Electronic Structure Studies of 1,2-Didehydrogenation of Arenes and Rearrangement of Arynes to Annelated Cyclopentadienylidenecarbenes

CCSD(T)/6-311G** electronic structure calculations confirm the experimentally postulated existence of cyclopentadienylidenecarbene (CPDC) and verify the reliability of the BLYP/6-311G** predictions for thermochemistry and kinetics of the 1,2-didehydrogenation of arenes and the ring contraction of the resulting arynes. The former process is found to possess low site specificity, its energetics being influenced mostly by steric overcrowding of hydrogens. On the other hand, the energetics and barriers of rearrangements of higher arynes to annelated CPDCs are determined by the nature of bonding in the reaction products. Three distinct types of isomerizations, of which only one is observed experimentally, are readily recognizable.