Non-cyclic conjugation

Non-cyclic interactions of two and three orbitals are described in the preceding chapters of this volume. We describe here cyclic interactions of three or more orbitals (Scheme 1). In 1982, cyclic orbital interaction was found in non-cyclic conjugation [15]. Interactions of bonds in molecules contain cyclic interactions of bond (bonding and antibonding) orbitals even if the molecular geometry is non-cyclic. The cyclic... 

The electron delocalizations in the linear and cross-conjugated hexatrienes serve as good models to show cyclic orbital interaction in non-cyclic conjugation (Schemes 2 and 3), to derive the orbital phase continuity conditions (Scheme 4), and to understand the relative stabilities (Scheme 5) [15]. 

The orbital phase is continuons in the linear conjugate triene and discontinuous in the cross-conjugate trine. The electron delocalization between the terminal bonds is favored in the linear triene and disfavored in the cross-conjugate triene. The linear triene is more stable. The continuity-discontinuity of orbital phase underlies the thermodynamic stabilities of non-cyclic conjugated molecules. 

Interactions polarize bonds. Trimethylenemethane (TMM) and 2-buten-l,4-diyl (BD) dianions (Scheme 6a, b) are chosen as models for hnear and cross-conjngated dianions. The bond polarization (Scheme 7) is shown to contain cyclic orbital interaction (Scheme 6c) even in non-cyclic conjugation [15]. The orbital phase continnity-discon-tinnity properties (Scheme 6d, e) control the relative thermodynamic stabihties. 

The finding of the cyclic orbital interactions in non-cyclic conjugation opens a way to systematic understanding and designing of molecules and reactions in a unified manner. Here, we apply the orbital phase theory to non-cychc interactions of bonds, groups, molecules, cationic, anionic, and radical centers, lone pairs, etc. 

Orbitals have been shown in the preceding section to interact in a cychc manner even in non-cyclic geometry. Cyclic conjugation contains the cyclic orbital interactions becanse of the cyclic geometry, in addition to those of the non-cyclic subsystems. 

With one more N-B bond, the cyclic conjugation is discontinuous in 1,3,2,4-diaza-diborine. The donors and acceptors are alternately disposed along the cyclic chain. Electrons cannot effectively delocalize in a cyclic manner, but between the adjacent donor-acceptor pairs in a non-cyclic manner. The diazadiborine is not predicted to be aromatic. 

Cyclobutadiene (l)15 is the first member of the series of cyclic conjugated hydrocarbons (Kekule compounds) cyclopropenylidene (2), of cyclic conjugated carbenes trimethylenemethane (3), of the non-Kekule hydrocarbons. 

In 2-pyrazolines [103] the conjugation involves N-1 and C-3 atoms. 2-Pyrazolines may be regarded as cyclic hydrazones, which have the advantage over non-cyclic products of being stable to hydrolysis. It has been shown by Elguero and Jacquier (1965) that protonation occurs at N-1, giving cation [104], while forms with the proton at N-2 and C-3 [105] may exist in amounts of less than 1-5%. 

Aromaticity has been long recognized as one of the most useful theoretical concepts in organic chemistry. It is essential in understanding the reactivity, structure and many physico-chemical characteristics of heterocyclic compounds. Aromaticity can be defined as a measure of the basic state of cyclic conjugated TT-electron systems, which is manifested in increased thermodynamic stability, planar geometry with non-localized cyclic bonds, and the ability to sustain an induced ring current. In contrast to aromatic compounds there exist nonaromatic and antiaromatic systems. Thus, pyrazine (69)... 

This widely used criterion is manifested in (a) bond length equalization due to cyclic delocalization and (b) planarity of the cyclic systems13-16. The planarity of cyclic conjugated systems is regarded as a manifestation of aromaticity, while the non-planarity of cyclic conjugated systems is often taken as an indication of antiaromaticity. 

The it energy of a non-classical conjugated hydrocarbon can be compared directly with that of a classical analogue by the PMO method.14 Consider an even monocyclic polyene. This can be formed by fusion of methyl with an odd AH with one atom less. These components can also be fused to form an acyclic polyene. Comparison gives the aromatic energy of the cyclic system by difference. In this way we find that rings with An + 2 atoms are more stable, and those with An atoms less stable, than analogous acyclic compounds. The same method can be used for the bicyclic systems XVII, XIX, XXI, XXII, XXIII. The procedure is indicated below... 

Another structural possibility for a hexadiene radical cation arises, when the two allylic moieties are linked in pairwise fashion to two- or three-carbon spacers. This structure type can be approached by oxidation of molecules such as semibullvalene [391-393] or barbaralane [394]. In the resulting radical cations, the two allylic moieties are held in close proximity model considerations suggest a non-bonding C—C distance of 2.2-2.3 A, considerably closer than for the previously discussed structure type. At this distance, a moderately strong interaction of the twin moieties cannot be excluded. Accordingly, we assigned cyclic conjugated structures to radical cations derived from semibullvalene (-> 138 cf. 

Interestingly, the radical cation 138 can be generated also by light-induced isomerization of cyclooctatetraene radical cation (140). The conversion of the red non-planar ion 140 (4n - 1 n electrons) upon irradiation with visible light had been observed previously [395], but the blue photo-product had not been recognized as the cyclic conjugated species 138 with 4n + 1 n electrons. This interconversion is one of only a few orbital symmetry allowed processes documented in radical cation chemistry [393]. 

The same difference can be shown to distinguish between cyclic systems with 4m and 4m + 2 carbon atoms respectively. The n = 0 energy level has degeneracies of m and 2m +1, for these respective systems. It is apparent that the zero level of 4m cyclics cannot accommodate more than 2m itinerant electrons, which means that the 2m odd couples cannot be delocalized to yield an aromatic system. The best known example of a non-aromatic conjugated cyclic molecule is cyclo-octatetraene, shown in Figure 6.5. 

Allenic hydrocarbons, including cyclic ones, underwent chlorine addition under radical conditions only one double bond reacted [12], A number of conjugated dienes added chlorine in their reaction with (dichloroiodo)benzene under radical conditions. Both 1,2- and 1,4-addition occurred their ratios varied, depending on the substitution of the non-cyclic substrate and the size of the cyclic ones trans products were normally favoured [13]. 1,6-Dienes were cyclized to 1,2-bis chloromethyl cyclopentanes [14] ... 

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