Iron enantiomerically pure

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. 

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). 

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. 

Having established a very effective method for the synthesis of tricar-bonyl(T74-vinylketene)iron(0) complexes, Thomas has subsequently undertaken the most comprehensive study on the reactivity of these complexes to date. The reactions of 221 with phosphoramidate anions,90134 coordinating ligands such as phosphines3 and isonitriles,69,87,89,135,142,143 a variety of nucleophiles,86,89135142 phosphonoacetate anions,88,89 alkynes,108,109,144,145 and al-kenes146,147 have ah been investigated. Crucially, Thomas has also developed a method138 for the kinetic resolution of the vinylketene complexes (221) that ultimately yields enantiomerically pure samples of the complex. This... 

Enantioselective cyclopropanations using enantiomerically pure tungsten [54], iron [458,483,630], and ruthenium [581] carbene complexes have also been at-... 

Most of the work in this area has concerned complexes racemic at iron. Section D.1.3.4.2.5.1.1. details methods for the preparation and resolution of enantiomerically pure iron acyl complexes. The details of alkylation reactions (see Section 1.1.1.3.4.1.3.) and aldol reactions (see Section 1.3.4.2.5.1.2.) of these and other iron acyl enolates are presented later with examples utilizing enantiomerically pure complexes indicated therein. Table 1 illustrates the scope of iron-acyl enolates prepared by deprotonation of complex 10 and its analogs. 

Moreover, reaction of racemic 15 with two equivalents of iron(III) chloride afforded pure 2-(l-hydroxy-2,2-dirncthylpropyl)cyclobutanone 16 in 70% yield. Carrying out this reaction on enantiomerically pure ( —)-15 gave optically pure cyclobutanone 16.138... 

In fact, the chemists working on these compounds wanted only one enantiomer of the irons epoxide—the top left stereoisomer. They were able to separate the trans epoxide from the cis epoxide by chromatography, because they are diastereoisomers. However, because they had made both diastereoisomers in the laboratory from achiral starting materials, both diastereoisomers were racemic mixtures of the two enantiomers. Separating the top enantiomer of the trans epoxide from the bottom one was much harder because enantiomers have identical physical and chemical properties. To get just the enantiomer they wanted the chemists had to develop some completely different chemistry, using enantiomerically pure compounds derived from nature. 

Iron porphyrin catalysts with TBHP have been used for the diastereoselective oxidation of sulfides affording up to 46% d.e.408 A series of manganese(salen) catalysts with hydrogen peroxide has been employed for the oxidation of aralkyl sulfides in 34-70% d.e. and 80-90% yield. The best catalyst was derived from enantiomerically pure trans-1,2-diaminocyclohexane (Figure 3.101).409... 

Chemists who wish to prepare enantiomerically pure phosphines should consult the excellent review by Pietrusiewics and Zablocka,45 which gives examples of most of the P-chiral phosphines that have been made efficiently. In the last 10 years, a number of effective asymmetric syntheses of P-chiral phosphines have been developed. Prior to this, many of the chiral phosphines prepared were synthesized in racemic form and then resolved using stoichiometric amounts of a chiral organic compound or chiral palladium, platinum, or iron complexes. Although some interesting phosphines have been prepared by this method, they are not mentioned here as the experimental procedures are not in widespread use. 

To prepare the enantiomerically pure iron acyl complex (R)-(39), a precursor diastereomeric menthoxyaUcyl complex was resolved and then manipulated (Scheme 14). More recently resolution of the chiral-at-metal acyl complexes themselves was achieved, and this has become the basis for a commercial preparation of the iron acyl developed for use as a chiral auxiliary (see below). Cationic iron complex (43) was treated with potassium L-mentholate to produce diastereomeric esters (44) that were not isolated but were reacted with LiBr/MeLi (Scheme 15). After chromatography and recrystallization the enantiomerically pure ironacyl complex (5 )-(39a) was obtained. It was suggested that only one diastereomeric ester can react (with inversion of configuration at iron, as shown) with the methyl nucleophile the unreactive diastereomer suffers from severe steric congestion about the electrophilic CO ligand. 

Alkylation of iron acyl complexes also provides access to iron carbenes. Thus, the neutral iron acyl complex will react with acid, with alkylating agents, or with trifluoromethanesul-fonic anhydride to afford cationic hydroxy- or aUcoxycarbene complexes (52) and (53) or the cationic vinylidene complex (54, L = CO) (Scheme 20). The vinylidene complex can be used to prepare a more substituted analog of (51) by treatment with a thiol. The enantiomerically pure iron acyl complex (R)-(45a) can be converted to the corresponding enantiopure methoxycarbene complex with Me30Bp4 as well. Finally,... 

