Endo- and Exo-Configured Cyclopropylidenes Incorporated into the Norbornadiene Skeleton: Generation, Rearrangement to Allenes, and the Effect of Remote Substituents on Carbene Stability

For the synthesis of endo-configured cyclopropylidenes annelated to benzonorbornadiene, first the exo-bridge hydrogen in benzonorbornadiene was blocked with ethyl, bromine, and methoxy groups. All efforts to add dichloro-, dibromo-, or fluorobromocarbenes to ethylbenzonorbornadiene failed. However, addition of fluorobromocarbene to bromo- or methoxybenzonorbornadiene gave the corresponding cyclopropane derivatives bearing two halogen atoms, which were submitted to the Doering-Moore-Skattebol reaction. The formed allene intermediates were trapped with furan. The reactivity of the double bonds in substituted benzonorbornadienes was analyzed by determination of the pyramidalization angles. Furthermore, the relative energies of various carbenes and their rearrangement to allene were studied at B3LYP/6-31G(d) level.