Partially and fully saturated compounds

The determination of the bond lengths of the fully saturated heterocycles has been complicated by their conformational mobility, which is considered in Section 2.3.4.3.2. The data which have been provided by electron diffraction are listed in Table 3 and show the expected trends consonant with increasing size of heteroatom. 

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In this chapter we summarize the reactivity of the six-membered heterocyclic compounds. We describe first the aromatic compounds, where aromatic is defined as fully conjugated round the ring, and then the partially and fully saturated compounds. Within each of these sections we discuss first the reactivity at the ring atoms, then the reactivity of substituent groups. 

Most compounds discussed in this chapter are partially and fully saturated bicycles and their benzo analogs. These groups of partially and fully saturated bicycles also include compounds having exomethylene, imino, and/or, most frequently, carbonyl groups on carbon atoms that have sp3 hybridization in the parent bicycles. 

Compounds are generally classified according to their fully unsaturated parent compound (but see below). Thus substituted, partially saturated, and fully saturated derivatives of, for example, pyrrole are all indexed under pyrrole. Benzo and similar derivatives are included under the most unsaturated parent system (e.g., quinoline, thienofuran, etc.). For any given heterocyclic parent only one indicated hydrogen isomer appears in the text, typically the most stable or the lowest numbered form thus all instances of pyran, whether of the 2H- or 4H-form, are indexed under 2H-pyran. The charges and additional valences for any heterocyclic parent structure are not indicated. 

Isoxazoles and their partially or fully saturated analogs have mainly been prepared, both in solution and on insoluble supports, by 1,3-dipolar cycloadditions of nitrile oxides or nitrones to alkenes or alkynes (Figure 15.10). Nitrile oxides can be generated in situ on insoluble supports by dehydration of nitroalkanes with isocyanates, or by conversion of aldehyde-derived oximes into a-chlorooximes and dehydrohalogenation of the latter. Nitrile oxides react smoothly with a wide variety of alkenes and alkynes to yield the corresponding isoxazoles. A less convergent approach to isoxazoles is the cyclocondensation of hydroxylamine with 1,3-dicarbonyl compounds or a,[3-unsatu-rated ketones. 

Significant amounts of partially or fully saturated ring compounds were found in the jet and diesel products. Mono- and dicyclic alkanes were a major portion of the saturate fraction... 

In vacuum gas oils and other 650°F-plus (343°C-plus) material, more than half of the carbon atoms may be found in ring compounds - polyaromatic hydrocarbons (PAH), partially saturated PAH (naphthene-aromatics) and fully saturated naphthenes. According to Qader and McOmber and Lapinas, et al., ° hydrocracking converts complex ring compounds into light products by the following sequence of reactions ... 

Consequently, the bond is fully saturated for A sp = 0 with a bond order of 1, but it is only partially saturated by the time the gap closes for AEap/2 h = 1 (cf eqn (7.92)) when the bond order equals 0.76. This simple second moment model has been extended to include the compound semiconductors. The resultant values of the bond order are given in Table 7.2. We see that the bonds in tetrahedral carbon and silicon are almost fully saturated, but those in zinc selenide and cadmium telluride are only about 75% saturated due partly to the mismatch in the sp orbitals between chemically distinct atoms. 

Hydrogenation—any reaction of hydrogen with an organic compound. In the case of fats and oils, this is the direct addition of hydrogen to double bonds of unsaturated molecules, resulting in a partially or a fully saturated product. 

The partially reduced thiopyrans are 3,4-dihydro-2//-thiopyran 20 and 3,6-dihydro-2//-thiopyran 21 and the fully reduced compound is tetrahydrothiopyran or thiane 22. The benzologues are 3,4-di hydro-2//-1 -benzothiopy ran or thiochroman 23 and 3,4-dihydro-l//-2-benzothiopyran or isothiochroman 24. The fully saturated derivatives of 23 and 24 are known as thiadecalin and isothiadecalin, respectively. 

As with the fully and partially conjugated compounds, the chemistry of the fully saturated ring is dictated by the heteroatom s chemistry. Typical reactivity, for example, formation of anions (Section 2.16.10.1) and 1,1 -biheteroles (Section 2.16.10.2), is displayed by these heterocyclic compounds. 

This chapter details the nomenclature, thermodynamics, reactions, and synthesis, and summarizes the experimental structural methods and applications in the following order (i) unsaturated, partially saturated, fully saturated monocyclic azocines, (ii) benzazocines, and (iii) dibenzazocines. The nitrogen bridged systems and unique compounds are not given detailed treatment owing to space limitations. 

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