Htickel array

The ene reaction can also be analyzed by using PMO method. As shown in Figure 6.10, in the transition state of ene reaction, addition to the double bond takes place from the same side (suprafacial), therefore, a supra—supra mode of addition leads to a Htickel array that is aromatic with (4n + 2) TT-electrons. Hence, the reaction is thermally allowed and photochemically forbidden. 

Scheme 2 Htickel or Mobius conjugation of the arrays of i-orbitals and radial (p ), tangential (p ), and perpendicular (pj p-orbitals...

Aromaticity is associated with cyclic arrays of mobile electrons with favourable symmetries. The mobile electron arrays may be n, o or mixed in character16. The 4n +2/4n Htickel rule provides a quantum mechanical framework which allows one to relate the stability and structure of jr-systcms to their --electron count, and is widely utilized for classifying jr-cyclic systems as aromatic or antiaromatic13. 

If the ring were sufficiently large that the twist between individual orbitals was small, such a system would not necessarily be less stable than the normal array of atomic orbitals. This same analysis points out that in such an array the Htickel rule is reversed and aromaticity is predicted for the An rr-electron systems. 

Htickel Orbital Array Mobius Orbital Array... 

The selection rules for [tt4 + tt2 ] and other cycloaddition reactions can also be derived from consideration of the aromaticity of the TS3 In this approach, the basis set p orbitals are aligned to correspond with the orbital overlaps that occur in the TS. The number of nodes in the array of orbitals is counted. If the number is zero or even, the system is classified as a Htickel system. If the number is odd, it is a Mobius system. Just as was the case for ground state molecules (see p. 716), Htickel systems are stabilized with 4 + 2 electrons, whereas Mobius systems are stabilized with 4n electrons. For the [tt4 + tt2] suprafacial-suprafacial cycloaddition the transition state is aromatic. 

The fact that there is at least one antibonding overlap in the cyclic array of orbitals does not reduce aromaticity. Mobius transition states are as aromatic as normal Htickel transition states. In contrast to stable Mobius annulenes, Mobius transition states in pericyclic reactions are quite frequent. They also include c aromatic... 

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