Bullvalenes, homoaromaticity

Earlier dynamic NMR studies had already demonstrated that the substituted semibullvalenes [87], [88] and [89] were indeed rapidly equilibrating Cope systems and not ground-state homoaromatics (Anet and Schenck, 1970 Moriarty et al., 1972 Russell et al., 1973). The bis(trifluoromethyl)semi-bullvalenes [90] and [91] had also been formulated as non-equilibrating Cope systems (Liu, 1968 Liu and Krespan, 1969). 

Following the suggestion that donor-acceptor (Dewar-Hoffmann) semi-bullvalenes [83a] would have a lower activation barrier for the Cope rearrangement, or even a homoaromatic ground state (Hoffmann and Stohrer, 1971 Dewar and Lo, 1971), numerous syntheses and studies of appropriately substituted semibullvalenes have been reported. In fact, this aspect of the search for homoaromatic semibullvalenes has been the most extensively investigated (for a partial summary of this work see Quast et al., 1985 Gompper et al., 1988, and references cited therein). 

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