Esters polydentate

Titanium chelates are formed from tetraalkyl titanates or haUdes and bi- or polydentate ligands. One of the functional groups is usually alcohoHc or enoHc hydroxyl, which interchanges with an alkoxy group, RO, on titanium to Hberate ROH. If the second function is hydroxyl or carboxyl, it may react similarly. Diols and polyols, a-hydroxycarboxyflc acids and oxaUc acid are all examples of this type. P-Keto esters, P-diketones, and alkanolamines are also excellent chelating ligands for titanium. 

Angelici and his coworkers have overcome some of the practical problems arising from the low formation constants of simple monoamino esters, by incorporating the ester moiety into a larger polydentate ligand. A typical example is the use of ethyl glycinate-N,N-diacetic acid (12) which hydrolyzes to give nitrilotriacetic acid (13). 

The Schiff-base complex 17 of ruthenium was developed by Scott et al. [51] and shows substantially high efficiency for ACP. The cyclopropanes derived from 4-nitrostyrene and EDA were obtained in 92% yield with a 99 1 trans-to-cis ratio and 98% ee for the trans form [51]. In most cases, in order to attain high trans-cis stereoselectivity, bulky ester groups of diazoesters were effective. Nevertheless, Nguyen et al. [52] reported in 2002 that the reaction of the smaller and common EDA with styrene assisted by Ru-salen-pyridine complexes 18 (1 mol %) at room temperature produced the cyclopropane products in high yield, 90-96%, and 98-99% ee for the trans form and 95-96% ee for the cis form. Zhang et al. [53] reported that a N,0-mixed polydentate ligand pro-... 

A combination of halide-free Pd salts in combination with polydentate phosphines causes the concomitant synthesis of pentenoate and nonadienoate esters, although conversions are low . Under the same conditions as used for the nonadienoate synthesis, except for substitution of bis( 1,2-diphenylphosphino)ethane(diphos) for the monodentate phosphine and isoquinoline for quinoline, the yield of C9 ester is 14 mole % and the C5 ester 12%. 

The polydentate trimethylamine theme has been extended to benzimidazole, benzothiazole, oxazoline, and pyridine substituents [101,102]. Several have provided significantly faster catalysis when quantitative rates are measured, particularly the pendant ester and water-soluble acid derivatives of the tris (benzimidazole) motif, 13a,b. 

More recently, a study by Lassaletta, Fernandez and co-workers showed that enantioselective NHC-Ag catalysis was not an impossible goal. Hence, the use of polydentate thiolato-NHCs 11-14 afforded good enantioselectivities in the 1,3-dipolar cycloaddition of azomethine ylides with acrylates (Scheme 11.4). ° Ligand 11 systematically produced the highest ee s (up to 80% ee) with a number of imino esters. The authors proposed that a tridentate SCS coordination of these C2-symmetric ligands onto the silver center was key for obtaining the good chiral induction observed in the cycloaddition. 

The dinuclear complex generated from Et2Zn and the polydentate ligand (301) has been shown to catalyse the Michael addition of A-substituted 3-hydroxyoxindoles to a,j0-unsaturated esters RCH=CHCOPh. Subsequent intramolecular transesterification provided the corresponding spirocyclic 5-lactones (<13 1 dr, <99% ee) ... 

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