Kinetics and Mechanism of Oxygen Transfer in the Reaction of p-Cyano-N,N-dimethylaniline N-Oxide With Metalloporphyrin Salts. 6. Oxygen Atom Transfer to and from the Iron(III) C2cap Porphyrin of Baldwin

Article abstract of DOI:10.1021/ja00245a040

The decomposition of p-cyano-N,N-dimethylaniline N-oxide (NO) is catalyzed (CH2Cl2 solvent at 25 deg C) by the SbF6^- salt of the C₂-capped (meso-tetraphenylporphinato)iron(III) of Baldwin ((TPPC₂cap)Fe^IIISbF6).Since neither NO nor the SbF6^- anion can fit under the cap, this result establishes that oxygen transfer from NO to the iron(III) center can occur without the intermediate iron(IV)-oxo porphyrin ?-cation being hexacoordinated.The products of NO decomposition (p-cyano-N,N-dimethylaniline (DA) 68percent, p-cyano-N-methylaniline (MA) 12percent, p-cyanoaniline (A) 1percent, N-formyl-p-cyano-N-methylaniline (FA) 6percent, N,N'-dimethyl-N,N'-bis(p-cyanophenyl)hydrazine (H) 7percent, and CH2O 3percent) account for 100percent of p-cyanodimethylaniline moiety of NO, but ca. 40percent of the oxide oxygen of NO remains unaccounted for by product isolation.This is due to some loss of the catalyst and oxidation of the CH2Cl2 solvent during turnover.Oxidation of solvent results in the gradual exchange of the SbF6^- axial ligand for Cl^-.Product formation on reaction of NO with (TPPC2cap)Fe^IIISbF6 can be approximated by the first-order rate law.Since the observed first-order rate constants at a given _i are a linear function of the iron(III) porphyrin, the reaction is first order in this catalyst and essentially so in NO.However, the finding that the initial rates are independent of _i establishes that the iron(III) porphyrin catalyst is saturated with NO on initiation of the reaction.The exchange of Cl^- for SbF6^- axial ligand and some loss of catalyst during turnovers are responsible for the reactions changing from initial zero order to essentially first order in _i.Trapping of the intermediate ((^{+._TPPC2cap)FeIV}O) by the 2,4,6-tri-tert-butylphenol (TBPH), 2,3-dimethyl-2-butene (TME), and p-cyano-N-methylaniline (MA) prevents solvent oxidation and porphyrin oxidative destruction.Under these trapping conditions the reaction is rapid and zero order in NO to completion.That the zero-order kinetics are due to saturation of the <(TPPC₂cap)Fe^III>⁺ catalysts by NO was established by following the time course of the reaction in the Soret and Q spectral regions.The products with TME are TME epoxide (100percent) and DA (100percent) while with TBPH the radical TBP^. (100percent) and DA (100percent) are formed.The rate is also increased, but to a lesser extent, when using the poorer substrates, cyclohexene and norbornylene.These substrates also impart a zero-orderness to the appearance of products.The kinetics for the reaction of (TPPC₂cap)Fe^IIISbF6 with NO using TME, TBPH, and MA as substrates have been computer simulated with the requirement of steady-state saturation in the intermediate (TPPC₂cap)Fe^IIION.For successful simulations it is required that both the formation of (^{+._TPPC2cap)FeIV}O + DA from (TPPC₂cap)Fe^IIION and the reaction of (^{+._TPPC2cap)FeIV}O with substrates (i.e., TME, TBPH, and MA) are partially rate limiting.At concentrations of added alkene less than that required for trapping of (^+.TPPC2cap)Fe^IVO there are obtained, ...