4n tt electron system

The cycloheptatrienide, or tropenide, anion (LII) has eight electrons, and, if these were delocalised over the ring as in (LIll) would provide a system which might be destabilised because of its being a 4n-Tt-electron system,... 

Sol 3. (c) The given transformation involves conrotatory cyclization of (2Z,4 )-hexa-2,4-diene (I) to give c/i-3,4-dimethylcyclobut-l-ene (II). For a 4n TT-electron system, conrotatory process is feasible only under thermal conditions. 

We classify compounds as aromatic [(4n + 2) ir-electron systems] or antiaromatic (2n TT-electron systems), if there is continuous conjugation around the ring, and as non-aromatic. Aromatic compounds are further subdivided into those without exocyclic double bonds and those in which canonical forms containing exocyclic double bonds contribute. 

For monocyclic conjugated polyenes, high stabilization is found for systems with (4n + 2) TT electrons but not for systems with (4n) tt electrons. The relationship is formulated as Hiickel s rule, which states that completely conjugated planar hydrocarbons are strongly stabilized (aromatic) when they have 4n -I- 2) tt electrons. Benzene (6 tt electrons) is aromatic but cyclobutadiene (4 tt electrons) and cyclooc-tatetraene (8 tt electrons) are not. 

The pattern for planar conjugated systems established for cyclobutadiene, benzene, and cyclooctatetraene persists for larger rings. All 4 - -2 systems are predicted to have all electrons paired in bonding MOs with net stabilization relative to isolated double bonds. In contrast, planar systems containing 4n tt electrons are predicted to have two degenerate orbitals, each with one unpaired electron. This pattern is the theoretical basis of the Hiickel rule. 

CT bonds. If it is assumed that the reactants approach one another face-to-face, as would be expected for reactions involving tt orbitals, the requirement for bonding interactions between the HOMO and LUMO are met for [2 - - 4] but not for [2 + 2] or [4-1-4] cycloadditions. (See Section 1.2 to review the MOs of conjugated systems.) More generally, systems involving 4 - - 2 tt electrons are favorable (allowed), whereas systems with 4n tt electrons are not. 

One of the main problems in assigning aromatic or antiaromatic character to a system is the relativity of these terms and consequently the need for proper reference compounds. Our approach overcomes this difficulty by using the neutral parent compounds as reference compounds they are good reference compounds because they differ merely in their electronic structure. Therefore, doubly charged 4n tt electron compounds are good model compounds for antiaromaticity studies. Because carbocyclic and heterocyclic dianions can be prepared from stable precursors and because they sustain moderate stability, they were selected for this study. 

If these are considered as bridged annulenes, then. s-indacenc (XLII) and <2s-indacene (XLIV) have peripheral 4n ir-electron systems while the s- and as-cycloheptindencs (Xl.IlI) and (XLV) have peripheral (4n + 2) TT-electron systems. 

The electrochemical reduction of (XXIX) [101], a 2,ll-bisdehydro[18]-annulene-1,10-dione [102], and of 2,4,11,13-tetrakisdehydro[18]-annulenc-1,6-diones and 2,4,11,13-tetrakisdehydro[18]annulene-l,10--diones [103,104] is consistent with the reduction behaviour of quinones. They are more easily reduced than is benzoquinone, possibly because there is less electrostatic repulsion between the resultant negatively charged oxygen atoms in the products, since they are further apart than in a six-membered ring. The [16]dione (XXX) is less easily reduced than the [18]diones this could either reflect that reduction in this case leads to a 4u TT-electron system rather than a (4n+2) system, or it may be due to the greater proximity of the oxygen atoms in (XXX) [104]. 

Aoepleiadylene, [XL), unlike pyracyclene, can have a (4n +2) TT-electron system in its periphery. The chemical shifts of the bridging carbon atoms in the C-n.m.r. spectrum suggest that a diamagnetic ring current is set up in the periphery [290], and the measured heat of hydrogenation is in accord with the idea that there is stabilisation due to the peripheral 14 TT-electron system [293]. 

