Synthesis, Characterization, and Reactivity of Oxomanganese(IV) Porphyrin Complexes

Article abstract of DOI:10.1021/ja00234a009

The preparation, isolation, and characterization of two types of oxomanganese(IV) porphyrin complexes are described.The reaction of chloro(5,10,15,20-tetramesitylporphyrinato)manganese(III) IIITPM(Cl), 1> with 1,2 equiv of tetramethylammonium hydroxyde (TMA(OH)) and 1.2 equiv of m-chloroperoxybenzoic acid (m-CPBA) in CH2Cl2 produced a second complex formulated as Mn^IVTPM(O) (2).The reaction of 1 in CH2Cl2 containing excess tetra-n-butylammonium hydroxide (TBA(OH)) at +1.20 V generated a stable oxomanganese(IV) porphyrin complex, IVTPM(O)(OH)> (3).When the reaction stoichiometry was altered, mixtures of complexes 2 and 3 could be prepared.The aerobic reaction of 1 in CHCl3 containing 6 N NaOH and a phase-transfer catalyst resulted in the formation of a similar complex, Mn^IVTMP(O)(X) (2a).The addition of excess TBA(OH) to CH2Cl2 solutions of either 2 or 2a resulted in the quantitative formation of 3.The EPR spectra of 2, 2a, and 3 all displayed a strong broad resonance at g ca. 4 and a weak unresolved signal at g ca. 2 consistent with a high-spin (S=3/2) assignment of the Mn^IV ions.The Mn^IV=O stretching frequency in 2 was identified at 754 cm^-1 by FT-IR spectroscopy.In the case of 3 the Mn^IV=O stretching frequency was at 712 cm^-1.The reaction of 2a with cis-β-methylstyrene under anaerobic conditions produced a mixture of cis- and trans-epoxide in ratio of 0.17.The reaction of 2 with cis-β-methylstyrene under aerobic conditions produced a different cis-epoxide/trans-epoxide ratio and product distribution than those of the identical reaction run under anaerobic conditions.In the presence of H2(18)O, the stereoisomeric epoxides showed a significantly different (18)O content.Further, (18)O was found to reside in the oxidation products when this reaction was carried out in the presence of (18)O2.Mechanisms for the epoxidation of olefins by 2 under anaerobic and aerobic conditions are discussed, which involve atom transfer from both oxomanganese(V) and oxomanganese(IV) species.