TT-electron delocalization

A second isomer of [lOJannulene (the cis trans cis cis trans stereoisomer) can have bond angles close to 120° but is destabilized by a close contact between two hydro gens directed toward the interior of the ring To minimize the van der Waals strain between these hydrogens the nng adopts a nonplanar geometry which limits its ability to be stabilized by tt electron delocalization It too has been prepared and is not very stable Similarly the next higher (4n + 2) system [14]annulene is also somewhat desta bilized by van der Waals strain and is nonplanar... 

The carbocation is aromatic the hydrocarbon is not Although cycloheptatriene has six TT electrons m a conjugated system the ends of the triene system are separated by an sp hybridized carbon which prevents continuous tt electron delocalization... 

Generally speaking, electrically conductive polymers are composed of conjugated polymer chains with TT-electrons delocalized along the backbone. 

The 1,2-dithiolylium and 1,3-dithiolylium ions (115) and (116) are iso-rr-electronic with the tropylium ion, from which they may be formally derived by replacing double bonds by sulfur atoms. Various calculations and structural data demonstrate that the rings are stabilized by tt-electron delocalization. 

TT-electron delocalization over the O—C—N system. No such delocalization is possible in the N-protonated form. 

FIGURE 10.6 Conformations and electron delocalization in 1,3-butadiene. The s-cis and the s-trans conformations permit the 2p orbitals to be aligned parallel to one another for maximum TT electron delocalization. The s-trans conformation is more stable than the s-cis. Stabilization resulting from tt electron de-localization is least in the perpendicular conformation, which is a transition state for rotation about the C-2—C-3 single bond. The green and yellow colors are meant to differentiate the orbitals and do not indicate their phases. 

During our discussion of benzene and its derivatives, it may have occurred to you that cyclobutadiene and cyclooctatetraene might be stabilized by cyclic tt electron delocalization in a manner analogous to that of benzene. 

Cycloheptatrienyl cation completely conjugated, six -TT electrons delocalized over seven carbons... 

Cp(CO)2Re(THF) forms the complex 105 upon reaction with thiophene (89JA8753, 910M2436). Similar species are known for 2- and 3-methyl-, 2,5-dimethyl, and tetramethylthiophene (91IC1417). Thiophene in 105 is S-coordi-nated, and the sulfur atom is pyramidal. Treatment of 105 with Fc2(CO)9 produces 106, where the thiophene ligand is bridge-coordinated via the sulfur atom to rhenium and four carbon atoms of the dienic system with iron (the coordination mode). The pyramidal nature of the sulfur atom is preserved. The -coordination of thiophene separates the dienic and sulfur counterparts of the ligand and decreases the TT-electron delocalization, which leads to the enhanced basicity of the sulfur atom. 

The aromaticity of naphthalene is explained by the orbital picture in Figure 15.12. Naphthalene has a cyclic, conjugated it electron system, with p orbital overlap both around the ten-carbon periphery of the molecule and across the central bond. Since ten 77 electrons is a Hiickel number, there is tt electron delocalization and consequent aromaticity in naphthalene. 

Antiaromatic (Section 15.3) Referring to a planar, conjugated molecule with 4n tt electrons. Delocalization of the t7 electrons leads to an increase in energy. 

X -phosphorins constitute a novel and very versatile class of heterocyclic compounds, the properties of which can be varied considerably by substituents at both the phosphorus and the ring C atoms. The ylid properties are fully suppressed by TT-electron delocalization over the entire ring. 

TT-Electron delocalization in isoxazole seems to be more effective than in oxazole however, isothiazole is less aromatic than thiazole thus it is not a general rule that 1,2-diazoles possess higher aromaticity in comparison with 1,3-diazoles. Oxygen-containing heterocycles are always less aromatic than their sulfur and nitrogen counterparts, e.g. thiazole > imidazole > > oxazole. At the same time, the relative aromaticity of S- and N-containing heterocycles can interchange (pyrazole > isothiazole > isoxazole). 

Molecules which are aromatic and thus possess substantial tt- electron delocalization exhibit large non-local contributions to their magnetic susceptibilities. By the absence of these contributions, 2- and 4-pyranones can be classed as non-aromatic. 

The intermediate shown for aromatic substitution no longer has an aromatic structure rather, it is a cation with four tt electrons delocalized over five carbon nuclei, the sixth carbon being saturated with s/ 3-hybrid bonds. It... 

It should be also mentioned that there are some cases when the cation selective solvation promotes greater TT-electron delocalization in remaining free anion radicals see Section 3.2.5. Staley and co-workers (1999 and Staley and Kehlbeck 2001) studied the phenomenon for the organic dianion too. Such a solvent effect leads to changes in the reactivity of these species. 

Do a Hiickel calculation on the conjugated four-carbon ring cyclobutadiene. Calculate the tt-electron delocalization energy. Comment on the applicablity of Hiickel s 4N -b 2 rule for aromaticity. 

The next cyclic alkadiene, 1,3-cyclopentadiene, has been experimentally studied by MW, GED and XR methods. The carbon skeleton is planar (C2v symmetry), and the small C=C—C angles compared to those in 1,3-butadiene (i24.3°) or cts-1-butene (l26.4°) do not seem to influence noticeably the lengths of the CC bonds, although other effects, such as TT-electron delocalization, might have an opposite effect. The apparently normal structure parameters observed for 1,3-cyclopentadiene might therefore be a result of different forces having opposite effects on the structure parameters. 

Steric interactions between the methyl groups in 2,2 -dimethylbiphenyl lead to a larger average angle between the planes of the two benzene rings, hence tt electron delocalization is reduced relative to the... 

In spite of the lack of a unique and precise definition, aromaticity is one of the most frequently used concepts in (organic) chemistry. This phenomenon, which is classically associated with a cyclic tt-electron delocalization, results in a stabilization of the molecular system considered. Benzene is the archetype of the phenomenon of aromaticity. Thus, a question of interest is to what extent the amino substituent influences the electron delocalization in the ring. There are several criteria to evaluate the aromaticity, including the geometry-based (HOMA), energy-based (ASE), magnetism-based (NICS) and electronic delocalization (PDI) models. Recent theoretical evaluations65 of these parameters... 

Roth and coworkcrs reported NMR data of the orthogonal butadiene (Z,Z)-3,4-dimethylhexa-2,4-diene. (Z,Z)-13 having the planes of the double bonds at a dihedral angle not far from 90°. This diene serves as the model for conjugated diene lacking TT-electron delocalization and for the transition state for interconversion of antiperiplanar (trans) and synpcriplanar (cis or gauche) butadiene. 

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