Homoconjugation cyclic

Fig. 5. [5]Pericyclines with conceived cyclic homoconjugation and neutral homoaromaticity...

When vinylcyclopropane is used as the key reference compound, the only examples of significant bond (electron) delocalization energies are cyclic homoconjugative systems with potentially aromatic electron ensembles (compare compounds 27-38 in Table 2). For example, norcaradienes 33 and 34 possess small, but significant bond delocalization energies RE of about 3 kcal mol1 (Table 2). Similarly, the RE values of the cyclohepta-trienes 30,31 and 32 are between 4 and 6 kcal moT1. 

In view of the clear homoaromatic character of 120 as demonstrated by the recent work on 123-127, the question arises as to whether cyclic homoconjugation is important for the ground state of semibullvalene, 120. 

We are, of course, aware of the strict Hiickel definition linking aromaticity to the presence of 4n + 2 rc-electrons, On the other hand, we note that the radical cations discussed below have SOMOs, which have one electron less than the Hiickel requirement and which show cyclic homoconjugation, even though the parent molecules assume alternative structures. In view of the precedence established for the radical anion (156) of bicyclo[6.1.0]nonatriene [416,417], in order to emphasize the dramatic difference between the structures of these radical cations and their precursors, and for reasons of convenience, we will refer to these species as homo- or bis-homoaromatic. The paradigm of these remarkable species is found in the barbaralane system. 

Of particular interest to us was the question of cyclic homoconjugation in the pericyclynes. The existence of strong orbital interactions among the acetylenic units in such molecules is... 

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