Hydrocarbons substituents, reaction site

The reasons for the ewrfo-selectivity of Diels-Alder reactions are only useful for the reactions of dienophiles bearing substituents with lone pairs without a Lewis basic site no secondary orbital interactions are possible. But even in reactions of pure hydrocarbons the ewrfo-selectivity is observed, requiring alternative explanations. For example, the ewrfo-preference of the reactions of cyclopropene with substituted butadienes have been rationalized on the basis of a special type of secondary orbital interactions70. Apart from secondary orbital interactions which are probably the most important reason for the selec-tivities of Diels-Alder reactions, recent literature also advocates other interpretations. 

A thoughtful reader would have noticed that, while plenty of methods are available for the reductive transformation of functionalized moieties into the parent saturated fragments, we have not referred to the reverse synthetic transformations, namely oxidative transformations of the C-H bond in hydrocarbons. This is not a fortuitous omission. The point is that the introduction of functional substituents in an alkane fragment (in a real sequence, not in the course of retrosynthetic analysis) is a problem of formidable complexity. The nature of the difficulty is not the lack of appropriate reactions - they do exist, like the classical homolytic processes, chlorination, nitration, or oxidation. However, as is typical for organic molecules, there are many C-H bonds capable of participating in these reactions in an indiscriminate fashion and the result is a problem of selective functionalization at a chosen site of the saturated hydrocarbon. At the same time, it is comparatively easy to introduce, selectively, an additional functionality at the saturated center, provided some function is already present in the molecule. Examples of this type of non-isohypsic (oxidative) transformation are given by the allylic oxidation of alkenes by Se02 into respective a,/3-unsaturated aldehydes, or a-bromination of ketones or carboxylic acids, as well as allylic bromination of alkenes with NBS (Scheme 2.64). 

The influence of substituents on equilibrium ratios of arenium ions differing in the site of proton attachment can be described with a modified Hammett equation using constants. The experimental data on the basicities of methylated benzenes (see Table 2) has shown (cf. the reaction constant q (0°) of protonation in HF at 0 °C is —11.3. As the protonation of ifromatic hydrocarbons is, for a first approximation, and isentropic reaction it requires the fulfilment of... 

Direct amination by amide ion is possible with aromatic molecules which are susceptible to nucleophilic attack. This group includes many heterocyclic compounds, particularly derivatives of pyridine, quinoline and related ring systems, and also a number of benzenoid compounds containing appropriate electron-attracting substituents. These reactions may be conducted at elevated temperatures in di-methylaniline or a hydrocarbon for example, or at lower temperatmes in liquid ammonia. In the pyridine series, it is the electron-deficient a-position which is preferentially aminated, though substitution may occur if no a-site is available (reaction 6). The second stage... 

The concentration of zinc accelerator-thiolate complexes in the rubber is not the only factor determining the balance of the two reactions in NR. Both the rate of desulfuration of polysulfide crosslinks and the rate of their thermal decomposition depend upon the positions of attachment of the sulfur chains to the backbone rubber chains and the detailed structure of the hydrocarbon at the ends of the crosslinks. In the course of normal accelerated vulcanization there are three different positions of attack on the polyisoprene backbone two of these are methylene groups in the main chain (labelled d and a in 3), and the third is the side chain methyl group (labelled b in 3). Direct analysis of the distribution of the sites of attack cannot yet be made on actual rubber vulcanizates, and information has had to be obtained solely by sulfuration of the model alkene 2-methyl-2-pentene and, more recently, 2,6-dimethyl-2,6-octadiene. The former (4) models the a-methylic site but only one of the two a-methylenic sites of polyisoprene the latter (5) models all three sites, but at the present time these are not all supported by the synthesis of relevant sulfides. Because allylic rearrangements are common in subsequent reactions of the sulfurated rubber, sulfur substituents appear not only on allylic carbon atoms but on isoallylic carbon atoms. Thus, from 2-methyl-2-pentene, the groups shown in Scheme 2 are formed. 

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