Alkyliron porphyrins

H NMR spectroscopy studies of iron(IIl) a-alkyl and o-aryl porphyrins have been very important in elucidating spin states. Alkyl and most aryl complexes with simple porphyrin ligands (OEP, TPP, or TTP) are low spin, S — I /2 species. NMR spectra for the tetraarylporphyrin derivatives show upheld resonances for the porphyrin pyrrole protons (ca. — 18 to —35 ppm), and alternating upfield and downfield hyperfine shifts for the axial alkyl or aryl resonances. For -alkyl complexes, the a-protons show dramatic downfield shifts (to ca. 600 ppm), upfield shifts for the /3-protons (—25 to — 160 ppm) and downfield shifts for the y-protons (12 ppm). The cr-protons of alkyliron porphyrins are not usually detected as a result of their large downfield shift and broad resonance. These protons were first detected by deuterium NMR in the dcuterated complexes Fe(TPP)CD3 (532 ppm) and Fe(TPP)CD2CDi (562, -117 ppm). ... 

Alkyliron porphyrin cotnplexes are not particularly stable in solution, and over period of several hours at room temperature the iron(II) porphyrin appears. 

Traylor et al. reached different conclusions in studies of alkene epoxidation. In homogeneous solutions in which iodosylbenzene was solubilized in dichloro-methane/alcohol/water, very rapid epoxidation (300 turnovers/sec) was found to be independent of substrates or concentration of alkane (124). No dependence on alkene concentrations was reported by Dicken et al. when they employed /7-cyanodimethylaniline 7V-oxide (725). Meanwhile, Traylor et al. found an accumulation of 7V-alkyliron porphyrin complexes (35) during the course of epoxidation of norbornene (126), and showed that it is the only intermediate accumulated in either the homogeneous system or the heterogeneous system 35 was found to form with a bimolecular rate constant of 450 M sec and decomposes at a rate of 0.07 sec. Epoxide formation occurs with a rate constant of at least 10 Af" sec in these conditions. Thus 35 is not an intermediate in the production of epoxide however, it could account for at least a small percentage of the products, since 35 was found to catalyze alkene epoxidation by utilizing PhIO (Scheme XIX). 

Alkyliron porphyrin complexes can also be formed by the attack on alkyl halides by Fe(I) systems, made by electroreduction as in, e.g., (48). 

Despite the successful determination of the crystal structures of some alkyliron porphyrins (see below), these compounds are in general, and notably in solution, quite fragile due to facile Fe-C bond homolysis (Equation (1)). Alkyliron(lll) porphyrin Fe-C bond cleavage at ambient temperature yields therefore a putative steady-state concentration of alkyl radicals. 

Alkyliron(lll) porphyrin complexes are air. sensitive, and when exposed to oxygen under ambient conditions the products are the very stable iron(IIl) /t-oxo dimers, [Fe(Por)]20. A more careful investigation revealed that the reaction of the alkyl complexes with oxygen proceeds via insertion of O2 into the Fe—C bond. " When a solution of Fe(Por)R (R = Me, Et, i-Pr) is exposed to O2 at —70 C, the characteristic H NMR spectrum of the low spin iron alkyl complex disappears and is replaced by a new, high spin species. The same species can be generated from the reaction of an alkyl hydroperoxide with Fe(Por)OH, and is formulated as... 

Insertion of SO2 into the Fe—C bond in FelPorfCHi was first reported in 1982, giving the sulfinato complexes Fe(Por)S02CH2, which are moderately air stable but can be further oxidized by O2 to give the sulfonato complexes FelPorfSOiCH. " Alkyliron(Ill) porphyrins insert CO to give the acyl complexes Fe(Por)C(0)R. For example, Fe(TPP)C(0)-n-Bu was formed either by this method or by the reaction of I Fe(TPP) r with ClC(0)-/ -Bu, and was characterized by an X-ray crystal structure... 

These complexes are easily reoxidized by dioxygen to TppFe R.i Alkyliron(III) porphyrins will undergo CO and CO2 insertion into the Fe-C bond, forming acyliron(III) (PFeC(O)R) and alkyl carboxylate (PFeOC(O)R) complexes, respectively. ... 

Battioni, P., J.-P. Mahy, G. Gillet, and D. Mansuy (1983). Iron porphyrin dependent oxidation of methyl- and phenylhydrazine Isolation of iron(II)-diazene and sigma-alkyliron (III) (or aryliron(III)) complexes. Relevance to the reactions of hemopro-teins with hydrazines. J. Am. Chem. Soc. 105, 1399-1401. 

---