Iron enolates

Experiment.—One drop of ferric chloride solution is added to a few drops of an aqueous solution of acetylacetone. The reaction characteristic of enols takes place. If now the solution is cooled in ice and dilute bromine water is rather quickly added, the red colour of the iron enolate disappears for a short time and then returns rapidly. 

Li and coworkers published addition reactions of ethers, sulfides, or tertiary amines 40 to p-dicarbonyl compounds 39 (Fig. 8) [96]. Fe2(CO)9 proved to be the catalyst of choice and di-tert-butyl peroxide the optimal oxidant. a-Functionalized p-dicarbonyl compounds 41 were isolated in 52-98% yield. Although the details of the catalytic cycle remain unclear, it seems to be likely that the peroxide is reductively cleaved by the Fe(0) catalyst leading to an Fe(I) complex and a ferf-butoxyl radical, which abstracts the a-hydrogen atom of 40. Addition of the resulting radical to the free enol form of 39 or the corresponding iron enolate of 39 may subsequently occur. It remains unclear, however, whether the main catalytic reaction proceeds on an Fe(0)-Fe(I) oxidation stage or whether further oxidation of initially formed Fe(I) rather leads to an Fe(II) catalyst. This cannot be excluded,... 

The iron carbonyl-mediated cycloaddition is also highly stereospecific despite a nonconcerted mechanism. In the reaction of dibromo ketone (57) with m-p-deuteriostyrene, only the cis stereoisomer of the adduct is detected at low conversion (equation 50). - This stereospecificity is attributed to the formation of a U-shaped intermediate, in which the bonds are fixed rigidly by charge-transfer or coulombic attraction between the iron enolate and the cationic center. The result can also be explained by rapid bond formation between the two termini before bond rotation can occur. Traru-anethole also couples stereos-pecifically with (57) (equation 51). ... 

According to a detailed mechanistic studythe reaction involves reversible, regiospecific addition of NaHFe2(CO)s to the C =C double bond of the enone, affording the corresponding binuclear iron enolate. Cleavage of the latter to the mononuclear iron enolate represents the rate-determining step, protonolysis of which provides the saturated ketone. 

Recently the isolation arid structure determination of the aldol product of the chiral iron enolate (161) with benzaldehyde was obtained as (162). Hiis structure is presumed to mimic closely the structure of the cyclic transition state for the aldol reaction. 

Li believed that a cyclic alkane radical was generated from the hydrogen abstraction by a tert-butoxy radical that arose from the iron-catalyzed decomposition of the tert-butyl peroxide. The cyclic radical 62 was then believed to react with the iron enolate 63 to form the alkylated product 61 (Scheme 34). 

Chiral enolates in which the auxiliary is in the ester portion provide still another route to optically active lactams. Early results indicated that little asymmetric induction was obtained with menthyl enolates. Use of the enolate obtained from 24 did lead to high levels of asymmetric induction. Treatment of 24 with lithium diisopropylamide in tetrahydrofuran, followed by addition of imine 25, gives cf -/(-lactam 26 in 79% yield and 91%ee98. Optically active /3-lactams can be prepared by addition of chiral iron enolates (see Section D.l. 1.1.3.2.) to imines99-101. Addition of aluminum enolate 27 to imine 28, followed by oxidative cyclization with iodine and an amine, affords /(-lactam 29 in 54% yield and >95% ee. 

Liebeskind and Davies " ° have independently developed the use of this chiral iron complex for enantioselective organic syntheses, particularly of a variety of optically active molecules. The reasons for this behaviour are that the complex is chiral-at-iron and one face is hindered by the PPha ligand reaction of these acyl-iron enolates occur with very high stereoselectivity (Scheme 3.7). 

Davies examined the same reaction.He found that when using an excess of Et2Al adducts of the enolate, the diastereoselectivity ratio became > 100 1. Further, if Cu(I) was used as counter ion the opposite stereochemistry was obtained.Moreover both Davies and Liebsekind used this chiral iron auxiliary in a stereoselective synthesis of S-lactams. " " Liebeskind reported that chiral iron enolate complex condensed with imines in the presence of Et2Al counter ion to give two isomers with a ratio up to 20 1. Oxidation with I2/R3N produced the racemic jS-lacatms (Scheme 3.9)... 

Therefore, the first step should proceed through a syn carbometallation to generate the cyclopropyliron species 138. The following step would be the (1-carbon elimination reaction keeping the cis-relationship between R and R, providing an iron enolate intermediate 137. Then, a transmetallation with an alkylaluminum species would generate the aluminum enolate 135, which would result, upon aqueous quench, in the corresponding tetrasubstituted alkene. Previous studies revealed that some neutral chromium(III) complexes are active in alkene polymerization... 

Ricci and coworkers [64] studied oxazoline moiety fused with a cyclopenta[P]thio-phene as ligands on the copper-catalyzed enantioselective addition of Et2Zn to chalcone. The structure of the active Cu species was determined by ESI-MS. Evans and coworkers [65] studied C2-symmetric copper(II) complexes as chiral Lewis acids. The catalyst-substrate species were probed using electrospray ionization mass spectrometry. Comelles and coworkers studied Cu(II)-catalyzed Michael additions of P-dicarbonyl compounds to 2-butenone in neutral media [66]. ESI-MS studies suggested that copper enolates of the a-dicarbonyl formed in situ are the active nucleophilic species. Schwarz and coworkers investigated by ESI-MS iron enolates formed in solutions of iron(III) salts and [3-ketoesters [67]. Studying the mechanism of palladium complex-catalyzed enantioselective Mannich-type reactions, Fujii and coworkers characterized a novel binuclear palladium enolate complex as intermediate by ESI-MS [68]. 

Reaction of a,a -dibromo ketones with iron carbonyls generates a reactive three-carbon unit. The first step of the reaction is the formation of the iron enolate (CLin) via a two-electron reduction of the dibromo ketone or through an oxidative addition of the carbon-bromine bond onto iron(0). The second step is an 8, 1 or Fe ion-assisted elimination of bromide ion to form the oxyallyl intermediate (CLIV). Evidence for the production of... 

Acyclic (ri -diene)iron complexes will give substituted cyclopentanones or y,5-unsaturated aldehydes upon treatment with nucleophiles under carbon monoxide atmosphere (Scheme 4-119). A proposed mechanism involves the addition of the nucleophile at an inner carbon atom of the t) -diene ligand followed by insertion of carbon monoxide. Subsequent protonation provides the unsaturated aldehyde. Alternatively, the acyliron intermediate can undergo carboferration to give an acyliron intermediate. Isomerization via (3-hydride elimination and readdition provides an iron-enolate anion that upon treatment with acid provides the cyclopentanone. ... 

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