Saturated systems

Data are available on the 13C spectra of saturated six-membered ring systems (B-79MI20101). The chemical shifts of the ot-, 3- and y-methylene carbon atoms (76JA3778) of compounds of type (41) are summarized in Table 15. 

A nitrogen atom at X results in a variable downfield shift of the a carbons, depending in its extent on what else is attached to the nitrogen. In piperidine (41 X = NH) the a carbon signal is shifted by about 20 ppm, to ca. 8 47.7, while in yV-methylpiperidine (41 X=NMe) it appears at 8 56.7. Quatemization at nitrogen produces further effects similar to replacement of NH by Aralkyl, but simple protonation has only a small effect. W-Acylpiperidines (41, X = NCOR) show two distinct a carbon atoms, because of restricted rotation about the amide bond. The chemical shift separation is about 6 ppm, and the mean shift is close to that of the unsubstituted amine (41 X = NH). The nitroso compound (41 X = N-NO) is similar, but the shift separation of the two a carbons is somewhat greater (ca. 12 ppm). The (3 and y carbon atoms of piperidines, W acyl piperidines and piperidinium salts are all upheld of the cyclohexane resonance, by 0-7 ppm. 

The ether oxygen of tetrahydropyran (41 X = 0) induces a large downfield shift of the a carbons, while the (3 and y carbons move slightly upfield, the y more noticeably. 

When two heteroatoms are present in a saturated six-membered ring their effects are approximately additive to within 5 ppm. Observed shifts for a few representative examples are shown in structures (44)-(48). 

The one-bond coupling ( 7C, ) for cyclohexane (an average of couplings to axial and equatorial protons) is 123 Hz, and is increased by substitution adjacent to the carbon by an electronegative element, as with the aromatic systems discussed above. 

3 SILICON-, GERMANIUM- AND TIN-NITROGEN RINGS 10.3.1 Saturated Systems 

A significant difference between cyclosilazanes, e.g. (Me2SiNH)3 and the isoelectronic cyclosiloxanes (Me2SiO)3, is the presence of the reactive N-H 

Both germanium and tin form saturated ring systems of the type (R2MNR ) that are synthesised in a similar manner to cyclosilazanes however, they are limited to four- and six-membered rings (n = 2 or 3, respectively). Transamination reaetions with the elimination of a volatile amine are also used to prepare eyelostannazanes [eqn (10.22)].  

The reactivity of the tin-nitrogen bond in eyelostannazanes towards orga-noelement halides may be used to prepare other inorganie heteroeyeles, e.g. B2N2 rings [eqn (10.23)].  

Current interest in saturated Si-N rings stems primarily from the industrial uses of silicon nitride (and related materials), which is a hard, chemically resistant insulator, and as precursors to silicon-nitrogen polymers (poly-silazanes). The formation and precursor chemistry of these polymers are discussed in Section 10.3.3. 

Groundwater can be found in the traditional sense at the water table below which the soil pore spaces are essentially saturated and the water is free to move, and in the unsaturated zone (or vadose zone) above the water table. It is possible for water to migrate through both of these zones, transporting dissolved components (or contaminants). The interaction of the various forces involved will determine the direction and rate of migration. 

Water table is a traditional term that describes the level at which water surface will stabilize when it freely enters a boring (or well) at atmospheric pressure. Below this level the water occupies all the pore spaces between mineral grains, cracks, and other openings (i.e., fractures, joints, cavities, etc.), and the formation is considered 

All voids in the subsurface medium are classified as porosity. When pore spaces are interconnected so that water can flow between them, the medium is said to be permeable. The actual openings that permit water flow are referred to as effective porosity. Effective porosity is calculated as the ratio of the void spaces through which water flow can occur to the bulk volume of the medium (expressed as a percentage) as follows  

Porosity that includes the voids between mineral (or soil) grains is referred to as primary porosity. When the porosity is the result of cracks, fractures, or solution channels, it is known as secondary porosity. The porosity of soft clay is often over 50%, but clay typically has low permeability because the pores are either not interconnected or are too small to permit easy passage of water. On the other extreme, nonfracture igneous rock often has a porosity of less than 0.1% but, again, low permeability. 

