Activation by Cations

In previous sections, numerous examples of anion activation by cationic micelles and polysoaps were presented. The extent of rate augmentation— 102—lO -fold—cannot be rationalized in terms of concentration effects alone. We believe that these observations are explained most reasonably by the concept of the hydrophobic ion pair (Kunitake et al., 1976a). According to this concept, anionic reagents are activated probably due to desolvation when they form ion pairs with an ammonium moiety in a hydrophobic microenvironment. The activation of anionic species in the cationic micellar phase... 

D. A. Chen, P. A Combined Gas-Phase, Solution-Phase, and Computational Study of C-H Activation by Cationic Iridium(III)... 

In contrast to the cation activation observed with the P. crispata enzyme, the preparation from D. desulfuricans 800-fold purified (184) was not activated by cations nor inhibited by EDTA, cysteine, and Na2S. Substrates other than those noted in Table VII were dATP, dADP,... 

Activation-Inhibition and Function In Vivo. When 0.15M NaCl was added to an orange PE-pectin reaction mixture at pH 7.5, activity was increased 5-fold, and at pH 5 it was increased 100-fold (17). As explained by Lineweaver and Ballou (19), NaCl caused the apparent activation by freeing the enzyme from the inactive ionic complex (pectin-carboxyl). They showed that at pH 5.7 pectic acid inhibited alfalfa PE activity 55% in 0.015M NaCl but only 17% in 0.2M NaCl. At pH 8.5 pectic acid inhibited PE activity only 9% in 0.015 NaCl. They concluded that the stimulation of activity by cations at low pH (17) did not show that cations were essential for activity, but, rather, that cations function by preventing product inhibition, which is greater at low pH. 

Scheme 3. Mechanism for the C-H activation by cationic iridium(III) complexes proposed by Bergman and co-workers...

Plant pectinesterases are activated by cations, and their pH optima shift to lower values in presence of cations 61, 77). Mayorga and Rolz (78) proposed equations predicting the activity of pectinesterases as a... 

Hinderling C, Feichtinger D, Plattner DA, Chen P (1997) A combined gas-phase, solution-phase, and computational study of C-H activation by cationic iridium(lll) complexes. J Am Chem Soc 119 10793... 

The effects of inorganic salts on plasma cholinesterase (E16) are largely contradictory. Fruentova (F9) reported that divalent cations are more effective inhibitors of horse serum cholinesterase than are monovalent ions, whereas divalent ions are frequently reported to have a marked activating effect (H38, T8, VI). Lithium and sodium nitrates have been shown by in vitro studies of the reaction of human plasma cholinesterase with benzoylcholine to have identical inhibition profiles (W21), while sodium and potassium chlorides had very similar inhibitory actions on the hydrolysis of acetylcholine by human plasma (H47). Silver nitrate, copper sulfate, and mercuric chloride are powerful inhibitors of F. polycolor butyrylcholinesterase (N2). Cohen and Oosterbaum (C12) concluded that activation by cations occurring at the usual substrate concentration is highly dependent on the experimental conditions. This supposition is very relevant to the somewhat random choice of buffers and substrates in the work reported above. 

In the anionic flotation of quartz, activators such as multivalent metal ions are required to provide adsorption sites for surfactant. Cationic polymer can also adsorb on silica and offer adsorption site for surfactant. It is found that quartz can be activated by cationic PAMD for flotation by the anionic dodecylsulfonate [7]. 

Scheme 6.12 Water activation by cationic (PSiP)ir " complexes.

Tellers DM, Bergman RG. An examination of C—H bond activation by cationic TpMe2lr(lll) complexes. Organometallics. 2001 20 4819-4832. 

Alkylbetaine esters [90-92], quaternized aspartic acid esters [23], and polyolesters of betaine [71] have been suggested for use in fabric care. The ester bond in betaine esters is strongly activated by cationic nitrogen in the a position, and therefore these materials can undergo rapid abiotic degradation and have very low aquatic toxicity. 

However, the increased steric demand of NHC ligands compared to phosphines can decrease the reactivity. For a DFT study on C-H bond activation by cationic Ir" NHC and PMCj complexes, see J. M. Meredith, R. Robinson Jr, K. I. Goldberg, W. Kaminsky and D. M. Heinekey, Organometallics, 2012,31,1879-1887. 

The role of the cyclopentadienyl ligand in the a-bond activation of methane by [Co(ri -C5H5)] has been studied and a reaction mechanism proposed which includes several intermediates containing Co-alkyl bonds.A series of Ir-Me species have been synthesised as part of an investigation into the effect of the hydridotris(pyrazolyl)borate ligand on C-H bond activation by cationic iridium complexes. The rhodium species 23 has been synthesised as part of a study into... 

A computational study of gold-catalysed annulations of 3-aryloxy alkynyl indoles has been reported Indole derivatives of electron-donating and electron-withdrawing protective groups undergo the 5-exo-dig cyclization upon activation by cationic [PR3AU-I-] species (Scheme 129). 

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