Highly Reactive Porphyrin-lron-oxo Derivatives Producedby Photolyses Of Metastable Porphyrin-lron(iv)rndiperchlorates

摘要

Photolyses of metastable porphyrin-iron(IV) diperchlorates in laser flash photolysis reactions gave highly reactive transients. The systems studied were 5,10,15,20-tetraphenylporphyrin (TPP), 5,10,15,20-tetramesitylporphyrin(TMP), and 2,3,7,8,12,13,17,18-octaethylporphyrin(OEP). The new species, which decayed within milliseconds in acetonitrile solutions, were shown to react with organic substrates by oxo-transfer reactions involving insertions into carbon-carbon double bonds of alkenes and styrenes or benzylic carbon-hydrogen bonds of arenes. The order of reactivity was OEP > TPP > TMP. Second-order rate constants for reactions with several substrates at 22 ℃ were determined; representative values of rate constants for the TPP derivative were k=8.6× 10~5 M~(-1) s~(-1) for styrene, k=2.5 × 10~6 M~(-1) s~(-1) for cyclohexene, and k= 7.7 × 10~4 M~(-1) s~(-1) for ethylbenzene. These porphyrin-iron-oxo transients reacted 4-5 orders of magnitude faster than the corresponding iron(IV)-oxo porphyrin radical cations with rate constants similar to those of porphyrin-manganese(V)-oxo derivatives. Rate constants for oxidations of benzylic C-H positions of arenes correlated with the C-H bond dissociation energies, and Hammett correlations for reactions with substituted styrenes had p~+ values ranging from -0.5 to -0.7, reflecting electrophilic character of the oxidants and their high reactivity. On the basis of their unique UV-visible spectra, high reactivities, and oxo-transfer properties, the new transients are tentatively identified as porphyrin-iron(V)-oxo perchlorates, electronic isomers (or valence tautomers) of well-known iron(IV)-oxo porphyrin radical cations.