FIGURE 3.9 Cubic packing of spheres with a porosity of 47.65% (A) vs. rhombohedral packing of spheres with a porosity of 25.95% (B). 

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The formation of the (X-delocalized norbornyl cation via ionization of 2-norbornyl precusors in low-nucleophilicity, superacidic media, as mentioned, can be considered an analog of an intramolecnlar Friedel-Crafts alkylation in a saturated system. Indeed, deprotonation gives nortricyclane,... 

Two air-saturation systems suited for use in water treatment are shown in Figure 15 (31). Such mechanisms faciHtate the release of air that generates much finer bubbles than mechanical air dispersion methods used in mineral flotation practice. 

Fig. 15. Ain-saturator systems used in the practice of water treatment by flotation (31). (a), Injection of air into suction line of recycle pump (b), packed...

Physical and chemical properties of the three most important forms of sodium sulfate are summarized ia Table 3. The solubiUty of sodium sulfate ia water from 0 to 360°C is shown ia Figure 1 (5). The solubiUty of the NaClNa2S04-H2 0-saturated system is also shown. The aqueous solubiUty of sodium sulfate changes rapidly from 0 to 40°C, and addition of NaCl to a saturated solution of Na2S04 dramatically suppresses this solubiUty. These two effects are exploited by all manufacturers of sodium sulfate. 

The method has also been applied to partially saturated systems. For instance, the dipole moments of a series of 1-acylpyrazolines (42) with R = H, Me, Et and Ph have been measured (72CHE445) they range from 3.46 to 4.81 D. When compared with values computed by the fragmentary calculation method, the conclusion was reached that here also the E form predominates. In all these examples the lone pair-lone pair repulsions determine the most stable conformation. 

Nucleophilic attack on ring atoms of large heterocycles is largely confined to saturated systems, saturated parts of partially unsaturated systems, and to carbonyl functions and the like. These reactions are not fundamentally different from those of corresponding acyclic systems, except for transannular reactions. 

The examples that have been presented in this section illustrate the approach that is used to describe structure and reactivity effects within the framework of MO description of structure. In the chapters that follow, both valence bond theory and MO theory will be used in the discussion of structure and reactivity. Qualitative valence bond terminology is normally most straightforward for saturated systems. MO theory provides useful insights into conjugated systems and into effects that depend upon the symmetry of the molecules under discussion. 

Quite a number of mixed sulfur-nitrogen macrocycles have been prepared, but these have largely been by the methods outlined in Chaps. 4 and 5 for the respective heteroatoms. An alternative method, involves the formation of a Schiff base, followed by reduction to the fully saturated system, if desired. An interesting example of the Schiff base formation is found in the reaction formulated in (6.12). Dialdehyde 14 is added to ethylenediamine in a solution containing ferrous ions. Although fully characterized, the yield for the reaction is not recorded. To avoid confusion with the original literature, we note the claim that the dialdehyde [14] was readily prepared in good yield by reaction of the disodium salt of 3-thiapentane-l, 5-diol . The latter must be the dithiol rather than the diol. 

The rotation of an axial CHjI group [cf. (3)] in a polycyclic saturated system is severely hindered by axial substituents on the same face therefore preferred orientations for both an 18- (Figure 12-1) and I9-CH2I (Figure 12-2) in steroids can be written ... 

The following discussion of hydroxamic acids includes saturated systems, e.g., 2, compounds such as 3, derived from aromatic systems, 7V-hydroxyimides such as 7V-hydroxyglutarimide (78), and certain of their derivatives including thiohydroxamic acids. Naturally occurring cyclic hydroxamic acids are discussed to show the range of structural types that has been found, hut macrocyclic polyhydroxamic acids are mentioned very briefly, because several comprehensive reviews of these compounds are already available. The main purpose of this review is to summarize the methods available for the synthesis of cyclic hydroxamic acids, to outline their characteristic reactions, and to present some useful physical data. Their synthesis and some biological properties have previously been reviewed by Coutts. ... 

