Ligand enediolate

It was found that nanosize Ti02 particles experience an adjustment in the coordination geometry of the Ti atoms near the particle surface from octahedral to square-pyramidal in order to accommodate the large surface curvature [57]. X-ray absorption near edge structure reveals that surface modification with enediol ligands (ascorbate, ortho-hydroxy cyclobutene dione, catechol, etc.) restores the pre-edge features of octahedrally coordinated Ti in the anatase crystal environment. Specific binding of the enediol modifiers to surface comer defects ... 

A more reactive cationic dimethyl tantalum derivative is produced from Ta(OEP)Me3 using one equivalent of the weak acid HNMe2Ph BPh4]. I Ta(OEP)Me2] reacts cleanly with CO to produce a cationic enediolate compound, containing the MeC(0 )==C(0 )Me ligand which results from both insertion and... 

Other studies have provided additional data on the relative stabilities of the lithium aldolates 14 and 15 derived from the condensation of dilithium enediolates 13 (Rj = alkyl, aryl) with representative aldehydes (eq. [ 10]) (16). Kinetic aldol ratios were also obtained for comparison in this and related studies (16,17). As summarized in Table 4, the diastereomeric aldol chelates 14a and ISa, derived from the enolate of phenylacetic acid 13 (R = Ph), reach equilibrium after 3 days at 25° C (entries A-D). The percentage of threo diastere-omer 15 increases with the increasing steric bulk of the aldehyde ligand R3 as expected. It is noteworthy that the diastereomeric aldol chelates 14a and 15a (Rj = CH3, C2HS, i-C3H7) do not equilibrate at room temperature over the 3 day period (16). In a related study directed at delineating the stereochemical control elements of the Reformatsky reaction, Kurtev examined the equilibration of both... 

DFT calculations indicate that an incoming CO2 displaces a water ligand at magnesium. Coordination of one of the oxygen atoms in CO2 to Mg " bends, and thus polarizes, the central carbon atom which becomes sufficiently electrophilic to attack the Si face (observed experimentally ) of the enediolate C2 atom, which leads to the formation... 

Figure 9-2. The co-ordination of copper(n) to ascorbic acid (9.1) generates a complex of the ligand in its enediolate form.

Since tropolones and 3-hydroxy-4-pyrones are taken in this chapter to be enols, we now cite their binding as enolato ligands to lanthanum and all the other trivalent lanthanides (save the radioactive promethium) " " . Likewise, we note such studies for complexes with enolato ligands derived from 3-acetyl-4-hydroxycoumarin, dehydroacetic acid and their oximes, and with the aromatic enediolates, squarate and croconate. Periodic trends in thermodynamic parameters were reported and analyzed in these studies. 

Another potential side reaction of the enediol(ate) intermediate is formation of the dicarbonyl compound, l-deoxy-D-glycero-2,3-pentodiulose 5-phosphate, resulting from p-elimination of the Cl-phosphate due to improper stabilization and/or premature dissociation of enediol(ate) from the enzyme active site. This compound has been characterized by reduction with borohydride, oxidation with H2O2, complexation with o-phenylenediamine, and 13C-NMR (23, 34). The p-elimination product is not detected in reactions with wild-type R. rubrum Rubisco but is formed in substantial amounts with mutants in which the Cl-phosphate ligands are substituted, demonstrating the required role of these amino acid side chains in stabilizing the enediol(ate) intermediate (34-35). 

Rather, the reactions will be interpreted as reverse aldol and then aldol reactions within the coordination shell of a metal, involving the transient existence of a metal complex of a cw-enediol and a hydroxyaldehyde (Figure 6.2). The isomerisation occurs when the carbonyl moves over the face of the enediol and recombines with the other carbon. The carbonyl group is a poor ligand and can thus break free of the complex simple rotation about C3-C4 while still attached to the metal will yield side-products epimeiised at C3. If the hydroxyaldehyde it is not held to the metal by other hydroxyls, particularly 04, it can escape completely. 

Excitation of the ligand field transitions of octacyanomolybdate(IV) by visible light leads to the photosubstitution of up to four cyanide anions (12) which effectively catalyze the dimerization of heterocyclic carb-2-aldehydes to the corresponding enediols (13) (<]>s/< )c > 10 ). Heterocyclic enediols have quite different properties compared with the substrate and can be used for unconventional image recording processes [24]. hv, H2O... 

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