Huckel system

The chemistry of the phosphirene system is attracting increasing attention, with the eventual goal of preparing the potentially aromatic Huckel system (214). A simple preparation of tervalent phosphirenes (215) is afforded by the reactions of dichloro-phosphines with alkynes in the presence of aluminium chloride,... 

Various geometries are possible for the transition state and they can be classified on whether each of the allyl systems interacts with lobes of the other system on the same side (suprafacially) or on opposite sides (antarafacially). Three transition states have been given. All have been classed on Huckels system, on the basis of aromatic transition state approach and so all are thermally allowed. The following picture gives the allowed transition state for thermal [3, 3] shifts. 

In the transition state a boat like structure appears and there will be a suprafacial cis addition to the termini of the n bond. The ene reaction does not have a symmetrical transition state and it is a thermally allowed concerted reaction. Its transition state involves a suprafacial interaction of six electrons (4 from the k bonds and two from the o bond) So it is a Huckel system and transition state is aromatic. In the terminatlogy of Woodward and Hoffmann it can be regarded as o2s + n2s + 7t2s reaction. 

The rule may then be stated A thermal pericyclic reaction involving a Huckel system is allowed only if the total number of electrons is4n + 2. A thermal pericyclic reaction involving a Mobius system is allowed only if the total number of electrons is 4n. For photochemical reactions these rules are reversed. Since both the 2 + 4 and 2 + 2 cycloadditions are Huckel systems, the Mobius-Hiickel method predicts that the 2 +4 reaction, with 6 electrons, is thermally allowed, but the 2 + 2 reaction is not. One the other hand, the 2 + 2 reaction is allowed photochemically, while the 2 + 4 reaction is forbidden. 

In the Mobius-Huckel approach, diagrams similar to Figure 18.4 can be drawn for this case. Here too, the disrotatory pathway is a Huckel system and the conrotatory pathway a Mobius system, but since six electrons are now involved, the thermal reaction follows the Huckel pathway and the photochemical reaction the Mobius pathway. 

The tangential pjp orbitals form a Huckel system for even-membered rings but a Mobius system for odd-membered rings. However, this seems to be of little consequence because it has been shown that both Huckel and Mobius orbital systems have always an aromatic... 

Another consequence of the donor/acceptor properties of the lithium atom is the dramatic stabilization by 37 kcal/mol (155 kJ/mol) of the cu-planar dilithiomethane 3b (cis) compared with the rra s-planar structure 3b (trans). In contrast to 3b (trans) the lithium atoms in 3b (cis) c n interact electronically to form together with the p orbital at the carbon atom a (4n + 2)-Huckel system (n = 0) isoelectronically to the cyclopropenyl cation 6 ... 

Figure 7.14 Reaction paths for 1 pericyclic reactions involving aromatic and non-aromatic numbers of electrons. Number of aromatic electrons is 4n + 2 for Huckel systems with no antara-facial components, or An electrons for Mobius systems with one antarafacial component...

As is often the case with simple HMO theory, it is the energy levels of the Mobius systems and not the MOs themselves that are of primary interest. Zimmerman developed a circle mnemonic, analogous to the circle mnemonic used with HMO theory in Chapter 4, which provides a shortcut to finding tt e Mobius energy levels. For Mobius MOs, the polygon corresponding to the cyclic molecule is inscribed in the circle of radius 26 with one side down (not with a comer down as was done for Huckel systems). This procedtue is illustrated in Figure 11.104 for both (a) Huckel MOs and (b) Mobius MOs for cyclopropenyl, cyclobutadiene, and benzene. 

T. S. for [1,3] suprafacial H shift, HUckel system, 0 node, 4 electrons, antiaromatic, hv allowed... 

T. S. for supra-retention, T. S. for antara-retention, T. S. for supra-inversion, T. S. for antara-inversion, Huckel system, MObius system, MObius system, HQckel system,... 

Here, with six electrons involved, it is in the disrotatory mode (Huckel system) that the transition state is stabilized. The chart that follows gives a general summary of the relationship between transition state topology, the number of electrons, and the stability of the transition state. 

While neither of these theoretical possibilities has been realized for stable 7i-electron systems, it must be remembered that delocalized systems in chemistry are not all of n type. There is no reason why delocalized systems should be two dimensional or involve exclusively Ti-type overlap of p or d AOs. In a three-dimensional delocalized system involving a- as well as retype overlap, we have much more flexibility and, as we shall see presently, examples of this alternative type of topology become important. Since the MO treatment of n systems was first introduced by Huckel, the normal class of delocalized systems with no required phase dislocations may be described as HUckel systems. The second topological type with one inevitable phase dislocation may then be called anti-Huckel systems. ... 

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