The thermal reaction of cyclopentadiene with ketenes yields in a [2+2]-cycloaddition cyclobutanones rather than Diels-Alder type [4+2]-cycloadducts. Although thermal [2+2]-cycloadditions in which the p-bonds of both reactants approach each other from the same side ("suprafacial", [2p_s + 2p_s]) are symmetry forbidden, the corresponding [2p_s + 2p_a] processes during which one of the p-bonds is attacked from opposite sides ("antarafacial") are thermally allowed. This mechanism is observed for ketenes or other linear molecules for which steric hindrance to such an antarafacial approach is minimal. These reactions are stereospecific and for cyclopentadiene and methylketene only the endo-product is formed. Most remarkably, the larger methyl group (compared to the smaller hydrogen atom on the other side of the ketene) is forced into the more crowded endo-position in a "masochistic steric" process.

This is induced by orbital symmetry (HOMO of the cyclopentadiene and LUMO of the ketene), and the necessity to attack the p-orbitals from opposite sides of the ketene. The animations visualize the molecular rearrangement and the orbital symmetry (basis set of atomic p-orbitals, not molecular orbitals) of this concerted cycloaddition.

Thermal ring opening of cyclobutenes proceed in a stereospecific manner, forcing the groups between which the s-bond is broken into a conrotatory motion The resulting p-orbitals have the symmetry of the highest occupied molecular orbital, HOMO, of the diene. Therefore cis-3,4-dimethyl-cyclobutene produces cis,trans-2,4-hexadiene and vice versa. The Möbius aromatic transition state is characterized by four delocalized p-electrons. The animations on the left visualize the atomic motions during ring opening and closure; the schematic p-orbitals shown are not molecular orbitals but a basis set of p-orbitals of appropriate symmetry (HOMO of the diene, and the lowest unoccupied molecular orbital, LUMO, of the cyclobutene). Data and transition state structures were taken from http://www.bluffton.edu/~bergerd/classes/CEM311/examples/conrotate.html

The thermal ring closure of 1,3,5-hexatrienes yields cyclohexadienes, a process in the course of which the head groups of the triene must undergo disrotatory motion, the same applies to the reverse ring opening reaction. Thus, trans,cis,trans-2,4,6-octatriene produces cis-5,6-dimethyl-1,3-cyclohexadiene only (both terminal methyl groups end up on the same side of the six-membered ring) and vice versa. The animations visualize the disrotatory motion of molecular fragments, which is required by the symmetry of the orbitals (highest occupied molecular orbital, HOMO, of the triene) forming the s-bond between p-orbitals at both ends of the p-system which are in "phase" (see animations of schematic drawings of atomic p-orbitals). Data and transition state structures were taken from http://www.bluffton.edu/~bergerd/classes/CEM311/examples/disrotate.html.

The thermal ring closure reaction of 1,3,5,7-octatetraene, has eight delocalized electrons, and therefore it is another example for a Möbius aromatic transition state (see also the conrotatory ring opening of cyclobutene above); for further details see H. Jiao, P. von R. Schleyer, "Aromaticity of pericyclic reaction transition structures: magnetic evidence.", J. Phys. Org. Chem. 1998, 11, 655-662.

At high temperatures (400-500°C) acetylene trimerizes to yield benzene, although the yield is low and many other aromatic hydrocarbons are formed as side products (in the presence of metal catalysts the trimerization and tetramerization may become the main reaction). Based on the experimental heats of formation of benzene and acetylene, the trimerization of acetylene should be extremely exothermic by about 600kJ/mol, but this thermally allowed reaction has a substantial activation barrier (estimated by computational methods to about 200kJ/mol). This barrier arises from the energy required to distort the three acetylenes to the transition state geometry, as well as electronic contributions dominated by the closed-shell repulsions between filled orbitals. Nevertheless, the corresponding transition state of the concerted cycloaddition is aromatic and of D_3h symmetry. The animations on the left visualize the transition between triple bonds and the aromatic ring system. The schematic orbital drawings are not molecular orbitals, but only atomic p-orbitals of the acetylene units.

For details see e.g. R. D. Bach, G, J. Wolber, H. B. Schlegel, "The origin of barriers to thermally allowed six-electron pericylcic reactions. The effects of HOMO-LUMO interactions on the trimerization of acetylene.", J. Am. Chem. Soc. 1985, 107, 2837-2841; and H. Jiao, P. von R. Schleyer, "Aromaticity of pericyclic reaction transition structures: magnetic evidence.", J. Phys. Org. Chem. 1998, 11, 655-662.