Aromatization transannular

Onium ions of small and large heterocyclics are usually produced by electrophilic attack on a heteroatom. In three- and four-membered rings nucleophilic attack on an adjacent carbon follows immediately, in most cases, and ring opening stabilizes the molecule. In large rings the onium ion behaves as would its acyclic analog, except where aromaticity or transannular reactions come into play (each with its electronic and steric pre-conditions). A wide diversity of reactions is observed. 

Anions of small heterocyclics are little known. They seem to be involved in some elimination reactions of oxetan-2-ones (80JA3620). Anions of large heterocycles often resemble their acyclic counterparts. However, anion formation can adjust the number of electrons in suitable systems so as to make a system conform to the Hiickel rule, and render it aromatic if flat geometry can be attained. Examples are found in Chapter 5.20. Anion formation in selected large heterocycles can also initiate transannular reactions (see also Section 5.02.7 below). 

W. Wang, J. Xu, Y.-H. Lai, and F.Wang, Alternating aromatic and transannular chromophores with and without linker effect of transannular tt-tt interaction on the optical property of dithia-paracyclophane-based copolymers, Macromolecules, 37 3546-3553, 2004. 

In this context transannular interactions must be mentioned, although there are very few authenticated reports of such effects, and they involve solely sp2 carbon atoms. Thus, Maciel and Nakashima (256) ascribed a shielding of the carbonyl atom in 129 of approximately 10 ppm relative to 128 (X = CH2, O, S) to a transannular interaction associated with a partial charge separation (Scheme 40). Less clear-cut results were obtained from the spectra of 3- and 4-thiacyclohexanone (199,257). For the sake of completeness we note that aromatic carbon atoms experience considerable deshielding (6-9 ppm) in bi- and multilayered [2.2]paracyclophanes (258,259). This was attributed to a decrease of the excitation-energy term in the o-p expression (eq. [3], p. 222). 

According to Lonsdale et al., the four aromatic bridgehead atoms C8, Cg, Cu and C14 are bent out of the plane of the remaining benzene carbon atoms by about 0.168 A (14°) at 291 °K (see Fig. 1) the aromatic nuclei are deformed into a boat conformation. The distance between the aromatic bridgehead atoms C3, C14 and Cg, Cu has shrunk to 2.751 A. The intramolecular distance between the plane formed by atoms C4, C5, C7, Cg and that formed by atoms C12, C13, C15, Ci6 is only 3.087 A the van der Waals distance between two parallel benzene nuclei is usually at least 3.4 A. There must therefore be considerable transannular -overlapping in compound 2. To compensate for this the CH2-CH2 bond length is unusually large 1.630 A at 291 °K at 93°K it is only 1.558 A. 

The proton resonance of 26 is also indicative of transannular interactions interspatial transannular coupling of each aromatic proton with all four fluorine nuclei of the other ring induces splitting of the aromatic protons (quintuplet r =3.18, /hf =0.8 Hz). An alternative... 

Depending upon substituents, transannular interactions in the [2.2]paracyclophane system are characterized by the steric or electronic effects of one aromatic nucleus on the physicochemical behavior of the other aromatic ring. The transannular reactions themselves, of course, are very dependent upon molecular geometry. 

A careful NMR study by the same group has shown that the 1-azaquinolizinium ion in neutral aqueous solution undergoes hydration to yield (241), but the equilibrium is shifted to the aromatic species (234) if the solution is made 0.1 M in acid (Scheme 118). Bicarbonate causes ring opening to afford j8-(2-pyridylamino)acrolein (242), which can be recyclized in acid. Deuteromethanol also adds to the 1-azaquinolizinium ring, in a transannular manner, and triethylamine converts the resulting salt (243) to 4-methoxy-l-aza-4 7-quinolizine (244). 

Derivatives of the linear polyacenes, naphthalene,46 anthracene,41,90,93 naphthacene,91,92 and pentacene,92 form stable photodimers M2 when irradiated in concentrated Oa-free solution or (exceptionally) in the crystalline state.94,95 Transannular a-bonding of the molecular dimer components results in a folding of the aromatic planes about the bonded atoms and a reduced -electron delocalization reflected in a shift of the absorption spectrum to much higher frequencies.46,92,96... 

The main aspects of the chemical reactivity of helicenes (e.g. electrophilic substitution) equally not deviate from those of planar aromatic compounds, and remarkable reactions of helicenes, which are incidentally found (e.g. the transannular bond formation between a C(l)-substituent and a part of the inner helix) can ultimately be reduced to known principles of aromatic reactivity. 

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