Mobius-Huckel method

We have emphasized that the Diels-Alder reaction generally takes place rapidly and conveniently. In sharp contrast, the apparently similar dimerization of olefins to cyclobutanes (5-49) gives very poor results in most cases, except when photochemically induced. Fukui, Woodward, and Hoffmann have shown that these contrasting results can be explained by the principle of conservation of orbital symmetry,895 which predicts that certain reactions are allowed and others forbidden. The orbital-symmetry rules (also called the Woodward-Hoffmann rules) apply only to concerted reactions, e.g., mechanism a, and are based on the principle that reactions take place in such a way as to maintain maximum bonding throughout the course of the reaction. There are several ways of applying the orbital-symmetry principle to cycloaddition reactions, three of which are used more frequently than others.896 Of these three we will discuss two the frontier-orbital method and the Mobius-Huckel method. The third, called the correlation diagram method,897 is less convenient to apply than the other two. 

As expected, the Mobius-Huckel method leads to the same predictions, Here we look at the basis set of orbitals shown in F and G for [1,3] and [1,5] rearrangements, respectively. 

We note that it is the Htickel systems with 4N electrons and the Mobius systems with 4N + 2 electrons which have a nonbonding degenerate pair of MO s and thus a facile mode of converting starting excited state to ground state of product. Finally, it should be noted the the Mobius-Huckel method is fully consistent with the Woodward-Hoffmann treatment, both for ground state and for photochemical reactions 40). 

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. 

The Mobius-Huckel approach now has become common to a large number of undergraduate textbooks because of its facile application to ground state chemistry. The method does not really differ in conclusions and derived rules from the Dewar method presented a year later 36). 

The Mobius-Huckel aromatic Iransition state method... 

In the PMO method, we analyze an electrocyclic reaction through the following steps (1) Define a basis set of 2p-atomic orbitals for all atoms involved (li for hydrogen atoms). (2) Then connect the orbital lobes that interact in the starting materials. (3) Now let the reaction start and then we identify the new interactions that are occurring at the transition state. (4) Depending upon the number of electrons in the cyclic array of orbitals and whether the orbital interaction topology corresponds to a Huckel-type system or Mobius-type system, we conclude about the feasibility of the reaction under thermal and photochemical conditions. 

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