Dialkyl Ether Formation at High-Valent Nickel

• Published in: Journal of the American Chemical Society Vol. 142; no. 46; pp. 19540 - 19550
• Main Authors: Le Vaillant, Franck, Reijerse, Edward J, Leutzsch, Markus, Cornella, Josep
• Format: Journal Article
• Language: English
• Published: United States American Chemical Society 18-11-2020
• ISSN: 0002-7863; 1520-5126; 1520-5126
• DOI: 10.1021/jacs.0c07381

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.