Benzene, 7r-system

This provides theoretical support for the concept of varying resonance demand, which reflects the varying degree of rr-overlap interaction between the benzylic p-orbital and the benzene 7r-system. 

Nakata et al, 1996, 1999). The agreement between the theoretical dihedral angles Ocaic and the empirical Oexpi of the twisted benzylic cations confirms that the observed decrease in the r value should be ascribed to a loss of resonance interaction caused by deviation from coplanarity of the carbocation centre and the benzene 7r-system. 

The above analysis illustrates why it is helpful to consider the enediyne moiety as two independent re-systems. As discussed above, the conjugated out-of-plane 7r-system of the reactant is smoothly transformed into the conjugate 7r-system of the product (e.g., the aromatic system of benzene) without an overall change in the number of bonds. We will refer to this group of electrons as out-of-plane MOs or simply as the re-system . In contrast, the two in-plane re-bonds are transformed in a more drastic way to the new cr-bond and a pair of radical centers. We will refer to this system of orbitals as in-plane re-bonds or as the a system . 

It seems that 15N chemical shifts of amide nitrogen are much less sensitive to electron withdrawal and donation than 15N shifts of amines because of the strong conjugation of the amide nitrogen lone-pair electrons with the carbonyl groups, which compete for conjugation with the 7r-systems of the benzene rings. 

Aniline and phenoxide ion have strong UV absorptions due to the overlap of the lone pair on the nitrogen (or oxygen) with the 7r-system of the benzene ring. This may be expressed in the usual Valenee Bond terms ... 

In this approach, the term union is used in the sense defined by Dewar union is a process in which two conjugated molecules combine in such a way that their two 7r-systems unite into one larger one. For example, biphenyl can be formed by the union of two molecules of benzene ... 

Acy 1-4-amino-1,5-dihydro-2-pyrrolones (6) (type Z in Scheme 1) possess the features of cyclic enaminediones. The push-pull-7r system decreases the nucleophilicity of the amino group. Therefore N-acylation with carboxylic acid chlorides requires relatively drastic conditions (dioxane, 100°C, K2C03). In particular, the reaction of the highly reactive DMF-acetal 8 to formamidine 9 succeeds only while refluxing in benzene (87TH1). (See Fig. 3.)... 

This equivalence of the valence bond and molecular orbital descriptions of the bonding in these complexes arises from the alternant1 properties of the metal-butadiene bonding network. A similar equivalence between the two theories occurs for benzene and other polyenes that have alternant 7r-systems (73, 140). 

The pyrrolo[3,2-6]pyrrole (118) is reported116 to result from thermolysis of the 12-7r-system 1,4,7-trimethyl-1,4,7-triazonine (117) in benzene or in the vapor phase. This interesting reaction proceeds with extrusion of methylamine, but the mechanism is uncertain. 

The first three observed ionization potentials for thieno[3,2-6]thiophene (3) (8.14, 8.66 and 10.02 eV) correlate favorably with those observed for the isoelectronic naphthalene (8.15, 8.80 and 10.00 eV) and benzo[6]thiophene (8.22, 8.77 and 10.05 eV) molecules, in contrast to the corresponding values for thiophene (8.87, 9.49 eV) and benzene (9.24, 9.24 eV) (73JCS(F1)93). This observation is explained by the fact that the delocalized 7r-electron cloud resulting from changing a benzene 7r-bond to a sulfur atom causes greater perturbation in the small framework involved compared to the same interchange in a more extended conjugated system such as naphthalene or benzo[6 jthiophene. 

If you compare the orbital energies of the Hiickel and Mobius cyclic 7r systems (Figures 21-13 and 21-16), you will see that the Hiickel systems have only one lowest-energy MO, whereas the Mobius systems have two. Hiickel systems have an odd number of bonding orbitals (which, when full, accommodate 2, 6, 10, 14, or An + 2 electrons) and the Mobius systems have an even number of bonding orbitals (which, when full, accommodate 4, 8, 12, or An electrons). The Hiickel molecular orbitals have zero or an even number of nodes (see, for example, the benzene MOs, Figure 21-5) the Mobius molecular orbitals are not shown, but they have one or an odd number of nodes. 

Delocalization of the negative charge by resonance is not possible in the anion of benzene. The pair of unshared electrons in phenyl anion is in an sp2 hybrid orbital that does not interact with the 7r system. 

Crown ethers selectively complex various alkali metal cations and can be thus used as model systems to study interactions between a macrocycle-bound cation and the 7r-system of a sidearm arene. Alkali-metal cation-7i interactions have recently received considerable attention because of the biological importance [88, 89, 175]. These studies have focused on Na+ and K+ interacting with benzene, phenol, and indole, which are the side chain arenes of phenylalanine, tyrosine, and tryptophan, respectively. Recent work [177-180] has demonstrated the formation of stable complexes between, for example, K+... 

Paracyclophanes, in which benzene units are linked at the 1,4-positions by ethylene bridges, show annulene characteristics upon reduction. The reduction of annulenes allows a straightforward alternating interconversion of [4n]- and [4n + 2]7r-systems, and so can be used as a good model for theory and spectroscopylb. 

There are molecular systems exhibiting 7r-bond fixation patterns that are entirely opposite to that induced by the Mills-Nixon effect [82,83,67]. Typical examples of this kind are provided by benzoborirene 33 and benzocyclopropenyl cation 34 (Fig. 19) These compounds represent extended 7r-systems relative to benzene itself since they encompass now empty 7r-orbitals at B and C+ atoms, respectively. The structural parameters offered by HF/6-31G [82] and MP2(fc)/6-31G [43] models are given in Table 10. Both molecules are planar. A salient feature of the aromatic CC bonds is their stretching relative to benzene at ortho and para positions. In contrast, meta bonds are more localized and shortened. Another striking property is a pronounced delocalization within the three-membered ring (aromatic pattern involving 27t electrons) as easily visualized by the resonance structures shown in Scheme 4. The same resonance mechanism is operative in benzocyclopropenyl cation. 

Mobile 7r-electrons in the benzene ring can be transferred to the o-orbitals or in the 7r-systems of acceptor molecules what gives rise to tt-o- or 7i-7i-complexes. 

Consider now the 7r-system in benzene. The MO approach will generate linear combinations of the atomic p-orbitals, producing six 7r-orbitals delocalized over the whole molecule with four different orbital energies (two sets of degenerate orbitals). 

Borin (12) with an sp hybridized boron atom has six 7r-eIectrons and is expected to be resonance stabilized. The boron atom has an empty sp orbital and should therefore be a strong Lewis acid. The ring system (12) is accordingly known only coordinated to Lewis bases, mostly to carbanions, to give a ring system isoelectronic with benzene. Thus systems such as the 1-arylborinate anions (83) and 5-aryldibenzo[6,e]borinate anions (84) are by far the best-known species. 

The above definition is common usage, but most chemists who apply it have restricted themselves to cases where only one p-doublet derived from a heteroatom contributes to the 7r-system. The problem of aromaticity in heterocycles possessing more than one p-doublet has received comparatively little attention. Scheme 1 shows two sequences, one homocyclic and the other heterocyclic, in which systems with three doublets are related to benzene. 

Paracyclophane 1 is the smallest stable member of the [m,n]paracyclophane series (Scheme 1). The close proximity of the two benzene rings leads to a strong interaction of the 7r-systems. In the dibenzoannelated analogue 2 two pairs of orthogonal 7r-systems can be found. Both molecules have been used to synthesize extended 7r-systems, in which 2 allows a mutually orthogonal arrangement of their subunits. 

---