Alkyliron complexes

Alkene synthesis. A regio- and stereoselective alkene synthesis is formulated in equations (I)—(III). The first step involves formation of the alkyliron complex 1. Treatment of 1 with trityl tetrafluoroborate abstracts a -hydrogen to give a cationic iron complex 2. Liberation of the free alkene is effected by Nal in acetone. This sequence is useful because 1- and 2-alkyliron complexes (1) are converted into 1-alkcncs exclusively 3-alkyliron complexes are converted exclusively into the less stable (Z)-2-alkenes. The paper includes some 20 examples of alkenes prepared in this way. 

The carbonyls Fe(CO)5 and [CpFe(CO)2]+ (2) form stable cationic complexes with alkenes, which are used for both protection and activation of alkenes [1]. [CpFe(CO)2]+ (2 abbreviated as Fp+) is prepared by the reaction of cyclopentadienyl anion (1) with Fe(CO)5, followed by oxidative cleavage with bromine, and used for the protection of alkenes. The electron density of the double bond is decreased by the coordination of [CpFe(CO)2]+ and hence this bond is activated to nucleophilic attacks. Introduction of nucleophiles, such as the carbon nucleophile of malonate, to cyclopentene becomes possible via the formation of the complex 3, and the stable tftmv-er-alkyliron complex 4 of cyclopentane is prepared. The vinyl ether complex 6 is obtained easily from the a-bromoacetal 5, and reacts with an enolate of ketone 7 as an... 

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... 

Carbonylferrate salts, such as K [HFe(CO)4] , which is readily prepared from [Fe(CO)s] and ethanolic KOH, react with alkyl bromides or iodides to form alkyliron complexes. Under ambient conditions these complexes undergo insertion of CO and, in the presence of excess CO, aldehyde is eliminated and [Fe(CO)s] is regenerated (Scheme 10). Unfortunately, the reaction is not catalytic and the hydridocar-bonylferrate salt must be prepared in a separate step. Using the commercially available salt Na2[Fe(CO)4] a similar reaction takes place, but in this case the intermediate acyl complex is anionic and acid treatment is necessary to liberate the aldehyde (Scheme 11). ... 

In a typical preparation 1.2 mol equiv of [Ph3C][BFJ in anhyd CHjClj is added dropwise to a stirred anhyd CH2CI2 solution of alkyliron complex cooled in an ice bath. After addition of the trityl cation is complete, the reaction is allowed to reach RT and continue to completion as determined by IR monitoring of the Vco region of the spectrum (ca. 1 h). The reaction solution is then cooled to at least 0°C and treated with diethyl ether to precipitate the product. After collection by vacuum filtration, washing with several small portions of ether and drying in vacuo, the yellow olefin complexes are recrystallized from CHjClj-ether with ca. 10% acetone at —78°C. 

Complexes derived from nucleophilic attack on [ / -C5H5Fe(CO)2(olefin)] complexes, hereafter abreviated [Fp(olefin)], are among the most common. Heteroatomic nucleophiles including methanol, dimethylamine, trimethylamine, benzyl amine, t-butylmercaptan, triphenylphosphine and triethylphosphite all react with [Fp(olefin)] complexes to produce stable >/ -alkyliron complexes. Monosubstituted olefin complexes react with methanol with high regiospecificity to produce air-sensitive amber oils, which, however, are well characterized ... 

Many carbanions react with [Fp(olefin)] complexes to form relatively stable / -alkyliron complexes. Stabilized enolates derived from cyanoacetates, ma-lonates, acetoacetates and nitromethane react with the [Fp(styrene)]" complex almost exclusively at the benzylic carbon but react with [Fp(propene)] complexes with low regioselectivity ... 

Enamines react with [Fp(olefin)] complexes at 0°C in acetonitrile in a similar fashion to give carbonyl-substituted j -alkyliron complexes. The reaction of the pyrrolidine enamine with the [Fp(ethylene)] complex is illustrative. 

Reaction of nonstabilized carbanions with [Fp(olefin)] complexes generally results in either displacement of the olefin or reduction of the metal rather than formation of stable (j -alkyliron complexes. This is especially true with simple, nonstabilized organo-magnesium halide or lithio reagents. However, allylmagnesium chloride and phenylmag-nesium chloride react in modest (20-40%) yield with the ethylene, propene and butadiene (1,4 addition) iron complexes. Lithium dimethylcuprate is even more efficient, reacting in up to 70% yield with Fp complexes of styrene, butadiene (1,4 addition) isoprene (1,4 addition) and allene. Complexes of cyclopentene and allene react in low... 

Nonstabilized ylids do not lead to isolable / -alkyliron complexes. 

Enol ethers aUo form [Fp(olefin)] complexes, which react with ketone enolates to form j -alkoxy-jj -alkyliron complexes ... 

This reaction is very rapid and produces air-stable, crystalline i -alkyliron complexes in excellent yields from a variety of substituted i/ -allyliron complexes. Other electrophilic olefins, including dichlorodicyanoquinone, dimethyl methylene malonate, and / ,/ -dicyano-o chlorpstyrene undergo similar cycloaddition reactions with // -allylFp complexes. jy vAllyl complexes of j/ -CsHjWCCOyj, j -C5H5Mo(CO)3, i/ -CjHj-Cr(NO)2 and (pyridine)bis(dimethylglyoximato)cobalt react in a similar fashion but are much less thoroughly studied . 

The tetracarbonyl iron complex of methyl acrylate, methyl 2-chloroacrylate ° and methyl crotonate react in a similar fashion. Although the resulting jj -alkyliron complexes are stable (requiring treatment with trifluoroacetic acid followed by a oxidant to free the organic group from the iron), they are not well characterized. 

Some of the classic studies of nucleophilic, attack on coordinated olefins were conducted with iron(II) species. Rosenblum reported the reactions of (ti -cyclopentadienyl) iron-olefin complexes with a wide range of carbanion and enamine nucleophiles. These reactions produce stable o-alkyliron complexes (Equation 11.22). The stereochemistry is cleanly trans. However, the regioselectivity of reactions of complexes of imsymmetrical olefins depended on the nucleophile. 

Scheme 4-20. Ti -Alkyliron complexes by reaction of (Ti -alkene)iron complexes with nucleophiles.

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