The epoxidation of olefins by iodosylbenzene is catalyzed by synthetic iron porphyrins. With (chloro-5,10,15,20-tetraphenylporphyrinato)iron(III) (FeTPPCl), cyclohexadiene oxide was produced in 93% yield and with (chloro-5,10,15,20-tetra-o-tolylporphyrinato)iron(III) (FeTTPCl), cyclooctene oxide was produced in 84% yield. Cis olefins were found to be more reactive than trans olefins. Thus, whereas cis-stilbene was epoxidized by FeTPPCl/iodosylbenzene in 77% yield, trans-stilbene was unreactive under these conditions. The degree of cis/trans selectivity was a sensitive function of substitution of the porphyrin periphery. With (chloro-5,10,15,20-tetramesitylporphyrrnato)iron(III), the relative reactivity of cis- and trans-cyclododecene was 8.92:1. Similarly, the less reactive cis double bond of trans, trans, cis-cyc\ododecatriene could be selectively epoxidized by this catalyst. Trans 1,2-disubstituted double bonds and 1,1-disubstituted double bonds showed similar reactivities. These results suggest an approach of the double bond from the side of the iron-bound oxygen and parallel to the porphyrin plane. A mechanism involving formation of and oxygen transfer from a reactive iron-oxo intermediate is proposed for this reaction. The preference for side-on approach of the olefin to the iron-bound oxygen has been attributed to a stereoelectronic effect involving parartially filled oxygen-iron pπ,dπ antibonding orbitals.
All Science Journal Classification (ASJC) codes
- Colloid and Surface Chemistry