Dialkyl Ether Formation at High-Valent Nickel

摘要

In this article, we investigated the I_2-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(sp~3)-OC(sp~3) using I_2 might not be operative. We isolated a paramagnetic bimetallic Ni~Ⅲ intermediate featuring a unique Ni_2(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(sp~3)-I bond reductive elimination occurs preferentially to any other challenging C-O bond reductive elimination. Formation of cychzed THF rings is then believed to occur through cyclization of an alcohol/alkoxide to the recently forged C(sp~3)-I bond. The results of this article indicate that the use of F~+ oxidants permits the challenging C(sp~3)-OC(sp~3) 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 Niln 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.