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Hydrogen‐Atom Abstraction Reactions by Manganese(V)– and Manganese(IV)–Oxo Porphyrin Complexes in Aqueous Solution
Author(s) -
Arunkumar Chellaiah,
Lee YongMin,
Lee Jung Yoon,
Fukuzumi Shunichi,
Nam Wonwoo
Publication year - 2009
Publication title -
chemistry – a european journal
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.687
H-Index - 242
eISSN - 1521-3765
pISSN - 0947-6539
DOI - 10.1002/chem.200901362
Subject(s) - chemistry , manganese , porphyrin , disproportionation , aqueous solution , hydrogen atom abstraction , reactivity (psychology) , xanthene , photochemistry , thioanisole , kinetic isotope effect , medicinal chemistry , catalysis , inorganic chemistry , hydrogen , organic chemistry , deuterium , medicine , physics , alternative medicine , pathology , quantum mechanics
High‐valent manganese(IV or V)–oxo porphyrins are considered as reactive intermediates in the oxidation of organic substrates by manganese porphyrin catalysts. We have generated Mn V – and Mn IV –oxo porphyrins in basic aqueous solution and investigated their reactivities in CH bond activation of hydrocarbons. We now report that Mn V – and Mn IV –oxo porphyrins are capable of activating CH bonds of alkylaromatics, with the reactivity order of Mn V –oxo>Mn IV –oxo; the reactivity of a Mn V –oxo complex is 150 times greater than that of a Mn IV –oxo complex in the oxidation of xanthene. The CH bond activation of alkylaromatics by the Mn V – and Mn IV –oxo porphyrins is proposed to occur through a hydrogen‐atom abstraction, based on the observations of a good linear correlation between the reaction rates and the CH bond dissociation energy (BDE) of substrates and high kinetic isotope effect (KIE) values in the oxidation of xanthene and dihydroanthracene (DHA). We have demonstrated that the disproportionation of Mn IV –oxo porphyrins to Mn V –oxo and Mn III porphyrins is not a feasible pathway in basic aqueous solution and that Mn IV –oxo porphyrins are able to abstract hydrogen atoms from alkylaromatics. The CH bond activation of alkylaromatics by Mn V – and Mn IV –oxo species proceeds through a one‐electron process, in which a Mn IV –‐oxo porphyrin is formed as a product in the CH bond activation by a Mn V –oxo porphyrin, followed by a further reaction of the Mn IV –oxo porphyrin with substrates that results in the formation of a Mn III porphyrin complex. This result is in contrast to the oxidation of sulfides by the Mn V –oxo porphyrin, in which the oxidation of thioanisole by the Mn V –oxo complex produces the starting Mn III porphyrin and thioanisole oxide. This result indicates that the oxidation of sulfides by the Mn V –oxo species occurs by means of a two‐electron oxidation process. In contrast, a Mn IV –oxo porphyrin complex is not capable of oxidizing sulfides due to a low oxidizing power in basic aqueous solution.