Heteroatom Bonds

Unsaturation corresponding to the maximum number of noncumulative double bonds. Heteroatoms have the normal valences given in Table 1.3. 

Both the LINK and the BOND constraints are ideal for specifying or excluding the chemistry at a particular bond. For example, one could easily exclude the formation of peptide bonds between specific components to generate peptidomimetics. However, the most prevalent use is to eliminate bonded heteroatoms or other unstable chemical species. 

Heteroatoms (such as O, S, N) that are double-bonded to other atoms in a ring can t donate lone-pair electrons to the pi system because their p-electrons are already involved in the double bond. Single-bonded heteroatoms can donate a single lone-pair to the pi system but not two, because one lone-pair must be in an unhybridized p orbital orthogonal (at 90 degrees) to the sp ring plane. 

The reaction (equation 76) of the hexenyl radical 47 forming cyclopentyl-methyl radical was discovered independently in several laboratories and has been of pervasive utility in both synthetic and mechanistic studyThe competition between formation of cyclopentylcarbinyl and cyclohexyl radicals favors the former even though the latter is more stable, and this kinetic preference is explained by more favourable transition state interaction. The effects of substituents on the double bond, heteroatoms in the chain, and many other factors on the partitioning between these two paths have been examined. In the gas phase above 300°C, methylcyclopentane has been observed to form cyclohexane via isomerization of cyclopentylmethyl radicals into the more stable cyclohexyl radicals. ... 

Polymers with saturated bonds, heteroatoms, heterostructures and poly-conjugated ones are available now as photosensitive materials. Really one cannot expect a single mechanism to be reponsible for photoconductivity in so many diverse systems. However, there are a lot of verified factors which permit us to explain the main features of the photoconductive processes in polymers. The status and prospects of the application of polymeric photoconductors as prospective new electronic materials will be also analyzed for various types of photoconductors. 

Polymers with Conjugated Bonds, Heteroatoms and Heterocycles in the Backbone Chain... 

Generation of electrophilic species from organic polyfluorinated materials usually requires direct interaction of a strong Lewis acid with a substrate, which can result in formation of a cation either by abstraction of a halogen anion (usually F ) from a substrate (Eq. 8) or by formation of a zwitterionic intermediate as a result of coordination of the Lewis acid with an unshared electron pair on a multiply bonded heteroatom (Eq. 9) [12] ... 

Wolfe23 defines the gauche effect as the tendency of molecules to adopt a structure maximizing the number of gauche interactions between adjacent lone pairs and/or polar bonds. Heteroatoms are assumed to be sp3 hybridized, thus having equivalent lone pairs (22). This phenomenological definition is unsatisfactory because it groups three different situations under the same title. 

B. Attack at Non-Carbonyl Multiple-Bonded Heteroatoms 19-42 Reduction of the Carbon-Nitrogen Double Bond... 

It is the maximum difference in charge between a hydrogen which can be donated and its covalently bonded heteroatom. 

In general, groups that have multiply bonded heteroatoms may act as electron sinks, because the multiple bond may cleave heterolytically and place the extra electron pair on the heteroatom. 

Heteroatom-Carbon Double Bonds , Heteroatom-Carbon Triple Bonds Heterocumulenes Conjugate Acceptors Triply Bonded and Allenic Conjugate Acceptors... 

Modification of organosilicon sorbents by metallic complexes in the gelation stage leads to the formation of a separate group of metal-containing FPS. Depending on the nature of the complex and a number of other factors the central metal atoms may or not become incorporated in the siloxane chain. In the first case a siloxane chain will contain chemically bonded heteroatoms, and the sorbents themselves can be of types A and B. In the second case, the nature of binding of the complex in a sorbent body may be quite different. 

Unsubstituted heteroatom-bonded heteroatom Nitrogen and sulfur mustard Polycyclic aromatic systems (PAH)... 

CEP matrix allows to differentiate multiple bonds, heteroatoms, and connectivities. Three molecular descriptors were derived from the CEP matrix ... 

The electrochemistry of o-bonded heteroatom dications (35) has been studied by cyclic voltammetry in acetonitrile. Only a one-reversible reduction peak was observed at - - 0.11 V <90TL6375>. 

The transformation of 1,2-azoles carrying, at C-3, a side-chain of three atoms terminating in a doubly-bonded heteroatom, into isomeric systems with a new five-membered ring is a general process, though there is no definitive view as to the details of its mechanism. 

Alkenes and Doubly Bonded Heteroatoms. One might expect that a double bond in a long hydrocarbon chain, such as a fatty acid, could be located by mass spectrometry since formation of allyl ions would be preferred ... 

The McLafferty Rearrangement. In many compounds containing a doubly bonded heteroatom (C=X), a hydrogen will transfer to X from the third carbon down the chain from C=X, with the loss of an olefin. The process is illustrated for 2-hexanone ... 

Figure 10.25 Twenty-nine specific toxicophores for mutagenicity as identified by Kazius el al. (Kazius, J-, et al. Derivation and validation of toxicophores for mutagenicity prediction. J. Med. Chem. 2005, 48, 312-320.) (A) Specific aromatic nitro, (B) specific aromatic amine, (C) aromatic nitroso, (D) alkyl nitrite, (E) nitrosamine, (F) epoxide, (G) aziridine, (H) azide, (I) diazo, (J) triazene, (K) aromatic azo, (L) unsubstituted heteroatom-bonded heteroatom, (M) aryl hydroxylamine, (N) alkyl halide, (O) acyl halide, (P) N- or 5-mustard, (Q) polycyclic aromatics, (R) bay-region, (S) K-region, (T) sulphonate-bonded C, (U) unsaturated aldehyde, (V) alkyl A-nitro, (W) diazonium, (X) p-propiolactone, (Y) unsubstituted a,p unsaturated alkoxy, (Z) l-aryl-2-monoalkyl hydrazine, (AA) aromatic methylamine, (AB) aryl hydroxylamine ester, and (AC) polycyclic planar system.

PE), the kinetic chain are transmitted preferably via a carbon atom (in y-position), in other polymers (with unsaturated bonds, heteroatoms, phenyl rings, etc.) the transfer of kinetic chains are statistically.In oxidizing polypropylene along with the main hydroperoxide ... 

Heteroatoms lone pairs and polar bonds The stereoelectronic component of conformational effects in the previous section was largely based on the relatively subtle differences between the donor and acceptor abilities of C-H and C-C bonds. Heteroatoms introduce a new array of stereoelectronic effects associated with the presence of polar bonds and lone pairs. These effects can be significantly stronger than the above C-H and C-C interactions, especially when lone pairs (good donors) and (good acceptors) can directly interact with each other. 

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