1,2-didehydro[10]annulenes: Structures, aromaticity, and cyclizations

摘要

The conformational space of C10H8 1,2-didehydro[10]annulenes, along with their unimolecular conversion to isonaphthalenes (cyclic allenes), has been studied computationally using DFT (B3LYP), single-reference [CCSD(T)], and multireference (MCQDPT2) post-HF methods. The introduction of the linear alkynyl moiety releases enough angle strain to make a nearly planar "heart" aromatic form the preferred conformer by more than 6 kcal/mol [CCSD(T)] over a localized C-2 "twist" structure, as opposed to the closely related C10H10 [10]annulene system. Computations also show that electrocyclic ring-opening of isonaphthalenes to the heart C10H8 annulene takes place through a low barrier of 15 kcal/mol, and this should be considered the working mechanism for the reported isomerizations during dehydro Diels-Alder reactions of phenyl-acetylenes.