Möbius aromaticity in [12]annulene: cis-trans isomerization via twist-coupled bond shifting.

Density functional and coupled cluster calculations show that facile thermal configuration change in [12]annulene occurs via a twist-coupled bond-shifting mechanism. The transition state for this process is highly aromatic with Möbius topology. At the CCSD(T)/cc-pVDZ//BH&HLYP/6-311+G** level, the isomerization of tri-trans-[12]annulene 1a (CTCTCT) to its di-trans isomer 2 (CCCTCT) via such a mechanism has a barrier of 18.0 kcal/mol, in good agreement with earlier experiments. Two other aromatic Möbius bond-shifting transition states were located that result in configuration change for other [12]annulene conformers. This mechanism contrasts sharply with diradical configuration change for acyclic polyenes and with planar bond-shifting mechanisms generally assumed for annulenes. This constitutes evidence that neutral Möbius aromatic annulenes play a role in the dynamic processes of neutral [4n]annulenes.

Computational evaluation of the evidence for tri-trans-[12]annulene.

[reaction: see text] Automerization in tri-trans-[12]annulene (1) was investigated by DFT, MP2, and coupled-cluster methods. Using the highest level of theory employed here, CCSD(T)/cc-pVDZ//BHandHLYP/6-311+G(d,p), we located two low-energy pathways for degenerate conformational change from the lowest-energy conformer of 1 (1a): one with E(a) = 4.5 kcal/mol that interconverts the three inner trans hydrogens with the three outer trans hydrogens and one with E(a) = 2.7 kcal/mol that interconverts the three inner hydrogens with each other. These results are consistent with the experimental results of Oth and co-workers on [12]annulene 1a (Oth, J. F. M.; Röttele, H.; Schröder, G. Tetrahedron Lett. 1970, 61). The conformational exchange of the inner trans hydrogens with the outer ones is predicted to occur via a one-step process involving a C(2)-symmetric transition state and not via the D(3)-symmetric transition state (1b) that was postulated earlier. Conformer 1b was found to be a shallow minimum 6.7 kcal/mol above 1a with a barrier of 0.4 kcal/mol for conversion to 1a. Finally, GIAO-B3LYP/6-311+G(d,p) and BHandHLYP/6-311+G(d,p) computed (1)H NMR chemical shifts of 1a and three other low-lying isomers support Oth's original assignment of observed (1)H NMR peaks to 1a at both low and high temperature.