Huckel aromatic transition state

The transition state in suprafacial attack is designated as of Huckel type in which no sign inversion of the cycle has taken place. The other type of migration involves one sign inversion. This is called mobius type inversion. The Huckel type of inversion occurs when the total number of electrons is 2, 6,. .., (4n + 2). This is also called aromatic transition state. In mobius type the participating electrons is 4, 8,. .. i.e. An. 

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. 

The terms aromatic and antiaromatic have been extended to describe the stabilization or destabilization of TRANSITION STATES of PERICYCLIC REACTIONS. The hypothetical reference structure is here less clearly defined, and use of the term is based on application of the Huckel (4n+2) rule and on consideration of the topology of orbital overlap in the transition state. Reactions of molecules in the ground state involving antiaromatic transition states proceed, if at all, much less easily than those involving aromatic transition states. 

We have now considered three viewpoints from which thermal electrocyclic processes can be analyzed symmetry characteristics of the frontier orbitals, orbital correlation diagrams, and transition-state aromaticity. All arrive at the same conclusions about stereochemistry of electrocyclic reactions. Reactions involving 4n + 2 electrons will be disrotatory and involve a Huckel-type transition state, whereas those involving 4n electrons will be conrotatory and the orbital array will be of the Mobius type. These general principles serve to explain and correlate many specific experimental observations made both before and after the orbital symmetry rules were formulated. We will discuss a few representative examples in the following paragraphs. 

In this method, the orbital symmetry rules are related to the Hiickel aromaticity rule discussed in Chapter 2. Huckel s mle, which states that a cyclic system of electrons is aromatic (hence, stable) when it consists of 4n + 2 electrons, applies of course to molecules in their ground states. In applying the orbital symmetry principle, we are not concerned with ground states, but with transition states. In the present method, we do not examine the molecular orbitals themselves but rather the p orbitals before they overlap to form the MO. Such a set of p orbitals is called a basis set (Fig. 15.5). In investigating the possibility of a concerted reaction, we put the basis sets into the position they would occupy in the transition state. Figure 15.6 shows this for both the... 

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. 

While the initial formulation of homoaromaticity pre-dated the introduction of orbital symmetry by some eight years the two concepts are inextricably linkedThis is most evident when pericyclic reactions are considered from the perspective of aromatic or antiaromatic transitions states and the Huckel/Mobius concept. The inter-relationship can be demonstrated by the electrocyclic reaction shown in Scheme 1. ... 

Transition state aromaticity (Huckel and Mobius topologies)... 

Huckel aromatics The benzene molecule has the suprafacial topology this means that the TT - electron density in benzene is continuous along the top or bottom face of the molecule. If the transition state for the pericyclic reaction has the same topology, it is said to resemble Huckel topology (Fig. 8.17). 

Day has given a careful account of the relationship between the Woodward-Hoffmann rules and Mobius/HUckel aromaticity, and has defined the terms supra-facial and antarafacial in terms of the nodal structure of the atomic basis functions. His approach makes quite explicit the assumption that the transition state involves a cyclic array of basis functions. Thus the interconversion of prismane (10) and benzene, apparently an allowed (B2g-t- 2s + 2s) process, is in fact forbidden because there are additional unfavourable overlaps across the ring. ... 

Moebius-Huckel Theory. The molecular orbital array of the transition state is analyzed in terms of aromaticity, which is determined by the number of TT electrons. 

We are dealing here with four-atom conjugated systems containing four electrons, i.e., the two pairs of electrons that form the C=C n bond and the CH2—CH2 (7 bond in (144). The disrotatory transition state (145), being of Huckel type, will then be isoconjugate with normal cyclobutadiene and so will be antiaromatic, whereas the conrotatory transition state will be isoconjugate with an anti-Hiickel analog of cyclobutadiene and so will be aromatic (see Table 4.2). 

This argument can obviously be extended to concerted pericyclic reactions of all kinds. The transition state for any such reaction will be isoconjugate with a normal Hiickel-type cyclic polyene or an anti-Hiickel analog of one. If the transition state is aromatic, the resulting stabilization will lower its energy and so accelerate the reaction. If it is antiaromatic, the converse will be true. Since, moreover, the rules for aromaticity in Huckel-type and anti-Huckel-type systems are diametrically opposite, in each case one will be aromatic and the other antiaromatic. If, then, a reaction can follow one of two alternative pericyclic paths, one involving a Hiickel-type transition state and the other an anti-Hiickel-type transition, the reaction will prefer to follow the path in which the transition state is aromatic. If, on the other hand, only one of the two alternatives is sterically possible, the reaction will take place relatively easily if the corresponding transition state is aromatic and with relative difficulty if it is antiaromatic. In the latter case, the antiaromatic transition state will, if possible, be bypassed by a two-step mechanism in which the transition state is linear instead of cyclic [e.g., equation (5.291)]. 

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