Anti-aromatic systems

Even of electron pairs exchanged Anti-aromatic system... 

Only a few reactions of benzodithiadiazines have been investigated. In common with dithiatriazines 12.8, the anti-aromatic system 12.12 (R = H) undergoes a reversible 5,5 -cycloaddition with norbornadiene. The reaction of 12.12 (R = F) with triphenylphosphine results in a ring contraction to give the imino 2 -phosphane 12.13. ... 

The bis-phosphonium salt (156) has been condensed with various dialdehydes to give potentially aromatic and anti-aromatic systems, while fully unsaturated 11- (158), 12- (159), and 13-membered (160) sulphur heterocycles have been prepared as shown. These showed no appreciable ring current and are presumably non-planar. 

Common examples of systems often mistaken as being aromatic (because of their alternating double and single bonds) are cyclobutadiene and cyclo-octatetraene (shown in Figure 6-9). In the case of cyclobutadiene, 4n + 2 = 4, giving n = 0.5, while for cyclooctatetraene, 4n + 2 = 8, so that n = 1.5. In these two compounds, n is not an integer, so these systems are anti-aromatic (nonaromatic). Anti-aromatic systems (non-Hilckel systems) are less stable than aromatic or normal systems. 

The CH3Y/CH2=Y- Systems 261 The NCCH2Y/NCCH=Y- Systems 280 Aromatic and anti-aromatic systems 282... 

A reaction of particular interest is that of Equation (35a) because it involves an anti-aromatic system. The barriers for Equation (35a) and its noncyclic reference system [Equation (35b)] are included in... 

In this and similar compounds the acetylene bond is supposed to donate only two jt-electrons to the conjugated system while the other jt-bond is located in the plane of the molecule and does not participate in the conjugation. Consequently, this compound satisfies the Hiickel rule for = 4. It indeed possesses aromatic properties. Anti-aromaticism. When a cyclic polyene system is studied it is important to know whether this system is nonaromatic, i.e.not stabilized by conjugation and sufficiently reactive due to the internal tension and other causes, or destabilized by conjugation, i.e. the cyclic delocalization increases the total energy of the system. In the latter case the molecule is called anti-aromatic. Here are typical examples of anti-aromatic systems cyclobutadiene, a cyclopropenyl anion, a cyclopentadi-enyl cation, and others. 

Unlike cyclopentadiene, cycloheptatriene is not an acidic hydrocarbon its pK is about 36. If a proton could be abstracted from cycloheptatriene, the resulting anion would have eight it-electrons and would be an unstable, anti-aromatic system. 

AM romatic systems are pretty stable they resist reacting. Nevertheless, W a number of reactions involving aromatic systems can be carried out. However, with the exception of combustion, the conditions required by the anti-aromatic systems for reactions that you studied in your first semester organic course are different than the conditions for aromatic systems. 

There are many other kinds of reactive intermediates, which do not fit into the previous classifications. Some are simply compounds that are unstable for various possible reasons, such as structural strain or an unusual oxidation state, and are discussed in Chapter 7. This book is concerned with the chemistry of carbocations, carbanions, radicals, carbenes, nitrenes (the nitrogen analogs of carbenes), and miscellaneous intermediates such as arynes, ortho-quinone methides, zwitterions and dipoles, anti-aromatic systems, and tetrahedral intermediates. This is not the place to describe in detail the experimental basis on which the involvement of reactive intermediates in specific reactions has been estabhshed but it is appropriate to mention briefly the sort of evidence that has been found useful in this respect. Transition states have no real hfetime, and there are no physical techniques by which they can be directly characterized. Probably one of the most direct ways in which reactive intermediates can be inferred in a particular reaction is by a kinetic study. Trapping the intermediate with an appropriate reagent can also be very valuable, particularly if it can be shown that the same products are produced in the same ratios when the same postulated intermediate is formed from different precursors. 

Whereas aromatic systems are defined by a positive resonance energy, anti-aromatic systems are characterized by a negative resonance energy. As a rule, antiaromatic compounds are unstable and contain 4n 7i-electrons in a cyclic planar, completely conjugated arrangement. Cyclobutadiene belongs to this category and is stable only in a solid matrix at very low temperatures (20 K). 

In anti-aromatic systems, on the other hand, a paramagnetic ring current is produced, in which the magnetic field strengthens the outer magnetic field inside, above and below the loop, and weakens it on the periphery. [16]Annulene, an example of an antiaromatic substance, therefore displays absorption signals at 5.33 ppm for the outer protons, and 9.44 ppm for the inner protons. 

The r-topology model of an annulene within an annulene was tested in 40, 41 and their anions. 40 can be viewed as having six 7c-electrons in the central 5MR and 22 71-electrons delocalized around the rim, thus explaining its aromahc character. On the other hand, 41 is predicted to be an anti-aromatic system because 24 n-... 

Photochemical [1,/] shifts are likewise readily analysed. If the interactions are assumed to be entirely supra-supra, then the low energy pathways are for [1,3] and [1,7] migrations (i.e. excited forms of anti-aromatic systems). 

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