Heteroatom influence

If the reactions of the same substituents on heteroaromatic azoles and on benzene rings are compared, the differences in the reactivities are a measure of the heteroatoms influence. Such influence by the mesomeric effect is smaller when the substituent is /3 to a heteroatom than when it is a or y. The influence by the inductive effect is largest when the substituent is a to a heteroatom. 

Atta, EM., Betz, R., Schmid, B., and Schmidt, R.R., Functionally substituted vinyl carbanions. Part 25. Heteroatom influence on vinyUc deprotonation, Chem. Ber., 119, 472, 1986. 

The Cram chelation model (sec. 4.7.B) is an example where the chelation effects of the heteroatom influence the rotamer population and, thereby, the selectivity of the reduction. Zinc borohydride [Zn(BH4)2], effectively chelates the carbonyl oxygen and alcohol oxygen atoms in the reduction of 42 and leads to intermediate 43. Transfer of hydride to the carbonyl gave primarily the anti diastereomer, 45 (4 96, 44/45). When the chelating hydroxyl group was blocked as a tert-butyldiphenylsilyl ether (in 46 - sec. 7.3.A.i), reduction with Red-Al (sec. 4.3) led to a reversal in selectivity (96 4, 47/48).The ability to chelate a heteroatom varies with the reagent used. Lithium aluminum hydride shows less selectivity, due in part to poorer coordination with the heteroatom and reduction of 42 gave a 27 73 mixture of 44 and 45,... 

The aromatic character of the phenol ring explains the higher stability of the enol form with respect to the keto form in the ground state.Variations in the aromatic character (Induced by substitution or heteroatoms) influence the forward and reverse energy barriers for the proton transfer between the keto and enol forms. Thus, it has been found for 2-(2 -hydroxy-phenyl)imidazole (Scheme 17) that the energy dif-... 

Much effort has been directed at asphaltenes flocculation and precipitation in petroleum crude mixtures that is a significant problem in oil production, transmission and processing facilities. Pressure, temperature, the chemical composition of the oil and the amount of dissolved gases affect this undesired phenomenon. In general, the prediction of asphaltenes precipitation is very difficult and suffers from the definition of an asphaltene. Usually, asphaltenes are defined as the part of the crude oil that is soluble in methylbenzene and benzene but insoluble in pentane or heptane. Asphaltenes consist of many thousands of species, differing in size and chemical structure. The aromatic character of the asphaltenes and their content of heteroatoms influence their solubility in different solvents and the tendency to flocculate. In most of the calculations of the phase equilibria the pseudo-component method has been... 

The complex nature of coal as a molecular entity (2,3,24,25,35,37,53) has resulted ia the chemical explanations of coal combustion being confined to the carbon ia the system. The hydrogen and other elements have received much less attention but the system is extremely complex and the heteroatoms, eg, nitrogen, oxygen, and sulfur, exert an influence on the combustion. It is this latter that influences environmental aspects. 

Intramolecular heteroatom coordination may also influence the stabilities or structures of catenated tellurium compounds. For example, a rare example of a tritelluride, bis[2-(2-pyridyl)phenyl]tritelluride, is stabilized by a Te N contact of 2.55 The ditelluride (2-MeOCelFtCOTe) has an unusual planar structure. Although the C=0 Te interaction is longer (3.11 A) than the Me 0 contact (2.76 A), ab initio molecular orbital calculations indicate that the planarity results predominantly from the former intramolecular connection. 

The hydrogen abstraction addition ratio is generally greater in reactions of heteroatom-centered radicals than it is with carbon-centered radicals. One factor is the relative strengths of the bonds being formed and broken in the two reactions (Table 1.6). The difference in exothermicity (A) between abstraction and addition reactions is much greater for heteroatom-centered radicals than it is for carbon-centered radicals. For example, for an alkoxy as opposed to an alkyl radical, abstraction is favored over addition by ca 30 kJ mol"1. The extent to which this is reflected in the rates of addition and abstraction will, however, depend on the particular substrate and the other influences discussed above. 

Alkyl radicals, when considered in relation to heteroatom-centered radicals (e.g. r-butoxy, benzoyloxy), show a high degree of chcmo- and rcgiospecificity in their reactions. A discussion of the factors influencing the rate and rcgiospecificity of addition appears in Section 2.3. Significant amounts of head addition arc observed only when addition to the tail-position is sterically inhibited as it is in a,p-disubstituted monomers. For example, with p-alkylacrylates, cyclohexyl... 

Nearly all of the polymers produced by step-growth polymerization contain heteroatoms and/or aromatic rings in the backbone. One exception is polymers produced from acyclic diene metathesis (ADMET) polymerization.22 Hydrocarbon polymers with carbon-carbon double bonds are readily produced using ADMET polymerization techniques. Polyesters, polycarbonates, polyamides, and polyurethanes can be produced from aliphatic monomers with appropriate functional groups (Fig. 1.1). In these aliphatic polymers, the concentration of the linking groups (ester, carbonate, amide, or urethane) in the backbone greatly influences the physical properties. 

In general compounds with heteroatoms (N, O, S and P) are more amenable to fluorescence reactions" than pure hydrocarbons. Under the influence of the catalytic sorbents substances rich in Jt-electrons are formed, that conjugate to rigid reaction products that are fluorescent when appropriately excited. The formation of fluorescent derivatives is frequently encouraged by gassing with nitrogen or carbon dioxide. 

Of all the known sites for metal-ion binding to the heteroatoms of DNA bases, G-N3 is the most elusive. The adjacent 2-amino group is often considered to offer steric hindrance to binding at this site. However, while this undoubtedly influences the chemistry it does not preclude binding. The tri-metalated [ [Pt(N]3(9-Et G N1,N3,N7)]5 compound has for many years been the only structurally characterized example of an N3-coordinated guanine (66). A second example has now been reported, the tetranuclear octacation 16 (56). In this complex both the N7 and N3 atoms are bound to Pd2+ (Fig. 22). The molecule presents an interesting new architecture for a guanine-tetramer. Such structures are well known in DNA chemistry and are almost inevitably metal-ion stabilized (67,68). 

The electronic structures of furan, thiophene, and selenophene, their protonated complexes, and their anions have been calculated by the extended Hiickel method.6 The results of these calculations have been used to determine the influence of the heteroatom on the degree of aromaticity and electron density. 

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