mechanism of Nickel-Catalyzed Dual C–O Bond Activation in the Deoxygenation of Ethers via Reductive Cross-Coupling Reaction: A DFT Study

Nickel-catalyzed C–O bond cleavage provides an
important
strategy to construct C–C bonds from oxygenated electrophiles.
The present study elucidated the mechanisms of ethers (R1OR1) deoxygenation via reductive cross-coupling reaction
using density functional theory (DFT) calculations. The results suggest
that, in the absence of B2pin2 or at the beginning
of the reaction, the most favorable mechanism occurs in six steps:
first oxidative addition, reduction, radical production, reduction,
radical addition, and reductive elimination. When a significant amount
of alkyl radical is formed from the parallel reaction of B2pin2 with the alkoxide anchored in the zinc cluster surface,
which contains the waste carbon fragment of ether, the only difference
in the reaction mechanism is that the intermediate before de reductive
elimination, R1–Ni(II)–R1, is formed directly through a radical addition in the Ni(I)–R1. The presence of B2pin2 is crucial to enable the C–C coupling between two carbon
fragments, which may originate from different ether molecules and
to ensure that the reaction stoichiometry becomes 1:1. In the presence
of B2pin2, the two catalytic cycles are kinetically
active and equally competitive (within the DFT error range) with rate
determining ΔG‡(THF/Tol.avg.) values of 31.0 and 31.5 kcal/mol, respectively, for the deoxygenation
of 2,2’-oxybis(methylene)-dinaphthalene in each cycle.