Substituents Substitution

Substituent, in. substituent substitute, substituierbar, a. replaceable, substituieren, t.i. substitute. 

Halobenzenes undergo nucleophilic aromatic substitution through either of two mechanisms. If the halobenzene has a strongly electron-withdrawing substituent in the ortho or para position, substitution occurs by addition of a nucleophile to the ring, followed by elimination of halide from the intermediate anion. If the halobenzene is not activated by an electron-withdrawing substituent, substitution can occur by elimination of HX to give a benzyne, followed by addition of a nucleophile. 

Indeed, nucleophilic attack at Ca is the most widely observed single reaction of Fischer carbenes. Substituent substitution is favored when one of the substituents on the carbene carbon is a good leaving group, e.g., halide, alkoxide, etc. Aminolysis of complex 41 typifies this mode of reaction (17),... 

The reactivity displayed by the heteroatom-substituted Ru, Os, and Ir carbene complexes discussed in this section toward nucleophilic reagents contrasts sharply with that described for the Fischer compounds. The reactions of these Group 8 complexes are almost exclusively restricted to the metal-ligand framework, with only two related substituent substitution reactions being reported (44) ... 

N-Substituted carbene ligands predominate amongst these Ru, Os, and Ir complexes, and thus substituent substitution reactions may be unfavorable on thermodynamic grounds. 

DISSOCIATION KINETICS SUBSITE MAPPING SUBSTITUENT SUBSTITUTION REACTION Sn REACTIONS... 

In an earlier discussion (254) polymers in which the chirality depends only on the presence of chiral side groups were said to be nonintrinsically chiral, in contrast with intrinsically chiral polymers where the chirality is independent of the internal structure of the substituent. Substituted carbon atoms in the polymers described in the next paragraphs are often indicated as true or classic asymmetric carbon atoms. In this way one can distinguish between carbon atoms whose four substituents are constitutionally different in the proximity of the atom under consideration, from the tertiary atoms of vinyl isotactic polymers. For these, only the different length of the two chain segments and/or the stmcture of tire end grmips make all the ligands different from each other. 

The name of the CRU is formed from the names of its subunits, including substituents (substituted subunits), and cited in order from left to right as they appear in the CRU. 

For compilation and arrangement of compounds, earlier reviews and surveys were taken as the basis. In comparison to the previously published reviews, the increasing number and complexity of structures observed is striking. Thus it became quite difficult to list all of these structures in a logical sequence, particularly prenylated derivatives with additional cyclized substituents. Substitution patterns used for grouping of the flavone and flavonol derivatives are as follows. 

Acrylate esters have been substituted with Ar-methyl-2-pyridone (5-position), furfural (5-position) and 2-thiophenecarbaldehyde (5-position).8 Benzo[b]furan reacts similarly with alkenes (2-position).9 Alkene substitutions with furan,10 thiophene,10 benzothiophene10 and N-acetylindoles,10 11 also, have been reported. Furan and thiophene yield mainly 2,5-dialkenylated products.9 a-Substituted chalcones with strongly electron-withdrawing substituents substitute with benzene.12 Ferrocene13 and t)4-cyclobuta-dienetricarbonyliron14 will substitute alkenes as well. 

Ab initio calculations [195] have been carried out to model the effect of some substituents substitution on the C2 carbon by a ti donor group stabilises the product more than the starting material and thus will be favourable for the reaction enthalpy. An NH2 group will provide more stabilisation than SH. This correlates well with the experimental results. 

In general, the tautomeric equilibrium (equation 1) is completely on the enamine side when an unsaturated electron-accepting substituent substitutes the /Tcarbon of 1 otherwise, the equilibrium lies far on the side of imine 1. However, apart from the two extreme cases above, some secondary enamines are sufficiently stable to exist in detectable amounts in equilibrium with the corresponding imines and thus it becomes possible to investigate the factors affecting the enamine-imine equilibrium51-53. 

This review describes the synthesis and properties of polyacetylenes with substituents (substituted polyacetylenes) mainly on the basis of our recent studies At first, Sections 2 and 3 survey the synthesis of substituted polyacetylenes with group 6 (Mo, W) and group 5 (Nb, Ta) transition metal catalysts respectively, putting emphasis on new, high-molecular-weight polyacetylenes. Then, Section 4 refers to the behavior and mechanism of the polymerization by these catalysts. Further, Section 5 explains the alternating double-bond structure, unique properties, and new functions of substituted polyacetylenes. Finally, Section 6 provides detailed synthetic procedures for substituted polyacetylenes. 

Many potentially useful phenothiazine derivatives have been synthe.sized and evaluated pharmacologically. Consequently. the large body of information permits accurate. statements about the structural features as.sociated with activity. Many of the features were summarized and interpreted by Gordon et al. The best position for substitution is the 2 position. Activity increases (with some exceptions) as electron-withdrawing ability of the sub.stitucnt incrcase.s. Another possibly important. structural feature in the more potent compounds is the pre.sencc of an unshared electron pair on an atom or atoms of the 2 substituent. Substitution at the 3 position can improve activity over nonsubslituled compounds but not as significantly os substitution at the 2 position. Substitution at po.sition I has a deleterious effect on antip.sychotic activity, us does (to a lesser extent) substitution at the 4 position. 

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