Delocalization Aromaticity

The stability of isothiazole derives from the fact that it has an aromatic delocalized ir-electron system. The NMR chemical shifts, which depend, inter alia, on ring currents, and the high stability of the molecular ions in mass spectrometry, are typical of aromatic compounds, and X-ray measurements confirm the partial double bond character of all the bonds of the ring. 

Replacing an a-alkyl snbstituent by an a-aryl group is expected to stabilize the cationic center by the p-Jt resonance that characterizes the benzyl carbocations. In order to analyze such interaction in detail, the cumyl cation was crystallized with hexafluoroantimonate by Laube et al. (Fig. 13) A simple analysis of cumyl cation suggests the potential contributions of aromatic delocalization (Scheme 7.3), which should be manifested in the X-ray structure in terms of a shortened cationic carbon—aromatic carbon bond distance (C Cat). Similarly, one should also consider the potential role of o-CH hyperconjugation, primarily observable in terms of shortened CH3 distances. Notably, it was found experimentally that the Cai distance is indeed shortened to a value of 1.41 A, which is between those of typical sp -sp single bonds (1.51 A) and sp -sp double bonds (1.32 A). In the meantime, a C -CH3 distance of 1.49 A is longer than that observed in the tert-butyl cation 1 (1.44 A), and very close to the normal value for an sp -sp single bond. 

Quantitative assessment of the electrophilic character of various types of phosphenium ions has been attempted using computational studies on hydride and halide exchange reactions, and the results attribute to 1,3,2-diazaphospholenium ions a lower electrophilicity (and thus higher stability) than other types of phosphenium ions [20, 66], The gain in stability due to aromatic -delocalization is predicted to be somewhat larger than inductive stabilization resulting from exhaustive A-alkylation of the parent diaminophosphenium ion, [P(NH2)2]+. 

Deprotonated I H- 1,2,4]diazaphospholes, exhibit aromatic delocalization by virtue of 6ir electrons like cyclopentadienyl ligand in the anionic ring, which, in addition to the donor lone pairs on N or/and P heteroatoms, makes them promising ligands with varied coordination modes. However, till the last decade, they had... 

Ring strain energies30) and aromatic delocalization energies must also be parameterized and calculated. This is handled automatically in EROS and makes use of the ring perception routine mentioned earlier15). 

Fragments in compounds 155—157 exhibit aromatic bond delocalization. The lowest aromaticity is calculated for Af-pyridinium cyclopentadienide 157, with the interfragmental C—N bond shorter than the corresponding one in 155 and 158. The phenolate moiety in 159 has a high NICS value (—4.6 ppm), in agreement with the one for deprotonated phenol (—6.2 ppm compared to —9.7 ppm for benzene, as cited),196 while the acceptor pyridinium counterpart has a NICS value of —5.5 ppm, showing aromatic delocalization. 

Azapyrylium (1,3-oxazinium) salts are known,217 218 and their stability suggests the existence of aromatic delocalization. 

It is also possible to construct seven-membered aromatic heterocycles with a jr-electron decet if the heteroatoms have sufficient electronegativity to produce seven bonding orbitals with 10 jr-electrons, as in compounds of type 209 (Scheme 79). For thio derivative 209 (X = S, Scheme 79), lH NMR data show deshielded NH protons compared to the system with X = CH2, a fact which was explained by aromatic delocalization.249... 

The 13-membered ring is available in a wide variety of unrestricted and restricted forms and undoubtedly represents one of the most thoroughly studied large-membered heterocycles. The size of the system s perimeter is sufficiently large to accommodate the presence of as many as three trans double bonds which, when strategically implanted, allow the molecular skeleton to adopt a rigidly flat or near-flat geometry in which to aromatically delocalize its 14 tr- electrons. 

Chemical characterization of 132 and 133 was obtained by subsequent treatment with PhBr and CH3I or deuteriolysis, respectively. As observed previously for [C4Me4GePh] (vide supra), 13 C NMR data give no significant evidence of aromatic delocalization in... 

Possible evidence for aromatic delocalization in 59 could come from proton NMR data. The chemical shifts of the vinyl protons in the model compounds 64 and 65 lie at 5.73 and 6.00 ppm. In 59 the vinyl proton resonance is shifted downfield to 6.75 ppm, consistent with deshielding due to an aromatic ring current. 

The characteristic structural features of two-coordinate low-valent germanium cation 276 are similar to those of the analogous Si cation 268 discussed above. The average C—N and C—C bond distances (1.348 A and 1.392 A, respectively) and the essentially planar ring indicate aromatic delocalization.691 In contrast, however, the y-CH proton (8 H 4.23) is more shielded. The aromatic substituents are perpendicular to the GeN2C3 plane. 

Another index of delocalization, devised and widely applied by Schleyer et al.,163 is the nuclear-independent chemical shielding (NICS) value. When calculated above the ring, this value corresponds to the -contribution which is a significantly negative quantity for situations with aromatic delocalization . Scheme 21 shows the A and NICS values computed by Schleyer et al.141 for benzene, hexasilabenzene, and hexazine. The three species are seen to possess equally aromatic electron sextets, as indeed anticipated from the appreciable vertical resonance ener-... 

Keywords Aromaticity, Delocalization, Phosphorus heterocycles, Structure... 

Substitution of the CH units in the ring by P or N results in some variation in the aromaticity. Azaphosphinines have been considered somewhat less aromatic than benzene, pyridine, or phosphinine according to NICS(0.5) values [284], With N atoms at the position adjacent to P, the aromatic delocalization is reduced and a [1,4] dipolar character is induced [284], The NICS(0) values of diphosphinines... 

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