Complexes iron alkyl

The enantiomerically pure chloromethyl complexes (-)-(/ )-9 and ( + )-(S)-9 (shown below as 10) can be converted to iron-alkyl complexes by treatment with sodium borohydride, Grignard reagents, or alkyllithium species, with no loss of enantiomeric purity16,17 (see also Houben-Weyl, Vol. 13/9 a, p 193). 

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... 

Nucleophilic attack on ( -alkene)Fp+ cations may be effected by heteroatom nucleophiles including amines, azide ion, cyanate ion (through N), alcohols, and thiols (Scheme 39). Carbon-based nucleophiles, such as the anions of active methylene compounds (malonic esters, /3-keto esters, cyanoac-etate), enamines, cyanide, cuprates, Grignard reagents, and ( l -allyl)Fe(Cp)(CO)2 complexes react similarly. In addition, several hydride sources, most notably NaBHsCN, deliver hydride ion to Fp(jj -alkene)+ complexes. Subjecting complexes of type (79) to Nal or NaBr in acetone, however, does not give nncleophilic attack, but instead results rehably in the displacement of the alkene from the iron residue. Cyclohexanone enolates or silyl enol ethers also may be added, and the iron alkyl complexes thus produced can give Robinson annulation-type products (Scheme 40). Vinyl ether-cationic Fp complexes as the electrophiles are nseful as vinyl cation equivalents. ... 

Fig. 2.1 Bipyridine-coordinated iron alkyl complex (A) for the polymerization of acetoni-...

The links between formation of an iron-alkyl complex and irreversible destruction of the heme moiety have not been forged, but model studies with diaryl- and carbethoxy-substituted carbene complexes suggest that the halogenated carbenes may shift to form a bond with a nitrogen of the porphyrin. The resulting A -haloaIkyl adduct are likely to undergo water-dependent hydrolysis and might therefore not be detected by the methods used to isolate other A -alkyl porphyrins. However, the formation of alternative reactive species that attack the protein or the heme cannot be ruled out. 

Tondreau AM, Milsmann C, Patrick AD, Hoyt HM, Lobkovsky E, Wieghardt K, Chirik PJ (2010) From synthesis and electronic structure of cationic, neutral and anionic bis(immo) pyridine iron alkyl complexes evaluation of redox activity in single-component ethylene polymerization catalysts. J Am Chem Soc 132(42) 15046-15059... 

Alkylation of the anion 2 with iodomethane or other haloalkanes provides alkyldicarbonyl(t/5-cyclopentadienyl)iron complexes such as 53,0 (see also Houben-Weyl, Vol. 13/9a, p 209). Migratory insertion of carbon monoxide occurs on treatment with phosphanes or phosphites9 -11 (see also Houben-Weyl, Vol. d3/9a, p257) to provide chiral iron-acyl complexes such as 6. This is the most commonly used preparation of racemic chiral iron-acyl complexes. 

Another route to enantiomcrically pure iron-acyl complexes depends on a resolution of diastereomeric substituted iron-alkyl complexes16,17. Reaction of enantiomerically pure chloromethyl menthyl ether (6) with the anion of 5 provides the menthyloxymethyl complex 7. Photolysis of 7 in the presence of triphenylphosphane induces migratory insertion of carbon monoxide to provide a racemic mixture of the diastereomeric phosphane-substituted menthyloxymethyl complexes (-)-(/ )-8 and ( + )-( )-8 which are resolved by fractional crystallization. Treatment of either diastereomer (—)-(/J)-8 or ( I )-(.V)-8 with gaseous hydrogen chloride (see also Houben-Weyl, Vol 13/9a, p437) affords the enantiomeric chloromethyl complexes (-)-(R)-9 or (+ )-(S)-9 without epimerization of the iron center. 

Subsequent carbonylation of the alkyl-iron complexes with carbon monoxide provides the desired chiral iron-acyl complexes, with essentially complete inversion of configuration at... 

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). ... 

The reduced alkyl complexes are reoxidized by O2 to the iron(lll) alkyls. The corresponding diamagnetic phthalocyanine iron(ll) alkyl complexes, rFe(Pc)R), were prepared by two-electron reduction of Fe(Pc) by LiAIFl4 to give [Fe(Pc) (actually the Fe(I) phthalocyanine radical anion) followed by reaction with Mel, Etl or i-PrBr. The methyl compound, [Fe(Pc)CHi] was characterized by X-ray crystallography. ... 

One-electron reduction of the iron(lll) alkyl complexes forms the diamagnetic iron(ll) alkyl anions [Fe(Por)R. The iron(ll) anions do not react with oxygen directly, but are first oxidized by O2 to the corresponding alkyliron(III) complexes, Fe(Por)R, which then insert O2 as described above. 

Iron(II) alkyl anions fFe(Por)R (R = Me, t-Bu) do not insert CO directly, but do upon one-electron oxidation to Fe(Por)R to give the acyl species Fe(Por)C(0)R, which can in turn be reduced to the iron(II) acyl Fe(Por)C(0)R]. This process competes with homolysis of Fe(Por)R, and the resulting iron(II) porphyrin is stabilized by formation of the carbonyl complex Fe(Por)(CO). Benzyl and phenyl iron(III) complexes do not insert CO, with the former undergoing decomposition and the latter forming a six-coordinate adduct, [Fe(Por)(Ph)(CO) upon reduction to iron(ll). The failure of Fe(Por)Ph to insert CO was attributed to the stronger Fe—C bond in the aryl complexes. The electrochemistry of the iron(lll) acyl complexes Fe(Por)C(0)R was investigated as part of this study, and showed two reversible reductions (to Fe(ll) and Fe(l) acyl complexes, formally) and one irreversible oxidation process."" ... 

Iron hydride complexes can be synthesized by many routes. Some typical methods are listed in Scheme 2. Protonation of an anionic iron complex or substitution of hydride for one electron donor ligands, such as halides, affords hydride complexes. NaBH4 and L1A1H4 are generally used as the hydride source for the latter transformation. Oxidative addition of H2 and E-H to a low valent and unsaturated iron complex gives a hydride complex. Furthermore, p-hydride abstraction from an alkyl iron complex affords a hydride complex with olefin coordination. The last two reactions are frequently involved in catalytic cycles. 

The proposed catalytic cycle is shown in Scheme 31. Hence, FeCl2 is reduced by magnesium and subsequently coordinates both to the 1,3-diene and a-olefin (I III). The oxidative coupling of the coordinated 1,3-diene and a-olefin yields the allyl alkyl iron(II) complex IV. Subsequently, the 7i-a rearrangement takes place (IV V). The syn-p-hydride elimination (Hz) gives the hydride complex VI from which the C-Hz bond in the 1,4-addition product is formed via reductive elimination with regeneration of the active species II to complete the catalytic cycle. Deuteration experiments support this mechanistic scenario (Scheme 32). 

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