Mutual activation

In comparing the reactivity at different positions in a heterocycle, a poly-substituted derivative is sometimes used with the idea that selective reaction of the same leaving group at different positions in a single molecule gives the most clear-cut answer. However, in a polychloroazine, the mutual activation of the chlorines by one another is not identical (unless the molecule is symmetrical, in which case the... 

An extensive study of the amination of halopyridines has been carried out by den Hertog and co-workers.A comparison of their results with studies in inert solvents using primary and tertiary amines should permit some evaluation of the postulated factors. 2,4-Dichloropyridine in concentrated aqueous ammonia (180°, 5 hr) resulted in the formation of 4-amino- (60% yield) and 2-amino-chloropyridines (20% yield). Under similar conditions, only 4-substitution of 3,4,6-trichloro- and of 2,3,4,5-tetrabromo- and -tetrachloro-pyridines was observed. However, in these and the other polyhalo pyridines, the appreciable and unequal mutual activation by the halogen substituents needs to be emphasized. 

The reactivity of cyanuric chloride (2,4,6-trichloro-s-triazine) as an indication of s-triazine activation is misleadingly high because of mutual activation of the chlorines meta activation > ortho or para activation) and its symmetry (cf. Section III,A, 1), However, the greatest variety of nucleophilic substitutions have been investigated with this substrate. 

The limited data available for 2,4-dichloroquinoline (Table X, line 9) show a substantially greater rate of methoxylation than for the 2- and 4-chloro analogs (Table X, line 6 and Table XI, line 2), as a result of activation (lowering of E ) by the additional chlorine substituent. Unequal mutual activation (cf. Section III, B, 2) by these substituents is indicated by the rate ratio of 1.9 1 for 4- to 2-substi-tution in the dichloro compound and of 25 1 for the two mono-chloro compounds. 

The relations 4- > 2-position in rate and 4- < 2-position in will apparently apply to reactions with anions, but the reverse relation is observed in piperidination, presumably due to 2-substitution being favored by hydrogen bonding in the zwitterionic transition state (cf. 47, 59, and 277) or by solvent-assisted proton removal from the intermediate complex (235). Substitutions of polychloroquino-lines (in which there is a combined effect of azine-nitrogen and unequal mutual activation of the chlorine substituents) also show 4- > 2-position in reactivity contrary statements are documented by these same references. Examples are cited below of the relation 2- > 4-position when a protonated substrate or a cyclic transition state is involved. 

As in blood coagulation (see p. 290), the early components in the complement system are serine proteinoses, which mutually activate each other through limited proteolysis. They create a self-reinforcing enzyme cascade. Factor C3, the products of which are involved in several functions, is central to the complement system. 

Osteoblasts (top) deposit collagen, as well as Ca "" and phosphate, and thereby create new bone matter, while osteoclasts (bottom) secrete H"" ions and collagenases that locally dissolve bone (bone remodeling). Osteoblasts and osteoclasts mutually activate each other by releasing cytokines (see p. 392) and growth factors. This helps keep bone formation and bone breakdown in balance. 

The diverse effects of insulin (see p. 160) are mediated by protein kinases that mutually activate each other in the form of enzyme cascades. At the end of this chain there are kinases that influence gene transcription in the nucleus by phosphorylating target proteins, or promote the uptake of glucose and its conversion into glycogen. The signal transduction pathways involved have not yet been fully explained. They are presented here in a simplified form. 

At the center of the apoptotic process lies a group of specialized cysteine-containing aspartate proteinases (see p. 176), known as cas-pases. These mutually activate one another, creating an enzyme cascade resembling the cascade involved in blood coagulation (see... 

Special properties are observed with mixtures of iron with other catalytically active metals such as cobalt, molybdenum, tungsten and uranium. These systems represent cases of mutual activations. 

Fig. 6.17. Plot of mutual activities for the binary mixture of water and...

Polymerisation carried out in the presence of a coordination catalyst is referred to as coordination polymerisation , when each polymerisation step involves the complexation of the monomer before its enchainment at the active site of the catalyst. The active site in each coordination catalyst comprises the metal atom (Mt), surrounded with ligands, one of which (X) forms a covalent active bond (Mt X) with this metal atom. This implies that the growing polymer chain is covalently bound to the metal atom. A characteristic feature of coordination polymerisation is the mutual activation of the reacting bonds of both the monomer (M) and the active site (Mt-X) through the complexation of the monomer with the metal atom at this site, which results in the cleavage of these bonds in the concerted reaction. 

The active bond (Mt-X) of the coordination catalyst is stable in the uncom-plexed state, but monomer coordination results in enhanced cleavage susceptibility of this bond as well as the respective bond in the coordinating monomer, which results in the occurrence of the polymerisation initiation step, followed by the subsequent chain propagation steps. Thus, the mutual activation of the monomer and the catalyst when they form the complex is to be emphasised as a characteristic feature of coordination polymerisation. 

These are the most important. The two double bonds mutually activate each other conjugation is essentially not destroyed by addition to the growing chain end. Therefore the conjugated dienes are difunctional monomers. They are polymerized by a relatively simple mechanism. Of all the polymers generated in living tissues, we have so far been able to imitate most closely natural rubber, poIy-cis-l,4-isoprene. Butadiene, isoprene and chloroprene are the dienes most often employed in macro-molecular chemistry. 

Sulfanilamido-6-(p-tolyl8ulfonyl)pyridazine, deactivated by anionization, seems to be about as reactive (150°, 12 hr) toward alkoxide ions as is the 6-chloro analog. In 3-chloro-6-methyl-sulfonylpyridazine the chloro group is preferentially displaced by sulfanilamide anion and by alkylamines and aniline. However, in this substrate as well as in 4-chloro-6-methylsulfonylpyrimidine, the relative reactivity involves their mutual activation as well as their mobilities as leaving groups. 

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