Covalent active species

For the discussion of the reactivity of covalent active species, see Section II.B.6.C.)... 

Thus, bromide-terminated macromolecules are considered covalent active species in polymerization of cyclic imines or oxazolines (cf., Section III.E, F). The same termini would be inactive, however, in the presence of much weaker nucleophiles, e.g., cyclic ethers. Recombination of active species with Br anion would thus result in termination. 

Thus, corresponding macroesters are the covalent active species in the polymerization of cyclic ethers. 

Table 7 Rate Constants of Propagation on Ionic and Covalent Active Species...

It has been found that decarboxylation may be completely eliminated if cationic polymerization of cyclic carbonates is initiated with alkyl iodide or bromide. It is believed that polymerization proceeds with the participation of covalent active species favoring propagation over side reactions leading to C02 elimination [204]. It is interesting to note, that BF3-initiated polymerization of some cyclic carbonates leads to high molecular weight polymers (M > 10s) [205]. 

The external ionization involves addition of the monomer molecule to the covalent active species and, thus, means the covalent propagation. 

As long as the spacer molecules are not covalently bound to the cluster surface and the spacer molecules have no delocalized rc-electron system along their backbone between their termini, this relation is valid. The respective activation energy drops as soon as covalently linking species, equipped with delocalized n electrons... 

Proposition 2 When the initiator solution is introduced into a monomer solution, the cations react with the double bond to give the active species which is an aluminated carbenium ion containing an Al-C covalent bond, as shown in Equation (iii) ... 

Whereas in Wmethyl-4,6-dimethylpyrimidinium ion the covalent addition takes place at C-6, it is assumed that 30 also undergoes covalent hydrazina-tion at C-6. However, the formation of dimer 32 shows the high sensitivity of C-2 in 30 for addition of nucleophiles, and it leads to the daring suggestion that it is the resonance-stabilized ylide 31 that probably is the active species undergoing addition at C-2 (Scheme III. 19). It was calculated (80UP1) that the reactivity at C-2 in the N-ylide 31 is greater than that at C-2 in the Waminopyrimidinium salt 30. 

The surest way to inhibit an enzyme is to block the active site irreversibly by chemical reaction with some active species to form a covalent bond. Thus, iodoacetate will irreversibly inactivate thiol proteases by forming the stable carboxymethyl mercaptan. lodoacetate is of course non-selective (many other enzymes would be inactivated), toxic (many sensitive sites would be alkylated) and moreover the drug itself is unstable due to its very reactivity. 

A catalytic cycle is composed of a series of elementary processes involving either ionic or nonionic intermediates. Formation of covalently bound species in the reaction with surface atoms may be a demanding process. In contrast to this, the formation of ionic species on the surface is a facile process. In fact, the isomerization reaction, the hydrogenation reaction, and the H2-D2 equilibration reaction via ionic intermediates such as alkyl cation, alkylallyl anion, and (H2D)+ or (HD2)+ are structure-nonrequirement type reactions, while these reactions via covalently bound intermediates are catalyzed by specific sites that fulfill the prerequisites for the formation of covalently bound species. Accordingly, the reactions via ionic intermediates are controlled by the thermodynamic activity of the protons on the surface and the proton affinity of the reactant molecules. On the other hand, the reactions via covalently bound intermediates are regulated by the structures of active sites. 

The covalent bonds to axial ligands L and L in 1 or 3 become stronger, hence the coordination sphere is more stable, and unstable intermediates or active species of reactions catalyzed by the heme systems may persist with the heavier homologs and easier lend themselves to chemical and physical investigations. 

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