Iron chiral

S. G. Davies, H. M. Kellie, and R. Polywka, Opening of carbohydrate 5,6-epoxides with chiral acetate and propionate enolate equivalents attached to the iron chiral auxiliary [(C5H5)Fe(CO)P(Ph)3], Tetrahedron Asymmetry, 5 (1994) 2563-2570. 

Examples of aldol condensations involving chiral aldehydes have also been reported. Condensation of the aluminum enolate derived from (2) with Boc-L-prolinal has been shown to proceed in a highly stereoselective manner and the iron chirality overpowered the latent stereoselectivity inherent in Boc-L-prolinal. ... 

Iron chiral auxiliary for asymmetric aldol reaction, Michael addition, p-amino acid and p-lactam synthesis. 

Davies employed the iron chiral auxiliary to establish unambiguously the absolute configuration of Winterstein s acid (3-iV,Ai-dimethylamino-3-phenylpropanoic acid) as (R). ° Winterstein s acid (3-A,A-dimethylamino-3-phenyl-propanoic acid) had been identified as the carboxylate side chain of the diterpene taxine This acid had... 

Chiral diene—iron tricarbonyl complexes were acylated using aluminum chloride to give acylated diene—iron complexes with high enantiomeric purity (>96% ee). For example, /ra/ j -piperjdene—iron tricarbonyl reacted with acyl haUdes under Friedel-Crafts conditions to give l-acyl-l,3-pentadiene—iron tricarbonyl complex without any racemization. These complexes can be converted to a variety of enantiomericaHy pure tertiary alcohols (180). 

Ironically, auxiliary-induced control via the alkene failed to generate synthetically useful selectivities, but direct substrate-induced control did. In particular, chiral silyl enol ethers with stereocenters in the y-position allowed the synthesis of enantiomerically... 

Catalytic, enantioselective cyclopropanation enjoys the unique distinction of being the first example of asymmetric catalysis with a transition metal complex. The landmark 1966 report by Nozaki et al. [1] of decomposition of ethyl diazoacetate 3 with a chiral copper (II) salicylamine complex 1 (Scheme 3.1) in the presence of styrene gave birth to a field of endeavor which still today represents one of the major enterprises in chemistry. In view of the enormous growth in the field of asymmetric catalysis over the past four decades, it is somewhat ironic that significant advances in cyclopropanation have only emerged in the past ten years. 

I.3.4.2.5. Chiral Enolates of Acyl-Metal Complexes J. S. McCallum and L. S. Liebeskind I.3.4.2.5.I. Chiral Iron-Acyl Complexes... 

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. 

Enantiomerically Pure Chiral Iron-Acyl Complexes... 

The reported preparations of enantiomerically pure chiral iron-acyl complexes have relied upon resolutions of diastereomers. One route1415 (see also Houben-Weyl, Vol. 13/9 a, p 421) employs a resolution of the diastereomeric acylmenlhyloxy complexes (Fe/ )-3 and (FeS )-3 prepared via nucleophilic attack of the chiral menlhyloxide ion of 2 at a carbon monoxide of the iron cation of 1. Subsequent nucleophilic displacement of menthyloxide occurs with inversion at iron to generate the enantiomerically pure iron-acyl complexes (i>)-4 and (f )-4. 

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

The dicarbonyl species thus produced undergo photolytically promoted carbon monoxide displacement by phosphine to give racemic mixtures of chiral a,/ -unsaturated iron-acyl complexes35. 

I.3.4.2.5.I.2. Aldol Additions of Enolates of Chiral Iron-Acetyl Complexes... 

The chiral lithium enolate 2 reacts with symmetrical ketones to produce /(,/i-dialkyl-/l-hydroxy-acyl complexes 3 which serve as precursors to oc,/1-unsaturated iron complexes (see Section 1.3.4.2.5.1.1.). 

Aldol Additions of Enolatcs of Chiral Iron-Propanoyl Complexes... 

Aldol reactions of a-substituted iron-acetyl enolates such as 1 generate a stcrcogenic center at the a-carbon, which engenders the possibility of two diastereomeric aldol adducts 2 and 3 on reaction with symmetrical ketones, and the possibility of four diastereomeric aldol adducts 4, 5, 6, and 7 on reaction with aldehydes or unsymmetrical ketones. The following sections describe the asymmetric aldol reactions of chiral enolate species such as 1. 

Table 5. Carboxylic Acids 5 by Removal of the Chiral Auxiliary from Iron-Acyl Complexes 4...

Woo et al. [54] prepared new chiral tetraaza macrocyclic hgands (48 in Scheme 23) and their corresponding iron(II) complexes and tested them, as well as chiral iron(II) porphyrin complexes such as Fe (D4 -TpAP) 49, in asymmetric cyclopropanation of styrene. 

There are very few examples of asymmetric synthesis using optically pure ions as chiral-inducing agents for the control of the configuration at the metal center. Chiral anions for such an apphcation have recently been reviewed by Lacour [19]. For example, the chiral enantiomerically pure Trisphat anion was successfully used for the stereoselective synthesis of tris-diimine-Fe(ll) complex, made configurationally stable because of the presence of a tetradentate bis(l,10-phenanthroline) ligand (Fig. 9) [29]. Excellent diastereoselectivity (>20 1) was demonstrated as a consequence of the preferred homochiral association of the anion and the iron(ll) complex and evidence for a thermodynamic control of the selectivity was obtained. The two diastereoisomers can be efficiently separated by ion-pair chromatography on silica gel plates with excellent yields. 

An iron complex-catalyzed asymmetric hydrosilylation of ketones was achieved by using chiral phosphoms ligands [68]. Among various ligands, the best enantios-electivities (up to 99% ee) were obtained using a combination of Fe(OAc)2/(5,5)-Me-Duphos in THF. This hydrosilylation works smoothly in other solvents (diethylether, n-hexane, dichloromethane, and toluene), but other iron sources are not effective. Surprisingly, this Fe catalyst (45% ee) was more efficient in the asymmetric hydrosilylation of cyclohexylmethylketone, a substrate that proved to be problematic in hydrosilylations using Ru [69] or Ti [70] catalysts (43 and 23% ee, respectively). 

Scheme 6 Chiral iron complexes for the asymmetric epoxidation of olefins...

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