Ligands hydroxyl groups

We have disclosed that the ligands 4c, 10, and 77, when complexed with a metal ion such as Zn2 +, Ni2+, or Co2+, become highly active toward the hydrolysis of p-nitrophenyl picolinate (7). The catalysis is most likely to occur through formation of a ternary complex in the transition state or in reactive intermediates. The metal ion in such a complex serves to activate the ligand hydroxyl group for nucleophilic attack and to orient the substrate into a favorable position to undergo the reaction. 

Rg. 1.24 Key interactions between the active enantiomer BMS270394 and RAR-y. A key interaction involves a hydrogen bond between the ligand hydroxyl group and the sulfur of Met-272. Notice that the linking amide group makes complementary interactions with the enzyme. 

Figure 27 Two-step deprotonaiions of axial ligand hydroxyl groups of [SbTPP(OH)2]Br in the excited triplet state [792],...

Thus in Fig. 5.22 the first outgassing at 25°C will have removed physisorbed water only, so that curve (1) is the isotherm of physical adsorption on the fully hydroxylated material. The 300°C outgassing, on the other hand, will have removed all the ligand water and the majority of the hydroxyl groups when isotherm (4) is determined, therefore, the Ti ions will chemisorb ligand water at low relative pressure, but the number of hydroxyl groups reformed will be very small. 

A hydroxyl group is situated ortho to a carboxyl group which as a bidentate ligand is terminally metallized on the fiber when aftertreated with dichromate. An example is Alizarine Yellow GG [584-42-9] (50) (Cl Mordant Yellow 1 Cl 14025). Cr(III) has a coordination number of six, and therefore normally two dye molecules of the sahcyhc type are chelated to the metal ion. 

The numerous surface hydroxyl groups provide attachment points for other functional groups and ligands. 

Nowadays, almost all commercially available HPLC stationary phases are also applicable to planar chromatography. In addition to the polar hydroxyl groups present on the surface of native silica, other polar functional groups attached to the silica skeleton can also enter into adsorptive interactions with suitable sample molecules (34). Silica with hydrophilic polar ligands, such as amino, cyano, and diol functions, attached to the silica skeleton by alkyl chains, all of which have been well proven in HPLC, have also been developed for TLC (34). 

Figure 1 shows the pH-rate profiles of some active complexes. Both Ni2 + and Zn2 + ion complexes of 8 afford saturation curves with inflection at around pH s 6 and 8, respectively, which represent, most likely, the ionization of the hydroxyl group complexed with a Ni2+ or a Zn2+ ion. The pKa = 8.6 was assigned for the ionization of the hydroxyl group of the latter complex 12). The lower pH for the ionization of the Ni2+ ion complex in respect to that of the Zn2+ ion complex indicates that the ligand 8 coordinates to Ni2+ ion more tightly than to Zn2+ ion, which is in conformity with a larger K value (1120 M) for the Ni2 + ion than for the Zn2 + ion complex (559 M) at pH 7.05 (Table 2). 

In the foregoing micellar reactions, it is likely that the reaction proceeds through the acylation of the hydroxyl group of the ligands, and the results indicate that the acylation step is greatly enhanced by complexation with Zn2 + ions under micellar... 

The ratios of these slopes for L- and D-esters are shown in Table 12. The kL/kD values of the acylation step in the CTAB micelle are very close to those in Table 9, as they should be. It is interesting to note that the second deacylation step also occurs enantioselectively. Presumably it is due to the deacylation ocurring by the attack of a zinc ion-coordinated hydroxide ion which, in principle, should be enantioselective as in the hydroxyl group of the ligand. Alternatively, the enantioselectivity is also expected when the free hydroxide ion attack the coordinated carbonyl groups of the acyl-intermediate with the zinc ion. At any rate, the rates of both steps of acylation and deacylation for the L-esters are larger than those for the D-esters in the CTAB micelle. However, in the Triton X-100 micelle, the deacylation step for the D-esters become faster than for the L-esters. 

The study on micellar models is still at the beginning. An amphiphilic ligand which can form micelles by itself has not yet been prepared. It is necessary to obtain complexes of higher stability in order to activate the hydroxyl group strong enough in the reactions of inactive esters or amides. Enantioselectivity must reach higher specifity. Nevertheless it seems to be clear that many features or some important clues have already been disclosed for further refinements of this micellar systems. More details about the present micellar reactions will be reported elsewhere in near future. 

Table IV presents the results of the determination of polyethylene radioactivity after the decomposition of the active bonds in one-component catalysts by methanol, labeled in different positions. In the case of TiCU (169) and the catalyst Cr -CjHsU/SiCU (8, 140) in the initial state the insertion of tritium of the alcohol hydroxyl group into the polymer corresponds to the expected polarization of the metal-carbon bond determined by the difference in electronegativity of these elements. The decomposition of active bonds in this case seems to follow the scheme (25) (see Section V). But in the case of the chromium oxide catalyst and the catalyst obtained by hydrogen reduction of the supported chromium ir-allyl complexes (ir-allyl ligands being removed from the active center) (140) C14 of the...

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