Dialkyl Ether Formation at High-Valent Nickel

In
this article, we investigated the I2-promoted cyclic
dialkyl ether formation from 6-membered oxanickelacycles originally
reported by Hillhouse. A detailed mechanistic investigation based
on spectroscopic and crystallographic analysis revealed that a putative
reductive elimination to forge C­(sp3)–OC­(sp3) using I2 might not be operative. We isolated
a paramagnetic bimetallic NiIII intermediate featuring
a unique Ni2(OR)2 (OR = alkoxide) diamond-like
core complemented by a μ-iodo bridge between the two Ni centers,
which remains stable at low temperatures, thus permitting its characterization
by NMR, EPR, X-ray, and HRMS. At higher temperatures (>−10
°C), such bimetallic intermediate thermally decomposes to afford
large amounts of elimination products together with iodoalkanols.
Observation of the latter suggests that a C­(sp3)–I
bond reductive elimination occurs preferentially to any other challenging
C–O bond reductive elimination. Formation of cyclized THF rings
is then believed to occur through cyclization of an alcohol/alkoxide
to the recently forged C­(sp3)–I bond. The results
of this article indicate that the use of F+ oxidants permits
the challenging C­(sp3)–OC­(sp3) bond formation
at a high-valent nickel center to proceed in good yields while minimizing
deleterious elimination reactions. Preliminary investigations suggest
the involvement of a high-valent bimetallic NiIII intermediate
which rapidly extrudes the C–O bond product at remarkably low
temperatures. The new set of conditions permitted the elusive synthesis
of diethyl ether through reductive elimination, a remarkable feature
currently beyond the scope of Ni.