Iron-Catalyzed Selective Etherification and Transetherification Reactions Using Alcohols

Simple
and readily available iron­(III) triflate turned out to be
a cheap, environmentally benign, and efficient catalyst for the direct
etherification of alcohols. The use of ammonium chloride as an additive
(5 mol %, 1 equiv relative to catalyst) suppressed the side reactions
completely and ensured the selective ether formation even on challenging
substrates containing electron-donating substituents. This method
allows the selective synthesis of symmetrical ethers from benzylic
secondary alcohols and unsymmetrical ethers directly from secondary
and primary alcohols. Moreover, transetherification of symmetrical
ethers using primary alcohols is attained. The reaction progress of
symmetrical ether and unsymmetrical ether formation followed zero-order
and first-order kinetics, respectively. Electron paramagnetic resonance
(EPR) measurements of the reaction mixture and simple iron­(III) triflate
clearly indicated that oxidation state of the metal center remains
same throughout the catalysis. Mechanistic studies confirmed that
the unsymmetrical ether formation occurs via the in situ formed symmetrical
ethers.