In case of [tt s + tu s] cycloaddition (4n 7r-electron system), a supra—supra mode of addition leads to a Huckel array, which is antiaromatic with 4n 7u-electrons (Figure 4.7). Therefore, the supra—supra mode of reaction is thermally forbidden and photochemically allowed. However, a supra—antara mode of addition uses a Mobius array, which is aromatic with 4n 7t-electrons. Therefore, [rt s + rc a] cycloaddition reaction is thermally allowed and photochemically forbidden. Similarly, we can analyze the [tu" s + Tt s] cycloaddition having (4n + 2) 7t-electrons (Figure 4.7). In this case, a supra—supra mode of addition leads to a Huckel array, which is aromatic with (4n + 2) 7C-electrons. Therefore, [7t" s + tu s] cycloaddition reaction now becomes thermally allowed and photochemically forbidden. However, a Itch s + Tu a] cycloaddition uses a Mobius array, which is antiaromatic with (4n + 2) 7u-electrons. Therefore, the reaction is thermally forbidden and photochemically allowed in this mode. 

When we recall the symmetry patterns for linear polyenes which were discussed in Chapter 1, we can further generalize the predictions based on the symmetry of the polyene HOMO. Systems with 4n tt electrons will undergo electrocyclic reactions by conrotatory motion, whereas systems with 4n + 2 tt electrons will react by the disrotatoiy mode. 

When the orbitals have been classified with respect to symmetry, they can be arranged according to energy and the correlation lines can be drawn as in Fig. 11.9. From the orbital correlation diagram it can be concluded that the thermal concerted cycloaddition reaction between butadiene and ethylene is allowed. All bonding levels of the reactants correlate with product ground state orbitals. Extension of orbital correlation analysis to cycloaddition reactions with other numbers of v electrons leads to the conclusion that the suprafacial-suprafacial addition is allowed for systems with 4n + 2 tt electrons but forbidden for systems with 4n tt electrons. 

An additional requirement for aromaticity is that the number of rr electrons in conjugated, planar, monocyclic species must be equal to 4n + 2, where n is an integer. This is called Hiickel s rule. Benzene, with six rr electrons, satisfies Huckel s rule for w = 1. Square cyclobutadiene (four rr electrons) and planar cyclooctatetraene (eight tt electrons) do not. Both are examples of systems with 4n TT electrons and are antiaromatic. 

Since the preparation of [I8]annulene, many other annulenes have been made. As long as they are (nearly) planar and delocalized, those with (4n + 2) tt electrons, such as benzene and [18]annulene, are aromatic, whereas those with 4n tt electrons, such as cyclobutadiene and [16]annulene, are antiaromatic. When cyclic delocalization is prohibited by angle or steric strain, such as in cyclooctatetraene or [10]annulene (Exercise 15-15), the systems are nonaromatic. Of course, cyclic polyenes in which there is no contiguous array of p orbitals are not annulenes and therefore also nonaromatic. 

Cyclic systems of 4n tt electrons can be converted into their aromatic counterparts by two-electron oxidations and reductions. For example, cyclooctatetraene is reduced by alkali metals to the corresponding aromatic dianion. This species is planar, contains fully delocalized electrons, and is relatively stable. It also exhibits an aromatic ring current in NMR. 

The nonbonding n level of allyl lies a low energy and consequently one expects that 12.45 should be stabilized. Calculations have shown this to be the case. The 4n + 2 tt electron systems will show this stabilizing feature. One needs to be careful here in that if the HOMO-LUMO gap is too large, then the stabilization may be negligible. The overlap in 12.44 is through-space and certainly not as large as that encountered between AOs on adjacent bonded atoms. 

H.6 (a) This is a (4n + 2) tt-electron system (where n = 1) a thermal reaction should take place with disrotatory motion ... 

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... 

Benzene, cyclobutadiene, and cyclooctatetraene provide clear- examples of Hiickel s rule. Benzene, with six tt electrons is a (4n + 2) system and is predicted to be aromatic by the rule. Square cyclobutadiene and planar- cyclooctatetraene are 4n systems with four and eight tt electrons, respectively, and are antiarornatic. 

Figure 11.14 shows a molecular orbital diagrfflTt for cycloheptatrienyl cation. There are seven tt MOs, three of which are bonding and contain the six tt electrons of the cation. Cycloheptatrienyl cation is a Hiickel (4n + 2) system and is an aromatic ion. 

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