Isolation of an Oxomanganese( V ) Porphyrin Intermediate in the Reaction of a Manganese( III ) Porphyrin Complex and H 2 O 2 in Aqueous Solution

The reaction of [Mn(TF 4 TMAP)](CF 3 SO 3 ) 5 (TF 4 TMAP= meso ‐tetrakis(2,3,5,6‐tetrafluoro‐ N , N , N ‐trimethyl‐4‐aniliniumyl)porphinato dianion) with H 2 O 2 (2 equiv) at pH 10.5 and 0 °C yielded an oxomanganese( V ) porphyrin complex 1 in aqueous solution, whereas an oxomanganese( IV ) porphyrin complex 2 was generated in the reactions of tert ‐alkyl hydroperoxides such as tert ‐butyl hydroperoxide and 2‐methyl‐1‐phenyl‐2‐propyl hydroperoxide. Complex 1 was capable of epoxidizing olefins and exchanging its oxygen with H 2 18 O, whereas 2 did not epoxidize olefins. From the reactions of [Mn(TF 4 TMAP)] 5+ with various oxidants in the pH range 3–11, the O−O bond cleavage of hydroperoxides was found to be sensitive to the hydroperoxide substituent and the pH of the reaction solution. Whereas the O−O bond of hydroperoxides containing an electron‐donating tert ‐alkyl group is cleaved homolytically, an electron‐withdrawing substituent such as an acyl group in m‐ chloroperoxybenzoic acid ( m ‐CPBA) facilitates O−O bond heterolysis. The mechanism of the O−O bond cleavage of H 2 O 2 depends on the pH of the reaction solution: O−O bond homolysis prevails at low pH and O−O bond heterolysis becomes a predominant pathway at high pH. The effect of pH on 18 O incorporation from H 2 18 O into oxygenated products was examined over a wide pH range, by carrying out the epoxidation of carbamazepine (CBZ) with [Mn(TF 4 TMAP)] 5+ and KHSO 5 in buffered H 2 18 O solutions. A high proportion of 18 O was incorporated into the CBZ‐10,11‐oxide product at all pH values but this proportion was not affected significantly by the pH of the reaction solution.