Potential carbene precursors have been considered for three tetracyclic benzene isomers: carbene 13.. for tetracyclo[3.1.0.01,3.03,5]hexane, 11; carbene 19.. for tetracyclo[3.1.0.01,4.02,3]hexane, 10, considered by Loschmidt as a structure for benzene; and carbene 20.. for Ladenburg's prismane, 4. The bicyclic (CH)6 isomer, 20.., avoids intramolecular cycloaddition; it is converted instead to Dewar benzene, 2; the system 13.. – 11.. is a “foiled” carbene system; finally, we were unable to characterize the putative precursor, 19.., for Loschmidt's hydrocarbon; an attempt to generate it by adding an electron to the corresponding carbene cation, 19.+, resulted in conversion to anti-10. The trajectory of this conversion indicates that “19..” lies at or below the inflection point of the trajectory connecting a putative transition state with anti-10. The free energies of all six species lie in the range 120–165 kcal/mol, well above any species that may be involved in the topomerization of benzene. Copyright © 2014 John Wiley & Sons, Ltd.
