New insights into the mechanisms of O-O bond cleavage of hydrogen peroxideand tert-alkyl hydroperoxides by iron(III) porphyrin complexes

The mechanisms of heterolytic versus homolytic O-O bond cleavage of H2O2. t ert-butyl hydroperoxide (t-BuOOH), 2-methyl-1-phenyl-2-propyl hydroperoxide (MPPH), and m-chloroperoxybenzoic acid (m-CPBA) by iron(III) porphyrin com plexes have been studied by carrying out catalytic epoxidations of cyclohex ene in protic solvent. In these reactions, various iron(III) porphyrin comp lexes containing electron-withdrawing and -donating substituents on phenyl groups at the meso position of the porphyrin ring were employed to study th e electronic effect of porphyrin ligands on the heterolytic versus homolyti c O-O bond cleavage of the hydroperoxides. In addition, various imidazoles were introduced as axial ligands to investigate the electronic effect of ax ial ligands on the pathways of hydroperoxide O-O bond cleavage. Unlike the previous suggestions by Traylor, Bruice, and co-workers, the hydroperoxide O-O bonds were found to be cleaved both heterolytically and homolytically a nd partitioning between heterolysis and homolysis was significantly affecte d by the electronic nature of the iron porphyrin complexes (i.e., electroni c properties of porphyrin and axial ligands). Electron-deficient iron porph yrin complexes show a tendency to cleave the hydroperoxide O-O bonds hetero lytically, whereas electron-rich iron porphyrin complexes cleave the hydrop eroxide O-O bonds homolytically. The heterolytic versus homolytic O-O bond cleavage of the hydroperoxides was also found to be significantly affected by the substituent of the hydroperoxides, ROOH (R = C(O)R', H, C(CH3)(3), a nd C(CH3)(2)CH2Ph for m-CPBA, H2O2, t-BuOOH, and MPPH, respectively), in wh ich the tendency of O-O bond heterolysis was in the order of m-CPBA > H2O2 > t-BuOOH > MPPH. This result indicates that the O-O bond of hydroperoxides containing electron-donating tert-alkyl groups such as t-BuOOH and MPPH te nds to be cleaved homolytically, whereas electron-withdrawing substituents such as an acyl group in m-CPBA facilitates O-O bond heterolysis. Since we have observed that the homolytic O-O bond cleavage of hydroperoxides prevai ls in the reactions performed with electron-rich iron porphyrin complexes a nd with hydroperoxides containing electron-donating substituents such as th e tert-alkyl group, we suggest that the homolytic O-O bond cleavage is faci litated when more electron density resides on the O-O bond of (Porp)Fe(III) -OOR intermediates. We also present convincing evidence that the previous a ssertion that the reactions of iron(III) porphyrin complexes with hydrogen peroxide and tert-alkyl hydroperoxides invariably proceed by heterolytic O- O bond cleavage in protic solvent and that the failure to obtain high epoxi de yields in iron porphyrin complex-catalyzed epoxidation of olefins by hyd roperoxides is due to the mechanism of heterolytic O-O bond cleavage follow ed by a fast hydroperoxide oxidation is highly unlike.