B3LYP calculations on bishomoaromaticity in substituted semibullvalenes

B3LYP/6-31G* calculations on the degenerate rearrangements of substituted s emibullvalenes spuriously predict the relative enthalpies of the bishomoaro matic TSs to be lower than the experimental values. However, the calculatio ns do make the useful and experimentally testable prediction that the two c yano and two phenyl substituents in 2,6-dicyano-4,8-diphenylsemibullvalene (9d) are more likely than four cyano substituents in 2,4,6,8-tetracyanosemi bullvalene (9f) or the four phenyl substituents in 2,4,6,8-tetraphenylsemib ullvalene (9g) to produce a semibullvalene that has a bishomoaromatic equil ibrium geometry in the gas phase. The major reason for the surprising findi ng that 9d is more likely to be bishomoaromatic than 9g is shown to be ster ic interactions between the phenyl groups at C-2 and C-8 and at C-4 and C-6 in bishomoaromatic structure log. These interactions inhibit the conjugati ve stabilization of log; but they are absent in bishomoaromatic structure 1 0d, where cyano groups replace the phenyl groups at C-2 and C-6 in log. (C) 2001 John Wiley & Sons, Inc.