Chiral iron complexes

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). 

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. 

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 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.). 

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...

Scheme 10 Asymmetric epoxidation of tra/ts-2-heptene with the chiral iron complex and H2O2...

Better results for the porphyrin complex-catalyzed asymmetric epoxidation of prochiral olefins were achieved by Naruta et al.98 using iron complexes of chiral binaphthalene or bitetralin-linked porphyrin 128 as chiral catalysts. As shown in Scheme 4-45, asymmetric epoxidation of styrene or its analogs provided the product with good ee. Even better results were obtained with substrates bearing electron-withdrawing substituents. 

The oxidative cyclization of chiral 2-pyrrolidino-l-ethanol derivatives is shown in the reaction of 251 with trimethyl-amine iV-oxide and a substoichiometric amount of cyclohexadiene iron tricarbonyl to produce the corresponding oxazolopyrrolidine ring 252. The mechanism of this reaction is unknown. Both amine oxide and iron complex are essential for the reaction (Equation 39) <2005TL3407>. 

As well as organic chiral auxiliaries, organometallic fragments have found some application as chiral auxiliaries in conjugate addition reactions. Particularly noteworthy are chiral molybdenum allyl complexes [69], chiral iron complexes [70], and planar chiral arene chromium species [71]. 

The enantiopure tricarbonyl(dienal)iron complex 61 suitably transfers chirality in the piperidine ring formation. Condensation to the Schiff base is followed by the intramolecular Mannich reaction catalyzed with p-TSA. The piperidine was converted to dienomycin C (62) in five additional steps <99EJOC1517>. 

Related Alkylations of Chiral a-Alkoxy Vinyl-Metal Complexes Chiral a-Alkoxy Vinyl-Iron Complexes... 

Chiral rhenium complexes, such as 1 and 4, are isoelectronic to the a-alkoxy vinyl-iron complexes discussed above and they exhibit analogous chemistry in many respects. Like the iron complexes, they are prepared as the Z-isomer and are readily alkylated by primary iodoalkanes and (bromomethyl)benzene with efficient 1,3-asymmetric induction97. Subsequent, spontaneous loss of halomethane produces the elaborated rhenium-acyl complexes. Two examples of the stereocontrolled preparation of diastereomeric rhenium-acyl complexes via this methodology are illustrated. 

A chiral metal center, as is found in a pseudotetrahedral iron complex with cyclopentudienyl. carbonyl, triphenylphosphine, and ethyl ligands, hus also beer used to address the question of alkyl migration versus carbonyl insertion. Inversion of... 

Applying the pseudo-atom convention to the iron complex in Figure 8, the iodine atom is priority 1, the t1S-C5H3R2 ligand is priority number 2, the phosphorus atom is priority number 3, and carbon is priority number 4.15 When the iron is viewed from the side opposite the priority number 4, the sequence is 1,2,3 in the anticlockwise or 5 direction. The highest priority carbon in the cyclopentadienyl ligand, indicated by an asterisk, is designated with the R chirality symbol by application of the extended CIP sequence rule. 

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. 

While sluggish under thermal conditions,274-275 the asymmetric conjugate addition of amines to alkyl crotonates is achieved at room temperature under high pressure (15 kbar).276 Thus, benzylamine can be added to the crotonate derived from 8-p-naphthyl menthol, with virtually complete diastereoselectivity. A related intramolecular 1,4-addition of an amine to a chiral enoate was used in a total synthesis of the alkaloid (-)-tylophorine.277 Additions of amines to chiral iron complexes of type (116) proceed with excellent selectivity and allow the preparation of homochiral p-lactams.l27128,l3() l32 In contrast, the addition of amine nucleophiles to chiral vinylic sulfoxides278-2811 and to chiral vinylsulfoximines281 proceeds with comparably low selectivities. 

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). 

P. Vogel s group studied exhaustively the 5,6,7,8-tetramethylidenebicyclo[2.2.2]octane system and its metal carbonyl complexes. The preparation and CD spectra of tricarbonyl-iron complexes (144-147) were reported333. The chirality of complexes 144 and 146 is due uniquely to the coordination of Fe(CO)3 moieties. The signs of the Cotton effects for (+)-144 and (+)-146 obey the octant rule, as the endo-Ft(CO)j, of 144 and 146 fall in a positive octant, while the second exo-Fe(CO)3 (syn to the carbonyl) lies almost on the XY nodal plane, and thus its contribution is expected to be small. The deuterium-substituted free tetraenone 148, however, showed an anti-octant behavior. The CD spectra of 144 and 146 are strongly temperature and solvent dependent. 

Scheme 4.26 Asymmetric hydrosilylation with chiral iron complexes.

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