Solvent Effects on Catalytic Epoxidation of Alkenes by Tetra-n-butylammonium Periodate and (Tetraarylporphyrinato)manganese(III)

Abstract

The epoxidation of cyclooctene and cis-stilbene was performed by tetra-n-butylammonium periodate (nBu₄NIO₄)in the presence of (tetraarylporphyrinato)manganese(III) [Mn(por)] and imidazole (im) in various CH₂Cl₂/alcohol solvents (alcohol = CH₃OH, C₂H₅OH, n-C₃H₇OH, i-C₃H₇OH, t-C₄H₉OH). In accord with the coordinating abilities of the alcohols to [Mn(por)], the epoxidation yields increased from CH₂Cl₂/CH₃OH to CH₂Cl₂/t-C₄H₉OH. In the epoxidation of cis-stilbene in the presence of (acetato)(tetraphenylporphyrinato)manganese(III) [Mn(tpp)(OAc)], the cis- to trans-stilbene oxide ratio increased consistently with the bulk of the alcohol in CH₂Cl₂/alcohol solvents. Also, it was found that the interaction of [Mn(tpp)(OAc)] and (acetato)(tetramesitylporphyrinato)manganese(III) [Mn(tmp)(OAc)] with nBu₄NIO₄ in the presence of imidazole but in the absence of alkenes yields [Mn(tpp)(O)(im)]⁺ and [Mn(tmp)(O)(im)]⁺ complexes. The formation of the Mn=O species for the small linear alcohols was faster than that for the larger ones, whereas the stability of the Mn=O complex was greater in the presence of the bulkier alcohols. Attempts were made to explain these effects.(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2008)