Cations strong chelation

The opportunity for chelation in the various enolate intermediates offers a possible explanation for the observed diastereoselectivities. In the dianions derived from l-acyl-2-pyrrolidinemethanols strong chelation of both of the lithium cations should lead to a rigid enolate structure 9. It is reasonable to assume that the pyrrolidine ring is locked in one conformation. Since, according to models, it is difficult to attribute the observed high diastereoselectivity to steric hindrance, it is probable that the lone pair on the nitrogen directs the facial selectivity of electrophilic attack (see Section 1.1.1.3.3.1.) to one side of the enolate a-carbon. 

Plasma prepared with strong chelators such as ethylene diamine tetraacetic acid (EDTA) may act as peptidase inhibitors for metalloproteases and peptidases using divalent cations as cofactors (28). 

Regardless of the initial source of rhodium, that is, whether it is added just as a halide or as a phosphine complex, under the reaction conditions 4.1 is formed. With phosphine complexes, the phosphine is converted to a phospho-nium (PRj" or 11 PR, ) counter-cation. As chelating phosphines bind more strongly than the monodentate ones, the induction time for the formation of 4.1 with chelating phosphines is longer. 

The quaternary stractures of He s are strongly dependent on pH and the presence or absence of divalent cations such as Ca + or Mg +. Thus, most arthropodan He s can be dissociated to functional units by simply raising the pH above 9 with the concomitant removal of divalent cations by chelating agents (e.g. EDTA). This dissociation can be reversed by dropping the pH and/or addition of divalent cations (> 10 mM). A very similar behavior is also observed for He s from molluscs. He s from both phyla typically exhibit cooperative oxygen binding. The extent of cooperativity is dependent upon several parameters such as pH, temperature, and the presence of divalent cations such as Ca + and... 

We note a very interesting manifestation of the strong chelating agent-cation interaction, i.e. the ESR linewidths observed for Chel Na+C10H8" in the presence of excess Ci0H8. We consider two processes which can broaden the ESR lines, anion-neutral molecule electron transfer... 

The adsorption of toxic elements in cationic form on metal oxides may he enhanced or inhibited in the presence of strongly chelating organic ligands, hut... 

We conclude that the inabihty of cationic Ni(II) complexes to act as CO/ethylene co-polymerization catalysts stems from the strong chelating Ni-0 bond as well as the tendency of the nickel system to form five-coordinated bis-carbonyl complexes. However, the nickel systems are not yet as well studied as their palladium homologues, and much more theoretical and e3q>erimental work has to be carried out before the Ni(ll) system is fiiUy imderstood. 

Amino polycarboxylic acids with the characteristic grouping -N(CH2COOH)2 form strong chelates with many polyvalent cations (Schwarzenbach et oL, 1945). Elements such as calcium, strontium or magnesium are therefore determined by titration with the solution of a chelating agent (complexone). 

The methylene group adjacent to the nitrogen can be metallated with strong base to give an a-amino anion in which the lithium cation is chelated by th two heteroatoms of the chiral auxiliary. It is interesting to compare the alternative route to chiral 2-alkylated tetrahydroisoquinolines described in section 6.4.1. 

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