Metal ions electrophilic interactions

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

Metal ions, because they are positively charged, act as electrophiles that is, they seek the possibility of sharing electron pairs so that a bond or a charge-charge interaction can be formed and electrical neutrality can thereby be attained. In these types of interactions, metal ions act like hydrogen ions (i.e., protons), except that they have a greater ionic volume... 

The in-out bicyclic amines prepared by Simmons and Park bear a remarkable semblance to the cryptands but lack the binding sites in the bridges. As a result, these molecules interact with electrophiles in a fashion similar to other tertiary amines and generally do not exhibit strong interactions with alkali or alkaline earth metal ions. The in-out bicyclic amines are prepared by reaction of the appropriate acid chlorides and amines in two stages to yield the macrobicyclic amine after reduction of the amidic linkages. A typical amine is shown above as compound 18. 

During the insertion mechanism, the metal is inserted into the carbon-oxygen bond. The insertion is promoted by a strong metal—oxygen interaction. It is thought that unreduced metal ions may play an important role in the insertion mechanism (electrophilic catalysis). The type of the catalyst, the method of preparation, and the additives can influence the concentration and stability of these ions. 

At the present, the most straightforward mechanism for the formation of J5 from 1 is via insertion of CO into the Th-C(acyl) bond to form a ketene (H, (eq. (4)) which subsequently dimerizes. Presumably, initial CO interaction could involve coordination either to the metal ion as shown or to the electrophilic vacant "carbene p atomic orbital. Considering the affinity of the Th(IV) ion for oxygenated ligands, interaction of the ketene oxygen atom with the metal ion seems reason-... 

HOMO-LUMO) interactions the LUMO being the antibonding cr x orbital [45], the HOMO a non-bonded electron pair, formally available at both 90° and about 180° to the C-X bond [46], Much similar work supports this interpretation. Contacts between halogens (X) and electrophilic centres E (all metal ions) [47] fall almost exclusively in the range 9O<0E<12O°, while, for better electron donors Nu, 0Nu generally lies between 150° and 180°. 

Subsequent to CO2 association in the hydrophobic pocket, the chemistry of turnover requires the intimate participation of zinc. The role of zinc is to promote a water molecule as a potent nucleophile, and this is a role which the zinc of carbonic anhydrase II shares with the metal ion of the zinc proteases (discussed in the next section). In fact, the zinc of carbonic anhydrase II promotes the ionization of its bound water so that the active enzyme is in the zinc-hydroxide form (Coleman, 1967 Lindskog and Coleman, 1973 Silverman and Lindskog, 1988). Studies of small-molecule complexes yield effective models of the carbonic anhydrase active site which are catalytically active in zinc-hydroxide forms (Woolley, 1975). In addition to its role in promoting a nucleophilic water molecule, the zinc of carbonic anhydrase II is a classical electrophilic catalyst that is, it stabilizes the developing negative charge of the transition state and product bicarbonate anion. This role does not require the inner-sphere interaction of zinc with the substrate C=0 in a precatalytic complex. 

Therefore mercury(II) acetate interacts as an electrophilic transition metal with the nucleophilic alkene to form the three-membered ring 52. This mercurinium ion is opened by relatively feeble nucleophiles like alcohols - or in this reaction water. Similar to a hydroboration the attack happened at the more substituted end of the mercurinium ion according to Markovnikov s rule. To get rid of the metal, solid potassium iodide is added. This means insoluble mercury(Il) iodide is formed, followed by loss of the methoxy group and formation of enol ether 54, which subsequently tautomerizes to the desired aldehyde 55. 

Although the direct interaction of metal complexes with alkanes under mild conditions has been demonstrated by the foregoing results, selective and catalytic functionalization of alkanes remains to be found. Success is probably more likely to be achieved by way of mild dehydrogenation (e.g., transfer of hydrogen to a suitable donor)6113 b or carbonylation rather than by oxidation. By analogy with Co(lII) oxidations of alkanes, these reactions should be facilitated by increasing the electrophilicity of the metal ion, e.g., by using the metal tri-... 

The cleavage of the C—H bond by direct participation of a transition metal ion proceeds via an oxidative addition mechanism or an electrophilic substitution mechanism. Metals in low oxidation states undergo oxidative addition while high oxidation state metals take part in electrophilic substitutions. Another function of the metal complex in these reactions consists of abstracting an electron or a hydrogen atom from the hydrocarbon, RH. The RH radical ions or R radicals which are formed then interact with other species, such as molecular oxygen which is present in the solution or in one of the ligands of the metal complex (21). 

Nowadays, activities of nucleic acids are controlled by interactions of Mg, Ca or Zn, but also by heavy-metal ions or electrophilic agents with certain specific sequences (e.g. in induction of metallothionein). Though this does imply NAs to act as ligands in physiological conditions, it need not imply that they could achieve the above sequence of steps, let alone the problem that up to now not even a hint of a prebiotic NA synthesis pathway was demonstrated, not even when using polyphosphoric acid in organic solvents. The E (L) values for NAs, nucleoside triphosphates or simpler species such as glycerinaldehyde-2,3-diphosphate are way too low to permit the sequence of events on their own. 

The possible assistance given to leaving groups by electrophiles in the form of solvent or metal ions represents one part of a ligand replacement reaction normally referred to as SE2(cyclic). Structure 19 shows the interaction at the transition state. It can be seen as simultaneous attack of the electrophile E at the leaving group X and of the... 

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