Metal ions electrophilic attack

Electrophilic attack, metal ions, 34 266-270 Electrophilic interactions, metal ions, 34 284 Electroreflning, of actinide metals, 31 13, 15, 17, 20, 21... 

Ion pairs and hydrogen-bonded species. Free tetrazolate anions 7 resulting from complete dissociation of salts of N-unsubstituted tetrazoles seldom exist under conditions of actual preparative experiments where ionic association must be taken into account, and the usual objects of an electrophilic attack are ion pairs of anion 7 and a metal cation, or ion pairs of the type 231 and 232 <2000H(53)1421>. 

Coordination of an olefin molecule to the metal leads to its activation. This facilitates reactions of alkenes with various compounds. The coordinated olefin may have nucleophilic as well as electrophilic properties, and therefore may react in the first case with electrophiles, and in the second case with nucleophiles. Thus, either the central atom or the alkene may undergo nucleophilic attack. In the case of weak TT-olefin Cu(I) and Ag(I) complexes, even weak nucleophiles attack metal ions. 

Certain metal cations are capable of electrophilic attack on alkenes. Addition is completed when a nucleophile adds to the alkene-cation complex. The nucleophile may be the solvent or a ligand from the metal ion s coordination sphere. 

Direct coordination of a metal ion to the non-bridging oxygen might render the phosphorus center more susceptible to nucleophilic attack (electrophilic catalysis Fig. 3e) or, alternatively, hydrogen bonding between a metal-... 

Fig. 3 Possible catalytic functions of metal ions in the cleavage of a phosphodiester bond. Metal ions can act as (a) a general acid catalyst, (b) a general base catalyst, (c) a Lewis acid that stabilizes the leaving group, (d) a Lewis acid that enhances the deprotonation of the attacking nucleophile, and (e) an electrophilic catalyst that increases the electrophilicity of the phosphorus atom...

Acid hydrolysis of an octahedral metal ion complex is typically a dissociative or SNl-type reaction. In the case of base hydrolysis, reactions tend to display SN2-type reaction mechanisms, although others take place by what is termed an SnI-conjugate base mechanism. The latter involves attack by an electrophile to abstract a proton... 

Presumably, these skeletal reorganization reactions start by coordination of a metal ion to an alkyne part in 78 to allow an alkene part to attack the resulting electrophilic alkyne carbon coordinated by the metal. However, the reaction mechanism remains yet to be clarified, and it is thought that each reaction mechanism differs depending on the metal used. 

Similarly, relevant electrophiles (Lewis acids) including A-type metal cations (hard), bivalent transition metal ions (borderline), and B-type metal ions (soft) can be categorized (see Stumm and Morgan 1996). Note that in organic molecules, the atom where a nucleophile attacks (i.e., the electrophilic site) may possess harder (e.g., C=0, P=0) or softer (e.g., CH3-X) character. 

One salient feature of this model of the active site with a bound substrate is that the carbonyl of the peptide bond that is to be broken is coordinated to Zn2+. This feature implies that the metal ion may serve as an electrophile to polarize the carbon-oxygen bond, thereby rendering the carbon succeptible to nucleophilic attack by water. In the x-ray structure a water molecule is... 

The role of the metal ion in hexokinase seems to be to coordinate the fi,y phosphate groups of ATP. In this manner the metal ion serves as an electrophilic center to neutralize the negative charges on the ionized phosphate groups of ATP. This then leads to facilitated catalytic attack by the 6-hydroxyl group of substrate on the y-phosphoryl of ATP. 

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