Ligands aminoalcohol

It appeared that the effieieney of the catalyst depends on the nature and the proportion of the crosslinking agent (eatalyst presenting 70% of EGDMA was more efficient than this with 30% of EGDMA and for DVB a decrease in conversion and ee was observed). It depended also on the specifie surfaee, the nature of the ligand (aminoalcohol or aminothiol) and on the... 

Products from aminoalcohols and TYZOR TPT were obtained by a2eotropiag the isopropyl alcohol with ben2ene (121,122). From trimethylethylenediamine, dimethylethanolamine, and dimethylisopropanolamine with TYZOR TPT, the orange (11), the yellow (12), and the pale-green (13) were obtained, respectively. The lithium salt of the ligand, derived from C H Li, combiaed with (RO)2TiCl ia hexane has also been used (123). 

Polynuclear complexes with aminoalcohols and iminoalcohols as ligands O-bridged and hydrogen bonded species. J. A. Bertrand and P. G. Eller, Prog. Inorg. Chem., 1976, 21, 29-53 (54). 

In 2004, ruthenium-catalysed asymmetric cyclopropanations of styrene derivatives with diazoesters were also performed by Masson et al., using chiral 2,6-bis(thiazolines)pyridines. These ligands were prepared from dithioesters and commercially available enantiopure 2-aminoalcohols. When the cyclopropanation of styrene with diazoethylacetate was performed with these ligands in the presence of ruthenium, enantioselectivities of up to 85% ee were obtained (Scheme 6.6). The scope of this methodology was extended to various styrene derivatives and to isopropyl diazomethylphosphonate with good yields and enantioselectivities. The comparative evaluation of enantiocontrol for cyclopropanation of styrene with chiral ruthenium-bis(oxazolines), Ru-Pybox, and chiral ruthenium-bis(thiazolines), Ru-thia-Pybox, have shown many similarities with, in some cases, good enantiomeric excesses. The modification... 

The enantioselectivity is the result of chelation of the chiral ligand to the zinc. The TS of the addition is believed to involve two zinc atoms. One zinc functions as a Lewis acid by coordination at the carbonyl oxygen and the other is the source of the nucleophilic carbon. The proposed TS for aminoalcohol A, for example, is shown below.149... 

The structures of the TSs have been explored computationally using combined B3LYP-MM methods.150 There are four stereochemically distinct TSs, as shown in Figure 7.4. For the aminoalcohol ligands, the anti-trans arrangement is preferred. Steric factors destabilize the other TSs. The substituents on the ligand determine the facial selectivity of the aldehydes. 

Fig. 7.4. Tricyclic transition structures for aminoalcohol catalysts syn and anti refer to the relationship between the transferring group and the bidentate ligand cis and trans refer to the relationship between the aldehyde substituent and the coordinating zinc. Reproduced from J. Am. Chem. Soc., 125, 5130 (2003), by permission of the American Chemical Society.

Synthetic approaches to silica-immobilized aminoalcohol ligands are reported in Scheme 58.310... 

In this work we have studied the preparation of electrocatalysts on the graphite matrix using tri-nuclear complexes of 3d-metals with aminoalcohol ligands. Tri-nuclear complexes, 2[Co(Etm)3] Me(N03)2, where Etm = ethanolamine, Me = Zn2+, Cu2+, Ni2+, Co2+, were investigated. 

The most successful ligands are unsymmetrical chiral diamines or aminoalcohols, perhaps because they influence the configuration of the ligated metal chir-... 

Fig. 35.2 S ome examples of 1,2-aminoalcohol and 1,2-dia-mine ligands and a readily prepared library of N-sulfonyldia-...

Since analogous ketoesters not containing halide could be hydrogenated in good yield, the acidity of the enolate seems to be the main reason for the replacement of the aminoalcohol ligand by the halogenated acetoacetate. Indeed,... 

