Hückel and Möbius bond-shifting routes to configuration change in dehydro[4n+2]annulenes

Computational investigation of the potential energy surfaces of dehydro[10]- and dehydro[14]annulenes revealed that mechanisms involving Hückel and Möbius π-bond shifting can explain the observed or proposed configuration change reactions. Unlike the case of annulenes, in which bond-shift midpoints correspond to transition states, for transformations of dehydroannulenes with Δtrans = 0, "hidden" Hückel bond shifts occur on the side of an energy hill, on the way to a cumulenic, purely conformational transition state. For example, interconversion between CTCCTC-dehydro[14]annulene (1a) and CCTCTC-dehydro[14]annulene (2a) has a CCSD(T)/cc-pVDZ//BHLYP/6-31G* barrier of 18.7 kcal/mol, consistent with experimental observations, and proceeds via a conformational transition state, with Hückel π-bond shifts occurring both before and after the transition state. However, when Δtrans = 1, a true Möbius π-bond shift transition state was located. The isomerization of CCTC-dehydro[10]annulene (10) to CCCC-dehydro[10]annulene (11) occurs by an initial "hidden" Hückel bond shift, followed by passage through a Möbius bond-shift transition state to 11, with an overall barrier of 29.8 kcal/mol at the CASPT2(12,12)/cc-pVDZ//(U)BHLYP/6-31G* level of theory. This is the lowest energy pathway between 10 and 11, in contrast to a cyclization/ring-opening route via a bicyclic allene described in previous reports.