Approach to Loschmidt's tetracyclic benzene isomer and related tetra‐ and tricyclic isomers by intramolecular carbene additions

Potential carbene precursors have been considered for three tetracyclic benzene isomers: carbene 13 .. for tetracyclo[3.1.0.0 1,3 .0 3,5 ]hexane, 11 ; carbene 19 .. for tetracyclo[3.1.0.0 1,4 .0 2,3 ]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.