posted on 2001-03-13, 00:00authored bySam P. de Visser, François Ogliaro, Nathan Harris, Sason Shaik
The epoxidation of ethene by a model for Compound I of cytochrome P450, studied by the use of
density functional B3LYP calculations, involves two-state reactivity (TSR) with multiple electromer species,
hence “multi-state epoxidation”. The reaction is found to proceed in stepwise and effectively concerted manners.
Several reactive states are involved; the reactant is an (oxo)iron(IV) porphyrin cation radical complex with
two closely lying spin states (quartet and doublet), both of which react with ethene to form intermediate
complexes with a covalent C−O bond and a carbon-centered radical (radical intermediates). The radical
intermediates exist in two electromers that differ in the oxidation state of iron; Por+•FeIIIOCH2CH2• and PorFeIVOCH2CH2• (Por = porphyrin). These radical intermediates exist in both the doublet- and quartet spin states.
The quartet spin intermediates have substantial barriers for transformation to the quartet spin PorFeIII−epoxide
complex (2.3 kcal mol-1 for PorFeIVOCH2CH2• and 7.2 kcal mol-1 for Por+•FeIIIOCH2CH2•). In contrast, the
doublet spin radicals collapse to the corresponding PorFeIII−epoxide complex with virtually no barriers.
Consequently, the lifetimes of the radical intermediates are much longer on the quartet- than on the doublet
spin surface. The loss of isomeric identity in the epoxide and rearrangements to other products arise therefore
mostly, if not only, from the quartet process, while the doublet state epoxidation is effectively concerted (Scheme
7). Experimental trends are discussed in the light of the computed mechanistic scheme, and a comparison is
made with closely related mechanistic schemes deduced from experiment.