A number of enantiomerically pure complexes have been made, and this chemistry has been used in several natural product syntheses. Enantiopure complexes are readily available from the corresponding vinylic epoxides, and in cases where diastereoselective complexation is possible, diastereoselectivities tend to be moderate (typically 3 1 -4 1). The rationale for the origin of this diastereoselectivity has been proposed to derive from a preferential complexation of a Fe(CO)4 fragment to the alkene anti to the epoxide. Since the initial vinyl epoxide is conformationally flexible, four diastereomeric itt-complexes would be produced as a consequence of anti or syn complexation to the s-trans or s-cis conformers. Isomerization of these initial 7r-complexes to alkoxy- 7r-allyl species would then enable interception of an iron-bound carbonyl ligand by the alkoxide to afford diastereomeric lactone complexes. Fortunately, equilibria between the two possible trans itt-allyl complexes and their more stable cis Tr-aUyl analogs simplifies the outcome significantly. Thus, for trans vinyl epoxides, the major diastereomer typically is the one designated as endo cis (the C-1 substituent points toward the iron atom) the minor diastereomer corresponds to the exo cis isomer (the C-1 substituent points away from the iron atom) (Scheme 51). For cis vinyl epoxides, this outcome is reversed - the exo cis isomer is the major product. 

A report of the X-ray crystallographic studies of enan-tiopure ( -allyl)Fe(CO)2(NO) complex (170) has appeared, though httle detail was provided. The same report described the CD spectrum of this complex in more detail the negative band at ca. 350 mn and the positive band at ca. 450 nm can be used to assign the configuration of the complex. Diastere-omeric complexes exhibit the opposite Cotton effect. The crystal structures of corresponding monophosphine complexes (145) have been determined. ft is possible to consider these complexes as either trigonal bipyramidal (bidentate allyl) or tetrahedral see Tetrahedral) (monodentate allyl), with the central carbon of the allyl closer to the iron atom (2.084 A) than the terminal carbon atoms (2.117 and 2.142 A). These complexes are chiral at the iron atom, and it has proved possible to separate the diastereomeric complexes formed by enantiomerically pure aminophosphines. 

Complex 376 can be prepared from enantiomerically pure rhenium precursor 381. The former can be deprotonated at low temperatures initiating the [2,3]-sigmatropic rearrangement to diastereomerically pure homoallylic sulfide complex 377. After S-alkylation, cyanide treatment releases the S ligand as product 379. As an extension of this work the authors showed that iron and ruthenium complexes can be used, too [219]. 

The enantiomerically pure secondary arsine-iron complex 2 has been used for assym-metric synthesis of a tertiary arsine (equation 61) . ... 

The biphosphole (370) has been obtained in enantiomerically pure form by spontaneous resolution in the crystallisation of a racemic mixture, without the use of chiral auxiliaries. A new approach to -functionalised phospholes is afforded by metallation at a methyl group of l-phenyl-3,4-dimethylphosphole(in which both phosphorus and the diene unit are protected by coordination to an iron carbonyl acceptor), followed by treatment with electrophiles, to give C-substituted products, e.g., (371). Copper(II) oxidation of the intermediate lith-iomethyl derivative leads to the formation of bridged systems, e.g., (372). ... 

Both UV and visible light have sufficient energy to initiate many processes in coordination complexes. Decarbonylation is one of the most typical photoreactions, because the dissociation energy of a common metal carbonyl oxide bond is as low as 200kJ mol 1.1048 Scheme 6.154 presents two examples the fission of (a) metal—CO1049 and (b) metal—CO—alkyl1050 bonds in some carbonyl complexes. In the latter case, irradiation of an enantiomerically pure iron complex 348 leads to decarbonylation, which is followed by alkyl migration. 

Miura and coworkers showed that the reaction could also be carried out using catalytic amounts of Cul in the presence of pyridine (95JOC4999). Asymmetric reactions were reported to occur with chiral bisoxazoline ligands producing p-lactams with moderate (40-68%) enantiomeric excesses. Use of an oxazolidinone with a chiral auxiliary appended to the alkyne also provided enantiomerically pure products (02TL5499). In all of these latter reports, mixtures of cis and irons lactam isomers were obtained in which the trans-product predominates. It was also shown that the c/s-isomer could easily be converted to the trans-product when exposed to base. 

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