Investigations into the Frustrated Lewis Pair-Catalyzed Hydrogenation of Epoxides

Abstract

Traditionally hydrogenation of unsaturated substrates is a transition metal (TM)-catalyzed or stoichiometric transformation. The report about heterolytic H2-activation via Frustrated Lewis Pairs (FLP) allows the performance of classic TM-catalyzed reactions in an organocatalytic fashion. In the last decade the substrate scope for FLP-catalyzed hydrogenation has grown steadily. However, to the best of our knowledge epoxides have not yet been successfully reduced. The epoxide motive is an important building block in organic synthesis or product in natural product synthesis. Currently, hydrogenation of epoxides to alcohols is a stoichiometric or a TM-complex catalyzed transformation. Here, attempts to FLP-catalyzed hydrogenations of epoxides to alcohols are described. The known FLPs B(C6F5)3/1,4-dioxane and B(C6F5)3/diethyl ether, which are used for the FLP-catalyzed hydrogenation of carbonyl moieties, were tested for possible hydrogenation of epoxides. Meinwald rearrangement of the epoxide to an aldehyde is observed. With this result a tandem isomerization-hydrogenation reaction was envisioned. Here an epoxide rearranges to an aldehyde, upon which it should be reduced to an alcohol. Unfortunately, this reaction was not successful. The reactivity of less electron-poor Lewis acids in FLP-catalyzed hydrogenation of epoxides was investigated. Upon investigations of adduct formation between the Lewis acids with trans- phenylpropane oxide, a Lewis acid dependent reactivity of the epoxide was found. trans-phenylpropane oxide dimerizes upon stoichiometric exposure to B(C6F5)3, but isomerizes to an aldehyde when exposed to B(C7H8)3. trans-phenylpropane oxide does not form a Lewis adduct with B(C6F5)2Mes. Therefore, it is proposed that a new FLP was found. Transfer hydrogenation (TH) of epoxides was attempted by B(C6F5)3-catalyzed hydride abstraction from a diene. However, hydride transfer was prevented most likely by the interaction of B(C6F5)3 with epoxides. The finding that B(C6F5)2Mes and trans-phenylpropane oxide form a FLP opens possibilities for future research, especially in combination with B(C6F5)2Mes -catalyzed TH.