The classical age of preparative organic chemistry saw the exploration of the extensive field of five-membered heterocyclic aromatic systems. The stability of these systems, in contrast to saturated systems, is not necessarily affected by the accumulation of neighboring heteroatoms. In the series pyrrole, pyrazole, triazole, and tetrazole an increasing stability is observed in the presence of electrophiles and oxidants, and a natural next step was to attempt the synthesis of pentazole (1). However, pentazole has eluded the manifold and continual efforts to synthesize and isolate it. 

When the reaction was followed in an nmr probe, the appearance and disappearance of two intermediates 66 and 67 was observed along with the buildup of product 65. The rate of reaction of 64 at 75° in formic acid, k = 3xl0" sec, is six times faster than the rate of the corresponding saturated system, 2,2-dimethyl-1-pentyl tosylate, k = 5xl0" sec, under identical conditions. If the inductive rate retarding effect of the triple bond is taken into account, then the calculated rate enhancement resulting from triple-bond participation in the solvolysis of 64 is about 3000(81). The... 

In the liver, the bilirubin is removed from albumin and taken up at the sinusoidal surface of the hepato-cytes by a carrier-mediated saturable system. This facilitated transport system has a very large capacity, so that even under pathologic conditions the system does not appear to be rate-limiting in the metabolism of bilirubin. 

The most problematic cases for the Stille reaction involve coupling saturated systems. The tendency for p-elimi nation of alkylpalladium compounds requires special conditions. Zh. v-(dialkylamino)cyclohexylphosphincs have shown considerable success... 

Too small a value of xc means too large a Kc and therefore saturation. System response is subject to imperfect pole-zero cancellation. 

Geminal chlorine or bromine substituents deshield vinylic fluorine significantly, whereas a vicinal chlorine substituent shields the fluorine, much as was the case for the saturated systems. Again similarly, a second vicinal chlorine substituent reverses the trend and shifts the fluorine signal downfield (Scheme 3.45). 

Heibst, E. In Atomic, Molecular, Optical Physics Handbook Drake, G., Ed. AIP Press New Yoik, 1996, p 429 Adams, N. G. In Atomic, Molecular, Optical Physics Handbook Drake, G., Ed. AIP Press New Yoik, 1996, p 441. For three-body systems, a slightly more complex temperature dependence is observed. For saturated systems, more complex treatments are needed —see Gilbert, R. G. Smith, S. C. Theory erf Unimolecular and Recombination Reactions Blackwell Oxford, 1990. 

Delocalisation takes place (cf. 1,3-dienes, p. 13), so that an electron-deficient atom results at C3, as well as at C, as in a simple carbonyl compound. The difference between this transmission via a conjugated system, and the inductive effect in saturated system, is that here the effect suffers much less diminution by its transmission, and the polarity at adjacent carbon atoms alternates. 

Note that (1) in saturated systems proton-proton couplings are seldom observed beyond three bonds, but (2) in aromatic and heterocyclic systems, four- and even five-bond coupling is commonplace. This is because spin coupling is transferred by electrons. Where you have extended conjugation, you can expect to observe coupling over a greater number of bonds. 

Reduced derivatives of all these parent structures (dihydro, tetrahydro, and fully saturated systems) increase greatly the number of possible compounds. Oxo compounds are also possible, and each of the nine parent structures has five different positions where the carbonyl group could be located. 

Several methods based on radical cyclizations have been developed and applied to the synthesis of the fully saturated system. An example (Scheme 11) is provided by the tandem radical cyclization of properly substituted imines 33 obtained from tu-benzeneselenylamines and aldehydes <1995TL5623>. 

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