Heteroatom-containing systems

Negative hyperconjugation lone pair/x-donor and a-acceplor 

Similar effects are also present in C=X moieties where X is a chalcogen (i.e. carbonyl and thiocarbonyl moieties for X=0, S) but, in these systems, such interactions do not have the stereodifferentiating consequences because of the symmetry of the p-lone pair of X. Such interactions will be discussed here only briefly. We will focus the next part on nitrogen-containing compounds. 

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Scheme 5 Example applications of spin parity to heteroatom-containing systems.

The concept of aromaticity is not restricted to hydrocarbons. Heterocyclic systems, whether of the pyrrole type 1.38 with trigonal nitrogen in place of one of the C=C double bonds, or of the pyridine type 1.39 with a trigonal nitrogen in place of a carbon atom, are well known. The n orbitals of pyrrole are like those of the cyclopentadienyl anion, and those of pyridine like benzene, but skewed by the presence of the electronegative heteroatom. The energies and coefficients of heteroatom-containing systems like these cannot be worked out with the simple... 

The first calculations of the frontier orbitals for acrolein gave the HOMO coefficients on the C=C double bond of acrolein, with the a carbon having the larger coefficient. This failed to explain the regiochemistry, but only because the simple Hiickel theory that was used is notoriously weak in dealing with electron distribution in heteroatom-containing systems. Later calculations gave a better set of coefficients, as shown in Fig. 6.29. 

As a final point, we note that what can be a small perturbation with an appropriate electron-donating or electron-withdrawing substituent can be an extreme perturbation when heteroatoms are part of the ji-system. It is tempting to apply these rules, for heteroatom-containing systems also. Typical examples would... 

It is also assumed that heterocyclic nitrogen plays an important role in thermolysis of resins and asphaltenes, by which the first reactions involve thermolysis of aromatic alkyl bonds. Secondary reactions are aromatization of naphthenic species and condensation of aromatic rings that activates the coke formation. Thus, the initial step in the coke formation from heavier fractions (resins and asphaltenes) is the formation of volatile hydrocarbons and nonvolatile heteroatom-containing systems. Products obtained are insoluble in the surrounding medium enhancing the carbonization to finally form coke (Magaril and Aksenova, 1968 ... 

Heteroatom-containing derivatives of pentalene and azulene are 8- and 10-jr-electron systems, respectively, and have been extensively studied by Hafner and co-workers.301-304 A few examples are presented in Schemes 30, 47, and 86. Electron-rich azapentalenes 247—249305 306 and various aza-azu-lenes 250—252300-302 have intense colors and are stable but oxygen-sensitive compounds. Azulenes with nitrogen heteroatoms placed in positions with high electron density, such as position 5, or positions 5 and 7, present a hypsochromic effect, while a heteroatom in position 6, with low electron density, exerts a bathochromic effect. When both types of positions are substituted by heteroatoms, their effect is canceled out.307 The bond lengths in 252 (Scheme 86), as determined by X-ray, indicate delocalization as in 252a and 252b.308 309... 

Lithiation of heteroaromatic compounds, especially nitrogen-containing systems, is an important methodology, because the lithiated intermediates can be easily functionalized in a regioselective manner by reaction with electrophiles. This process can be performed by deprotonation only at the ortho position of the heteroatom, but obviously the reaction is not possible at other positions . 

NMR spectroscopy ( Se, /= 1/2, 7%) is a powerful technique for identifying cyclic selenium molecules, especially the heteroatomic ring systems that contain sulfur or tellurium in addition to selenium, for which several isomers are possible for most compositions (Section 12.1.2). Solutions of monoclinic selenium in CS2 have been shown by high-performance liquid chromatography to form an equilibrium mixture of cyclo-Seg, cyclo-Sey and cyclo-Se6. The Se NMR spectra of such solutions show two singlets that are attributable to cyclo-Seg and cyclo-Se with relative intensities that correspond to a molar ratio of ca 6 No resonance is observed for cyclo-Sey presumably as a result of the fluxional behaviour (pseudorotation) of the seven-membered ring (Section 12.1.2). 

Fig. 4.1 illustrates the first few members of the series of neutral polyenes the equilibria between butadiene 4.1 and cyclobutene 4,2, between hexadiene 4,3 and cyclohexadiene 4.4, and between octatetraene 4.5 and cyclooctatriene 4,6. There are of course heteroatom-containing analogues, with nitrogen or oxygen in the chain of atoms, and the systems can be decked out with substituents and other rings. To appreciate what the fundamental reaction is, it is only necessary to tease out the components—the longer conjugated... 

Fig. 4.2 illustrates the first few members of the series of equilibria of conjugated ions. In cations, they are the equilibria between the allyl 4.11 and the cyclopropyl cation 4.12, the pentadienyl 4.13 and the cyclopentenyl cation 4.14, and the heptatrienyl 4.15 and cycloheptadienyl cation 4.16, In anions, they are between the allyl 4.17 and the cyclopropyl anion 4.18, the pentadienyl 4.19 and the cyclopentenyl anion 4.20, and the heptatrienyl 4.21 and cycloheptadienyl anion 4.22. There are heteroatom-containing analogues, with nitrogen and oxygen lone pairs rather than a carbanion centre, and the systems can again have substituents and fused rings. 

An efficient and convenient methodology for the aerobic oxidation of alcohols catalysed by sol-gel trapped perruthenate and promoted by an encapsulated ionic liquid in supercritical carbon dioxide solution has been reported. The reaction is highly selective and useful for substrates otherwise difficult to oxidize.263 A four-component system consisting of acetamido-TEMPO-Cu(C104)2-TMDP-DABCO has been developed for aerobic alcohol oxidation at room temperature. The catalytic system shows excellent selectivity towards the oxidation of benzylic and allylic alcohols and is not deactivated by heteroatom-containing (S, N) compounds. The use of DMSO as the reaction medium allows the catalysts to be recycled and reused for three runs with no significant loss of catalytic activity.264... 

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