Synthesis of benzonorbornadienes: Regioselective benzyne formation

This report details the synthesis of several benzonorbornadienes by Diels-Alder cycloaddition of cyclopentadiene derivatives with substituted benzyne intermediates, which were generated by low-temperature metal-halogen exchange of halobenzenes. General conditions were developed, allowing synthesis of most benzonorbornadienes described herein at the multigram scale with isolated yield approaching 90% in some cases. Cycloaddition of the benzyne produced by substitution of a c;chlorodifluorobenzene for a bromodifluorobenzene in the metal-halogen exchange reaction unexpectedly gave a different benzonorbornadiene, The benzyne? which resulted by a deprotonation pathway rather than by metal-halogen exchange, formed in a highly regioselective elimination step.