Six-membered rings, characteristics

Many common explosives are organic molecules that contain nitro (NO2) or nitrate (NO3) groups attached to a carbon skeleton. The Lewis structures of two of the most familiar explosives, nitroglycerin and trinitrotoluene (TNT), are shown here (resonance structures are not shown for clarity). TNT contains the six-membered ring characteristic of benzene. 

The bonding arrangement in benzene confers special stabihty to the molecule. As a result, millions of organic compounds contain the six-membered ring characteristic of benzene. Many of these compounds are important in biochemistry, in pharmaceuticals, and in the production of modern materials. 

The replacement of carbon by other elements produces changes in several structural parameters and consequently affects the conformational characteristics of the molecule. In this section, we will first describe some stereochemical features of heterocyclic analogs of cycloalkanes. For the purpose of elaborating conformational principles, the discussion will focus on six-membered rings, so that the properties may be considered in the context of a ring system possessing a limited number of low-energy conformations. 

Fluorinated cyclobutanes and cyclobutenes are relatively easy to prepare because of the propensity of many gem-difluoroolefins to thermally cyclodimerize and cycloadd to alkenes and alkynes. Even with dienes, fluoroolefins commonly prefer to form cyclobutane rather than six-membered-ring Diels-Alder adducts. Tetrafluoroethylene, chlorotrifluoroethylene, and l,l-dichloro-2,2-difluoroethyl-ene are especially reactive in this context. Most evidence favors a stepwise diradical or, less often, a dipolar mechanism for [2+2] cycloadditions of fluoroalkenes [S5, (5], although arguments for a symmetry-allowed, concerted [2j-t-2J process persist [87], The scope, characteristic features, and mechanistic studies of fluoroolefin... 

The Se NMR chemical shifts of Se-N compounds cover a range of >1500 ppm and the value of the shift is characteristic of the local environment of the selenium atom. As a result, Se NMR spectra can be used to analyse the composition of a complex mixture of Se-N compounds. For example, Se NMR provides a convenient probe for analyzing the decomposition of selenium(IV) diimides RN=Se=NR e.g., R = Bu). By this method it was shown that the major decomposition products are the six-membered ring (SeN Bujs, the five-membered ring Sc3(N Bu)2 and fifteen-membered ring Sc9(N Bu)6 (Figure 3.2 and Section 6.3). 

Sulfanuric halides contain the characteristic group -N=S(0)X- (X = Cl, F). Unlike the isoelectronic cyclophosphazenes (NPClajx (x = 3-17), " only six-membered rings have been well characterized. The sulfanuric halides are colourless solids (X = Cl) or liquids (X = F), which are stable in dry air. Sulfanuric chloride [NS(0)C1]3 is best prepared by treatment of SOCI2 with sodium azide in acetonitrile at -35°C (Eq. 8.16). It may also be obtained as a mixture of a- and yS-isomers in a two-stage reaction from H2NSO3H and PCls. The fluoride [NS(0)F]3 is formed as a mixture of isomers by the fluorination of [NS(0)C1]3 with Sbp3. ... 

Unlike nickel Catalysts, palladium complexes do not catalyze the homo-cyclization reaction to give CDT or COD. The difference seems to be due to a different degree of hydride shift and atomic volume. With palladium catalysts, the hydride shift is easier, and hence linear oligomers are formed. The characteristic reaction catalyzed by palladium is the cocyclization of two moles of butadiene with one-hetero atom double bonds such as C=N and C=0 bonds to give six-membered rings with two vinyl groups (19) ... 

The monosaccharides have a couple of characteristics that prove to be important in terms of their structure and function. They all have at least one carbon that is bonded to four different groups (a chiral carbon), and most form five-and six-membered rings easily. The presence of chiral carbons allows these compounds to exist as two different optical isomers that are nonsuperimposable mirror images of each other. (Your feet are nonsuperimposable mirror images. Try putting a left shoe on a right foot )... 

Five- and six-membered rings formed by coordination of diamines with a metal ion have the stereochemical characteristics of cyclopentane and cyclohexane. The ethylenediamine complexes have puckered rings and the trimethylenediamine complexes have chair conformations. The methylene protons are nonequivalent in these nonplanar conformations, taking on the character of equatorial and axial substituents. They are made equivalent as the result of rapid conformational inversion at room temperature, just as in the alicyclic compounds (Fig. 7.1). This has been observed in nmr studies of planar and octahedral complexes of ethylenediamine-type ligands with a number of metals. 

In addition to the characteristic CH stretching VCD, a number of molecules that are strocturally similar to 3-methylcyclohexanone exhibit characteristic ROA features in the skeletal region below 700 cm. Figure 6 (11). In particular, a bisignate couplet near 500 cm is observed in six-membered ring compounds with a carbonyl substituent [(-F)-pulegone(73), (-F)-camphor (74), (-t-)-3-brom-ocamphor (74), (+)-nopinone (75)] and a broad low frequency couplet is observed in ketones with a 3-methyl substituent (75). The ROA of 3-methylcyclohexanone exhibits a third characteristic couplet near 400 cm. In all cases the sense of the observed couplets correlates with the absolute configuration of the most stable chair conformation. 

As we prcx eed further with the process of generating hydrocarbons from carbon frameworks represented on the buckminsterfullerene surface, we encounter a new class of compounds known as semibuckminsterfullerenes. These Cjq hydrocarbons (Figure 5), which we will also refer to as buckybowls , represent one-half of the buckminsterfullerene Cgo surface, and share several common characteristics including 30 sp carbon frameworks consisting of multiple fused five- and six-membered rings and bowl-shaped geometries. 

Another effect of the high steric hindrance in dithiin 25 due to the 3,3 -dibornane skeleton is the fact that this dithiin can be oxidized readily (with 1 mol of t-chloroperbenzoic acid (MCPBA) at 0°C) to sulfoxide 26 <1995T13247, 1994TL1973> (Scheme 30), a stmcture which is remarkably stable in contrast with the usual characteristics of sulfoxides in the dithiin series. The six-membered ring interconversion of the sulfoxide 26, slowed obviously by the steric bulk hindrance, was investigated by dynamic NMR (AG ca. 10-11 kcalmoP ). With an excess of MCPBA at elevated temperature, the sulfoxide 26 is oxidized to the sulfone 27 parallel treatment with oxygen, however, failed to yield the same compound. 

The insertion of acetylene derivatives might also be utilised in the preparation of six membered rings. A characteristic distinction between such processes and olefin insertion is the fact, that the intermediate formed by the insertion of an acetylene into the palladium-carbon bond is unable to undergo /2-hydride elimination, therefore the concluding step of these processes is usually reductive elimination. 

Halogen atoms attached to ring carbon of heteroaromatic six-membered rings show both reactions typical of aryl halides and their own characteristic reactions. 

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