Homoaromaticity in aza- and phosphasemibullvalenes. A computational study

The enthalpies of activation Delta H double dagger for the Cope rearrangement in several aza- and phosphasemibullvalenes have been investigated by MP4/cc-pVDZ//MP2/cc-pVDZ and CCSD(T)/cc-pVDZ//MP2/cc-pVDZ calculations. One tetraazasemibullvalene and several phosphasemibullvalenes were found to have vanishing Delta H double dagger values, which together with calculated large negative Nucleus Independent Chemical Shift (NICS) values and geometrical data show that these molecules have delocalized and bishomoaromatic minima. Furthermore, three azasemibullvalenes were found to have small Delta H double dagger values (<= 2 kcal/mol) combined with large negative NICS, suggesting that they could also have bishomoaromatic minima. Copyright (C) 2010 John Wiley & Sons, Ltd.