Initiation substituents

The ortho position is one removed from the initial substituent s position. The meta position is two removed, and the para is three removed. 

In the case of catalysts for the polymerisation of conjugated dienes, the initiating substituent or the ultimate monomer unit of the growing polymer... 

Catalysts for the polymerisation of heterocyclic and heterounsaturated monomers contain a heteroatom, such as, for example, Cl (i.e. with an Mt-Cl active bond), or the group bound to the metal atom via the heteroatom (Mt X for example, X = 0, S, N) as the initiating substituent. However, there are also catalysts that possess an alkyl initiating substituent at the metal atom, especially when they are used for the polymerisation of heterounsaturated monomers. 

The coordination polymerisation of heterocyclic and heterounsaturated monomers consists in the nucleophilic attack of the metal initiating substituent (or the growing polymer chain) on the carbon atom of the coordinated monomer. Scheme 2.6 shows initiation and propagation steps in the coordination polymerisation of epoxides, as the most representative heterocyclic monomers with an endocyclic heteroatom, with catalysts containing an Mt-X active bond [68,114,115] ... 

When considering this process at a bond at which the carbon atom carrying the initial substituent is attached, it is obvious that besides the ortho-nitrobenzenium ions also the so-called "ipso" substituted ion should be formed. Ipso attack thus is in my view not a special case, but, indeed an integral part of all aromatic substitution reactions. The only difference is that in order to de facto observe ipso substitution, the original substituent must be of a good electrophilic leaving group nature. In the case of toluene this is not the case, as would be the most unsuitable... 

Some substituents increase the rate of a subsequent substitution and generally direct it to the ortho and para positions. Other substituents slow the rate and direct further substitution to the meta position. Understanding the dependence of substitution position and rate on the structure of the initial substituent is essential. 

Of course, the number and location of bromine atoms fixed on the organic rest depend on the nature, location, and number of the initial substituents of the compound under study. 

Doubina N, Paniagua SA, Soldatova AV, Jen AKY, Marder SR, Luscombe CK (2011) Steric effects of the initiator substituent position on the externally initiated polymerization of 2-Bromo-5-iodo-3-hexylthiophene. Macromolecules 44 512-520... 

Fig. 9.2 shows some of the more likely structures occurring in the vulcanization of the natural rubber. The approximate chemical shifts for carbons in these structures have been calculated by using chemical shift substituent effects. The initial substituent effect values used are taken from Wehrli and Wirthlin [13] but were slightly modified based on model compound studies. The values are monosulfidic, O = +18 ppm, = +6 ppm, y = —A ppm di- and polysulfide, a = +25 ppm, yS = +6 ppm, / = —4 ppm. 

Figure 2 40. To illustrate the isomorphism problem, phenylalanine is simplified to a core without representing the substituents. Then every core atom is numbered arbitrarily (first line). On this basis, the substituents of the molecule can be permuted without changing the constitution (second line). Each permutation can be represented through a permutation group (third line). Thus the first line of the mapping characterizes the numbering of the atoms before changing the numbering, and the second line characterizes the numbering afterwards. In the initial structure (/) the two lines are identical. Then, for example, the substituent number 6 takes the place of substituent number 4 in the second permutation (P2), when compared with the reference molecule.

Gassman and co-workers developed a synthetic route from anilines to indoles and oxindoles which involves [2.3]-sigmatropic rearrangement of anilinosul-fonium ylides. These can be prepared from Ai-chloroanilines and ot-thiomcthyl-ketones or from an aniline and a chlorosulfonium salt[l]. The latter sequence is preferable for anilines with ER substituents. Rearrangement and cyclizalion occurs on treatment of the anilinosulfonium salts with EtjN. The initial cyclization product is a 3-(methylthio)indole and these can be desulfurized with Raney nickel. Use of 2-(methylthio)acetaldehyde generates 2,3-unsubstituled indoles after desulfurization[2]. Treatment of 3-methylthioindoles with tri-fiuoroacetic acid/thiosalieylie acid is a possible alternative to Raney nickel for desulfurization[3]. 

These two methods are closely related but differ in the point of initial attachment of the substituent from which the carbocyclic indole ring is constructed. One strategy for building up 2-substituted pyrroles capable of... 

Best Synthetic Methods is now 10 years old, is a family of 16 volumes and has been well received by the majority of chemists as a valuable aid in their synthetic endeavours, be they academic or commercial. The focus of the series so far has been on special methods, reagents or techniques. This volume is the first of a new sub-series with a focus on heterocycles and their synthesis. It is amazing the extent to which each heterocyclic type has its own specialized synthetic methodology. Whether the chemist is endeavouring to make a heterocycle by ring synthesis or wishes to introduce specific substituents, it is the intention that this new development will serve their needs in a practical, authoritative, fully illustrative and compact manner. Richard Sundberg is an authority on indole chemistry and it is a pleasure to have such a noted heterocyclist to initiate this venture. 

