2,5‐dichloropentacyclo[4.3.0.02,5.03,8.04,7]non‐3‐ene: Generation and trapping of a homocubene and ab initio mo calculations on this and related molecules

Reaction of bromotrichlorohomocubane 8 with MeLi in the presence of anthracene, 9,10‐dimethoxyanthracene, 1,2,3‐tri‐methylisoindole, diphenylisobenzofuran, and 2,5‐dimethyl‐furan furnished Diels‐Alder adducts of the title compound 2,5‐dichlorohomocubene 7 in reasonable yields. Adduct 14a was dechlorinated by reduction with Na in tert‐butyl alcohol to afford 14b. This compound could be selectively isomerized by AgBF4 to 20 and by the rhodium(I) complex 22 to a sensitive intermediate which could be reduced to 25. The structures of adduct 15a and of 14b were confirmed by X‐ray analysis, ensuring that bridgehead olefin 7 had been trapped. Ab initio MO calculations show that 7 is lower in energy than 11 and 12. The extension of the MO calculations to all parent homocubenes (2, 3, 26, 27, 28) and to dehydrohomocubanes 6 and 29 was carried out to obtain information on their relative energies, their structures, and their singlet‐triplet energy separation.