Homoaromaticity in aza‐ and phosphasemibullvalenes. A computational study

The enthalpies of activation ΔH‡ 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 ΔH‡ 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 ΔH‡ values (≤2 kcal/mol) combined with large negative NICS, suggesting that they could also have bishomoaromatic minima.