Iron complexes, cationic chiral

Access to alkyl substituted derivatives of the homotropylidene complexes is provided via the >j4-tropone iron complex by reaction with diazoalkanes, followed by mild thermolysis of the 3+2 py razoline adduct to give the corresponding homotropone complexes (equation 149)217,218. The 8,8-dimethyl derivative was used as starting material for the preparation of the fluxional ( 5-2,8,8-trimethylbicyclo[5.1.0]octa-2,4-dienylium)Fe(CO)3 cation complex219. More recently (homotropone)Fe(CO)3 was used for the synthesis of unique chiral 1,2-homoheptafulvene iron complexes220 221. 

A titanium complex derived from chiral /V-arencsulfonyl-2-amino-1 -indanol [20], a cationic chiral iron complex [21], and a chiral oxo(salen)manganese(V) complex [22] have been developed for the asymmetric Diels-Alder reaction of oc,P-unsaturated aldehydes with high asymmetric induction (Eq. 8A.11). In addition, a stable, chiral diaquo titanocene complex is utilized for the enantioselective Diels-Alder reaction of cyclopentadiene and a series of a.P Unsaturated aldehydes at low temperature, where catalysis occurs at the metal center rather than through activation of the dienophile by protonation. The high endo/exo selectivity is observed for a-substituted aldehydes, but the asymmetric induction is only moderate [23] (Eq. 8A. 12). 

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. 

Cyclopropanation reactions of nonheteroatom-stabilized carbenes have also been developed. The most versatile are the cationic iron carbenes that cyclopropanate alkenes with high stereospecificity under very mild reaction conditions. The cyclopropanation reagents are available from a number of iron complexes, for example, (9-alkylation of cyclopentadienyl dicarbonyliron alkyl or acyl complexes using Meerwein salts affords cationic Fischer carbenes. Cationic iron carbene intermediates can also be prepared by reaction of CpFe(CO)2 with aldehydes followed by treatment with TMS-chloride. Chiral intermolecular cyclopropanation using a chiral iron carbene having a complexed chromium tricarbonyl unit is observed (Scheme 61). 

Synthesis of Chiral Lewis Acids. BIPHOP-F is used in the synthesis of chiral transition metal Lewis acids. Because of its electronic properties, it enhances the acidity of the metal. Coordination of the bidentate ligand to the metal is accomplished by CO substitution (eq 1 and 2 ). The cationic ruthenium or iron complexes are obtained after one or two additional steps (L is a labile ligand and X the counter anion). 

The complexes are isolated, characterized and used as chiral Lewis acids. Dissociation of the labile ligand liberates a single coordination site at the metal center. These Lewis acids catalyze enantioselective Diels-Alder reactions. For instance, reaction of methacrolein with cyclopentadiene in the presence of the cationic iron complex (L = acrolein) occurs with exo selectivity and an enantiomeric excess of the same order of magnitude as those obtained with the successful boron and copper catalysts (eq 3). ... 

Aromatic ketimines are reduced enantioselectively to amines (50 atm H2/toluene/65°C/24h), using a cooperative catalysis involving Knolker s iron complex and a BINOL-derived hydrogen phosphate auxiliary, with P-NMR evidence supporting the bifunctional catalysis. A phosphine-free chiral cationic ruthenium complex catalyses enantioselective hydrogenation of IV-alkyl ketimines, including many heretofore problematic substrates. 0... 

The iron complexes have found a particular value in rendering allyl cations chiral. An ordinary allyl cation is planar and, therefore, achiral. Coordination to iron can retain the chirality as coordination of the metal distinguishes the two faces. It has been shown that the rf-sulfonyl alkene complex 9.23 can be formed in good d.e. and recrystallized to complete purity (Scheme 9.8). Acid treatment yields the desired cationic T -allyl complex 9.24, and treatment of this with suitable nucleophiles yields the allylated products, after... 

Two main classes of T -dienyliron complexes are known, namely the cationic tricarbonyliron complexes and neutral cyclopentadienyliron compounds. The cyclopentadienyl (Cp) ligand is relatively inert to a broad variety of reaction conditions. It is often introduced as a ligand to tune the properties of the iron complex, as a chiral auxiliary,or in material science in organoiron pol5miers. However, it is only rarely transformed itself into a more elaborate organic product. For this reason, the chemistry of the cyclopentadienyl ligand will not be discussed in more detail in this chapter. Tricarbonyl( n -dienylium)iron complexes, on the other hand, represent versatile electrophilic building blocks for the attachment of 1,3-butadiene, cyclohexadiene, or aromatic moieties to nucleophilic molecules. 

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. 

The cationic aqua complexes of the C2-symmetric trans-chelating tridentate ligand 447 proved also highly effective chiral catalysts. The complexes involving the metal(II) perchlorates of iron, cobalt, nickel, copper and zinc produced the main endo adduct of cyclopentadiene and N - aery loy 1-1,3 -oxazo I i din -2 -one with very high ee values281. 

Stereoselectivity) is observed however, for ethylidene complexes of Fe(CO)(PR3)Cp (69) the products reflect trans selectivity. This difference in stereoselectivity has been suggested to be dependent upon which conformer is more reactive. The reaction of a chiral-at-iron cationic carbene complex (70) with styrene or vinyl acetate affords optically active cyclopropane products with high enantioselectivity (Scheme 24). h >3 intramolecular cyclopropanation, as in the case of (71), proceeds moderately well for the formation of norcarane-type ring systems however, intramolecular C-H insertion is a competing pathway when the alkene is highly... 

Some progress has been made in performing asymmetric variations of this chemistry. Addition of the anion of sulfoximine ester (272) to achiral (cycloheptadienyl)iron cation complex (271, L = P(OPh)3), followed by desullu-rization, affords enantiomerically enriched (273, 50% ee) (Scheme 77). In a similar manner, addition of the enolate derived from a chiral iV-acyloxazolidinone (274) to achiral (cyclohexadienyl)iron cation complex (275) and subsequent auxiliary removal affords (276, 70% ee). 

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