Heme compound II models in chemoselectivity and disproportionation reactions

Heme compound II models bearing electron-deficient and -rich porphyrins, [Fe IV (O)(TPFPP)(Cl)] − ( 1a ) and [Fe IV (O)(TMP)(Cl)] − ( 2a ), respectively, are synthesized, spectroscopically characterized, and investigated in chemoselectivity and disproportionation reactions using cyclohexene as a mechanistic probe. Interestingly, cyclohexene oxidation by 1a occurs at the allylic C-H bonds with a high kinetic isotope effect (KIE) of 41, yielding 2-cyclohexen-1-ol product; this chemoselectivity is the same as that of nonheme iron( iv )-oxo intermediates. In contrast, as observed in heme compound I models, 2a yields cyclohexene oxide product with a KIE of 1, demonstrating a preference for C&z.dbd;C epoxidation. The latter result is interpreted as 2a disproportionating to form [Fe IV (O)(TMP + &z.rad;)] + ( 2b ) and Fe III (OH)(TMP), and 2b becoming the active oxidant to conduct the cyclohexene epoxidation. In contrast to 2a , 1a does not disproportionate under the present reaction conditions. DFT calculations confirm that compound II models prefer C-H bond hydroxylation and that disproportionation of compound II models is controlled thermodynamically by the porphyrin ligands. Other aspects, such as acid and base effects on the disproportionation of compound II models, have been discussed as well. Disproportionation of Cpd II models depends on the electron-richness of the porphyrin ligand; Cpd II with an electron-deficient ligand is difficult to disproportionate, whereas Cpd II with an electron-rich ligand readily disproportionates to form Cpd I as a true oxidant.