Nucleophiles ferrocene derivative

Quite efficient nucleophilic catalysts with planar (21a-c) and axial (22a-d) chirality were recently developed by Fu et al. [17-22] and Spivey et al. [23-25], The ferrocene-derived catalysts developed by Fu (21a-c) were first tested in the kinetic resolution of aryl alkyl carbinols with diketene as the acyl donor. 

Chung et al. reported the enantioselective synthesis of chiral NHCs, such as 6, using a chiral ferrocene derivative (Scheme 8) [28]. The nucleophilic substitution of the hydroxy function by an imidazole in an acidic medium gives the imidazolium salt with retention of the configuration at the chiral C-atom. 

Reaction of (284) with an aldehyde, ketone, or enol ether in the presence of acid results in an electrophilic substitution that produces a -ferrocenylalkyl carbocations that may be trapped by nucleophiles (azides, amines, thiols). This chemistry may be used to prepare enantiomerically pure ferrocene derivatives in a maimer that avoids resolution procedures (Scheme 86)." For example, the enol ether from (-)-menthone affords a kinetic carbocation (302) that may be trapped or allowed to rearrange to the more thermodynamically stable cation (303) and then trapped, thus offering a means of controlling the configuration of the stereocenter adjacent to the ferrocene unit. Use of an enantiomerically pure aldehyde derived from Q -pinene (304) affords a 1 1 carbocationic mixture that similarly isomerizes to a single cation. 

Ugi has coined the term stereorelating synthesis for the sequence lithiation/reac-tion with electrophiles [62,118], and used this technique as a method for the chemical correlation of the structure and for the determination of the enantiomeric purity of many 1,2-disubstituted ferrocene derivatives obtained either by resolution or by asymmetric synthesis (for a compilation, see [118]). It is important to note that all stereochemical features discussed above for central chiral compounds, such as retentive nucleophilic substitution, remain valid when more substituents are present at the ferrocene ring and the conversion of functional groups in planar chiral ferrocenes can be achieved by the same methods as described. 

A mixture of ferrocenyl alcohol (1 0.5 mmol), nucleophile with activated hydrogen atom (2 0.5 mmol) and ceric ammonium nitrate (CAN 5 mol %) in acetonitrile (2 mL) was stirred at room temperature for an appropriate time (0.25-120 h for varying substrates). Upon completion, monitored by TLC, the solvent was evaporated under the reduced pressure. The crude product was purified by flash column chromatography with ethyl acetate and petroleum ether as eluents to afford ferrocene derivative 3 with good yield (52-99%). Each of the products was characterized by means of spectral studies including IR, H NMR, NMR and HRMS. 

The synthesis of polyphosphazenes derivatized with ferrocenyl units has also been achieved by functionalization of a preformed polyphosphazene. Wisian-Neilson and Ford showed that when polymethylphenylphosphazene (66) was treated with n-BuLi, approximately half of the methyl substituents on the polymer deprotonated. These nucleophilic groups could subsequently be reacted with ferrocene derivatives as shown in Scheme 14. Polymer 68 had approximately 45% of its methyl groups converted to the organometallic groups, whereas substitution occurred at only 36% of the methyl groups in polymer 70. The presence of the ferrocenyl groups resulted in increased glass transition temperatures for these polymers. For example, when 23% substitution occurred in 70, the was 65°C, whereas a 36% substitution level resulted in a T of 87°C. ... 

In a similar manner, certain bis(arene)iron(II) salts react easily with nucleophiles and may be used in organic synthesis. For example, (mesityIene)2Fe(PF6)2 reacts with phenyl, terf-butyl, and vinyllithium to give either the compound XLVIII or the pseudo-ferrocene derivative XLIX, according to the stoichiometry of the reaction (Helling and Braitsch, 1970). 

Metallocene chemistry is presently a very promising field of inorganic chemistry. E.g., metallocene complexes of titanium and zirconium play an important role in the production of polymers. The role of ferrocene derivatives as effective nucleophilic catalysts in kinetic resolutions of many useful... 

In 1999 Trost and Schroder reported on the first asymmetric allylic alkylation of nonstabilized ketone enolates of 2-substituted cyclohexanone derivatives, e.g. 2-methyl-1-tetralone (45), by using a catalytic amount of a chiral palladium complex formed from TT-allylpaUadium chloride dimer and the chiral cyclohexyldiamine derivative 47 (equation 14). The addition of tin chloride helped to soften the lithium enolate by transmetala-tion and a slight increase in enantioselectivity and yield for the alkylated product 46 was observed. Besides allyl acetate also linearly substituted or 1,3-dialkyl substituted allylic carbonates functioned well as electrophiles. A variety of cyclohexanones or cyclopen-tanones could be employed as nucleophiles with comparable results . Hon, Dai and coworkers reported comparable results for 45, using ferrocene-modified chiral ligands similar to 47. Their results were comparable to those obtained by Trost. 

---