Chiral ligands ferrocenes

Asymmetric catalysis using chiral ligands, including cyclic phosphine or pyra-zole fragments covalent-bonded with ferrocene system 98PAC1477. 

The same group of workers has prepared ferrocene derivatives 39 as chiral ligands for asymmetric catalysis. Again, cyclic sulfates of optically pure 1,3-diols were used as reagents in the preparation <99SL1975>. 

Figure 4.19. Ferrocene derived Cj asynunetric chiral ligands...

Despite the resolution which was required to produce the enantiomerically pure starting materials (which fortunately is highly efficient—recrystalhzation of the mother liquors allows isolation of both enantiomers) , Ugi s lithiation provided the basis for a rapid growth in the use of enantiomerically pure, planar chiral ferrocenes which has continued since. Several reviews have covered applications of enantiomerically pure ferrocenes as chiral ligands, which until the 1990 s were all made using Ugi s method... 

A new family of chiral ligands for asymmetric homogeneous hydrogenation has been developed. The performance of mono- and bis-rhodium complexes of these chiral ferrocene tetraphosphine ligands in the hydrogenation of model substrates was surveyed in comparison to their ferrocene bis-phosphine analogs. 

Aminocarbonylation has also been applied to the synthesis of unsymmetrical ferrocene-1,1 -bis-carboxamides. Ferrocene-based chiral ligands are very useful in asymmetric catalysis, and enantiomerically pure ferrocenyl ligands can be obtained by optical resolution of unsymmetrically substituted ferrocenes. However, the synthesis of such unsymmetrical ferrocenes is not an easy task. The use of aminocarbonylation gave a solution to this challenge. For example, the Pd-catalyzed reaction of symmetrical ferrocenyl diiodide 144 with two different amines, morpholine and diethylamine (5 equiv. each) under 39.5 atm of CO, gave the desired unsymmetrically disubstituted ferrocene-biscarboxamide 145 in 85% yield (Equation (11)). ... 

In recent years we have developed a new type of planar-chiral ligand system which is based on a phosphaferrocene skeleton equipped with an additional donor function Y [8]. This structure is closely related to the well-known ferrocene-type ligands in that a CH unit of the latter has been replaced by a P atom. The phosphaferrocene moiety serves as both a chiral metallocene-type backbone and as a donor group via the phosphorus atom lone pair. These new ligands are unique in their topological architecture and show interesting ligand properties. 

With other electrophiles, ferrocenes 12 and 13 could be obtained, bearing a selenium group [19] or a silanol moiety [20], respectively, in the ortho position. Those compounds proved to be catalytically active as well, and in particular 13 was of interest, since - to the best of our knowledge - it was the first silanol ever used as a chiral ligand in asymmetric catalysis. Details of this study will be discussed below. 

FIGURE 15.2 Starting compound and structures of the first ferrocene-based chiral ligands. 

The first example of a C C bond-forming reaction catalysed by gold was the asymmetric aldol condensation developed in 1986.30 The addition of an isocyano acetate to an aldehyde produces the A -oxazole as the major and Z-oxazole as the minor product in excellent enantiomeric excess (ee) in the presence of a cationic gold catalyst, [Au(CyNC)2]BF4, and a chiral diphosphanyl ferrocene ligand (see Scheme 12.5). 

Chiral moieties have been tethered onto MCM-41 surface in order to obtain optically active supported catalysts possessing the intrinsic texture brought by this particular support.2 >23 24 26 41-43 Thomas et al26,41 have developed a very interesting methodology to anchor a chiral ligand l,r-bis(diphenylphosphino)ferrocene to the inner walls of MCM-41 silica (Figure 1). 

Togni s synthetic route to a planar chiral (trimethylsilyl group on ferrocene) and central chiral (asymmetric carbon in the NHC-Cp alkyl linker) carbene ligand starts from a central chiral aminomethyl ferrocene (see Figure 5.29) [9]. Lithiation and subsequent reaction with trimethylsilyl chloride introduces planar chirality on ferrocene. Quartemisation of the dimethylamino group with methyl iodide enables reaction with imidazole to the double Fc substituted imidazolium salt which can then be deprotonated to the free carbene with potassium rerf-butylate. 

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. 

As chiral ligands for transition metal complex-catalyzed asymmetric reactions, a variety of novel chiral ferrocenylchalcogen compounds, which possess a planar chirality due to the 1,2-unsymmetrically disubstituted ferrocene structure, have been prepared from chiral ferrocenes (Scheme 1). Thus, chiral diferrocenyl dichalcogenides bearing an optically active dimethylaminoethyl or p-tolyl-sulfoxide moiety 1-10 were prepared by lithiation of the corresponding chiral ferrocenes, highly diastereoselectively, in moderate to high chemical yields. 

Modifications. A diverse array of chiral ligands related to [(R)-a-(2-naphthyl)aminomethyl]ferrocene was screened (see those mentioned in eq 1).4 It was found that when the 2-naphthyl group was replaced with the 3,5-di-rert-butyl phenyl group, the ee s improved from 82% to 92% in the case of cinnamyl chloride when Zn(neo-pentyl)2 was used. It is equally significant to note that the temperature could be elevated to —30°C (eq 4).4... 

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