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 la 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 = C epoxidation. The latter result is interpreted as 2a disproportionating to form [Fe-IV(O)(TMP+.)](+) (2b) and Fe-III(OH)(TMP), and 2b becoming the active oxidant to conduct the cyclohexene epoxidation. In contrast to 2a, la 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.

Automated summary

In this study, heme compound II models with electron-deficient and -rich porphyrins were synthesized and investigated for their activity in chemoselectivity and disproportionation reactions using cyclohexene as a probe. The results showed that different compound II models exhibited different chemoselectivity towards the oxidation of cyclohexene, which may be attributed to the disproportionation process and the formation of active oxidants.