Bertrand, J. A. and Eller, P. G., Polynuclear Complexes with Aminoalcohols and Iminoalcohols as Ligands Oxygen-Bridged and Hydrogen-Bonded Species 21 29... 

A number of groups have reported the preparation and in situ application of several types of dendrimers with chiral auxiliaries at their periphery in asymmetric catalysis. These chiral dendrimer ligands can be subdivided into three different classes based on the specific position of the chiral auxiliary in the dendrimer structure. The chiral positions may be located at, (1) the periphery, (2) the dendritic core (in the case of a dendron), or (3) throughout the structure. An example of the first class was reported by Meijer et al. [22] who prepared different generations of polypropylene imine) dendrimers which were substituted at the periphery of the dendrimer with chiral aminoalcohols. These surface functionalities act as chiral ligand sites from which chiral alkylzinc aminoalcoholate catalysts can be generated in situ at the dendrimer periphery. These dendrimer systems were tested as catalyst precursors in the catalytic 1,2-addition of diethylzinc to benzaldehyde (see e.g. 13, Scheme 14). 

Since 1980 the interest in this reaction increased because enantiospecificity was introduced and much more valuable products could be made. A wide variety of ligands was tested, such as chiral dipyridines, phenanthrolines, diphosphines, aminoalcohols, bis-oxazolines, bis-oxazolines with a third donor atom in the centre, bis-thioureas, diamines, etc [33], In 1981 the highest e.e. reported was still only 20%. For many years the best results were obtained with chiral diimines and phenanthrolines but e.e. s were below 70% [34], Pfaltz introduced bis-oxazolines for this reaction and obtained e.e. s as high as 91% [35] in 1991. 

As bidentates seemed to give more stable complexes, and because trans configurations seemed to be favoured by some of the monodentates, Thomas and Suss-Fink investigated the use of ligands that are bidentate and can coordinate in a trans fashion [11,16], This has led to the development of new diphosphines based on condensation reactions of 2-diphenylphos-phinobenzoic acid with aminoalcohols or diols, reminiscent of the wide bite angle ligands used by Trost for the asymmetric allylic alkylation (Chapter 13.2). The best... 

In 1996, Enders and coworkers reported the asymmetric epoxidation of ( )-enones 91 in the presence of stoichiometric amounts of diethylzinc and (lR,2R)-A-methylpseudo-ephedrine (120) under an oxygen atmosphere to give fraw -epoxides 92 with excellent yields (94-99%), almost complete diastereoselectivity (>98% de) and with very good enantioselectivities (61-92%) (Scheme 54) . For the same reaction Pu and coworkers utilized achiral polybinaphthyl 121 as ligand (in excess) instead of the chiral aminoalcohol. For each substrate, only one diastereomer was formed, but in most cases yields were lower than observed with the Enders system. Enders catalyst shows high asymmetric induction for alkyl-substituted enones (ee 82-92%), but for substrates bearing only aromatic substituents only modest enantioselectivity was obtained (R = R = Ph ... 

Because vicinal diamines and 2-aminoalcohols are important components of natural products and medicinal agents, and used as ligands for metal-catalyzed reactions, especially in asymmetric synthesis, efficient methods for the compounds have been extensively investigated over the past decade. " ... 

The chiral ligand (44) was prepared starting from the cyclic a-amino acid (S)-proline80). Recently, similar chiral catalysts and related molybdenum complexes involving optically active N-alkyl-P-aminoalcohols as stable chiral ligands and acetylacetone as a replaceable bidentate ligand, were designed for the epoxidation of allylic alcohols with alkyl hydroperoxides which could be catalyzed by such metal complexes 8,). 

The most promising results are offered by trifluoromethyl aminoalcohols as chiral ligands (entry 10). Cinchona alkaloids in the presence of pyridine (entry 7) and cinchona-derived surfactants (entry 6), which provide an asymmetric micellar microenvironment in aqueous solvents, are also worthy of note. 

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