Oxygen or Nitrogen Substitution

To study the effect of the introduction of heteroatoms, and therefore of increasing bond polarities, a database with 590 oxohydrocarbons [4] and one with 458 

When potential energy functions accounting only for dispersion interactions are used, the calculated amounts of (PE) are much smaller than the experimental sublimation energies at the same time, many short contacts in the first shell around O 

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The oxygen- or nitrogen-substituted allene adducts as described in the previous section are of major interest because of the possible subsequent cyclization leading to very valuable oxygen- or nitrogen-containing heterocydes (Scheme 8.25). The syn-... 

These reactions occur with similar rates to those carried out in dipolar aprotic solvents such as DMF or DMSO. An advantage of using the room-temperature ionic liquid for this reaction is that the lower reaction temperatures result in higher selec-tivities for substitution on the oxygen or nitrogen atoms. The by-product (sodium or potassium halide) of the reaction can be extracted with water and the ionic liquid recycled. 

The nonaromatic character of oxepins makes this structure susceptible to the reduction of one or more double bonds. 10-[(2-Aminoethyl)sulfanyl]-substituted dibenz[b,/]oxepins with various substituents in position 2 smoothly react to give the 10,1 l-dihydrodibenz[b,/]oxepin system 1 on treatment with magnesium in methanol.71 202 The advantage of this method is the possibility of reducing dibenz[b,/]oxepins with various heteroatoms such as sulfur, oxygen or nitrogen in the side chain. 

The methodology is useful for a variety of synthetic purposes. The cycloadditions are not subject to steric hindrance. Thus diyne cycloadditions to 2,5-disubstituted furans or pyrroles, followed by elimination of the oxygen or nitrogen bridges, provides an excellent, short route to peri-substituted arenes, as in the following examples 4 6 8... 

Xanthines substituted at the 8-position with an oxygen or nitrogen atom are oxidized with peracids to 4,5-dihydro-1,2,4-oxadiazoles (200) (Scheme 87) <940PP353>. 

Carbohydrate analogs in which a carbon atom substitutes the glycosidic oxygen (or nitrogen in the case of nucleosides) are defined as C-glycosides (Fig. 1). 

The characteristic SO2 stretch of thiadiazetidine dioxides occurs at 1160 and 1330 cm-1 (70BCJ3543, 62HCA717). Carbonyls in the 3-position absorb at 1750 cm-1, which is approximately the same place as (3 -lactams (62HCA717,79CB1012). Sulfur, unlike oxygen or nitrogen, does not raise the frequency of absorption when substituted for carbon in the /3 -lactam ring. 

The mechanism of the thermal ROP of (Cl2PN)3 has been proposed to involve a cationic mechanism (see Scheme 8.2 in Section 8.1.2.2). The unique reaction sequence involving ROP followed by nucleophilic substitution with oxygen- or nitrogen-based nucleophiles (generally alkoxides, aryloxides or primary amines) permits a diverse range of polyorganophosphazenes to be... 

The topic of nucleophilic attack at an sp2 carbon would be too wide a field to review in one chapter, if one were to discuss all facets and outcomes, some of which are shown in Figure 1. If the nucleophile is anionic an anion is formed, but if the nucleophile is neutral a zwitterion is produced. If X is oxygen or nitrogen, then the outcome can be carbonyl addition, substitution or a Darzens-type reaction. If the X is another carbon then addition leads to conjugate additions or polymerization. Alternatively the nucleophile could be expelled after rotation about the C—C bond to give overall isomerization. Both substitution and cyclization are also observed. 

The O atom and the NH group are iso-electronic this implies a certain similarity between their physico-chemical characteristics. In nitrogen-doped vitreous silicas, the =Si-0 and =Si-N -H (=Si-N -Si=) PC can be stabilized, in which an unpaired electron is localized on an electronegative atom (oxygen or nitrogen). Which of these states is thermodynamically more stable The enthalpies AH (OK) (kcal/mol) of reactions with the participation of fluorine-substituted compounds, which simulated groups in vitreous silica were calculated ... 

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