Attack on the electrophilic C-2 may occur as in the 2-aminothiazoles series, which probably explains the rearrangements observed in acidic medium (121, 711, 712, 723, 724), in aqueous medium with NaOAc (725), or with aqueous NaHCOj (725) (Scheme 232). That the initial attack probably involves the C-2 atom is substantiated by the fact that this rearrangement occurs under extremely mild conditions for 2-iinino-3-substituted-5-nitro-4-thiazolines (725). As the whole mechanism proposed (see p. 92) is reversible, when imino derivatives are submitted to such rearrangement conditions the rearrangement is expected to occur faster if steric interaction between 3- and 4-substituents exists in the 2-imino isomer. Another reaction may occur in acidic medium phenylimino-2-bipheny]-3,4-4-thiazoline hydrolyzed with hydrochloric acid gives the corresponding 4-thiazoline-2-one and aniline (717). 

Curiously enough, bulky substituents on nitrogen increase this reactivity towards methyl iodide (119). This has been related to a steric decompression of the thiocarbonyl group in the transition state. Furthermore, knowledge of the ratio of conformers in the starting 4-alkyl-3-i-Pr-A-4-thiazoline-2-thiones and in the resulting 4-alkyl-3-i-Pr-2-methylthiothi-azolium iodides combined with a Winstein-Holness treatment of the kinetic data indicates that in the transition state, the thiocarbonyl bond is approximately 65% along the reaction coordinate from the initial state... 

Treatment of 192 with dimethyl acetylenedicarboxylate yields a thiophene derivative (195) when R = Ph and a 2-p3Tidone (1S>6) derivative when R = H (Scheme 100). The proposed mechanism involves the formation of a mesoionic derivative (193) initially further dipolar addition yields adduct 194, the decomposition of which is dependent on the R substituent as described for related compounds (435). ... 

Because we have come to associate activating substituents with ortho para directing effects and deactivating substituents with meta the properties of the halogen substituents appear on initial inspection to be unusual... 

In cationic polymerization the active species is the ion which is formed by the addition of a proton from the initiator system to a monomer. For vinyl monomers the type of substituents which promote this type of polymerization are those which are electron supplying, like alkyl, 1,1-dialkyl, aryl, and alkoxy. Isobutylene and a-methyl styrene are examples of monomers which have been polymerized via cationic intermediates. 

Triazines pose rather more of a problem, probably because the carbons are in an effectively oxidized state so that no metaboHc energy is obtained by their metaboHsm. Very few pure cultures of microorganisms are able to degrade triazines such as Atrazine, although some Pseudomonads are able to use the compound as sole source of nitrogen in the presence of citrate or other simple carbon substrates. The initial reactions seem to be the removal of the ethyl or isopropyl substituents on the ring (41), followed by complete mineralization of the triazine ring. 

CX-Aminonitriles are compounds containing both cyano and amine substituents attached to the same carbon atom. They are versatile synthetic intermediates that are used to make aminoacids, agrichemicals, chelants, radical initiators, and water-treatment chemicals. In some cases, aminonitriles produced as intermediates are not isolated, but immediately further reacted, for example by hydrolysis, as is the case in producing... 

Applications. Among the P—O- and P—N-substituted polymers, the fluoroalkoxy- and aryloxy-substituted polymers have so far shown the greatest commercial promise (14—16). Both poly[bis(2,2,2-trifluoroethoxy)phosphazene] [27290-40-0] and poly(diphenoxyphosphazene) [28212-48-8] are microcrystalline, thermoplastic polymers. However, when the substituent symmetry is dismpted with a randomly placed second substituent of different length, the polymers become amorphous and serve as good elastomers. Following initial development of the fluorophosphazene elastomers by the Firestone Tire and Rubber Co., both the fluoroalkoxy (EYPEL-F) and aryloxy (EYPEL-A) elastomers were manufactured by the Ethyl Corp. in the United States from the mid-1980s until 1993 (see ELASTOLffiRS,SYNTHETic-PHOSPHAZENEs). 

The first-order decomposition rates of alkyl peroxycarbamates are strongly influenced by stmcture, eg, electron-donating substituents on nitrogen increase the rate of decomposition, and some substituents increase sensitivity to induced decomposition (20). Alkyl peroxycarbamates have been used to initiate vinyl monomer polymerizations and to cure mbbers (244). They Hberate iodine quantitatively from hydriodic acid solutions. Decomposition products include carbon dioxide, hydrazo and azo compounds, amines, imines, and O-alkyUiydroxylarnines. Many peroxycarbamates are stable at ca 20°C but decompose rapidly and sometimes violently above 80°C (20,44). 

By varying the molecular stmcture, it is possible to synthesize dye initiators with the requited characteristics. The synthesis of polymethine dyes with different chain length, end groups, and substituents, or with other variations of the chromophore, has been summarized (3,4,9,21,73,74) (see also Cyanine dyes). 

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