Iron, reaction with alkyl halides

Fig. 2 Catalysts and ligands used in iron-catalyzed cross-coupling reactions with alkyl halides...

Cross coii ing of Grignard reagents with 1-alkenyl halides, in marked contrast to alkyl halides, occurs readily with the reduced iron catalyst, as described above. The iron-catalyzed reaction of Grignard reagents with 1-alkenyl halides can, however, be differentiated from the reaction with alkyl halides. Thus, a mixture of propenyl bromide and ethyl bromide on reaction with methylmagnesium bromide afforded butene-2 but no cross-over products such as pentene-2 or propylene. The latter certainly would have resulted if a propenyliron species per se were involved in the catalytic process. Cross coupling under tEese circumstances clearly merits further study. 

The a-protons of iron acyl complexes are acidic and these can be deprotonated with Lithium diisopropylamide (LDA) or with n-butyllithimn. Thus the corresponding enolates are readily functionalized and undergo reaction with alkyl halides, aldehydes, disulfides, trimethylsilyl chloride, and epoxides to afford the corresponding a-derivatized products. " Early work on racemic complexes revealed that these transformations occur in a highly diastereoselective fashion,... 

The nucleophilic character of the complexes is apparent from the reactions with alkyl halides or acyl halides. With the iron complex, unsymmetrical ketones are formed 91). 

Scheme 4-2. In situ formation of Collman s reagent from pentacarbonyliron or iron(III) chloride and reaction with alkyl halides.

The most common representatives of alkenyliron complexes are iron-substituted enones and enals. They are prepared from Fp-anions and P-halovinyl ketones and aldehydes (Scheme 4-30), ° and they can be further functionalized via their enolates and reaction with alkyl halides or mesylates. ... 

Alternatively, acyliron complexes can be obtained in a two-step sequence either from dicarbonyl(cyclopentadienyl)ferrates or from dicarbonyl(cyclopentadienyl)iron halides via alkyl-Fp complexes. The first method makes use of the nucleophilicity of [CpFe(CO)2] anions. Their reaction with alkyl halides provides alkyl-Fp complexes that, upon treatment with phosphanes or phosphites, undergo a migratory insertion of a carbonyl ligand into the metal-alkyl bond leading to an acyliron complex (Scheme 4-38). " ... 

Iron-acyl enolates such as 1, 2, and 3 react readily with electrophiles such as alkyl halides and carbonyl compounds (see Houben-Weyl, Vol. 13/9a p418). The reactions of these enolatc species with alkyl halides and similar electrophiles are discussed in Section D.1.1.1.3.4.1.3. To date, only the simple enolates prepared by a-deprotonation of acetyl and propanoyl complexes have been reacted with ketones or aldehydes. 

Recently, Fu and coworkers have shown that secondary alkyl halides do not react under palladium catalysis since the oxidative addition is too slow. They have demonstrated that this lack of reactivity is mainly due to steric effects. Under iron catalysis, the coupling reaction is clearly less sensitive to such steric influences since cyclic and acyclic secondary alkyl bromides were used successfully. Such a difference could be explained by the mechanism proposed by Cahiez and coworkers (Figure 2). Contrary to Pd°, which reacts with alkyl halides according to a concerted oxidative addition mechanism, the iron-catalyzed reaction could involve a two-step monoelectronic transfer. 

Iron-Catalyzed Cross-Coupling Reactions of Alkyl Halides with Organometallic Reagents... 

Fig. 3 Catalytic manifolds in iron-catalyzed cross-coupling reactions of alkyl halides 1 with organometallic species 2...

Neutral a-alkyliron complexes are obtained upon reaction of Na[Cp(CO)2pe] (5) with alkyl halides (9) (Scheme 6), and as with Collman s reagent this occurs in an Sn2 fashion with inversion of coirfiguration at the carbon atom. Epoxides also participate in this reaction, but tertiary alkyl halides are poor substrates. Alternatively, complexes (9) may be prepared by reaction of an appropriate metal alkyl with Cp(CO)2PeX (6). Typically complexes of this type are prepared in order to gain access to the synthetically nseful cationic rf--alkene iron complexes (Section 4.1.2). Also, nucleophilic addition of (5) to heteroatom-snbstituted alkyl halides (snch as methoxymethylchloride or chloromethyl methyl snllide) affords complexes of type (9) that can be converted to cationic... 

As already mentioned above, iron(I) porphyrins can undergo various reactions. They are of synthetic value for preparation of a-alkyl iron porphyrins, which can be obtained by direct alkylation of the corresponding electrochemically generated iron(I) porphyrins with alkyl halides. The highly reduced species can also be oxidized by molecular oxygen or hydrogen peroxide to yield the iron oxyporphyrin (oxophlorin), which is further oxidized to verdoheme by molecular oxygen, " as discussed in Section 6.1.5. 

Reaction of 74 with alkyl halides gives 75 (equation 285 °). Triorganoantimony compounds containing allyl groups react with the iron complex 76 with elimination of the allyl moiety to give 77 (equation 286 ). Reaction of the tetracoordinate antimony ate complex 78 with the iron complex 79 gives 80, which has been characterized by X-ray analysis (equation 287 ). 

Applications of carbonylaie reactions in organic synthesis are numerous. Particularly noteworthy are schemes involving tetracarbonylferrate( - ID (referred to as Collman s reagent), which can be isolated as a. sodium salt, NanFe(CO)4-1.5 dioxane, and is commercially available. The highly nucleophilic [Fe(CO)4p reacts readily with alkyl halides to yield alkyl iron carbonylates ... 

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