Catalytic etherification of alcohols in Shilov system: C-O versus C-H bond activation

A novel catalytic reaction of alcohol etherification in the system ROH - PtCl42- - PtCl62- was found. Methanol easily transforms into dimethyl ether in the presence of catalytic amounts of Pt-II chloro complexes at 70 degrees C. Under the same conditions reaction of ethanol affords diethyl ether (catalytic) and pi-ethylene Pt-II complex (stoichiometric). The reactions are accompanied by multiple HAD exchange, which is indicative of intermediacy of corresponding alkyl platinum derivatives. The plausible reaction mechanism involves oxidative addition of alcohol forming intermediate alkyl platinum(IV) derivative followed by decomposition of it via reductive elimination step under the action of alcohol giving the ether and regenerating catalyst. In the case of ethyl alcohol reaction, beta-hydrogen abstraction from the intermediate Pt-ethyl species yields pi-ethylene platinum(II) complex. Although it seems that the reaction does not involve the initial breaking of C-H bonds of an alcohol, this system can be regarded as a model for studying of some peculiarities of Shilov chemistry, in particular, of isotope scrambling mechanisms in Shilov alkane activation. In contrast to reactions of dimethyl and diethyl ethers formation, tert-butyl ethers formation in CD3OH/t-BuOH medium is catalyzed by Pt-IV chloro complexes also and is not accompanied by isotope scrambling. These observations argue against intermediacy of alkyl platinum.derivatives suggesting that acid-catalyzed mechanism operates in tert-butyl alcohol etherification. (C) 2016 Elsevier B.V. All rights reserved.