Esters, coordinated

In contrast to diphenylacetylene, the activated alkyne, ethyne dicarboxylic methyl ester, coordinates to OsHCl(CO)(P Pr3)2 tram to the hydride. Then rearrangement to the cw-isomer takes place, followed by insertion to yield the... 

Dimerisation and ester coordination restricts the number of sites available for alkoxides to two only, while maintaining a comparatively Lewis-acidic titanium centre, as needed for the reaction. In the dimer, the methine protons, alkoxide groups, and ester groups are inequivalent, but they show a rapid exchange on the H NMR timescale at room temperature, as the AG for the process is only 64 kl.mol. This process is much faster than the catalytic reaction, but due to the C2-symmetry of the tartaric esters the resulting structures of the dimers are the same. 

Hydrolysis of peptides,84 amides,85 phosphate esters,86 sulfonate esters87 and acetals88 can also be metal catalyzed. The hydrolysis of a phosphate ester coordinated to cobalt(III) also occurs at an increased rate (Scheme 19).89 A rather similar reaction occurs in the amine exchange of coordinated dithiocarbamates (equation 21).90 The conversion of imidates to amidines has been mentioned previously and is a similar type of reaction (see Section 7.4.2.2.1). 

The insertion of carbon dioxide into a transition metal-oxygen bond, e.g., a metal alkoxide, results in an organic carbonate ester, coordinated in either a monodentate or bidentate manner. Only a limited number of such reactions have been observed, and little mechanistic information is available. The reactions may proceed by interaction of C02 with ROH or RO in solution followed by metal coordination, in a manner similar to the C02 reactions with the early transition metal dialkylamides. Alternatively, direct attack of C02 on the alkoxide oxygen might occur, or a C02 adduct may form as an intermediate. 

As pointed out earlier, propargylic esters coordinate to gold to form complexes that can undergo 1,2- or 1,3-acyl migrations to form a-acyloxy-o ,jS-unsaturated carbenes or allene-gold complexes (equation 23). ... 

Figure 1. Crystal structures for esters coordinating to TiCl4.

Figure 2. Crystal structures for aldehydes, ketones, and thiol esters coordinating to TiCU.

The synthesis and characterization of the neutral homoenolate mono-Gp CpTiC CF CF COOEt) and cationic bis-Cp titanium [Cp2TiCFl2CF[2COOEt]+Znl3 derivatives have been reported. The molecular structure of the cationic complex shows ester coordination to give a five-membered metallacycle (Scheme 499).53... 

Figure 17-10 An alternate mechanism for substrate binding to the dinuclear metal center, in which the phosphate ester coordinates to both Fe and Fe ions. Subsequent nucleophilic attack ensues from a /i-hydroxo ligand.

The aminolysis of esters catalyzed by complex 8 is possibly initiated by N-H activation of the amine by metal-ligand cooperation involving 8. Ester coordination followed by intramolecular nucleophilic attack by the amido ligand at the acyl functionality is thought to be a key step [15]. Overall, in one catalytic cycle, two... 

SO that cyclic, chelated products are formed. Carbomethoxycobalt tetra-carbonyl, for example, prepared from ethyl bromoacetate and sodium cobalt tetracarbonyl, appears to exist, under 1 atm of carbon monoxide at 0°C, largely as an ester-coordinated cyclic complex (7). 

Industrially this process is catalysed with tin(ii) octanoate and there is a desire to move away from tin and use less-toxic metals or even metal-free conditions. The polymer itself is used in a diverse range of applications from commodity plastics (packaging) to high-end biological uses (tissue engineering, sutures and stents). " In the mechanism initially the ester coordinates to the Lewis-acidic metal centre, the alkoxide then attacks the carbonyl carbon and the lactide ring opens and new alkoxide is formed and... 

Solid-state NMR spectroscopy was utilised to characterize and identify the metal-ester coordination in active fourth generation Ziegler-Natta catalysts. C CSA studies of the donor metal ligand were inestigated. 

Scheme 7.2 Ester coordination observed in the B3LYP/LANL2DZ optimized structure of an ester carbene formed from a 1-substituted cyclobutene ester [35].

A range of substituents on the diene and the dienophile are tolerated and reactions proceed in high yields with high enantioselectivities. The authors have proposed a square-planar model (275) with acyl phosphonate or acyl ester coordinated to the Cu(II)/t-Bu-BOX (13) complex to account for the observed stereoselectivity. Stavenger and Schreiber also applied this approach to enan-tioselective hetero Diels-Alder reactions in a diversity-oriented synthesis of a library of dihydropyrancarboxamides [89]. Jorgensen and coworkers also reported Cu(II)/bisoxazoline-catalyzed hetero Diels-Alder reactions employing acyl esters... 

H-Bonding and Phosphate Ester Coordination to a Metal Center... 

Fig. 8.17 Mononuclear zinc complexes used to evaluate the influence of H-bonding on phosphate ester coordination to a zinc cation [111].

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