Ester complexes

Reaction with Lactones. Hydroxycarboxyhc acid ester complexes of titanium are formed by reaction of a tetraalkyl titanate with a lactone, such as P-propiolactone, y-butyrolactone, or valerolactone (35). For example. 

Trichlorotitanium monoacylates form, dufing thermal decomposition of TiCl, ester complexes (128) ... 

The first approach applied for [cinchonidine (CD) - a-keto ester] complex was also unsuccessful. In the open conformation CD cannot provide the required steric shielding. In open form either the quinuclidine or the quinoline moiety of CD will interact with the substrate. It has already been demonstrated that the quinuclidine moiety has a crucial role both in the rate acceleration and the induction of ED [13]. 

In earlier kinetic and computer modeling [1,2, 14] the open form of CD (CDopen) was used to illustrate the adsorbed [CD - a-keto ester] complex. In this complex the quinuclidine nitrogen was involved in the interaction with the substrate directly or via a proton bridge. 

We have modelled the [CDopen - methyl pyruvate] complex. The result is shown in Figure 2. In this complex there is no steric hindrance to prevent the free rotation of the substrate around the quinuclidine nitrogen. Thus, in complex shown in Figure 2. there is no preferential stabilization of the substrate. In earlier computer modeling it was suggested that Pt is involved in the stabilization of the [CDopew-a-lfeto ester] complex, i.e. the Pt surface prevent the free rotation of the substrate, however the driving force for enantio-differentiation, i.e. for preferential adsorption of the substrate, was not discussed [14]. 

Variety of a-keto esters, such as methyl and ethyl pyruvate, methyl mandalate, dihydro-4,4 - dimethyl-2,3 fiiranedione were used to calculate the shielded form of [CDdosed - a-keto ester] complexes leading to the formation of ( R) or (S) product, respectively. The details of these results will be a subject of a subsequent paper [17]. As emerges from these calculations the favourable directionality is maintained in complexes (R), even for dihydro-4,4 - dimethyl-2,3 fiiranedione. 

The amino acid complexes [TcNCl(L)(PPh3)] (HL=L-cysteine, L-cysteine ethyl ester, cysteamine) have been prepared from [TcNC PPlfj ] or ASPI14 [TcNCU]/PPh3 [88]. The crystal structure of the L-cysteine ethyl ester complex 21 shows a Tc=N bond length of 1.605(3) A and the Tc atom displaced by 0.594(1) A above the square basal plane [88]. Other structurally characterized examples are the square pyramidal 22 with ONSP coordination and Tc=N 1.611(3) A [81], and 23 with NSPC1 coordination and Tc=N 1.615(7) A [89],... 

Fig. 30. Pig liver esterase-catalyzed hydrolysis of anionic DMS-ester complexes...

This facilitates intramolecular hydride transfer resulting in a Ru-hydroxy ester complex (66) which readily releases the chiral product. When an (R)-BINAP-Ru catalyst is used, the R enantiomer is obtained in >99% ee. The chirality of the BINAP ligand accounts for the difference in energy between the possible transition states TS and TS. ... 

In the case of hydrogenation using [Ru(BINAP)Cl2]n as the catalyst precursor, the reaction seems to occur by a monohydride mechanism as shown in Scheme 6-31. On exposure to hydrogen, RuC12 loses chloride to form RuHCl species A, which in turn reversibly forms the keto ester complex B. Hydride transfer occurs in B from the Ru center to the coordinated ketone to form C. The reaction of D with hydrogen completes the catalytic cycle.67... 

The N,0-[Co(en)2(AA-AA OR)]3+ dipeptide ester complexes are formed rapidly (seconds to minutes) at room temperature on treating... 

Table VI lists a number of dipeptide ester complexes prepared via aminolysis in Me2SO and isolated using ion-exchange chromatography many others have been obtained from similar syntheses. Crystal structures are available for A-[Co(en)2((S)-Ala-CR)-Phe)]Br3 H20 (26), obtained from reaction of A-[Co(en)2((S)-AlaOMe]3+ with (i )-PheOMe and acid hydrolysis (Fig. 1), and for A-[Co(en)2((S)-Leu-(S)-Leu OMe)]Cl3 4H20 (24), A-[Co(en)2((S,fl)-Ala-(S)-ValOMe)](C104)3 (24), and /3-[Co(trien)(Gly-GlyOEt)](C104)3 H20 (10). These show considerable variation in chelate 0-Ci-C2-N dihedral angles (0-35°) (10) and it remains to be seen whether this property is important to epimerization (at C2) in these species.

Typical syntheses of Co(III)-amino acid, amino acid ester, and dipeptide ester chelates are described below. The NMR spectra of the isolated products were in accord with expectation. The procedures given here are generally applicable, except for that given for [Co(en)2((iS)-GluOBzl)]I2. If this method is used to coordinate amino acids that are only partially soluble in Me2SO, more forcing conditions (extended reaction times, 1-5 h, 50-60°C) may be required. Dipeptide ester complexes are not always as amenable as [Co(en)2 (Val-GlyOEt)]I3 to crystallization from water. 

As discussed in more detail elsewhere (Tee and Du, 1992), the transition state [27] probably arises from a 2 1 ternary complex (20). If such is the case, kc2 = k JK2 and if Af2 = 50mM, then k c kc, meaning that the 1 1 and 2 1 CD-ester complexes have virtually the same reactivities. On the other hand, if K2>50mM (as is quite likely) then k c> kc, and the 2 1 complexes would be more reactive. 

Fig. S.4. Ubcl/ubiquitin thiol ester complex model (1 FXT). The surface of Ubcl is shown with residues implicated in ubiquitin binding colored purple and the active-site cysteine colored yellow. Ubiquitin is colored green.

Complexation with caffeine and theophylline-7-acetate depresses the rate of alkaline hydrolysis of substituted phenyl benzoates and is consistent with the formation of molecular complexes with 1 1 stoichiometry between the hosts and esters stacking of the xanthines is excluded as an explanation in the range of concentrations studied. Inhibition of hydrolysis is attributed to repulsion of the hydroxide ion from the host-ester complex by the extra hydrophobicity engendered by the xanthine host, as well as by the weaker binding of the transition state to the host compared with that in the host-ester complex. ... 

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