Transition structures Mobius

Aromatic (top) and antiaromatic (bottom) Mobius transition structures. 

To explain the increase in the rate of the cyclobutene opening 6.292 —> 6.293, we need to remember that the conrotatory pathway will have a Mobius-like aromatic transition structure, not the anti-aromatic Hiickel cyclobutadiene that we saw in Fig. 1.38. We have not seen the energies for this system expressed in 8 terms, nor can we do it easily here, but the numbers are in Fig. 6.42, where we can see that a... 

Whereas methylenecyclopropanes only react with highly electron-deficient dienophiles in a [ 2n + 2fT) + 2n] fashion, alkenylidenecyclopropanes 1 readily undergo this cycloaddition type. A number of comprehensive and elaborate investigations with various alkenylidenecyclopropanes and 4-phenyl-l,2,4-triazoline-3,5-dione indicate that these reactions are concerted and proceed via [( 2j+,25+ 2 J -I-, 2 J transition states, involving the terminal double bond in an eight-electron Mobius aromatic transition structure 4. 

We have considered three viewpoints from which thermal electrocyclic processes can be analyzed symmetry characteristics of the frontier orbital, orbital correlation diagrams, and transition state aromaticity. All arrive at the same conclusions about the stereochemistry of electrocyclic reactions. Reactions involving 4n + 2 electrons are disrotatory and involve a HUckel-type transition structure, whereas those involving 4n electrons are conrotatory and the orbital array are of the Mobius type. These general principles serve to explain and correlate many specific experimental observations. The chart that follows summarizes the relationship between transition stmcture topology, the number of electrons, and the feasibility of the reaction. 

The concept of Mobius aromaticity has been extended from transition structures to discussions of the stability of organic molecules. Extensive reviews of Mobius aromaticity and of Mobius structures were provided by Rzepa and by Herges. A computational study of the (CH)i2, (CH)i6, and (CH)2o annulenes found twisted Mobius structures to be energy minima, but in each case steric strain caused such structures to be higher in energy than their nontwisted Hiickel coimterparts. Similar results from another study are shown in Table 11.7. In order to test the stabilizing effect of Mobius... 

Zimmerman has chosen to describe the di-n-methane rearrangement as in Mobius array which corresponds to a delocalised transition structure for an allowed photochenucal reaction, but not for ground state reaction. 

Day21 has given a careful account of the relationship between the Woodward-Hoffmann rules and Mobius/Hiickel 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 (n2s+ 2S+ 2S) process, is in fact forbidden because there are additional unfavourable overlaps across the ring.2... 

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