Dialkyl Ether Formation at High-Valent Nickel.

Area of Science:

• Organometallic Chemistry
• Catalysis
• Organic Synthesis

Background:

• Investigated I2-promoted cyclic dialkyl ether formation from 6-membered oxanickelacycles.
• Previous reports suggested direct C(sp3)-OC(sp3) bond formation via reductive elimination.

Purpose of the Study:

• To elucidate the mechanism of I2-promoted cyclic dialkyl ether formation.
• To identify reactive intermediates and understand the role of iodine in the catalytic cycle.

Main Methods:

• Detailed mechanistic investigation using spectroscopic (NMR, EPR) and crystallographic analysis.
• Isolation and characterization of a paramagnetic bimetallic Ni(III) intermediate.
• Thermal decomposition studies of the intermediate at varying temperatures.

Main Results:

• A stable, paramagnetic bimetallic Ni(III) intermediate with a Ni2(OR)2 core and a μ-iodo bridge was isolated and characterized.
• The intermediate decomposes above -10 °C, yielding elimination products and iodoalkanols, indicating preferential C(sp3)-I bond reductive elimination.
• Cyclic THF ring formation occurs via cyclization of an alcohol/alkoxide to the C(sp3)-I bond.

Conclusions:

• The direct C(sp3)-OC(sp3) bond formation via reductive elimination using I2 is likely not operative.
• The use of F+ oxidants enables challenging C(sp3)-OC(sp3) bond formation at high-valent nickel centers, minimizing side reactions.
• The study enabled the synthesis of diethyl ether via reductive elimination, a feat previously challenging for nickel catalysis.