Weak chelator

The fact that with acetal 1 (R1 = H R2 = CH3) a lower stereoselectivity is observed than with the acetals where R1 = Ft or C6II5 suggests that the bulkiness of the substituent at the acetal center also plays an important role in fixing the conformation of the transition state. With 1 bearing a hydrogen atom at the acetal center (R1 = II), the acetyl group is allowed to occupy the quasiequatorial position (3B) and the addition reaction therefore proceeds with no or only a weak chelation control. The same presumably holds for the elyoxal monoacetal 1 (Ri = R2 = M). 

For a-benzyloxycyclohexaneacelaldehyde and 2-butenylstannanes, good chelation control was observed using zinc iodide and titanium(IV) chloride, but only weak synjanti selectivity. Better syn/anti selectivity was found using boron trifluoride-diethyl ether complex, but weak chelation control. Magnesium bromide gave excellent chelation control and acceptable syn/anli selectivity90. 

Polymers may act as weak chelating agents and thus retain hardness salts in solution in BW. In this event the hardness as determined by EDTA titration is measurable, particular attention should be given to maintaining the recommended level of carbonate or phosphate. In appropriate circumstances, chelating agents may be used in place of phosphate. 

Thus, if zinc 2-ethoxybenzoate is a weak chelate it will be preferentially extracted and the reaction will move to the left. Eventually the matrix will contain only eugenol, as, indeed, Brauer (1972) found. 

Similar to their spontaneous 1,4-hydration in water, heterocycles 49a, 50a-e, and 50g all react simply upon dissolution in warm methanol to form his-addition products. The X-ray crystal structure of the bis-methanol adduct (51) derived from the diazaborine 50b clearly showed it to be a zwitterion comprised of tetrahedral borate anion and formamidinium cation fragments. Once again, 50f proved to be the only exception A weakly chelated dimethylborate ester (not shown) was found to be its structure in MeOH by B NMR. 

The molecular complexation constant expresses the strength of the bond. A weak chelation results in an unsteady and labile complex. A high constant results in a very stable compound that will not tend to dissociate. 

Metal Ion Effect Complexation of PMPS with Zn2+ ion could be observed by the decrease of absorption intensity of the pyridine ring (Figure 20b). A similar decrease was observed for the parent pyridyl-methylphosphonate (Figure 20b). Therefore, the 1 1 complex in which a metal ion is bridged between pyridyl nitrogen and phosphoryl oxyan-ion likely is involved (Figure 16a). Meanwhile, no detectable change of spectrum was observed in the case of Mg2+ ion, in accordance with very weak chelation of Mg2+ ion with a pyridine base. 

The interaction of an N-donor ligand with a metal ion also covers a wide range, from the very weak chelate or macrocycle-assisted association of an alkali metal cation to the very robust Co and Ru metal-nitrogen bonds. In aqueous solution, this interaction (equation 2) is essentially a substitution... 

Ku, Y. Peters, R.W. "The Effect of Weak Chelating Agents on the Removal of Heavy Metals by Precipitation Processes," Environ. Prog., 19S6,5, pp 147-153. 

In virtually all cases, the driving force for metal-directed reactions is the stabilization associated with the formation of a chelating ligand from monodentate ligands, or the conversion of a weakly chelating ligand to a stronger chelator. Often, the major product in the presence of the metal ion is not even detected from the same reaction in the absence of the metal ion. The metal has either caused an extreme displacement of an equilibrium, or promoted a new and rapid reaction pathway by complexation and stabilization of an otherwise inaccessible transition state. 

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