Iron alkyl

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. 

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... 

Scheme 8.5 Addition of iron alkyl- and vinyl-intermediates to ketones and aldehydes.

Sulfur dioxide readily inserts into metal-carbon tr bonds (136, 137). The reaction is stereospecific with respect to the configuration at the a-carbon atom of the alkyl group (138) and proceeds with inversion of configuration at the a-carbon atom (139-142). The stereospecificity of the S02 insertion with respect to the metal center was demonstrated with diastereoisomeric pairs of enantiomers of Fe compounds (143-146). Also, the stereochemistry of the S02 insertion at the iron atom was studied by starting with the optically active iron alkyl derivative 26a (see Scheme 18). Retention of configuration was concluded from the similarity in the... 

Stereochemical changes occurring at the metal have been investigated for the iron alkyl (VII). The reaction shown in Eq. (16) was carried out with each of the two diastereomerically related pairs of enantiomers (i.e., RR—SS and RS—SR) (7). Complete (> 95%) stereospecificity. 

Tin(II) chloride also reacts with CpFe(CO)2Me to afford CpFe(CO)2-Sn(Me)Cl2 (55). However, SnBrg behaves differently toward the above iron alkyl, yielding CpFe(CO)2SnBr3 and CpFe(CO)2Br. 

E. Catalysis of Alkyl Disproportionation by Iron. Alkyl disproportionation is the sole reaction observed during the iron-catalyzed reaction of ethylmagnesium bromide and ethyl bromide.(47) The catalyst is a reduced iron species formed in situ by the reaction of iron(lI,III) with Grignard reagent, and ective... 

Iron acyl complexes are among the most widely studied of the organometallic iron species, especially as applied to organic synthesis. As previonsly mentioned they are prepared to provide access to iron alkyls (via decarbonylation), and because iron acyls can be deprotonated to form enolates much like any carbonyl they have been utilized as chiral auxiliaries in asymmetric synthesis. Also, iron acyls are an important entry point for the preparation of iron carbenes. 

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. ... 

Closely related are the -acyl complexes of iron (115). These have been prepared by three methods. Loss of N2 from (112) in the presence of an alkyl halide gives oxidative addition see Oxidative Addition) to yield ) -acyl complexes for some phosphine ligands. Sodium amalgam reduction of Fe halide complexes (116), and exposme of the resulting (117) to an alkyl halide, gives similar complexes (equation 24). Finally, placement of iron alkyl (aryl) complexes (118) under a CO atmosphere results in insertion to afford complexes (119) (equation 25). ... 

In method (a), an alkyl halide (or tosylate) reacts with [Fe(CO)4] " in N-melhyl-2-pyrrolidone to give an anionic iron alkyl (1), some of which have been isolated and characterized. Further reaction with another alkyl halide gives a complex tentatively formulated as (2), which may decompose to the ketone via the complex (3) formed by solvent-assisted migratory insertion. No alkyl coupling products are detected. 

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

In addition, the degree of saturation of the polyolefin increases with the ratio of Al/Fe (Fig. 3.12) depending on the reaction time with increasing conversion and hence decreasing number of methyl groups on aluminum, the formation of olefins increases. This shows that olefin elimination from an iron alkyl species is... 

Photochemical reaction of bifunctional ketoenamines in CCI4 with maleic and fumaric acids and esters can be used for stereoselective synthesis of cis and irons alkyl 7-azabicyclo[2.2.1]heptanecarboxylates in good yield (equation 248). 

The anomalous products may be obtained by conversion of enamines 85 and 91 to the corresponding enolates 86 and 92 due to the effect of external base or the tetrasubstituted enamine, followed by attack on a second molecule of methyl vinyl ketone affording isomeric enolates 87 and 93. Protonation of these enolates would afford precursors to enones 88 cis alkyl groups) and 94 irons alkyl groups). 

All of the steps in this mechanism have precedent in stoichiometric reaction chemistry. It is assumed that alkene adds before silane, but these steps may be reversed in some cases. The reversibility of two steps in the mechanism accounts for both the observed isotopic exchange between the alkene and silane, and the accompanying isomerization of alkenes. The fact that hydrosilylation occurs with retention of configuration at silicon266 is consistent with this mechanism, since oxidative addition of silane to a metal center is known to proceed in a cis manner and with retention199,267. The product-releasing step (elimination from an alkyl/silyl complex) has recently been observed in the thermal decomposition of an iron alkyl/silyl derivative (equation 100)268. Hydrosilylation as catalyzed by Co2(CO)8 appears to proceed by a somewhat different pathway8,35,262. 

Lehnert, N., K. Fujisawa, and E.I. Solomon (2003). Electronic structure and reactivity of high-spin iron-alkyl- and -pterinperoxo complexes. Inorg. Chem. 42,469-481. 

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. 

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